Polymer Nanomechanical Cantilever Sensors with Novel Electrical Transduction Schemes for

Bio/chemical Sensing Submitted in partial fulfilment of the requirements

for the degree of

Doctor of Philosophy

by

Seena.V (Roll Number: 06407601)

Supervisors:

Prof. V. Ramgopal Rao

Prof. Soumyo Mukherji

Department of Electrical Engineering

INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY

2011

ii

Dedicated to

My Parents, In-Laws, Husband Dr. Pradeep Kumar, Son Jagdeep and

Brother Sanith

A humble offering at the lotus feet of my AMMA,

Sadguru Sri Mata Amritanandamayi Devi

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Declaration

I declare that this written submission represents my ideas in my own words and where

others’ ideas or words have been included, I have adequately cited and referenced the original

sources. I also declare that I have adhered to all principles of academic honesty and integrity

and have not misrepresented or fabricated or falsified any idea/data/fact/source in my

submission. I understand that any violation of the above will be cause for disciplinary action

by the institute and can evoke penal action from the sources which have thus not been

properly cited or from whom proper permission has not been taken when needed.

_________________________________

Seena.V

Roll No. 06407601

Date: ________________

\

v

Indian Institute of Technology Bombay, India

Certificate of Course Work

This is to certify that Seena.V was admitted to the candidacy of the Ph.D. degree in the

Department of Electrical Engineering after successfully completing all the courses required for the

Ph.D. degree program. The details of the coursework done are given below.

S. No. Course No. Course Name Credits

1 EE 661 Physical Electronics 6

2 EE 669 VLSI Technology 6

3 EE 671 VLSI Design 6

4 EES 801 Seminar 4

5 HS 699 Communication and Presentation Skills 4

6 EE 672 Physics of Transistors 6

7 EE 620 Microelectronics Lab 6

8 BM 658 Biomedical Microsystems 6

9 EE 724 Nanoelectronics 6

10 MM 669 Mechanical Behaviour of Thin Films 6

11 EE 661 Electronic System Design 6

IIT Bombay Dy. Registrar (Academic)

Date:

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Acknowledgments

I am highly indebted to my guide Prof. V. Ramgopal Rao, for his constant and

valuable guidance at every stage of my work. It was through him that I got inducted to the

field of microelectronics. His positive approach in solving problems, and the faith and

confidence he has shown on me, motivated me in pursuing my Ph.D.

I am grateful to my co-guide Prof. Soumyo Mukherji for his support and timely

suggestions in the course of my research work. He has been a constant source of

encouragement throughout my Ph.D. I am extremely thankful to him for the guidance he gave

me in writing thesis.

I owe my sincere gratitude to Prof. Prita Pant for all the guidance and support she gave

me in characterizing the mechanical behavior of films and devices. I also owe my sincere

thanks Prof. Prakash.R.Apte for boosting my interest in systematically analyzing and solving

problems and for the valuable suggestions he gave me during the final stages of my work.

My sincere acknowledgement to Prof. Rudra Pratap, Department of Mechanical

Engineering, Indian Institute of Science, and Bangalore, India for the LDV characterization

experiments on polymer cantilevers and microaccelerometers performed at IISC under his

guidance.

I express my sincere thanks to my Research progress Committee members Prof.

D.Bahadur and Prof. Anilkumar for their acceptance to be in my panel and having spent their

valuable time in reviewing my annual progress reports and for providing valuable inputs.

I take this opportunity to acknowledge the partial funding received from the

Department of Information Technology, Government of India, through the Centre of

Excellence in Nanoelectronics. I sincerely thank Tata Consultancy Services, India for the

fellowship for my doctoral studies at IIT Bombay. I sincerely acknowledge Prof. Dinesh. K.

Sharma for his support through TCS fellowship scheme.

I hereby thank the staff and students of Centre for Excellence in Nanoelectronics for

their constant and efficient services. I thank Prof. Pinto for his support in terms of guidance

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for staff and students of CEN in maintaining the fabrication facility. I wish to acknowledge

the Nanoindenter facility and Central SPM facility of IIT Bombay.

I acknowledge to the project support received from Department of Science and

Technology, Govt. of India for developing the explosive sensors using microcantilevers. The

project helped in bringing focus to my research in developing low cost polymer

microcantilevers for explosive sensing. I thank Dr Pramod Soni from TBRL India for

providing the calibrated TNT vapour generator for the explosive vapour experiments.

I would also like to thank my colleagues Prajakta, Prasenjit, Anukool, Avil,

Ravishankar, Nidhi, Akash, Ramesh, Rohit, Harshil, Bijesh and Nikhil for all kinds of co-

operation, support and discussions during experimental work. I appreciate Prajakta for her

contributions to microaccelerometer developments. My special thanks to Ramesh for all his

help for pentacene depositions using his optimized parameters. I thank Karuna for all her

support during nanoindentation experiments. I take this opportunity to thank my friends,

Naveen, Rajashree and Harshil for proof reading my thesis. I owe my sincere thanks to

Naveen who has been very sincere in helping me by discussing the results. My special thanks

to Shweta Deora who has been a wonderful friend right from beginning of my Ph.D. I would

like to thank Ms. Tanvi Shelatkar, Ms. Arti, Mr. Santosh, Ms. Madhu, Ms. Vaishali and others

in EE Dept., IRCC and administrative staff of IIT Bombay for their timely help in various

official matters.

I am indebted to my in-laws, who came to support me and my husband during really

hard times of our Ph.D. I am ever indebted to my parents, who have always been my source

of motivation for studies throughout my life. I render my appreciation to my mother and

mother in-law who did their best at different times to support me by taking care of my son.

My heartfelt appreciation is extended to my husband, Dr. Pradeep Kumar, who motivated me

to join for Ph.D. and constantly supported me in stressful times during my research work.

With his experiences in doing Ph.D. at IIT Bombay, he could guide me in many aspects

throughout my Ph.D. My son, Master Jagdeep, has been a perennial source of enthusiasm and

cheer at all times through his constant inquiries and innocent playful tricks.

Seena.V

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Abstract

Bio/chemical sensing applications in the areas such as environmental monitoring,

healthcare, biomedical technology, clinical analysis and food processing demand fast, hand-

held, easy to use, inexpensive and highly sensitive methods for detection of very low levels of

chemical or biological substances. The demand for miniaturized high throughput sensors

supported by advancements in micro electro mechanical systems (MEMS) and

nanotechnology had led to the development of simple, versatile and promising class of

sensors known as “nanomechanical cantilever based bio/chemical sensors”.

The nanomechanical cantilever based bio/chemical sensors translate molecular

interactions into nanomechanical motions that can be measured by different external (optical)

and integrated transduction (electrical) techniques. Self-sensing microcantilevers with

integrated electrical transduction mechanism overcome the practical limitation with optical

microcantilevers pertaining to the field deployment of sensors. The conventional

microcantilever sensors are mostly silicon-based with their design and performances limited

by their high stiffness structures. Polymers such as SU-8, on the other hand have a much

lower Young’s modulus compared to silicon based materials which can be exploited for

achieving improvement in sensitivity. The cost of fabrication of polymer devices are known

to be much lower compared to conventional silicon microfabrication processes. This research

work focuses on the development of ultra-sensitive and cost effective polymer

nanomechanical cantilever sensors with novel integrated electrical transduction schemes for

bio/chemical sensing applications.

The first part of this research work aimed at the development of an optimized and

highly sensitive SU-8 nanomechanical cantilever bio/chemical sensor with embedded polymer

nanocomposite of SU-8 and Carbon Black (CB) as the piezoresistive layer. The optimization

targeted improving its electrical, mechanical and transduction characteristics. An optimum

concentration of CB in SU-8 in the range of 8–9 vol. % was arrived at by performing

systematic mechanical and electrical characterizations of the SU-8/CB nanocomposite.

Mechanical characterization of SU-8/CB nanocomposite thin films was performed using the

nanoindentation technique with an appropriate substrate effect analysis. Piezoresistive SU-8

nanocomposite microcantilevers having an optimum CB concentration were fabricated with a

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design aimed at surface stress measurements and reduced fabrication process complexity.

With the optimized material formulations and fabrication processes, stress free SU-8

nanocomposite microcantilevers with an average beam thickness of just 3 μm were achieved

with an improved sensitivity, low device variability and a low noise level. The devices were

characterized for their mechanical, electromechanical and noise performances. The devices

exhibited the highest surface stress sensitivity of 7.6 ppm (mN m−1) −1 among polymer

piezoresistive microcantilevers reported in literature and the noise characterization results

support their suitability for biochemical sensing applications.

A chemical sensing application for polymer nanocomposite microcantilevers for the

detection of explosive vapours has been developed for the first time. These polymer devices

functionalized with 4-MBA were found to be suitable for detection of explosives like TNT

and RDX. The controlled experiments that had been carried out for detection of different

concentrations of TNT vapours verified their potential to detect TNT vapour concentrations

down to less than 6 ppb with an approximate sensitivity value of 1 mV/ppb of TNT. The

sensor was found to be reusable for multiple sensing operations and sensitivity and response

time were found to be adequate for detection of explosive vapours even in ambient.

The final part of this research work introduces an ultra-sensitive SU-8 nanomechanical

cantilever sensor with integrated electrical transduction using a strain sensitive organic field

effect transistor (OFET) inside SU-8 cantilever structure. This novel device was named as

‘Organic CantiFET’. A novel and simple fabrication process was developed that yielded low

cost Organic CantiFET chip. Organic CantiFET devices were characterized to evaluate their

mechanical, electrical, noise and electromechanical performances and thereby benchmark

them with other available microcantilever sensors. These devices exhibit higher deflection

and surface stress sensitivities (15.6 ppm/nm and 400 ppm compared to that of SU-8

nanocomposite cantilevers that has been presented in the initial parts of this research work.

The characterization results proved their candidature as efficient and cost effective

biochemical sensors having a minimum detectable surface stress in the range of 0.18 mN/m.

Key words: polymer microcantilever, polymer nanocomposite,SU-8 nanocomposite, SU-

8/CB nanocomposite, nanoindentation, Young’s modulus, spring constant, SU-8, carbon

black, explosive detection, OFET, pentacene, nanomechanical cantilever, Organic CantiFET,

SU-8 dielectric

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Table of Contents

1.1 Introduction to the topic ..................................................................................22

1.2 Motivation .........................................................................................................3

1.3 Microcantilevers as bio/chemical sensors-Literature review............................5

1.3.1 Sensing modes and principles.............................................................5

1.3.2 Microcantilever materials .................................................................10

1.3.3 Transduction principles ....................................................................11

1.3.4 Polymeric piezoresistive microcantilevers .......................................18

1.3.5 Applications of microcantilever as bio/chemical sensors.................21

1.3.6 Microcantilevers for detection of explosive vapours........................23

1.4 Objectives and scope of the work ...................................................................25

1.5 Thesis Organization ........................................................................................28

2.1 Introduction .....................................................................................................30

2.2 SU-8 material properties and processing ........................................................31

2.3 Single layer SU-8 microcantilevers for optical transduction ..........................34

2.3.1 Design of single layer SU-8 microcantilevers ..................................34

2.3.2 Fabrication process development for SU-8 microcantilevers...........37

2.3.3 Challenges in fabrication ..................................................................41

2.3.4 Optimized fabrication process for SU-8 microcantilevers ...............43

2.3.5 Characterization ................................................................................45

2.3.6 Application in explosive vapour detection .......................................47

Acknowledgments ................................................................................ vi

Abstract.............................................................................................. viii

List of figures ..................................................................................... xiii

List of tables ........................................................................................xxiChapter 1 Introduction and Literature Review ................................. 22

Chapter 2 SU-8 Microcantilevers for Optical Transduction.............. 30

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2.4 Summary .........................................................................................................51

3.1 Introduction .....................................................................................................53

3.2 SU-8/CB nanocomposite thin film preparation and patterning ......................54

3.3 Electrical characterization: Percolation study on SU-8/CB nanocomposite...60

3.4 Development of SU-8/CB nanocomposite microcantilevers ..........................62

3.4.1 Fabrication process ...........................................................................64

3.4.2 Characterization ................................................................................68

3.4.3 Shortcomings and scope for improvement .......................................69

3.5 Optimization of SU-8/CB nanocomposite microcantilevers ..........................70

3.5.1 Characterization of dispersion of Carbon black in SU-8 ..................70

3.5.2 Nanoindentation for mechanical characterization of SU-8/CB

nanocomposites.................................................................................73

3.5.3 Electrical characterization of SU-8/CB nanocomposite:

Conduction mechanism and temperature dependence......................78

3.5.4 Design and fabrication of SU-8/CB nanocomposite

microcantilevers with improved sensor performance .......................82

3.5.5 Mechanical characterization: Spring constant and resonant

frequency measurements ..................................................................88

3.5.6 Electromechanical characterization: Sensitivity ...............................92

3.5.7 Noise characterization ......................................................................93

3.6 Summary .........................................................................................................96

4.1 Introduction .....................................................................................................98

4.2 Explosive Vapour Experiments ......................................................................99

Chapter 3 Polymer Nanocomposite Microcantilever Sensor with Integrated Electrical Transduction ..................................................... 53

Chapter 4 Detection of Explosive Vapours using Polymer

Nanocomposite Microcantilevers ........................................................ 98

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4.2.1 Controlled experiments with a flow cell on PCB ...........................105

4.3 Effect of humidity on microcantilever response ...........................................107

4.4 Summary .......................................................................................................110

5.1 Introduction ...................................................................................................112

5.2 Organic CantiFET device design and development......................................114

5.2.1 Material selection............................................................................114

5.2.2 Organic CantiFET device designs ..................................................122

5.2.3 Process integration for organic CantiFETs .....................................123

5.3 Characterization of Organic CantiFET .........................................................128

5.3.1 Electrical characterization ..............................................................129

5.3.2 Mechanical characterization ...........................................................129

5.3.3 Electromechanical characterization ................................................132

5.3.4 1/f noise characterization ................................................................136

5.4 Summary .......................................................................................................139

Chapter 5 Polymer nanomechanical sensor with integrated OFET for electrical transduction....................................................................... 112

Chapter 6 Conclusion and future recommendations ....................... 140

Appendix A ....................................................................................... 145

Appendix B ....................................................................................... 147

Appendix C ....................................................................................... 152

Appendix D ....................................................................................... 158

Appendix E ....................................................................................... 162

References164

List of publications............................................................................ 175

List of figures

Figure 1.1 Schematic representation of selectivity in bio/chemical sensors..................... 2

Figure 1.2 Principle of microcantilever based bio/chemical sensor.................................. 3

Figure 1.3 Microcantilever operation modes [12]............................................................. 6

Figure 1.4 Lateral view of a beam subjected to surface stress changes of Δσ1 and Δσ2 leading to a bending with constant radius of curvature of R. The dotted line corresponds to the neutral axis [14] ...................................................................... 7

Figure 1.5 Schematic depiction of (a) chemisorption of straight-chain thiol molecules on a gold coated cantilever. (b) analyte-induced cantilever deformation due to the swelling of thick analyte permeable coating during analyte interaction. (c) analyte- induced cantilever deformation in the case of a nanostructured surface coating [15]. .......................................................................................................... 8

Figure 1.6 Schematic of (A)"optical lever" readout that is commonly used to measure the deflection of microcantilever probes in standard AFM [8] (B) interferometric readout [39] ......................................................................................................... 12

Figure 1.7 (A) A schematic of a simplified piezoresistive microcantilever model consisting of single layer of structural material with thickness h and a piezoresistor with thickness → 0. (B) Schematic of the microcantilever in Wheatstone bridge configuration. ....................................................................... 15

Figure 1.8 (A) Schematic of the interaction between probe and target molecules on an embedded MOSFET microcantilever [50] .......................................................... 16

Figure 1.9 SU-8 microcantilever with Au as strain gauge (A) Schematic of the device chip (B) Optical micrograph of the microcantilever with integrated meander-type Au resistor [34]............................................................................................ 18

Figure 1.10 Optical image of a micromachined SU-8 cantilever with integrated SU-8/carbon piezoresistor (Length = 200 m, Width = 200 m and thickness = 7 m) [57] .............................................................................................................. 19

Figure 2.1 Typical photolithographic processing sequence for microfabrication of SU-8 structures. (1) Dehydration baking step for removing the moisture on the sample surface (2) SU-8 resist is spun onto a sample (3) Prebake on a hotplate for evaporating the solvent. (4) UV-light exposure through a mask. (5) Post

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exposure bake for completion of crosslinking process (6) Development in SU-8 developer to remove the non-cross- linked areas in SU-8 ................................... 33

Figure 2.2 Resonant frequency and spring constant as function of length of the microcantilever .................................................................................................... 36

Figure 2.3 Schematic of SU-8 microcantilever die for optical transduction (A) Single cantilever die (B) Schematic of the cantilever die attached to the SU-8 frame. . 37

Figure 2.4 Schematic of fabrication process flow for SU-8 microcantilevers (A) Sacrificial layer (B) Structural layer SU-8 for the microcantilever (C) Thick SU-8 for anchor and frames of microcantilever die (D) Release of the microcantilever devices attached to frame. ......................................................... 37

Figure 2.5 (A) SU-8 patterns on substrate indicating the stress lines (B ) and (C) SEM micrograph of the fabricated SU-8 microcantilever after release from the substrate............................................................................................................... 41

Figure 2.6(A)160 µm thick SU-8 structure before hard bake. Stress indicated in corners (B) The sample after hard bake. Stress lines got vanished after hard bake......... 44

Figure 2.7 SU-8 microcantilevers fabricated after optimization process (A) SU-8 frames holding arrays of SU-8 microcantilevers. These devices were coated with gold. (B) Optical micrograph of one of the microcantilevers (C) SEM micrograph indicating the stress free nature of the microcantilevers ..................................... 45

Figure 2.8 Frequency response of gold coated SU-8 microcantilever ............................ 46

Figure 2.9 Simulated data of resonant frequency with scaling of length of the SU-8 microcantilever coated with Cr/Au. Inset : Mesh model for the microcantilever structure. .............................................................................................................. 47

Figure 2.10 Experimental setup for explosive vapour experiment (A) Schematic of the setup (B) Explosive vapour generator ................................................................. 49

Figure 2.11 Response of the microcantilever to TNT vapour stream generated at 65oC with a flow rate of 30 SCCM .............................................................................. 50

Figure 2.12 (A) Conceptual diagram of microcantilever coated with benzene thiol (eg. 4-MBA) receptors on the gold side and undergoing compressive stress and bending due to interaction with aromatic gas (B) Interaction between DNT and 4 MBA leading to compressive stress in the gold film (C) interaction between DNT and 4-MBA at SAM grain boundaries leading to compressive strain in gold film [97] ...................................................................................................... 51

Figure 3.1 (A) Optical micrograph of SU-8/CB nanocomposite with lot of CB clusters on a pattern of Au on silicon dioxide. (B) SEM micrograph showing a CB cluster in SU-8..................................................................................................... 56

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Figure 3.2 Optical micrograph of photo-lithographically patterened SU-8/CB with 5 CB vol. % (A) before and (B) after ultra-sonication clean step in IPA..................... 57

Figure 3.3 SU-8/CB (8.4 vol%) strips patterned using different UV exposure doses (A) 204 mJ/cm2 (B) 408 mJ/cm2 (C) 530 mJ/cm2 (D) SEM image of SU-8/CB strip with 530 mJ/cm2. .......................................... 57

Figure 3.4 Microfabrication processing steps for SU-8/CB nanocomposite .................. 58

Figure 3.5 Thickness and RMS roughness as a function of CB concentration ............... 58

Figure 3.6 (A & B) AFM topographical images of SU-8/CB Composite films (A) 6 vol% (B) 7.4 vol% (C) SEM micrograph of SU-8/CB composite with 8 vol.%59

Figure 3.7 Percolation theory applied to conductive composites. The formation of the first complete particle linkage occurs at Vc that resulting in a sharp decrease in resistivity [101] ................................................................................................... 60

Figure 3.8 Percolation characteristics of SU-8/CB composite........................................ 61

Figure 3.9 Optical micrograph (20X) of SU-8/CB film on SU-8 for different CB concentration showing the density of conductive filler network. ....................... 62

Figure 3.10 Schematic of SU-8/CB nanocomposite microcantilever die ....................... 63

Figure 3.11 Fabrication process flow for SU-8/CB nanocomposite microcantilevers (1) Sacrificial or release layer (2) First layer of SU-8 (3) Cr/Au for contact pads (4) Ti/Au contact wire (5)SU-8/CB nanocomposite layer (6) encapsulating SU-8 (7) Thick SU-8 die base (8) Release of cantilever die from the substrate ....... 66

Figure 3.12 Photographs and micrographs of fabricated polymer nanocomposite microcantilever devices at different stages of fabrication (A) Processed 2" silicon wafer after final lithography step (B) Released devices soaked in IPA after etching of Cr layer on contact pads (C) Released and dried SU-8 frames holding the microcantilever devices (D and E) SEM and optical micrograph of on of the microcantilever die............................................................................... 67

Figure 3.13 Electromechanical characterization plots of two SU-8 nanocomposite microcantilevers with different resistances ......................................................... 68

Figure 3.14 Dispersion characterization results from DLS (A & B) Size distribution of CB for samples prepared with energy of sonication of 3kJ and 0.6 kJ. (C) Mean diameter of carbon black as a function of duration of sonication. ...................... 71

Figure 3.15 . SEM mage of SU-8/CB Composite films prepared with sonication energy of (A) 0.6 kJ (B) 3 kJ in pulse ............................................................................. 72

Figure 3.16 SEM micrograph of SU-8/CB nanocomposite pattern with reduced line edge roughness .................................................................................................... 72

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Figure 3.17 Typical load vs. depth of indentation plot obtained in nanoindentation experiment ........................................................................................................... 73

Figure 3.18 Load versus indentation depth for different loads. Inset: Scanned image of a set of indents in SU-8 .......................................................................................... 75

Figure 3.19 Young’s modulus and hardness of SU-8 as a function of indentation depth. The discontinuous line indicates the indentation depth of 10% of film thickness ............................................................................................................................. 76

Figure 3.20 (A) Young’s modulus of SU-8 as a function of indentation depth after modified King’s analysis. (B)Young’s Modulus of SU-8/CB composite as a function of CB Vol %. Here all the samples were indented with Pmax = 450 N ............................................................................................................................. 77

Figure 3.21 (A) I-V characteristics for SU-8/CB composites with 4.9 CB vol.% and 9.4 CB vol.% along with the theoretical curve fit (B) Value of power term ‘n’ and exponential term ‘B’ for different CB resistors .................................................. 78

Figure 3.22 Electrical characterization of nanocomposite resistors showing the variability in resistance as a function of CB concentration................................. 80

Figure 3.23 Temperature dependent resistance of SU-8/CB composites with CB concentration of (A) 6 vol. % (B) 7.8 vol. % ...................................................... 81

Figure 3.24 (A) Contact current mechanism in polymer PTC composite (B) Tunneling current mechanism in polymer [102] .................................................................. 81

Figure 3.25 Device design schematic. (A) Planar schematic (B).Cross sectional schematic of SU-8 cantilever with embedded SU-8/CB composite. .................. 83

Figure 3.26 Fabrication process flow (1) First layer of SU-8 (2) Cr/Au for contacts (3) SU-8/CB composite layer (4) encapsulating SU-8 (5) Thick SU-8 die base (6) Release of cantilever die from the substrate........................................................ 84

Figure 3.27 SU-8/CB resistor patterns on “V” shaped and “U” shaped cantilever areas after lithography process 4 .................................................................................. 85

Figure 3.28 (A) Photograph of the processed wafer after the final lithographic step. Arrays of polymer devices attached to the dummy substrate just before the release can be seen (B) optical micrograph of one of the devices on the wafer. (C) Arrays of polymer nanocomposite microcantilever device chips after release process (D) SEM image of one of the device chips containing 4 cantilevers. .... 86

Figure 3.29 Modified fabrication process flow (1) Sacrificial layer (2) First layer of SU-8. But not developed (3) SU-8/CB composite layer before development(4)SU-8/CB nanocomposite layer and SU-8 layer 1 after development with sonication

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(5) Cr/Au for contacts (6) encapsulating SU-8 (7) Thick SU-8 die base (8) Release of cantilever die from the substrate........................................................ 87

Figure 3.30 Load and unload segment of indentation on cantilever ............................... 89

Figure 3.31 Load displacement characteristics of SU-8 nanocomposite microcantilever obtained from nanoindenter. Insets: (1)Schematic of measurement.(2) Optical image indicating the place of indentation on the SU-8 nanocomposite microcantilever .................................................................................................... 90

Figure 3.32 Frequency plot from Laser Doppler Vibrometer (A) “U” shaped cantilever(B) “V” shaped cantilever ................................................................... 91

Figure 3.33 Electromechanical characterization plot. Inset : I-V characteristics of the polymer nanocomposite microcantilever with different bending conditions for a voltage span of 100 mV (200 data points). ......................................................... 92

Figure 3.34 Noise measurement scheme for polymer nanocomposite microcantilever . 94

Figure 3.35 (A) Noise spectral density of SU-8/CB nanocomposite microcantilevers with 2 different concentrations (B) Noise spectral density at different bias voltage for devices with 8.4 CB vol%................................................................. 94

Figure 4.1 (A) Wheatstone bridge circuit indicating the microcantilever positions (B) Block diagram schematic of ADS1232REF board to which the bridge output is fed ...................................................................................................................... 100

Figure 4.2 Microcantilever response recorded during deflection using calibrated micromanipulator .............................................................................................. 101

Figure 4.3 Schematic of the experimental set up for explosive vapour experiments.... 101

Figure 4.4 (A & B) PTFE gas flow cell (FC) incubating the microcantilever PCB that is connected to DC bridge circuit, all inside a shielded enclosure. ADS1232 REF circuit from TI can be seen here (C & D) Microcantilever PCB and a microcantilever die (E) Complete experimental setup. VG: Vapour generator ; FC : Flow cell .................................................................................................... 102

Figure 4.5 Calibration of TNT vapour generator at different temperature ................... 103

Figure 4.6 Response of a 4-MBA coated polymer nanocomposite microcantilever for consecutive cycles of TNT and nitrogen ........................................................... 104

Figure 4.7 Responses of 4-MBA coated Microcantilever to RDX ............................... 105

Figure 4.8. Microcantilever PCB and 10 mm diameter PTFE gas flow cell arrangement ........................................................................................................................... 106

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Figure 4.9 Microcantilever response to different flow rates of nitrogen ...................... 106

Figure 4.10 . (A) Response of a 4-MBA coated polymer nanocomposite microcantilever to different concentrations of TNT vapour in nitrogen (B) TNT vapour detection sensitivity plot. .................................................................................................. 107

Figure 4.11 Schematic of experimental setup for studying the response of microcantilevers to humidity............................................................................. 108

Figure 4.12 Response of 4-MBA coated Microcantilever to humidity......................... 108

Figure 4.13 Response of 4-MBAcoated polymer microcantilever to TNT and RDX vapours in ambient conditions........................................................................... 110

Figure 5.1 Schematic of the concept of an ‘Organic CantiFET’ device ....................... 114

Figure 5.2 (A) Pentacene molecule (B) P-type OFET schematic (C) Illustration of working principle of an OFET with respect to applied gate bias, VGS. ............ 115

Figure 5.3 Schematic of fabrication process for pentacene OFET with SU-8 as dielectric. (A) RCA cleaning of n+ silicon wafer (B) SU-8 spin coating and processing for gate dielectric (C) Cr/Au deposition and patterning for source and drain electrodes (D) Pentacene.......................................................................... 118

Figure 5.4 I-V characteristics of OFETs with SU-8 as (A) Transfer characteristics of OFETs with different dielectric thicknesses along with gate leakage (B) Output characteristics of OFET #1................................................................................ 120

Figure 5.5 Optical microscopic image of SU-8 films (A) SU-8 450 nm film on silicon dioxide. Lot of pin holes being observed. (B) SU-8 950 nm film on silicon dioxide indicating better quality compared to SU-8 450 nm ............................ 121

Figure 5.6 Pentacene on dielectric surfaces (A) SEM micrograph showing pentacene grain boundaries on SU-8. (B & C) AFM topographical image of pentacene on SU-8 and silicon dioxide indicating better grain size for the case of silicon dioxide dielectric ............................................................................................... 122

Figure 5.7 (A& B) Schematic (not to scale) of organic CantiFET (A) planar schematic of the device chip. S, D and G are the source, drain and gate contacts (B) cross section (position of cross section indicated using dotted line) of the device illustrating an SU-8 cantilever with integrated pentacene OFET. (C1-3) Different CantiFET device designs. (1) Strain and current directions are parallel ie longitudinal case (2) Strain and current directions are perpendicular ie transverse case (3) Simple comb like inter-digitated structure which contains the current components parallel and perpendicular to strain .................................. 123

Figure 5.8 Schematic of fabrication process for polymer CantiFET (1) Sacrificial layer (2)First layer of SU-8 defining the cantilever and contact vias.(3) Au electrode patterning for gate of transistor (4) SU-8 as gate dielectric (5) Au electrodes

xix

defining the source and drain of OFET (6) Thick SU-8 layer defined for anchor or chip of the cantiFET device. A photograph of the processed silicon wafer after this final lithography step is on right side (7) Release of the CantiFET device from substrate and pentacene deposition (8) final device structure showing cantilevers , source drain and gate contacts through vias ................... 125

Figure 5.9 (A) Photographs of released arrays of organic CantiFETs (B) SEM micrograph of the fabricated CantiFET device (C) Bottom and enlarged view of cantilever portion of the CantiFET from SEM showing the inter digitated source drain electrode configuration (Type 1 & 3 CantiFET) (D) Top and enlarged view of cantilever portion of the CantiFET from SEM ............................................. 128

Figure 5.10 I-V characteristics of organic CantiFETs after silicon nitride encapsulation. (A) Transfer characteristics indicating good ION/IOFF ratio (2.2 x 103). Saturation field effect mobility and threshold voltage extracted from |IDS|1/2 vs. VGS characteristics as shown were 2.9 x 10-4 cm2/Vs and -11.5 V. (B) Output characteristics of silicon nitride encapsulated organic CantiFETs.................... 130

Figure 5.11 Load segment of beam bending experiment on organic CantiFET using nanoindenter ...................................................................................................... 130

Figure 5.12 Load displacement characteristics of Organic CantiFET. Inset showing the optical micrograph of the image indicating the location of indenter placement. ........................................................................................................................... 131

Figure 5.13 I-V characteristics of organic CantiFET obtained from electromechanical characterization. (A) Transfer characteristics of an organic CantiFET (CantiFET # 3) under different amount of bending. (B) IDS-VDS characteristics (@VDS = -40 V) under different amounts of bending ........................................................ 132

Figure 5.14 Percentage change in drain current, saturation field effect mobility and threshold voltage of the organic CantiFET # 3 as a function of percentage strain ........................................................................................................................... 134

Figure 5.15 Percentage change in drain of as a function of percentage strain (A) CantiFET # 1 The average value of Idsat at strain %= 0 was 11 nA (B) CantiFET # 2. The average value of Idsat at strain %= 0 was 6 nA .................................... 135

Figure 5.16 Mean and standard deviation of percentage change in drain current as a function of percentage strain for 5 cantiFET devices. ...................................... 136

Figure 5.17(A) Block diagram schematic of the setup used for measuring the 1/f noise. Device under test (DUT) is the cantiFET device. (B) 1/f like noise characteristics of Organic CantiFET. ................................................................ 137

Figure A1.1 (a) Case 1 considered in King’s analysis. The schematic of flat punch indenter indenting a film of thickness t. (b) Case 2 considered in modified King’s analysis. The schematic of Berkovich indenter indenting a film of thickness t to a depth of h [109]. ....................................................................... 145

xx

Figure A1.2 Plot of α as a function of normalized punch size [109]. ........................... 146

Figure A2.1 Schematic of polymer MEMS accelerometer (A) Planar schematic of SU-8 accelerometer with embedded SU-8/CB composite. (B) Cross sectional schematic of one of the legs of beams indicating individual layer thicknesses 148

Figure A2.2. Schematic of fabrication process for polymer composite MEMS accelerometer .................................................................................................... 149

Figure A2.3SEM micrographs of the fabricated SU-8 nanocomposite MEMS accelerometer .................................................................................................... 149

Figure A2.4Resonance frequency plot for polymer composite MEMS accelerometer 150

Figure A2.5 Microaccelerometer proof mass displacement vs. acceleration................ 150

Figure A2.6 Static electromechanical characterization of SU-8 nanocomposite beam connected in Wheatstone bridge. Refer Chapter 4 for measurement scheme. .. 151

List of tables

Table 1-1 Comparison of main classes of transduction schemes.................................... 17

Table 1-2 Applications of microcantilever-based sensors [77]....................................... 22

Table 1-3 Vapour pressures and molecular weights of some explosives [24] ................ 24

Table 2-1 SU-8 material properties ................................................................................. 36

Table 2-2 SU-8 microcantilever specifications ............................................................... 36

Table 2-3 Optimized process parameters for SU-8 microcantilevers ............................. 44

Table 3-1 Dimensional parameters of polymer nanocomposite microcantilever ........... 64

Table 3-2 Details of polymer nanocomposite thin film samples prepared for nanoindentation experiments............................................................................... 74

Table 3-3 Summary of performances of SU-8 /CB nanocomposite microcantilevers compared with other existing polymer microcantilever sensors ......................... 95

Table 5-1 Layer details of OFETs fabricated for performance evaluation of pentacene OFETs with SU-8 dielectric. ............................................................................. 119

Table 5-2 Specifications of OFETs with SU-8 as dielectric ......................................... 119

Table 5-3 Specifications of organic CantiFET device chips......................................... 128

Table 5-4 Comparison of performances of organic CantiFET and other nanomechanical cantilever sensors .............................................................................................. 138

Table 6-1Performance of the developed nanomechanical cantilever sensors compared to existing polymer nanomechanical cantilever sensors ....................................... 143

Table A3-1 Process details of different SU-8 layers in SU-8 nanocomposite microcantilevers ................................................................................................ 154

Table A3-2 Process details of different SU-8 layers in optimized SU-8 nanocomposite microcantilevers ................................................................................................ 155

Table A3-3 Process details of different SU-8 layers in Organic CantiFET .................. 156

xxii

Chapter 1

Introduction and Literature Review

1.1 Introduction to the topic

A sensor may be defined as a device that converts a nonelectrical, physical or chemical input

into an electrical output signal. According to Middlehoek, sensors can be classified according

to the energy domain of its primary input/output as electrical, thermal, radiation, mechanical,

magnetic and bio/chemical sensors [1]. The evolution of today’s microsensors with at least

one physical dimension at the sub-millimetre level happened due to the revolutions in the

field of microelectronics. The well-established integrated circuit industry played an

indispensable role in fostering an environment suitable for the development of microsystems

known as microelectromechanical systems (MEMS) [2]. MEMS sensors are used in various

industrial, consumer, defence and biomedical applications. Micro accelerometers, pressure

sensors and microarrays are some of the commercially available MEMS sensors. MEMS

being a technology derived from microelectronics, these miniature MEMS sensors hold

advantages such as, low cost of production due to very large production volume and fewer

materials, easy integration with required instrumentation on microelectronics chips, arraying

capability enabling multiplexed measurements, greater portability, robustness and low power

consumption. Many tools used in the design and manufacturing of MEMS devices are

borrowed from the conventional IC industry. Hence silicon is considered to be the primary

2

material even in MEMS though MEMS systems using other materials like polymers, metals

and ceramics have been demonstrated.

