MEMS ATOMIC FORCE MICROSCOPE (AFM)

MEMS ATOMIC FORCE MICROSCOPE (AFM)

BY

Surya Kolluru, B.Tech.,

A Thesis

In

ELECTRICAL ENGINEERING

Submitted to the Graduate faculty

Of Texas Tech University in

Partial Fulfillment of

The requirements of

The degree of

MASTER OF SCIENCE

In

ELECTRICAL ENGINEERING

Approved

Dr. Tim Dallas

Chairperson of the Committee

Dr. Stephen Bayne

Ralph Ferguson

Dean of the Graduate School

December, 2010

Copyright 2010,

Surya Kiran Kolluru

Texas Tech University, Surya Kolluru, December 2010

ii

ACKNOWLEDGMENTS

I am thankful to my advisor Dr. Tim Dallas for giving me the opportunity to

work in the field of Microelectromechanical systems. He has given me the liberty to

work with different projects in the laboratory. Since 2008, MEMS has given me

knowledge to, develop test setups using LabView, and create an automated test setup

for characterization.

My special thanks to Kim Zinsmeyer and Phil Cruzan (TTU Physics). I had

valuable lessons in the mechanical design of construction plates. Without their help

my project would have taken more time to be completed. I am particularly thankful for

their hospitality. I used to walk to their office and get all my doubts clarified.

Ganapathy Sivakumar has been a great mentor for me in lab. His critiques,

suggestions, and clarifications were all perfect. I would also like to thank Ashwin

VijaySai for his support and his valuable suggestions.

My sincere thanks to Dr. Brandon Weeks (TTU HSC), for letting me use the

Commercial AFM, and giving valuable suggestions in the design of the Atomic Force

Microscope. I would also like to thank his students Gengxin Zhang and Li Yao for

teaching me how to use the commercial Atomic Force Microscope.

I would like to thank my other committee member Dr. Stephen Bayne. I sincerely

thank him for his appreciation for my project. His valuable suggestions are vital for my

future publications in this work.


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TABLE OF CONTENTS

ACKNOWLEDGMENTS ........................................................................................... II

ABSTRACT .................................................................................................................. V

LIST OF FIGURES .................................................................................................... VII

I. INTRODUCTION .................................................................................................... 1

1.1 Applications of the Atomic Force Microscope ................................................. 3

1.2 Objective ........................................................................................................... 6

II. THEORETICAL BACKGROUND ....................................................................... 7

2.1 Atomic Force Microscope ................................................................................ 7

2.1.1 AFM Probe ............................................................................................ 8

2.1.2 Optical Lever ....................................................................................... 10

2.1.3 Optical (Beam-Deflection) Cantilever Amplification ......................... 11

2.1.4 Piezoelectric Scanner .......................................................................... 13

2.1.4.1 Hysteresis ..................................................................................... 16

2.1.4.2 Creep ............................................................................................ 16

2.1.4.3 Aging ............................................................................................ 17

2.1.4.4 Bow .............................................................................................. 17

2.1.5 Feedback System ................................................................................. 18

2.2 Imaging Modes .............................................................................................. 19

2.2.1 Contact Mode Imaging ........................................................................ 19

2.2.2 Tapping Mode Imaging ....................................................................... 20

2.2.3 Friction Imaging .................................................................................. 22

2.2.4 Dynamic Force Spectroscopy mode.................................................... 22

III. EXPERIMENTAL SETUP ................................................................................. 24

3.1 Mechanical Design ......................................................................................... 24

3.1.1 Z Assembly ......................................................................................... 27

3.1.2 Cantilever Holder Assembly ............................................................... 28

3.2 MEMS XY stage ............................................................................................ 29


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3.2.1 SUMMIT-V Process ........................................................................... 29

3.2.2 AutoCAD designs for MEMS XY stage ............................................. 32

3.3 Experimental Set-up ....................................................................................... 32

3.4 Op-Amp Design ............................................................................................. 39

3.4.1 Piezo Amplifier ................................................................................... 39

3.4.2 Stability for Op-Amp .......................................................................... 40

3.4.2.1 Feedback Zero Compensation ...................................................... 41

3.4.2.2 Noise Gain Compensation.............................................................42

3.4.2.3 Isolation Resistor Compensation ................................................403

3.5 PCB Design .................................................................................................... 44

3.5.1 Power Supply Bypassing .................................................................... 45

3.5.2 Ground Loops ..................................................................................... 46

3.5.3 Grounding and Shielding .................................................................... 48

3.6 Conclusions .................................................................................................... 50

IV. EXPERIMENTAL RESULTS AND FUTURE WORK ................................... 51

4.1 Calibration of Cantilever Deflection .............................................................. 51

4.2 Adhesion Force Measurements for Hydrophobic Coatings ........................... 54

4.3 Adhesion Force Measurements for Silicon Substrate .................................... 56

4.4 Signal to Noise ratio calculation for MEMS AFM System ........................... 58

4.5 Future Work ................................................................................................... 60

V. CONCLUSION ...................................................................................................... 61

APPENDIX ................................................................................................................ 75

MATLAB CODE FOR THE FFT ANALYSIS OF THE MEMS AFM ................. 75


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ABSTRACT

This thesis presents the design, development, and testing MEMS based Atomic

Force Microscope (AFM). The system is capable of measuring the pull-off force

measurements on different surfaces. The design is capable of producing a vertical

resolution of ~ 2nm.

The MEMS AFM consists of a stand-alone optical head, stepper motor

controlled Z stage assembly incorporated with a MEMS XY stage. The shaft end of

the stepper motor has a worm gear attached, which takes care of the Z movement. A

commercial piezo actuator from Noliacs used. A LabView-based system is used to

control the entire assembly. Custom made piezo amplifiers and transimpedance

amplifiers are developed.

The MEMS-based AFM has significance in finding the adhesion forces of a

surface estimating the surface energy present on the surface. Surface forces in the

order of ~21nN and ~ 11nN are measured for a bare silicon substrate and hydrophobic

coated MEMS XY stage. In addition, the results obtained are compared with the

results of the force measurements performed on a commercial AFM system (Park XE-

100). The signal to noise ratio for the system is calculated and is found to be 40 dB.


vi

LIST OF FIGURES

1. Cantilever geometry: a) Bottom View b) Side View[78]

............................................ 8

2. Schematic of Laser Beam Deflection in AFM. ....................................................... 10

3. Optical Lever as a mechanical amplifier. ................................................................ 12

4. Piezo Scanner .......................................................................................................... 14

5. Tripod Scanner. ....................................................................................................... 15

6. Hysteresis Curves of a Piezo Actuator[83]

. .............................................................. 16

7. Piezo scanner depicting the bow motion. ................................................................ 17

8. Feedback loop of an AFM system. .......................................................................... 18

9. Sample tip forces 84

. ................................................................................................. 21

10. Schematic of the vertical tip movement during the approach and retract

cycles of a force spectroscopy experiment. [85]

. .......................................... 23

11. AFM diagram with optical and mechanical components shown. .......................... 25

12. 3-D side angle View of the MEMS AFM. ............................................................. 26

13. 3-D view of stepper motor controlled Z assembly. ................................................ 27

14. Illustration of the cantilever holder assembly. ....................................................... 28

15. Cross-section of SUMMIT V TM process layers showing

Polysilicon and Sacrifical Oxide Layers in µm. 85

...................................... 31

16. (a)AutoCAD layout of the device with important components

highlighted.(b)SEM of the fabricated device 87

. ......................................... 32


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17. Experimental setup for performing Force-Distance Analysis. ............................... 33

18. Screenshot of Labview VI illustrating various controls of the

stepper motor. .............................................................................................. 35

19. Screenshot of Labview VI illustrating various controls in the

Piezo approach segment. ............................................................................. 35

20. Flow-chart illustrating of the flow of sequence in Labview VI ............................. 37

21. Continuation of Flow-chart illustrating of the flow of

sequence in Labview VI. ............................................................................. 38

22. Screenshot of the equivalent circuit of the Piezo Amplifier

drawn in P-Spice. ........................................................................................ 40

23. Frequency Response for the Feedback Zero

Compensation Technique. 88

........................................... 41

24. Screenshot of the equivalent circuit of the Noise

Compensation technique. ............................................................................ 42

25. Frequency response for the Noise Gain compensation

technique.88

.................................................................................................. 43

26. Screenshot of the equivalent circuit of the piezo amplifier

drawn in P-Spice for isolation resistor compensation. ................................ 44

27. Frequency response for the noise gain compensation

technique.88

.................................................................................................. 44

28. Power supply by-passing[89]

. .................................................................................. 46

29. Illustration of ground loops[89]

............................................................................... 47

30. Circuit implementing star-ground approach. [90]

..................................................... 47

31. Illustration of grounding and shielding by using guard rings. ............................... 48


viii

32. Board layout for transimpedance amplifier............................................................ 49

33. Board layout for the piezo amplifier. ..................................................................... 50

34. Plot of approach and retract curves for contact mode

cantilever and clean mica in air. .................................................................. 52

35. Table indicting the slope of the above linear fit. .................................................... 53

36. Linear fit of the repulsive contact regime. ............................................................. 53


cantilever and hydrophobic coated surface using MEMS

AFM. ........................................................................................................... 54


cantilever and hydrophobic coated surface using PARK

XE100 AFM. ............................................................................................... 55

39. Line scan showing pull-off measurements for multiple trials. ............................... 56

40. Pull-off force measurement for a bare silicon substrate using

the MEMS AFM system. ............................................................................ 57


cantilever and hydrophobic coated surface using Park-

XE100 AFM. ............................................................................................... 58

42. FFT response of the variations of the cantilever at fixed set

point force with the DC component included. ............................................ 59

43. FFT response of the variations of the cantilever at fixed set

point force with the DC component removed. ............................................ 59


1

CHAPTER I

INTRODUCTION

The technology that deals with the science and engineering of materials at the

dimensions of roughly 1 to 100nm (1 billion nm = 1 m) is called nanotechnology1.

