Micro and Smart Systems

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Micro and Smart Systems


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Micro and Smart SystemsTechnology and Modeling

G.K. ANANTHASURESH

K.J. VINOY

S. GOPALAKRISHNAN

K.N. BHAT

V.K. AATRE

Indian Institute of Science

Bangalore INDIA


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VP AND EXECUTIVE PUBLISHE R Don Fowley

ASSISTANT PUBLISHER Daniel Sayr e

SENIOR EDITORIAL ASSISTANT Katie Singleton

EXECUTI VE MARKETING MANAGER Christopher Ruel

MARKETI NG ASSISTANT Ashley Tomeck

SENIOR PRODUCTION MANAGER Janis Soo

ASSISTANT PRODUCTION EDITOR Elaine S. Chew

EXECUTI VE MEDIA EDITOR Tom Kulesa

MEDIA EDITOR Wendy Ashenberg

MEDIA SPECIALIST Jennif er Mullin

COVER DESIGNER Wendy Lai

COVER I MAGE Sambuddha Khan

T h i s b o o k w a s s e t i n T i m e s b y T h o m s o n D i g i t a l , N o i d a , I n d i a a n d p r i n t e d a n d b o u n d b y C o u r i e r W e s t f o r d , I n c .

T h e c o v e r w a s p r i n t e d b y C o u r i e r W e s t f o r d , I n c .

T h i s b o o k i s p r i n t e d o n a c i d f r e e p a p e r . 1

F o u n d e d i n 1 8 0 7 , J o h n W i l e y & S o n s , I n c . h a s b e e n a v a l u e d s o u r c e o f k n o w l e d g e a n d u n d e r s t a n d i n g f o r m o r e t h a n2 0 0 y e a r s , h e l p i n g p e o p l e a r o u n d t h e w o r l d m e e t t h e i r n e e d s a n d f u l fi l l t h e i r a s p i r a t i o n s . O u r c o m p a n y i s b u i l t o n a

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Dedicated to

The Institute of Smart Structures and Systems (ISSS)without whose initiative and support this book would not have materialized


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cPrefaceIf we trace the history of electronics technology over the last six decades, we see that the

discovery of the transistor and the development of the integrated circuit (IC) are the key

milestones. However, it is miniaturization and the ensuing very-large-scale-integration

(VLSI) technologies that really created the electronics and computer revolutions. It is only

more recently, within the last couple of decades, that the technology of miniaturization has

been extended to mechanical devices and systems; we now have the microelectromechan-

ical system (MEMS) revolution. Complemented by the advances in smart materials, this

has led to highly application-oriented microsystems and smart systems.

A microsystem is a system that integrates, on a chip or in a package, one or more of

many microdevices: sensors, actuators, electronics, computation, communication, control,power generation, chemical processing, biological reactions, etc. It is now clear that the

functionality of such an integrated system will not only be far superior to any other

engineered system that we know at the macroscale but will also be able to achieve things

well beyond what macroscale integrated systems can do. Smart microelectromechanical

systems are collections of microsensors and actuators that can sense their environment and

can respond intelligently to changes in that environment by using microcircuit controls.

Such microsystems include, in addition to the conventional microelectronics, packaging,

integrated antenna structures for command signals, and microelectromechanical structures

for desired sensing and actuating functions.

However, micromachined actuators may not be powerful enough to respond to the

environment. Using macroscale actuators would defeat the purpose of miniaturization,

cost-effective batch-processing, etc. Hence, there is a need to integrate smart material-

based actuators with microsystems. This trend is currently being witnessed as this field

moves beyond microsensors, which have been the main emphasis in microsystems so far.

Microsystems and smart system technologies have immense application potential in

many fields, and in the coming decades, scientists and engineers will be required to design

and develop such systems for a variety of applications. It is essential, then, that graduating

engineers be exposed to the underlying science and technology of microsystems and smart

systems. There are numerous books that cover both microsystems and smart systems

separately and a few that cover both. Many of them are suitable for practicing professionals

or for advanced-level courses. However, they assume certain fundamentals in various topicsof this multidisciplinary field and thus serve the function of a reference book rather than a

primary textbook. Many do not emphasize modeling at the fundamental level necessary to be

useful at the undergraduate level or for self-study by a reader with background in other

disciplines.

This book essentially deals with the basics of microsystem technology and is intended

principally as a textbook at the undergraduate level; it can also be used as a background

book at the postgraduate level. The book provides an introduction to smart materials and

systems. We have tried to present the material without assuming much prior disciplinary

background. The aim of this book is to present adequate modeling details so that readers

can appreciate the analysis involved in microsystems (and to some extent, smart systems),

thereby giving them an in-depth understanding about simulation and design. Therefore, the

book will also be useful to practicing researchers in all branches of science and engineering

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who are interested in applications where they can use this technology. The book presents

adequate details on modeling of microsystems and also addresses their fabrication and

integration. The engineering of practical applications of microsystems provides areas for

multidisciplinary research, already laden with myriad technological issues, and books

presently available do not address many of these aspects in sufficient depth. We believe

that this book gives a unified treatment of the necessary concepts under a single title.Anticipating the need for such a technology, the Institute of Smart Structures and

Systems (ISSS), an organization dedicated to promoting smart materials and micro-

systems, was established. This Institute was instrumental not only in mounting a national

program and triggering R&D activities in this field in India, but also in creating the

required human resources through training courses and workshops. Furthermore, ISSS also

initiated a dialogue with Visvesveraya Technological University (VTU), Belgaum,

Karnataka, a conglomerate of over 170 Karnataka engineering colleges, to introduce

an undergraduate-level course in microsystems and MEMS and to set in motion the

creation of a potential syllabus for this course. The culmination of this dialogue is the

present book. The material for this book has been taken from several advanced workshops

and short courses conducted by the authors over last few years for faculty and students of

VTU. A preliminary version of book was used at VTU colleges, where a course on

microsystems was first introduced in 2009, and very helpful feedback was received from

teachers of this course, who patiently used the draft to teach about 500 students at various

colleges. In a sense this book has been class- and student-tested and is a substantially

enhanced version of the original draft.

This book has ten chapters covering various topics in microsystems and smart systems

including sensors and actuators, microfabrication, modeling, finite-element analysis,

modeling and analysis of coupled systems (of great importance in microsystems),

electronics and control for microsystems, integration and packaging, and scaling effects

in microsystems. The book also includes case studies on a few microsensor systems toillustrate the applications aspects.

In the authors opinion, the material of the book can be covered in a standard

undergraduate one-semester course. The content of Chapters 5 and 6 may be considered

optional. The entire book can be covered in a single-semester postgraduate course or a

two-semester undergraduate course supplemented by a design and case-study oriented

laboratory.

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cAcknowledgmentsAny project like writing a book depends on the help, advice, and consent of a large number

of people. The authors have received such help from many people and it is a pleasure to

acknowledge all of them. The trigger for the book was provided by the initiative taken by

the Institute of Smart Structures and Systems. The authors acknowledge their indebtedness

to ISSS and its presidents, and indeed dedicate the book to ISSS. The authors would like to

specially thank Prof. S. Mohan of Indian Institute of Science and Dr. A.R. Upadhya,

Director, National Aerospace Laboratories, for their support and encouragement.

While ISSS initiated the writing of the book, it was the support and enthusiasm of the

Visvesvaraya Technological University (VTU) that sustained its writing. The authors

gratefully acknowledge this support, especially from the former vice-chancellors, Prof. K.Balaveer Reddy and Prof. H.P. Khincha.

A number of VTU faculty and students who attended the workshops based on the

preliminary versions of the book provided the all-important feedback necessary to finalize

the book. While thanking them all, the authors would like to mention in particular Prof.

Premila Manohar (MSR Institute of Technology, Bangalore) and Prof. K. Venkatesh

(presently at Jain University, Bangalore). In addition, the significant contributions of Prof.

N.G. Kurahatti (presently at East Point College of Engineering and Technology,

Bangalore), who compiled part of the contents of Chapter 3 during the initial stages of

manuscript preparation, are gratefully acknowledged. The writing of the book would not

have been possible without the work put in by several of our post-graduate students.

Contributions of P.V, Aman, Santosh Bhargav, A.V. Harikrishnan, Shyamsananth

Madhavan, Ipe Mathew, Rizuwana Parveen, Pakeeruraju Podugu, and Jayaprakash Reddy,

who collected much of the information presented in Chapter 2, are gratefully appreciated.

Also acknowledged for their help are: Subhajit Banerjee, Varun Bollapragada, Vivek

Jayabalan, Shymasananth Madhavan, Fatih Mert Ozkeskin, Krishna Pavan, Sudhanshu

Shekhar, and Puneet Singh who ran simulations and provided material for Chapter 10.

Assistance given by M. S. Deepika and R. Manoj Kumar in creating some of the

illustrations is also gratefully acknowledged. The authors thank all their students who

read the early manuscripts of this book and provided useful feedback. The credit for the

cover image goes to Sambuddha Khan. The image shows a part of the bulk-micromachined

accelerometer with a mechanical amplifier developed by him as part of his PhDdissertation.

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cA N o t e t o t h e R e a d e rMost chapters include worked-out examples, problems given within the text, and end-of-

the-chapter exercises.

Some chapters also include exploratory questions marked as Your Turn. They urge

the reader to think beyond the scope of the book. They are intended to stimulate the interest

of the reader.

Acronyms and notation used in the book are included in separate lists at the beginning

of the book. Additionally, a glossary of important terms appears at the end of the book.

An Appendix that appears at the end of the book provides supplementary material for

the convenience of the reader.

Typographical oversights, technical mistakes, or any other discrepancies may pleasebe broughtto the attentionof the authors by sendinge-mail to: [email protected].