There have always been demands for the detection of very low levels of a large number of

chemical and biological substances in application areas such as environmental monitoring,

healthcare, biomedical technology, clinical analysis and food processing. For example in the

case of homeland security applications, recent increase in security concerns in public places

like airports and public transports have increased the demand for low-cost portable, efficient,

and easy to use explosive sensing technology. The emergence of MEMS and nanotechnology

along with these demands enabled the development of a class of microsensor systems called

as microfabricated bio/chemical sensors. Bio/chemical sensors combine a bio/chemical

recognition element coupled to a physical transducer [3]. A biological or chemical

recognition element recognizes a specific target analyte and does not recognize other analytes,

which imparts selectivity to the sensor (Figure 1.1). The transducer translates the

bio/chemical-recognition event into measurable quantities such as change in electrical signal,

an optical emission, a mechanical motion etc. In the case of biosensors, the recognition

element may be an enzyme, antibody, antigen, living cells, tissues, etc. and in the case of

chemical sensors, these can be any chemical substance specific for the target analyte.

Figure 1.1 Schematic representation of selectivity in bio/chemical sensors

A class of MEMS sensors known as microcantilever sensors (nanomechanical cantilever

sensors) came into existence as atomic force microscopy (AFM) probes [4] and they have a

huge potential as platforms for the development of many physical [5], chemical [6], and

biological sensors [7-9].

3

1.2 Motivation

Development of bio/chemical sensing application using microcantilevers for a wide variety of

applications opens challenging and interesting research problems as these have a definite edge

over other sensors owing to its versatility, sensitivity, scope for miniaturization and

parallelization and low cost. Microcantilever based bio/chemical sensors work on the

principle of conversion of the bio/chemical recognition event into nanomechanical motion

[10]. The cause for nanomechanical motion can be due the free energy change on the surface

of the sensor due to the reaction of the target analyte with the receptor molecule or due to the

mass change on addition of the target analyte bound to the microcantilever surface (Figure

1.2).

Figure 1.2 Principle of microcantilever based bio/chemical sensor

These nanomechanical cantilever sensors offer many orders of magnitude higher sensitivity in

comparison to other commonly used bio/chemical sensors such as quartz crystal

microbalances (QCM), flexural plate wave oscillators (FPW), and surface acoustic wave

devices (SAW) [11]. The distinct advantages of the nanomechanical cantilever sensors that

support their candidature for bio/chemical sensing applications can be enlisted as given

below:-

Smaller size with surface area of the order of 10-5 cm2, orders of magnitude smaller

compared to the QCM and SAW devices [11].

Miniaturization and mass production at a relatively low cost using standard

semiconductor manufacturing processes.

4

Several modes of operation (static mode used for measuring surface stress change,

resonant mode used for detecting mass loading, heat mode etc.) in comparison to

single mode of operation (gravimetric) for other sensors.

Large surface to volume ratio: - Hence the change in the Gibbs surface free energy

induced by surface-analyte interactions lead to large surface forces. Mass loading also

leads to larger amplitude of displacement or larger changes in resonant frequencies in

comparison to other sensors. Microcantilevers possess superior sensitivities for

detection of many chemical and biological analytes. This high sensitivity is attributed

to large surface to volume ratio and ability to detect cantilever motion with sub-

nanometre precision

Bio/chemical selectivity: - Selectivity can be imparted to microcantilever sensors

using biological or chemically selective layers like in the cases of SAW and QCM

based sensors.

Improved dynamic response.

Label free detection, an important characteristics in the case of biosensors.

Increased reliability and precision compared to other conventional sensors.

Feasibility for fabrication of multi-element sensors arrays supporting high degree of

parallelization

Ease of integration of microcantilever sensor with on-chip electronic circuitry.

Considering the demand for development of ultra-sensitive, versatile and cost effective sensor

platforms along with the above mentioned advantages of microcantilever sensors, a detailed

literature survey has been carried out in different aspects of research in microcantilever based

bio/chemical sensors. Objectives and scope of this thesis were formulated based on the

literature review being presented in the next section.

5

1.3 Microcantilevers as bio/chemical sensors-Literature review

Different research aspects in the development of microcantilevers based bio/chemical sensors

are covered in this literature survey. The key aspects include the different sensing modes of

operation of microcantilevers, various structural materials for microcantilevers, transduction

schemes that are used to measure and convert the changes in mechanical parameters of the

microcantilevers to useful signals and an overview of different applications of

microcantilevers in the field of bio/chemical sensing. Advancements in one of the chemical

sensing applications, i.e., microcantilever for sensing explosive vapours is covered as a

separate subsection, as this is known to be a challenging and an important application and is

also one of the on-going research topics of interest in this field.

1.3.1 Sensing modes and principles

The sensing operation modes of microcantilevers could be broadly classified based on their

principles in translating the recognition event into nanomechanical motion. Typically there

are three modes of operation such as static mode, dynamic mode and heat mode [12] . In static

mode, the bending of the microcantilever upon the molecular adsorption is measured. In

dynamic mode, the dependence of resonant frequency of the microcantilever on the mass of

the microcantilever is exploited. The heat mode, takes advantage of the bimetallic or bimorph

effect that leads to a bending of a biomaterial microcantilever with change in temperature.

Figure 1.3 pictorially depicts these modes of operation in different scenarios. It would be

appropriate to discuss the details of these modes of operation [12].

(1) Static mode

The static deflection of a microcantilever is related to the difference in surface stress of the

two faces of microcantilever. When one of the surfaces of the microcantilever is

functionalized with a receptor layer, the adsorption or binding of target molecule or material

to the receptor layer leads to a differential surface stress between top and bottom of the

microcantilever. Uniform compressive or tensile stress acting on an isotropic material tends to

increase or decrease the surface area.

6

Figure 1.3 Microcantilever operation modes [12]

(a) Static bending of a microcantilever on adsorption of a molecular layer. (b) Diffusion of molecules into polymer layer leads to swelling of the polymer and hence to the bending of the cantilever. (c) Highly specific molecular recognition of biomolecules by receptors changes the surface stress on the upper surface of the microcantilever that leads to bending of the microcantilever. (d) Oscillation of a microcantilever at its resonance frequency (dynamic mode) allows information on mass changes taking place on the cantilever surface to be obtained (application as microbalance). (e) Changing the temperature while a sample is attached to the apex of the cantilever allows information to be gathered on decomposition or oxidation process. (f)Dynamic mode measurements in liquids yield details on mass changes during biochemical processes . (g) In the heat mode, a bimetallic cantilever is employed. Here bending is due to the difference in the thermal expansion coefficients of two materials. (h) A bimetallic microcantilever with catalytically active surface bends due to heat production during a catalytic reaction. (i) A tiny sample attached to the apex of the cantilever is investigated, taking advantage of the bimetallic effect. Tracking the deflection as a function of temperature allows observation of phase transitions in the sample in a calorimetric mode [12].

In the case of a differential surface stress between top and bottom surface of the

microcantilever, the microcantilever will bend with a constant radius of curvature as depicted

in Figure 1.4. A plane in the microcantilever which is not deformed is known as neutral plane.

The radius curvature is related to the differential surface stress as given in equation 1.1 which

was developed by Stoney in 1909 for measuring the surface stress of a thin film deposited on

to a sheet of metal [13].

1푅≅ 6

1− 휈퐸푡

(Δ휎 − Δ휎 ) (1.1)

7

Where R radius curvature, E: Young’s modulus, t: film thickness, ν: Poisson’s ratio,

(Δσ1= Δσ2): differential surface stress.

Figure 1.4 Lateral view of a beam subjected to surface stress changes of Δσ1 and Δσ2 leading to a bending with constant radius of curvature of R. The dotted line corresponds to the neutral

axis [14]

Stoney’s equation was further modified for the case of simply supported rectangular

microcantilever beams. The modified equation related the differential surface stress to the end

point displacement of the microcantilever as given in equation 1.2 [15]. Here L is the length

of the microcantilever, and Z is the endpoint displacement of the microcantilever.

∆푧 ≅

퐿2푅

=3(1− 휈)퐿

퐸푡(Δ휎 − Δ휎 ) (1.2)

The deflection sensitivity can be improved by decreasing the thickness or by increasing the

length of the microcantilever or by using microcantilever structural material with low

Young’s modulus.

One could typically categorise the origin of adsorption induced surface stress change on the

microcantilevers functionalised with different coatings using three different models [15]. The

first model considers the interactions between the cantilever coatings and the environment

containing the target species to be predominantly a surface phenomenon. Adsorption of

analyte species on microcantilever surface may involve physisorption which is a weak

8

bonding with binding energy less than 0.1 eV or chemisorption which is a stronger bonding

with typical binding energy more than 0.3 eV [15]. The surface stress changes typically can

be attributed to the Gibbs free energy changes associated with the adsorption process. An

example of a chemisorption process of thiol molecules on gold coated microcantilever leading

to the bending of the microcantilever is illustrated in Figure 1.3 (a) and Figure 1.5 (a). It has

been reported that this model can be well correlated to the experimental observation of

compressive stress on gold side of gold coated microcantilevers exposed to alkanethiols [14,

15]. It is in general applicable to the microcantilevers for sensing bio-molecular and chemical

interactions like antibody-antigen reaction in point of care diagnostics [7,18], DNA

hybridization [19,20], explosive vapour detection [21-25] etc.

The second model considers the interaction of target analyte with the cantilever coating to be

a bulk phenomenon. This is applicable to microcantilevers modified with a thick analyte

permeable coating. In this case, the predominant mechanism for microcantilever bending can

be the analyte induced swelling of the coating on the microcantilever (Figure 1.3 (b) and

Figure 1.5 (b)).

Figure 1.5 Schematic depiction of (a) chemisorption of straight-chain thiol molecules on a

gold coated cantilever. (b) analyte-induced cantilever deformation due to the swelling of thick analyte permeable coating during analyte interaction. (c) analyte- induced cantilever

deformation in the case of a nanostructured surface coating [15].

The third model considers the interaction of the target analyte with cantilever coatings that are

modified as nanostructures. This model is appropriate for nanostructured interfaces and

coatings such as surface immobilized colloids. The nanostructured surfaces of

9

microcantilevers facilitate an efficient conversion of the energy of target-receptor interactions

which is a combination of bulk, surface, and inter-surface interactions to mechanical energy.

Nanostructured coatings substantially increase the number of binding sites per cantilever

surface and hence sensitivity of the sensor.

(2) Dynamic mode

The resonance frequency, f of an oscillating microcantilever can be expressed as equation 1.3.

The equation is used for first mode of vibration.

푓 =

12휋

퐾푚∗ (1.3)

where K is the spring constant and m* is the effective mass of the microcantilever [26].

Resonance frequency may change due to changes in mass or changes in spring constant and

the change in frequency can be expressed as equation 1.4 [26].

푑푓(푚∗ ,퐾) =

휕푓휕푚∗ 푑푚∗ +

휕푓휕퐾

푑퐾 = 푓2푑퐾퐾

−푑푚∗

푚∗ (1.4)

The contribution from spring constant (dK/K term) can be minimized by using appropriate

designs for microcantilevers with localized adsorption areas at the terminal end of the

cantilever (Figure 1.3(d)). Hence variation in resonance frequency is direct indication of mass

loading as expressed in equation 1.5.

∆푚 =

퐾4휋

1푓

−1푓

(1.5)

where f0 and f1 are microcantilever resonance frequencies before and after molecular

adsorption. The sensitivity and hence the minimum detectable mass depends on the ratio of

the mass and the resonant frequency of the microcantilever. Thus the sensitivity can be

enhanced by decreasing the dimensions of the microcantilever. By coupling the mass change

10

with varying environment temperature conditions, resonant mode of operation could be

extended to micromechanical thermogravimetry applications (Figure 1.3(e)).

The resolution of mass sensing of microcantilevers operating in dynamic mode in liquid

medium is expected to get affected due to damping effects. Hence dynamic mode of

microcantilevers might not be appropriate for sensing applications with liquid medium that

would demand very high resolution.

(3) Heat mode

Microcantilevers made of two material layers with different thermal expansion coefficients

undergo thermally induced stress and deformation. Microcantilever bending could be due to

external temperature changes (Figure 1.3 (g)) due to the heat generated on the microcantilever

surface either by exothermal reactions (Figure 1.3 (h)) or due to the heat associated with the

analyte adsorption (micromechanical calorimetry Figure 1.3 (i)). The last two cases are

directly related to cantilever based calorimetry for bio/ chemical sensors.

1.3.2 Microcantilever materials

Most commonly used microcantilever structural materials are single crystalline silicon,

polycrystalline silicon, silicon nitride, silicon dioxide and mechanically stable polymers like

SU-8, TOPAS and Parylene [27-29].

There has always been a demand for bio/chemical sensors with very high sensitivity so as to

detect bio-molecular interactions in the case of medical diagnostics or to sniff very small

amount of hazardous gases or explosive vapours for security applications.

As per the working principle of microcantilever based surface stress sensor, the sensitivity is

determined by the stiffness of the cantilever structure (dictated by the Young's modulus of the

material). Though the commercial cantilevers are made out of silicon derived materials that

have a higher Young's modulus, the need for highly sensitive and inexpensively fabricated

microcantilever sensors lead to a diversion from conventional silicon micromachining

technologies towards polymer based microcantilever technologies. Among the polymers

reported for microfabrication, SU-8 which is an epoxy based polymer developed by IBM, is

the most commonly used polymer structural material in MEMS [30-32]. Since 1999 [33], the

11

use of SU-8 polymer which is considered as a high aspect ratio negative photoresist for

MEMS applications has been exponentially growing during the last couple of years [34-

36][35]. Due to its ability in forming patterns with wide range of thickness varying from few

hundred of nanometres to a few millimetres with high aspect ratios, SU-8 has been a popular

and cost effective alternative to silicon for the fabrication of micro scale components such as

micro channels, micro-moulds for electroplating or masters for hot embossing.

Considering its applicability as a structural material for microcantilevers, SU-8 seems to be a

good candidate with its inherent advantages such as

Lower Young's modulus (Expected improvement in sensitivity)

Inexpensive and less complex fabrication process.

SU-8 is a well understood UV and e-beam resist

Fabrication processes consume lesser amount of chemicals and gases and low

temperature processing making it cost effective.

Mechanical and thermal stability

Compatibility for integrating sensor with microfluidics (important for biosensor

applications)

1.3.3 Transduction principles

The performance of a microcantilever sensor relies on real-time measurements and the

resolution of measurements of cantilever mechanical parameters during sensing operation.

These parameters include microcantilever tip displacement, spatial orientation, radius of

curvature, and intrinsic stress. Transduction schemes can be chosen based on mode of

operation i.e., static or dynamic; microcantilever design, microcantilever material and the

magnitude of expected response from the microcantilever. Microcantilever transduction

schemes can be broadly classified as optical and electrical. There are a variety of electrical

transduction schemes such as piezoresistive, piezoelectric, capacitive, electron tunnelling

technique and embedded MOSFET technique. The principles of operation of some of the

12

selected techniques are briefly presented here. The inherent advantages and disadvantages of

each transduction scheme are provided as Table 1-1.

(1) Optical lever method

Optical read-out is one of the most common and simplest schemes for detecting the

movement of microcantilevers and is derived from the standard AFM arrangement. In optical

lever method, the laser is focused at the free end of the microcantilever and the reflected laser

beam is detected using a position-sensitive photo detector (PSD) and the measurement can be

correlated to get the displacement information as shown in Figure 1.6 (A).

This method is expected to provide sub-angstrom resolution and can be implemented easily.

Referring to Table 1-1, implementation of this technique for readout of microcantilever arrays

is technologically challenging as (i) it requires an array of laser sources with the same number

of elements as that of the microcantilever array, and with the same pitch distance. (ii)

Sequential switching (on and off) of each laser source is also necessary to avoid overlapping

of the reflected beams on the photo-detector. Another optical transduction scheme developed

by IBM [8,37,38], allows the measurement of deflection of eight microcantilevers using an

array of vertical cavity surface emitting lasers (VCSELs). The same pitch has to be

maintained in both microcantilever and VCSEL arrays. A single photo-detector tracks the

movement of the spots reflected from respective cantilever.

Figure 1.6 Schematic of (A)"optical lever" readout that is commonly used to measure the

deflection of microcantilever probes in standard AFM [8] (B) interferometric readout [39]

Another optical approach using CCD camera in place of PSD has been developed by a

research group in University of California, Berkeley, USA. In this method, a two dimensional

array of microcantilevers is illuminated simultaneously resulting in a two-dimensional array

of reflected laser spots that are captured by a high resolution CCD camera. Such an approach

13

has been demonstrated to be used in microcantilever array based bio/chemical sensors for

multiple analyte detections [39, 40].

(2) Interferometric transduction

This method is based on the interference that occurs between a reference laser beam and the

reflected beam from the microcantilever [39]. Differential measurement between two

microcantilevers is preferred over single microcantilever measurements for eliminating the

environmental disturbances. A schematic of an interferometric sensor that inherently

measures the differential bending between two adjacent microcantilevers is shown in Figure

1.6(B). The cantilevers are supported by L-shaped structures, with interdigitated fingers

between the supports and the microcantilevers to form diffraction gratings. This allows

measurements of absolute deflection of each individual microcantilever. The interdigitated

fingers between the two microcantilevers enable differential measurement.

(3) Piezoresistive

Piezoresistive read out is based on the surface stress induced changes observed in the

resistivity of a piezoresistive material layer embedded inside the cantilever [42].

Piezoresistivity is a material property where the bulk resistivity of a material changes with

strain. The ratio of the relative change in resistance per unit strain is called gauge factor, K

which is the product of piezoresistive coefficient and Young’s modulus of the material. The

relation between the strain, ε and the relative change in resistance for a piezoresistor is given

by equation 1.6.

∆푅푅

= 퐾휀 (1.6)

∆푅푅

= (1 + 2휈)휀 +Δ휌휌

(1.7)

Here R : resistance, ν :Poisson’s ratio, ε: strain, ρ : resistivity of the piezoresistor. The change

in resistance might be due to both geometric effects (1+ 2ν) and the fractional change in

resistivity (Δρ/ρ) of the material with strain and hence can be represented as equation 1.7 [43].

14

The gauge factor contribution from geometric effects alone could be approximately in the

range of 1.4-2.0. In the case of metals, the Δρ/ρ term is very negligible. However, in the case

of semiconductor materials such as silicon, the Δρ/ρ can be 50–100 times larger than the

geometric term [43].

Consider a single layer microcantilever of thickness h with an infinitely thin piezoresistor on

top (Figure 1.7). In the case of surface stress induced on top of the cantilever, the surface

stress sensitivity can be expressed as equation 1.8 [34]. The assumption made is that, the

piezo-layer thickness tends to zero and neutral axis is placed at the middle of the

microcantilever structure.

∆푅푅

= 4퐾퐸ℎ

휎 (1.8)

Here, E: Young’s modulus of the structural material, h: the thickness of the microcantilever,

K: gauge factor of the thin piezoresistor and휎 : surface stress. This equation implies that for a

given thickness h of a microcantilever, the surface stress sensitivity depends on the ratio K/E

which is the ratio of gauge factor of the piezoresistor and the Young's modulus of the

structural material. Since the strain is distributed in such a way that it changes its direction at

the neutral axis with zero strain along the neutral axis plane, in order to get good sensitivity, it

is advisable to keep the piezoresistor away from the neutral axis.

The most common electrical circuit configuration for measuring the change in resistance with

a provision for temperature compensation is a Wheatstone bridge circuit and the schematic of

the configuration with a piezoresistive microcantilever placed in one of the legs of the bridge

is as shown in Figure 1.7 (B). The output signal Vout is given by equation 1.9. Here, 푉 is the

bridge bias.

푉 =

14∆푅푅푉 (1.9)

15

Figure 1.7 (A) A schematic of a simplified piezoresistive microcantilever model consisting of single layer of structural material with thickness h and a piezoresistor with thickness → 0. (B)

Schematic of the microcantilever in Wheatstone bridge configuration. Usually, the resistors 1 and 2 are microcantilevers. The variable resistor 2 is the measurements microcantilever and resistor 1 is the reference microcantilever. Resistors 3 and 4 need not be microcantilevers

The implementation of the first piezoresistive microcantilever for bio/chemical sensing

application was reported in the year 2000 [44] and since then a lot of applications have been

developed using piezoresistive microcantilevers. The arrays of piezoresistive microcantilevers

have been reported to be in use for electronics nose based gas sensing applications [45]. In all

these sensors, the typical configuration followed was that of a Wheatstone bridge with

reference and measurements cantilevers along with external resistors. Analytical models and

optimization study for improving the performance of piezoresistive microcantilever for

different sensing applications have also been reported [46-48].

(4) Piezoelectric

Piezoelectric properties have been used for both sensing and actuation for microcantilever

based sensors. Mechanical stress generates electrical potential across a piezoelectric material

for sensing and vice versa for actuation. Piezoelectric read-out is commonly used in

microcantilevers in dynamic mode [29]. This is because, electric potential generated by static

force cannot be retained by the thin film piezoelectric material and hence the resolution in

transduction can be expected to lower in the case of microcantilevers operated in static mode

compared to dynamic mode

(5) Capacitive

Capacitance between two separated electrodes changes when the distance between them

changes. Hence, in the case of a cantilever placed close to an electrode, the displacement of

16

the cantilever would cause a change in capacitance of this parallel plate arrangement. This

electric read-out was initially introduced in the case of AFM probes [49] and was

subsequently implemented for microcantilever based sensors. This readout is known to be

sensitive to changes in refractive index of the medium and is not suitable for liquid media.

(6) Embedded MOSFET readout

The need for a very sensitive transduction scheme with improved resolution has led to the

implementation of metal oxide semiconductor field effect transistors (MOSFET) embedded

micro cantilevers as this could be a low noise detection scheme.

Figure 1.8 (A) Schematic of the interaction between probe and target molecules on an embedded MOSFET microcantilever [50]

The silicon nitride cantilever is a reference, and the gold-coated one is used as a sensing cantilever. Specific biomolecular interactions between receptor and target bend the cantilever. Magnified view of embedded

MOSFET in cross section shows stressed gate region when cantilever bends, resulting in change of drain current due to conductivity modulation of the channel underneath the gate [50].

In this scheme, a MOSFET is embedded inside the microcantilever and on occurrences of

surface stress changes on the microcantilever surfaces, the channel mobility of the embedded

MOSFET changes due to the induced stress (Figure 1.8). This modulation of the channel

mobility results in change in the drain current of the transistor which can be measured easily.

Differential measurements using measurement and reference microcantilever have been

17

reported. The performance of the microcantilevers using this scheme has been reported to be

better compared to those of piezoresistive scheme in terms of sensitivity and noise [50].

Table 1-1 Comparison of main classes of transduction schemes

Transduction scheme

Advantages Disadvantages

Optical lever Easy to implement, Sub-angstrom resolution, well studied technique in AFM.

Difficult to implement arrays, prone to optical artefacts such as change in refractive index. Hence not suitable for translucent/opaque solutions, Difficulty in laser alignment and in implementing hand-held system with allied electronics.

Interferometric Very sensitive, provides a direct and absolute measurement of the displacement, minimal laser alignments.

Works only for small deflections, might not be suitable for portable sensor systems.

Piezoresistive Suitable for all media and all modes of operation, facilitates sensor array implementation, supports on-chip read-out electronics and temperature compensation, appropriate candidate for hand-held bio/chemical sensors.

Piezoresistive layer can affect the mechanical properties of the structure, requires proper isolation from the solution, inherent noise in piezoresistor might limit resolution in sensing.

Piezoelectric Most appropriate for dynamic measurements as sensing and actuation can be done.

Not suitable for static mode, most piezoelectric materials are not CMOS compatible, requires proper isolation from the solution.

Capacitive Suitable for small size cantilevers, CMOS integration, mechanical properties of the cantilevers are not affected.

Electromagnetic interference, not suitable for liquid media.

Embedded

MOSFET

Very good sensitivity, low noise detection scheme, works for all media, facilitates on-chip CMOS circuit integration.

Fabrication complexity, proper isolation from the solution.

18

1.3.4 Polymeric piezoresistive microcantilevers

Piezoresistive microcantilevers have been known to exhibit lower sensitivity in comparison to

the optical lever based devices. The sensitivity of piezoresistive microcantilevers is expected

to improve with increase in K/E, as per the expression given in equation 1.8. So the right

direction in developing piezoresistive microcantilevers with improved sensitivities might be

to use polymer structural material with lower Young’s modulus, E and piezoresistive material

with high gauge factor, K. As presented in the previous sections, SU-8 which is a

mechanically stable polymer with much lower E (at least one order of magnitude times

smaller compared to silicon based materials) was considered as microcantilever structural

material.

SU-8 microcantilevers with gold as strain gauge were reported in [34,36,51]. The design of

the cantilever chip and the fabricated device is shown in Figure 1.9. The chip consists of two

identical cantilevers with integrated resistors and two resistors on the substrate forming a

whole Wheatstone bridge. Gold has a lower gauge factor (KAu = 2) compared to that of silicon

(KSi = 140). However, the sensitivity of SU-8 microcantilevers with Au as strain gauge could

be in the same order as that of piezoresistive silicon microcantilevers as (K/E) SU-8/Au =

0.4GPa−1 is only slightly smaller than (K/E) Si = 0.77 GPa−1. These polymer devices

exhibited a deflection sensitivity of 0.3 x 10-6 (ΔR/R [nm]-1) and surface stress sensitivity of 3

x 10-4 (ΔR/R [N/m]-1)

Figure 1.9 SU-8 microcantilever with Au as strain gauge (A) Schematic of the device chip (B)

Optical micrograph of the microcantilever with integrated meander-type Au resistor [34]

The sensitivity of piezoresistive microcantilevers could be enhanced further by integrating

piezoresistive materials with higher gauge factor, such as polysilicon. The piezoresistive

polysilicon had to be deposited at room temperature using hotwire chemical vapour

19

deposition technique (HWCVD) [52,53]. However as polysilicon is a stiffer material than SU-

8, the film should be thin enough such that it does not add to the stiffness of the structure

while ensuring that the thin polysilicon does not lead to increased noise in measurements.

Piezoresistive materials with good gauge factor like that of polysilicon and modulus similar to

that of SU-8 would be the right choice. So the main goal of improvement in sensitivity

without compromising on the process complexity and thereby ensuring cost effectiveness can

only be accomplished by a proper choice of piezoresistive thin films containing similar

mechanical characteristics and process compatibility as that of SU-8. Composites of polymers

and conducting fillers such as carbon black (CB) and carbon nanotubes are known to exhibit

piezoresistive behaviour as reported in[54,55]. The suspension of different conducting

nanoparticles in SU-8 was reported in [56]. Hence polymer composites based on SU-8 with

conductive fillers could be a viable option for the embedded piezoresistive material in SU-8

microcantilevers. In such a device, the strain in the microcantilever is measured by the change

in resistance of the embedded piezoresistive layer containing conducting nanoparticles. When

the microcantilever structure is deformed or strained during a sensing operation, the

nanocomposite layer responds to the strain by increasing the distance (in the case of tensile

strain) between the individual conductive fillers. This eventually disturbs the conducting

network and leads to an increase in resistivity of the composite layer. A research work in this

direction was reported where piezoresistive polymer conductive filler composite made of SU-

8 and carbon black was integrated inside an SU-8 microcantilever [57].

Figure 1.10 Optical image of a micromachined SU-8 cantilever with integrated SU-8/carbon piezoresistor (Length = 200 m, Width = 200 m and thickness = 7 m) [57]

SU-8/CB nanocomposite piezoresistor patterned on an SU-8 layer for the microcantilever

structure is given in Figure 1.10 (B). Gamelgard et.al. [57], used piezoresistors that were

20

prepared out of CB nanoparticle, Conductex 975 and SU-8 2002 with concentration of CB

being 16 wt.% whereas the percolation threshold was 12 wt.%. The thickness of the

composite layer was 1.4 m and the overall microcantilever thickness was 7 m. The gauge

factor of SU-8/CB composite was approximately 15-20. From the data given in [57], it was

found that surface stress sensitivity for these microcantilevers was found to be about 4 ppm

[N/m]-1. These SU-8 microcantilever exhibited better sensitivity compared to that of

previously discussed configurations. Though this work established the proof-of-concept of

integrating a SU-8/CB composite as piezoresistor, further improvements in device or process

development as well as detailed characterization of the device for evaluating the performances

of these polymer devices and application developments using such devices have not been

reported.

1.3.4.1 Piezoresistive conducting polymers and other organic molecules

Intrinsically conducting polymers were also reported to exhibit piezoresistive effect. The

feasibility and process compatibility of some of these conducting polymers as piezoresistive

layers for SU-8 microcantilevers were reported by some research groups.

Ramona Mateiu et al [58] reported the piezoresistive behaviour of poly 3,4-

ethylenedioxythiophene (PEDT). PEDT films were patterned using conventional UV

lithography followed by reactive ion etching. The gauge factor for PEDT thin films was

experimentally determined to be 3.41 ± 0.42 which is close to metal strain gauges like gold.

So this could be a potential candidate for all polymer piezoresistive polymer MEMS devices.

Another conducting polymer, polyanyline (PANI) with a slightly higher gauge factor was also

reported [59]. Processing and patterning of PANI is reported to be compatible with SU-8

processing. PANI polymer, Panipol T (Panipol Oy, Finland) was found to exhibit a gauge

factor of K= -4.5. The negative gauge factor was explained as the effect of ordering of

polymer chains that improved the inter-chain conduction and hence conductivity.

An organic semiconductor, pentacene, had also been reported to show piezoresistive

behaviour [60,61]. Pentacene is a well understood organic semiconductor for organic field

effect transistors and organic FETs have a huge potential in realizing large-area, mechanically

flexible, lightweight and low-cost devices and circuits such as in paper-like displays, radio

21

frequency identification tags, and large area sensors on flexible substrates. However, the

effect of strain on the electrical behaviour of pentacene based OFETs has always been a

concern while discussing the reliability of OFETs for flexible electronics applications and

different device design ideas to overcome this effect is an on-going research for realizing

ultra-flexible OFET devices and circuits [62-64]. There are a very few reports available in

literature that investigated the effect of bending induced strain on change in current of

pentacene OFETs [61,65][66]. This negative aspect of dependence of electrical behaviour on

bending induced strain in pentacene OFETs for flexible electronics can be utilized in a

positive way in realizing sensor applications using pentacene. The reported applications of

pentacene as strain sensor include flexible strain sensors[60], large area force sensors[67],

tactile sensors and pressure sensors [68,69] all of them being large area sensors. The most

common deposition process for Pentacene is vacuum sublimation. This allows uniform

deposition of pentacene layers and the deposition process is done at room temperature making

it suitable for polymer substrates [70] . As pentacene is a well understood organic

semiconductor for organic field effect transistors, most of these reported sensor applications

employed pentacene based OFETs as strain sensor. This was highly beneficial as the

resistivity of pentacene thin films were known be very high (~2x103 Ωm [71] and the

transistor configuration would support the arrays of sensors with built-in switching matrix

using these integrated OFETs.

1.3.5 Applications of microcantilever as bio/chemical sensors

Microcantilevers have been employed for various bio/chemical sensing applications that

could be categorized as (1) sensors in medicine and biology and (2) sensors in chemistry and

environmental monitoring. The most common microcantilevers based bio/chemical

applications demonstrated so far in literature can be listed as Table 1-2. In addition to these

categories, microcantilever based sensors can also be categorized as gas sensors and

biosensors.

22

Table 1-2 Applications of microcantilever-based sensors [77]

One of the earliest microcantilever based gas sensors were used in detection of mercury

vapours as reported by Thundat et.al.[72] Commercially available silicon nitride

microcantilevers coated with gold were employed for this purpose. Gold coated

microcantilevers were also operated in dual (static and dynamic) mode for detection of several

other gaseous phase analytes, such as 2-mercaptoethanol [73] . Microcantilevers operated in

static mode were able to detect mercaptoethanol vapour concentrations down to 50 parts per

billion (ppb). As per earlier studies, microcantilevers coated with certain metals as active

coating could bring high sensitivity and selectivity. These metals included gold (Au) that has

high reactivity toward mercaptans (also known as thiols) and palladium (Pd) and palladium

based alloys in which hydrogen is known to show high solubility. This good sensitivity of Au

and Pd coated cantilevers to mercury and hydrogen respectively was explored to implement a

palm-sized, self-contained sensor module with spread-spectrum-telemetry reporting [74].

Microcantilevers functionalized with chemically selective organic molecules also came into

existence when gelatine coated microcantilevers were used for the first time for humidity

23

sensing [75]. Since then a number of sensors were reported using organic layer coated

microcantilevers. Subsequently, the concept of ‘chemical nose’, which was available with

other conventional sensors such as SAW etc., was also being implemented using

microcantilever arrays. Berger et al. [76] reported microcantilever based chemical nose using

single row microcantilever array containing eight cantilevers each functionalized with

different coating to distinguish between different volatile organic compounds.

In early days, biosensor applications using microcantilevers were based on an indirect method

of detection of bio-molecular interactions. For example, in [78], a glucose sensor developed

using microcantilevers took advantage of ultrahigh calorimetric sensitivity of a bimaterial

microcantilever. In this case, bimaterial microcantilever functionalized with glucose oxidize

performed the glucose sensing by responding to enzyme-induced exothermic processes on its

surface.

Microcantilever based biosensors with direct conversion of biological receptor-ligand

interactions into mechanical response of the microcantilever were also developed for different

biosensing applications [79]. The concept of using reference and measurements

microcantilevers was implemented to minimize the unwanted responses to environmental

effects such as temperature changes, vibrations and effect of fluid flow and this was a

significant milestone in developing microcantilever based biosensors. Since then

microcantilevers were in use for biomolecular recognition of streptavidin (Biotin-streptavidin

interaction), myoglobin an early marker for acute myocardial infarction (AMI), prostate

specific antigen (PSA) etc.

1.3.6 Microcantilevers for detection of explosive vapours

The rise in the number of deadly terrorist attacks over the last decade has encouraged the

technologists to come up with new solutions to quickly detect the concealed explosives. Most

of these explosives are usually very difficult to detect as they have very low vapour pressures

(Ref. Table 1-3). In addition to this requirement of high sensitivity, the desired sensor is also

expected to provide very good selectivity. Also there exist demands for explosive sensors

with minimum production and deployment costs [11] especially since cost effective

technologies can globally revolutionize the battle against terrorist attacks. Though optical

24

techniques such as Raman spectroscopy possess very high selectivity, sufficient sensitivity for

such an application had not been reported.

Table 1-3 Vapour pressures and molecular weights of some explosives [24]

Explosive Molecular weight

Vapour pressure at 20oC (Torr)

Ethylene glycol dinitrate (EGDN) 152.1 5.2 x 10–2

2,4,6-trinitrotoluene (TNT) 227.1 4.8 x 10–6

Pentaerythritol tetranitrate (PETN) 316.1 6.2 x 10–8

2,4,6-trinitrophenol (picric acid) 229.1 3.1 x 10–8

Tetranitro-triazacyclohexane (RDX) 222. 8.3 x 10–10

Tetranitro-N-methylamine (Tetryl) 287.1 3.7 x 10–10

Several decades of research for the development of sensors for explosives and hazardous

gases helped the research community to identify different popular detection techniques such

as the ion mobility spectrometry (IMS) and mass spectrometry. However these conventional

explosive detection techniques tend to be expensive and bulky besides having a longer

response time. However, in order to efficiently detect explosives at different locations,

deployment of multiple sensors is necessary and hence the recent demands are for

development of extremely sensitive and cost effective sensors that can be mass produced.

Miniature gravimetric sensors such as SAW sensors and QCM sensors are not small enough

to be deployed as arrays and these sensing technologies require frequency measuring systems

that are known to be expensive and large in size. Hence MEMS systems such as

microcantilevers have been found to be suitable for explosive detection because of their

advantages such as small size, high sensitivity, low power consumption and versatility to

integrate multiple explosive detectors in a single miniature package.