Nanotechnology is a multidisciplinary approach that employs a vast and diverse array

of tools and techniques derived from engineering, physics, chemistry, and biology2.

Recent years have witnessed a substantial growth in the field of nanotechnology, with

increasing need for tools for imaging, measuring and manipulating matter at the

microscale and nanoscale3, 4

. Microscopic imaging has a gigantic influence in industry

and scientific research, especially in the semiconductor industry, surface physics and

various engineering disciplines. Because of these growing demands for investigating

micro-scale and nano-scale samples and the ongoing quest for miniaturization, there

are numerous research methods and instruments for profile and surface inspection.

The significance of this research has been recognized through a number of Nobel

prizes.

The demonstration of the phase contrast method and the subsequent invention

of the Phase Contrast microscope by Zernike earned him the Nobel Prize in 1953

5, 6, 7.

Ruska received the Nobel Prize in 1986 “for his fundamental work in electron optics,

and for the design of the first electron microscope”8, 9

. Scanning tunneling

microscopy10, 11, 12

, invented by Binnig- Rohrer in 1982 which earned them the Nobel

Prize in 1988, is one such tool that has revolutionized micro-scale and nano-scale


2

imaging and has had a great impact on the technical community by providing a new

and unique tool to advance fundamental science and technology. However, the basic

drawback that it can only image conducting and semiconducting surfaces led to the

invention (Binnig, Quate, and Gerber) of the Atomic Force Microscope (AFM)

through collaboration between IBM and Stanford University in 198613

. Unlike the

electron microscope, which provides a two-dimensional projection or a two-

dimensional image of a sample, the AFM provides a true three-dimensional surface

profile. Since its invention, the atomic force microscope has become a powerful

instrument to measure the surface topography for nanometer scale science and

technology. The ability of the AFM to image at atomic resolution, and the ability to

image a wide variety of specimens (metal semiconductors, soft biological samples,

conductive and non-conductive materials) under wide variety of conditions (air14,15

,

liquid16, 17, 18

, vacuum19,20,21

) have made the AFM a sought after tool in the industry.

The AFM has generated broad technical appeal to the industry with its many powerful,

yet turnkey features.

The operating principle of AFM is based on the measurement of forces of the

order of 10–13

to 10–4

N between the sharp tip attached to the cantilever and the

sample. The forces acting on the tip causes the deflection of the cantilever. A feedback

mechanism is employed to adjust the tip-to-sample distance to maintain a constant

force between the tip and the sample. The different methods that can measure the

deflection of the cantilever include optical-lever22, 23

; electron tunneling 11, 13,24

,

capacitive sensing25

and piezoresistive26, 27

AFM cantilevers. However, the optical


3

sensor technique exhibits stability and high sensitivity and is simple, robust and small

enough to be integrated into the instrument. The Optical-lever technique proposed by

Meyer and Amer22

in 1988 is most commonly used to measure the deflection of the

probe's cantilever in the AFM. The optical lever operates by reflecting a laser beam

off the back of the cantilever. Angular deflection of the cantilever causes a twofold

larger angular deflection of the laser beam.

1.1 Applications of the Atomic Force Microscope

The remarkable feature of an Atomic Force Microscope (AFM) to “view”

details at the atomic and molecular level has created a niche for itself among

contemporary technologies. New scanning and measurement technologies have

increased the utility of AFM for a broad variety of applications like life science28, 29, 30

,

polymer science31, 32

, electrochemistry33

, biophysics34

, nanotechnology35, 36

and

biotechnology37,38

. These developments have revolutionized AFM in a few short

years, from a laboratory curiosity to one of the most potent, flexible, and widely used

technologies for surface characterization.

The atomic force microscope has the unique capability of imaging biological

samples with molecular resolution and in addition, the forces between single

molecules and the mechanical properties of biological samples can be investigated.

The application of Atomic Force Microscopy(AFM) to biology has provided unique

opportunities to visualize39, 40, 41

and manipulate42, 43, 44

functional biological structures

with nanometer resolution and has enabled researchers to address biological questions

that could not otherwise be investigated. Parot, Dufrene, Hinterdorfer et al.45

reviewed


4

the history of the AFM in biology and the recent progress made using AFM for

studying cells and cellular interactions, probing single molecules and imaging bio-

surfaces at high resolution. Goksu, Vanegas, Blanchette et al.46

reported the structure

and dynamic measurements of the bio-membranes by AFM. He also designed

experiments, which reveal quantitative details of the mechanisms and factors

controlling vesicle rupture, domain shape and size, phase transformations, and some

model biological interactions in the bio-membranes. Kaminskyj et al.47

showed the use

of AFM for probing the ultra structure, chemistry, physical characteristics and motion

of fungal cells. Lu, Li, An et al.48

focused on focus on the recent development of the

manipulation of single DNA molecules based on atomic force microscopy.

The AFM is fast becoming a powerful characterization tool for polymer

science to visualize and image various materials and their respective surface structures

with superior spatial resolution. The AFM can be used for patterning polymers49,50,51,52

in nanoscale, selective removal of polymer thin films53

and material analysis54

.

Schonherr, Tocha and Vancso55

reported how the atomic force microscope can

quantitatively address friction and in particular surface dynamics of the polymers at

the nanoscale. Hua, King and Henderson 56

showed the patterning of PMMA thin

films using a heated atomic force microscope probe for use as a masking layer in area

selective atomic layer deposition process. Nelson, King et al.57

has described the

deposition of continuous metal nanostructures onto glass and silicon using a heated

atomic force microscope cantilever.

The AFM also finds various applications in electrochemistry58

. It can be used


5

to study surface structure, properties, and reactivity of the electrode surface at the

electrochemical interface. Studies may be conducted in situ; imaging surfaces in an

electrochemical solution or ex situ, imaging surfaces before and after the electrode is

exposed to the electrolyte solution. In situ AFM measurement has been used to study

the electrochemical deposition of conducting polymers such as poly (o-

methoxyaniline)59

, polybithiophene60

and polypyrrole61

, but mainly focused on the

polymer morphology changes under different polymerization conditions 62

.

The relentless miniaturization of electronic devices and circuits advances

toward the nanometer length scale has demanded the industry to look for alternative

characterization and failure analysis tools. The remarkable feature AFM to offer

enhanced spatial resolution combined with limitations of the other optical

characterization techniques 63, 64

has allowed the AFM to be used for patterning very

discreet features in the field of micro/nano electronics and real space profiling of

electrostatic potential variations in nanoscale electronic devices. Nakamura 65

et al.

reported that AFM conductive probes can be used to study the field-effect transistor

(FET) characteristics of isolated copper phthalocyanine (CuPc) nano-crystals. Koet

al.66

reported the fabrication of single-channel superconducting flux flow transistor

(SSFT) using AFM lithography.

Data storage requires creating structures with nanometer-sized indentations.

The AFM technique finds applications in lot of probe based storage technologies.

Various probe-based techniques have been brought forward ((Cooper et

al.67

,Vettigeret al.68

, Shin et al69

, Bhushan and Kwak70,71

).Various recording


6

techniques using thermo mechanical (Mamin and Rugar72

,Vettigeret al.68

), phase

change (Gidon et al.73

, Wright et al.74

), magnetic (Ohkubo et al.75

), electrical (Barrett

and Quate76

) and ferroelectric (Franke et al.77

) methods have been demonstrated. The

AFM can be used for high density data storage by writing information as nanometer-

sized indentations into a polymer substrate. IBM has developed a high-density data

storage technology that uses an array of scanning probes (Millepede) for thermo

mechanical recording and playback on an about40 nm thick polymer with a harder Si

substrate (Vettiger et al.68

).

The science, technology, and applications of AFM will continue to be

advanced in the future. The path to commercial success or even laboratory success is

not straightforward. The most significant barriers include the design of high speed

AFM (enhancing the scanning speed of the scanner and increasing the bandwidth of

the feedback loop) that leads to high-throughput and nanometer-resolution imaging.

1.2 Objective

The main objective of this project is to develop a MEMS Atomic Force

Microscope; utilizing the raster scanning feature of the two-axis XY-stage developed

using Sandia‟s SUMMIT V fabrication process. The MEMS Atomic Force

Microscope consists of a standalone optical head, stepper motor controlled Z stage

assembly for bringing the XY stage underneath the cantilever. The feedback for the

AFM, the approach mechanism for the stepper motor and the data acquisition from the

photodetector are all done through National Instruments LabView software.


7

CHAPTER II

THEORETICAL BACKGROUND

In this chapter we discuss the background of Atomic Force Microscope and its various

modes of operation. We shall also see the various sensing mechanisms used for

quantifying the cantilever deflection.

2.1 Atomic Force Microscope

The Atomic Force Microscope is a characterization tool that can be used to

manipulate and image matter at the nanoscale. The principle of AFM is based on an

atomically sharp tip mounted on a soft cantilever spring that scans over the surface.

The sharp tip interacts with the sample surface sensing the local forces between the tip

and the sample. The most characteristic property of the AFM is that the images are

acquired by 'feeling' the sample surface and the sample

Topography can be recorded with good resolution. The main components of AFM are:

1) AFM probe- a sharp tip mounted on a cantilever.

2) Optical lever system that measures cantilever deflections

3) Feedback loop that allows for monitoring the interaction forces between the tip

and the surface.

4) The piezoelectric scanner that moves the tip relative to the sample in a 3D

pattern.

5) The data acquisition system for acquiring the data and converting the raw data

into topography.


8

2.1.1 AFM Probes

In this section the material and geometrical properties of a cantilever used in

Atomic Force Microscopy are explained. Being the most sensitive part of the AFM,

the tip that interacts with the surface plays a crucial role in the system. The most

common probes are micro-fabricated using Silicon and Silicon Nitride. Another

material used in cantilever manufacturing is a polymer called SU-8. IBM invented the

near-UV negative photoresist SU-8 78

. The properties and the dimensions of the probe

determine the sensitivity and the resolution of the system. High flexibility and low

stiffness play a crucial role as cantilever with these characteristics exert lesser

downward forces on the sample, and as a result, cause less contortion and damage to

the sample. The resonant frequency of a cantilever is given by

(2.1)

Where k is the spring constant and mo is the effective mass.