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cAcronymsmBGA Microball-grid array

mCP Microcontact printing

mTM Microtransfer molding

mTAS Micro-total analysis system

1D One-dimensional

2D Two-dimensional

3D Three-dimensional

ADC or A/D Analog-to-digital converterAFM Atomic force microscopy

APCVD Atmospheric pressure chemical vapor

deposition

ASIC Application-specific integrated circuits

ASIC Application-specific-integrated circuit

BEM Boundary element method

BGA Ball-grid array

BiCMOS Bipolar CMOS

bio-MEMS bio-microelectromechanical systems

BJT Bipolar junction transistor

BSG Borosilicate glass

BST Barium strontium titanate

BW Bandwidth

CMOS Complementary metal-oxide-

semiconductor

CMP Chemicalmechanical planarization

CMRR Common-mode rejection ratio

COC Cyclic olefin copolymer

COF Chip-on-flex

CPD Critical point drying

CRT Cathode ray tube

CTE Coefficient of thermal expansion

CVD Chemical vapor deposition

DAC or D/A Digital-to-analog converter

DFT Discrete Fourier transform

DLC Diamond-like carbon

DLP Digital light processor

DMD Digital Mirror Device

DoD Drop-on-demand

DOF Degree of freedom

DRIE Deep reactive-ion etching

DSP Digital signal processing

EDP Ethylene diamine pyrocatechol

EDM Electrical discharge machining

EEPROM Erasable programmable read-onlymemory

EGS Electronic grade silicon

EMI Electromagnetic interference

ER Electrorheological

ETC Electro-thermal-compliant

ER Electro rheological

FBG Fiber Bragg grating

FCC Face-centered cubic

FCP Few-chip package

FDM Finite difference method

FE Finite element

FEA Finite element analysis

FEM Finite element method

FFT Fast Fourier transform

FPI Fabry Perot interferometer

HDTV High-definition television

HF HydrofluoricHVAC Heat, ventilation, and air-conditioning

I/O Input/output

IBE Ion beam etching

IC Integrated circuit

ICP Intracranial pressure

IF Intermediate frequency

IR Infrared

ISR Interrupt service routine

LCD Liquid crystal display

LED Light-emitting diode

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LIGA Lithographie Galvanoformung

& Abformung

LPCVD Low-pressure chemical vapor deposition

LPF Low-pass filter

LSB Least significant bit

LTCC Low-temperature cofired ceramics

LVDT Linear variable differential transformer

MAP Manifold-absolute pressure

MBE Molecular beam epitaxy

MCM Multichip module

MCM-D Multichip module deposited

MEMS Microelectromechanical systems

MGS Metallurgical grade silicon

Micro-EDM Microelectrical discharge machining

MMF Magneto motive force

MMIC Monolithic microwave integrated circuits

MOCVD Metal-organic chemical vapor

deposition

MOEMS Micro-opto-electromechanical systems

MOS Metal-oxide-semiconductor

MOSFET Metal oxide semiconductor field-effect

transistor

MR Magnetorheological

MS Metal semiconductor

nMOS n-channel MOSFETs

pMOS p-channel MOSFET

ODE Ordinary differential equation

Op-amp Operational amplifier

PCB Printed circuit board

PCR polymerase chain reaction

PDE Partial differential equationPBGA Plastic-ball-grid array

PDMS Polydimethylsiloxane

PECVD Plasma-enhanced chemical vapor

deposition

PFC Piezofiber composite

PHET Photovoltaic electrochemical etch-stop

technique

PID Proportional-integral-derivative

PLC Programmable logic controller

PLL Phase-locked loop

PMMA Polymethyl methacrylate

pMOS p-channel MOSFETs

PMPE Principle of minimum

potential energy

PSG Phosphosilicate glass

PTFE Polytetra-fluoroethylene

PVC Polyvinyl chloride

PVD Physical vapor deposition

PVDF Polyvinylidene fluoride

PVW Principle of virtual work

PZT Lead zirconate titanate

R&D Research and development

RF Radio frequency

RIE Reactive-ion etching

RTA Rapid thermal annealing

SAC Successive-approximation converter

SAW Surface acoustic wave

SCS Single-crystal silicon

SEM Scanning electron microscope

SFB Silicon fusion bonding

SFEM Spectral FEM

SI unit International standard unitSIP System-on-a-chip

SISO Single-inputsingle-output

SMA Shape-memory-alloy

SNR Signal-to-noise ratio

SOI Silicon-on-insulator

SOP System-on-a-package

sPROMs Structurally programmable microfluidic

system

SuMMiT Santia ultra multi-layer microfabricationtechnology

TCR Temperature coefficient of resistivity

UV Ultraviolet

VCO Voltage-controlled oscillator

VED Vacuum electron devices

VLSI Very large-scale integration

VPE Vapor phase epitaxy

WRT Weighted residual technique

XFEM Extended FEM

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cNotat ionThere are 26 letters in the English alphabet and 24 in the Greek alphabet. Using both lower

and upper case, we have 100 symbols to denote various quantities. Traditionally, every

discipline reserves certain symbols for certain quantities. When we mix disciplines, as

happens in interdisciplinary subjects, there are bound to be clashes: the same symbol is

used for different quantities in different disciplines (e.g., Rfor reaction force in mechanics

and resistance in electronics). We have made an effort to minimize the overlap of such

quantities when they are used in the same chapter. As a result, we use nontraditional

symbols for certain quantities. For example, Yis used for Youngs modulus instead ofE

sinceEis used for the magnitude of the electric field, because they both appear in the same

chapter.Occasionally, we also use subscripts to relate a certain symbol to a discipline (e.g.,kth

for thermal conductivity). Boldface symbols are used for vectors (e.g., E for the electric

field vector).

The symbols in the list below are arranged in this order: upper-case English, lower-

case English, upper-case Greek, and lower-case Greek, all in alphabetical order. For each

symbol, boldface symbols appear first and symbols with subscripts or superscripts appear

afterwards. If the same symbol is used in two different disciplines, the descriptions are

separated by OR; if the same symbol is used within the same discipline, or is used.

A Cross-sectional area of a bar or beam ORarea of a parallel-plate capacitor or a proof

mass

B Magnetic flux density vector

A0 Difference mode gain

AC Common mode gain

Bo Bond number

C Capacitance OR a constant

Cox Gate oxide capacitance per unit area

D Coil diameter of a helical spring OR

magnitude of the electric displacement

vector OR diffusion constant

Dn Normal component of the electric

displacement vector

DE Dissipated energy

E Electric field vector

En Normal component of the electric field

ESE Electrostatic energy

ESEc Electrostatic complementary energy

F Force; occasionally, also the transverse force

on a beam or a point force on a body

Fe Electrostatic forceFd Damping force

G Gauge factorH Magnetic field

I Inertia in general or area of moment of

inertia of a beam OR electric current

Ic0 Reverse saturation current

Kd Derivative controller gain

KP Proportional controller gain

KI Integral controller gain

J Polar moment of inertia OR the magnitude

of the electric current density

K Bulk modulus of a material

KB Boltzmann constant

Kn Knudsen number

KE Kinetic energy

L Length or size OR inductance OR

Lagrangian

M Magnetization

M Bending moment in a beam or a column OR

mass of the proof mass

MSEc Magnetostatic coenergy

n Number of turns

NA Acceptor dopant concentrationND Donor dopant concentration

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No

Intrinsic concentration

P Axial force in a bar or a beam or a point force

on a body OR magnitude of an electric

polarization vector

PE Potential energy

Q Electric chargeR Reaction force OR electrical resistance

Re Reynolds number

S Sensitivity

SE Strain energy

SEc Complementary strain energy

T Torque OR temperature

U Velocity vector

V Volume OR vertical shear force OR voltage

W Work

Vth Threshold voltage

Vbi Built in potential

Y Youngs modulus, a material property usually

denoted byain mechanics; we useabecausea

is used for the magnitude of the electric field

and for acceleration

n Unit vector usually normal to a surface and

directed outward

a Acceleration

b Width of a beam or damping coefficient

dl A differential vector tangential to a path at a

pointds A differential vector normal to a surface

dV Differential volume

g Acceleration due to gravity OR gap in parallel-

plate capacitor

g0 Initial gap in parallel-plate capacitor

h Convective heat transfer coefficient

i Electric current

k Spring constant or stiffness in general

kB Boltsmann constant

ke Electrical conductivity

kth Thermal conductivityl Length

p Perimeter OR pressure

q Distributed transverse load OR electric charge

qe

Charge of electron

r Position or distance vector

r Unit vector in the direction of a position or

distance vector

se Strain energy per unit volume

t Surface force on an elastic body (also called

traction)

te Electrostatic force on a conductor

t Time OR thickness OR force (traction) on a

surface

u Displacement in general or onlyx-displacementin 2D or 3D objects

v y-displacement in 2D or 3D objects

w Width OR transverse displacement of a beam or

z-displacement in a 3D object

D Deflection OR an increment in a quantity (if

followed by another symbol)

a Coefficient of thermal expansion OR a constant

of proportionality

xe Electrical susceptibility

d Deflection OR an increment in a quantity (if

followed by another symbol)

2 Normal strain

e Permittivity

e0 Permittivity of free space

er Relative permittivity

h Viscosity

f Twist OR electric potential

g Shear strain OR surface tension

k Torsional spring constant

l Wave length

m Permeabilitymn

electron mobility

m0 Permeability of free space

n Poisson ratio

u Slope of a bent beam

r Radius of curvature of a straight beam that is

bent

re Electrical resistivity

rm Mass density

se Maxwells stress tensor

s Normal stress

t Shear stress or time constantv Frequency of applied stimulus (force, voltage,

etc.)

vn Natural frequency (also called resonance

frequency)

cL Line charge density

cs Surface charge density

cv Volumetric charge density

xvi c Notation


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cContentsPreface viiAcknowledgments ixA Note to the Reader xiAcronyms xiiiNotation xv

c CHAPTER 1

Introduction 11.1. Why Miniaturization? 2

1.2. Microsystems Versus MEMS 4

1.3. Why Microfabrication? 5

1.4. Smart Materials, Structures and Systems 7

1.5. Integrated Microsystems 9

1.5.1. Micromechanical Structures 10

1.5.2. Microsensors 11

1.5.3. Microactuators 12

1.6. Applications of Smart Materials and

Microsystems 131.7. Summary 15

c CHAPTER 2

Micro Sensors, Actuators, Systems andSmart Materials: An Overview 172.1. Silicon Capacitive Accelerometer 18