Different techniques for the detection of explosives in vapour phase using microcantilevers

have been reported in the literature [21,24,25,40,80-82] which includes receptor based

detection and receptor free detection. In receptor based detection, microcantilevers are

functionalized with receptors for molecular recognition. The receptors commonly used for the

most popular explosives such as TNT, RDX and PETN are 4-MBA (4-mercaptobenzoic acid),

25

SXFA (poly(1-(4-hydroxy-4-trifluoromethyl-5,5,5-trifluoro)pent-1-enyl)methylsiloxane)etc.

[24,83,84] and the functionalization procedure is based on the receptor and microcantilever

surface chemistry. The molecular recognition mechanisms in these cases are based on the

weak interactions that can be reversed making the microcantilever sensors reversible. Self-

assembled siloxane-based TNT-sensitive bi-layer are also reported to be in use with silicon

dioxide microcantilevers and are seen to be more stable than thiol based SAM such as 4-MBA

[85]. Another novel coating material, hexa-fluoro-isopropanol-functionalized mesoporous-

silica (HFMS) possessing ultra-high specific-surface-area was also been reported. This seems

to be one of the effective coating materials as it allows more specific-capture of a large

number of the very small TNT molecules [86]. All these receptors based on weak molecular

interactions enable sensitive and reversible mechanisms for detection of explosive vapours.

However, reversible receptor-based approaches are known to lack selectivity and therefore

identifying highly specific receptor molecules to improve selectivity in detection of explosive

is an on-going research. Receptor free techniques such as , differential thermal analysis that

produces unique and reproducible thermal response patterns for different molecules [80][81]

and photothermal deflection spectroscopy (PDS) [22] were introduced to bring selectivity

along with sensitivity and sensor reversibility. Both these techniques utilized extremely high

thermal sensitivity of a bimaterial cantilever.

1.4 Objectives and scope of the work

Based on the literature survey that has been carried out on microcantilever based bio/chemical

sensors, it has been found that the demand for low cost, highly sensitive and handheld

portable microcantilever bio/chemical sensor platform might be fulfilled by using SU-8

microcantilevers with integrated piezoresistive transduction. The selection of the

piezoresistive layer should be in such a way that it possesses high gauge factor, low Young’s

modulus(comparable to that of SU-8), process compatibility with SU-8 and patternability

using standard processes followed in microfabrication.

As per the available literature on piezoresistive SU-8 microcantilevers summarized in section

1.2.4, SU-8 microcantilevers with SU-8/CB strain sensor could offer very good sensitivity due

to large value of K/E for this composite structure. As the piezoresistor is SU-8 based, the

26

process compatibility and cost effectiveness could be additional merits. However, the

literature could only convey a basic proof of concept of integrating SU-8/CB nanocomposite

as piezoresistor. Hence, investigating the scope of improving the sensitivity of these sensors

and evaluating their performance for bio/chemical sensing applications could be an interesting

topic of research. It can be hypothesized that the performance of SU-8 nanocomposite

microcantilevers can be improved by using well dispersed and optimized composite of CB in

SU-8 with desirable electrical, mechanical and piezoresistive properties and by improving the

design of the microcantilever.

SU-8 being an organic material, integration of a strain sensitive conducting polymer or

organic layer with very good gauge factor for electrical transduction in SU-8 microcantilevers

could also be explored. As discussed section 1.2.4.1, pentacene is an organic semiconductor

with proven capability as strain sensor though not in use for MEMS based sensors. Its low

temperature deposition process would make it compatible for deposition on polymer

structures. In line with the previously reported configuration (pentacene as channel material

in OFET) of large area strain sensors using pentacene, pentacene based OFET could be

integrated with SU-8 microcantilevers. This might be a cost effective counterpart of the

MOSFET embedded silicon nitride microcantilevers that were demonstrated to show better

performances over piezoresistive microcantilevers [50].

Based on these hypotheses, the objectives and scope of this thesis could be listed as below:-

1. Explore the different possibilities of improving the performances of SU-8

nanomechanical cantilever sensors with integrated nanocomposite as piezoresistor.

(i) Prepare SU-8/CB nanocomposites to study their thin film and electrical conduction

behaviour in order to integrate the nanocomposite as piezoresistive layer with SU-8

microcantilevers.

(ii) Fabrication process development for polymer nanocomposite microcantilevers with

minimum possible beam and piezoresistive layer thicknesses. This approach could

lead to fabrication of SU-8/CB nanocomposite microcantilevers with improved

sensitivity.

(iii) A systematic characterization of the SU-8/CB nanocomposite in order to

understand its electrical and mechanical characteristics. This is required to decide

the optimal range of concentration of CB filler in SU-8 in order to improve the

27

overall performance of the polymer nanocomposite microcantilever sensor in terms

of its sensitivity and device variability

(iv) Fabrication of SU-8 microcantilevers (using a microcantilever design with

minimum possible fabrication process complexity chosen for surface stress

measurements) with integrated SU-8 nanocomposite prepared with optimum filler

concentration and improved dispersion technique.

(v) Systematic characterization of fabricated nanocomposite microcantilevers for their

mechanical, electrical and electromechanical and noise parameters so as to

benchmark their performances with other microcantilever based bio/chemical

sensors.

(vi) Application development for these nanocomposite microcantilevers in the field of

chemical sensing, more specifically for detection of explosive vapours.

2. Explore a novel electrical transduction technique for SU-8 nanomechanical cantilever

sensors by integrating a strain sensitive organic field effect transistor (OFET) with a

polymer nanomechanical cantilever. This device could be thought of as a low cost

polymer counterpart of a similar reported device in silicon, MOSFET embedded

microcantilever that was discussed in the literature review. The ways of integrating the

OFET with an SU-8 microcantilever could be evolved by evaluating the performance of

OFET with polymer dielectric compatible with SU-8 processes and by analysing process

compatibility for fabricating these two devices (OFET and SU-8 microcantilever). With

optimizations in the unit process level, an SU-8 nanomechanical cantilever device with

embedded OFET (here after named as ‘Organic CantiFET’) could be fabricated and

characterized.

Though the focus of this thesis has been on the development of SU-8 nanomechanical

cantilever sensors with integrated electrical read out, simple single layer SU-8

microcantilevers for optical transduction could also be developed in order to get familiarized

with SU-8 microcantilever fabrication process and to optimize the unit processes involved in

the fabrication of SU-8 based microcantilevers. In addition to this, due to ease in

implementing such a simple sensor in a laboratory environment, this would also aid in

application development using SU-8 microcantilever platforms.

28

1.5 Thesis Organization

This thesis is organized in six chapters.

Chapter 1 i.e. the present chapter provided a detailed introduction and motivation for this research

supported by literature survey along with the list of objectives and scope of the research work

presented this thesis.

Chapter 2 gives a basic understanding of the polymer SU-8 and microfabrication processing.

Design and fabrication of single layer SU-8 microcantilevers for optical lever transduction is

also covered in this chapter. The microfabrication process development for SU-8

microcantilevers presented in this chapter forms the base for the development of multilayer

SU-8 microcantilevers that are discussed in the following chapters. Mechanical

characterization of SU-8 microcantilevers and application development for explosive vapour

detections using a standard atomic force microscope (AFM) for optical transduction also has

been discussed in this chapter.

Chapter 3 deals with the development of polymer nanomechanical cantilevers with embedded

polymer/nanoparticle composite for electrical transduction. This chapter covers the (i)

systematic characterization of the piezoresistor material, SU-8/Carbon black composite for its

thin film, mechanical and electrical characterizations (ii) design and fabrication process

development for SU-8 nanocomposite microcantilevers and (iii) various characterizations of

the fabricated nanocomposite nanomechanical cantilever devices for evaluating their

performances for bio/chemical sensing applications.

In Chapter 4, application development for explosive detection using polymer nanocomposite

microcantilevers is discussed.

In Chapter 5, polymer nanomechanical cantilever sensor with a novel transduction scheme

using an embedded strain sensitive organic field effect transistor (OFET) inside an SU-8

microcantilever structure is explored. In this chapter, fabrication process development for this

device and various characterizations of these devices to benchmark their performances with

other existing polymer microcantilever devices have been covered in detail.

29

Chapter 6 finally concludes this research work. Results obtained in the previous chapters are

summarized based on which some future recommendations are made is 8also presented in this

chapter.

30

Chapter 2

SU-8 Microcantilevers for Optical Transduction

2.1 Introduction

The focus of the research work in this thesis has been on the development of novel SU-8

nanomechanical cantilever sensors with integrated electrical readout for bio/chemical sensing

applications. However, the microfabrication process integration for such SU-8

microcantilevers could be complex as these microcantilevers would be multilayer composite

structure that accommodates the in-situ electrical transduction scheme. The unit processes had

to be optimized before the final process integration for these SU-8 microcantilevers.

Development of a fabrication process for single layer SU-8 microcantilevers could be

considered as the first milestone as this would aid in establishing the process scheme and the

main unit processes for SU-8 microcantilevers using the available materials in the given

laboratory conditions. In principle, these single layer SU-8 microcantilevers could be used as

sensors with optical transduction that are very similar to atomic force microscope probes.

Moreover, the development of these simple SU-8 cantilever sensors might also help in testing

the feasibility of using SU-8 microcantilevers for various sensing applications.

This chapter provides an overview of SU-8 material, processing, fabrication of single layer

SU-8 microcantilevers for optical transduction, device characterization and finally application

31

development using these SU-8 microcantilevers for detection of explosive molecules in

vapour phase.

2.2 SU-8 material properties and processing

SU-8 is an epoxy based, negative tone, near-UV sensitive photoresist developed by IBM

Research [30,32]. It has a great potential for high aspect ratio structures [31] . It can be spin

coated to thicknesses in range of few hundreds of nm to few mm. The very low Young's

modulus (~ 4-5 GPa) and the ease of processing makes this photoresist a very promising

material for the fabrication of microcantilever sensor with very good sensitivity.

SU-8 monomer has eight epoxy groups . Such molecules are capable of being converted to a

thermoset form or three-dimensional network structure. This process is called curing or

crosslinking. So these eight epoxy groups are responsible for the crosslinking with other

monomers to form a polymer with the highest known crosslinking density.

SU-8 is commercially available from Microchem [87] and comes in two series which differs

from each other by the type of solvent present. The SU-8 formulations used in this research

work belong to SU-8 2000 series with cyclopentanone as the solvent. SU-8 resist contains a

monomer, a solvent and a photo-initiator which is triarylsulfonium salt that has the property

of disintegrating when exposed to light. One of the disintegration products is strong Lewis

acid that initiates the crosslinking chain reaction between SU-8 monomers and this

mechanism is termed as cationic polymerization [88,89].

The typical processing sequence for microfabrication of SU-8 structures is illustrated in

Figure 2.1.

1. Dehydration bake

The cleaned or pre-treated (cleaning or pre-treatment decided based on the history of the

sample) sample is kept for dehydration bake in an oven or a hotplate at 110oC. This ensures

removal of moisture from the sample surface.

32

2. Spin coating

The formation of SU-8 layer is done by spin coating of SU-8 on the sample using a standard

spinner. The thicknesses of the spin coated SU-8 layers are determined by the viscosity of the

resist and the spin coating parameters such as speed (rpm) and time. In the case of thick SU-8

layers, the thickness depends on the quantity of photoresist initially placed on the substrate for

spin coating. SU-8 2000 series comes with different formulations that have different solvent

content and hence different viscosities. For example, SU-8 2000.5 ,SU-8 2002 and SU-8 2100

spin coated at 3000 rpm for 35 seconds are expected to form SU-8 layers of thicknesses of

500 nm, 2 m and 100 m respectively. In the case of high viscous SU-8 formulations, spin

coating process imparts edge bead. This can be minimized by leaving the spin coated sample

for a few minutes on a levelled surface at room temperature. As this is done at room

temperature, the solvent present in the resist allows reflow of SU-8 and hence minimises the

edge bead.

3. Prebake

Prebake or soft bake step is performed for removing solvent from the SU-8 layer after spin

coating. It is performed by slowly heating the substrate on a levelled hotplate. Microchem

recommends a two-step baking process at 65oC and 95oC with the baking times decided based

on the SU-8 thickness. As reported earlier in [90], over baking is less critical than under

baking. The glass transition temperature (Tg) of SU-8 is about 50oC, and so the prebake

temperature of SU-8 being greater than this, reflow of the resist occurs during initial few

minutes of prebake. In order to produce smooth, uniform coating of SU-8 on the substrate, it

is very critical to have a levelled hotplate.

4. UV Exposure

As per the catalytic polymerization of SU-8, the basic source of crosslinking of SU-8 is the

Lewis acid that has been generated during UV exposure. So this is one of the most critical

steps that can decide the quality of the final SU-8 structure aimed for. Exposure doses

(defined as product of UV lamp intensity and duration of exposure) need to be optimized

depending upon the formulation of SU-8 and prebaking history. Optical mask aligners with

UV sources having wavelengths near to 365 nm are used for this process. Light absorbance in

33

SU-8 is minimal at this wavelength and hence it is advisable to use optical filters that mask all

wavelengths other than 365 nm in order to get good aspect ratio SU-8 structures.

Figure 2.1 Typical photolithographic processing sequence for microfabrication of SU-8 structures. (1) Dehydration baking step for removing the moisture on the sample surface (2) SU-8 resist is spun onto a sample (3) Prebake on a hotplate for evaporating the solvent. (4) UV-light exposure through a mask. (5) Post exposure bake for completion of crosslinking process (6) Development in SU-8 developer to remove the non-cross-linked areas in SU-8

5. Post Exposure Bake (PEB)

SU-8 crosslinking process occurs during the post exposure bake (PEB) in the regions that

contain Lewis acid generated during UV exposure. PEB at temperatures greater than Tg is

necessary because very little reaction takes place in the SU-8 resist in solid state in which

molecular motion is effectively frozen. The crosslinking process changes the SU-8 thin film

properties mainly in two ways. (1) Shrinkage of the resist happens due to the densification

imparted due to crosslinking and degassing of the solvent (2) Glass transition temperature

rises with progressing crosslinking process during PEB. In order to minimize the stress in the

coating due to thermal mismatch between coating and substrate, the hotplate is allowed to

slowly cool to room temperature before the samples are removed from the hotplate.

6. Development

Development of SU-8 is performed in propylene glycol methyl ether acetate (PGMEA) based

developer solution from Microchem. It is advisable to perform the development process with

agitation so as to reduce the development time and to increase the efficiency. After

34

completing the development, the sample is given a rinse in fresh PGMEA to remove residues.

Final rinsing is performed with iso propyl alcohol (IPA) in order to replace the relatively high

surface tension PGMEA with a lower surface tension solvent and thus reduces collapse of

high aspect ratio structures. Finally the sample is blow dried using nitrogen.

7. Hard bake (Optional)

Hard baking of the developed structure at temperatures higher than the prebake and PEB is an

optional step that ensures complete crosslinking of SU-8. Hard bake step is essential if the

SU-8 structures are expected to be exposed to higher temperatures (greater than the PEB

temperature) after fabrication.

2.3 Single layer SU-8 microcantilevers for optical transduction

The optical read-out is one of the simplest and sensitive schemes for detecting the deflection

of microcantilevers. This scheme is commonly employed in a standard AFM. nAs the

transduction of the microcantilever motion is done with external laser and PSD arrangement,

microcantilever structures for this scheme do not require any additional specific layer for

transduction. The design, fabrication process development and an application of such a single

layer SU-8 microcantilever for detection of explosive vapours are covered in this section.

2.3.1 Design of single layer SU-8 microcantilevers

The SU-8 microcantilevers discussed here are single layer microcantilever sensors with the

anchor dimensions appropriate for inserting into a standard AFM nose so that they can be

characterized optically with much ease. The microcantilevers are meant for bio/chemical

sensing applications with the microcantilever operating in the static mode. For a typical

(bio)chemical sensing application using microcantilevers, the deflection resulting from

surface stress change because of the target analyte-probe molecular interactions typically in

the range of 1mN/m to few N/m need to be measured [77]. Geometrical dimensions of

microcantilevers are decided based on surface stress sensitivity and mechanical stability. One

of the most important mechanical parameters of the microcantilevers, resonance frequency

has to be kept above 5 kHz for minimizing the effect of external mechanical vibrations.

35

The design for SU-8 microcantilever sensor in the optical lever scheme discussed here is

chosen based on three criteria (i) ability to detect a minimum surface stress <5 mN/m, (ii)

resonance frequency greater than 5 kHz and finally (iii) microcantilever die dimensions to fit

into an atomic force microscope (AFM) probe. The resonant frequency and spring constant as

a function of microcantilever length were analytically estimated using equations 2.1 to 2.3 as

shown in Figure 2.2. Here the width of the microcantilever was fixed based on the minimum

resolvable feature in the mask printer and the size of the laser spot diameter. The

microcantilever thickness was chosen slightly higher (2 m) than the minimum achievable

thickness using SU-8 2002 which is 1.1 m. The material properties of SU-8 2002 considered

for this purpose are given in Table 2-1. The SU-8 microcantilever dimensions chosen based

on the requirements mentioned above and the estimated resonance frequency and spring

constant are given in Table 2-2. The displacements of these microcantilevers in response to

surface stress change in the range 1 mN/m to 1 N/m could be expected to be in the range of

5.3 nm to 5.3 m and hence the device design was appropriate for typical bio/chemical

applications.

푆푝푟푖푛푔푐표푛푠푡푎푛푡, 푘 =3퐸퐼퐿

(2.1)

푀표푚푒푛푡표푓푖푛푒푟푡푖푎, 퐼 =

푤ℎ12

(2.2)

푅푒푠표푛푎푛푐푒푓푟푒푞푢푒푛푐푦, 푓 = 0.32

푘푚

(2.3)

The schematic of the designed SU-8 microcantilever structure is given in Figure 2.3(A). An

array of such microcantilever chips are attached to SU-8 frames for easy handling of the

fabricated microcantilevers (Figure 2.3(B)) during release process. An opening is provided in

the anchor of the microcantilever in order to enable faster release of microcantilever structure.

36

Figure 2.2 Resonant frequency and spring constant as function of length of the

microcantilever

Table 2-1 SU-8 material properties

Property Value

Density (Kg/m3) 1123 [87]

Young’s Modulus

(GPa)

5[34]

Poisson’s Ratio (υ) 0.22

Table 2-2 SU-8 microcantilever

specifications

Parameter Value

Length 300 µm

Width 40 µm

Thickness 2 µm

Die area 3.4 mm x 1.5 mm

Spring constant 14.8 mN/m

Resonance frequency

7.5 kHz

Deflection range for Δσ (1mN/ to 1N/m)

5.3 nm to 5.3 m

37

Figure 2.3 Schematic of SU-8 microcantilever die for optical transduction (A) Single

cantilever die (B) Schematic of the cantilever die attached to the SU-8 frame.

2.3.2 Fabrication process development for SU-8 microcantilevers

The fabrication process allowed the development of fully polymer (SU-8) structures on a

dummy substrate and final separation of the SU-8 structure from the dummy substrate. The

fabrication process had three levels of lithography. The schematic of the fabrication process

sequence is given in Figure 2.4. The mask layout is provided in Appendix D.

Figure 2.4 Schematic of fabrication process flow for SU-8 microcantilevers (A) Sacrificial

layer (B) Structural layer SU-8 for the microcantilever (C) Thick SU-8 for anchor and frames of microcantilever die (D) Release of the microcantilever devices attached to frame.

The first step in the fabrication process was cleaning of the silicon wafer which can be either

with RCA or piranha cleaning. The silicon wafer is the dummy substrate with a release layer

or sacrificial layer so that polymer structures fabricated on the substrate can be removed by

38

etching a sacrificial layer. Each of the unit processes in the fabrication of these

microcantilevers is detailed below. Though the dummy substrate considered in this research

work was silicon wafer, in principle the substrate could be glass substrate or polymeric

substrates. However, the substrates should possess certain prerequisites such as the substrate

surface roughness as minimal as possible, substrate dimensions compatible for the processing

equipment to be used for the fabrication process and the stability of the substrate material

during the entire processing.

(A) Sacrificial layer

The essential qualities of release or sacrificial layers are:-

1. It should be possible to coat a sufficiently thick layer of the release layer material so

that it ensures the easy access for the etchant chemical to perform under etch reaction

that would not be diffusion limited. An efficiently progressing under etch process aids

in easy release of the structure from the substrate.

2. The sacrificial layer should possess uniform thickness and minimum surface

roughness. This is important because any non-uniformity or roughness on the

sacrificial layer could get transferred to the microcantilever structural layer.

3. The surface properties of the sacrificial layer should enable uniform spin coating of

SU-8 layer and provide good adhesion for SU-8 layer without delamination or peeling

off.

4. The etching reaction should be fast enough for easy release of the SU-8 structure from

the underlying substrate.

5. The etchant for the sacrificial layer should not attack the crosslinked SU-8.

A comparative study on different sacrificial layer materials such as metals (Al,Cr/Au and Cu)

and polymers ( polymethylmethacrylate, polyimide and poly-styrene) were reported [91]. Out

of the metals, Cu provided better results. However, they were the best choice as sacrificial

layers for SU-8 structures with planar dimensions up to 200 m. Polymers were shown to be

good as sacrificial layers for SU-8 structures up to 600 m. But there are certain issues that

were reported such as poor adhesion between the polymeric sacrificial layer and SU-8 and the

39

tendency of SU-8 to attack photoresist sacrificial layers like the polyimide resist [91]. A

combination of multiple metal layers, Cr/Au/Cr of 5/50/250 nm was reported as one of the

best candidate for release layer [90] and was named as enhanced sacrificial layer technique.

Here Au with thick Cr layer acts as galvanic cell that speeds up the etching process. But the

relative thicknesses values were very critical. While all the above mentioned release

processes were based on wet chemical etching, there existed certain dry methods for release

of SU-8 structures using polysilicon or fluorocarbon layers as sacrificial layers [92,93].

However, these release processes were equally time consuming and required complex plasma

systems like reactive ion etching (RIE). Hence, there was a requirement for an easy and fast

process for the release of SU-8 structures. A non-organic spin-on dielectric material named,

hydrogen silsesquioxane (HSQ) which takes a highly porous silicon dioxide structure when

cured at high temperature was considered for this purpose. The etchant used was Buffered

Oxide Etch (BHF 5:1) or Hydrofluoric Acid (49%). Due to the porous nature of the oxide, the

release process was expected to be faster in comparison to etching of thermally grown or

chemical vapour deposited silicon dioxide and other existing release methods mentioned

above.

The process sequence and the parameters for HSQ layer deposition are as follows.

1. Dehydration bake of Pirahna cleaned silicon wafer at 130oC for 45 minutes

2. Spin coat 1:2 of HSQ: MIBK (Dowcorning FOX 12) at 1500 rpm for 30 seconds. Repeat

the process three times to get thick layer of HSQ (200 nm).

3. Curing process for the HSQ layer on a Hot plate at 100oC

4. Furnace anneal in nitrogen ambient at 450oC in order to convert the HSQ layer to a porous

silicon dioxide like structure.

In addition to HSQ, RF sputtered silicon dioxide was also tried out as sacrificial layer.

(B) Alignment mark

The first process after sacrificial layer deposition is the deposition and patterning of alignment

marks. Alignment marks made of any reflecting materials such as metals were essential in

order to align the two consecutive layers of SU-8. This is because, SU-8 being transparent, the

40

patterns on first layer of SU-8 of thickness 2 m could be very difficult to resolve after the

subsequent spin coat and bake processes for thick SU-8 2100. Here, alignment marks made of

30 nm of gold were used. A thin layer (5 nm) of chromium or titanium could be used as

adhesion promoting layer for gold. The patterning process followed was standard lift-off

process (mask # 1). This step is not shown in the process sequence as the alignment marks

were global alignment marks for the wafer and not part of the device structure.

(C) SU-8 layer for cantilever structural layer

Next step is the patterning of SU-8 layer of 2 m that defined the structural layer of the

microcantilever along with an extension of the layer to microcantilever anchors and frames.

For this, the wafer was kept for dehydration bake step at 110oC for 30 minutes. Then SU-8

2002 was spin coated at 500 rpm for 5 seconds and then at 3000 rpm for 35 seconds with

proper acceleration. Subsequently the sample was kept for prebake (PB) which was a two-step

baking on a hotplate maintained at 70oC for 3 minutes and 90oC for 7 minutes. The SU-8

layer was subjected to UV exposure through mask # 2 for 10 seconds in Karl Suss MJB 3

standard mask aligner. This was followed by post exposure bake (PEB) which was again a

two-step bake at 70oC for 2 minutes and 90oC for 5 minutes. Then the development in SU-8

developer for 1 minute followed by rinse in IPA was performed. The wafer was blow dried

using nitrogen gun.

(D) Thick layer of SU-8 for microcantilever anchor and holding frame

SU-8 2100 was used for forming anchor and frame layers with thickness greater than 220

microns. The processes were very similar to SU-8 2002 process explained earlier with an

increase in bake and exposure timings.

The optical micrograph of the SU-8 structures on wafer after this process is shown in Figure

2.5(A). Lot of stress lines were observed in the structure.

41

Figure 2.5 (A) SU-8 patterns on substrate indicating the stress lines (B ) and (C) SEM micrograph of the fabricated SU-8 microcantilever after release from the substrate.

(E) Release of SU-8 microcantilever devices

The release process for SU-8 microcantilever structures was performed by isotropic etching of

sacrificial layer in BHF 5:1. The release process was faster (40 minutes) in the case of

samples with HSQ as sacrificial layer in comparison to that (90 minutes) of samples with

sputtered silicon dioxide as sacrificial layer. The micrographs of the devices after release are

given in Figure 2.5 (B and C). The curvature observed in the microcantilevers could be

attributed to the stress developed in SU-8 layers. The stress in SU-8 layers was evident from

the stress lines observed in the SU-8 structures before release.

2.3.3 Challenges in fabrication

There were certain issues faced while fabricating SU-8 microcantilevers and it seemed very

important to include discussions on these issues along with modifications made on the

fabrication process to solve these issues.

Stress induced curvature

The stress in SU-8 films leads to residual curvature of free hanging SU-8 structures. The

stress in the SU-8 films may be of two types, intrinsic or extrinsic, where extrinsic stresses are

those that arise due to substrate interaction while intrinsic stresses are present in the free-

standing film as well. Residual stress in the cantilevers after fabrication is very undesirable

and understanding the mechanisms and causes of stress in the polymer is very important to

improve the fabrication process.

42

The thermal stress may be due to difference in coefficient of thermal expansion between the

substrate Si and SU-8. So during cooling stage after PEB, it develops tensile stress [94].

Before crosslinking, SU-8 is stress free since the monomers are not constrained to move by

the polymer lattice. So stress is developed mainly in the thermal cycle after exposure. This

can be reduced by reducing the PEB temperature and slow ramp down to room temperature

after the second step of PEB. However if thermal stresses are uniform across the thickness of

SU-8 layer, it should not curve after being released.

The stress in thick SU-8 anchor layer could be the major cause of curvature seen in

microcantilevers. This origin of stress could be mainly controlled by optimizing the

combination of UV exposure dose and PEB. When SU-8 is baked on a hot plate, a linear

temperature gradient through the SU-8 film could be expected to exist [95]. As a result, the

bottom layers of the SU-8 film will be curing at higher temperatures than the top surface. As

per the theory given in [95], with moderate UV doses, the crosslinking density through the

SU-8 film and hence stress can be altered with baking parameters. Compliant structures may

have a negative, positive or zero curvature. However, with very large UV doses, the catalyst

concentration through the film is at or near a maximum, and the thermal gradient from baking

on a hot plate dominates stress behaviour of the film. The bottom layers of film are expected

to be at a higher temperature than the top layers. So the crosslinking density would be higher

near the bottom than at the top. Feng et.al.in [96] reported that thermal expansion of epoxies

depends on the extent of crosslinking. Hence when an SU-8 film with a through thickness

gradient of crosslinking density is cooled, the bottom contracts less than the top, resulting in

less tensile strain at the bottom and more at the top. This gradient in strains causes the SU-8

hanging structures to curl up for all baking times.

Another observation was that the bending of the microcantilevers and the frames were more

in the case of those with thicker SU-8 pad layer. This may also be due to non-uniform

shrinkage of thick SU-8 along the thickness during cross-linking. And this effect is more

severe in thicker pads. This happens because the bottom side is getting heated up more during

hot plate bake. So a possible solution may be to reduce the thickness of anchor layer of SU-8.

However in order to have uniform crosslinking of SU-8, it is good to reduce the PEB

temperature and increase the time of PEB.

43

Non uniform layer of sacrificial layer

Lot of striations were observed in the spin coated layer of HSQ. Also the HSQ layer was

found to rough with roughness values in the order of tens of nanometres. Different ratios of

HSQ and MIBK were tried out to improve the quality of the HSQ layer. However, sputter

deposited silicon dioxide was found to better than HSQ in terms of uniformity and roughness

(RMS roughness ~ 3-5 nm) and hence was considered to be a better candidate as a sacrificial

layer.

2.3.4 Optimized fabrication process for SU-8 microcantilevers

The residual stress leads to the curvature of microcantilevers after release. As mentioned

earlier, this is a thermal stress generated due to the (i) difference in thermal expansion

coefficient of substrate and SU-8 and (ii) gradient in crosslinking density throughout the

thickness of SU-8 2100 layer during post exposure bake. This was addressed by three

modifications in the process:-

1. SU-8 2100 spin parameters changed to reduce the thickness to less than 170 microns

2. Post exposure bake (PEB) step was modified. Highest exposure temperature was reduced

and PEB was performed for longer time duration. To reduce the thermal induced stress, the

temperature was ramped slowly and finally ramped down to room temperature.

3. A hard bake step of 30 minutes in 95oC -110 oC was also required to eliminate the stress.

The sample before after hard bake step is shown in Figure 2.6.

Based on these analyses, the process recipe for fabricating microcantilevers was modified as

summarized in Table 2-3. The devices fabricated following this modified processes were

stress free as shown in Figure 2.7

44

Figure 2.6(A)160 µm thick SU-8 structure before hard bake. Stress indicated in corners (B)

The sample after hard bake. Stress lines got vanished after hard bake

Table 2-3 Optimized process parameters for SU-8 microcantilevers SU-8 layer

Spin coat

Prebake Exposure

PEB Development

Hard bake

SU-8 2002 (2 µm)

300 rpm 4s 500 rpm 4s 3000 rpm 30s 300 rpm 2s

@70oC 3 min Ramp to 90oC in 6min @90oC 7 min Ramp down to 30oC

10sec

@70oC 3 min Ramp to 90oC in 6min @90oC 7 min Ramp down to 30oC

1 min in SU-8 developer and 1-2 min rinse in IPA and blow dry

@100oC 10 min

SU-8 2100 (180 µm)

300 rpm 7s 500 rpm 10s 1500 rpm 30s 300 rpm 2s

@65oC 15 min Ramp to 85oC in 10 min @85oC 45 min Ramp down to 30oC

2.5min

@65oC 15 min Ramp to 85oC in 10 min @85oC 45 min Ramp down to 30oC

20 min in SU-8 developer and 1-2 min rinse in IPA and blow dry.

@100oC 30 min

45

Figure 2.7 SU-8 microcantilevers fabricated after optimization process (A) SU-8 frames holding arrays of SU-8 microcantilevers. These devices were coated with gold. (B) Optical micrograph of one of the microcantilevers (C) SEM micrograph indicating the stress free

nature of the microcantilevers

2.3.5 Characterization

The dynamic characterization of these microcantilevers had to be performed in order to find

out their resonant frequency. For this purpose, a PicoSPM II, (Agilent) AFM system was

employed. The typical laser diameter is approximately 35 μm and the minimum detectable

deflection of cantilevers is in the sub-nanometer range.

SU-8 microcantilevers to be characterized using AFM were coated with 45 nm of gold for

providing a reflecting surface for laser beam. 10 nm of chromium was used as the adhesion

promoting layer. Microcantilever chip was mounted on acoustic AC mode nose of the AFM

scanner. The laser was focused near the free end of the cantilever and the position of the

photo detector of AFM was adjusted to get maximum output signal. In acoustic AC mode, the

piezoelectric stack inside the scanner excites the microcantilever to oscillate and the laser

beam reflected from the oscillating microcantilever gets deflected in a regular pattern over a

photodiode array, generating a sinusoidal electronic signal. The root mean square voltage

value for this signal is displayed on the display unit. Microcantilever was excited with signals

of different frequencies ranging from 5 kHz to 100 kHz. The drive signal was gradually

increased from 20% of the maximum possible value, in order to maximize the response

46

without going into saturation. The gain was also adjusted in order to improve the amplitude

and signal to noise ratio (SNR) in the frequency response curve.

Figure 2.8 shows the frequency response of SU-8 microcantilever obtained from the AFM.

Resonance frequency was approximately 12 kHz. The Q-factor, an important parameter that

gives a measure of dissipation mechanisms that damp the oscillations of the microcantilever

was also computed. Q-factor was calculated as the ratio of resonant frequency to frequency

difference between -3dB points (Q = fres/(Δf3dB ) and it was found to be 20. The value of Q

factor is too low to use these microcantilevers for dynamic mode of operation for sensing

mass changes. However, as the resonant frequency is higher than 5 kHz, these

microcantilevers can be operated in static mode for surface stress change measurements

without any external mechanical vibrational noise.

Figure 2.8 Frequency response of gold coated SU-8 microcantilever

The experimentally determined value of resonant frequency is higher than the analytically

estimated value of resonant frequency single layer SU-8 microcantilever as given in table

Table 2-2. However, the resonant frequency calculated for SU-8 microcantilever with 10 nm/

45 nm of Cr/Au (similar to the characterized structure) using the equations for multilayer

microcantilevers was 9 kHz (Refer Appendix E for the procedure followed). This multilayer

structure was also simulated using CoventorWare. The plot obtained from simulation data of

resonant frequency as a function of length of this Cr/Au coated SU-8 microcantilever is given

47

in Figure 2.9. The simulation yielded a resonant frequency of 9.5 kHz for SU-8

microcantilever with these geometrical dimensions.

Figure 2.9 Simulated data of resonant frequency with scaling of length of the SU-8 microcantilever coated with Cr/Au. Inset : Mesh model for the microcantilever structure.

2.3.6 Application in explosive vapour detection

In recent years, microcantilever technology for explosive detection has been found to be

extremely suitable because of its advantages such as small size, high sensitivity, low power

consumption and versatility to integrate multiple explosive detectors in a single miniature

package as explained in chapter 1. Application of SU-8 microcantilevers for receptor based

explosive detection is explores here. The receptor coating material considered for this purpose

was 4-mercaptobenzoic acid (4-MBA, SH- C6H4-COOH) is acidic in nature due to the

presence of the -COOH group, that can dissociate into a -COO- charged group. Due to the

presence of a sulfur group at the end, this benzene thiol can bind easily to a gold surface. In

48

order to functionalize the microcantilever with the receptor 4-MBA, one of the surfaces of the

microcantilever surface had to be coated with Au. As the gold-thiol (Au-SH) bond is known

to be covalent in nature and there exist π-π stacking interaction between aromatic groups of

adjacent receptors, the 4-MBA receptor coating is quite stable, and forms a closely packed

self-assembled monolayer (SAM). This allows efficient transduction of surface stresses to

bending of the cantilever beam. 4-MBA SAM provides the well oriented –COOH groups for

efficient binding of vapour phase bases like explosives [11].

The steps involved in functionalizing SU-8 microcantilevers with 4-MBA are as follows:-

1. Sputtering of 5nm of Ti and 30nm of Au.

2. Clean the cantilevers with ethanol

3. 4-MBA SAM was achieved by immersing/incubating the gold coated microcantlevers

in 6mM solution of 4-MBA (97%, from Sigma-Aldrich.) in ethanol for 24 hours.

4. Rinse the cantilevers with ethanol in order to wash-off the physically adsorbed 4-

MBA receptor molecules and thus leaving behind stable SAM of 4-MBA on the gold

coated surface of the SU-8 microcantilever.