The equation states that cantilever can have high resonant frequency and low spring

constant if it has lower mass. The geometry of the cantilever is shown in Figure 1.

(a) (b)

Figure 1Cantilever geometry: a) Bottom View b) Side View[78]


9

The effective mass for the rectangular prism cantilever can be calculated using

Raleigh‟s method as follows 79

:

(2.2)

Where = Density of the material

w = Width of the cantilever

l = Length of the cantilever

t = Thickness of the cantilever.

The k for a rectangular cantilever is given by

(2.3)

Where E is the Young‟s Modulus of the cantilever.

Substituting the expressions for mo and k yields

(2.4)

The resonance frequency of the cantilever can be maximized by maximizing

E /

and

t / l2 . E and are material dependent, and

t / l2 is a geometric requirement.


10

2.1.2 Optical Lever

The Atomic Force Microscope measures the deflection of the cantilever at each

point of the scanning area to estimate the changes in surface topography. The optical

lever method proposed by Meyer and Amer is the most commonly used technique of

detecting the cantilever deflection22

. A laser beam is reflected off the back end of the

cantilever onto a quadrant photodetector. The quadrant photodetector senses the

changes in intensity from the reflected laser beam due to the bending of the cantilever

caused by the tip-sample interactions. The difference between the photodiode signals

indicates the position of the laser spot, and thus the angular deflection of the

cantilever.

The system works on the principle of optical-lever where a small deflection of

the cantilever is amplified to a larger displacement on the photodetector. The detection

method is shown in Figure 2.

Deflection of

Figure 2 Schematic of Laser Beam Deflection in

AFM.


11

the cantilever can be calculated from the individual photocurrents or voltages of the

quadrant photodetector. If the light moves between the upper and lower

or the left and right pairs of the quadrant photodetector, the voltages can be calculated

as follows:

VVertical= (Vupperleft + Vupperright) - (Vlowerleft + Vlowerright )

VHorizontal= (Vupperleft + Vlowerleft) - (Vupperright + Vlowerright)

When the cantilever is in contact with the surface during the scan, the vertical

deflections are used to measure the surface changes in the vertical direction, and if the

horizontal deflections are considered; a more accurate image of the surface topography

is obtained.

2.1.3 Optical (Beam-Deflection) Cantilever Amplification

The relationship between the cantilever bending and the photodetector voltage

is derived 80

. The schematic of the bending of the cantilever is shown in the figure 3.

Displacement of the tip in x and y direction (x and y), as well as alteration in laser

beam reflection angle are the result of cantilever curvature.


12

The radius of curvature R of a cantilever is a measure of bending is given by

Where E = Young‟s Modulus

ν = Poisson‟s ratio

τ = Thickness of the cantilever

R = Bending radius of the cantilever.

From the above figure, the actual cantilever displacement x is transformed into

displacement d on the photodetector. As all the angles are very small, it can be

assumed the bending angle of the cantilever is equal to half of the angle of the

deflected laser beam, /2. Therefore, the bending angle of the cantilever is given as

(2.8)

Figure 3 Optical Lever as a mechanical amplifier.


13

Where L is the distance between the photodetector and the cantilever.

The actual cantilever deflection

x is calculated from the cantilever length

l and the

bending angle of the cantilever is given by

(

Combining the above two equations, the actual cantilever deflection is given by

The relation between the radius of curvature and the deflection angle is

And after substitution,

2.1.4 Piezoelectric Scanner

The piezoelectric scanners are made from piezoelectric crystals that are used

for precise relative motion between the tip and the sample. The piezoelectric crystals

expand or contract proportionally with an applied voltage. The scanner is constructed

By stacking three independent piezoelectric crystals, each of them being responsible

for movements on the X, Y & Z axes respectively. The piezoelectric scanners produce

good reproducibility for smaller displacements, but for larger displacements

(displacements greater than 70%) the piezo response is non-linear (hysteresis). The

Hysteresis can be corrected by closed loop' feedback systems which independently

monitor the scanner movement and correct its motion.


14

One commonly used design solution is a scanner configuration consisting of a

hollow tube with X & Y axis controlled by two segments of piezoelectric crystal, and

the other segment for moving the tip in Z direction81,82

. Another solution is using a

single piezo cylinder responsible for moving in all the three directions (X, Y & Z

directions) as shown in Figure 4. An X-Y-Z piezo cylinder has five electrodes: four

segment electrodes on the outer surface and one electrode on the inner side of the tube.

There are two types of scanning configurations: Scanned tip AFM where the piezo

scanner is rigidly attached to the probe and is scanned over the sample which remains

stationary and scanned sample AFM where the scanner is attached to the sample and is

moved under the tip. The advantage with the scanned tip AFM is to equip the

system with temperature controlled and liquid controlled stages. The drawback for this

configuration is that the noise level is higher compared to the scanned sample

Figure 4 Piezo Scanner


15

AFM. For the scanned sample AFM, the weight of the sample has to be controlled

because it sits on top of the scanner.

A tripod scanner can be used for three-dimensional movement of the tip as

shown in figure 5. The tripod scanner consists of three independently controlled piezo

transducers. Each transducer is made of rectangular piece of PZT metalized on two

sides. The three transducers are often called X piezo, Y piezo and Z piezo. By

applying voltages on the piezo, the rods deform in X, Y & Z linear directions.

The material properties of piezo ceramics cause errors that influence the successful

imaging of the AFM systems 82

.

Figure 5 Tripod Scanner.


16

2.1.4.1 Hysteresis

The differences in the material properties and dimensions of each

piezoelectric element cause each scanner to respond differently to the applied voltage.

This phenomenon is based on the crystalline polarization effects and molecular effects

within the piezoelectric material. The result of this is the non-linearity of molecular

dipoles in the ceramic materials.

Piezo scanners are sensitive at the end of the travel than at the beginning. This

causes the relationship of displacement vs. force to be nonlinear. Thus, the return path

of the scanner deviates from the extension path. The hysteresis curve for a piezo

Actuator is shown in figure6.

2.1.4.2 Creep

Large changes in X and Y offset cause drift in the displacement of the

piezo actuators. A DC voltage is applied to the piezo to move the requested offset

distance. 95% of the total expansion occurs quickly, but the remaining movement is

Figure 6 Hysteresis Curves of a Piezo

Actuator[83]

.


17

slow. The slow portion of the travel is called piezo creep. This will interfere with the

scanning process and distort the image. One way to get around this is to integrate a

capacitor into the drive electronics, which slowly discharges against the trend, keeping

he piezo more or less stationary.

2.1.4.3 Aging

The sensitivity of the piezoelectric materials decreases exponentially

with time. The scanner often needs to be calibrated in the beginning stages as mostly

the changes occur in the scanners early life.

2.1.4.4 Bow

The piezoelectric scanners that move the probe in Atomic Force

Microscope typically move the probe in a curved motion over the surface. This results

in a “Bow” in the AFM image as shown in figure 7. In the tube scanning instruments,

the XY scanner is accomplished by moving the end of the scanner in X & Y

directions.

Figure 7 Piezo scanner depicting the bow

motion.


18

Translating the Z piezo compensates the Z displacement inherent in swinging an arc.

“Leveling” or “Flattening” the image can compensate the bow in the image.

2.1.5 Feedback System

The main purpose of the feedback system is to regulate the tip-sample distance

by keeping the probe and sample in a fixed relationship while scanning. The constant

contact between the tip and the sample allows for the images to be obtained at lower

forces and without the tip getting damaged.

Typically, a feedback system consists of a three-dimensional piezo crystal that

regulates the movement between the cantilever and the sample, the cantilever and the

photodetector system, and a feedback circuit. The main purpose of the feedback circuit

is to maintain a predefined setpoint that is determined by the instrument gains. The

instrument gains are identified experimentally, the optimal Proportional, Integral, and

Derivative (PID) parameters for each application. Figure 8 shows the schematic of

typical feedback loop in an AFM system.

Figure 8 Feedback loop of an AFM system.


19

Calculating the difference between the photodetector voltage and pre-defined set point

voltage generates the error signal. The error value is sent as an input for the feedback

calculation. The output of the feedback is given as an input to the high voltage piezo

amplifier, which amplifies the input voltage and sends it to the piezo actuator. The

piezo actuator adjusts accordingly and regulates the tip-sample separation. The

feedback calculation is used to keep the error signal closed to zero. The sample

topography is represented by output of the feedback calculation.

2.2. Imaging Modes

Over the years, different modes of AFM operation have been developed. The different

modes are the following:

2.2.1 Contact Mode Imaging

The most common mode of imaging is the constant force mode. In this mode

the tip is brought in contact with the sample surface, and set to scan the sample in XY

raster pattern. A feedback loop maintains a pre-set constant deflection of the cantilever

with respect to the sample surface by moving the Z scanner for each point of X&Y co-

ordinate. The deflection of the cantilever is caused due to the forces between the tip

and the sample. These forces cause the cantilever to bend according to Hooke‟s law.

(

is the deflection if the cantilever

F is the force acting normal to the cantilever

k is the spring constant of the cantilever.


20

The scanning speed is limited due to the poor mechanical response of the piezo tube as

it has a resonance frequency of 1 KHz along the Z-axis and resonant frequency of 200-

300 Hz when actuated along the XY plane.

The advantages of this mode are simple operation and direct relation between

the output and the tip-sample forces. The difficulty of measurements taken by this

mode is that the system is more sensitive to noise such as mechanical vibrations and

electrical noise.

The topographical height at the sample at each given point is obtained by the

change in the Z-axis coordinate. By combining the information on all the three axes in

an image called the „height image‟ the 3D image of the sample is obtained.