2.1.1. Overview 18

2.1.2. Advantages of Silicon Capacitive

Accelerometers 19

2.1.3. Typical Applications 192.1.4. An Example Prototype 19

2.1.5. Materials Used 19

2.1.6. Fabrication Process 19

2.1.7. Key Definitions 20

2.1.8. Principle of Operation 21

2.2. Piezoresistive Pressure Sensor 22

2.2.1. Overview 22

2.2.2. Advantages of Piezoresistive Pressure

Sensors 22

2.2.3. Typical Applications 22

2.2.4. An Example Commercial Product 23





2.3. Conductometric Gas Sensor 24

2.3.1. Overview 24


2.3.3. An Example Product Line 25





2.4. Fiber-Optic Sensors 26

2.4.1. Overview 26

2.4.2. Advantages of Fiber-Optic Sensors 27

2.4.3. An Example Prototype 27




2.4.7. Principle of Operation 282.5. Electrostatic Comb-Drive 29

2.5.1. Overview 29






2.6. Magnetic Microrelay 32

2.6.1. Overview 32


2.6.3. Materials Used 332.6.4. Fabrication Process 33



2.7. Microsystems at Radio Frequencies 34

2.7.1. Overview 34

2.7.2. Advantages of RF MEMS 34




2.7.6. Fabrication Process 362.7.7. Key Definitions 36


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2.8. Portable Blood Analyzer 37

2.8.1. Overview 37

2.8.2. Advantages of Portable Blood

Analyzer 39


2.8.4. Fabrication Process 392.8.5. Key Definitions 39


2.9. Piezoelectric Inkjet Print Head 40

2.9.1. Overview 40

2.9.2. An Example Product 40





2.10. Micromirror Array for Video Projection 42

2.10.1. Overview 42

2.10.2. An Example Product 43





2.11. Micro-PCR Systems 45

2.11.1. Overview 45

2.11.2. Advantages of Micro-PCR

Systems 45

2.11.3. Typical Applications 462.11.4. An Example Prototype 46





2.12. Smart Materials and Systems 48

2.12.1. Thermoresponsive Materials 49

2.12.2. Piezoelectic Materials 50

2.12.3. Electrostrictive/Magnetostrictive

Materials 50

2.12.4. Rheological Materials 512.12.5. Electrochromic Materials 51

2.12.6. Biomimetic Materials 51

2.12.7. Smart Gels 51

2.13. Summary 52

c CHAPTER 3

Micromachining Technologies 553.1. Silicon as a Material for Micromachining 56

3.1.1. Crystal Structure of Silicon 563.1.2. Silicon Wafer Preparation 59

3.2. Thin-film Deposition 60

3.2.1. Evaporation 60

3.2.2. Sputtering 61

3.2.3. Chemical Vapor Deposition 62

3.2.4. Epitaxial Growth of Silicon 64

3.2.5. Thermal Oxidation for Silicon

Dioxide 653.3. Lithography 65

3.3.1. Photolithography 66

3.3.2. Lift-Off Technique 68

3.4. Doping the Silicon Wafer: Diffusion and Ion

Implantation of Dopants 69

3.4.1. Doping by Diffusion 70

3.4.2. Doping by Ion Implantation 72

3.5. Etching 75

3.5.1. Isotropic Etching 75

3.5.2. Anisotropic Etching 76

3.5.3. Etch Stops 81

3.6. Dry Etching 82

3.6.1. Dry Etching Based on Physical

Removal (Sputter Etching) 84

3.6.2. Dry Etching Based on Chemical

Reaction (Plasma Etching) 84

3.6.3. Reactive Ion Etching 85

3.6.4. Deep Reactive Ion Etching (DRIE) 87

3.7. Silicon Micromachining 89

3.7.1. Bulk Micromachining 91

3.7.2. Surface Micromachining 923.8. Specialized Materials for Microsystems 97

3.8.1. Polymers 97

3.8.2. Ceramic Materials 98

3.9. Advanced Microfabrication Processes 99

3.9.1. Wafer Bonding Techniques 99

3.9.2. Dissolved Wafer Process 101

3.9.3. Special Microfabrication

Techniques 102

3.10. Summary 106

c CHAPTER 4

Mechanics of Slender Solids inMicrosystems 111

4.1. The Simplest Deformable Element: A

Bar 112

4.2. Transversely Deformable Element: A

Beam 115

4.3. Energy Methods for Elastic Bodies 124

4.4. Examples and Problems 127

4.5. Heterogeneous Layered Beams 1324.6. Bimorph Effect 135

4.7. Residual Stresses and Stress Gradients 136

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4.7.1. Effect of Residual Stress 137

4.7.2. Effect of the Residual Stress

Gradient 140

4.8. Poisson Effect and the Anticlastic Curvature

of Beams 141

4.9. Torsion of Beams and Shear Stresses 1444.10. Dealing with Large Displacements 151

4.11. In-Plane Stresses 153

4.12. Dynamics 159

4.12.1. A Micromachined Gyroscope:

Two-Degree-of-Freedom Dynamic

Model for a Single Mass 160

4.12.2. A Micromechanical Filter:

Two-Degree-of-Freedom Dynamic

Model with Two Masses 166

4.12.3. Dynamics of Continuous Elastic

Systems 171

4.12.4. A Note on the Lumped Modeling of

Inertia and Damping 172

4.13. Summary 173

c CHAPTER 5

The Finite Element Method 1775.1. Need for Numerical Methods for Solution of

Equations 177

5.1.1. Numerical Methods for Solution ofDifferential Equations 178

5.1.2. What is the Finite Element Method?

179

5.2. Variational Principles 182

5.2.1. Work and Complementary Work 182

5.2.2. Strain Energy and Kinetic

Energy 184

5.2.3. Weighted Residual Technique 185

5.2.4. Variational Symbol 190

5.3. Weak Form of the Governing Differential

Equation 1915.4. Finite Element Method 192

5.4.1. Shape Functions 193

5.4.2. Derivation of the Finite Element

Equation 199

5.4.3. Isoparametric Formulation and

Numerical Integration 204

5.4.4. One-Dimensional Isoparametric Rod

Element 204

5.4.5. One-Dimensional Beam Element

Formulation 2075.4.6. Two-Dimensional Plane Isoparametric

Element Formulation 209

5.4.7. Numerical Integration and Gauss

Quadrature 210

5.5. Numerical Examples 212

5.5.1. Example 1: Analysis of a Stepped Bar

(Rod) 212

5.5.2. Example 2: Analysis of a Fixed RodSubjected to Support Movement 214

5.5.3. Example 3: A Spring-Supported Beam

Structure 215

5.6. Finite Element Formulation for

Time-Dependent Problems 217

5.6.1. Mass and Damping Matrix

Formulation 218

5.6.2. Free Vibration Analysis 223

5.6.3. Free Vibration Analysis of a Fixed

Rod 225

5.6.4. Free-Vibration Analysis of Proof-

Mass Accelerometer 229

5.6.5. Forced Vibration Analysis 230

5.6.6. Normal Mode Method 231

5.7. Finite Element Model for Structures with

Piezoelectric Sensors and Actuators 233

5.8. Analysis of a Piezoelectric Bimorph

Cantilever Beam 235

5.8.1. Exact Solution 236

5.8.2. Finite Element Solution 238

5.9. Summary 239

c CHAPTER 6

Modeling of Coupled ElectromechanicalSystems 245

6.1. Electrostatics 246

6.1.1. Multiple Point Charges 247

6.1.2. Electric Potential 248

6.1.3. Electric Field and Potential Due to

Continuous Charge 252

6.1.4. Conductors and Dielectrics 2536.1.5. Gausss Law 254

6.1.6. Charge Distribution on the

Conductors Surfaces 257

6.1.7. Electrostatic Forces on the

Conductors 258

6.2. Coupled Electromechanics: Statics 259

6.2.1. An Alternative Method for Solving the

Coupled Problem 264

6.2.2. Spring-Restrained Parallel-Plate

Capacitor 2676.3. Coupled Electromechanics: Stability and

Pull-In Phenomenon 276

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6.3.1. Computing the Pull-In and Pull-Up

Voltages for Full Models 282

6.4. Coupled Electromechanics: Dynamics 283

6.4.1. Dynamics of the Simplest Lumped

Electromechanical Model 285

6.4.2. Estimating the Lumped Inertia of anElastic System 287

6.4.3. Estimating the Lumped Damping

Coefficient for the In-Plane

Accelerometer 290

6.5. Squeezed Film Effects in

Electromechanics 294

6.6. Electro-Thermal-Mechanics 295

6.6.1. Lumped Modeling of the Coupled

Electro-Thermal-Compliant

Actuators 297

6.6.2. General Modeling of the Coupled ETC

Actuators 304

6.7. Coupled Electromagnet-Elastic Problem 306

6.8. Summary 308

c CHAPTER 7

Electronics Circuits and Control for Microand Smart Systems 313

7.1. Semiconductor Devices 314

7.1.1. The Semiconductor Diode 3147.1.2. The Bipolar Junction Transistor 317

7.1.3. MOSFET 320

7.1.4. CMOS Circuits 323

7.2. Electronics Amplifiers 325

7.2.1. Operational Amplifiers 325

7.2.2. Basic Op-Amp Circuits 327

7.3. Signal Conditioning Circuits 330

7.3.1. Difference Amplifier 331

7.3.2. Instrumentation Amplifier as a

Differential Voltage Amplifier 332

7.3.3. Wheatstone Bridge for Measurementof Change in Resistance 334

7.3.4. Phase-Locked Loop 336

7.3.5. Analog-to-Digital Converter 337

7.4. Practical Signal conditioning Circuits for

Microsystems 341

7.4.1. Differential Charge

Measurement 341

7.4.2. Switched-Capacitor Circuits for

Capacitance Measurement 343

7.4.3. Circuits for Measuring FrequencyShift 343

7.5. Introduction to Control Theory 344

7.5.1. Simplified Mathematical

Description 344

7.5.2. Representation of Control

Systems 345

7.5.3. State-Space Modeling 346

7.5.4. Stability of Control Systems 3517.6. Implementation of Controllers 354

7.6.1. Design Methodology 354

7.6.2. Circuit Implementation 356

7.6.3. Digital Controllers 357

7.7. Summary 360

c CHAPTER 8

Integration of Micro and SmartSystems 363

8.1. Integration of Microsystems andMicroelectronics 364

8.1.1. CMOS First 364

8.1.2. MEMS First 365

8.1.3. Other Approaches of Integration 365

8.2. Microsystems Packaging 366

8.2.1. Objectives of Packaging 366

8.2.2. Special Issues in Microsystem

Packaging 367

8.2.3. Types of Microsystem Packages 369

8.2.4. Packaging Technologies 3708.2.5. Reliability and Key Failure

Mechanisms 374

8.3. Case Studies of Integrated

Microsystems 375

8.3.1. Pressure Sensor 376

8.3.2. Micromachined Accelerometer 388

8.4. Case Study of a Smart Structure in Vibration

Control 401

8.4.1. PZT Transducers 402

8.4.2. Vibrations in Beams 403

8.5. Summary 404

c CHAPTER 9

Scaling Effects in Microsystems 4099.1. Scaling in the Mechanical Domain 410

9.2. Scaling in the Electrostatic Domain 413

9.3. Scaling in the Magnetic Domain 414

9.4. Scaling in the Thermal Domain 415

9.5. Scaling in Diffusion 417

9.6. Scaling in Fluids 4189.7. Scaling Effects in the Optical Domain 420

9.8. Scaling in Biochemical Phenomena 422

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9.9. Scaling in Design and Simulation 423