5. Store the dried cantilevers in vacuum desiccators to avoid moisture.

4-MBA coated microcantilever was loaded in the AFM scanner and the whole assembly was

incubated in AFM chamber with Teflon tubes for inlet and outlet for gas flow. Humidity

changes in the environment can affect the target-receptor interactions and hence significantly

complicate the explosive vapour experiments. So in order to eliminate these secondary effects

and to simplify the experimental analysis, the humidity levels in the AFM scanner chamber

were brought to minimal by purging with high purity( Excellent grade 99.999 % purity) dry

nitrogen.

The schematic of the experimental setup is shown in Figure 2.10. TNT vapour stream was

generated by flowing dry nitrogen carrier gas through the vapour generator module. Vapour

generator contains the explosive reservoir quartz “U” tube that can be maintained at a specific

temperature using a temperature controller. The flow rate of nitrogen carrier gas through the

explosive (TNT in this case) vapour source was controlled using a mass flow controller (MFC

49

from SIERRA). The gas flow rate was maintained at 30 SCCM (Standard Cubic Centimetre

per Minute) using the MFC. When the TNT vapour stream had to be stopped, the same carrier

gas flow was maintained through the microcantilever gas flow cell by redirecting the nitrogen

flow stream through a bypass line. This switching between TNT vapour stream and dry

nitrogen stream was performed using three way valves V1 and V2.

Initial experiments were carried out with TNT vapour generated at 65oC. AFM chamber was

purged with dry nitrogen till the signal got stabilized. Purging was done for 1-2 hours before

starting the experiment. This helped in reducing the humidity levels. The response of the

microcantilever for TNT vapour is shown in Figure 2.11.

Figure 2.10 Experimental setup for explosive vapour experiment (A) Schematic of the setup

(B) Explosive vapour generator

50

Figure 2.11 Response of the microcantilever to TNT vapour stream generated at 65oC with a

flow rate of 30 SCCM

The microcantilever response curve verifies the proof of concept of detection of TNT vapours

using 4-MBA coated SU-8 microcantilevers. The response curve indicates the direction of

bending of the microcantilever the side in which laser is focussed which is the gold coated

surface. Curvature of the microcantilever in this direction could indicate presence of

compressive stress in the gold coating. This experimental result could be supported by the

theoretical analysis for the interaction chemistry between 4-MBA and a similar molecule,

dinitrotoluene (DNT) which is a decomposition product of TNT [97] which is also a

nitroaromatic with two nitro groups. The origin of compressive stress could be either of the

two possible interactions given below:-

1. Considering the case of closely packed 4-MBA SAM without any defect. The TNT

molecule can bring the adjacent thiol molecules of the 4-MBA SAM closer to each other

via hydrogen bonding. This leads to a compressive stress in the gold film (Figure 2.12(B).

2. Considering the case of microcantilever functionalized with 4-MBA SAM with certain

defects leading to grain boundaries in the SAM. In this case the TNT molecule can

occupy the grain boundaries and generate attractive forces between adjascent receptor

molecules via weak pi-pi interactions and hydrogen bonding. This can also lead to

compressive stress in the gold film (Figure 2.12(C)).

51

Both these cases could be illustrated using the example of interaction between DNT and 4-

MBA as depicted in Figure 2.12. These weak interactions between TNT molecules and 4-

MBA were reversible as depicted in the microcantilever response curve in which the sensor

output signal got reduced with nitrogen purging. The functionalized SU-8 microcantilever

sensors could be regenerated with nitrogen gas and hence these sensors were found to be

reusable for multiple operations. Due to the large volume of the AFM scanner chamber which

served as the gas flow cell, the response time of these sensors seemed to be long. However,

these experiments served as a proof of concept and a base for detection of explosive using the

SU-8 microcantilevers.

Figure 2.12 (A) Conceptual diagram of microcantilever coated with benzene thiol (eg. 4-MBA) receptors on the gold side and undergoing compressive stress and bending due to

interaction with aromatic gas (B) Interaction between DNT and 4 MBA leading to compressive stress in the gold film (C) interaction between DNT and 4-MBA at SAM grain

boundaries leading to compressive strain in gold film [97]

2.4 Summary

This chapter provided a basic understanding of the microfabrication processes involved in

developing structures using SU-8; the polymeric structural material considered for

52

microcantilevers. A simple fully polymeric microcantilever sensor device with a single layer

of SU-8 in the microcantilever structure with a capability to optically detect surface stress of

few mN/m was designed and fabricated. The process followed allowed the fabrication of all

polymer structures on a dummy substrate and release of the structures from the substrate by

removing the sacrificial layer. The capabilities of thin films of HSQ and sputtered silicon

dioxide as sacrificial layers were explored and were found to be better candidates compared to

the release layers reported in the literature. The unit processes in fabricating single layer SU-8

microcantilevers were optimized and this could serve as a base work for fabrication of

multilayer SU-8 microcantilevers that has been developed in the lab as part of this thesis

work. The fabricated SU-8 microcantilevers for optical transduction were characterized using

standard AFM. An application for these microcantilevers for detection of explosive vapours,

TNT has been successfully implemented. The functionalization protocol and the explosive

vapour experiments could also serve as a background work in developing sensors for

explosive detection using SU-8 microcantilever with integrated electrical transduction

mechanism to be discussed in chapter 4.

53

Chapter 3

Polymer Nanocomposite Microcantilever Sensor with

Integrated Electrical Transduction

3.1 Introduction

Microcantilevers with integrated piezoresistors perform electrical transduction of strain by

resultant change in resistance. Therefore, when a microcantilever with its surface

functionalised with a coating that is selective to target molecules is exposed to the analyte, the

molecular adsorption induces a differential surface stress between top and bottom surface of

the microcantilevers. This differential surface stress results in a change in resistance of the

piezoresistive layer as represented in equation1.8 in Chapter 1, which says that surface stress

sensitivity depends on the ratio of gauge factor (K) of the piezoresistive film to the Young’s

modulus (E) of the structural material. The development of SU-8 (low Young’s modulus)

microcantilever with SU-8 nanocomposite piezoresistor (aimed at providing good gauge

factor and low young’s modulus) is presented in this chapter. Added to the benefit of expected

improvement in sensitivity, unlike the previously reported piezoresistive layers used in SU-8

54

microcantilevers, SU-8 nanocomposite piezoresistor would exhibit compliance compatibility

with that of the structural material, SU-8.

The device concept of a polymer nanocomposite microcantilever is to embed a piezoresistive

nanocomposite made of SU-8 and Carbon Black (CB) conducting nanoparticles inside an SU-

8 microcantilever. A similar idea was reported in [57] in which SU-8 microcantilevers of

thickness 7 m with embedded SU-8/CB nanocomposite (16 wt. % of CB in SU-8)

piezoresistor with a gauge factor in the range of 15-20 was presented. However further

improvements in device or process development or detailed characterization of the device for

evaluating and optimizing the performances of these polymer devices for any kind of sensing

applications have not been reported. There is still a lot of room for improvement in this

technology and developing a platform based on this technology for many applications in

bio/chemical sensing.

The performance of SU-8 nanocomposite microcantilevers could be improved on first hand

by improving the dispersion of CB in SU-8 and reducing the thickness of the cantilever

structure which is discussed in this chapter. The other focus points discussed in this chapter

pertain to the improvement in performance of polymer nanocomposite microcantilever

sensors that could be achieved by (i) performing a systematic characterization of the SU-8/CB

nanocomposite in order to understand its electrical and mechanical characteristics. This was

required to decide the optimal range of concentration of CB filler in SU-8 in order to improve

the overall performance of the polymer nanocomposite microcantilever sensors in terms of

sensitivity and device variability. (ii) Improving the microcantilever design aimed for surface

stress measurement based nanomechanical cantilever. (iii) Systematic characterization of the

fabricated microcantilever devices for analysing their electrical, mechanical,

electromechanical and noise characteristics. These analyses are of prime importance in

verifying the capability of these polymer nanocomposite microcantilevers for bio/chemical

sensing applications.

3.2 SU-8/CB nanocomposite thin film preparation and patterning

The polymer nanocomposite was prepared by ultrasonic mixing of a high-structured Carbon

Black (CB), Conductex 7067 Ultra (Columbian Chemicals) in SU-8 and Nanothinner

55

(Microchem). The diameter of carbon black nanoparticles was approximately 40 nm. In order

to use SU-8/Carbon black as piezoresistor layer, the composite material should satisfy certain

prerequisites such as (i) possess proper dispersion of CB in SU-8 (ii) it should be spin

coatable and photo-lithographically patternable with compatible process close to that of SU-8

(iii) it should be conductive and piezoresistive (v) possess thermal and mechanical stability

similar to SU-8 (vi) possess compliance compatibility with SU-8 (similar Young’ modulus

value as that of SU-8). SU-8/CB nanocomposites were prepared and characterized in order to

investigate in these aspects.

The preparation of SU-8/CB nanocomposite layer involves the following steps:-

1. Carbon Black being hygroscopic, it was required to pre-heat the CB powder in a baking

oven at a temperature between 100oC and 110oC to remove the trapped moisture.

2. Mechanically grind the CB powder and weigh the quantity as per the required

concentration of CB in SU-8 typically quantified as CB weight percentage or CB volume

percentage. CB weight % and CB volume % can be defined as

퐶퐵푤푒푖푔ℎ푡% =

(푀 )(푀 ) + 푉 × 휌

× 100 (3.1)

퐶퐵푉표푙푢푚푒% =

⎩⎪⎨

⎪⎧ 푀

휌

푀휌 + 푉

⎭⎪⎬

⎪⎫

× 100 (3.2)

Where M,휌 and V are mass, density and volume respectively. 휌 = 1.12 g/mL ;

휌 =1.2 g/mL

3. Ultrasonic mixing process of CB in SU-8.

The mixing processes of CB in SU-8 2002 were tried out with bath ultra-sonication as

well as probe sonication out of which the probe sonication provided better dispersion than

the other. Sonication was carried out at energy of 600 J. Sonication generated lot of heat

because of which the solvent in the SU-8 was found to get evaporated, leaving a highly

viscous SU-8 /CB mixtures which were very difficult to be spin coated. This problem was

56

solved by performing probe sonication in ice bath. However, the viscosity of the SU-

8/CB mixture seemed to be higher compared to SU-8 2002 and this would lead to higher

layer thickness. In addition to this observation, the spin coated film was highly non-

uniform with CB clusters formed (Figure 3.1). This non-uniformity is clearly due to the

poor dispersion of CB in SU-8 and the effect of centrifugal force during spin coating. In

order to improve the dispersion of CB in SU-8 and to compensate for the expected

increase in viscosity with CB loading, a mixture of Nanothinner 2000 from Microchem

and SU-8 2002 in 1:1 ratio was tried out in place of SU-8 2002 in all the subsequent

experiments. This has resulted in an improvement in dispersion and hence quality of SU-

8/CB film as presented in later part of this section.

Figure 3.1 (A) Optical micrograph of SU-8/CB nanocomposite with lot of CB clusters on a pattern of Au on silicon dioxide. (B) SEM micrograph showing a CB cluster in SU-8

4. Microfabrication process for SU-8/CB nanocomposite thin films

The as-prepared SU-8 /CB nanocomposites were spin coatable. Experiments were carried

out to investigate the patternability of SU-8/CB nanocomposite using the standard

photolithographic process for SU-8. It was observed that even with the addition of CB,

the photosensitive nature of SU-8 had been retained. However because of the presence of

CB which is known to absorb UV, the exposure doses required for proper patterning of

SU-8/CB nanocomposites were found to be more compared to that of SU-8 and the

required UV dose increased with increase in CB concentration. Other than this difference,

it was also observed that CB residues were seen even in the non-patterned areas after

development in SU-8 developer solution. The optical micrograph of SU-8/CB resistor

patterns (Figure 3.2(A)) before development on an oxidized silicon wafer with gold

57

contacts illustrates this observation. So, a separate ultra-sonication process in isopropyl

alcohol (IPA) was carried out in order to remove these CB residues. With this cleaning

process, CB residues were getting dispersed in IPA leaving the non-patterned area as

clean as before spin coat process of SU-8/CB (Figure 3.2(B)).

Figure 3.2 Optical micrograph of photo-lithographically patterened SU-8/CB with 5 CB vol. % (A) before and (B) after ultra-sonication clean step in IPA

This figure illustrates the effect of ultrasonication step in removing the CB residues after lithographic process.

The UV dose required for plain SU-8 sample with thicknesses varying from 500 nm to 2

microns was typically between 55 mJ/cm2 and 70 mJ/cm2. In order to optimize the UV

doses required for SU-8/CB nanocomposites with a given CB concentrations, the samples

were photo-lithographically patterned with different UV doses and the quality of SU-

8/CB film patterns were analysed.

Figure 3.3 SU-8/CB (8.4 vol%) strips patterned using different UV exposure doses (A) 204 mJ/cm2 (B) 408 mJ/cm2 (C) 530 mJ/cm2

(D) SEM image of SU-8/CB strip with 530 mJ/cm2. This figure illustrates that the UV exposure dose of 530 mJ/cm2 was required for patterning SU-8/CB nanocomposite (8.4 vol.%) sample in comparison to 60 mJ/cm2 for SU-8.

58

Micrographs of SU-8/CB nanocomposite (8.4 CB vol. %) patterns formed with different

UV doses is provided in Figure 3.3 and this could illustrate the need for higher dose for

achieving the crosslinking density in SU-8/CB nanocomposite.

The microfabrication process for SU-8/CB nanocomposite can be summarized as Figure 3.4 .

Figure 3.4 Microfabrication processing steps for SU-8/CB nanocomposite

SU-8/CB samples with different CB concentrations were prepared in order to analyse the thin

film morphology and thickness of these nanocomposite films.

Figure 3.5 Thickness and RMS roughness as a function of CB concentration

59

As given in the plot (Figure 3.5), it was observed that the thickness and roughness (Figure 3.6

(A & B)) of SU-8/CB film increased with increase in CB loading and the increase in thickness

is due to increase in viscosity of SU-8/CB spin coatable resist. SU-8/CB nanocomposite

samples were coated with 10 nm of gold and subsequently observed under Raith 150 scanning

electron microscope (SEM). The SEM micrograph of SU-8/CB nanocomposite with 8 vol.%

of CB is shown in Figure 3.6 (C).

Figure 3.6 (A & B) AFM topographical images of SU-8/CB Composite films (A) 6 vol% (B) 7.4 vol% (C) SEM micrograph of SU-8/CB composite with 8 vol.%

RMS roughness of 7.4 vol.% SU-8/CB nanocomposite film (74 nm) is depicted to be greater than the RMS roughness of 6 vol.% SU-8/CB (45 nm).

60

3.3 Electrical characterization: Percolation study on SU-8/CB

nanocomposite

The conduction behaviour in composite systems in general are understood in terms of

percolation phenomena which states that, when a critical amount of conductive filler is loaded

into an insulating polymer matrix, continuous linkages or network of filler particles are

formed which result in transformation of the composite from insulator to a conductor [98-

100]. This phenomenon is depicted graphically in Figure 3.7 which shows that, as the volume

fraction of the conductive filler is increased, the probability of formation of continuous

conducting network increases until the critical volume fraction, Vc, beyond which the

electrical conductivity suddenly becomes high.

Figure 3.7 Percolation theory applied to conductive composites. The formation of the first complete particle linkage occurs at Vc that resulting in a sharp decrease in resistivity [101]

In order to measure the conductivity of SU-8/CB nanocomposites, the resistor strips of these

nanocomposites were patterned on samples with gold (Au) contact wires and contact pads.

For this, oxidized silicon wafers were patterned with Cr/Au as contact pads. These samples

were dehydrated at 120oC for 30 minutes. SU-8/CB mixture with different weight percentages

from 5 wt. % to 10 wt. % were prepared and spin coated at 3000 rpm for 30 seconds with a

spread cycle at 500 rpm for 5 seconds. Then prebake step at 68oC and 90oC was performed

followed by a UV exposure in Karl Suss MJB3 standard aligner. Post exposure bake similar

to prebake was done in order to complete the UV initiated cross linking. The samples were

61

developed in SU-8 developer and finally ultrasonicated in IPA leaving SU-8/CB strip patterns

between the Au electrodes (Figure 3.2(B)).

Figure 3.8 Percolation characteristics of SU-8/CB composite. Power law fit parameters indicate a good fit with R2 = 0.99 and percolation threshold, fc = 6.5 wt.% ( or 6.1 vol.%)

The resistors were probed inside standard Suss Microtech probe station and their resistances

were measured using Keithley 2602 source measuring unit. The resistivity of these composite

resistors decreased with increase in CB wt. % (Figure 3.8). This obeyed a power law equation

for percolation, given by resistivity, ρ ~ (f - fc)-t where f is the fraction of the conductive filler

and fc is the critical fraction known as the percolation threshold and t is the critical exponent

[102]. This power law fit on the experimental data yielded a typical percolation threshold of

6.5 wt%. The conduction in these films was due to the formation of carbon black conductive

network which became denser with increase in conductive particle concentration as depicted

in Figure 3.9.

62

Figure 3.9 Optical micrograph (20X) of SU-8/CB film on SU-8 for different CB concentration

showing the density of conductive filler network.

3.4 Development of SU-8/CB nanocomposite microcantilevers

As mentioned earlier, microcantilevers with integrated piezoresistors enable electrical

transduction of strain by a change in resistance. The resulting change in resistance can be

represented in terms of deflection and surface stress with the help of equations 3.3 and 3.4

respectively [103].

∆푅푅

= 3K1 −퐿

2. 퐿 푑

퐿. Z (3.3)

where K is the gauge factor of the piezoresistive material, Pie zoL is the length of piezoresistive

film, L is the length of the cantilever, d is the distance of piezoresistive layer from the neutral

axis and Z is the amount of downward deflection.

63

22

1 .

13 2

T RS

i ii ii i ici

R Z ZKR E h hE h Z

(3.4)

where S is the surface stress, TZ is the position of top layer, RZ is the position of

piezoresistive layer, and iE , ih and icZ are the Young’s modulus , thickness and position of

the ith layer with respect to neutral axis.

According to equation 3.3, the deflection sensitivity increases with increase in gauge factor of

the piezoresistive layer, the distance of piezoresistive layer from neutral axis and with

decrease in ratio of length of piezoresistor film to the length of the cantilever. The design

presented here was chosen satisfying these conditions.

Figure 3.10 Schematic of SU-8/CB nanocomposite microcantilever die

The planar schematic of the SU-8/CB nanocomposite microcantilever is given in Figure 3.10. The SU-8 die holds one measurement microcantilever and a reference microcantilever so as to

support differential measurements. The design parameters chosen for the SU-8/CB nanocomposite microcantilevers are listed in

Table 3-1.

64

Table 3-1 Dimensional parameters of polymer nanocomposite microcantilever

Parameter Value (m)

Cantilever length 250 Width 120 Cantilever thickness 3

Upper SU-8 layer thickness 0.4 Lower SU-8 layer thickness 1.8

SU-8/CB piezo-layer thickness 0.8

3.4.1 Fabrication process

The cantilevers were fabricated following the flip-chip approach which was discussed in

Chapter 2. This was done by defining of individual layers on a dummy substrate and release

of the whole polymer structure from the substrate. The detailed process flow for polymer

nanocomposite microcantilever is shown in Figure 3.11. The layout of the mask set used for

fabricating these devices is given in Appendix D and process recipe is given in Appendix C.

2” Silicon wafers were piranha cleaned and a sacrificial layer of sputtered silicon dioxide with

thickness >200 nm was deposited using RF sputtering system (Figure 3.11(1)).

Layer 1: Mask #1: SU-8 for first layer of microcantilever and the die with contact vias

First layer of the device structure to be defined was the first layer of the microcantilever and

the first layer of the SU-8 die or the chip with contact vias. This layer should be as thin as

possible in order to (1) support good step coverage for contact lines and (2) to aid in shifting

the position of piezoresistive layer as far away as possible from the neutral axis with the

minimum possible overall thickness of the microcantilever. Since the minimum thickness that

could be achieved by spin coating the lowest viscous formulation of SU-8 available in the lab,

i.e., SU-8 2002 was 900 nm, a set of experiments were conducted by mixing SU-8 2002 and

nanothinner in order to find out the minimum thickness of SU-8 that could be achieved. A

65

mixture of SU-8 2002 and nanothinner in the ratio of 1:1 could give a thickness of 400 nm.

This was lithographically patterned (Mask#1) using the regular SU-8 processing steps (Figure

3.11(2).

66

Figure 3.11 Fabrication process flow for SU-8/CB nanocomposite microcantilevers (1) Sacrificial or release layer (2) First layer of SU-8 (3) Cr/Au for contact pads (4) Ti/Au contact wire (5)SU-8/CB nanocomposite layer (6) encapsulating SU-8 (7) Thick SU-8 die

base (8) Release of cantilever die from the substrate Part (8) in this figure shows the released SU-8 microcantilever die after flipping. Two microcantilevers with their respective contact pads accessible through the contact vias can also been seen.

Layer 2: Mask #2: Contact pads

Cr/Au (20 nm/250 nm) was thermally evaporated and was subsequently patterned for contact

pads (Mask # 2, Figure 3.11(3)). Etching technique was chosen instead of lift-off process for

patterning the contact pads as the Cr/Au contact layers were found to break near the edges of

contact vias while performing the lift-off process. Also, Cr was used as the adhesion

promoting layer instead of Ti as the etchant of Ti is HF based. So the etch patterning process

for Ti might even affect the silicon dioxide sacrificial layer.

Layer 3: Mask #3: Wires or electrodes for contacting the piezoresistive layer

Contact wires or the electrode for the piezoresistive layer was also defined using Au. Though

the electrode material (Au) was same as the contact pad material defined using mask #2, this

was done as a separate step with a reduced thickness (60 nm), as some parts of these gold

electrodes extended into the microcantilever structure. So RF sputtered Ti/Au wires were

pattered using a standard lift-off technique (Mask#3, Figure 3.11(4)).

Layer 4: Mask #4: SU-8/CB nanocomposite piezoresistive layer

Then SU-8/CB nanocomposite layer with CB concentration of 10 wt. % was patterned on

these gold electrodes (Mask#4, Figure 3.11(5)) by following the process explained in previous

section.

Layer 5: Mask #5: SU-8 encapsulation layer for the cantilever

The encapsulation layer of SU-8 2002 of thickness 1.8 microns was patterned (Mask#5,

Figure 3.11(6)).

Layer 6: Mask #6: Thick SU-8 layer for anchor or die

Then a thick layer of SU-8, (SU-8 2100) was spin coated and pattered for SU-8 die and

frames of thickness 150 m which hold these dies together (Mask#6, Figure 3.11(7)). A

67

photograph of the processed silicon wafer after final lithography step is shown in Figure

3.12(A).

Release of structure

The SU-8 frames holding arrays of microcantilever dies were released from the substrate by

soaking the sample in BHF 5:1. The SU-8 frames were seen to be floating on the HF solution

and they were slowly transferred to DI water. Then the Cr layer on the gold contact pads were

etched by using chromium etchant (22% Cerric ammonium nitrate and 8% acetic acid in 70%

DI water) at room temperature. The cantilevers were slowly soaked again in DI water and

then in IPA for 2 minutes and then dried (Figure 3.12(B, C). The average release time of

cantilever die was about 90 minutes.

Figure 3.12 Photographs and micrographs of fabricated polymer nanocomposite microcantilever devices at different stages of fabrication (A) Processed 2" silicon wafer after

final lithography step (B) Released devices soaked in IPA after etching of Cr layer on contact pads (C) Released and dried SU-8 frames holding the microcantilever devices (D and

E) SEM and optical micrograph of on of the microcantilever die In (E & F), the black patch on the microcantilever is the SU-8/CB piezoresistor pattern. SEM micrograph in (D) shows straight microcantilevers confirming the stress free nature of the composite structure

68

The schematic of the final device is shown in Figure 3.11(8). The device micrographs are

shown in Figure 3.12(D, E, and F). The overall thickness of the microcantilevers was 3 m

and SEM micrograph indicates that the composite microcantilever structure is straight and

hence free of residual stress.

3.4.2 Characterization

The fabricated cantilevers were electromechanically characterized to demonstrate the

piezoresistive behaviour. Calibrated micromanipulator probe needle was used to deflect the

cantilever tip. Current voltage characteristics were performed on these cantilevers using

Keithley 4200 source measuring unit. The minimum measurable downward movement of this

micromanipulator was 10 m.

The electromechanical characterization results of these cantilevers are presented as R/R

versus deflection plot in Figure 3.13.

Figure 3.13 Electromechanical characterization plots of two SU-8 nanocomposite

microcantilevers with different resistances

It was observed that deflection sensitivity was better for those cantilevers with a higher

nominal resistance, as can be observed from the slopes of the linear fit curve for two different

69

samples of varying resistances. The highest deflection sensitivity achieved with these devices

was 0.55 ppm/nm. Using this experimental result on deflection sensitivity, the gauge factor of

the 10 wt. % SU-8/CB nanocomposite piezoresistor was extracted using equation 3.3. The

gauge factor was found to be 20 that is approximately 10 times higher compared to Au strain

gauges. The surface stress sensitivity of these cantilevers was extracted using equation 3.4 and

was found to be 4.1x10-3 [N/m]-1. This is 10 times the sensitivity of the SU-8 cantilever with

Au [34] and at least a factor of two higher compared to the previously reported results on SU-

8/CB composite cantilevers [57] . This could be attributed primarily to the lower thicknesses

of the piezoresistive layer and the microcantilever structure achieved with this fabrication

process.

3.4.3 Shortcomings and scope for improvement

(i) The fabricated devices exhibited good deflection sensitivity and surface stress

sensitivity. The design helped in investigating the feasibility of fabricating polymer

nanocomposite microcantilevers with a minimum thickness of 3 m and aided in

extracting the gauge factor of SU-8/CB nanocomposite. However, the design of the

device is not optimum for surface stress measurements based microcantilevers for

bio/chemical sensors, as the SU-8/CB nanocomposite piezoresistive layer did not

encompass the entire length of the cantilever.

(ii) Process and device variability due to statistical distribution of CB in SU-8 matrix

and edge roughness and random breakages of SU-8/CB pattern formed during the

final ultra-sonication cleaning step. A systematic characterization experiment to

study the dispersion of CB in SU-8 was required in order to understand the source of

variability and to optimize the SU-8/CB nanocomposite preparation process.

(iii) Though the sensor concept with improved sensitivity was demonstrated, a systematic

characterization study of the SU-8/CB nanocomposite was necessary to decide the

optimal range of concentration of CB filler in SU-8.

(iv) The fabrication process was complex with 6 levels of lithography which obviously

affect the process yield and increases the cost of fabrication. So the possibility of

reducing the number of mask levels had to be investigated.

(v) Sensitivity of the SU-8/CB nanocomposite sensor was as high as 4 ppm [mN/m]-1.

But in order to evaluate the sensor for any application, along with the

70

characterization of sensitivity, an estimate of minimum detectable limit was required.

This could be done by measuring the built-in noise levels in the device.

3.5 Optimization of SU-8/CB nanocomposite microcantilevers

Systematic experiments were carried out in order to address the shortcomings with the

initially developed polymer nanocomposite microcantilevers. The different aspects covered

here are as follows :-

(i) Systematic characterization of the SU-8/CB nanocomposite in order to understand its

electrical and mechanical characteristics. This was required to decide the optimal

range of concentration of CB filler in SU-8 in order to improve the overall

performance of the polymer nanocomposite microcantilever sensor in terms of its

sensitivity and device variability.

(ii) Improvements in the device design aiming for nanomechanical cantilevers for surface

stress measurements and for reducing the fabrication process complexity.

(iii) Characterization of fabricated polymer nanocomposite microcantilevers for their

mechanical, electrical, electromechanical and noise performances which could aid in

benchmarking their performances with other existing piezoresistive microcantilever

devices.

3.5.1 Characterization of dispersion of Carbon black in SU-8

In order to address the variability of polymer nanocomposite microcantilevers arising from

the statistical distribution of CB nanoparticles in polymer matrix, the quality of dispersion of

CB in the polymer needs to be characterized. This characterization was done using Dynamic

Light Scattering (DLS) technique. In the case of particles suspended in a liquid medium, DLS

measures the Brownian motion of particles and relates it to the size of particles [104]. The

particle diameter obtained in DLS is hydrodynamic diameter since that refers to how a

particle diffuses within a liquid medium. As SU-8 being photo sensitive, the solvent of SU-8

i.e., cyclopentanone was used as the liquid medium. CB samples were prepared with different

71

sonication energies for DLS analysis. The system used was BI-200SM from Brookhaven

instruments [105]. The size distributions of CB nanoparticles in different samples were

analysed. The mean diameter of CB in cyclopentanone is plotted as a function of sonication

energy as given in Figure 3.14(C). The reduction in diameter tends to saturate with increase in

sonication energy. The size distribution of CB in cyclopentanone for sonication energy of 3 kJ

and 0.6 kJ are given as inset in Figure 3.14 (A) and (B) respectively.

Figure 3.14 Dispersion characterization results from DLS (A & B) Size distribution of CB for samples prepared with energy of sonication of 3kJ and 0.6 kJ. (C) Mean diameter of carbon

black as a function of duration of sonication.

SU-8/CB nanocomposite samples with different sonication parameters were prepared and

were coated with 10 nm of gold for subsequent evaluation under Raith 150 scanning electron

microscope (SEM). SEM micrographs (Figure 3.15) confirm the DLS measurement results on

72

better dispersion of CB nanoparticles in a SU-8 matrix prepared with sonication energy of

3kJ.

Figure 3.15 . SEM mage of SU-8/CB Composite films prepared with sonication energy of (A) 0.6 kJ (B) 3 kJ in pulse

SU-8/CB nanocomposite sample was prepared using the sonication energy of 3 kJ and was

spin coated and patterned using the photolithographic process explained in previous sections.

It was observed that the line edge roughness, another important parameter leading to device

variability, was also considerably reduced as shown in Figure 3.16.

Figure 3.16 SEM micrograph of SU-8/CB nanocomposite pattern with reduced line edge roughness

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3.5.2 Nanoindentation for mechanical characterization of SU-8/CB

nanocomposites

The basic mechanical characterization of these thin film nanocomposites was performed by

using nanoindentation technique with Hysitron Triboscope. This characterization was

required to check whether nanoparticle filler loading would potentially change the Young’s

modulus and hardness of SU-8 nanocomposite. Samples with varying CB concentrations were

prepared on a silicon substrate with the specifications given in Table 3-2.

Figure 3.17 Typical load vs. depth of indentation plot obtained in nanoindentation experiment

The nanoindentation technique is based on driving an indenter into the material whose

mechanical characterization has to be done [106]. This leads to elastic and plastic deformation

in the material. When the indenter is withdrawn from the material, the elastic deformation is

recovered. During the loading & unloading processes, load is recorded as a function of depth

of indentation as illustrated in Figure 3.17. Olivar Pharr method [107] can be used to analyse

the unloading part of the load-penetration depth curve in order to extract the reduced modulus,

Er and hardness, H using equations 3.5 and 3.6.

74

2rc

SEA

(3.5)

max

c

PHA

(3.6)

Here S, contact stiffness is the slope of unloading curve as shown in Figure 3.17 , Ac the

projected contact area, Pmax the maximum load and β is a constant that depends upon the

indenter geometry. Young’s modulus of the composite thin film is calculated from Er using

equation 4.7.

22 11 1 i

r iE E E

(3.7)

Here Ei and νi are the Young’s modulus and Poisson’s ratio of the indenter.

Table 3-2 Details of polymer nanocomposite thin film samples prepared for nanoindentation experiments

Sample ID CB vol. % Thickness RMS Roughness

SU-8 0 2 µm 7 nm

SU-8_CB_2.4 2.4 0.5 µm 25 nm

SU-8_CB_4.9 4.9 0.75 µm 36 nm

SU-8_CB_6 6 0.8 µm 45 nm

SU-8_CB_7.8 7.8 0.95 µm 75 nm

SU-8_CB_9.4 9.4 1 µm 102 nm

Indentations were carried out using a Berkovich diamond indenter (Ei = 1141 GPa and νi=

0.07) with maximum load (Pmax) varying from 80 µN to 600 µN. Maximum loads applied to

the thin film samples were varied so that the indentation depth was limited to within 10% of

film thickness. The lower limit of Pmax is decided in such a way that indentation depth is at

least four times more than the roughness of the sample. Polymers are known to show their

75

peculiar viscoelastic behavior while indenting and in order to minimize the effect of

viscoelasticity high loading and unloading rates were used [108]. The viscoelastic behaviour

if present would be indicated with the presence of a “nose” in the force curve due to the

increase in penetration depth even during the unloading portion of the load curve.

Figure 3.18 Load versus indentation depth for different loads. Inset: Scanned image of a set of

indents in SU-8

Load-indentation depth curves for different Pmax values are shown in Figure 3.18 along with

an array of Berkovich indents on the SU-8 surface. It can be noticed that viscoelastic

behaviour is not seen in these load curves.

Young’s moduli and hardness values for different load-depth curves were extracted using the

method explained above and are shown as a function of maximum indentation depth in Figure

3.19. Young’s modulus tends to increase for higher depth of indentation, whereas hardness

does not show a significant variation with indentation depths. It is widely accepted that for

depths of indentation greater than 10% of film thickness, substrate interaction effects are

observed. This can be clearly seen in Figure 3.19, where the dotted line indicates 10% of film

76

thickness. For hmax values beyond that, there is a sharp increase in the modulus values, which

could be attributed to the effect of the stiffer substrate, silicon (E = 170 GPa).

Figure 3.19 Young’s modulus and hardness of SU-8 as a function of indentation depth. The discontinuous line indicates the indentation depth of 10% of film thickness

A better analysis of indentation of compliant films on stiffer substrates can be carried out

using a modified King’s analysis as given in equation 3.8 [109]. In this method the effect of

substrate modulus is compensated appropriately.

2 ( ) ( )2 211 11 1t h t h

fi sa a

r i f s

e eE E E E

(3.8)

where Es and Ei are the moduli of the substrate and the indenter respectively. Parameters a, t

and h are square root of projected area, film thickness and indenter depth respectively. ‘α’ is a

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fitting parameter which is obtained from King’s plot [109] based on the value of normalized

punch size, a / (t-h). The calculated modulus as a function of indentation depth based on the

modified King’s analysis is given in Figure 3.20 (A).

Figure 3.20 (A) Young’s modulus of SU-8 as a function of indentation depth after modified King’s analysis. (B)Young’s Modulus of SU-8/CB composite as a function of CB Vol %.

Here all the samples were indented with Pmax = 450 N

Similarly Young’s moduli and hardness for other samples with different CB concentrations

were also calculated. Here the data shown is for indentations carried out for an array of

indents carried out at a Pmax of 450 N. Modulus values were found to increase with

increasing CB filler loading (Figure 3.20 (B)). This agrees reasonably well with two existing

theories for particulate filled polymer nanocomposites with spherical fillers, namely parallel

mixing model (ref. equation 3.9) and the modified Guth’s model (ref. equation

3.10)[110,111].

m m f fE V E V E (3.9)

21 2.5 14.1m f fE E V V

(3.10)

78

where V and E are volume fraction and Young’s modulus respectively. Subscripts m and f

refer to matrix (SU-8) and filler (CB). The value of Em used here, for SU-8 is 6 GPa as

obtained from nanoindentation results and Ef for CB is 15 GPa [110].

Hardness values for different samples did not vary much with CB filler concentration and all

the values were within 10% of the hardness of pure SU-8 sample.