2.2.2 Tapping Mode Imaging

Contact mode imaging is not an ideal choice for soft samples due to friction

between the AFM tip and the sample. In tapping mode, the cantilever vibrates at its

resonant frequency under an external electrical excitation with amplitude of a few tens

to hundreds of Angstroms. The AFM tip briefly touches the sample at the bottom of

the swing, producing a decrease in oscillation amplitude while raster scanning the

sample in the X -Y directions.

The relationship between the resonant frequency of the cantilever and

variations in sample topography is as follows:

The resonant frequency of the cantilever varies as the square root of its spring

constant.


21

The spring constant of the cantilever varies with the force gradient experienced

by the cantilever.

The force gradient changes with tip-sample separation.

Thus, the sample topography or changes in the tip-sample spacing are represented

by changes in resonant frequency of the cantilever that can be used to measure

changes in the force gradient. The interacting forces plotted against the tip-sample

distance in the contact and non-contact modes are shown in the figure 9.

Similar to the constant force mode in contact mode, the decrease in oscillation

amplitude is maintained at a constant value by the feedback loop, moving the

piezoelectric element and the height image can be recorded. In addition, the phase

image can also be recorded using the tapping mode. The phase image can be recorded,

when the tip touches the sample at the bottom of the swing, the phase of oscillation is

disturbed, including a phase difference between the tip and the electrical oscillator

driving it.

Figure 9 Sample tip forces 84

.


22

2.2.3 Friction Imaging

This mode of imaging uses torsional deflection of the cantilever. The

cantilever can measure the friction between the tip and the sample. When the tip

touches the sample, friction occurs and it causes the cantilever to twist. The twisting

motion of the cantilever causes the laser beam to move left and right. The quadrant

photodetector can distinguish the resulting left-and-right motion of the reflected laser

beam from the up-and-down motion from topographic variations. The measured lateral

force provides information on the sample properties, as well as the tip-sample

interaction.

2.2.4 Dynamic Force Spectroscopy mode

In this mode, the cantilever is made to touch the sample and retracted back

over a pre-defined distance in the Z direction. The scan parameters are turned off and

the sample is held at a fixed XY position. As the piezo expands and retracts, the Z

movement of the piezo and the deflection signal from the cantilever are recorded in a

force curve. This may then be repeated at different locations to build up a map of the

tip-surface interaction, or can be repeated at the same point to give a full statistical

understanding of the interaction. These measurements are used to measure nanoscale

contacts, atomic bonding, Vander Waals forces, and single molecule stretching and

rupture forces. The schematic diagram in Figure 10 shows the movement of the

cantilever and the tip during the force spectroscopy experiment- towards the sample


23

and away from the sample. The diagram below shows the position of the cantilever at

various positions in the approach and retracts cycle.

The forces experienced by the cantilever as it approaches the cantilever give

information about long-range interactions, such as electrostatic forces. As the

cantilever comes closer to the surface (nanometers or atomic distances), shorter range

forces such as capillary or Van der Waals forces can be measured. An effect known as

“snap-in” occurs when the sum of the forces is attractive and the tip may jump in

contact with the surface. Once the cantilever is in contact with the surface, it is pushed

into the surface with some force, in which the viscoelastic properties of the sample can

be investigated. When the tip is pulled away from the surface, the adhesion forces can

be measured.

Figure 10 Schematic of the vertical tip movement during the

approach and retract cycles of a force spectroscopy

experiment. [85]

.


24

CHAPTER III

EXPERIMENTAL SETUP

In this chapter, we present the experimental setup used in the design of a

MEMS enabled Atomic Force Microscope. The experimental setup consists of four

main components – Mechanical Assembly, Z Assembly, MEMS XY Stage, and the

electronics for the system. Each of these four components has a detailed description in

this chapter. The chapter also focuses on the design, characterization, and

implementation of these components. Finally, an assembly view of the entire setup is

presented along with an algorithm to operate the system.

3.1 Mechanical Design

The mechanical elements of the MEMS enabled Atomic Force Microscope

(AFM) are documented in this design. The elements of the mechanical assembly of the

instrument are illustrated using three-dimensional solid models made in Autodesk

Inventor. First, the design elements of the Z assembly are addressed and then the

accommodation of the optical head is described. Finally the additional design elements

are presented.

The mechanical assembly primarily consists of four rectangular plates joined

together with set-screws as shown in Figures 11 and 12. The laser diode with the XY


25

stage is connected to the top plate of the assembly. An angular mount connected to the

sidewall of the mechanical assembly holds the photodetector. The cantilever holder

assembly is connected to the sidewall of the mechanical assembly.

The Z platform sits on a linear translation XY stage and is connected to the

bottom plate of the assembly. The design offers easy access to the optical head and for

replacing the cantilever.

Figure 11 AFM diagram with optical and mechanical components shown.


26

Figure 12 3-D side angle View of the MEMS AFM.


27

3.1.1 Z Assembly

The Z stage assembly consists of a stepper motor connected to a worm gear

assembly as shown in Figure 13. The worm gear in turn is connected to a non-rotating

spindle micrometer head, which moves the sample platform in Z direction. A piezo

tube is mounted on the sample platform and the MEMS chip is magnetically attached

to the piezo tube. The stepper motor can be operated in 1/2, 1/4, 1/8, 1/16, 1/32, 1/64

stepping modes. 50 rotations of the motor/worm gear yield 0.5mm movement of the

micrometer. The minimum resolution of the stepper motor controlled Z assembly is

3.3 nm. The ASCII based commands for the stepper motor are given through Labview

interface using the RS 232 communication. The coarser motion (mm) of the stepper

motor brings the MEMS-chip near the vicinity of the cantilever tip. The finer

movement (nm) is accomplished by the Piezo tube.

Figure 13 3-D view of stepper motor controlled Z assembly.


28

3.1.2 Cantilever Holder Assembly

The cantilever holder consists of three sub-assemblies: A rectangular mount, Z

translation stage and a cantilever angular mount as shown in Figure 14. The

rectangular mount is attached to the main mechanical assembly using precision

screws. The rectangular mount holds the Z translation stage from Newport

Corporation (MS-500-X) with a sensitivity of 0.5 m. The underside of the cantilever

angular mount is etched to accommodate a magnet. The magnet attracts the cantilever

plate and keeps it in place.

Figure 14 Illustration of the cantilever holder assembly.


29

3.2 MEMS XY stage

Commercially available Scanning Probe Microscopes use piezo scanners for

scanning the sample in 2-axis (X & Y directions). The scan speed is usually limited by

two factors. The first is the fundamental resonance frequency of the scanning element

(usually a piezo element). Secondly, the motion of the piezo element can excite

resonances in the mechanical loop of the scanning probe microscope as a whole. Since

the resolution is very high (10-10

m), any vibration will lower the image quality

significantly.

Microelectromechanical Systems (MEMS) are devices with micrometer-sized

features that perform electronic and mechanical actions. These devices can be

designed to perform SPM-like motion and MEMS SPM devices have been designed

previously by a number of groups. In this project, I have incorporated a bi-directional

Two-Axis Electrostatic MEMS Positioning System, fabricated using the SUMMIT V

fabrication process.

3.2.1 SUMMIT-V Process

SUMMIT- V process is the acronym for Sandia Ultra-planar Multi-level

MEMS fabrication Technology. It is a surface-micromachining process developed by

Sandia National Laboratories featuring four mechanical layers of polysilicon

fabricated above a thin highly doped polysilicon electrical interconnect and ground

plane layer. Figure 15 shows the cross section view of the various structural and

sacrificial layers available in the SUMMiT V process along with their thickness.


30

The process begins with an n-type, (100) silicon wafer on which a 0.63-μm

layer of silicon dioxide (SiO2) is thermally grown. This layer creates electrical

isolation for the MMPOLY0 layer with the wafer. A 0.8-μm thick layer of low-stress

silicon nitride (Si3N4) is then deposited on top of the oxide layer. The silicon nitride is

used as an etch stop during the wet etch step and it can also be used as extra layer for

electrical isolation. This nitride layer acts like an electrical insulator and also as an

etch stop thus protecting the underlying oxide from wet etchants during processing.

Above the nitride layer, deposit 0.3 m doped poly Si (Poly 0, 28Ω/sq), pattern/etch

with MMPOLY0mask, for mechanical anchor, ground or wiring layer.

Above MMPOLY0, the 1st sacrificial oxide layer (SacOx 1) is deposited,

which is a 2 μm thick conformal layer. A pattern/etch with DIMPLE_CUT mask is

used to form dimples (dimple depth controlled using timed etch 1.5 μm deep). The

SacOx1 layer is patterned a 2nd

time with SACOX1_CUT mask to form anchor sites to

the MMPOLY0 layer. Above MMPOLY0, 1 m of doped poly Si is deposited

conformally (Poly 1, 23Ω/sq).

Above MMPOLY1 is sacrificial oxide layer (SACOX2), this is a thin layer

(0.3μm) that separates MMPOLY1 from MMPOLY2.A pattern/etch with SACOX2

mask is used to define a structure in the layer or with PIN_JOINT_CUT to define a

hub for a rotating member. The gear hubs provide a rotational degree of freedom to

mechanical parts (gears) developed in the SUMMiT process.

Above SACOX2, deposit 1.5 μm of doped poly Si (Poly 2, 22Ω/sq). Some

goes directly onto MMPOLY1 being permanently bonded (~ 2.5 μm thick);


31

pattern/etch with MMPOLY2. For the layers above MMPOLY2, chemical mechanical

polishing (CMP) is done to planarize the sacrificial oxide layers.

Above MMPOLY2, deposit ~ 6 m of oxide (TEOS process) (SacOx 3)

followed by CMP to planarize. It is Patterned/etched with DIMPLE3_CUT for

dimples (etch through SacOx3) and SACOX3_CUT for anchors. Approximately, 0.4

m poly Si is deposited to backfill dimple holes.

Above the SACOX3 layer, approximately 2 m of doped poly (Poly 3, 8Ω/sq) is

deposited on SACOX3 layer, and patterned/etched with MMPOLY3. Above

MMPOLY3 layer, the oxide layer (SacOx 4) is deposited & patterned/etched with

SACOX4_CUT and DIMPLE4_CUT. Finally, Deposit ~ 2 m doped poly Si

layer(Poly 4) and pattern/etch with MMPOLY4.