9.10. Summary 426

c CHAPTER 10

Simulation of Microsystems Using FEA

Software 42910.1. Background 429

10.2. Force-Deflection of a Tapering Helical Spring

Using ABAQUS 430

10.3. Natural Frequencies of an Accelerometer in

ANSYS 432

10.4. Deflection of an Electro-Thermal-Compliant

(ETC) Microactuator in COMSOL

MultiPhysics 436

10.5. Lumped Stiffness Constant of a Comb-Drive

Suspension in NISA 438

10.6. Piezoelectric Bimorph Beam in a

Customized FEA Program 440

10.7. Resonant Micro-Accelerometer in

ABAQUS 442

10.8. Pull-In Voltage of an RF-MEMS Switch in

IntelliSuite 44410.9. A Capacitive Pressure Sensor in

Coventorware 447

10.10. Summary 449

Appendix 451Glossary 459Index 463About the Authors 473

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cCHAPTER 1I n t r o d u c t i o n

L E A R N I N G O B J E C T I V E S

After completing this chapter, you will be able to:

c Get an overview of microsystems and smart systems.c Understand the need for miniaturization.

c Understand the role of microfabrication.

c Learn about smart materials and systems.

c Learn about typical applications of microsystems and smart systems.

M yt ho lo gy a nd f ol k t al es i n a ll c ul tu re s h av e f as ci na t in g s to ri e s i nv ol vi n g m ag i c a nd

m i n i a t u r i z a t i o n . A l i B a b a , i n a s t o r y i n1001 Arabian Nights, h a d t o s a y Open Sesame to

m ak e t he c av e d oo r o pe n b y i ts el f. W e n ow h av e a ut o ma ti c d oo rs i n s up er ma r ke ts t ha t o pe na s y o u m o v e t o w a r d t h e m w i t h o u t e v e n u t t e r i n g a w o r d . J o n a t h a n S w i f t s fi c t i t i o u s B r i t i s h

h e r o , L e m u e l G u l l i v e r , t r a v e l e d t o t h e i s l a n d o f L i l l i p u t a n d w a s a m a z e d a t t h e m i n i a t u r e

w o r l d h e s a w t h e r e . G u l l i v e r w o u l d p r o b a b l y b e e q u a l l y a m a z e d i f h e w e r e w a s h e d a s h o r e

i n t o t h e 2 1st- c en t ur y w or l d b e ca u se w e n o w h a ve f a bu l ou s m i ni a tu r e m a rv e ls u n dr e am t

o f i n S wi ft s t im e . M ag i c a nd m in ia t ur iz at io n a re r ea li t ie s t od ay . A rt hu r C . C la rk e, a

f am ou s s ci en ce fi ct i on w ri te r, o nc e s ai d t ha t a s uf fic i en tl y a dv an ce d t ec hn ol og y i s

i n di s ti n gu i sh a bl e f r om m a gi c . W h at m a ke s t h is m a gi c a r e al i ty ? T h e a n sw e r l i e s i n e x ot i c

a n d s m ar t m a te r ia l s, s e ns o rs a n d a c tu a to r s, c o nt r ol a n d m i ni a t ur i za t io n . S m ar t ne s s a n d

s m a l l n e s s g o h a n d i n h a n d . S m a r t s y s t e m s a r e i n c r e a s i n g l y b e c o m i n g s m a l l e r , l e a d i n g t o a

m a g i c a l r e a l i t y . S m a l l s y s t e m s a r e i n c r e a s i n g l y b e c o m i n g s m a r t e r b y i n t e g r a t i n g s e n s i n g ,

a c t ua t io n , c o mp u ta t io n , c o mm u ni c at i on , p o we r g e ne r at i on , c o nt r ol , a n d m o re . L i ke t h e

I n d i a n m y t h o l o g i c a l i n c a r n a t i o n V a m a n a , d e s c r i b e d a s b e i n g s m a l l a n d s m a r t , y e t a b l e t o

c ov er t he E ar th , s ky , a nd t he w or ld b en ea t h i n t hr ee f oo ts te ps , t he c om bi na t io n o f s ma ll ne ss

a nd s ma rt ne ss h as l im it le ss p os si bi l it ie s. T hi s b oo k i s a bo ut m ic ro sy st em s a nd s ma rt

s ys te ms , n ot a bo ut m ag ic . A s N ob el L au re at e p hy si c is t R ic ha rd F ey nm an n ot ed i n h is

l e c t u r e s , t h e l a w s o f s c i e n c e a s w e k n o w t h e m t o d a y d o n o t p r e c l u d e m i n i a t u r i z a t i o n , a n d

t h e r e i s s u f fi c i e n t r o o m a t t h e b o t t o m [ 1 , 2 ] . I t i s o n l y a q u e s t i o n o f d e v e l o p i n g t h e r e q u i s i t e

t e c h n o l o g i e s a n d p u t t i n g t h e m t o g e t h e r t o m a k e i t a l l h a p p e n . L e t u s b e g i n w i t h t h e n e e d

f o r m i n i a tu r i z at i o n .

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c 1.1 WHY MINIATURIZATION?

A m i c r o s y s t e m i s a s y s t e m t h a t i n t e g r a t e s , o n a c h i p o r i n a p a c k a g e , o n e o r m o r e o f m a n y

t h in g s: s e ns o rs , a c tu a to r s, e l ec t r on i cs , c o mp u ta t io n , c o mm u ni c at i on , c o nt r ol , p o we rg e n e r a t i o n , c h e m i c a l p r o c e s s i n g , b i o l o g i c a l r e a c t i o n s , a n d m o r e . Y o u m a y fi n d i t i n t e r e s t -

i n g t h a t t h i s d e fi n i t i o n d o e s n o t e x p l i c i t l y m e n t i o n s i z e o t h e r t h a n a l l u d i n g t o a c h i p o r a

p a c k a g e d s y s t e m c o n s i s t i n g o f c h i p s a n d o t h e r a c c e s s o r i e s . M i n i a t u r i z a t i o n i s e s s e n t i a l i na c hi e vi n g t h is l e ve l o f i n te g ra t io n o f a d i sp a ra t e a r ra y o f c o mp o ne n ts .

T h e r e i s n o d o u b t t h a t t h e f u n c t i o n a l i t y o f s u c h a n i n t e g r a t e d s y s t e m w i l l n o t o n l y b e

f a r s u p e r i o r t o a n y o t h e r e n g i n e e r e d s y s t e m t h a t w e k n o w a t t h e m a c r o s c a l e b u t w i l l a l s o

a c h i e v e t h i n g s b e y o n d t h o s e a c h i e v a b l e b y m a c r o s y s t e m s . T h i n k o f a b i g s h i p , a n a i r c r a f t ,

o r a p ow er p la n t: t he y a ll s er ve o ne p ri ma r y p ur po se . B ut a c hi p t ha t i nt eg ra te s s ev er al

c o m po n e n t s, a s a l r e a dy m e n t i on e d ,can serve multiple functions. T hi s i s o ne r ea so n f or

m i ni a tu r iz a t io n . T h is d o es n o t i m p l y t h at w e c a nn o t i n te g ra t e m a ny t h in g s a t t h e m a cr o

s ca le ; i t i s a q ue st io n o f e co no my a nd t o s om e e xt en t f un ct io na li ty . M ic ro sy st em s

t e ch n ol o gy , b y f o ll o wi n g t h e l a rg e l y s u cc e ss f ul p a ra d ig m o f m i cr o el e ct r on i cs , r e ma i ns

economical due to the batch production o f m i cr o fa b ri c at i on p r oc e ss e s. Y o u c a n m a kep l e n t y o f t h i n g s o n o n e s i n g l e s i l i c o n w a f e r , a n d t h u s , t h e c o s t p e r i n d i v i d u a l t h i n g c o m e s

d o wn d r as t i ca l ly . T h is i s a n ot h er r e as o n f o r m i ni a tu r iz a t io n . N o th i ng w o ul d b e tt e r i l l us t ra t e

t hi s t ha n a dv an ce s i n c om pu te r t ec hn ol og y. C om pu ti ng s ys te ms o f t od ay a re m uc h

m o re p o we r fu l , h a ve m a ny m o re f e at u re s , a r e f a r l e ss p o we r -c o ns u mi n g a n d, o f c o ur s e,

a re s ig ni fic an tl y c he ap er t ha n t ho se a va il a bl e 2 0 o r 3 0 y ea rs a go . M in i at ur iz at i on a nd

i n te g ra t io n a p pr o ac h es h a ve p l ay e d a s i gn i fic a nt r o le i n a c hi e vi n g t h es e ( s ee F i gu r e 1 . 1) .

C om mo n o bj ec ts i n v ar io us s iz e s ca le s a re c om pa re d i n F ig ur e 1 .2 . I n a l ig ht e r v ei n, a

p o p u l a r a c r o n y m f o r t h e m i c r o s y s t e m s t e c h n o l o g y i s s p e l l e d MDM $ : i t r e a d s m i l l i o n s o f

e ur os a nd m il li on s o f d ol la rs ! T he m ar ke t s ha re o f microelectromechanical systems

( ME MS ) h as e xc ee de d t he m il li on -d ol la r m ar k s om e t im e b ac k a nd i s w el l p oi se d t o

c r o ss t h e b i l l i on - d ol l a r m a r k.E n e r g y h a s a l w a y s b e e n a p r e c i o u s c o m m o d i t y . T o d a y i t i s i n c r e a s i n g l y s o b e c a u s e o f

e v e r - i n c r e a s i n g d e m a n d c o u p l e d w i t h r a p i d l y d e p l e t i n g e n e r g y r e s o u r c e s . W i t h t h e h u m a n

p ro pe ns it y f or e le c tr on ic g ad ge ts , t he r eq ui re m en ts f or b at te ri es a re a ls o o n t he r is e,

t r i g g e r i n g r e s e a r c h e f f o r t s o n h i g h - e n e r g y m i n i a t u r e b a t t e r i e s . A n d t h e s m a l l e r t h e g a d g e t s

( a n d c o n s t i t u e n t d e v i c e s ) , t h elower theenergy requirements, t h u s a d d i n g a f u r t h e r r e a s o n

f o r m i n i at u r i z in g d e v i ce s a n d s y s te m s .