3.5.3 Electrical characterization of SU-8/CB nanocomposite: Conduction

mechanism and temperature dependence

The aim of electrical characterization was to understand the conduction behaviour of SU-

8/CB composites at different CB loading and find out the usable range of concentrations of

CB in SU8. For this, SU-8/CB resistors of varying CB concentration with gold contacts were

fabricated. Current voltage (I-V) characteristics of SU-8/CB resistors with different CB

loadings were analysed to understand the conduction mechanism. I-V characteristics of SU-

8/CB resistors with lower concentration samples exhibited symmetric and a non-ohmic

behaviour and the characteristics tend to become linear for samples with CB loading well

above percolation threshold Figure 3.21(A) shows IV characteristics for SU-8/CB composites

with 4.9 CB vol. % and 9.4 CB vol. %.

Figure 3.21 (A) I-V characteristics for SU-8/CB composites with 4.9 CB vol.% and 9.4 CB vol.% along with the theoretical curve fit (B) Value of power term ‘n’ and exponential term

‘B’ for different CB resistors

79

A theoretical model explains the conduction behaviour of nanocomposites in non-ohmic

regime by modelling conduction as an electron emission process, probably a tunnelling of

electrons from one particle to next particle inside the polymer matrix. The emission current

density, J is related to bias voltage through a combination of power term and exponential

term, given by equation 3.11 [100].

nJ=AV expBV

(3.11)

where, A B and n are constants. A is proportional to tunnelling frequency, n varies between 1

and 3, and B is proportional to the average inter-particle separation. I-V characteristics for

different CB concentrations fit well with this model (Figure 3.21 (A)) for curve fit details).

This is illustrated in the details of curve fit in Figure 3.21 (A) (Adj.R-Square = 0.99) using

equation 6 for the sample with 4.9 CB vol. %. From the extracted constants, it was observed

that value of power term ‘n’ moves towards 1 and value of ‘B’ vanishes for increasing loads

of CB (Figure 3.21 (B)). The tunnelling of electrons through the gaps separating the carbon

aggregates can reasonably justify the observation of translation of non-ohmic to nearly ohmic

(quasi-ohmic) behaviours in IV characteristics with increase in CB vol%.

SU-8/CB resistors with different CB concentrations (10 devices from each group) were

characterized to investigate the variability in the resistance values. The information is as

plotted in Figure 3.22. It can be observed that the percentage variability in resistance

decreases with increase the CB vol. % and the variability is < 30 % for samples with CB

concentrations > 8 vol. %.

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Figure 3.22 Electrical characterization of nanocomposite resistors showing the variability in

resistance as a function of CB concentration.

3.5.3.1 Temperature dependent resistivity behaviour of SU-8/CB

nanocomposite

Electrical characterization experiments were carried out to understand the temperature

dependance of resistance of these composites with two different CB loading of 6 vol% and

7.8 vol % which are just above and below percolation threshold (Figure 3.23).

One can observe that in both the samples, the resistance gradually increases with rise in

temperature followed by rapid increase in resistance in a short span between between 90oC

and 105oC. Subsequently the resistance was found to decrease with increase in temperature

resistance after a certain temperature giving a peak in temperature depedence of resistance at

temperature values between 100 oC and 105 oC. The increase and decrease in resistivity with

temperature are in general termed as positive temperature coefficient of resistivity (PTC)

effect and negative temperature coefficient of resistivity (NTC) effect respectively.

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Figure 3.23 Temperature dependent resistance of SU-8/CB composites with CB concentration

of (A) 6 vol. % (B) 7.8 vol. %

PTC effect could be attributed to the larger thermal expansion of polymer matrix compared to

that of conductive filler CB that might lead to the disruption of CB conductive network

(Figure 3.24(A)). PTC effect could also be explained by theory of electron tunnel effect as

depicted in Figure 3.24(B) which says that at low temperature there exists uniform

distribution of interparticle gaps with the gaps width small enough to allow electron

tunnelling and with increase in temperature these interpartcile gaps become random and wider

leading to reduction in probability of tunnelling and hence increase in

resistivity[102,112,113].

Figure 3.24 (A) Contact current mechanism in polymer PTC composite (B) Tunneling current

mechanism in polymer [102] A

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The NTC effect might be due to densification of SU-8 polymer with progressive crosslinking

above the highest temperature of the latest bake cycle during the processing (~95oC). This

densification leads to decrease in distance between the conductive CB particles and hence

decrease in resistivity of the SU-8/CB nanocomposite. It was also observed that samples with

lower filler, CB loadings, exhibited strong temperature dependent resistivity behaviour.

3.5.4 Design and fabrication of SU-8/CB nanocomposite microcantilevers

with improved sensor performance

Second generation SU-8 nanocomposite microcantilevers were designed with new parameters

and were fabricated based on the knowhow obtained from the systematic characterization

experiments detailed in the previous sections. The design for polymer nanocomposite

microcantilever is chosen in order to improve the surface stress sensitivity, common mode

rejection, mechanical stability and packaging compatibility of the sensor. Dimensions of

microcantilevers are decided based on stiffness and resonance frequency of microcantilever

which are the two important mechanical properties of beams for mechanical sensitivity and

stability under external vibrations. The resonance frequency should be greater than 5 kHz so

that mechanical noise would not excite the microcantilever. The thicknesses of individual SU-

8 layers were chosen in order to keep the piezoresistive SU-8 nanocomposite layer away from

neutral axis to improve the piezoresistive sensitivity, while considering the microfabrication

constraints for SU-8. SU-8 cantilever die contains two sets of measurement and reference

cantilevers as depicted in the planar and cross sectional schematic of the polymer device die

(Figure 3.25).

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Figure 3.25 Device design schematic. (A) Planar schematic (B).Cross sectional schematic of SU-8 cantilever with embedded SU-8/CB composite.

Other than the “U” shaped cantilevers, “V” shaped cantilevers were also designed as part of

the same mask. Mechanical stability of “V” shaped cantilevers was known to better compared

to that of simple rectangular cantilevers and it has got inherent advantage of reducing the

lateral torsion.

For the SU-8/CB nanocomposite piezoresistor, optimum carbon black filler loading in the

range of 8 – 9 vol. % was chosen based on the following requirements and the justification

provided in the previous section based on the material characterization data. (i) Low Young’s

modulus: for this, the CB concentration is kept as low as possible. (ii) UV patternability of the

SU-8/CB nanocomposite with minimum possible edge roughness: the quality of patterns

worsens for SU-8/CB samples for CB filler loadings > 9 vol.%. (iii) Good strain sensitivity:

SU-8/CB composites with CB concentrations just above the percolation threshold are

expected to give the maximum strain sensitivity. (iv) Ohmic conduction behaviour and

minimum variability in conduction: the variability in SU-8/CB resistance values decreases

with increase in CB concentration.

The fabrication of this device structure involves the defining of individual layers on a dummy

substrate, silicon and release of the whole polymer structure from the silicon substrate.

The detailed process sequence with individual layer patterns is illustrated in Figure 3.26.

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Figure 3.26 Fabrication process flow (1) First layer of SU-8 (2) Cr/Au for contacts (3) SU-8/CB composite layer (4) encapsulating SU-8 (5) Thick SU-8 die base (6) Release of

cantilever die from the substrate.

The fabrication of these devices started with a pirahna cleaned silicon substrate with a 200 nm

of silicon dioxide that is indented to be the released layer. A 500 nm of SU-8 (SU-8 layer 1)

was patterned to define the microcantilever structure and the die with contact vias. Here a

formulation of SU-8 from Microchem (SU-8 2000.5) that can give SU-8 layer thickness of <

500 nm was used in place of a mixture of SU-8 2002 and nanothinner that was used in

Generation I devices. This was followed by the formation of gold (200 nm) wires and contact

pads with chrome (10 nm) as the adhesion support layer. Subsequently SU-8/CB

nanocomposite strain sensitive layer was patterned followed by the patterning of SU-8 (1.6

um) encapsulation layer (SU-8 layer 2). In order to improve the process yield of SU-8/CB

nanocomposite layer pattern the process sequence was slightly modified. After the UV

exposure and post exposure baking step for SU-8/CB nanocomposite layer, the development

of the layer and sonication in IPA was not performed. Instead, the photolithography step for

SU-8 layer 2 was performed using the same mask as that of SU-8/CB layer. After this step,

development of both the layers together followed by sonication in IPA was performed to

remove the carbon black residues. Instead of final sonication in IPA, development of both the

layers can also be performed in sonication. The SU-8 encapsulation layer protected the SU-

8/CB layers from unwanted breakages and edge roughness while sonication. The device

85

patterns formed after SU-8/CB resistor and encapsulation lithography steps is shown in

Figure 3.27.

Figure 3.27 SU-8/CB resistor patterns on “V” shaped and “U” shaped cantilever areas after

lithography process 4

Final lithographic step was for forming the SU-8 anchor die (~150 µm) for the cantilevers.

The photograph of a processed silicon wafer after this final lithographic step is shown in

Figure 3.28. The figure shows arrays of device dies attached to frames on a silicon wafer. The

release of polymer device chips from the dummy substrate was performed by wet etching of

the silicon dioxide layer in buffered oxide etch for an approximate duration of 30 minutes.

The process recipe followed for fabricating these devices and the design of mask sets are

given in Appendix C and Appendix D respectively.

With this new device design and process, the numbers of photolithography levels were

reduced to 5 in comparison to six mask levels in previous device. This reduction in

photolithographic steps became possible due to the change in the device design that obviated

the need for additional gold wire for the piezoresistor. The fabricated SU-8 cantilever

structures on a 2” silicon wafer before releasing from the dummy substrate is shown in Figure

3.28. The released device chips and SEM image of one of the fabricated devices are also

shown in Figure 3.28.

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Figure 3.28 (A) Photograph of the processed wafer after the final lithographic step. Arrays of polymer devices attached to the dummy substrate just before the release can be seen (B)

optical micrograph of one of the devices on the wafer. (C) Arrays of polymer nanocomposite microcantilever device chips after release process (D) SEM image of one of the device chips

containing 4 cantilevers.

These polymer cantilevers are about 3 µm thick and the SEM micrograph confirms the stress

free nature of these free standing polymer nanocomposite structures achieved through an

optimization of baking parameters for individual layers of SU-8 at different levels of

lithography.

3.5.4.1 Modified microfabrication process for SU-8/CB nanocomposite microcantilevers with improved process yield

The device variability in general observed in polymer nanocomposite microcantilevers and

the low yield in fabricating these devices can be attributed to the following facts. (1) Poor

dispersion of CB in SU-8 (2) Non-uniform distribution of CB in SU-8/CB layer after spin

coating the nanocomposite solution (though well dispersed) on wafers with some layers being

already patterned. If these previous patterns present on the wafers are thicker (> 1 m), lot of

striations were seen on the spin coated film of SU-8/CB on such a sample and this is a clear

indication of non-homogeneity. (3) Breakages of SU-8/CB patterns during final sonication

step which is the main reason for low process yield. Out of these, the problem (1) has been

87

addressed appropriately as discussed in previous sections. A common solution for addressing

(2) and (3) was arrived at by modifying the process flow.

Figure 3.29 Modified fabrication process flow (1) Sacrificial layer (2) First layer of SU-8. But not developed (3) SU-8/CB composite layer before development(4)SU-8/CB nanocomposite

layer and SU-8 layer 1 after development with sonication (5) Cr/Au for contacts (6) encapsulating SU-8 (7) Thick SU-8 die base (8) Release of cantilever die from the substrate.

The modified process is as given in Figure 3.29. In this process, the first layer and SU-8 and

SU-8/CB layer are patterned together. The photolithographic steps for both these layers were

88

done separately using the same mask, whereas the development of both the layers in SU-8

developer solution was done together with sonication. In this process, the presence of an

undeveloped bottom SU-8 layer acts as the lift-off layer for easy removal the undeveloped

SU-8/CB layer. So the samples were patterned with no residues of carbon black with lesser

time of the sonication in comparison to previous process. The yield of SU-8/CB patterning

process was improved to 40 %.

3.5.5 Mechanical characterization: Spring constant and resonant

frequency measurements

Experiments were carried out to characterize the two important mechanical properties, spring

constant and resonance frequency for these polymer nanocomposite microcantilevers.

3.5.5.1 Spring constant measurement: Beam bending technique using nanoindentation

The spring constant of the SU-8 nanocomposite microcantilever was extracted using

microcantilever beam bending technique using a nanoindenter, which provides high load and

displacement measurement sensitivity. In addition to this, the spring constant of the indenter

(diamond) is very high in comparison to the microcantilever structure. So it is a direct and

simple measurement since one need not consider the case of two springs in series as done in

spring constant measurement using standard AFM [53][114].

The nanoindenter was used to apply load to the tip of the microcantilever leading to the

displacement of the microcantilever. Hysitron Triboscope nanoindenter system was used for

this experiment and this instrument records force versus time and the displacement versus

time data simultaneously.

89

Figure 3.30 Load and unload segment of indentation on cantilever

Few experiments were conducted to tune the indenter for this experiment by changing the

parameters like preload gain, integral gain, set point, preload and load rate. Indenter can be

used in (1) load controlled mode where we specify the load with which we need to indent with

a particular load rate and (2) displacement control mode where we specify the displacement of

the indenter in the segment time. Based on the experiments we have performed, displacement

control mode gave reliable and reproducible results without much problem of drift error.

The procedure for performing the spring constant measurement using nanoidneneter is as

enlisted below.

1. Load microcantilever and define the co-ordinates of the sample.

2. Define the point for indent. Since the indenter tip cannot be placed exactly at tip of the

cantilever, the point was at a few microns away from the tip of the microcantilever.

3. Define the set point and preload, preload gain, integral gain.

4. Define the load function. The sample was loaded and unloaded without any holding time

at the maximum loading point.

5. Using the indenter software itself, the indenter was engaged with polymer nanocomposite

microcantilever and the indentation was performed. The load and unload segments

90

showing the load and displacement as a function of time is shown in Figure 3.30. As the

indentation was performed in displacement control mode, there is relatively less scatter in

displacement data and more scatter in the load data. For reliable spring constant

measurements, the peak load or displacement should be such that the cantilever does not

undergo plastic deformation.

6. From this, the load versus displacement curve was plotted for the microcantilever

structure as given in Figure 3.31.

7. Since the indenter tip was placed at a distance 60 µm away from the tip; the actual spring

constant of the cantilever structure was calculated from the slope of the curve using

equation 3.12.

3L lk slope

L

(3.12)

Here L is the length of the cantilever structure, l is the distance from the tip of the cantilever

where load is applied as indicated in the inset of the plot. The spring constant extracted from

measurements was 0.44 N/m whereas the analytically calculated value is 0.26 N/m (Refer

Appendix E for the procedure).

Figure 3.31 Load displacement characteristics of SU-8 nanocomposite microcantilever obtained from nanoindenter. Insets: (1)Schematic of measurement.(2) Optical image

indicating the place of indentation on the SU-8 nanocomposite microcantilever

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3.5.5.2 Resonance Frequency using LDV

For resonance frequency measurements, the experimentation was performed with Polytec

Laser Doppler Vibrometer1(LDV). Microcantilever die was attached to a piezo buzzzer which

provided the actuation. The laser beam from the LDV was directed to the cantilever surface.

The vibration amplitude and frequency were extracted from the doppler shift of the laser

beam frequency due to the motion of the cantilever. The output of an LDV is a continuous

analog voltage that is proportional to the target velocity component along the direction of the

laser beam. When the structure is excited at its natural frequencies, the structure will start to

show its mode shapes. Hence from the frequency response obtained from LDV, the resonant

frequency of the structure can be obtained. The resonant frequency plot obtained from the

LDV for SU-8 nanocomposite microcantilevers (CB concentration of 8.4 vol. %) is given in

Figure 3.32. These microcantilevers were coated with 30 nm of Au.

Figure 3.32 Frequency plot from Laser Doppler Vibrometer (A) “U” shaped cantilever(B)

“V” shaped cantilever

Measured resonant frequency for “U shaped and “V” shaped microcantilevers were 22.6 kHz

and 38.8 kHz respectively. This is closer to the analytically calculated value (~ 20 kHz for

“U” shaped microcantilever) obtained by following the case of multilayer composite

cantilever beam given in Appendix E. Since the resonance frequency of the device is higher

than 5 kHz, it would not be affected by external mechanical vibrations.

1 The LDV measurements performed at IISC Bangalore.

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3.5.6 Electromechanical characterization: Sensitivity

The fabricated microcantilevers were electromechanically characterized to demonstrate the

piezoresistive behaviour. This was performed by deflecting the tip of the microcantilever with

a calibrated micromanipulator needle from Suss Microtech with simultaneous measurement of

resistance using Keithley 4200 source measuring unit. Minimum possible vertical deflection

using these manipulators was 10 µm. ΔR/R for a polymer nanocomposite microcantilever

(8.4 CB vol. %) plotted as a function of deflection is given in.

Figure 3.33 Electromechanical characterization plot. Inset : I-V characteristics of the polymer nanocomposite microcantilever with different bending conditions for a voltage span of 100

mV (200 data points).

The deflection sensitivity (ΔR/R for unit deflection) calculated using this data is 1.1 ppm/nm

and the gauge factor was extracted to be approximately 90. This gauge factor and hence the

deflection sensitivity is higher than that of the previously fabricated devices discussed in

section 3.4. This improvement in the sensitivity was achieved basically due to the

incorporation of a well dispersed SU-8/CB nanocomposite strain sensitive layer with optimum

CB concentration achieved through systematic characterization experiments. The surface

stress sensitivity was calculated using equation (1). The extracted surface stress sensitivity is

7.6 x 10-3 [N/m] -1 which is greater than that of an optimized silicon microcantilever and one

93

order of magnitude higher than that of polymer microcantilevers with Au as the strain gauge

[115] reported in literature. Further improvement can be achieved by tuning the thickness and

shape of the polymer microcantilever structure.

Polymer nanocomposite microcantilevers with lower concentration were also

electromechanically characterized. It was observed that for devices with 7 vol.% CB, the

deflection sensitivity was found to be around 20 ppm /nm and this increased sensitivity is

attributed to the CB concentration nearing percolation threshold. But because of the same

reason, the variability of these low CB concentration devices is also higher (Figure 3.22). So

until the availability of ‘device variability aware design’ for the interface electronics, these

lower concentration devices might not be considered for bio/chemical sensing applications.

3.5.7 Noise characterization

The performance evaluation of a piezoresistive microcantilever sensor is not complete just by

estimating their sensitivity. The resolution of the measurement signal decides the minimum

detectable surface stress or minimum detectable deflection using these polymer

nanomechanical microcantilever sensors. The measurement signal resolution is limited by the

noise originating from the piezoresistor. The two major and important sources of noise in

piezoresistors are (1) Johnson Noise and (2) Hooge noise which is also known as 1/f noise

[42]. Thermal fluctuations of charge carriers cause the Johnson noise and this noise is

frequency independent. The Hooge noise or 1/f noise is an electrical noise that dominates at

smaller frequencies and falls off at high frequencies. According to Hooge’s model [116], the

power spectral density of this noise is inversely proportional to the number of carriers in the

resistor and it increases with increase in bias voltage of the resistor. The total noise power

spectral density can be obtained by adding the all the noise components. The noise level is

obtained by integrating the noise power spectral density over the measurement bandwidth.

The noise spectrum of SU-8/CB nanocomposite microcantilevers with different CB

concentrations were recorded using a noise measurement set up with the schematic shown in

Figure 3.34 .

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Figure 3.34 Noise measurement scheme for polymer nanocomposite microcantilever

A battery operated low noise trans-impedance preamplifier (Stanford Research 570) with gain

varying from 10-3 to 10-12 A/V was used to bias the resistors and to measure and amplify noise

levels in current. A spectrum analyser (SR 750) was used to record the noise power spectrum

in frequencies ranging from 1Hz to a few KHz. By using the battery operated SR 570, the

noise from AC lines was prevented.

Figure 3.35 (A) Noise spectral density of SU-8/CB nanocomposite microcantilevers with 2 different concentrations (B) Noise spectral density at different bias voltage for devices with

8.4 CB vol%

The noise spectra for SU-8/CB nanocomposite microcantilevers with CB concentration of 7

vol. % and 8.4 vol. % are shown in Figure 3.35 (A). The noise levels decrease with increase in

the filler concentration and this trend is attributed to the increase in number of charge carriers

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with CB filler loading. Bias dependence of noise spectral density is also illustrated in Figure

3.35 (B). It can be observed that the major component of noise in these SU-8/CB

nanocomposite microcantilevers is 1/f noise. The noise level for polymer nanocomposite

microcantilever (8.4 vol %) was calculated assuming a bandwidth 80 Hz. The noise level (in

mV calculated as the product of noise current and base resistance) in SU-8/CB nanocomposite

microcantilevers with 8.4 vol. % of CB was found to be approximately 1.89 mV. Based on

this noise level, the estimated minimum detectable surface stress value came to be about 39

mN/m.

Based on the electromechanical and noise characterization results, the performance of these

polymer nanocomposite microcantilevers can be quantified as given in Table 3-3. As can be

noted in this table, the surface stress sensitivity of the fabricated SU-8 nanocomposite

microcantilever is greater than that of SU-8 microcantilever with composite piezoresistor [57]

and one order of magnitude higher than that of polymer microcantilevers with Au as the strain

gauge [34] reported earlier. However, the estimated minimum detectable surface stress value

for SU-8 nanocomposite microcantilevers was higher than that of SU-8 microcantilevers with

Au as the strain gauge. Further improvements in performances can be achieved by tuning the

thickness and shape of SU-8 nanocomposite microcantilever structure.

Table 3-3 Summary of performances of SU-8 /CB nanocomposite microcantilevers compared with other existing polymer microcantilever sensors

Property

SU-8 microcantilever with composite

piezoresistor[57]

SU-8/Au/SU-8 microcantilever

[34]

SU-8/CB nanocomposite microcantilever

Gauge factor of integrated piezoresistor

15-20 2 90

Surface stress sensitivity (ppm/[mN/m])

4 (calculated based on the data given in paper)

0.3 7.6

Lower limit of detection (surface stress mN/m)

Not known 0.1 39

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The SU-8 nanocomposite devices exhibited high noise levels which lead to lower resolution

in sensing as compared that of silicon based piezoresistive microcantilevers. However, they

possess higher sensitivity and the polymer nanocomposite microcantilever fabrication process

tends to be simple and cost effective compared to that of silicon based microcantilevers and

this makes them suitable for a variety of low cost bio/chemical sensing applications which

could make this technology extremely attractive.

3.6 Summary

In summary, the development of a polymer nanomechanical cantilever bio/chemical sensors

with embedded polymer nanocomposite as the piezoresistive layer has been presented. The

summary of the key aspects covered in this chapter are enlisted below.

[1] SU-8/CB composites were prepared by ultrasonic mixing of SU-8 2002, Nanothinner and

Carbon Black, Conductex 7067 Ultra. Experimental results on photolithographic

patterning and basic conductivity studies the composite with different CB filler

concentrations were presented. The percolation threshold achieved with the process was

6.5 wt.% which is less than that reported for similar composite in [57].

[2] Design and fabrication of SU-8 microcantilevers with embedded polymer nanocomposite

(SU-8/CB) piezoresistor was presented. The microcantilevers had an overall stack

thickness of just 3 µm compared to 7 µm in [57].

[3] Characterization of these devices was performed to measure the deflection and surface

stress sensitivity. The deflection sensitivity of these cantilevers was found to be 0.55

ppm/nm. The surface stress sensitivity for these devices is 4.1x10-3 [N/m]-1 which is

more than 10 times the sensitivity of the SU-8 cantilever with Au and at least a factor of

two higher compared to the reported result on SU-8/CB cantilevers.

[4] Systematic and more detailed material characterizations were performed for improving

the device performance and efficiency of fabrication process. Improvement in dispersion

characteristics of SU-8/CB composites was achieved with characterization study

conducted with dynamic light scattering technique. SU-8/CB composites with different

CB concentrations were prepared with improved CB dispersion in order to find out the

effect of varying CB filler concentration on the mechanical and electrical characteristics

97

of the polymer nanocomposite. This has been done for identifying the optimal range of

CB concentrations for improved device performance. Using nanoindentation experiments

for SU-8/CB composite, effect of CB filler loading on the mechanical properties of the

composites were studied. Young’s modulus was observed to increase with CB loading

and the values for 8 vol% samples were about 30-40% higher compared to a pure SU-8

sample. Electrical conduction behaviour and the device to device resistance variability of

SU-8/CB nanocomposite with different CB loading were analysed. Device variability was

observed to be decreasing with increasing concentrations of CB and the variability was <

30 % for samples with CB concentrations > 8 vol. %. Based on these characterization

results, the preferable range of concentration of CB in SU-8 turned out to be 8 – 9 vol. %

.

[5] SU-8 microcantilevers with embedded SU-8/CB nanocomposite piezoresistor having

optimum carbon black filler loading in the range of 8 – 9 vol. % were designed and

fabricated. The design for polymer nanocomposite microcantilever was chosen in order to

improve the surface stress sensitivity, common mode rejection, mechanical stability and

packaging compatibility of the sensor with an additional benefit of reduction in process

complexity.

[6] The experimentally determined resonance frequency and spring constant for the

fabricated polymer nanocomposite cantilever of thickness 3 µm were 22.6 kHz and

0.44N/m respectively. These devices exhibited surface stress sensitivity of 7.6 x 10-3

[N/m]-1 which is greater than that of an optimized silicon microcantilever and one order

of magnitude higher than that of polymer microcantilevers with Au as the strain gauge.

The noise spectrum of polymer nanocomposite microcantilevers indicated that the major

component of noise in these devices was 1/f noise. Though the polymer nanocomposite

devices showed higher noise levels compared to that of silicon based piezoresistive

microcantilevers, their higher sensitivity supports their suitability for low cost

bio/chemical sensing applications.

[7] A novel polymer MEMS accelerometer with SU-8/Carbon black as a piezoresistor was

also designed and fabricated. This was the first time demonstration of a piezoresistive

polymer accelerometer. The device design, fabrication and characterization of the device

is presented in Appendix 2.

98

Chapter 4

Detection of Explosive Vapours using Polymer

Nanocomposite Microcantilevers

4.1 Introduction

Receptor based detection of explosive vapours using microcantilevers works on the principle

of translation of target molecule-receptor binding into nanomechanical motion of the

microcantilever. As per the current literature available, microcantilever that have been used

for these applications are mostly silicon or derived materials such as silicon dioxide and

silicon nitride. The major challenge in explosive vapour detection is that sensor should be able

to detect the explosive molecules in very low concentrations. So there is always a need for

more sensitive microcantilever sensing platforms. So polymer nanocomposite microcantilever

with their very good surface stress sensitivity might be a good candidate for these

applications. Also developing an application using these polymer nanocomposite

microcantilevers would help in understanding the real time challenges using these

microcantilevers as a platform technology for different sensing applications.

99

In order to use polymer nanocomposite microcantilevers for explosive detection, the

cantilever surface needs to be coated or functionalized with a receptor ie., a chemically

selective layer for the target molecule. As mentioned in Chapter 2, 4-MBA (4-

Mercaptobenzoic acid, also known as thiosalicylic acid) is chosen as the receptor layer as the

most popular explosives such as TNT, RDX and PETN are known to bind with 4-MBA

[117,118]. 4-MBA forms stable self-assembled monolayers (SAM) on Au. 4-MBA is acidic in

nature due to the presence of –COOH group that can dissociate into a –COO- charged group.

The SAM provides well oriented –COOH groups for efficient binding of vapour phase bases

like explosives.

Polymer nanocomposite microcantilevers (CB concentrations of 8.4 vol. %) were

functionalized using 4-MBA by following the functionalization procedure detailed in Chapter

2 for single layer SU-8 microcantilevers.

4.2 Explosive Vapour Experiments

Functionalized microcantilevers had to be attached to a custom designed PCB for further

characterization. Wire-bonding technique was tried out for making connections between

device contact pads and PCB contacts. But the device contact pads were getting damaged

during wire bonding. This is basically because of the thin contact pads of 180 nm of PVD

deposited gold on SU-8. Electroplated Au contact pads could have been a viable option to

solve this issue. However the process and masks had to be adapted in order to accommodate

the gold electroplating step. So in place of wire-bonding, a low temperature curable 2-part

epoxy, Epotek H20E (Epoxy Technologies) was used to make the contact lines from device to

PCB. The curing was performed in an oven at 80oC for 3 hours.

The functionalized microcantilever along with non-functionalized microcantilever as

reference microcantilever were attached to a PCB using insulating epoxy (EPO-TEK H70E)

with contacts made using conducting silver epoxy (EPO-TEK H20E)). The microcantilevers

were kept inside a PTFE gas flow cell (25 mm diameter) with a proper ‘O’ ring seal. The PCB

was partially placed inside the flow cell, so that the microcantilevers are inside the flow cell

and the PCB contacts were available outside the flow cell for making connections to the

circuit. The microcantilever PCB and the arrangement of PTFE gas flow cell and the circuit

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are shown in Figure 4.4 (A-D). These microcantilevers were then connected in a DC bridge

circuit with a provision for data logging into a computer. ADS 1232 multipurpose board from

Texas Instruments (TI) which had a USB interface and Lab view based software for data

acquisition was used for this purpose [119]. The DC bridge configuration of microcantilevers

along with external resistors and the block diagram of the ADS1232REF circuit are given in

Figure 4.1.

Figure 4.1 (A) Wheatstone bridge circuit indicating the microcantilever positions (B) Block diagram schematic of ADS1232REF board to which the bridge output is fed

Here C1 is the measurements microcantilever which is functionalized with 4-MBA and C2 is

the reference microcantilever which is coated with only gold. R1 and R2 are external

resistances whose values are chosen based on the microcantilever resistance values.

The ADS1232 is a precision highly integrated 24-bit analog-to-digital converter (ADC) that

include an input multiplexer, low-noise programmable gain amplifier (PGA), precision third-

order delta-sigma ADC and fourth-order digital filter. With the input-referred RMS noise

down to 17nV the ADS1232 provide a complete front-end solution for bridge sensor

applications. The input multiplexer accepts two differential inputs. The on-board, low-noise

PGA has a selectable gain of 1, 2, 64, or 128 supporting full-scale differential input of 2.5v,

1.25v, 39mv, or 19.5mv respectively. It supports two data rates: 10 samples per second (sps)

(with both 50Hz and 60hz rejection) and 80 sps.

In order to test the instrumentation scheme, electromechanical characterization one of the test

microcantilevers was performed. The microcantilever was subjected to displacements in

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multiples of 10 µm using a standard calibrated micromanipulator. The voltage signal from the

TI board was recorded during this experiment and is as shown in. This corresponded to a

sensitivity of 2 mV per µm of deflection

Figure 4.2 Microcantilever response recorded during deflection using calibrated micromanipulator

The schematic of the experimental set up used for explosive vapour detection experiment is

given in Figure 4.3 .

Figure 4.3 Schematic of the experimental set up for explosive vapour experiments

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The vapour generator contained a “U” shaped quartz tube reservoir of TNT or RDX

maintained at a constant temperature using a calibrated temperature controller. During the

experiment, explosive vapour streams were generated by flowing carrier gas (dry nitrogen)

through the explosive reservoir “U” tube. . The gas flow rate was maintained at 30 SCCM

using a mass flow controller (MFC).

Figure 4.4 (A & B) PTFE gas flow cell (FC) incubating the microcantilever PCB that is

connected to DC bridge circuit, all inside a shielded enclosure. ADS1232 REF circuit from TI

can be seen here (C & D) Microcantilever PCB and a microcantilever die (E) Complete

experimental setup. VG: Vapour generator ; FC : Flow cell

The photograph of the complete experimental setup is as shown in Figure 4.4 (E). Dry

Nitrogen purging was done for an hour before starting the experiment in order to bring down

the humidity levels. Experiments were carried out for detection of TNT vapours. The vapour

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generator had been calibrated for TNT concentrations at different temperatures using Gas

chromatography/Mass spectroscopy (GC/MS) and the calibration data is as given in Figure

4.5.

Figure 4.5 Calibration of TNT vapour generator at different temperature2

For TNT vapour exposure experiment, TNT “U” tube reservoir was maintained at 65oC.

During the experiment, TNT vapour streams were generated by flowing dry nitrogen through

the TNT reservoir. In order to switch from TNT vapour, the same nitrogen gas flow was

maintained through the microcantilever gas flow cell by redirecting the nitrogen flow stream

through a bypass line. This switching between TNT vapour stream and dry nitrogen stream

was performed using 3-way valves V1 and V2 indicated in the schematic (Figure 4.3).

Extrapolating the calibration curve (Figure 4.5) for 65oC, TNT vapour concentration was

approximately 30 ppb. Microcantilever response for alternating cycles of TNT (~30 ppb) and

2 TNT concentrations were calculated from the TNT vapour generator calibration data obtained from Dr. Pramod Soni, TBRL.

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dry nitrogen for three minutes and five minutes respectively was recorded as shown in Figure

4.6.

Figure 4.6 Response of a 4-MBA coated polymer nanocomposite microcantilever for consecutive cycles of TNT and nitrogen

It was observed that the polymer nanocomposite cantilever sensor responded to the TNT

vapour with an output response of 22 mV which corresponded to a compressive stress on the

Au coated surface of the cantilever. Also, the sensor could be regenerated with a post purging

step using nitrogen. We can thus conclude that these functionalized polymer nanocomposite

microcantilevers were reusable even after multiple exposure cycles. The binding chemistry

responsible for this easily reversible adsorption of TNT molecules in 4-MBA coated

microcantilever is the hydrogen bonding between the carboxyl groups of 4-MBA and nitro

groups of TNT molecule.

In the same way, experiments were carried out for the detection of RDX vapours. The RDX

vapours were generated at temperature of 85oC and nitrogen carrier gas flow at 30 SCCM was

used. As per the experimental result given in Figure 4.7, 4-MBA coated polymer

nanocomposite microcantilevers were found to be suitable for the detection of RDX vapours

also.

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Figure 4.7 Responses of 4-MBA coated Microcantilever to RDX

As can be observed from the microcantilever response curve for RDX, the response was

slower towards RDX vapours in comparison to TNT vapours and this difference could be

attributed to the lower vapour pressure of RDX compared to that of TNT.

4.2.1 Controlled experiments with a flow cell on PCB

In order to improve the response time (rapidity of sensing) of the sensor and to reduce the

vapour adsorption to the walls of the flow cell, a small flow cell of 10 mm diameter made out

of PTFE was integrated with the device PCB as shown in Figure 4.8. The tubings and fittings

were also changed in order to make them compatible with the newly designed flow cell. A

nozzle arrangement with an internal gradual tapering at an angle was used to connect 6 mm

diametric PTFE tubes to 2 mm diameter PTFE tubings that were connected at the inlet and

outlet of the flow cell. The photographs of the new flow cell and is given in Figure 4.8.

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Figure 4.8. Microcantilever PCB and 10 mm diameter PTFE gas flow cell arrangement

Before doing the explosive vapour experiment using this arrangement, a controlled

experiment was conducted in which a test microcantilever’s (which was also 4-MBA coated)

response to different flow rates of dry nitrogen were recorded as shown in Figure 4.9 and this

confirms that the microcantilever is insensitive to flow changes of nitrogen gas.

Figure 4.9 Microcantilever response to different flow rates of nitrogen

Controlled experiments were carried out for the detection of TNT vapours using this new flow

cell arrangement. Microcantilever responses to different concentrations of TNT vapours

generated by varying the temperature of the vapour generator were recorded. As bimaterial

microcantilevers might respond to temperature changes (bimorph effect), differential

measurements using both functionalized and non-functionalized gold coated microcantilevers

kept at two arms of the Wheatstone bridge circuit (Figure 4.1(A)) were necessary. As the

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selective functionalization of 4-MBA was difficult for microcantilevers in the same die, the

reference microcantilever belonged to a different SU-8 die. This arrangement ensured the

recording of the true response of the microcantilevers towards TNT vapours. The output

voltage was found to increase with increase in concentration (Figure 4.10(A)). The baselines

for these plots during only the nitrogen gas exposure were stable with a peak to peak noise

value within 2 mV. 4-MBA coated polymer nanocomposite microcantilevers should be able

to detect TNT vapours down to a few ppb (~ 6 ppb) concentrations with an approximate

sensitivity value of 1 mV/ppb of TNT (Figure 4.10 (B).