Finally, the release of all the exposed oxides is carried out by a 100:1 HF:HCl

Figure 15 Cross-section of SUMMIT V TM process layers showing

Polysilicon and Sacrifical Oxide Layers in µm. 85


32

wet etch solution. Simple air evaporation or supercritical carbon-dioxide (SCCO)

drying is done to minimize the capillary forces [86].

3.2.2 AutoCAD designs for MEMS XY stage

The figure 16 below shows a MEMS bidirectional Two-Axis Electrostatic

MEMS Positioning System fabricated using Sandia‟s SUMMiT V design process.

3.3 Experimental Set-up

Figure 17 shows the experimental set-up for measuring the adhesion forces of

different surfaces. A computer with LabView is used for integrating all the

communication between the hardware in the set-up.

Figure 16 (a) AutoCAD layout of the device with important components

highlighted.(b) SEM of the fabricated device 87

.


33

A custom developed optical characterization system is used to observe the laser light

on the cantilever and the positioning of the sample under the cantilever. The optical

set-up is comprised of a Basler – A601f c-mount camera with 100x optical lens

system. National Instruments data acquisition board (USB-6281) is used in the setup.

USB-6281 is used to generate control signals to the stepper motor driver circuit, piezo

Figure 17 Experimental setup for performing Force-Distance Analysis.


34

amplifier and to acquire the analog voltage readout from the transimpedance amplifier.

The acquired analog voltage signal is fed to the piezo actuator.

Figure 18 shows the front panel of the LabView Virtual Instrument (VI)

program. This VI is comprised of several segments. In the STEPPER MOTOR

segment, the VI performs the movement of the sample in the Z direction. Another

important feature of the VI is the Step Mode control for the user. The user has the

liberty to choose the step mode from among the microstepping mode options

available. Based on the relative position of the sample with respect to the cantilever,

the user can use any mode based on his/ her needs on the approach mechanism.

Figure 19 shows the front panel of the LabView Virtual Instrument (VI)

program. This VI includes the XY graph for photodiode signal in a quadrant format

replicating the movement of the laser on the photodiode. The other controls include

the SUM signal indicating the laser intensity on the photodiode and the Vpiezo which

indicates the voltage being applied to the piezo tube. The Vpiezo and the

DIFFERENCE signal of the photodiode are written into an excel file for plotting the

data.


35

The sequence of steps for performing force-distance analysis is given in Figure 20.

Initially, we set the set point voltage to the desired value and the maximum

Figure 18 Screenshot of Labview VI illustrating various controls of the

stepper motor.

Figure 19 Screenshot of Labview VI illustrating various controls in the

Piezo approach segment.


36

displacement in X direction (Xmax) and maximum displacement in Y direction (Ymax)

values respectively, for the XY stage.

We move the sample towards the cantilever by selecting the coarse approach

mechanism using the stepper motor. Then the control checks if the sample is close

enough to the cantilever or not. If the sample is not close enough, we continue with the

coarse approach, else the user can switch over to the fine approach mode using the

piezo. In the fine approach mode, the piezo displaces by 5 nm and every time the

piezo is displaced, the user can monitor if the Va-b signal on the photodiode is equal to

the set point or not. If the condition is not met the piezo voltage is incremented. If the

condition is met, then the piezo voltage is decremented. Once the decremental voltage

reaches the minimum condition of the piezo, then the stage is displaced by 0.2m.

Then the whole process is repeated again for each value of X & Y until X = Xmax and

Y = Ymax. The data is collected and the force-vs -distance curves are plotted.


37

Figure 20 Flow-chart illustrating of the flow of sequence in Labview VI.


38

Figure 21 Continuation of Flow-chart illustrating of the flow

of sequence in Labview VI.


39

3.4 Op-Amp Design

In this section the design considerations of the piezo amplifier and photodiode

amplifier are discussed.

3.4.1 Piezo Amplifier

A piezo amplifier with a peak voltage rating of 200 V and a peak current of

500 mA was designed. The Apex precision power‟s PB50 and Linear Technologies

LT1007 operational amplifiers are used in this design. Both the amplifiers are

cascaded to form a composite amplifier configuration. The PB50 and LT 1007 are

used as booster amplifier and driver amplifier, respectively. The main advantage with

the power booster included within the feedback loop of the driver amplifier results in

a composite amplifier with the accuracy of the driver and the extended output voltage

range and current capability of the booster.

The design of composite amplifier requires careful consideration of several

amplifier and system parameters. The most important of these are gain, stability, slew

rate, and output swing of the driver. Operating the booster amplifier in higher gain

results in a higher slew rate and lower output swing requirement for the driver, but

makes stability more difficult to achieve.


40

3.4.2 Stability for Op-Amp

The stability of the op-amp depends on the phase relationship between the

input and output for the amplifier. If the phase crosses -180° at a frequency where the

gain of the amplifier is less than unity, the amplifier is “unconditionally stable”. To

compensate the amplifier, we add some additional components that shape its

frequency response so that its gain is less than unity by the time the phase hits -

180°.This does, however, compromise the high-frequency response of the amplifier.

Most of the op-amps are internally compensated. However, some op-amps require

external components to make sure that the op-amp remains stable. In this section we

will discuss the various compensation techniques used in designing the Piezo amplifier

to make it stable.

Figure 22 Screenshot of the equivalent circuit of the Piezo Amplifier drawn

in P-Spice.


41

3.4.2.1 Feedback Zero Compensation

The goal of this compensation technique is to alter the loop gain (1/)

by adding a feedback capacitor CF, which places a pole in the loop so that it intersects

the open loop gain (AOL) line at 20db per decade instead of at 40dB per decade as

shown in Figure 23.

Figure 23 Frequency Response for the Feedback Zero

Compensation Technique. 88


42

3.4.2.2 Noise Gain Compensation

The objective of this technique is to close the loop gain early by adding

a parallel ZI path through RI and CI, as shown in Figure 24. When the capacitive

reactance of CI is large, the loop gain (1/ß1) line is governed by RIN. However, when

the Capacitive reactance of CI becomes small, and then RI supplants RIN as the input

resistor ac- counting for the shift to the loop gain (1/ß2) line crosses the open loop

(AOL) line at 20dB per decade as shown in Figure 25.

Figure 24 Screenshot of the equivalent circuit of the Noise Compensation

technique.


43

3.4.2.3 Isolation Resistor Compensation

The loop gain (1/), if uncompensated would intersect with the (Aol)

where the phase shift is 40 db/decade, so that the circuit would become unstable. The

resistor RISO adds a corner frequency or zero in the open loop (AOL) plot, restoring a

20dB per decade slope before the 1/ß line intercepts it as shown in Figure 27. The

usable bandwidth is approximately 3 kHz.

Figure 25 Frequency response for the Noise Gain compensation technique.

88


44

3.5 PCB Design

In a system that measures deflections in the low nanometer range, the design of

the printed circuit board (PCB) poses a challenge. The results in the above discussion

are based on PSPICE simulations and do not take into account on-board parasitic that

Figure 26 Screenshot of the equivalent circuit of the piezo amplifier

drawn in P-Spice for isolation resistor compensation.

Figure 27 Frequency response for the noise gain compensation

technique.88


45

play a vital role in morphing the actual signal. Beside circuit simulations, prototyping

on a copper board or a breadboard is of utmost importance to get a measure of the

parasitic capacitances and noise effects introduces due to circuit components and trace

lines. The following steps were included in the final PCB design to reduce the effect

of parasitic and to reduce unwanted oscillations in the circuit:

3.5.1 Power Supply Bypassing

Bypassing the power supply at the amplifier‟s supply terminals is a critical

aspect of PCB design. Each supply pin must be bypassed to common with a “high

frequency bypass” 0.1uF to 0.22uF ceramic capacitor. These capacitors must be

located directly at the power op amp supply pins. In rare cases where power supply

line inductance is high, it may be necessary to add 1 to 10 ohms of resistance in series

with the high frequency bypass capacitor to dampen the Q of the resultant LC tank

circuit as shown in figure 28.

In addition, a “low frequency bypass” capacitor, minimum value of 10uF per

Ampere of peak output current, should be added in parallel with the high frequency

bypass capacitors from each supply rail to common. Tantalum capacitors should be

used when possible due to their low leakage, low ESR and good thermal

characteristics. Aluminum Electrolytic capacitors are acceptable for operating

temperatures above 0°C. These capacitors should be located within 2" of the power op

amp supply pins.


46

3.5.2 Ground Loops

The star ground is a point that all grounds are referenced to. It is a common

point for load ground, amplifier ground, signal ground and power supply ground. If

the star ground approach is not followed, load current flows through parasitic layout

resistances and wiring as shown in Figure 29. This causes the output signal to be in

phase with the signal at the node to which it is fed back. This will result in positive

feedback and cause oscillations in the circuit. By implementing the star ground

approach as shown in Figure 30, the parasitic resistances in the load current return line

is made to appear as common mode signal to the amplifier.

Figure 28 Power supply by-passing[89]

.


47

Figure 29 Illustration of ground loops[89]

Figure 30 Circuit implementing star-ground

approach. [90]


48

3.5.3 Grounding and Shielding

In any circuit where high impedances are involved, careful shielding is

required to reduce “hum” pickup in input leads. To avoid leakage, utmost care must be

used in planning the board layout. A “guard” pattern should completely surround the

high impedance input leads and should be connected to a low-impedance point, which

is at the signal input potential as shown in Figure 31.

Figure 31 Illustration of grounding and shielding by using guard rings. [90]


49

The board layout for the amplifiers was done using the 4PCB artist software.

The boards include 2 layer boards for the transimpedance amplifier and piezo

amplifier.

Figure 32 Board layout for transimpedance amplifier.