T h er e a r e, o f c o ur s e, m o re t e ch n ic a l r e as on s f o r m i ni a t ur i za t io n .Some phenomena

favor miniaturization. T a k e o p t i c s , f o r e x a m p l e . I f w e h a v e a m i c r o m e c h a n i c a l d e v i c e t h a t

Figure 1.1 M i n i a t u r i z a t i o n o f c o m p u t e r h a r d w a r e t e c h n o l o g y .

2 c 1 Introduction


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c a n m o v e a m i cr o n- s iz e d c o m p on e nt a n d c o n t ro l i t s m o v e me n t t o a f r ac t io n o f a m i cr o n

( t h e r a n g e o f l i g h t w a v e l e n g t h v i s i b l e t o h u m a n s ) , t h i s o p e n s u p n u m e r o u s n e w p o s s i b i l i t -

i e s . T h e r e a r e a l r e a d y c o m m e r c i a l p r o d u c t s t h a t u s e o p t i c a l m i c r o s y s t e m s , a l s o k n o w n a s

m i c r o -o p t o- e l e c tr o m e c ha n i c al s y s t em s ( M OE M S ) .

T h in k o f t h e b i ol o gi c al c e ll s t h at a r e t h e b a si c b u il d in g b l oc k s o f l i vi n g o r g a ni s ms .T h e y a r e t h e w o r k s h o p s w h e r e a m a z i n g m a n u f a c t u r i n g , a s s e m b l i e s a n d d i s a s s e m b l i e s t a k e

p l a c e m o s t e f fi c i e n t l y . T h e s e c e l l s t o o h a v e f e a t u r e s , t h a t i s , s i z e , m o t i o n , a n d f o r c e s , t h a t

Atoms

Molecules

NanostructuresViruses

Smallestmicroelectronic

features

BacteriaBiological cellsDust particles

Diameter of human hairMicrosystem devices

Optical fibers

Packaged ICsPackaged MEMS

Lab-on-a-chip

Plain old machinesHumansAnimalsPlants

Planes, trains, automobiles

1 nm

0.1 m

10 m

1 mm

100 mm

10 nm

1 m

100 m

10 mm

1 m

10 m

Figure 1.2 I l l u s t r a t i o n o f o b j e c t s a t v a r i o u s s i z e s c a l e s .

1.1 Why Miniaturization? b 3


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a re c om pa ra bl e t o t ho se o f m ic ro me ch an ic al s tr uc tu re s. S o, t he re i s a s ub fie ld w it hi n

m i c ro s ys t e ms k n ow n a s b io - m ic r o el e c t ro m ec h a ni c a l s y st e m s ( b i o- M EM S ). F u r th e r -

m o re , t h e re a r e r e a so n s f o r m i n ia t u ri z i ng c h e mi c a l p r oc e s si n g . C o nt r o ll i n g p r o ce s s c o n di -

t io ns o ve r a s ma ll v ol um e i s m uc h e as ie r t ha n o ve r a l ar ge v ol um e. H en ce , t he efficiency of a

chemical reaction is greater i n m i n i a t u r i z e d s y s t e m s . I t i s c l e a r f r o m t h i s p e r s p e c t i v e w h y

l i v in g o r g an i s ms a r e c o m pa r t me n t al i z e d i n t o m i c ro n - si z e d u n i ts t he c e l ls .M in ia tu r iz at io n c an r es ul t i n f as te r d ev i ce s w it h improved thermal management.

E n e r g y a n d m a t e r i a l s r e q u i r e m e n t d u r i n g f a b r i c a t i o n c a n b e r e d u c e d s i g n i fi c a n t l y , t h e r e b yr e s ul t i n g i n cost/performance advantages. A rr ay s o f d ev ic es a re p os si bl e w it hi n a s ma ll

s p ac e . T h is h a s t h e p o te n ti a l f o r i m pr o ve d r e du n da n cy . A n ot h er i m po r ta n t a d va n ta g e o f

m in ia tu ri z at io n i s t he p os si b il it y o f i nt eg ra ti on o f m ec ha ni ca l a nd fl ui di c p ar ts w it h

e l e c tr o n i c s, t h e r eb y s i m p li f y i n g s y s t e m s a n d r e d u c in g p o w e r r e q u i r e m e nt s . M i c r of a b ri -

c a t i o n e m p l o y e d f o r r e a l i z i n g s u c h d e v i c e s h a s improved reproducibility, a n d d e v i c e s t h u s

p r od u ce d h a ve increased selectivity and sensitivity, wider dynamic range, improved

accuracy and reliability.

I n t e g r a t e d m i c r o s y s t e m s a r e a c o l l e c t i o n o f m i c r o s e n s o r s a n d a c t u a t o r s t h a t c a n s e n s e

t he ir e nv i ro nm en t a nd r ea ct t o c ha ng es i n t ha t e nv ir on me nt b y u se o f a m ic ro ci rc ui t c on tr ol .S u c h m i c r o s y st e m s i n c l ud e , i n a d d i ti o n t o t h e c o n ve n t i o na l m i c r o el e c t r on i c s p a c k ag i n g ,

i n t e gr a t e d a n t e nn a s t r uc t u r e s f o r c o m m an d s i g n al s , a n d m i c r oe l e c t ro m e c ha n i c a l c o n fig u -

r a t i o n s f o r d e s i r e d s e n s i n g a n d a c t u a t i n g f u n c t i o n s . T h e s y s t e m m a y a l s o n e e d m i c r o p o w e r

s u pp l y, m i cr o re l ay , a n d m i cr o si g na l p r oc e ss i ng u n it s . S uc h s y st e ms w i th m i cr o co m po -

n e n t s a r e f a s t e r , m o r e r e l i a b l e , m o r e a c c u r a t e , c h e a p e r , a n d c a p a b l e o f i n c o r p o r a t i n g m o r e

c o mp l ex a n d v e rs a ti l e f u nc t i on s t h an s y st e m s u s ed t o da y .

A m i ni a tu r iz e d l o w- p ow e r t r an s ce i ve r i s a n e x ce l le n t e x am p le o f a m i cr o sy s te mt ec hn ol og y. F ig ur e 1 .3 (a ) s ho ws a s im pl ifi ed b lo ck d ia gr am o f a t ra ns ce iv er a nd a

b o a rd - l e ve l i m p le m e n ta t i o n o f t h e s a m e c o n s i s ti n g o f s e v er a l c h i ps t h a t a r e b a s ic a l l y

c o mp o ne n ts w i th h i gh q u al i ty - fa c to r s s uc h a s r a di o f r eq u en c y ( R F) fi l te r s, s ur f ac ea c ou st i c w a ve ( SA W ) i n te r me d ia t e- f re q ue n cy ( I F) fi l te r s, c r ys t al o sc i ll a to rs , a n d

t r a n s i s t o r c i r c u i t s [ 3 ] . A p o s s i b l e s i n g l e - c h i p i m p l e m e n t a t i o n o f t h i s t r a n s c e i v e r u s i n g

m i c r o s y s t e m s t e c h n o l o g y i s s h o w n i n F i g . 1 . 3 ( b ) . T h i s e n a b l e s m i n i a t u r i z a t i o n a s w e l l

a s l o w p o w er c o n su m p ti o n .

c 1.2 MICROSYSTEMS VERSUS MEMS

W e h a ve p r es e nt e d s e ve r al r e as o ns f o r m i ni a tu r iz a ti o n f r om d i ff e re n t p o in t s o f v i ew . L e t u s

n o w e x a m i ne h o w m i cr o sy s te m s t e ch n ol o gy c a me i n to e x is t en c e . I n t h e l a te 1 9 60 s a n d

e a r l y 1 9 7 0 s , n o t l o n g a f t e r t h e e m e r g e n c e o f t h e i n t e g r a t e d c h i p , r e s e a r c h e r s a n d i n v e n t o r si n a c a d e m i a a n d i n d u s t r y b e g a n t o e x p e r i m e n t w i t h m i c r o f a b r i c a t i o n p r o c e s s e s b y m a k i n g

m o v ab l e m e c h an i c a l e l e m e nt s . A c c e l er o m et e r s , m i c r om i r r o rs , g a s -c h r o ma t o g r ap h y i n s t ru -m e n t s , e t c . , w e r e m i n i a t u r i z e d d u r i n g t h i s p e r i o d . S i l i c o n w a s t h e m a t e r i a l o f c h o i c e a t t h a t

t i m e . A s e m i na l p a p e r b y P e t e r se n [ 4 ] s u m m a r i ze s t h e s e d e v e l o pm e n t s. I n c r ea s e d a t t e n t i o n

t o t h e m i cr o sy s te m s fi e ld c a me i n l a te 1 9 80 s w h en a m i cr o ma c hi n ed e l ec t ro s ta t i c m o to r

w as m ad e a t t he U ni ve rs it y o f C al if or ni a, B er ke l ey a nd M as sa ch us et ts I ns ti tu te o f

T e ch n ol o gy , C a mb r id g e. T h is m o vi n g m i cr o me c ha n ic a l e n ti t y f a sc i na t e d e v er y on e a n d

s h o w e d t h e w a y f o r w a r d f o r m a n y o t h e r d e v e l o p m e n t s . T h e a c r o n y m M E M S w a s c o i n e d

d u ri n g t h at p e ri o d. H o we v er , t h is a c ro n ym i s i n ad e qu a te t o da y b e ca u se o f t h e n u me r ou s

d is ci pl i ne s b ey on d j us t m ec ha ni ca l a nd e le ct ri c al t ha t h av e j oi ne d t he l ea gu e. A m or es u it a bl e t e r m i s mi c ro s ys t em s ; h e nc e t h e t i tl e f o r t h is b o ok .

4 c 1 Introduction


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c 1.3 WHY MICROFABRICATION?