Figure 4.10 . (A) Response of a 4-MBA coated polymer nanocomposite microcantilever to different concentrations of TNT vapour in nitrogen (B) TNT vapour detection sensitivity plot.

4.3 Effect of humidity on microcantilever response

Since the microcantilevers were functionalized with 4-MBA with which the explosive

molecules bind using hydrogen bonds, the effect of humidity on 4-MBAcoated polymer

composite also had to be characterized. Experimental setup that used a humidity generator for

carrying out the study is shown in Figure 4.11.

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Figure 4.11 Schematic of experimental setup for studying the response of microcantilevers to humidity

The response of the 4-MBA coated polymer nanocomposite microcantilever to change in

humidity is shown in Figure 4.12.

Figure 4.12 Response of 4-MBA coated Microcantilever to humidity.

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The microcantilever output signal was found to be increasing with increase in humidity levels

and the signal went back to near the original value as humidity levels were decreased with dry

nitrogen stream. It was observed that there was a change in voltage of 200 mV for a relative

humidity change of 13 % (Figure 4.12). Water is a polar solvent and so it forms hydrogen

bonds with other polar molecules and with itself. In the case of exposure of a 4-MBA coated

microcantilever, water molecules form hydrogen bonds with –COOH group of 4-MBA which

causes a surface stress change on the microcantilever. As the water molecule can form

hydrogen bonds with itself, this adsorption induced surface stress increases with increase in

humidity levels.

From this study on response of 4-MBA coated polymer nanocomposite microcantilevers

towards variation in humidity levels, one can realize that selectivity could be a major concern

in detecting explosive vapour molecules using microcantilevers functionalized with molecules

such as 4-MBA with which unwanted molecules such as water molecules that adsorb easily

by formation of hydrogen bond. However this selectivity problem related to humidity has

been partially addressed for these 4-MBA functionalized microcantilevers. For this,

experiments were conducted to study the response of the microcantilever to trace TNT in

normal ambient in place of controlled experiments using vapour generator and dry nitrogen

source arrangement. Microcantilevers were placed in the flow cell in the same way as

explained in controlled explosive vapour experiments. A DC pump [1.4 litre/min] was

incorporated at the outlet of cantilever flow cell in order to sample the air with forced

circulation. This was used in place of MFC. Microcantilever response was recorded while

sampling the ambient air using the pump and this was continued for stabilizing the signal

(Figure 4.13). TNT trace sample was brought near the inlet of the flow cell and the sensor

responded indicating the presence of TNT. As per the literature available, the approximate

TNT concentration in air can be 4 ppb at room temperature [120]. During this part of the

cycle the pump takes in the TNT along with the ambient air. When the TNT source was taken

away (by closing the vial containing TNT), the sensor signal went back to the original level

(Figure 4.13). The pump was on throughout the experiment. The ambient air took the role of

purging the adsorbed TNT on 4-MBA and bringing the sensor back to the status where only

ambient air was present which might be at a specific level of humidity. Another cycle of

experiment was carried out with exposure to trace RDX source as depicted in Figure 4.13. As

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the 4-MBA coated microcantilevers could detect TNT and RDX in normal ambient under the

condition that the ambient humidity does not vary throughout the experiment.

Figure 4.13 Response of 4-MBAcoated polymer microcantilever to TNT and RDX vapours in ambient conditions3

4.4 Summary

A chemical sensing application for SU-8/CB nanocomposite microcantilevers for detection of

explosive vapours TNT and RDX has been successfully implemented. The microcantilever

sensitivity and response time were found to be adequate for such an application. Control

experiments were conducted to detect TNT vapours with different concentrations. 4-MBA

coated polymer nanocomposite microcantilevers could detect TNT vapour concentrations

down to a 6 ppb with an approximate sensitivity value of 1 mV/ppb of TNT. As the reaction

between the explosive molecules and 4-MBA was reversible, these sensors were found to be

reusable. It seemed very difficult to compare this work with the the previously published

results for explosive detection using silicon microcantilevers as the device geometries,

3 The result presented in Figure 4.13 is based on the experiment conducted by Nikhil, M-Tech, 2011 batch, Department of Electrical Engineering, IIT Bombay [142].

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functionalization materials and methods and the electronic instrumentation schemes and

systems were different. However the main focus of this thesis was to optimize the material

and the fabrication aspects for the polymer nanocomposite cantilever platform in order to

improve their surface stress sensitivity so as to demonstrate their potential for bio/chemical

sensing applications. With the help of this part of the work, the potential of this cost effective

and ultra-sensitive polymer nanocomposite microcantilever sensing platform for a typical, yet

challenging chemical sensing application was demonstrated.

For a sensor, selectivity is as important as sensitivity. Explosive vapour exposure experiments

presented in this Chapter were conducted by isolating secondary effects such as ambient

humidity changes. However, a set of dedicated experiments were conducted in order to the

test the effect of ambient humidity. The results obtained based on the experiments conducted

to test the effect of humidity changes on 4-MBA coated SU-8 nanocomposite

microcantilevers were alarming as the sensor was responding to humidity changes with a

better sensitivity compared to that of explosive vapours. The hydrogen bond between 4-MBA

and water molecules was responsible for this. However, the experiments conducted using

microcantilevers functionalized with 4-MBA for detection of TNT and RDX in ambient

conditions could support their candidature for detection of explosive vapours in ambient

where the humidity levels do not vary within a measurement cycle. In addition, such

secondary effects can be solved either by using microcantilever array approach with separate

coatings that support different types of interactions with the analyte molecules or by using a

very specific coating materials and both these approaches are part of the on-going research

activities in our group. Also, as per the available literatures, selectivity is considered to be a

major bottleneck faced in developing receptor based micro fabricated explosive sensors.

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Chapter 5

Polymer nanomechanical sensor with integrated OFET

for electrical transduction

5.1 Introduction

Polymer nanomechanical cantilever sensors with one of the most popular electrical

transduction schemes using integrated piezoresistors were discussed in previous chapters.

Piezoresistive SU-8 microcantilevers with different strain sensitive layers such as gold (Au)

[34,36,51] and polysilicon [52,53] have been reported earlier. These approaches have their

own advantages and disadvantages which were also discussed in Chapter 3. The limitations of

using such strain sensitive or piezoresisitive materials (lower gauge factor for gold and

compliance compatibility and noise related issues for polysilicon) with SU-8 nanomechanical

devices have been overcome by integrating a compliant strain sensitive material SU-8/Carbon

Black nanocomposite as the strain sensitive layer which was presented in Chapter 3 and

Chapter 4. Polymer nanomechanical sensor platforms using SU-8 nanocomposite

microcantilevers find applications in low cost chemical sensing owing to their high sensitivity

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and low cost of fabrication. However, it was found that there exist certain issues with polymer

nanocomposite microcantilevers which were discussed in Chapter 3. To enlist them:-

(1) Process variability issues attributed to the processing of SU-8/CB nanocomposite that

affect the process yield in fabricating these devices. The main reason for this is the statistical

distribution of conducting nanoparticles, carbon black in the insulating matrix SU-8.

(2) Larger noise levels compared to reported SU-8 microcantilevers with gold as strain gauge

and this affected the resolution (minimum detectable surface stress/ deflection) of sensing. .

Considering these limitations of the former technology and as part of the efforts in finding out

better electrical transduction methods for polymer microcantilevers with higher sensitivities,

the scope of integrating a strain sensitive conducting/semiconducting and compliant material

with SU-8 microcantilevers were being explored and presented in this chapter. One such

strain sensitive organic material, pentacene is a well-studied organic semiconductor and is

commonly used as a channel material in organic field effect transistors (OFET). Pentacene is

reported to exhibit good strain sensitivity [60,61] with its Young’s modulus nearly matching

with that of SU-8 [121]. Pentacene based large area strain and pressure sensors on flexible

substrates like polyethylene naphthalate (PEN) have also been reported earlier as mentioned

in Chapter 1. The common and simple process followed for deposition of pentacene is

vacuum sublimation (resistive thermal evaporation or electron beam evaporation) which

allows uniform deposition of these layers and so process variability can be expected to be

lower compared to spin coated polymer/nanoparticle composite layers. Pentacene films can be

deposited at low substrate temperatures and hence the integration with polymer substrates

becomes possible [70]. Since the resistivity of as deposited pentacene is known to be high,

instead of using pentacene as a piezoresistor, an OFET with pentacene as a strain sensitive

channel material were explored in this study for electrical transduction in an SU-8

nanomechanical cantilever sensor.

The device concept is to embed a pentacene based OFET inside a SU-8 microcantilever as

shown in Figure 5.1. This device named as ‘Organic CantiFET’ in this work, can be thought

of as a low cost polymer counterpart of a similar device in silicon, a MOSFET embedded

microcantilever reported earlier [50,122]. When SU-8 microcantilevers with embedded

pentacene OFET undergo nanomechanical motion during sensing events, the strain sensitive

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organic semiconductor pentacene responds to this by changing its mobility and hence the

drain current of the transistor. The change in drain current can be measured and recorded

using appropriate signal conditioning circuitry in order to perform the sensing operation. A

simple differential amplifier circuit with sensing and reference transistors could be used in a

complete sensor configuration. Such an arrangement of organic CantiFET retains the

advantages offered by Wheatstone bridge used for piezoresistive sensors. The integrated

OFET eases the implementation of nanomechanical sensor arrays with integrated signal

conditioning circuitry.

Figure 5.1 Schematic of the concept of an ‘Organic CantiFET’ device

5.2 Organic CantiFET device design and development

The realization of a conceptualized Organic CantiFET device demands some basic

understanding of OFETs to aid in material selection and device design; to conduct systematic

experimental studies for unit process development and the final process integration.

5.2.1 Material selection

The materials for the integrated OFET were chosen based on the underlying physics of

operation of OFETs and the additional requirements specific to the process integration for the

final device aimed here.

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A Organic semiconductor and some basic facts about OFETs

The strain sensitive organic material, pentacene is known to be a p-type organic

semiconductor, since positive charge carriers dominate transport in pentacene thin films.

Organic semiconductors are unsaturated carbon based materials and they can be organized

into two categories, plastics or polymers and small organic molecules or oligomers.

Pentacene which belongs to oligomers is an aromatic hydrocarbon containing five benzene

rings as shown in Figure 5.2 (A). Pentacene is one of the promising materials widely and

intensively studied as the channel material for OFET and it possesses high mobility [70].

Pentacene thin films can be deposited either by thermal vacuum evaporation or by solution

processes. The organic materials have π orbitals, which play primary roles in the

semiconductor properties and carrier transport. Lot of theoretical and experimental studies

had been carried out by a large number of researchers to investigate the physics involved in

the charge transport in organic semiconductors like pentacene. The transport mechanisms can

be described in terms of hopping between adjacent localized states [123] or in terms of

polarons where the presence of a charge distorted the structure around it [124][125].

Figure 5.2 (A) Pentacene molecule (B) P-type OFET schematic (C) Illustration of working principle of an OFET with respect to applied gate bias, VGS.

When VGS = 0 V, no charges are injected. When a negative gate bias is applied, positive charges are accumulated at the organic semiconductor/organic insulator interface and if the Fermi levels of source/drain electrodes are closer to the HOMO level of organic semiconductor, holes can be injected from the electrodes to the HOMO of the semiconductor or vice versa.

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Organic field-effect transistors (OFETs) are 3-terminal devices like MOSFETs in which the

carrier density in the channel region between source and drain can be controlled by the

voltage applied to the third terminal i.e., gate voltage. The schematic of an OFET is shown in

Figure 5.2 (B). The core structure of OFET is the metal insulator semiconductor (MIS)

structure constituted by gate electrode, gate dielectric and the organic semiconductor. The

operating principle of OFET can be imagined similar to that of enhancement MOSFET with a

main difference that OFETs work in accumulation mode whereas MOSFETs work in

inversion mode. The working of an OFET could be described using a simplified energy level

diagram for Fermi level of source/drain electrodes and the energy levels, HOMO (highest

occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of the organic

semiconductor. Considering the case where gate voltage is not applied (Figure 5.2 (C)), direct

injection of charge carriers from the source/drain electrodes creates current flow in the

organic semiconductor. Such currents will be relatively small due to high resistance of the

organic semiconductors (the organic semiconductor behaves like an insulator as it is

intrinsically undoped) and large distance between the source and drain electrodes. When a

negative gate voltage is applied (Figure 4b), positive charges are induced or accumulated near

the organic semiconductor/gate dielectric interface to form a p-type conducting channel. If the

Fermi level of source/drain metal is close to the HOMO level of the organic semiconductor,

these positive charges can be extracted by these electrodes by applying appropriate voltage

between these electrodes. The operation of OFETs requires low energetic barriers at the

metal-organic interfaces for both source and drain contacts for proper injection and extraction

of carriers.

Despite the fundamental differences between the modes of operation of OFETs and

MOSFETs, the characteristic equations (eqn. for linear regime and eqn. for saturation regime)

of MOSFET transistors can be applied to the OFETs also [126] as given in equation 5.1 and

5.2.

퐼퐷푠 = μ퐶푂푋

푊퐿

(푉퐺푆 −푉푇퐻)푉퐷푆 −푉퐷푆

2 (5.1)

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퐼퐷푠 = μ퐶푂푋푊퐿

(푉퐺푆 −푉푇퐻)2 (5.2)

B Source/drain and gate electrodes

In the case of pentacene based OFETS, source and drain regions are not usually doped. So

charge carriers that form the conducting channel of the transistor must get injected from

contacts. The charge injected into the pentacene film depends on the schottky barrier between

the source/ drain metal contacts and the active layer. Thus the charge injection is determined

by the work function of the metal contacts. So for a p-type semiconductor like pentacene, high

work function metals like gold (Au) are commonly used as source/ drain electrodes [127]. The

work function of Au (~5.1 eV)) is closer to the HOMO level of pentacene (~5.2 eV) and

hence reduces the contact resistance. Gate electrode chosen was also Au.

C Gate dielectric

There are a certain number of aspects to be considered for choosing the gate dielectric

material and they include leakage, patterning convenience and compatibility, semiconductor

compatibility, achievable capacitance, hysteresis etc. [127]. The gate dielectric material to be

considered for the organic CantiFET should possess certain qualities other than these basic

qualities required for a good dielectric material for OFETs. To enlist them, the material

should be easily patternable with good process compatibility with SU-8 to and compliance

compatibility with SU-8. Various dielectric materials such as silicon dioxide, HfO2 [126] and

some polymer materials [128][129] are reported to be in use in Pentacene OFETs. Reported

dielectric constant value for Microchem SU-8 2000 series is 4.1 which is close to the value for

silicon dioxide. Hence the feasibility of using SU-8 itself as the dielectric material for

pentacene OFETs was explored.

OFETS with SU-8 as dielectric have been scarcely reported in the literature. A preliminary

study is required in order to analyse the performance of pentacene OFETs with the gate

dielectric made of SU-8 processed in the present laboratory conditions and hence to optimize

the process and layer thickness of SU-8. For this purpose, bottom gate bottom contact

(BGBC) [126] pentacene OFETs were fabricated with different SU-8 dielectric thicknesses

varying from 300 nm to 1µm. The schematic of this BGBC OFET fabrication process

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followed is given in Figure 5.3. The dielectric films of thickness below 400 nm were found to

be too leaky.

Figure 5.3 Schematic of fabrication process for pentacene OFET with SU-8 as dielectric. (A)

RCA cleaning of n+ silicon wafer (B) SU-8 spin coating and processing for gate dielectric (C) Cr/Au deposition and patterning for source and drain electrodes (D) Pentacene

N-type silicon wafer (low resistivity; 0.01 ohm cm) was RCA cleaned. After a sufficient

dehydration bake step, SU-8 was spin coated and subjected to a two-step bake at 70oC and

90oC for optimized timings. This was followed by ultra violet (UV) exposure in Karl SUSS

MJB3 mask aligner. After a post exposure bake cycle, the sample was developed in standard

SU-8 developer with a rinse step in iso propyl alcohol (IPA) to yield the SU-8 layer to be used

as gate dielectric. The SU-8 process parameters such as spin coating speed, baking time and

UV exposure dose are decided based on the thickness of the SU-8 layer and the process recipe

is given in Appendix C. Gold (Au) was chosen as the gate metal with a thin layer of chrome

(Cr) as the adhesion layer. Cr/Au (7nm /80nm) was deposited using sputtering and then

patterned using standard optical lithography followed by wet etching of Au and Cr to form

source/drain electrodes. The final step in the fabrication of OFET is deposition of pentacene.

Pentacene deposition was done using thermal evaporation of purified pentacene (Sigma

Aldrich. The OFETs fabricated were having comb type inter-digitated source drain electrodes

and the width to length ratio (W/L) of the transistors was 600/20. The summary of

specifications of different layers of the OFETs described here is given in Table 5-1.

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Table 5-1 Layer details of OFETs fabricated for performance evaluation of pentacene OFETs with SU-8 dielectric.

Layer Material Layer thickness (OFET#1)

Layer thickness (OFET#2)

Gate N+ Si 275 ± 25 m 275 ± 25 m

Gate Dielectric SU-8 940 nm 455 nm

Organic semiconductor Pentacene 40-50 nm 40-50 nm

Source/ Drain Gold 80 nm 80 nm

The fabricated devices were characterized in order to analyse their performances. The

electrical measurements were performed at room temperature under ambient atmospheric

conditions using Agilent 4156C semiconductor parameter analyser. The transfer (IDS vs. VGS)

and output (IDS vs. VDS) characteristics for these devices were recorded and are as shown in

Figure 5.4(A) and (B) respectively.

The devices exhibit typical p-channel characteristics (Figure 5.4(A)). The saturation field

effect mobility (μ) and threshold voltage (VTH) were extracted from the highest slope of |IDS|1/2

vs. VGS plots using the standard saturation region current equation for OFETs. The typical set

of parameters extracted from the characterization results of the fabricated OFETs with SU-8

dielectric is given in Table 5-2

Table 5-2 Specifications of OFETs with SU-8 as dielectric

Parameter OFET#1 OFET#2

Mobility [cm2/Vs] 8.09 10-4 4.91x 10-4

Threshold voltage [V] -6.28 -3.02

ION/IOFF 1x103 2x102

Gate current density (@VDS = VGS=-40V)[A/cm2]

1.53x10-8 1.2x10-4

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Figure 5.4 I-V characteristics of OFETs with SU-8 as (A) Transfer characteristics of OFETs

with different dielectric thicknesses along with gate leakage (B) Output characteristics of OFET #1

OFET# 1 exhibited low gate leakage and good switching characteristics (Gate current density

=1.53x10-8 A/cm2 @ VGS=-40V and ION/IOFF = 1 x 103). Gate leakage was typically in the

range of pA for OFET #1 with SU-8 dielectric thickness of 900 nm. This is three orders of

magnitude lower than the drain current, where as in the case of OFET #2, the gate leakage

(Gate current density =1.2x10-4 A/cm2 @ VGS=-40V) was typically in the range of tens of

nano amperes which is in the same order as that of drain current. This higher gate leakage

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could be attributed to the poor quality of SU-8 with thickness below 450 nm as shown in

Figure 5.5.

The output characteristics of encapsulated OFET #1 exhibited a linear increase in the drain

current at low drain bias (Figure 5.4(B)). This is a clear indication of the existence of good

ohmic contact at the interface of source drain electrodes and the organic semiconductor. At

high drain biases, proper saturation of drain current is also observed.

Figure 5.5 Optical microscopic image of SU-8 films (A) SU-8 450 nm film on silicon dioxide.

Lot of pin holes being observed. (B) SU-8 950 nm film on silicon dioxide indicating better quality compared to SU-8 450 nm

So based on these characterization results the SU-8 dielectric thickness in the range of 950 nm

– 1 µm was chosen, considering the present lab conditions and the available SU-8

formulations.

The mobility of OFETs with SU-8 as dielectric were found to be one to two orders of

magnitude less compared to similar OFETs fabricated with silicon dioxide as dielectric. This

difference in field effect mobility could be attributed to grain size and grain orientation of

pentacene on SU-8 and silicon dioxide. Grain size of pentacene films significantly influences

carrier transport in the films. The SEM micrograph (Figure 5.6 (A)) of pentacene on SU-8

shows that the grains of pentacene on SU-8 are random in nature with varying grain sizes. The

AFM topographical images (Figure 5.6 (B &C) of pentacene on SU-8 and silicon dioxide are

shown. The pentacene gain sizes were larger on silicon dioxide in comparison to that on SU-

8. The surface energy and roughness of the dielectric material underneath the pentacene layer

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is known to influence the grain sizes of pentacene films [70]. The value of RMS roughness of

SU-8 was approximately 3 nm where as that of silicon dioxide was less than 1 nm. The

analysis of morphology of pentacene films deposited on SiO2 with different surface roughness

presented in [130] supports the fact that the grain size decreases with increasing surface

roughness.

Figure 5.6 Pentacene on dielectric surfaces (A) SEM micrograph showing pentacene grain boundaries on SU-8. (B & C) AFM topographical image of pentacene on SU-8 and silicon

dioxide indicating better grain size for the case of silicon dioxide dielectric

5.2.2 Organic CantiFET device designs

Having studied the pentacene OFET characteristics with SU-8 as dielectric, the next task was

to realize the organic CantiFET by integrating the OFET with an SU-8 microcantilever. The

schematic of the designed CantiFET device chip is given in Figure 5.7. The CantiFET device

chip consisted of two CantiFET devices for differential current measurement scheme, in

which one of the devices was considered as the measurement cantilever and other being the

reference cantilever.

Arrays of such CantiFET device chips had to be fabricated. The transistor covered the whole

length of cantilever so that the design is appropriate for surface stress measurements based

nanomechanical cantilever sensors. The thickness of the gate dielectric, SU-8 has been chosen

based on the preliminary study conducted on the current voltage (I-V) characteristics of

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BGBC pentacene OFETs using SU-8 as dielectric as discussed in the previous section. The

transistor configuration is again BGBC. However, different source drain electrode layouts of

OFETs were designed in order to study the dependence of strain direction on change in

current. Pentacene thin films deposited through thermal evaporation can be considered as

amorphous to polycrystalline in nature and thus expecting them to be isotropic in nature.

However, the anisotropic nature of strain sensitivity in pentacene is reported in literature . The

different designs of CantiFET are as given in Figure 5.7[C].

Figure 5.7 (A& B) Schematic (not to scale) of organic CantiFET (A) planar schematic of the device chip. S, D and G are the source, drain and gate contacts (B) cross section (position of cross section indicated using dotted line) of the device illustrating an SU-8 cantilever with

integrated pentacene OFET. (C1-3) Different CantiFET device designs. (1) Strain and current directions are parallel ie longitudinal case (2) Strain and current directions are perpendicular ie transverse case (3) Simple comb like inter-digitated structure which contains the current

components parallel and perpendicular to strain

5.2.3 Process integration for organic CantiFETs

Optimization of unit process for pentacene OFET and SU-8 microcantilevers were performed

independently. Now in order to integrate the OFET inside an SU-8 microcantilever, the

process compatibility of individual layers/materials of OFET and SU-8 needs to be ensured.

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The process integration of organic FET with solution processed polymer materials like SU-8

is a real challenge as the organic semiconductor, pentacene is known to get degraded by

organic solvents. Effect of SU-8 or its solvent cyclopentanone and developer PGMEA on

pentacene is not yet reported. So simple and quick test experiments were conducted in which

pentacene OFETs were fabricated and characterized. SU-8 was spin coated and then removed

immediately using PGMEA developer. Then the transistor was characterized and it was found

that the OFET did not show transistor behaviour and this is clear indication that pentacene

semiconductor was degraded in exposure to SU-8 or its developer. In order to separate the

two effects one more set of experiments were conducted in which the pentacene OFET was

dipped inside the solvent for SU-8 ie, cyclopentanone and then dried and the transistor was

characterized. OFET in this case also did not work. With these two sets of experiments, it

became clear that SU-8 and SU-8 developer would degrade the pentacene layer and hence the

process integration of OFET with SU-8 cantilever needs to be planned to avoid the exposure

of pentacene layer to organic solvents used in SU-8 lithography.

For this, a novel, yet simple process sequence is followed, in which, one develops all the

required layers except the pentacene layer for OFET on the SU-8 cantilever and then deposits

pentacene layer after the releasing the device chips from the substrate to form the final

organic cantiFET. This process has inherent advantages such as (1) this avoids the exposure

of pentacene layer to organic solvents used in SU-8 lithography (2) obviates the need for

patterning of pentacene layer which is known to be a difficult task. The schematic of this

fabrication sequence is given in figure and process steps are detailed as below. The process

recipe followed while fabricating organic CantiFET and the design of mask sets are given in

Appendix C and Appendix D respectively.

Step 1: Release/Sacrificial Layer

An RCA /Pirahna cleaned silicon wafer that was used as a reusable dummy substrate. The

first layer to be developed was sacrificial layer used as the release layer for separating out the

polymer cantiFET chips from the dummy substrate. The sacrificial layer chosen here was

silicon dioxide layer (sputtered oxide with thickness more than 200 nm or thermally grown

silicon dioxide with thickness = 700 nm to 800 nm) and the arguments for choosing this

material remains the same as described in Chapter 2 and Chapter 3.

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Figure 5.8 Schematic of fabrication process for polymer CantiFET (1) Sacrificial layer

(2)First layer of SU-8 defining the cantilever and contact vias.(3) Au electrode patterning for gate of transistor (4) SU-8 as gate dielectric (5) Au electrodes defining the source and drain of OFET (6) Thick SU-8 layer defined for anchor or chip of the cantiFET device. A photograph of the processed silicon wafer after this final lithography step is on right side (7) Release of

the CantiFET device from substrate and pentacene deposition (8) final device structure showing cantilevers , source drain and gate contacts through vias

Step 2: SU-8 layer for microcantilever on the die with contact vias

The first layer to be patterned is the cantilever die with structural cantilever layer and contact

vias. The thickness of this layer is decided in such a way that the step coverage of metals used

for contacts is ensured through the contact vias. After a sufficient dehydration bake step, SU-8

2002 was spin coated and subjected to a two-step bake at 70oC and 90oC for optimized

126

timings with a slow ramp up and ramp down to reduce the stress in structural layer. This was

followed by ultra violet (UV) exposure in Karl SUSS MJB3 mask aligner using the respective

mask (Mask#1). After a post exposure bake cycle, the sample was developed in standard SU-

8 developer with a rinse step in iso propyl alcohol (IPA) to yield the first SU-8 layer as shown

in step 2 of Figure 5.8. This layer was subjected to a post development baking step (hard bake

at low temperature) in order to improve the stability of the layer and to remove built-in

thermal stress developed if any.

Step 3: Gate electrode and contact pads for integrated OFET

Gate electrode and contact pads of the OFET are defined using gold (Au). Au was chosen as

the gate metal with a thin layer of chrome (Cr) as the adhesion layer. Cr/Au (7nm /80nm) was

deposited using sputtering and then patterned using standard optical lithography (Mask #2)

followed by wet etching of Au and Cr to form the gate electrode and contact pads as given in

in step 3 of Figure 5.8.

Step 4: Gate dielectric (SU-8)

Gate dielectric of the OFET is defined using SU-8 2002. The process as well as the mask is

very similar to the first SU-8 layer (mask used was Mask #1).

Step 5: Source and drain electrodes and contact pads

Cr/Au layers were sputter deposited and patterned using the respective mask (Mask #3) for

the formation of source, drain electrodes and the contacts, by following the same process

recipe for gate electrode/contacts.

Step 6: Anchor or polymer device die

A thick SU-8 layer of thickness more than 100 microns is to be patterned to form the anchor

or the device chip and the frames for holding arrays of device chips for ease in handling. For

this SU-8 2100 is spin coated followed by a two-step baking process on a hot plate at

temperatures 65oC and 85oC and subsequent lithography steps using an optimized process

recipe for 150 micron thick SU-8. Photograph of a processed 2” silicon wafer after the final

lithography step (in step 6 of Figure 5.8) is also shown in Figure 5.8, where arrays of SU-8

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device chips attached to frames can be seen. This is just before releasing of these polymer

device chips from the dummy substrate.

Step 7: Release of the polymer device chips

Finally the polymer device chips were released from the substrate using isotropic etching of

silicon dioxide using buffered hydrofluoric acid (BHF 5:1). This step is followed by rinse in

DI water and in IPA and then these SU-8 device chips are allowed to dry.

Step 8: Deposition of pentacene

The next step in the fabrication of organic CantiFET is deposition of pentacene on the

released SU-8 nanomechanical device to complete the integration of an OFET. Pentacene

layer of 50 nm was deposited using thermal evaporation technique. For this, the released

polymer devices were loaded upside down to the substrate holder so that source drain contacts

are exposed for pentacene evaporation. Purified pentacene (Sigma Aldrich) was thermally

evaporated at a deposition rate 1 Ǻ/second.

Photograph of an array of released organic CantiFET chips along with the scanning electron

micrographs (SEM) of one of a CantiFET device is shown in figure (A), (B), (C) and (D). The

dimensions of the fabricated organic CantiFET device chips are enlisted in Table 5-3.

Step 9: Encapsulation of organic CantiFET

As these organic cantiFET devices are developed aiming for bio/chemical sensing

applications, ensuring the stability of the integrated OFET in ambient atmospheric conditions

is an important measure. For this, the pentacene OFET in the SU-8 cantilever has been

encapsulated using a very thin (~ 15 -20 nm) silicon nitride layer deposited using hot wire

chemical vapour deposition (HWCVD) process. The HWCVD technique was chosen because

the nitride layer can be deposited at close to room temperatures, which is suitable for polymer

materials. HWCVD silicon nitride has also been reported as an effective encapsulation layer

for OFETs [131]. The silicon nitride encapsulation layer was kept thin enough so as not to

increase the stiffness of the cantilever structure.

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Figure 5.9 (A) Photographs of released arrays of organic CantiFETs (B) SEM micrograph of

the fabricated CantiFET device (C) Bottom and enlarged view of cantilever portion of the CantiFET from SEM showing the inter digitated source drain electrode configuration (Type 1 & 3 CantiFET) (D) Top and enlarged view of cantilever portion of the CantiFET from SEM

Table 5-3 Specifications of organic CantiFET device chips

Property CantiFET#1

(Unit : µm)

CantiFET#2

(Unit : µm)

CantiFET#3

(Unit : µm)

Cantilever length 312 312 312

Cantilever width 168 168 168

Cantilever thickness 2 2 2

SU-8 dielectric thickness 0.95 0.95 0.95

OFET W/L 300/17 300/27 523/17

Pentacene thickness

Silicon nitride encapsulation

0.05

0.015

0.05

0.015

0.05

0.015

CantiFET chip area 4000 x 4000 4000 x 4000 4000 x 4000

5.3 Characterization of Organic CantiFET

The as-fabricated organic CantiFET devices were systematically characterized with respect to

their electrical, mechanical and electromechanical and noise parameters.

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5.3.1 Electrical characterization

For electrical characterization, the organic CantiFETs were probed inside a shielded, vibration

isolated probe station. The electrical measurements were performed at room temperature

under ambient atmospheric conditions using an Agilent 4156C semiconductor parameter

analyser. Output (IDS vs. VDS) and transfer (IDS vs. VGS) characteristics for these devices were

recorded. Organic CantiFET devices were characterized before and after the silicon nitride

encapsulation process. It was observed that the integrated OFET inside the nanomechanical

cantilever retained its transistor behaviour after silicon nitride encapsulation. The devices

exhibit typical p-channel characteristics (Figure 5.10 (A)). The saturation field effect mobility

(µ) and threshold voltage (VTH) extracted from the highest slope of |IDS|1/2 versus VGS plots

using the standard saturation region current equation for OFETs were found to be 3.7 x 10-4

cm2/Vs and -11.5 V respectively. These integrated transistors exhibited low gate leakage and

good switching characteristics (Gate current density =1.2 x 10-7 A/cm2 @ VGS=-40V and

ION/IOFF = 2.2 x 103). The output characteristics of encapsulated Organic CantiFET (Figure

5.10 (B)) also verifies the typical transistor behaviour of the pentacene OFET that is

embedded inside the microcantilever.

5.3.2 Mechanical characterization

The spring constant the organic CantiFET, which is considered to be an important mechanical

design parameter for nanomechanical cantilevers, was extracted using microcantilever beam

bending technique.

These experiments were carried out using a nanoindenter following the procedure detailed in

Chapter 3 for characterizing polymer nanocomposite microcantilever. The indenter was

placed at a distance 55 m away from the tip of the cantilever. Few experiments were

conducted to tune the indenter parameters such as preload gain, integral gain, and set point,

preload and load rate.

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Figure 5.10 I-V characteristics of organic CantiFETs after silicon nitride encapsulation. (A) Transfer characteristics indicating good ION/IOFF ratio (2.2 x 103). Saturation field effect

mobility and threshold voltage extracted from |IDS|1/2 vs. VGS characteristics as shown were 2.9 x 10-4 cm2/Vs and -11.5 V. (B) Output characteristics of silicon nitride encapsulated

organic CantiFETs.

Figure 5.11 Load segment of beam bending experiment on organic CantiFET using nanoindenter

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The load segment showing the load and displacement as a function of time for the optimized

load rate is shown in Figure 5.11. The load vs. displacement curve for the CantiFET structure

can be plotted as given in Figure 5.12, with the slope being 0.6 N/m. The actual spring

constant of the cantilever structure was calculated from the slope of the curve using equation

given in the graph. Here L is the length of the cantilever structure, l is the distance from the

tip of the cantilever where indenter tip is placed as indicated in the inset of the plot. The

spring constant obtained using this method was 0.4 N/m.

Figure 5.12 Load displacement characteristics of Organic CantiFET. Inset showing the optical

micrograph of the image indicating the location of indenter placement.

From this the resonance frequency was calculated using the equation (equation 5.3) used for

first mode of vibration of rectangular microcantilever beams

*

12res

kfm

(5.3)

where k is the spring constant and m* is the effective mass of the microcantilever. Here the

value of the spring constant obtained from previous beam bending technique (0.4 N/m) was

used. The effective mass was calculated considering the layers of SU-8 and gold. Given the

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negligible thickness and low density of pentacene, it was not considered for effective mass

calculation. The density of SU-8 used for effective mass calculation was 1.2 g/cm3. The

extracted value of resonance frequency was found to be 12.9 kHz and this ensures minimum

interference from external mechanical vibrational noise.

5.3.3 Electromechanical characterization

In order to ensure the suitability of CantiFET devices for bio/chemical sensing applications,

the fabricated devices were electromechanically characterized for evaluating their strain

sensitive behaviour and thereby to extracting their surface stress sensitivity.

Electromechanical characterization of organic CantiFET devices was performed by bending

the cantilever using a calibrated micromanipulator and measuring the I-V characteristics of

CantiFET after each bending step. As the transistor is located at bottom side of the cantilever,

this bending would result in compressive strain in the pentacene layer.

The transfer characteristics (@ VDS = -40V) and output characteristics (@ VGS = -40V) of an

organic CantiFET (CantiFET # 3) under different levels of strain are plotted in Figure 5.13. It

can be observed that, these organic CantiFET devices exhibited a good strain sensitive

behaviour with the drain current increasing with compressive strain.

Figure 5.13 I-V characteristics of organic CantiFET obtained from electromechanical

characterization. (A) Transfer characteristics of an organic CantiFET (CantiFET # 3) under different amount of bending. (B) IDS-VDS characteristics (@VDS = -40 V) under different

amounts of bending

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The average strain, ε on the strain sensitive layer, pentacene, for a given amount of bending

was calculated using equation 5.4.