50

3.6 Conclusions

This chapter describes the mechanical and electrical design of the MEMS Atomic

Force Microscope (AFM). The experimental setup consists of four main components -

Mechanical Assembly, Z Assembly, MEMS XY Stage, and the electronics for the

system. Each of the subsystems has been explained in detail. The sequence of steps

involved in the operation of the stepper motor controlled Z assembly and the

photodiode acquisition are explained in detail using flow charts.

Figure 33 Board layout for the piezo amplifier.


51

CHAPTER IV

EXPERIMENTAL RESULTS AND FUTURE WORK

This chapter discusses the results of adhesion force measurements of various samples.

First, the optical lever sensitivity of the MEMS AFM is characterized and explained.

Second, the adhesion force measurements of hydrophobic and silicon-substrate are

reported. In the force measurements for silicon substrate and mica, the sample is

placed on a glass substrate, which is glued to the MEMS packaged chip. The

mechanical XY stage is used to position the sample underneath the cantilever.

After the discussions of various measurements, possible future work for improving the

setup to obtain 3-D topography is explained.

4.1 Calibration of Cantilever Deflection

The conversion of the photodetector signal into a quantitative of force requires

two measurements. The first stage is to calibrate the distance that the cantilever

actually deflects for measured change in photodiode voltage. This value depends on

the dimensions of the cantilever, and the optical path of the AFM detection laser. The

deflection of the cantilever is given by δ; the spring constant of the cantilever can be

used to convert this value into force using the Hooke‟s Law formula:

(

Where

F = Force measured in nN


52

k = Spring constant of the Cantilever

δ = Sensitivity (V/nm) of the Optical Lever

A force curve between a plain cantilever tip and a hard mica substrate is used to

determine the sensitivity of the experimental set-up as shown in Figure 34. The

sensitivity is the measurement of the deflection of the tip in nanometers for a given

moment of detection of laser on the photodiode. The sensitivity is calculated by

taking into account the gradient of the repulsive contact region.

Figure 34 Plot of approach and retract curves for contact mode

cantilever and clean mica in air.


53

The gradient of the repulsive contact region is marked in Figure 34 and the linear fit

for the curve and the slope equations are shown in Figure 35 and Figure 36.

respectively. The calculation in this case gives us a sensitivity of 17mV/nm. This

Figure 35 Linear fit of the repulsive contact regime.

Figure 36 Table indicting the slope of the above linear fit.


54

value can be used to convert the deflection data along the whole curve into

nanometers.

4.2 Adhesion Force Measurements for Hydrophobic Coatings

Adhesion force measurements are performed on a hydrophobic coated MEMS XY

stage as shown in Figure 37.

Figure 37 shows the force-distance plot for a hydrophobic coated MEMS XY stage.

The MEMS XY stage is brought in contact with the cantilever and then retracted back.

The deflection (nm) vs. height (nm) values is stored in an Excel file in LabView. The

deflection values are converted to suitable forces using equation 4.1. The cantilever


cantilever and hydrophobic coated surface using MEMS AFM.


55

used in this experiment is CSG10 which has a spring constant of 0.974 N/m. The true

interaction force value is found by subtracting the baseline from all deflection data.

The least value of the force on the force-distance curve indicates the adhesion force

present on the hydrophobic coated MEMS stage. In order to make sure that the

obtained reading is valid, the force-distance curve analysis is performed on a

commercial AFM system (Park-XE 100) as shown in Figure 38. The force values

obtained from the commercial AFM system is found to be ~ 11nN.

The repeatability of the measurements is performed at multiple locations by moving

the MEMS stage in the X direction in 0.5µm incremental steps as shown in Figure 4.6.

Initially, the MEMS stage is at its rest position. The Z piezo is moved such that the

surface of stage is in contact with the cantilever tip. Force-distance curves are taken

five times and the MEMS stage is displaced by 0.5 µm. The surface of the stage is


cantilever and hydrophobic coated surface using PARK

XE100 AFM.


56

again approached and retracted from the cantilever tip. This whole process is repeated

until the stage displaces by 2.5µm in the X direction as shown in Figure 39.

4.3 Adhesion Force Measurements for Silicon Substrate

The pull-off force measurements for a bare silicon substrate are performed as

shown in Figure 40. For a bare silicon substrate, the adhesion forces are stronger than

the hydrophobic coated MEMS stage. The experiment is performed on a glass

substrate which is glued to the MEMS packaged chip. The positioning of the silicon

substrate under the cantilever is done using the Newport Precision 460p series manual

XY stage. The adhesion values of the silicon substrate is found to be ~ 21 nN, twice

the value of the hydrophobic coated MEMS stage.

Figure 39 Line scan showing pull-off measurements for multiple

trials.


57

Figure 41 shows the pull off force measurements using a Park-XE 100 AFM.

The adhesion force for the silicon substrate is found to be 18nN, which closely

matches with the value, obtained using the MEMS AFM system.

Figure 40 Pull-off force measurement for a bare silicon

substrate using the MEMS AFM system.


58

4.4 Signal to Noise ratio calculation for MEMS AFM System

The signal to noise ratio (S/N) is calculated by touching the cantilever tip to

the mica substrate with a pre-defined set point force. Once the tip makes contact with

the sample with the defined set-point force, the voltage on the Z piezo is held constant

for 2 seconds. During this time, 2000 sample points of the Va-b signal of the

photodiode are collected. The data points are stored in an Excel file and are later

analyzed using the FFT analysis in Matlab. After the sample points are collected, the

piezo is retracted back to zero position.

Figure 41 Plot of approach and retract curves for contact

mode cantilever and hydrophobic coated surface using

Park- XE100 AFM.


59

Figure 42 shows the FFT response of the FFT response of the variations of the

cantilever at fixed set point force with the DC component included. The DC

component gives the signal magnitude and is found to be 262 units.

Figure 42 FFT response of the variations of the cantilever at fixed set

point force with the DC component included.

Figure 43 FFT response of the variations of the cantilever at fixed set

point force with the DC component removed.


60

Figure 43 shows the FFT response of the variations of the cantilever at fixed set point

force with the DC component removed. The DC component is removed by subtracting

the mean of the sample points from each individual point. The maximum noise peak is

found to be 7.8 units. The noise spectrum includes all non-fundamental spectral

components in the Nyquist frequency range (sampling frequency / 2) without the DC

component, the DC component, and the harmonics: The signal to noise ratio is

calculated using the formula:

(4.2)

The SNR for the MEMS AFM systems was found to be 40dB.

The RMS variations of the cantilever deflection are calculated by the square root of

the arithmetic mean (average) of the squares of the original values. The RMS variation

is found to be 2nm.

4.5 Future Work

1. Obtain a 3-D topography by utilizing an electrothermal MEMS stage as the

electrostatic actuator causes the cantilever to snap down due to the potential

difference between the MEMS stage and the cantilever.

2. Pull-off force measurements for other various self-assembled mono layer

coated MEMS devices.

3. Reducing the overall size of the system such that there would be minimum

mechanical vibrations in the system.


61

CHAPTER V

CONCLUSION

This thesis demonstrates the design, development, and operation of MEMS Atomic

Force- Microscope integrated with a bi-directional two-axis MEMS positioning

system. The MEMS AFM consists of a stand-alone optical head, stepper motor

controlled Z assembly. Commercial Piezo actuator is used along with stepper motor

coupled worm gear assembly to provide micro-step motion. National Instruments data

acquisition cards (USB-6281) are utilized in conjunction with LabView to provide

control of the stepper motor. The signals from the photodiode are collected using the

data acquisition card and sent to the Labview Interface.

The pull-off force measurements for different surfaces are demonstrated.

Surface forces in the order of 21 nN and 11 nN are measured for a bare silicon

substrate and hydrophobic coated MEMS stage. The repeatability of the force

measurements is performed to see the variations in the adhesion force values.

The signal to noise ratio for the system is calculated to be 40dB. The RMS variations

of the cantilever deflection are found to be 2 nm. Possible improvements are specified

that can be implemented in the system to obtain 3-D topography.


62

REFERENCES

[1] "Nanotechnology: Big Things from a Tiny World", Nano Technology

Initiative [Online]. Available from http://www.nano.gov/

[2] S. K. Sahoo, S. Parveen, and J. J. Panda, “The present and future of

nanotechnology in human health care”, Nanomedicine-Nanotechnology

Biology and Medicine, vol. 3, no. 1, pp. 20-31, Mar, 2007.

[3] J.A. Dagata, J. Schneir, H.H. Harary, C.J. Evans, M.T. Postek, J.Bennett,

“Modification of hydrogen‐passivated silicon by a scanning tunneling

microscope operating in air”, Appl. Phys. Lett, vol. 56 , pp. 2001-2003. 1990.

[4] E.S. Snow, P.M. Campbell, “Fabrication of Si nanostructures with an atomic

force microscope”, Appl. Phys. Lett. vol. 64, pp. 1932-1934, 1994.

[5] F. Zernike, “Diffraction theory of the knife-edge test and its improved form,

the phase-contrast method”, Royal Astronomy Society Monthly Notices, vol.

94, pp. 377-384, 1934.

[6] F. Zernike, “Phase-contrast, a new method for microscopic observation of

transparent objects. Part I”, Physica, vol. 9, pp. 686-698, 1942.

[7] F. Zernike, “Phase-contrast, a new method for microscopic observation of

transparent objects. Part II”, Physica, vol. 9, pp. 974-986, 1942.

[8] E. Ruska, “Die elektronenmikroskopische Abbildung elektronenbestrahlter

Oberflächen.”, Z Phys, vol. 83, pp. 492-497, 1933.

http://www.nano.gov/


63

[9] E. Ruska, H.O. Muller, “Über Fortschritte bei der Abbildung

elektronenbestrahlter Oberflächen”, Z Phys, vol. 116, pp. 366-369, 1940.

[10] G. Binnig, H. Rohrer, C.H. Gerber, and E. Weibel, “Tunneling through a

controllable vacuum gap”, Appl. Phys. Lett., Vol. 40, Issue 2, pp. 178-180

1982.