M i c r o s y s t e m s t e c h n o l o g y e m e r g e d a s a n e w d i s c i p l i n e b a s e d o n t h e a d v a n c e s i n i n t e g r a t e dc i r c ui t ( I C ) f a b r i c a ti o n p r o c es s e s, b y w h i c h s e n s o rs , a c t u at o r s a n d c o n t ro l f u n ct i o n s w e r e

c o- fa br ic at ed i n s il ic on . T he c on ce pt s a nd f ea si bi l it y o f m or e c om pl e x m ic ro sy st em sd e vi c e s h a ve b e en p r op o se d a n d d e mo n st r at e d f o r a p pl i ca t io n s i n s u ch v a ri e d fi e ld s a s

m i c r o flu i d i cs , a e r os p a c e , a u t o mo b i l e, b i o m ed i c a l , c h e m ic a l a n a l ys i s , w i r el e s s c o m m un i -

c a t io n s, d a ta s t or a ge , a n d o p ti c s.

I n m i c r o s y s t e m s , m i n i a t u r i z a t i o n i s a c h i e v e d b y a f a b r i c a t i o n a p p r o a c h s i m i l a r t o t h a t

f o l l o w e d i n I C s , c o m m o n l y k n o w n a s micromachining. A s i n I C s , m u c h o f t h e p r o c e s s i n g

i s d o ne b y c h em i ca l p r oc e ss i ng r a th e r t h an m e ch a ni c al m o di fi ca t io n s. H e nc e ,machiningh er e d oe s n ot r ef er t o c on ve nt io na l a pp ro ac he s ( su c h a s d ri l li ng , m il li ng , e tc .) u se d i n

r e al i z in g m a cr o me c h an i ca l p a rt s , a l th o ug h t h e o b je c ti v e i s t o r e al i z e s u ch p a rt s . A s w i th

s e m i c o n d u c t o r p r o c e s s i n g i n I C f a b r i c a t i o n , m i c r o m a c h i n i n g h a s b e c o m e t h e f u n d a m e n t a l

t e c hn o lo g y f o r t h e f a br i ca t io n o f m i cr o sy s te m s d e vi c es a n d, i n p a rt i cu l a r, m i ni a tu r iz e ds e ns o rs a n d a c t ua t or s . S i li c on m i cr o ma c hi n i ng , t h e m o st m a tu r e o f t h e m i cr o ma c hi n i ng

(a)

Surfaceacoustic

wave(SAW)

RF filter LNA

VCOVCO

BPF

Mixer Mixer

IF Amp.Basebandprocessing

circuits

IF filter

Off oron-boardantenna

Crystaloscillator

Off-chip

Ceramicor boardlevel MIC

(b)

RF filter LNA

Micromachinedfilters

Micromachinedfilter

Micromachined

antennas

Micromachinedresonator

VCOVCO

BPF

Mixer Mixer

IF Amp.Basebandprocessing

circuits

IF filter

Crystaloscillator

Figure 1.3 I n t e g r a t e d r a d i o f r e q u e n c y t r a n s c e i v e r s : a t y p i c a l a p p l i c a t i o n o f m i c r o s y s t e m s [ 3 ] . ( a) C u r r e n t a p p r o a c h e s

( s h a d e d r e m a r k s i n d i c a t e c o m p o n e n t s o u t s i d e t h e e l e c t r o n i c s d i e ) . (b) I n t e g r a t e d M E M S - b a s e d R F r e c e i v e r o n a s i n g l e

c h i p . L N A , l o w n o i s e a m p l i fi e r ; R F , r a d i o f r e q u e n c y ; V C O , v o l t a g e - c o n t r o l l e d o s c i l l a t o r ; I F , i n t e r m e d i a t e f r e q u e n c y ;

B P F , b a n d p a s s fi l t e r .

1.3 Why Microfabrication? b 5


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t e ch n ol o gi e s, a l lo ws t h e f a br i ca t io n o f m i cr o sy s te m s t h at h a ve d i me n si o ns i n t h e s u b-

m i ll i me t e r t o m i cr o n r a ng e . I t r e fe r s t o f a sh i on i ng m i cr o sc o pi c m e ch a ni c al p a rt s o u t o f a

s il ic on s ub st ra t e o r o n a s il i co n s ub st ra te t o m ak e t hr ee -d im en si on al ( 3D ) s tr uc tu re s,

t h er e by c r ea t in g a n e w p a ra d i gm i n t h e d e si g n o f m i ni a tu r iz e d s y st e m s. B y e m pl o yi n g

m a te r ia l s s u ch a s c r ys t al l in e s i li c on , p o ly c ry s ta l li n e s i li c on , s i li c on n i tr i de , e t c. , a v a ri e ty o f

m e ch a ni c al m i cr o st r uc t ur e s i n cl u di n g b e am s , d i ap h ra g ms , g r oo v es , o r ifi c es , s p ri n gs ,g e ar s , s u sp e ns i on s , a n d a g r ea t d i ve r si t y o f o t he r c o mp l ex m e c ha n ic a l s t ru c tu r es h a ve

b e e n c o n ce i v e d , i m p l em e n t e d, a n d c o m m e rc i a l l y d e m o ns t r a te d .S il ic on m ic ro ma ch in i ng h as b ee n t he k ey f ac to r i n t he f as t p ro gr es s o f m ic ro sy st em s i n

t he l as t d ec ad e o f t he 2 0t h c en tu ry . T hi s i s t he f as hi on in g o f m ic ro sc op ic m ec ha ni c al

p ar ts o ut o f s il i co n s ub st ra te s a nd

m o re r e ce n tl y o t he r m a te r ia l s. T h is

t e ch n iq u e i s u se d t o f a br i c at e s u ch

f ea tu re s a s c la mp ed b ea ms , m em -

b ra ne s, c an ti le ve r s, g ro ov e s, o ri -

fi ce s, s pr in gs , g ea rs , c ha mb er s,

e tc ., t ha t c an t he n b e s ui t ab ly c om -b i n e d t o c r e a t e a v a r i e t y o f s e n s o r s .

Bulk micromachining, which in-

vo lve s c ar vi ng o ut t he r eq ui re d

s t ru c tu r e b y e t ch i ng o u t t h e s i li c on

s ub st ra te , i s t he c om mo nl y u se d

m et ho d. F or i ns ta nc e, a t hi n d ia -

p hr ag m o f p re ci se t hi c kn es s i n t hef e w- m ic r on r a ng e s u it a bl e f o r i n te -

g ra ti ng s en si ng e le me nt s i s v er y

o ft en r ea li z ed w it h t hi s a pp ro ac h.F i g u r e 1 . 4 s h o w s t h e s c a n n i n g e l e c -

t ro n m ic ro sc op e ( SE M) i ma ge o f a c an t il ev er b ea m o f S iO2 f a br i c at e d b y b u lk m i cr o -

m a ch i ni n g ( w et c h em i ca l e t ch i ng ) o f s i li c on . T h e d i me n si o ns o f t h is c a nt i l ev e r b e am a r e

length65mm , w i dt h15mm , a n d t h ic k ne s s 0.52mm (1mm 106 m).

Surface micromachining i s a n a lt er na te m ic ro ma ch in in g a pp ro ac h. I t i s b as ed o np a t t e r n i n g l a y e r s d e p o s i t e d o n t h e s u r f a c e o f s i l i c o n o r a n y o t h e r s u b s t r a t e . T h i s a p p r o a c h

o ff er s t he a tt ra c ti ve p os si bi l it y o f i nt eg ra ti ng t he m ic ro ma ch in ed d ev ic e w it h m ic ro -

e l ec t ro n ic s p a tt e r ne d a n d a s se m bl e d o n t h e s a m e w a fe r . I n a d di t io n , t h e t h ic k ne s s o f t h e

s t r u c t u r a l l a y e r i n t h i s c a s e i s p r e c i s e l y d e t e r m i n e d b y t h e t h i c k n e s s o f t h e d e p o s i t e d l a y e r

a n d h e nc e c a n b e c o nt r ol l ed t o s u bm i cr o n t h ic k ne s s l e ve l s.

F i g u r e 1 . 5 s h o w s a s c h e m a t i c o f a p o l y c r y s t a l l i n e s i l i c o n r e s o n a t i n g b e a m t h a t c a n b em a d e b y t h e s u r f a c e m i c r o m a c h i n i n g t e c h n i q u e . N o t e t h a t t h e r e s o n a t o r b e a m i s a n c h o r e d

a t i t s e n d s a n d t h a t a g a p o f d 0.1mm e x i s t sb e t w e e n t h e r e s o n a t o r a n d t h e r i g i d b e a m l a i d

p e rp e nd i cu l ar t o i t . A s a r e su l t, t h e r e so n at o r

v ibr at es a t t he f re que ncy of t he a c si gna l

a pp li ed t o i t w it h r es pe ct t o t he r ig id b ea m

a nd a c ur re nt fl ow s d ue t o t he c ap ac it an ce

v a r i a t i o n w i t h t i m e . T h i s c u r r e n t p e a k s w h e n

t he f re qu en c y o f t he i np ut s ig na l m at ch e s w it h

t he m ec ha ni ca l r es on an ce f re qu en cy o f t hev ib ra ti ng b ea m. T hu s t he r es on at or c an b e

u se d a s a fi lt er .

Figure 1.4 S E M i m a g e o f S i O2 microcantilever beam

p r e p a r e d b y b u l k m i c r o m a c h i n i n g ( w e t c h e m i c a l

e t c h i n g ) o f s i l i c o n .

Anchor Resonator

Rigid beamElectrodes

Figure 1.5 S c h e m a t i c o f a r e s o n a t o r b e a m [ 4 ] .

6 c 1 Introduction


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U n ti l a b o ut t h e 1 9 90 s , m o st m i c ro s ys t em s d e v ic e s w i th v a r io u s s e ns i ng o r a c tu a t in g

mechanismswerefabricatedusingsiliconbulkmicromachining,surfacemicromachining,and

micromolding processes. More recently, 3D microfabrication processes incorporating other

materials(suchaspolymers)havebeenintroducedinmicrosystemstomeetspecificapplication

requirements (e.g. biomedical devices and microfluidics).