2

3 12

pnr

LZ Z

LL

(5.4)

Here L: length of beam, Lp: length of pentacene layer, Znr: distance of pentacene layer from

neutral axis and Z : the vertical deflection.

In the case of the organic CantiFET structure, Lp= L as the pentacene layer encompasses the

whole length of the microcantilever. Znr =h/2, the thickness of the beam as the assumption of

of a very thin strain sensitive layer of pentacene on top of the microcantilever is valid. With

these assumptions, the equation 5.4 for the average strain ε on the strain sensitive layer,

pentacene can be modified as equation 5.5.

2

34

h ZL

(5.5)

Here L: length of beam, h: thickness of beam and Z : the vertical deflection

Based on this calculation and I-V characteristics of organic CantiFET under different amounts

of bending, the percentage change in drain current, field effect mobility and threshold voltage

were plotted as a function of change in the strain (Figure 5.14). From this analysis, it can be

observed that the transistor parameter that is strongly influenced by the strain is, field effect

mobility and not the threshold voltage. This is in agreement with the present understanding of

hopping transport in pentacene, where the compressive strain reduces the hole hopping energy

barrier, due to the decrease in hopping distance [61].

The devices exhibited very high strain sensitivity (ΔI/I per unit strain) of the order of 103 and

deflection sensitivity (ΔI/I in ppm per nm of deflection) of 15.6 ppm/nm.

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Figure 5.14 Percentage change in drain current, saturation field effect mobility and threshold

voltage of the organic CantiFET # 3 as a function of percentage strain

The sensitivity was extracted from the minimum slope of the electromechanical

characterization plot. The term strain sensitivity is analogous to gauge factor in the case of

simple piezoresistors. Hence the equation for surface stress sensitivity for organic CantiFET

can be written with reference to that of a piezoresistive microcantilever with very thin

piezoresistive layer on top of the microcantilever structure which was represented in

equation1.8 in Chapter 1. Hence, the surfaces stress sensitivity ( 1( / ) sI I ) of an organic

CantiFETs can be related to its strain sensitivity 1( / )I I by equation 5.6.

1 1 4( / ) ( / )sI I I IE t

(5.6)

where, I : Drain current, s : surface stress, E : Young’s modulus of structural material and t:

thickness of the cantilever.

The surface stress sensitivity (ΔI/I in ppm per unit surface stress in mN/m) for organic

CantiFET # 3 device extracted from the electromechanical characterization using equation 5.6

was 401 ppm [mN/m]-1.

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Figure 5.15 Percentage change in drain of as a function of percentage strain (A) CantiFET # 1

The average value of Idsat at strain %= 0 was 11 nA (B) CantiFET # 2. The average value of Idsat at strain %= 0 was 6 nA

Other two types of organic CantiFET devices were also characterized for evaluating their

strain sensitive behaviours. The characterization method and the extraction of parameters

remains the same. The percentage change in drain current as a function of strain for CantiFET

#1 and CantiFET #2 are given in Figure 5.15. It was observed that, CantiFET #1 devices

showed increase in drain current with compressive strain (Figure 5.15 (A)). But CantiFET #2

devices did not show any clear trend in their strain sensitivity behaviour as is illustrated in

Figure 5.15 (B).

Organic CantiFET # 1 and CantiFET # 3 exhibited similar levels of strain sensitivity. So a set

of devices of CantiFET # 1 and CantiFET # 3 were fabricated and characterized together to

investigate the device variability pertaining to strain sensitivity. A plot of mean and standard

deviation of percentage change in current as a function of strain is shown in Figure 5.16. The

standard deviation in sensitivity was found to be within 40 %.

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Figure 5.16 Mean and standard deviation of percentage change in drain current as a function

of percentage strain for 5 cantiFET devices.

5.3.4 1/f noise characterization

In order to predict the minimum detectable surface stress for such sensors, the noise levels in

these devices need to be measured. The noise spectrum of the organic CantiFET device was

recorded using a noise measurement set up with the schematic as shown in Figure 5.17(A).

The organic CantiFET device was probed inside a shielded and vibration isolated probe

station. A battery operated low noise trans-impedance preamplifier (Stanford Research 570)

with gain varying from 10-3 to 10-12 A/V was used to provide gate bias. The output or the

drain terminal was connected to another SR570 in order to provide drain bias and to measure

and amplify noise levels in drain current. A spectrum analyzer (SR 750) was used to record

the noise power spectrum in frequencies ranging from 1Hz to a few KHz. Using programmed

low noise preamplifiers (SR 570) that were battery operated for drain and gate biases, the

noise from AC lines was prevented [132].

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Figure 5.17(A) Block diagram schematic of the setup used for measuring the 1/f noise. Device

under test (DUT) is the cantiFET device. (B) 1/f like noise characteristics of Organic CantiFET.

As the maximum output drive voltages from SR 570 was +/- 5V, the noise measurements

were carried out with device bias condition ,VGS = VDS = -5V. The noise power spectrum in

drain current of organic CantiFET is given in Figure 5.17 (B). 1/f noise in pentacene OFET

follow mobility fluctuation model governed by the empirical Hooge’s relation which says that

noise power density, SID is proportional to the square of drain current and inversely

proportional to (VGS -VTH ) [133]. Using this approximation, the noise current level for this

organic CantiFET was calculated for the 1/f noise frequency range. With the low noise level

of 1.46 pA and the high surface stress sensitivity, it should therefore be possible to achieve

the minimum detectable surface stress of 0.18 mN/m.

Organic CantiFET characterization results can be summarized by quantifying the performance

of organic CantiFET nanomechanical sensors in terms of deflection sensitivity, surface stress

sensitivity and minimum detectable surface stress. The performance of these sensors was

compared with other relevant nanomechanical sensors reported in literature such as optimized

silicon microcantilever, MOSFET embedded silicon cantilevers, SU-8 microcantilevers with

138

integrated gold as strain gauge, and the SU-8 microcantilever with SU-8/CB nanocomposite

as strain sensor which was presented in the previous chapter. The summary of this analysis is

given in Table 5-4.

Table 5-4 Comparison of performances of organic CantiFET and other nanomechanical cantilever sensors

Property SU-8

microcantilever with Au as strain gauge

SU-8 nanocomposite microcantilever

Organic CantiFET

Deflection sensitivity (ppm/nm) 0.3 1.1 15.66

Surface stress sensitivity (ppm/[mN/m])

0.3 7.6 401

Minimum detectable surface stress (mN/m) 0.14 39 0.18

The deflection sensitivity of organic CantiFET sensors (15.6 ppm per nm of deflection) was

50 times better compared to SU-8 microcantilevers with gold as strain gauge, 15 times better

compared to that of SU-8 nanocomposite cantilevers. The extracted surface stress sensitivity

value for organic CaniFET is at least three orders of magnitude higher in comparison to that

of SU-8 microcantilevers with integrated Au strain gauge (0.3 ppm [mN/m]-1) and 50 times

higher than that of SU-8 nanocomposite cantilevers. With these higher surface stress

sensitivity and lower noise levels, it should therefore be possible to achieve the minimum

detectable surface stress of 0.18 mN/m, which is at least two orders of magnitude lower

compared to the detection limit of SU-8 nanocomposite cantilevers. The significant

improvement in performance in such a nanomechanical cantilever sensor can be attributed to

the integration of OFET as strain sensor with a very thin (~50 nm) low young’s modulus and

strain sensitive organic semiconductor, pentacene with a polymer microcantilever. This

capability of detecting low surface stress makes the organic CantiFET a suitable candidate for

bio/chemical sensor applications, since the reported surface stress values due to molecular

interactions can be as low as 5mN/m [7].

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5.4 Summary

In summary, polymer nanomechanical cantilever sensors with a novel electrical transduction

scheme by integrating a strain sensitive pentacene OFET inside an SU-8 cantilever have been

explored. This novel device was named as ‘Organic CantiFET’. The ways of integrating the

pentacene OFET with an SU-8 microcantilever were explored by evaluating the performance

of OFET with SU-8 dielectric and by analysing process compatibility for fabricating these

two devices (OFET and SU-8 microcantilever). With optimizations at the unit process level,

organic CantiFET devices with different configurations were designed and fabricated. The

fabrication process followed is a novel yet simple process that avoided the exposure of

pentacene to organic solvents used in SU-8 processing with an additional benefit of omitting

the patterning step of the pentacene layer. These devices have been subsequently

characterized for their mechanical, electrical, electromechanical and noise characteristics and

thereby evaluating their performance parameters. It has been observed that these devices

exhibit a very high deflection sensitivity (15.6 ppm/nm) and their surface stress sensitivity

(401 ppm [mN/m]-1) is at least 50 times higher as compared to that of a polymer

nanocomposite cantilever while the lower limit surface stress detection is in the range of 0.18

mN/m. The cost of fabrication of such polymer devices is typically much lower than the

conventional silicon based microcantilevers. Because of these advantages, organic CantiFETs

could pave the way for development of low cost and highly sensitive bio/chemical sensors for

a variety of applications. Further, the concept of integrating a strain sensitive OFET on a

suspended polymer structure can open up new avenues for realizing ultra-sensitive strain

sensors for pressure and inertial sensing applications.

140

Chapter 6

Conclusion and future recommendations

SU-8 nanomechanical cantilever sensors with two efficient electrical transduction schemes

have been successfully developed. The highlights of the major contributions from this

research work discussed in various chapters are enlisted below:-

[1] Design and fabrication of polymer nanocomposite (SU-8/CB) microcantilevers with

overall stack thickness of just 3 µm. The deflection sensitivity of these cantilevers was

found to be 0.55 ppm/nm. The surface stress sensitivity for these devices is 4.1x10-3

[N/m]-1 which is more than 10 times the sensitivity of the SU-8 cantilever with Au and at

least a factor of two higher compared to the reported result on SU-8/CB cantilevers.

[2] Characterization and optimization of dispersion characteristics of CB in polymer matrix

to address the variability issue with the fabrication of polymer nanocomposite

microcantilevers. Standard deviation of device resistances values were brought down to

25 % for SU-8/CB with CB concentration of 8.5 vol. %.

[3] Systematic mechanical characterization of these polymer nanocomposite thin films was

performed using nanoindentation experiments in order to analyse the effect of CB filler

loading on the Young’s modulus and hardness of the polymer composite. It was very

important to estimate the Young’s modulus of this piezoresistor layer in order to predict

the performance parameters of the sensor, to be more specific, the sensitivity.

[4] Electrical characterization of these nanocomposites was performed to study the

conduction behaviour of these composites in order to arrive at an optimum range of CB

141

filler concentration in the composite for the strain sensitive layer. Polymer

nanocomposite thin film resistors were also characterized for temperature dependence.

The composite showed both PTC and NTC behaviour.

[5] After successful material characterization, SU-8 microcantilevers with embedded SU-

8/CB nanocomposite piezoresistor having optimum carbon black filler loading in the

range of 8 – 9 vol. % were designed and fabricated. The improved version of polymer

nanocomposite microcantilever was designed to maximize the surface stress sensitivity

and reduce one mask level in the fabrication process. The experimentally determined

resonance frequency and spring constant for the fabricated polymer nanocomposite

cantilever of thickness 3 µm were 22.6 kHz and 0.44N/m respectively. These devices

exhibited surface stress sensitivity of 7.6 x 10-3 [N/m]-1 which is greater than that of an

optimized silicon microcantilever and one order of magnitude higher than that of polymer

microcantilevers with Au as the strain gauge. The noise spectrum of polymer

nanocomposite microcantilevers indicated that the major component of noise in these

devices was 1/f noise. Though the polymer nanocomposite devices showed higher noise

levels compared to that of silicon based piezoresistive microcantilevers, their higher

sensitivity supports their suitability for low cost bio/chemical sensing applications.

[6] For the first time, a chemical sensing application for SU-8/CB nanocomposite

microcantilevers for the detection of explosive vapours has been successfully

implemented. These SU-8 nanocomposite microcantilevers functionalized with 4-MBA

were found to be suitable for detection of explosives in vapour phase. The controlled

experiments that had been carried out for detection of different concentrations of TNT

vapours generated from a calibrated vapour generator verified their potential to detect

TNT vapour concentrations down to less than 6 ppb with an approximate sensitivity value

of 1 mV/ppb of TNT. The microcantilever sensitivity and response time were found to be

adequate for such an application. The sensor was found to be reusable for multiple

sensing operations.

[7] A novel polymer MEMS accelerometer with SU-8/Carbon black as a piezoresistor was

also designed and fabricated. This was the first time demonstration of a piezoresistive

polymer accelerometer. The device design, fabrication and characterization of the device

are presented in Appendix 2.

142

[8] A novel device concept of an ultra-sensitive polymer nanomechanical cantilever sensor

with integrated electrical transduction using a strain sensitive organic field effect

transistor (OFET) inside a polymer cantilever structure had been introduced in as the final

contribution from this research work. This device named as ‘Organic CantiFET’ can be

thought of as a low cost polymer counterpart of a similar device in silicon, a metal oxide

semiconductor field effect transistor (MOSFET) embedded microcantilever reported

earlier. This nanomechanical device combines the benefits of having a compliant (low

Young’s modulus) structural material (SU-8) for the cantilever and a compliant integrated

strain sensitive polymer (pentacene) in transistor configuration with very good strain

sensitivity. The device design, fabrication and a detailed characterization of these sensors

for their mechanical, electrical, electromechanical and noise parameters were presented. .

A novel and simple fabrication process was developed that allowed the integration of an

OFET with pentacene as the channel material inside an SU-8 nanomechanical cantilever.

The characterization results show that the deflection sensitivity of organic CantiFET

sensors was 15.6 ppm per nm of deflection and the extracted surface stress sensitivity was

401 ppm/[mN/m]-1. The surface stress sensitivity value for organic CantiFET is at least

three orders of magnitude higher in comparison to that of SU-8 microcantilevers with

integrated Au strain gauge and 50 times higher than that of SU-8 nanocomposite

cantilevers. Thus these fabricated organic CantiFETs exhibited excellent performance

that aid in categorizing them as efficient and cost effective bio/chemical sensors having a

minimum detectable surface stress in the range of 0.18 mN/m. The concept of integrating

a strain sensitive OFET on a suspended polymer thin film structure can also open up new

avenues for realizing ultra-sensitive strain sensors for pressure and inertial sensing

applications.

In summary, SU-8 nanomechanical microcantilever sensors with two efficient electrical

transduction schemes have been successfully designed, fabricated and characterized. A

chemical sensing application of SU-8 nanocomposite microcantilevers for the detection of

explosive vapours has been successfully implemented. The a comparison of the performances

of SU-8/CB nanocomposite microcantilevers and organic CantiFETs with that of other

polymer microcantilevers reported in the literature is summarized in Table 6-1

143

Table 6-1Performance of the developed nanomechanical cantilever sensors compared to existing polymer nanomechanical cantilever sensors

Property

SU-8 microcantilever with composite

piezoresistor

SU-8 microcantilever

with Au as strain gauge

1. SU-8 nanocomposite microcantilever

2. Organic CantiFET

Surface stress sensitivity (ppm/[mN/m])

4 0.3 7.6 401

Minimum detectable surface stress (mN/m)

(Not reported) 0.14 39 0.18

The developed SU-8 nanocomposite microcantilevers possess higher sensitivity than SU-8

microcantilevers with Au as strain gauge as well as the previously reported SU-8

microcantilever with composite piezoresistor. As can be seen from the table, organic

CantiFETs possess the highest sensitivity.

Future Recommendations

[1] Explore other conducting nanoparticles with good dispersion characteristics as

conductive fillers. Highly conductive and smaller size nanoparticles would lead to lower

percolation threshold for the polymer nanocomposite and hence improvement in ease of

patterning. Carbon nanotubes (CNT) might be a good candidate for this purpose.

However, it has been reported that CNT tend to agglomerate and it would be very

difficult to get well dispersed CNT in SU-8. In order to address this issue, CNT could be

functionalized to bind to the epoxide groups in SU-8.

[2] Minimum detectable surface stress is the key factor that decides the suitability of such

sensors for different applications. To bring down the LOD, explore different

piezoresistive layers with low noise levels and that could be compatible with SU-8.

[3] As the particulate nature of conducting fillers in SU-8 leads to heterogeneity of the

composite and hence the variability, the feasibility of using conducting polymers that can

be mixed with SU-8 or intrinsically strain sensitive conducting polymers with good gauge

factors could be explored.

144

[4] In the case of detection of explosive vapours, in order to improve upon selectivity aspect,

three approaches could be adopted. (1) Follow microcantilever array approach with

separate coatings that support different types of interactions with the analyte molecules

and then selectively detect the explosives by performing pattern recognition (2)

Functionalize microcantilevers with coating materials very specific (probably

Calixarenes, molecular imprinted polymers) to explosive molecules (3) Use

microcantilevers with other potential explosive sensors such as OFET based explosive

sensors which together could bring orthogonality to the sensing.

[5] In the case of organic CantiFETs, the bias voltage range can still be reduced by using

polymer dielectric with dielectric constant higher than SU-8. Though SU-8 was used as

the dielectric material in the developed organic CantiFETs, better dielectric materials

could be incorporated into the integrated OFET to improve the mobility and to reduce the

bias stress instability.

[6] Polymer materials such as parylene that can be easily deposited at low temperature could

replace the HWCVD silicon nitride encapsulation layer for these CantiFETs.

145

Appendix A

Analysis of Nanoindentation data using King’s method of substrate effect analysis

In order to consider the substrate effect, Young’s modulus values for thin films were re-

calculated from reduced modulus using a modified King’s analysis [109] as given in equation

below.

2 ( ) ( )2 211 11 1t h t h

fi sa a

r i f s

e eE E E E

(1)

where Es and Ei are the moduli of substrate and indenter respectively. Parameters a, t and h

are square root of projected area, film thickness and indenter depth respectively. α is a

numerically determined scaling parameter and it is a function of normalized punch size,

a / (t-h), where (t-h) is the effective film thickness below the punch. The basic assumption

made in King’s analysis is a flat punch indenting a film of thickness t (Figure A1.1(a)); where

as in the modified King’s analysis applicable for Berkovich indenter, the effective film

thickness below the punch is (t-h) as can be seen in Figure A1.1(b).

Figure A1.1 (a) Case 1 considered in King’s analysis. The schematic of flat punch indenter indenting a film of thickness t. (b) Case 2 considered in modified King’s analysis. The schematic of Berkovich indenter indenting a film of thickness t to a depth of h [109].

146

The procedure for calculation of Film modulus from reduced modulus is as follows.

1. Calculate the normalized punch size, a / (t-h) where a = Square root of projected area, (t-h) is

the effective film thickness for Berkovich type indenter. t=Film thickness, h= maximum

depth of indent.

2. From King’s plot (Figure A1.2) find out the value for fitting parameter ‘α’.

3. Calculate Young’s modulus of film Ef using equation (1).

Figure A1.2 Plot of α as a function of normalized punch size [109].

147

Appendix B

SU-8/CB nanocomposite for a novel all polymer piezoresistive microaccelerometer4

MEMS accelerometers use a variety of transduction principles, such as capacitive [134],

piezoelectric[135], tunnelling [136] and piezoresistive [137-140] . Among these,

piezoresistive accelerometers seem to be especially attractive due to their structural

simplicity, easier fabrication process and their immunity to parasitic capacitance and

electromagnetic interference. Various research groups have worked on the design, fabrication

and optimization of piezoresistive MEMS accelerometers. However, the reported

piezoresistive micro-accelerometers are silicon based with the associated complex and

expensive fabrication steps such as the SOI wafer processing, double sided alignment, silicon

bulk micromachining, etc. Piezoresistive accelerometers are also considered to be less

sensitive compared to the capacitive accelerometers. However, it has been shown that

sensitivity of the piezoresistive accelerometers can be improved by novel device designs or by

introducing compliant structural materials and high gauge factor piezoresistive elements. The

latter direction very similar to the polymer nanocomposite piezoresistive microcantilevers

presented in this chapter would be an optimum solution for improving the performance and

reducing the cost of production.

SU-8 based piezoresistive MEMS accerometers were developed for the first time by

integrating SU-8/CB nanocomposite strain sensitive layer. The polymer nanocomposite

microaccelerometer has a quad beam structure with a suspended proof mass, surrounded by a

thick frame; all fabricated using SU-8. The outer frame provides support for the beams and a

region for contact pads. Individual beams contain polymer nanocomposite (composite of SU-

8 and Carbon Black) layer sandwiched between two SU-8 layers. The polymer nanocomposite

4 MEMS accelerometer developed in collaboration with Prajakta S Vaidya, M-Tech Batch 2009, Department of Electrical Engineering, IIT Bombay [143].

148

acts as a strain sensitive layer which changes its resistance under strain. A planar schematic of

the whole device structure and a cross sectional schematic of the composite beam is illustrated

in Figure A2.1. The proposed polymer accelerometer is not designed for a specific application

but the emphasis in this work is more on the demonstration of technology.

Figure A2.1 Schematic of polymer MEMS accelerometer (A) Planar schematic of SU-8 accelerometer with embedded SU-8/CB composite. (B) Cross sectional schematic of one of

the legs of beams indicating individual layer thicknesses

Basic principle of operation of these accelerometers is based on the transduction of vertical

force into a resistance change. Acceleration imparted upon this microaccelerometer acts like a

vertical force and causes a displacement of the proof mass. The displacement of proof mass

leads to a strain in the beams which results in a change in the resistance of the embedded

polymer nanocomposite, that can be easily measured using interface electronics. During the

actual measurements, MEMS accelerometers are connected to appropriate electronic circuitry

to give voltage output. Thus a linear relationship between acceleration and sensor output

voltage can be obtained. The maximum displacement, for a given acceleration decreases with

an increase of stiffness of the structure which is proportional to the Young’s modulus of the

structural material. The deflection induced resistance change ΔR/R is proportional to the

gauge factor of the piezoresistive layer. Thus the acceleration sensitivity is proportional to the

ratio of the gauge factor of piezoresistive material and the Young’s modulus of the structural

material.

The device fabrication process as given in Figure A2.2 was very similar to that of SU-8

nanocomposite microcantilevers with an additional lithographic step for proof mass.

149

Figure A2.2. Schematic of fabrication process for polymer composite MEMS accelerometer

An SEM image of the fabricated device is shown in Figure A2.3. The straight beams holding

the proof mass indicates that with the optimized process being followed for fabricating these

devices, the built-in stress in SU-8 layers were minimized. The proof mass thickness in this

device was just 12-15 m and it could be increased to improve the sensitivity.

Figure A2.3SEM micrographs of the fabricated SU-8 nanocomposite MEMS accelerometer

The characterization of one of the microaccelerometers done using LDV is shown in Figure

A2.4 which indicates a resonance frequency of 10.8 kHz.

150

Figure A2.4Resonance frequency plot for polymer composite MEMS accelerometer

Based on the dynamic characterization experiments done using LDV, the relative

displacements of the proof mass of the accelerometers were plotted as a function of

acceleration (

Figure A2.5). As expected, the response sensitivity at resonance is larger (280 nm/g) for the

device with the lower resonant frequency (f=10.86 kHz). However, the response of the device

with the higher resonant frequency is better in terms of linearity. The visible scatter in the

response of the more sensitive device needs to be investigated further.

Figure A2.5 Microaccelerometer proof mass displacement vs. acceleration

151

In order to extract the piezoresistive response of SU-8 nanocomposite MEMS accelerometers,

the static electromechanical characterization was performed (Figure A2.6). Based on this and

the dynamic characterization results (figure A2.5), the sensitivity of the MEMS

accelerometers could be extracted as 0.56 mV/g.

Figure A2.6 Static electromechanical characterization of SU-8 nanocomposite beam

connected in Wheatstone bridge. Refer Chapter 4 for measurement scheme.

152

Appendix C

I. Common processes

A. Release layers or sacrificial layers for releasing SU-8 microcantilevers

1. RF Sputtered silicon dioxide

Base Vacuum 2 x 10-6 mbar

Process Vacuum 0.023 mbar

Sputter Gas Argon

Substrate Temperature Room temperature

Thickness (For 55 min) 200 nm

2. Thermally grown silicon dioxide Pyrogenic oxidation @ 1050oC (for 2 hr. 30 min) to yield silicon dioxide thickness

between 800 nm.

B. Physical vapour deposition of metals (Cr, Ti and Au)

1. Thermal evapouration

Method : Resistive thermal evaporation.

Vacuum : 2 x 10-6 ; Deposition rate for Cr = 0.16 nm/sec at filament current of 81 Amps

Deposition rate for Au = 0.1-0.3 nm/sec for filament current of 70 Amps.

2. RF Sputtering

Parameter Value Cr Ti Au

Base Vacuum 1.3 x 10-5 mbar

Process Vacuum 1.2 x 10-2 mbar

Sputter Gas Argon

Substrate Temperature Room temperature

RF Power 150 W 150 W 80 W

Thickness (Dur.: 1min.) 15 nm 3.6 nm 55 nm

153

C. Lift-off patterning of metals (Ti/Au)

1. Take 2-3 ml Positive Resist (PPR) S1813; spin coat @1500 rpm for 35 seconds with a

spread cycle of 500 rpm for 15 seconds

2. Prebake on a hot plate for 12 min at 70oC

3. UV exposure in Karl Suss MJB 3 std. aligner for 1 minute

(@Lamp intensity = 3.4 mW/cm2 )

4. Develop in MF 319 developer for 24 sec.

5. Rinse in water, blow dry. DO NOT HARD BAKE.

6. Deposit Ti/Au film with substrate at room temperature.

Ensure that substrate is not heated for extended periods of time

7. Stir or ultrasonicate in Acetone to lift-off the deposited film from unexposed areas

D. Etching of metals (Cr/Au)

1. Deposit Cr/Au film

2. Take 2 ml Positive Resist (PPR) S1813; spin coat @3000 rpm for 35 seconds with a

spread cycle of 500 rpm for 15 seconds

3. Prebake on a hot plate for 13 min at 70oC

4. UV exposure in Karl Suss MJB 3 std. aligner for 45 sec.

(@Lamp intensity = 3.4 mW/cm2 )

5. Develop in MF 319 developer for 18 sec.

6. Rinse in water, blow dry.

7. Perform etching in respective etchants for Au and Cr. After etching Au, remove PPR

with Acetone. This is very important as the Cr etchant hardens the PPR5. Then perform

the Cr etching. Finally rinse in DI water and blow dry the sample in nitrogen.

5 Private communication with Nidhi Maheswari, Ph.D. student, IIT Bombay

154

II. Device specific processes

1. SU-8/CB nanocomposite microcantilever (Device fabrication :Figure 3.11)

Table A3-1 Process details of different SU-8 layers in SU-8 nanocomposite microcantilevers SU-8 layer

Spin coat

Prebake Exposure (Unknown intensity)

PEB Development

Hard bake

SU-8 2002 + nanothinner (1:1) (400 nm)

300 rpm 4s 500 rpm 4s 3000 rpm 30s 300 rpm 2s

@70oC 3 min Ramp to 90oC in 6min @90oC 2 min Ramp down to 30oC

8 sec

@70oC 3 min Ramp to 90oC in 6min @90oC 2 min Ramp down to 30oC

1 min in SU-8 developer and 1-2 min rinse in IPA and blow dry

@100oC 10 min

SU-8/CB (10 wt.% )

300 rpm 4s 500 rpm 4s 3000 rpm 20s 300 rpm 2s

2 min. @70oC 3 min Ramp to 90oC in 6min @90oC 7 min Ramp down to 30oC

(Not done) (Not done)

SU-8 2002 (1.8 µm)

300 rpm 4s 500 rpm 4s 3000 rpm 30s 300 rpm 2s

@70oC 3 min Ramp to 90oC in 6min @90oC 7 min Ramp down to 30oC

12 sec

@70oC 3 min Ramp to 90oC in 6min @90oC 7 min Ramp down to 30oC

2-3 min in SU-8 developer and sonication in IPA and blow dry

@100oC 10 min

SU-8 2100 (150 µm)

300 rpm 7s 500 rpm 10s 2300 rpm 30s 300 rpm 2s

@65oC 15 min Ramp to 85oC in 10 min @85oC 45 min Ramp down to 30oC

1.8 min

@65oC 15 min Ramp to 85oC in 10 min @85oC 45 min Ramp down to 30oC

20 min in SU-8 developer and 1-2 min rinse in IPA and blow dry.

@100oC 30 min. Ramp down to 50oC.

155

2. Optimized SU-8/CB nanocomposite microcantilever (Device fabrication : Figure 3.26)

Table A3-2 Process details of different SU-8 layers in optimized SU-8 nanocomposite microcantilevers

SU-8 layer

Spin coat

Prebake Exposure (@Lamp intensity = 3.9 mW/cm2)

PEB Development

Hard bake*

SU-8 2000.5 (500 nm)

300 rpm 4s 500 rpm 4s 3000 rpm 30s 300 rpm 2s

@70oC 3 min Ramp to 90oC @90oC 3 min Ramp down to 30oC

16 sec

@70oC 3 min Ramp to 90oC @90oC 5 min Ramp down to 30oC

1 min in SU-8 developer and 1-2 min rinse in IPA and blow dry

@100oC 10 min Ramp down to 50oC

SU-8/CB (8.4 vol.% )

300 rpm 4s 500 rpm 5s 2000 rpm 20s 300 rpm 2s

@70oC 3 min Ramp to 90oC @90oC 5 min Ramp down to 30oC

2.3-2.4 min (Dose : 530 mJ/cm2 (Figure 3.3)

@70oC 3 min Ramp to 90oC in 6min @90oC 6 min Ramp down to 30oC

(Not done)

(Not done)

SU-8 2002 (1.6 µm)

300 rpm 4s 500 rpm 4s 2300 rpm 30s 300 rpm 2s

@70oC 3 min Ramp to 90oC @90oC 7 min Ramp down to 30oC

20 sec

@70oC 3 min Ramp to 90oC @90oC 7 min Ramp down to 30oC

1 min in SU-8 developer and 1-2 min rinse in IPA and blow dry

@100oC 10 min Ramp down to 50oC

SU-8 2100 (148 µm)

300 rpm 7s 500 rpm 10s 2200 rpm 30s 300 rpm 2s

@65oC 15 min Ramp to 85oC in 10 min @85oC 1 hr. 10 min Ramp down to 30oC

Double exposure. Each for 0.65 min with 30 sec. gap

@65oC 10 min Ramp to 85oC @85oC 20 min Ramp down to 30oC

12-20 min in SU-8 developer and 1-2 min rinse in IPA and blow dry.

@100oC 30 min. Ramp down to 50oC. Very important step to remove stress in SU-8

156

*In the recent process discussed in Figure 3.29, the hard bake temperature at different stages of processing was different. After the first SU-8 layer and SU-8/CB layer, the hard bake was performed at 130oC for 10-15 minutes, in the second SU-8 layer, hard bake was performed at 120oC for 10 minutes and in the final SU-8 2100 layer, the hard bake was performed at 110oC for 30 minutes.

3. Organic CantiFET (Device fabrication : Figure 5.8)

Table A3-3 Process details of different SU-8 layers in Organic CantiFET SU-8 layer

Spin coat

Prebake Exposure PEB Development

Hard bake

SU-8 2002 (980 nm)

300 rpm 4s 500 rpm 5s 5500 rpm 30s 300 rpm 2s

@70oC 3 min Ramp to 90oC @90oC 5 min Ramp down to 30oC

18sec

@70oC 3 min Ramp to 90oC @90oC 5 min Ramp down to 30oC

1 min in SU-8 developer and 1-2 min rinse in IPA and blow dry

@100oC 10 min Ramp down to 50oC

SU-8 2002 (940-950 nm)

300 rpm 4s 500 rpm 5s 6200 rpm 30s 300 rpm 2s

@70oC 3 min Ramp to 90oC @90oC 5 min Ramp down to 30oC

18sec

@70oC 3 min Ramp to 90oC @90oC 5 min Ramp down to 30oC

1 min in SU-8 developer and 1-2 min rinse in IPA and blow dry

@100oC 10 min Ramp down to 50oC

SU-8 2100 (148 µm)

300 rpm 7s 500 rpm 10s 2200 rpm 30s 300 rpm 2s

@65oC 15 min Ramp to 85oC in 10 min @85oC 1 hr. 10 min Ramp down to 30oC

Double exposure. Each for 0.65 min with 30 sec. gap

@65oC 10 min Ramp to 85oC @85oC 20 min Ramp down to 30oC

12-20 min in SU-8 developer and 1-2 min rinse in IPA and blow dry.

@100oC 30 min. Ramp down to 50oC. Very important step to remove stress in SU-8

Pentacene deposition:

Vacuum 2 x 10-6 mbar

Substrate temperature 65oC

157

Rate of deposition 0.1 nm/sec.

Encapsulation layer: HWCVD Silicon nitride

Base pressure 2 x 10-6 mbar

Process pressure 6 x 10-2 mbar

Silane : Ammonia flow rate ratio 1: 20

Substrate temperature R.T.

Filament temperature 1850oC

Rate of deposition 0.1 nm/sec.

Expected thickness (for 5 min) 15- 20 nm

158

Appendix D

Mask layouts

Mask layout designed for different devices developed as part of this Ph.D. work is provided

here. All the designs are for 2” substrates. For SU-8 layers, the negative (dark field

counterpart ) of the masks provided were used. Along with the mask set, enlarged design of

one of the devices in each mask is also provided for easy understanding.

1. Single layer SU-8 microcantilevers (Device fabrication : Figure 2.1)

159

2. SU-8/CB nanocomposite microcantilever (Device fabrication :Figure 3.11)

SU-8/CB strips

160

3. Optimized SU-8/CB nanocomposite microcantilever (Device fabrication : Figure 3.26)

(“V” shaped devices in the mask design)

SU-8/CB strips

161

4. Organic CantiFET (Device fabrication : Figure 5.8)

162

Appendix E

Spring constant and resonant frequency

Resonant frequency of a cantilever beam in first mode of vibration could be written as

*1

2eff

resk

fm

Here, effk : Effective spring constant, m* : effective mass of the beam.

Case 1: Single layer cantilever beam

33

effEIkL

Here, I: Area moment of inertia; E: Young’s modulus of the cantilever structural material; L: Length of the cantilever beam

For a rectangular cantilever beam, 3

12WhI ;

* 0.243m L W h

Here, W: Width of the rectangular cantilever beam; h: Thickness of the beam; : Density

For a “U” shaped cantilever beam, 3

6WhI ;

* 2 0.243m L W h

Here, W: Width of the rectangular cantilever beam; h: Thickness of the beam; : Density

For a “V” shaped cantilever beam[141] 3

31 1 46

WI bhb

;

* 0.163m L b h

163

Case 2: Multi layer cantilever beam

Consider the case of a multilayer composite beam.

The effective spring constant, 3(3 )eff

effEI

kL

Here, eff i ii

EI E I is for the composite structure.

Ei : Young’s modulus of the ith layer; Ii : Area moment of inertia of the ith layer.

For a rectangular beam, 3

2

12i

i i i NWhI Wh h h

Here hi : Thickness of the ith layer ; hN : Position of the neutral axis.

164

References

[1] J. w Gardner and V.K. Varadan, Osama O. Awadelkarim, Microsensors, MEMS and Smart devices, John Wiley & Sons, 2002.

[2] Bharat Bhushan, “Introduction to Nanotechnology,” Springer Handbook of Nanotechnology, B. Bhushan, ed., Springer, , pp. 1-10.

[3] S.P. Mohanty and E. Kougianos, “Biosensors : A Tutorial Review,” IEEE Potentials, 2006, pp. 1-16.

[4] King W P et.al., “Design of atomic force microscope cantilevers for combined thermomechanical writing and thermal reading in array operation,” J. Microelectromech. Syst., vol. 11, p. 765–74.