[11] G. Binnig, H. Rohrer, C.H. Gerber et al., “Surface Studies by Scanning

Tunneling Microscopy,” Physical Review Letters, vol. 49, no. 1, pp. 57, 1982.

[12] G. Binnig, H. Rohrer, C.H. Gerber, and E. Weibel, “7 × 7 Reconstruction on

Si(111) Resolved in Real Space”, Physical Review Letters, vol. 50, pp. 120 –

123, 1983.

[13] G. Binnig, C.F. Quate, C.H. Gerber, “Atomic Force Microscope”, Physical

Review Letters, vol. 56, pp. 930-933, 1986.

[14] F. J Giessibl, “Advances in Atomic Force Microscopy”, Rev. Mod. Phys. Vol.

75, pp. 949–983. 2003.

[15] T. Schimmel, T. Koch,. J. Kuppers, M. Lux-Steiner, “True atomic resolution

under ambient conditions obtained by atomic force microscopy in the contact

mode”, Appl. Phys. A, vol. 68, pp. 399-402, 1999.

[16] R. Sonenfield, J. Schneir, B. Drake, P.K. Hansma, and D.E. Aspnes,

“Semiconductor topography in aqueous environments: Tunneling microscopy

of chemomechanically polished (001) GaAs.” Appl. Phys. Lett. vol. 50, pp.

1742, 1987.


64

[17] T. Fukuma, T. Ichii, K. Kobayashi, H. Yamad and K. Matsushige, “True-

molecular resolution imaging by frequency modulation atomic force

microscopy in various environments”, Appl. Phys. Lett., vol. 86, pp. 034103,

2005.

[18] T. Ando, N. Kodera, D. Maruyama, E. Takai, K. Saito and A. Toda, “A high-

speed atomic force microscope for studying biological macromolecules in

action”, J. Appl. Phys., vol. 41, pp. 4851-4856, 2002.

[19] J.K Gimzewski and A. Humbert, “Scanning tunneling microscopy of surface

microstructure on rough surfaces”, IBM J.Res. Develop, vol. 30, pp. 472-477,

1987.

[20] N. Oyabu, Y. Sugimoto, M. Abe, O. Custance and S. Morita, “Lateral

manipulation of single atoms at semiconductor surfaces using atomic force

microscopy”, Nanotechnology, vol. 16, pp.S112-S117, 2005.

[21] F.J Giessibl, “Atomic Resolution of the Silicon (111)-(7x7) Surface by Atomic

Force Microscopy”, Science, vol. 267, vol. 68-71, 1995.

[22] G. Meyer and N. M. Amer, “Novel optical approach to atomic force

microscopy”, Appl. Phys. Lett., vol 53, pp. 1045-1047, 1988.

[23] S. Alexander, L. Hellemans, O. Marti, J. Schneir, V. Elings, P. K.Hansma, M.

Longmire, and J. Gurley, “An Atomic Resolution AFM Implemented Using an

Optical Lever.” Appl. Phys. Lett. vol. 65, pp. pp 164, 1989.


65

[24] O. Marti, B.Drake, and P.K. Hansma, “Atomic force microscopy of liquid-

covered surfaces: atomic resolution images”, Appl. Phys. Lett., vol. 51, pp.

484-486, 1987.

[25] R.Erlandsson, G.M. McClelland, C.M Mate, and S.Chiang, J.Vac.Sci. Technol.

A 6, 266 (1988).

[26] Y. Su, A. G. R. Evans, A. Brunnschweiler et al., "Fabrication of improved

piezoresistive silicon cantilever probes for the atomic force microscope."

Sensors And Actuators A-Physical, vol. 60, pp. 163-167, 1996.

[27] J. Otero, M. Puig-Vidal, "Low-noise Instrumentation for the Measurement of

Piezoresistive AFM Cantilever Deflection in Robotic Nanobiocharacterization

Applications." IEEE Instrumentation & Measurement Technology Conference,

pp. 1392-1396, 2008.

[28] Y. F. Dufrene, “Recent progress in the application of atomic force microscopy

imaging and force spectroscopy to microbiology,” Current Opinion in

Microbiology, vol. 6, no. 3, pp. 317-323, 2003.

[29] P. Frederix, P. D. Bosshart, and A. Engel, “Atomic Force Microscopy of

Biological Membranes,” Biophysical Journal, vol. 96, no. 2, pp. 329-338, Jan,

2009.

[30] G. S. Trindade, J. M. C. Vilela, J. M. S. Ferreira et al., “Use of atomic force

microscopy as a diagnostic tool to identify orthopoxvirus,” Journal of

Virological Methods, vol. 141, no. 2, pp. 198-204, 2007.


66

[31] H. Schonherr, E. Tocha, and G. J. Vancso, "Friction and surface dynamics of

polymers on the nanoscale by AFM," STM and AFM Studies On, Topics in

Current Chemistry, pp. 103-156, Berlin: Springer-Verlag Berlin, 2008.

[32] S. Kassavetis, K. Mitsakakis, and S. Logothetidis, "Nanoscale patterning and

deformation of soft matter by scanning probe microscopy." pp. 1456-1460.

[33] N. Knorr, S. Rosselli, T. Miteva et al., “Biased-probe-induced water ion

injection into amorphous polymers investigated by electric force microscopy,”

Journal of Applied Physics, vol. 105, no. 11, pp. 9, Jun, 2009.

[34] L. A. Wade, I. R. Shapiro, C. P. Collier et al., “Selective functionalization of

carbon nanotube tips allowing fabrication of new classes of nanoscale sensing

and manipulation tools”, US 07514214,to California Institute of Technology,

2009.

[35] F. J. Rubio-Sierra, S. Burghardt, A. Kempe et al., "Atomic force microscope

based nanomanipulator for mechanical and optical lithography.", 2004 4th

IEEE conference on Nanotechnology, pp. 468-470. 2004.

[36] L. S. C. Pingree, E. F. Martin, K. R. Shull et al., “Nanoscale impedance

microscopy - A characterization tool for nanoelectronic devices and circuits,”

IEEE Transactions on Nanotechnology, vol. 4, no. 2, pp. 255-259, Mar, 2005.

[37] J. H. Lu, H. K. Li, H. J. An et al., “Nano-manipulation of single DNA

molecules based on atomic force microscopy,” 2005 27th Annual International

Conference of the IEEE Engineering in Medicine and Biology Society, Vol 1-7,

pp. 7478-7481, 2005.


67

[38] K. G. Wang, L. Wang, J. Li et al., “The morphology of DNA solution in an

open fluidic channel studied by non-contact AFM,” Micron, vol. 39, no. 4, pp.

481-485, 2008.

[39] A. Engel, and D. J. Muller, “Observing single biomolecules at work with the

atomic force microscope,” Nature Structural Biology, vol. 7, no. 9, pp. 715-

718, Sep, 2000.

[40] S. Scheuring, J. N. Sturgis, V. Prima et al., “Watching the photosynthetic

apparatus in native membranes,” Proceedings of the National Academy of

Sciences of the United States of America, vol. 101, no. 31, pp. 11293-11297,

Aug, 2004.

[41] B. Maco, B. Fahrenkrog, N.-P. Huang et al., "Nuclear pore complex structure

and plasticity revealed by electron and atomic force microscopy," Methods in

Molecular Biology, Methods in Molecular Biology : 322 X. J. Liu, ed., pp.

273-288: Humana Press Inc, 2006.

[42] M. Rief, J. Pascual, M. Saraste et al., “Single molecule force spectroscopy of

spectrin repeats: Low unfolding forces in helix bundles,” Journal of Molecular

Biology, vol. 286, no. 2, pp. 553-561, Feb, 1999.

[43] H. B. Li, M. Carrion-Vazquez, A. F. Oberhauser et al., "Stretching single

molecules into novel conformations using the atomic force microscope.",

Abstracts of papers of the American Chemical Society pp. 164-PMSE, 2003.

[44] T. Marshall, M. Long, J. W. Piper et al., “Direct observation of catch bonds

involving cell-adhesion molecules,” Nature, vol. 423, no. 6936, pp. 190-193,

May, 2003.


68

[45] P. Parot, Y. F. Dufrene, P. Hinterdorfer et al., "Past, present and future of

atomic force microscopy in life sciences and medicine." Journal of Molecular

Recognition, vol. 20, pp. 418-431. 2007.

[46] E. I. Goksu, J. M. Vanegas, C. D. Blanchette et al., “AFM for structure and

dynamics of biomembranes,” Biochimica Et BiophysicaActa-Biomembranes,

vol. 1788, no. 1, pp. 254-266, Jan, 2009.

[47] S. G. W. Kaminskyj, and T. E. S. Dahms, “High spatial resolution surface

imaging and analysis of fungal cells using SEM and AFM,” Micron, vol. 39,

no. 4, pp. 349-361, 2008.

[48] J. H. Lu, H. K. Li, H. J. An et al., “Nano-manipulation of single DNA

molecules based on atomic force microscopy,” 2005 27th Annual International

Conference of the IEEE Engineering in Medicine and Biology Society, Vols 1-

7, pp. 7478-7481, 2005.

[49] M. Möltau, S. Welheim, J. Mlynek, G. Drausch, U. Steiner, Nature 391,

877(1998).

[50] L.A. Kung, L. Kam, J.S. Hovis, S.G. Boxer, ““Patterning Hybrid Surfaces of

Proteins and Supported Lipid Bilayers”, Langmuir, vol. 16 , pp. 6773-6776,

2000.

[51] X. Cheng, Y. Hong, J. Kanicki, L.J. Guo, “High-resolution organic polymer

light-emitting pixels fabricated by imprinting technique”, J. Vac. Sci.

Technol.,B, vol. 20, pp. 2877-2880, 2002.


69

[52] J.R. Lawrence, P. Andrew, W.L. Barnes, M. Buck, G.A. Tumbull, I.D.W.

Samuel, “Optical properties of a light-emitting polymer directly patterned by

soft lithography”, Appl. Phys. Lett. vol 81, pp. 1955-1957, 2002.