I t i s i n t e r e s t i n g t o n o t e t h a t a l m o s t a l l t h e m i c r o s y s t e m s d e v i c e s e m p l o y o n e o r m o r eo f t h e t h re e b a si c s t ru c tu r es n a m el y , a d i ap h ra g m, a m i cr o br i dg e , o r a b e am th a t a r e

r ea li z ed u si ng m ic ro ma ch in in g o f s il ic on i n m os t c as es a nd o th er m at er i al s i nc lu di ngp ol ym e rs , m et al s, a nd c er am ic s. T he se t hr ee s tr uc tu re s p ro vi d e f ea si bl e d es ig ns f or

m ic ro se ns or s a nd a ct ua to rs t ha t e ve nt u al ly p er fo rm t he d es ir ed t as k i n m an y s ma rt

s t ru c tu r es . T h e m a in i s su e s w i th r e sp e ct t o i m pl e me n ti n g t h es e s t ru c tu r es a r e t h e c h oi c e

o f m a te r ia l s a n d t h e m i cr o ma c hi n in g t e ch n ol o gi e s u s ed t o f a br i c at e t h es e d e vi c e s.

c 1.4 SMART MATERIALS, STRUCTURES AND SYSTEMS

T he a re a o f s ma r t m at er ia l s ys te ms h as e vo lv ed f ro m t he u ne nd in g q ue st o f m an ki nd t o

m im ic s ys te ms o f n at ur al o ri gi n. T he o bj e ct iv e o f s uc h i ni ti at iv es i s t o d ev el o p t ec hn ol og ie s

t o p r o du c e n o n bi o l og i c a l s y s t e m s t h a t a c h ie v e

o p ti m um f u nc t io na l it y a s o b se r ve d i n n at u ra l

b i ol og i ca l s y st e ms a n d e mu l at e t h ei r a d ap t iv e

c a p a bi l i ti e s b y a n i n t e gr a t ed d e si g n a p pr o a ch .I n t h e p r e s en t c o nt e xt , a s ma r t m a te ri a l i s o n e

w h o se e l e ct r i ca l , m e c ha n ic a l , o r a c o us t i c p r o -

p e r t ie s o r w h os e s t r uc t u re , c o m po s it i o n, o r f u n c-

t i on s c h an g e i n a s p ec i fie d m a nn e r i n r es p on se t o

s o m e s t i m u l u s f r o m t h e e n v i r o n m e n t . I n a s i m i -

l a r w a y , o n e m a y e n v i s a g e s m a r t s t r u c t u r e s t h a tr e qu ir e t he a d di ti o n o f p r op e rl y d e si g ne d s e n-

s o r s, a c t ua t or s , a n d c o n tr o ll e r s t o a n o t h er w i se d um b s t r uc t u re ( s e e s c he m a ti c i n F i g ur e 1 . 6 ).

A s s m ar t m a te r ia l s s y st e ms s h ou l d m i mi c

n at ur al ly o cc ur ri n g s ys te ms , t he g en er a l r e-

q u i r e me n t s e x p e c te d f r o m t h e s e n o n bi o l o g ic a l

s y s t em s i n c l ud e :

1. F u l l i n te g ra t io n o f a l l f u nc t io n s o f t h e s y st e m.

2. C o n t i nu o u s h e a l t h a n d i n t e gr i t y m o n it o r i ng .3. D a m a g e d e t e ct i o n a n d s e l f -r e c o v er y .

4. I n t e l l i g e nt o p e r at i o n a l m a n a ge m e n t .

5. H i g h d e g re e s o f s e c u ri t y , r e l i ab i l i t y , e f fi c i e n c y a n d s u s t ai n a b il i t y .

A s o ne c an n ot e, t he m at er i al s i nv ol ve d i n i mp le me nt in g t hi s t ec hn ol o gy a re n ot

n e c e s s a r i l y n o v e l . Y e t t h e t e c h n o l o g y h a s b e e n a c c e l e r a t i n g a t a t r e m e n d o u s p a c e i n r e c e n t

y e ar s a n d h a s i n de e d b e en i n sp i re d b y s e ve r al i n no v at i ve c o nc e pt s .

A s m en ti on ed e ar li er , t he s tr uc tu ra l , p hy si ca l, o r f un ct io na l p ro pe rt ie s o f s ma rt

m at er ia ls r es po nd t o s om e s ti mu lu s f ro m t he e nv ir on m en t a nd t hi s r es po ns e s ho ul d b e

r ep et it iv e i n t he s en se t ha t t he s am e c ha ng e i n t he e nv i ro nm en t m us t p ro du ce t he s am er e s p o n s e . W e k n o w t h a t e v e n w i t h o u t d e s i g n , m o s t m a t e r i a l s d o r e s p o n d t o t h e i r e n v i r o n -

m en t. F or e xa mp le , n ot e t he c ha ng e i n d im en si on s o f m os t m at er ia l s w he n h ea te d o r

Structure

Sensor Actuator

Controlunit

Figure 1.6 B u i l d i n g b l o c k s o f a t y p i c a l

s m a r t s y s t e m .

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c o o l e d . H o w e v e r , w h a t d i s t i n g u i s h e s a s m a r t m a t e r i a l f r o m t h e r e s t i s t h a t w e design the

m a t e r i a l s o t h a t s u c h c h a n g e s o c c u r i n a s p e c i fi c m a n n e r f o r s o m e d e fi n e d o b j e c t i v e t o b e

a cc om pl is he d. H en ce , t he m ai n f ea tu re t ha t d is ti ng ui sh es s ma rt m at er ia l s i s t ha t t he y

respondsignificantly t o s o me e x te r na l s t im u li t o w hi c h m o st m a te r ia l s a r e u n re s po n si v e.

F u r t h e r m o r e , o n e w o u l d l i k e t o e n h a n c e s u c h a r e s p o n s e b y a t l e a s t o n e o r t w o o r d e r s o f

m a g ni t u d e o v e r o t h e r m a t e r i a l s.B ot h a ct i ve a n d p a ss iv e a p pr oa c he s h a ve b e en a t te mp t ed i n t hi s c o nt e xt . T h is d i st i nc t io n

i s b a se d o n t he r e qu ir e me n t t o g e ne ra t e p o we r r e qu ir e d t o p e rf o rm r e sp on s es . H en c e, a na ct i ve s ys t em h as a n i n bu i lt p o we r s o ur c e. T y pi ca l ly , a c ti v e s e ns or s a n d a ct u at or s h a ve b e en

f av o re d i n d e si gn i ng s ma r t s tr u ct u re s. H ow ev e r, i n r e ce nt y e ar s t h e c on c ep t o f p a ss iv e

s m a r t n e s s h a s c o m e t o t h e f o r e . P a s s i v e s m a r t n e s s c a n b e p e r v a s i v e a n d e v e n c o n t i n u o u s i n

t he s t ru c tu r e. S uc h s t ru c tu re s d o n o t n e ed e x te r na l i n te r ve nt i on f or t h ei r o p er a ti on . I n

a d d i t i o n , t h e r e i s n o r e q u i r e m e n t f o r a p o w e r s o u r c e . T h i s i s p a r t i c u l a r l y r e l e v a n t i n l a r g e -

s ca l e c i vi l e n gi ne e ri n g s tr uc t ur es . P a ss i ve s ma r tn e ss c a n b e d e ri ve d f ro m t h e u n iq u e i n tr in si c

p r o p e r t i e s o f t h e m a t e r i a l u s e d t o b u i l d s u c h s t r u c t u r e s . O n e c o m m o n e x a m p l e i s t h e s h a p e -

m e m o r y - a l l o y ( S M A ) m a t e r i a l e m b e d d e d i n a e r o s p a c e c o m p o s i t e s , d e s i g n e d s o t h a t c r a c k s

d o n o t p r op a ga t e. S u ch s m ar t m at e ri a ls a r e d i sc us se d b ri e fly i n C ha p te r 2 .B e si d e s sm a rt s y st e m o r sm a rt m a te r ia l , a n ot h er w i de l y u s ed t e rm i s smart

structure. O n e d i s t i nc t iv e f e at u re o f s m ar t s t ru c tu r es i s t h at a c tu a to r s a n d s e ns o rs c a n b e

e m be d de d a t d i sc r et e l o ca t io n s i n si d e t h e s t ru c tu r e w i th o ut a f fe c ti n g t h e s t ru c tu r al i n te g ri t y

o f t he m ai n s tr uc tu re . A n e xa mp le i s t he e mb ed de d s ma rt s tr uc tu re i n a l am in at ed

c o mp o si t e s t ru c tu r e. F u rt h er m or e , i n m a ny a p pl i ca t io n s, t h e b e ha v io r o f t h e e n ti r e s t ru c tu r e

i ts el f i s c ou pl ed w it h t he s ur ro un di ng m ed iu m. T he se f ac to rs n ec es si ta te a c ou pl e d

m o de l in g a p pr o ac h i n a n al y zi n g s uc h s m ar t s t ru c tu r es . T h e f u nc t i on a n d d e sc r ip t io n o f v ar io us c om po ne nt s o f t he s ma rt s ys te m i n T ab l e 1 .1 a re s um ma ri ze d i n T ab le 1 .1 [ 5] .

C i vi l e n gi n ee ri n g s y st e ms s uc h a s b u il di n g f ra m es , t ru s se s , o r b ri d ge s c o mp r is e a

c om p le x n e tw or k o f t r us s, b e am , c o lu m n, p la t e, a n d s h el l e l em en t s. M o ni t or i ng t he s tr u ct u ra lh ea lt h o f b ri dg e s tr uc tu re s f or d if fe re nt m ov in g l oa ds i s a n a re a o f g re at i mp or ta nc e i n

i nc r ea s in g t h e s t ru ct u ra l i nt e gr i ty o f i n fr a st ru c tu re s a n d h as b e en p u rs ue d i n m an y c o un tr i es .

D a m p i n g o f e a r t h q u a k e m o t i o n s i n s t r u c t u r e s i s y e t a n o t h e r a r e a t h a t h a s b e e n t a k e n u p f o r

a ct i ve r e se a rc h i n m an y s ei s mi c al l y a c ti ve c o un tr i es . S ma r t d ev i ce s d e ri v ed f ro m s ma rt

m at e ri a ls a r e e x te n si ve l y u s ed f o r s uc h a p pl i ca t io n s. A b r id g e w ho s e s t ru c tu ra l h e al t h c a n b em on i to r ed i s s h ow n i n F i gu r e 1 . 7. I n s u ch a b ri d ge , fi b er o pt i c s en s or s a r e u s ed a s s e ns in g

d ev i ce s, w he re a s l e ad z i rc o na t e t i ta n at e ( P ZT ) o r T e rf e no l- D a c tu at o rs a r e n o rm al l y u s ed f or

p e rf o r mi n g a c t ua t i on s s u ch a s v i b ra t i on i s o la t i on a n d c o n tr o l.