[5] B. R, L.H. P, G. Ch, G.J. K, F.J. H, S. L, and M.E. and G¨. H-J, “Micromechanical thermogravimetry,” Chem. Phys. Lett., vol. 294, p. 363–9.

[6] S.-hyung S. Lim, D. Raorane, S. Satyanarayana, and A. Majumdar, “Nano-chemo-mechanical sensor array platform for high-throughput chemical analysis,” Sensors And Actuators, vol. 119, 2006, pp. 466-474.

[7] Y. Arntz, J.D. Seelig, H.P. Lang, J. Zhang, P. Hunziker, J.P. Ramseyer, E. Meyer, M. Hegner, and C. Gerber, “Label-free protein assay based on a nanomechanical cantilever array,” vol. 14, 2003, pp. 86-90.

[8] L.G. Carrascosa and M. Moreno, “Nanomechanical biosensors : a new sensing tool,” Trends Anal. Chem., vol. 25, 2005, p. 196–206.

[9] C. Ziegler, “Cantilever-based biosensors,” Analytical and Bioanalytical Chemistry, vol. 379, 2004, pp. 946-959.

[10] J. Fritz, M.K. Baller, H.P. Lang, and H. Rothuizen, “Translating Biomolecular Recognition into Nanomechanics,” Science, vol. 288, 2000, pp. 316-318.

[11] J.W. Gardner, JehudaYinon, ed., Electronic Noses & Sensors for the Detection of Explosives, NATO Science series, .

[12] H.-peter Lang, M. Hegner, and C. Gerber, “Nanomechanical Cantilever Array Sensors,” Handbook of Nanotechnology, Springer, , pp. 443-459.

[13] Stoney G G, “The tension of metallic films deposited by electrolysis,” Proc. R. Soc. Lond. A, vol. 82, 1909, p. 172.

[14] R. Raiteri, “Micromechanical cantilever-based biosensors,” Sensors and Actuators B: Chemical, 2001.

165

[15] N.V. Lavrik and M.J. Sepaniak, “Cantilever transducers as a platform for chemical and biological sensors,” Review of Scientific Instruments, vol. 75, 2004, pp. 29-33.

[16] R. Berger, E. Delamarche, H.P. Lang, C. Gerber, J.K. Gimzewski, E. Meyer, and H. Güntherodt, “Surface stress in the self-assembly of alkanethiols on gold probed by a force microscopy technique,” Applied Physics A: Mater.Sci.Process, vol. 66, 1998, pp. 55-59.

[17] R. Berger, E. Delamarche, H.P. Lang, C. Gerber, J.K. Gimzewski, E. Meyer, and H.-joachim Gu, “Surface Stress in the Self-Assembly of Alkanethiols on Gold Ru,” Science, vol. 276, 1997, pp. 2021-2024.

[18] K.W. Wee, “Novel electrical detection of label-free disease marker proteins using piezoresistive self-sensing micro-cantilevers,” Biosensors and Bioelectronics,, 2005.

[19] G. Wu, H. Ji, K. Hansen, T. Thundat, R. Datar, R. Cote, M.F. Hagan, A.K. Chakraborty, and A. Majumdar, “Origin of nanomechanical cantilever motion generated from biomolecular interactions,” Materials Science, 2001, pp. 18-22.

[20] K.M. Hansen and T. Thundat, “Microcantilever biosensors,” Methods, vol. 37, 2008, pp. 57-64.

[21] L.A. Pinnaduwage, D.L. Hedden, A. Gehl, V.I. Boiadjiev, J.E. Hawk, R.H. Farahi, and T. Thundat, “A sensitive , handheld vapor sensor based on microcantilevers,” Review of Scientific Instruments, vol. 75, 2008, pp. 4554-4557.

[22] A.R. Krause, C.V. Neste, L. Senesac, T. Thundat, and E. Finot, “Trace explosive detection using photothermal deflection spectroscopy,” Journal of Applied Physics, vol. 103, 2008, p. 094906.

[23] C.W.V. Neste, L.R. Senesac, D. Yi, and T. Thundat, “Standoff detection of explosive residues using photothermal microcantilevers,” 2008, pp. 2008-2010.

[24] L. Senesac and T.G. Thundat, “Nanosensors for trace explosive detection,” Materialstoday, vol. 11, 2008, pp. 28-36.

[25] L.A. Pinnaduwage, A. Wig, D.L. Hedden, A. Gehl, D. Yi, T. Thundat, R.T. Lareau, and I. Introduction, “Detection of trinitrotoluene via deflagration on a microcantilever,” Journal of Applied Physics, vol. 95, 2004, pp. 5871-5875.

[26] K.M. Hansen and T. Thundat, “Microcantilever biosensors,” Methods, vol. 37, 2008, pp. 57-64.

[27] M.A. and B.A. Greve A, Keller S, Vig A L, Kristensen A, Larsson D, Yvind K, Hvam J M, Cerruti M, “Thermoplastic microcantilevers fabricated by nanoimprint lithography,” J. Micromech. Microeng., vol. 20, 2010, p. 015009.

166

[28] L.Y. and X.Y. Katragadda R, Wang Z, Khalid W, “Parylene cantilevers integrated with polycrystalline silicon piezoresistors for surface stress sensing,” Appl. Phys. Lett., vol. 91, 2007, p. 083505.

[29] A. Boisen, S. Dohn, S.S. Keller, S. Schmid, and M. Tenje, “Cantilever- like micromechanical sensors,” Rep. Prog. Phys., vol. 74, 2011, pp. 1-30.

[30] J.M. Shaw, J.D. Gelorme, N.C. LaBianca, W.E. Conley, and S.J. Holmes, “Negative photoresists for optical lithography,” IBM Journal of Research and Development, vol. 41, 1997, pp. 81-94.

[31] J. Brugger, P. Renaud, and P. Vettiger, with M. Despont and H. Lorenz, N.Fahrni, “High aspect ratio ultrathick, negative-tone near-UV photoresist for MEMS applications,” IEEE MEMS, 1997, pp. 518-522.

[32] H. Lorenz, M. Despont, M. Fahrni, N. LaBianca, P. Vettiger, and P. Renaud, “SU-8: a low-cost negative resist for MEMS,” J. Micromech. Microeng, vol. 7, 1997, pp. 121-124.

[33] de R.N.F. and A.D. Genolet G, Brugger J, Despont M, Drechsler U, Vettiger P, “Soft, entirely photoplastic probes for scanning force microscopy,” Rev. Sci. Instrum., vol. 70, 1999, p. 2398.

[34] J. Theysen, A.D. Yalcinkaya, P. Vettiger, and A. Menon, “Polymer-based stress sensor with integrated readout,” Journal of Physics D: Applied Physics, vol. 35, 2002, pp. 2698-2703.

[35] J.H.T. Ransley, M. Watari, D. Sukumaran, and R.A. McKendry, A.A. Seshia, “SU8 bio-chemical sensor microarrays,” Microelectronic Engineering, vol. 83, 2006, pp. 1621-1625.

[36] M. Nordström, S. Keller, M. Lillemose, A. Johansson, S. Dohn, D. Haefliger, G. Blagoi, M. Havsteen-jakobsen, and A. Boisen, “SU-8 Cantilevers for Bio/chemical Sensing; Fabrication, Characterisation and Development of Novel Read-out Methods,” Sensors, 2008, pp. 1595-1612.

[37] H P Lang et.al, “Sequential position readout from arrays of micromechanical cantilever sensors,” Appl. Phys. Lett., vol. 72, 1998, p. 383.

[38] H P Lang, “A chemical sensor based on a micromechanical cantilever array for the identification of gases and vapors,” Appl. Phys. A, vol. 66, 1998.

[39] C.A. Savran, T.P. Burg, J. Fritz, and S.R. Manalis, “Microfabricated mechanical biosensor with inherently differential readout,” Applied Physics Letters, vol. 83, 2003, pp. 1659-1661.

[40] S.-hyung S. Lim, D. Raorane, S. Satyanarayana, and A. Majumdar, “Nano-chemo-mechanical sensor array platform for high-throughput chemical analysis,” Sensors And Actuators, vol. 119, 2006, pp. 466-474.

167

[41] M. Yue, H. Lin, D.E. Dedrick, S. Satyanarayana, A. Majumdar, A.S. Bedekar, J.W. Jenkins, and S. Sundaram, “A 2-D Microcantilever Array for Multiplexed Biomolecular Analysis,” Journal Of Microelectromechanical Systems, vol. 13, 2004, pp. 290-299.

[42] B.A.A. Barlian, W.-tae Park, J.R. Mallon, A.J. Rastegar, and B.L. Pruitt, “Review : Semiconductor Piezoresistance for Microsystems,” Proceedings of the IEEE, vol. 97, 2009.

[43] A.A. Barlian, W.-tae Park, J.R.M. Jr, A.J. Rastegar, B.L. Pruitt, and M. Engineering, “Review: Semiconductor Piezoresistance for Microsystems,” Proc IEEE Inst Electr Electron Eng, vol. 97, 2009, pp. 513-552.

[44] J.H. and H.O. Boisen A, Thaysen J, “Environmental sensors based on micromachined cantilevers with integrated read-out,” Ultramicroscopy, vol. 82, 2000, p. 11.

[45] S.T. and G.C. Yoshikawa G, Lang H P, Akiyama T, Aeschimann L, Staufer U, Vettiger P, Aono M, “Sub-ppm detection of vapors using piezoresistive microcantilever array sensors,” Nanotechnology, vol. 20, 2009, p. 15501.

[46] F.T. Goericke and W.P. King, “Modeling Piezoresistive Microcantilever Sensor Response to Surface Stress for Biochemical Sensors,” IEEE Sensors, vol. 8, 2008, pp. 1404-1410.

[47] P.S. J, D.J. C, and B.L. Pruitt, “Piezoresistive cantilever performance: I. Analytical model for sensitivity.,” J. Microelectromech. Syst, vol. 19, 2010, p. 137.

[48] S.-J. Park, J.C. Doll, A.J. Rastegar, and B.L. Pruitt, “Piezoresistive Cantilever Performance—Part II: Optimization,” J Microelectromech Syst., vol. 19, 2010, pp. 149-161.

[49] B. N, B. J, de R.N. F, and D. U, “Scanning force microscopy in the dynamic mode using microfabricated capacitive sensors,” J. Vac. Sci. Technol. B, vol. 14, 1996, p. 901.

[50] G. Shekhawat, S.-H. Tark, and V.P. Dravid, “MOSFET-Embedded Microcantilevers for Measuring Deflection in Biomolecular Sensors,” Science, vol. 311, 2006, pp. 1592-1595.

[51] J. Thaysen, A.D. Yalçinkaya, R.K. Vestergaard, S. Jensen, M.W. Mortensen, P. Vettiger, and A. Menon, “SU-8 BASED PIEZORESISTIVE MECHANICAL SENSOR,” IEEE MEMS, 2002, pp. 320-323.

[52] N.S. Kale, S. Nag, R. Pinto, V.R. Rao, and S. Member, “Fabrication and Characterization of a Polymeric Microcantilever With an Encapsulated Hotwire CVD Polysilicon Piezoresistor,” Journal Of Microelectromechanical Systems, 2008, pp. 1-9.

[53] N.S. Kale, “Making Hotwire CVD a Viable Technology Alternative for BioMEMS Applications : Device Optimization , Fabrication and Characterization,” 2007.

168

[54] X.-wu Zhang, Y.I. Pan, Q. Zheng, and X.-su Yi, “Time Dependence of Piezoresistance for the Conductor-,” Polymer, 2000, pp. 2739-2749.

[55] M.H.G. Wichmann, S.T. Buschhorn, J. Gehrmann, and K. Schulte, “Piezoresistive response of epoxy composites with carbon nanoparticles under tensile load,” vol. c, 2009, pp. 1-8.

[56] H.C. Chiamori, J.W. Brown, E.V. Adhiprakasha, and E.T. Hantsoo, “Suspension of nanoparticles in SU-8 : Processing and characterization of nanocomposite polymers,” Microelectronics Journal, vol. 39, 2008, pp. 228-236.

[57] L. Gammelgaard, P.A. Rasmussen, M. Calleja, P. Vettiger, and A. Boisen, “Microfabricated photoplastic cantilever with integrated photoplastic/carbon based piezoresistive strain sensor,” Applied Physics Letters, vol. 88, 2006, p. 113508.

[58] R. Mateiu, M. Lillemose, T. Steen, A. Boisen, and O. Geschke, “Reliability of poly 3 , 4-ethylenedioxythiophene strain gauge,” Microelectronic Engineering, vol. 84, 2007, pp. 1270-1273.

[59] M. Lillemose, M. Spieser, N.O. Christiansen, A. Christensen, and A. Boisen, “Intrinsically conductive polymer thin film piezoresistors,” Microelectronic Engineering, vol. 85, 2008, pp. 969-971.

[60] T. Ji, S. Jung, and V.K. Varadan, “Field-Controllable Flexible Strain Sensors Using Pentacene Semiconductors,” IEEE Electron Device Letters, vol. 28, 2007, pp. 1105-1107.

[61] T. Sekitani, Y. Kato, S. Iba, H. Shinaoka, and T. Someya, “Bending experiment on pentacene field-effect transistors on plastic films,” Applied Physics Letters, 2005, pp. 3-5.

[62] T. Sekitani, U. Zschieschang, H. Klauk, and T. Someya, “Flexible organic transistors and circuits with extreme bending stability,” Nature Materials, vol. 9, 2010, p. 1015.

[63] A. Jedaa and M. Halik, “Toward strain resistant flexible organic thin film transistors,” Applied Physics Letters, vol. 95, 2009, p. 103309.

[64] T. Sekitani, S. Iba, Y. Kato, Y. Noguchi, and T. Someya, “Ultraflexible organic field-effect transistors embedded at a neutral strain position,” Applied Physics Letters, vol. 87, 2005, p. 173502.

[65] C. Yang, J. Yoon, S.H. Kim, K. Hong, D.S. Chung, K. Heo, C.E. Park, and M. Ree, “Bending-stress-driven phase transitions in pentacene thin films for flexible organic field-effect transistors,” Applied Physics Letters, vol. 92, 2008, p. 243305.

[66] M. Kanari, M. Kunimoto, T. Wakamatsu, and I. Ihara, “Critical bending radius and electrical behaviors of organic field effect transistors under elastoplastic bending strain,” Thin Solid Films, vol. 518, 2010, pp. 2764-2768.

169

[67] G. Darlinski, U. Böttger, R. Waser, H. Klauk, M. Halik, U. Zschieschang, G. Schmid, and C. Dehm, “Mechanical force sensors using organic thin-film transistors,” Journal of Applied Physics, vol. 97, 2005, p. 093708.

[68] I. Manunza, A. Sulis, and A. Bonfiglio, “Pressure sensing by flexible, organic, field effect transistors,” Applied Physics Letters, vol. 89, 2006, p. 143502.

[69] I. Manunza and A. Bonfiglio, “Pressure sensing using a completely flexible organic transistor,” Biosensors and Bioelectronics, vol. 22, 2007, pp. 2775-2779.

[70] M. Kitamura and Y. Arakawa, “Pentacene-based organic field-effect,” J. Phys.: Condens. Matter, vol. 20, 2008, p. 184011.

[71] S. Jung, T. Ji, and V.K. Varadan, “Pentacene-Based Low-Voltage Strain Sensors With PVP/Ta2O5 Hybrid Gate Dielectrics,” IEEE Transactions on Electron Devices, vol. 57, 2010, pp. 391-396.

[72] T. Thundat, E. A. Wachter and S.L. SharpR. J. Warmack, “""” Appl. Phys. Lett., vol. 66, 1995, p. 1695.

[73] P.G. Datskos, I. Sauers, “""” Sens. Actuators B, vol. 61, 1999, p. 75.

[74] C. L. Britton et.al, “C. L. Britton et. al,” Ultramicroscopy, vol. 82, 2000, p. 17.

[75] T. Thundat, G.Y. Chen, R.J. Warmack, and D.P. Allison, E. A. Wachter, “""” Anal. Chem., vol. 67, 1995, p. 519.

[76] R. Berger, C. Gerber, and H.P. Lang, J. K. Gimzewski, “""” Microelectron. Eng., vol. 35, 1997, p. 373.

[77] S.K. Vashist, “A Review of Microcantilevers for Sensing Applications,” Journal of Nanotechnology Online, 2007, p. http://www.azonano.com/Details.asp?ArticleID=1927.

[78] A. Subramanian, P.I. Oden, S.J. Kennel, K.B. Jacobson, R.J. Warmack, T. Thundat, and M.J. Doktycz, “""” Appl. Phys. Lett., vol. 81, 2002, p. 385.

[79] N.V. Lavrik and M.J. Sepaniak, “Cantilever transducers as a platform for chemical and biological sensors,” Review of Scientific Instruments, vol. 75, 2004, pp. 29-33.

[80] D. Yi, A. Greve, J.H. Hales, L.R. Senesac, Z.J. Davis, D.M. Nicholson, A. Boisen, and T. Thundat, “Detection of adsorbed explosive molecules using thermal response of suspended microfabricated bridges,” Applied Physics Letters, vol. 93, 2008, p. 154102.

[81] L.R. Senesac, D. Yi, A. Greve, J.H. Hales, Z.J. Davis, D.M. Nicholson, A. Boisen, and T. Thundat, “Micro-differential thermal analysis detection of adsorbed explosive molecules using microfabricated bridges,” Review of Scientific Instruments, vol. 80, 2009, p. 035102.

170

[82] P.L. Edmiston, D.P. Campbell, D.S. Gottfried, J. Baughman, and M.M. Timmers, “Sensors and Actuators B : Chemical Detection of vapor phase trinitrotoluene in the parts-per-trillion range using waveguide interferometry,” Response, vol. 143, 2010, pp. 574-582.

[83] L.A. Pinnaduwage, A.C. Gehl, and S.L. Allman, “Miniature sensor suitable for electronic nose applications,” Review of Scientific Instruments, vol. 78, 2007, p. 055101.

[84] 2010L.X., with C. Y, Xu P, “Self-assembling siloxane bilayer directly on SiO2 surface of micro-cantilevers for long-term highly repeatable sensing to trace explosives,” Nanotechnology, vol. 21, 2010, p. 26550.

[85] Y. Chen, P.C. Xu, Y. Liu, T.T. Yang, Y.L. Yang, and X.X. Li, “A NOVEL METHOD OF SELF-ASSEMBLING SILOXANE BI-LAYER DIRECTLY ON S I O 2 SURFACE OF MICRO-CANTILEVER FOR LONG-TERM REPEATABLE SENSING TO TRACE EXPLOSIVES State Key Lab of Transducer Technology , Shanghai Institute of Microsystem and Information Technology,” Transducers, 2009, pp. 600-603.

[86] P. Xu, H. Yu, X. Gan, M. Liu, X. Li, and C. Academy, “RESONANT CANTILEVERS WITH ULTRA-DENSELY INWALL- FUNCTIONALIZED MESOPOROUS-SILICA SENSING-LAYER FOR DETECTION OF ppt-LEVEL TNT State Key Lab of Transducer technology , Shanghai Institute of Microsystem and Information ( a ) ( b ) ( c ) ( d ),” 23rd International Conference on MicroElectroMechanical Systems (IEEE MEMS), 2010, pp. 128-131.

[87] “www.microchem.com.”

[88] “http://www.geocities.ws/guerinlj/.”

[89] “http://memscyclopedia.org/su8.html.”

[90] Grégoire GENOLET, “NEW PHOTOPLASTIC FABRICATION TECHNIQUES AND DEVICES BASED ON HIGH ASPECT RATIO PHOTORESIST,” 2001.

[91] S.D. Psoma and D.W.K. Jenkins, “COMPARATIVE ASSESSMENT OF DIFFERENT SACRIFICIAL MATERIALS FOR RELEASING SU-8 STRUCTURES,” Rev.Adv.Mat.Sci, vol. 10, 2005, pp. 149-155.

[92] P.A. Rasmussen and A. Boisen, “Dry release of all-polymer structures,” Microelectronic Engineering, vol. 79, 2005, pp. 88-92.

[93] S. Mouaziz, G. Boero, and R.S. Popovic, “Polymer-Based Cantilevers With Integrated Electrodes,” Journal Of Microelectromechanical Systems, vol. 15, 2006, pp. 890-895.

[94] S.J.H. I, S. Choi, Y. Choi, M. Alien, and G.S. May, “Characterization of low-temperature SU-8 photoresist processing for MEMS applications,” ASMCʼ04, 2004, pp. 404-408.

171

[95] D. Sameoto, S.-h Tsang, I.G. Foulds, S.-w Lee, and M. Parameswaran, “Control of the out-of-plane curvature in SU-8 compliant microstructures by exposure dose and baking times,” Journal of Micromechanics and Microengineering, vol. 17, 2007, pp. 1093-1098.

[96] R. Feng and R.J. Farris, “Influence of processing conditions on the thermal andmechanical properties of SU8 negative photoresist coatings,” J.Micromech.Microeng., vol. 13, 2003, pp. 80-88.

[97] D. Raorane, S.-hyung S. Lim, and A. Majumdar, “Nanomechanical Assay to Investigate the Selectivity of Binding Interactions,” Nano Letters, 2008, pp. 1-7.

[98] V. Panwar, V.K. Sachdev, and R.M. Mehra, “POLYMER Insulator conductor transition in low-density polyethylene – graphite composites,” European Polymer Journal, vol. 43, 2007, pp. 573-585.

[99] E. Station, “Percolation phenomena in polymer / carbon composites,” Journal of material science letters, vol. 7, 1988, pp. 459-462.

[100] T. Blythe and D. Bloor, Electrical Properties of Polymers, Cambridge: Cambridge University Press, .

[101] G.R. Ruschau, S. Yoshikawa, and R.E. Newnham, “Resistivities of conductive composites,” J. Appl. Phys., vol. 72, 1992, pp. 953-959.

[102] H.P. Xu, Y.H. Wu, D.D. Yang, J.R. Wang, and H.Q. Xie, “STUDY ON THEORIES AND INFLUENCE FACTORS OF PTC PROPERTY IN POLYMER-BASED CONDUCTIVE COMPOSITES,” Rev.Adv.Mat.Sci, vol. 27, 2011, pp. 173-183.

[103] J. Thaysen, “Cantilever for biochemical sensing integrated in a microliquid handling system,” 2001.

[104] “http://www.bic.com/literature/lit_theory_01.html.”

[105] “http://www.bic.com/products/particle_sizing/p_PS_BI-200SM.html.”

[106] W.D. Nix, “Elastic and plastic properties of thin films on substrates : nanoindentation techniques,” Materials Science and engineering, vol. A234-236, 1997, pp. 37-44.

[107] W.C. Oliver and G.M. Pharr, “Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology,” J. Mat. Res., vol. 19, 2004, pp. 3-20.

[108] D. Tranchida, S. Piccarolo, J. Loos, A. Alexeev, D. Tranchida, S. Piccarolo, J. Loos, and A. Alexeev, “Mechanical Characterization of Polymers on a Nanometer Scale through Nanoindentation . A Study on Pile-up and Viscoelasticity Mechanical Characterization of Polymers on a Nanometer Scale through Nanoindentation . A Study on Pile-up and Viscoelasticity,” Macromolecules, vol. 40, 2007, pp. 1259-1267.

172

[109] R. Saha and W.D. Nix, “Effects of the substrate on the determination of thin film mechanical properties by nanoindentation,” Acta Materialia, vol. 50, 2002, pp. 23-38.

[110] J.C. Grunlan, D. Rowenhorst, L.F. Francis, and W.W. Gerberich, “Modulus Determination of Polymer Matrix Composites: Comparison of Nanoindentation and Dynamic Mechanical Analysis,” Mat. Res. Soc. Symp, 2001, pp. 1-6.

[111] N. Abdel-Aal, F. El-Tantawy, A. Al-Hajry, and M. Bououdina, “Epoxy Resin / Plasticized Carbon Black Composites . Part II . Correlation Among Network Structure and Mechanical Properties,” Polymer Composites, 2008, pp. 804-808.

[112] F. El-tantawy, K. Kamada, and H. Ohnabe, “Electrical properties and stability of epoxy reinforced carbon black composites,” vol. 57, 2002, pp. 242 - 251.

[113] E.K. Sichel and J.I. Gittleman, “ELECTRICAL PROPERTIES OF CARBON - POLYMER COMPOSITES,” Journal of Electronic Materials, vol. 11, 1982, pp. 699-747.

[114] C. Serre, P. Gorostiza, A. Prez-rodriguez, F. Sanz, and J.R. Morante, “Measurement of micromechanical properties of polysilicon microstructures with an atomic force microscope,” Sensors and Actuators, vol. 67, 1998, pp. 215-219.

[115] A.D. Yalcinkaya, P. Vettiger, and A. Menon, with J. Thaysen, “Polymer-based stress sensor with integrated readout,” J. Phys. D: Appl. Phys., vol. 35, 2002, p. 2698–2703.

[116] F.N. Hooge, T.G.M. Kleinpenning, and L.K.J. Vandamme, “Experimental studies on l/f noise,” Rep. Prog. Phys., vol. 44, 1981.

[117] L.A. Pinnaduwage, V. Boiadjiev, J.E. Hawk, and T. Thundat, “Sensitive detection of plastic explosives with self-assembled monolayer-coated microcantilevers,” Applied Physics Letters, vol. 83, 2008, pp. 1471-1473.

[118] L.A. Pinnaduwage, J.E. Hawk, V. Boiadjiev, D. Yi, and T. Thundat, “Use of Microcantilevers for the Monitoring of Molecular Binding to Self-Assembled Monolayers,” Langmuir, vol. 19, 2008, pp. 7841-7844.

[119] T. Instruments, “http://datasheet.octopart.com/ADS1232-Texas-Instruments-datasheet-152680.pdf.”

[120] G.A.S. John, “Determination of the concentration of explosives in air by isotope dilution analysis,” Forensic Science, vol. 6, 1975.

[121] L.F. Drummy, P.K. Miska, and D.C. Martin, “Plasticity in pentacene thin films,” JOURNAL OF MATERIALS SCIENCE, vol. 39, 2004, pp. 4465 - 4474.

[122] S.-hyun Tark, A. Srivastava, S. Chou, and G. Shekhawat, “Nanomechanoelectronic signal transduction scheme with metal-oxide- semiconductor field-effect transistor-embedded microcantilevers,” APPLIED PHYSICS LETTERS, vol. 94, 2009, p. 104101.

173

[123] D. D.H., “Explanation for the E -dependent mobilities of charge transport in molecularly doped polymers,” Physical Review B, vol. 52, pp. 939-954.

[124] H.A. J, “Semiconducting and metallic polymers : The fourth generation of polymeic metals,” Synthetic metals, vol. 125, pp. 23-42.

[125] S. E.A. and C. V., “Organic molecualr crystals : Their electronic states,” 1980.

[126] S.P. Tiwari, “FABRICATION AND CHARACTERIZATION OF LOW VOLTAGE SOLUTION PROCESSED ORGANIC,” 2008.

[127] I. Kymissis, Organic Field Effect Transistors Theory , Fabrication and, Springer, 2008.

[128] J. Puigdollers, C. Voz, A. Orpella, R. Quidant, I.M. A, M. Vetter, and R. Alcubilla, “Pentacene thin-film transistors with polymeric gate dielectric,” Organic Electronics, vol. 5, 2004, pp. 67-71.

[129] A. Wang, I. Kymissis, V. Bulović, and A.I. Akinwande, “Tunable threshold voltage and flatband voltage in pentacene field effect transistors,” Applied Physics Letters, vol. 89, 2006, p. 112109.

[130] S. Steudel, S. De Vusser, S. De Jonge, D. Janssen, S. Verlaak, J. Genoe, and P. Heremans, “No Title,” Appl. Phys. Lett., vol. 85, 2004, p. 4400–2.

[131] S.P.Tiwari et.al., “Organic FETs with HWCVD silicon nitride as a passivation and gate dielecritc layer,” Thin Solid Films, vol. 516, 2008, p. 770–772.

[132] R. Bijesh, “Study of flicker noise and RTN for characterization of gate dielectrics in MOS systems,” 2010.

[133] Z. Jia, S. Member, I. Meric, and S. Member, “Doping and Illumination Dependence of 1 / f Noise in Pentacene Thin-Film Transistors,” IEEE Electron Device Letters, vol. 31, 2010, pp. 1050-1052.

[134] B.S. Butefisch S, Schoft A, “Three-axes monolithic low-g accelerometer,” Journal Of Microelectromechanical Systems, vol. 9, 2000, p. 551–556.

[135] K Okada, “Tri-axial piezoelectric accelerometer T IX), Stockholm. Sweden , pp.,” Technical Digest, 8th Intl.conf.solid-state sens. and act.(Transducers ’95/Eurosensors, p. 566–569.

[136] S.S. Dong H, Jia Y, Hao Y, “A novel out-of-plane MEMS tunneling accelerometer,” Sens Actuators A Phys, vol. 120, 2005, p. 360–364.

[137] A.J. Roylance LM, “A batch fabricated silicon accelerometer. IEEE Trans Electron Dev 26:1911–1917. doi:10.1109/ T-ED.1979.19795,” 1979.

174

[138] M.G. Ning Y, Loke Y, “Fabrication and characterization of high g- force, silicon piezoresistive accelerometers,” Sens Actuators A Phys., vol. 48, 1995, pp. 55-61.

[139] L.J. Pak JJ, Kabir AE, Neudeck GW, “A bridge-type piezoresistive accelerometer using merged epitaxial lateral overgrowth for thin silicon beam formation,” Sens Actuators A Phys, vol. 56, 1996, p. 267–271.

[140] L.S. Kal S, Das S, Maurya DK, Biswas K, Ravi Sankar A, “CMOS compatible bulk micromachined silicon piezoresistive accelerometer with low off-axis sensitivity,” Microelectron J, vol. 37, 2006, pp. 22-30.

[141] E. Finot, A. Passian, and T. Thundat, “Measurement of Mechanical Properties of Cantilever Shaped,” Sensors, vol. 8, 2008, pp. 3497-3541.

[142] N.M. Duragkar, “Approach towards Cantilever Array for Explosive Detection,” 2011.

[143] P.S. Vaidya, M.Tech Thesis 2009.

175

List of publications

INTERNATIONAL JOURNALS

1. V.Seena, A.Fernandes, P.Pant, S.Mukherji and V.R.Rao “Development of polymer

nanocomposite nicrocantilever sensors: Material characterization, fabrication and

application in explosive detection”, Nanotechnology (IOP), Vol 22, 2011

2. V.Seena, Akash Nigam, Prita Pant, Soumyo Mukherji and V.Ramgopal Rao, “Organic

CantiFET”: A Nanomechanical Polymer Cantilever Sensor with Integrated OFET”,

Accepted to appear in Journal of Microelectromechanical Systems (IEEE/ASME) 2011.

3. V.Seena, Ravishankar S. Dudhe, Harshil N. Raval, Sheetal Patil, Anil Kumar, Soumyo

Mukherji and V. Ramgopal Rao, "Organic Sensor Platforms for Environmental and

Security Applications", Electro-Chemical-Society (ECS) Transactions - Volume 35,

"Tutorials in Nanotechnology:Dielectrics in Nanosystems", April, 2011

4. Seena.V, Avil Fernandes, Soumyo Mukherji, V.Ramgopal Rao, "Photoplastic

Microcantilever Sensor Platform for Explosive Vapor Detection", Accepted to appear in

International Journal of Nanoscience 2011.

5. Seena.V, Anukool Rajorya, Prita Pant, Soumyo Mukherji, V.Ramgopal Rao “Polymer

microcantilever biochemical sensors with integrated polymer composites for electrical

detection”, Solid State Sci. (Elsevier)), v.11, p.1606-1611, 2009

6. Seena.V, Nitin.S.Kale, Sudip Nag, Soumyo Mukherji, V.Ramgopal Rao “Development of

polymeric microcantilever platform technology for biosensing applications”, International

Journal for Micro and Nano systems, 2009, pp.65-70.

7. V.Seena, Prajakta Vaidya, Soumyo Mukherji, Rudra Pratap, V.Ramgopal Rao “A Novel

SU-8 Composite Piezo-resistive MEMS Accelerometer ” , JMEMS Letters (IEEE/ASME)

(To be resubmitted)

INTERNATIONAL CONFERENCES

8. V.Seena, Prasenjit Ray, Rohit V. Pandharipande, Prakash.R.Apte and V. Ramgopal Rao,

“A novel high yield microfabrication process and Porphyrin SAM functionalization for

polymeric piezoresistive microcantilevers for biochemical sensing” Accepted for oral

presentation in MRS Fall meeting 2011.

176

9. V. Seena, Anukool Rajorya, Avil Fernaundus,Karuna, Prita Pant, Soumyo Mukherji, V

Ramgopal Rao, “Fabrication and Characterization of a Novel Polymer Composite

Microcantilever Sensors for Explosive Detection ”, Technical digest 23rd IEEE

International Conference on MEMS (IEEE MEMS 2010), Hong Kong, Jan 2010.

10. V.Seena, Sudip Nag, Sheetal Patil, Soumyo Mukherji, V Ramgopal Rao, "An Ultra-

Sensitive Polymer Composite Microcantilever Platform for Explosive Detection", 7 th

International Workshop on Nanomechanical Cantilever Sensors, May 26-28, 2010

Banff, Canada.

11. V.Ramgopal Rao, V. Seena, Nitin Kale, Abhinav Prasad, Sheetal Patil, Deepika Reddy,

Dilip Agarwal, Sahir Gandhi, Soumyo Mukherji, "An Ultra-Sensitive Polymer Composite

Lab-on-Chip Platform for Disposable Cardiac Diagnostic Applications", 2010 MRS Fall

Meeting, November 29-December 3, 2010 Boston, Massachusetts, USA.

12. V.Seena, Avil Fernandes, Soumyo Mukherji, V.Ramgopal Rao, "Photoplastic

Microcantilever Sensor Platform for Explosive Vapor Detection", International

Conference on Nano Science and Technology, Mumbai, India, February 17-20, 2010.

13. Ravishankar.S.Dudhe, Seena. V, Soumyo Mukherji, Anil Kumar, and Ramgopal Rao,

"Organic Sensors for Explosive Detection", Proceedings of the International Conference on

Computers and Devices for Communication (CODEC’09), December 14- 16, 2009, Hyatt

Regency, Kolkata .

14. Seena.V, Nitin.S.Kale, Anukool Rajoriya, Akash Nigam, Soumyo Mukherji and

V.Ramgopal Rao,"Nano-Electro-Mechanical-systems for Cardiac Diagnostics", 2nd

Bangalore Nano, Dec 11-13, 2008, Bangalore

15. Seena.V, Nitin.S.Kale, Soumyo Mukherji, V.Ramgopal Rao, “Development of polymeric

microcantilevers with novel transduction schemes for biosensing applications”, 5 th

International Workshop on Nanomechanical Cantilever Sensors, May 19 - 21,

2008,Mainz, Germany 2008.

16. Seena.V, Nitin.S.Kale, Sudip Nag, Soumyo Mukherji, V.Ramgopal Rao “Development of

polymeric microcantilever platform technology for biosensing applications”, Proceedings

of the International Conference on Smart materials Structures and Systems, July 24-26

2008, Bangalore, India

17. V. Seena, P. Nageswararao, S.Mukherji, Anil Kumar, V.Ramgopal Rao, "Polymer

microcantilever biochemical sensors with integrated polymer composites for electrical

177

detection", Proceedings of the European Materials Research Society Symposium, E-MRS

spring meeting, Strasbourg, 25-29, May 2008.

BOOK CHAPTER

18. H. N. Raval, R. S. Dudhe, V. Seena, Ramesh R.Navan, Anil Kumar, and V. Ramgopal

Rao, "Solution Processed Polymers: Properties, Fabrication and Applications", book

chapter for the series "Organic Semiconductors: Properties, Fabrication and Applications",

Nova Science Publishers, Inc.2010.