[53] Y. Hua, S. Saxena, C.L. Henderson, W.P. King, “Nanoscale Thermal

Lithography by Local Polymer Decomposition Using a Heated Atomic Force

Microscope Cantilever Tip”, J. Micro/Nanolithogr.MEMS MOEMS, vol. 6 ,

pp. 023012, 2007.

[54] Sergei N. Magonov, Darrell H. Reneker, “Characterization of polymer surfaces

with Atomic Force Microscopy”, Annual Review of Materials Science., Vol.

27, pp 175-222, 1997.

[55] H. Schonherr, E. Tocha, and G. J. Vancso, "Friction and surface dynamics of

polymers on the nanoscale by AFM," STM and AFM Studies On, Topics in

Current Chemistry, pp. 103-156, Berlin: Springer-Verlag Berlin, 2008.

[56] Y. M. Hua, W. P. King, and C. L. Henderson, "Nanopatterning materials using

area selective atomic layer deposition in conjunction with thermochemical

surface modification via heated AFM cantilever probe lithography."

Microelectronic Engineering, vol. 85, pp. 934-936, 2008.

[57] B. A. Nelson and W. P. Kinget al, “Direct deposition of continuous metal

nanostructures by thermal dip-pen nanolithography”, Appl. Phys. Lett88, pp.

033104, 2006.

[58] N. Knorr, S. Rosselli, T. Miteva et al., “Biased-probe-induced water ion

injection into amorphous polymers investigated by electric force microscopy,”

Journal of Applied Physics, vol. 105, no. 11, pp. 9, Jun, 2009.


70

[59] M.F. Suárez, R.G. Compton, “In situ atomic force microscopy study of

polypyrrole synthesis and the volume changes induced by oxidation and

reduction of the polymer.” J. Electroanal. Chem. Vol. 462, pp. 211-221, 1999.

[60] R. Vidu, P. Stroeve, “Improvement of the Thermal Stability of Li-Ion Batteries

by Polymer Coating of LiMn2O4 “, Ind. Eng. Chem. Res. Vol. 43, pp. 3314-

3324, 2004.

[61] M. Skompska, A. Szkurlat, A. Kowal, M. Szklarczyk, “Spectroelectrochemical

and AFM studies of doping-undoping of poly(3-hexylthiophene) films in

propylene carbonate and aqueous solutions of LiClO4", Langmuir, vol. 19, pp.

2318-2324, 2003.

[62] M. Péter, M.A. Hempenius, R.G.H. Lammertink, J.G. Vancso,

“Electrochemical AFM on surface grafted poly(ferrocenylsilanes)”, Macromol.

Symp. Vol. 166, pp. 285-296, 2001.

[63] Z. H. Levine, A. R. Kalukin, S. P. Frigo et al., “Tomographic reconstruction of

an integrated circuit interconnect,” Applied Physics Letters, vol. 74, no. 1, pp.

150-152, Jan, 1999.

[64] Lewis A, Shambrot E, Radko A, Lieberman K, Ezekiel S, Veinger D, et al.

“Failure analysis of integrated circuits beyond the diffraction limit: Contact

mode near-field scanning optical microscopy with integrated resistance,

capacitance, and UV confocal imaging.”, Proceedings of the IEEE, vol. 88(9),

pp.1471-1479; 2000 Sep.


71

[65] Nakamura M, Yanagisawa H, Kuratani S, Iizuka M, Kudo K. Characterization

of organic nano-transistors using a conductive AFM probe. 5th International

Conference on Nano-Molecular Electronics (ICNME2002); Kobe, Japan:

Elsevier Science Sa; 2002. p. 360-4; 2002 Dec.

[66] S. Ko, and S. J. Kim, “Characteristics of a single-channel superconducting flux

flow transistor fabricated by an AFM modification technique,” Physica C-

Superconductivity and Its Applications, vol. 466, no. 1-2, pp. 16-22, Nov,

2007.

[67] E. B. Cooper, S. R. Manalis, H. Fang et al., “Terabit-per-square-inch data

storage with the atomic force microscope,” Applied Physics Letters, vol. 75,

no. 22, pp. 3566-3568, Nov, 1999.

[68] P. Vettiger, J. Brugger, M. Despont et al., "Ultrahigh density, high-data-rate

NEMS-based AFM data storage system." pp. 11-17. 1999.

[69] H. Shin, K. M. Lee, W. K. Moon et al., “An application of polarized domains

in ferroelectric thin films using scanning probe microscope,” IEEE

Transactions on Ultrasonics Ferroelectrics and Frequency Control, vol. 47,

no. 4, pp. 801-807, Jul, 2000.

[70] B. Bhushan, and K. J. Kwak, “Platinum-coated probes sliding at up to 100 mm

s(-1) against coated silicon wafers for AFM probe-based recording

technology,” Nanotechnology, vol. 18, no. 34, pp. 12, Aug, 2007.


72

[71] B. Bhushan, and K. J. Kwak, "Noble metal-coated probes sliding at up to 100

mm s(-1) against PZT films for AFM probe-based ferroelectric recording

technology." (Invited) J. Phys.: Condens. Matter, vol. 20, May 2008.

[72] H. J. Mamin, and D. Rugar, “Thermomechanical Writing with an Atomic

Force Microscope Tip,” Applied Physics Letters, vol. 61, no. 8, pp. 1003-1005,

Aug, 1992.

[73] S. Gidon, O. Lemonnier, B. Rolland et al., “Electrical probe storage using

Joule heating in phase change media,” Applied Physics Letters, vol. 85, no. 26,

pp. 6392-6394, Dec, 2004.

[74] C. D. Wright, M. Armand, and M. M. Aziz, “Terabit-per-square-inch data

storage using phase-change media and scanning electrical nanoprobes,” IEEE

Transactions on Nanotechnology, vol. 5, no. 1, pp. 50-61, Jan, 2006.

[75] T. Ohkubo, J. Kishigami, K. Yanagisawa et al., "Submicron magnetizing and

its detection based on the point magnetic recording concept." IEEE Trans.

Magn. Pp. 5286-5288.

[76] R. C. Barrett, and C. F. Quate, “charge storage in a nitride-oxide-silicon

medium by Scanning Capacitance Microscopy,” Journal of Applied Physics,

vol. 70, no. 5, pp. 2725-2733, Sep, 1991.

[77] K. Franke, J. Besold, W. Haessler et al., “Modification and detection of

domains on ferroelectric pzt films by Scanning Force Microscopy,” Surface

Science, vol. 302, no. 1-2, pp. L283-L288, Jan, 1994.


73

[78] M. Hopcroft, T. Kramer, G. Kim et al “Micromechanical testing of SU-8

cantilevers”, ATEM-03, JSME-MMD, Vol. 28, pp. 735-742, Sep 10-12, 2003.

[79] Mechanical Design, University of Ulm [Online]. Available:

http://wwwex.physik.uniulm.de/lehre/physikalischeelektronik/phys_elektr/nod

e235.html

[80] Bharat Bhushan "Springer Handbook of Nanotechnology. Second Edition", pp.

530-537, 2006.

[81] Hong-Qiang Li. Atomic Force Microscopy Student Module [Online].

Available:

http://www.chembio.uoguelph.ca/educmat/chm729/afm/firstpag.htm

[82] Atomic Force Microscope for Biological Applications, Texas A & M Health

Sciences Center [Online]. Available:

http://medicine.tamhsc.edu/basicsciences/sbtm/afm/principles.php

[83] Displacement of Piezo Actuators, Physik Instrumente [Online]. Available:

http://www.physikinstrumente.com/tutorial/4_20.html

[84] G.Schitter, P.Menold, H.F.Knappet al “High Performance feedback for fast

scanning Atomic Force Microscopes”, Review of Scientific Instruments, vol 72,

number 8, Aug 2001.

[85] Tutorials-Force Spectroscopy, JPK Instruments [Online]. Available:

http://www.jpk.com/introduction-to-force-spectroscopy.437.html

http://wwwex.physik.uniulm.de/lehre/physikalischeelektronik/phys_elektr/node235.html



http://www.chembio.uoguelph.ca/educmat/chm729/afm/firstpag.htm

http://medicine.tamhsc.edu/basicsciences/sbtm/afm/principles.php

http://www.physikinstrumente.com/tutorial/4_20.html

http://www.jpk.com/introduction-to-force-spectroscopy.437.html


74

[86] SUMMiT (Sandia ultra-planar, multi-level MEMS technology), Sandia

National Laboratories [Online]. Available:

http://mems.sandia.gov

[87] J. Allen, Micro Electro Mechanical System Design. Boca Raton: Taylor &

Francis, 2005, pp. 65-95.

[88] G.Sivakumar et al “A Bidirectional Two-Axis Electrostatic MEMS

Positioning System”, Journal of MicroelectroMechnaical Systems, Vol. 19,

No. 3, June 2010.

[89] Application Note Library - Apex Operational Amplifiers [online]. Available:

http://www.cirrus.com/en/pubs/appNote/Apex_AN19U_D.pdf

[90] Quad High Speed Precision Difet® Operational Amplifier (September 2000)

[Online] Available:

http://focus.ti.com/lit/ds/symlink/opa404.pdf

http://mems.sandia.gov/

http://www.cirrus.com/en/pubs/appNote/Apex_AN19U_D.pdf

http://focus.ti.com/lit/ds/symlink/opa404.pdf


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APPENDIX A

MATLAB CODE FOR THE FFT ANALYSIS OF THE MEMS AFM

clc;

clear all;

fp = fopen('sample1.txt','r');

x = fscanf(fp,'%f ');

x1 = x - mean(x);

plot(abs(fft(x(1:1024))), 'Linewidth',4);

withdc = abs(fft(x(1:1024)));

withoutdc = abs(fft(x1(1:1024)));

temp = 0;

fori = 1:512,

temp = temp + (withoutdc(i)* withoutdc(i));

end

noisepower = sqrt(temp);

signalpower = withdc(1);

SNR = 20* log10(signalpower/noisepower);

MEMS ATOMIC FORCE MICROSCOPE (AFM)

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