Table 1.1 Purpose of various components of a smart system

Unit

Equivalent in

Biological Systems Purpose Description

Sensor Tactile sen sing Data acquisition Collect required raw data needed for appropriate

s e n s i n g a n d m o n i t o r i n g

Data bus 1 Sensory nerves Data tran smission Forward raw data to local and/or central command

a n d c o n t r o l u n i t s

Control system Brain Command and

c o n t r o l u n i t

M a n a g e a n d c o n t r o l t h e w h o l e s y s t e m b y a n a l y z i n g

d a t a , r e a c h i n g t h e a p p r o p r i a t e c o n c l u s i o n s , a n d

d e t e r m i n i n g t h e a c t i o n s r e q u i r e d

Data bus 2 Moto r n erves Data instruction s Transmit decisions and asso ciated instructions tom e m b e r s o f s t r u c t u r e

Actuator Muscles Action d ev ices Take action by triggering controlling devices/u nits

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T h e b e n e fi c i a r i e s a n d s u p p o r t e r s o f t h e s m a r t s y s t e m s t e c h n o l o g y h a v e b e e n m i l i t a r y

a n d a e ro s pa c e i n du s tr i e s. S o me o f t h e p r oo f -o f -c o nc e pt p r og r am s h a ve a d dr e ss e d s t ru c -

t u ra l h e al t h m o ni t or i ng , v i br a ti o n s u pp r es s io n , s h ap e c o nt r ol , a n d m u lt i fu n ct i on a l s t ru c -

t u ra l c o nc e pt s f o r s p ac e cr a ft s , l a un c h v e hi c le s , a i rc r af t s a n d r o to r cr a ft s . T h e s t ru c tu r es b u il t

s o f a r h a v e f o c u se d o n d e m o ns t r a ti n g p o t e nt i a l s y s te m - l e ve l p e r f or m a n c e i m p r o v e m en t s

u s in g s m ar t t e ch n ol o gi e s i n r e al i st i c a e ro s pa c e s y st e ms . C i vi l e n gi n ee r in g s t ru c tu r es

i nc lu di n g b ri dg es , r un wa ys a nd b ui l di ng s t ha t i nc or po ra te t hi s t ec hn ol og y h av e a ls ob e en d e mo n st r at e d. S m ar t s y st e m d e si g n e n vi s ag e s t h e i n te g ra t io n o f t h e c o nv e nt i on a l

fi e l d s o f m e c h an i c a l e n g i n ee r i n g, e l e c t ri c a l e n g i n ee r i n g, a n d c o m pu t e r s c i e nc e / i n fo r m a -

t i on t ec hn ol o gy a t t he d es ig n s ta ge o f a p ro du ct o r a s ys te m.

A s d i s c u s s e d e a r l i e r , s m a r t s y s t e m s s h o u l d r e s p o n d t o i n t e r n a l ( i n t r i n s i c ) a n d e n v i r o n -

m e nt a l ( e xt r i ns i c) s t im u li . T o t h i s e n d , t h ey s h o u ld h a v e s e ns o rs a n d a c tu a to r s e m be d de d i n

t h em . S o me o f t h es e d e vi c es c o mm o nl y e n co u nt e re d i n t h e c o nt e xt o f s m ar t s ys t em s a r e

l i st e d i n T ab le 1 .2 .

c 1.5 INTEGRATED MICROSYSTEMS

I n te g ra t ed m i cr o sy s te m s c a n b e c l as si fi ed i n to t h re e m a jo r g r ou p s a s f o ll o ws :

1. Micromechanical structures: T h e s e a r e n o n - m o v i n g s t r u c t u r e s , s u c h a s m i c r o b e -

a m s a n d m i c r oc h a n ne l s .

Figure 1.7 T h e h i s t o r i c G o l d e n B r i d g e b u i l t i n 1 8 8 1 t o c o n n e c t A n k l e s h w a r a n d B h a r u c h i n G u j a r a t ,

I n d i a . T h e i n s e t s h o w s t h e d e t a i l s o f o n e - t i m e s t r u c t u r a l h e a l t h m o n i t o r i n g o f a s e c t i o n o f t h e b r i d g e .

I t h a d 6 6 fi b e r o p t i c s t r a i n g a u g e s a n d m i c r o m a c h i n e d a c c e l e r o m e t e r s . C o u r t e s y : I n s t r u m e n t a t i o n

Scientific Technologies Pvt. Ltd., Bangalore, www.inscitechnologies.com.

Table 1.2 Some sensors and actuators used in smart systems

Device Physical Quantity Example Successful Technologies

Sensor Acceleration Accelerometer PZT , Microfabrication

Angular rate Gyroscope Fiber o ptic, Micro fabricatio n

Position Linear variab le d ifferential

transformer (LVDT)

Electromagnetic

Tran sd ucer Crack detection Ultrasonic transducer PZT

Actuator Movement Thermal Shape memory alloy

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2. Microsensors: T h e s e r e s p o n d t o p h y s i c a l a n d c h e m i c a l s i g n a l s ( s u c h a s p r e s s u r e ,

a c ce l er a ti o n, p H, g l uc o se l e ve l , e t c. ) a n d c o nv e rt t h em t o e l ec t ri c al s i gn a ls .

3. Microactuators: T h e se c o nv e rt e l ec t ri c al o r m a gn e ti c i n pu t t o m e ch a ni c a l f o rm so f e n er g y ( e .g . r e so n at i ng b e am s , s wi t c he s , a n d m i cr o pu m ps ) .

Microsystems i n te g ra t e s e ns o rs , a c tu a t or s a n d e l ec t ro n ic s t o p r ov i de s o me u s ef u l

f un ct io n. T he A DX L5 0, w hi ch w as r el ea se d i n 1 99 1 a nd w as A na lo g D ev ic es fi rs t

c o m m e r c i a l d e v i c e , i s a n e x c e l l e n t e x a m p l e o f s u c h a m i c r o s y s t e m . T h e b l o c k d i a g r a m o f

t hi s s ys te m i s s ho wn i n F ig ur e 1 .8 [ 6] . T hi s m ic ro sy st em i s b as ed o n a s ur fa ce m ic ro -

m ac hi ni ng t ec hn ol og y w it h s en si ng e le ct ro ni cs i nt eg ra te d o n t he s am e c hi p a s t he

a cc el e ro me te r. H er e, t he a cc el er om et e r i s a s en so r t ha t r es po nd s t o t he a cc el er at io n o r

d e ce l er a ti o n a n d g i ve s a n o u tp u t v o lt a ge t o t h e c o nt r ol c i rc u it , w h ic h i n t u rn t r ig g er s a n

a c t u a t o r t o d e p l o y t h e a i r b a g d u r i n g a c r a s h , s o t h a t t h e p e r s o n s s e a t e d i n t h e f r o n t s e a t a r ep r ot e ct e d f r om c r as h in g i n to t h e f r on t w i nd s hi e l d o r t h e d a sh b oa r d.

1.5.1 Micromechanical Structures

M ic ro ma c hi ni ng i s u se d c om me r ci al ly t o p ro du ce c ha nn el s f or m ic ro flu id ic d ev ic es

a nd a ls o t o f ab ri ca te s ys te ms r ef er re d t o a s l ab s o n a c hi p f or c he mi al a na ly si s a nd

a n a l y s i s o f b i o m e d i c a l m a t e r i a l s [ 7 ] . U s u a l l y s u c h c h a n n e l s a r e m a d e o n p l a s t i c s o r g l a s s

s ub st ra t es . F ig ur e 1 .9 ( a) s ho ws o ne s uc h d ev ic e r ep or te d i n t he l it er at ur e [ 8] . M ic ro -

m a ch i ni n g i s a l so u s ed t o m a ke a v a ri e t y o f m e ch a ni c al s t ru c tu r es . F i gu r e 1 . 9 ( b ) s h o ws a n

S E M i m a g e o f a s i li c on n a no t i p f a br i ca t e d [ 9 ] u s in g a bu l k m i cr o ma c hi n in g p r oc e ss .

S u c h m i c r o t i p s fi n d a p p l i c a t i o n s i n a t o m i c f o r c e m i c r o s c o p y ( A F M ) t e c h n o l o g y a n d fi e l de m is s io n a r ra y f o r f u tu r is t i c v a cu u m e l ec t r on d e vi c es c a pa b l e o f o p er a t io n i n t e ra h er t z

f r e qu e n c y r a n g e [ 1 0 ] .

Outputvoltage

PreampBufferDemodulatorand low-pass

filter

Square waveoscillator

Feedback voltage

Anchor

Polysilicon proofmass and moving

electrodes

Fixed polysiliconcapacitor plates

Suspensionsystem

Figure 1.8 A s c h e m a t i c d i a g r a m o f A D X L 5 0 a c c e l e r o m e t e r .

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1.5.2 MicrosensorsS e v e r a l m i c r o m a c h i n e d s e n s o r s h a v e e v o l v e d o v e r t h e l a s t t w o d e c a d e s . A m o n g t h e m t h e

p r e s s u r e s e n s o r s o c c u p y a l m o s t 6 0 % o f t h e m a r k e t . A s c h e m a t i c i s o m e t r i c c u t - a w a y v i e w

o f a p i ez o re s is t iv e p r e s su r e s e n so r d i e i s s h o w n i n F i g. 1 . 10 ( a ). H e re , w e c a n s e e t h e f o ur

Figure 1.9 Miniature mechanicals t r u c t u r e s s h o w i n g ( a ) p o l y m e r

m e s o p u m p ; ( b ) s i l i c o n n a n o t i p

f a b r i c a t e d u s i n g b u l k

micromachining.(a) (b)

Figure 1.10 Schematic diagrams

o f m i c r o s e n s o r s : ( a ) c u t - a w a y

v i e w o f a p i e z o r e s i s t i v e p r e s s u r es e n s o r; ( b ) c a p a c it i v e -s e n s in g

accelerometer.

(a)

Piezoresistor

Pad

Diaphragm

Pressure port

Glass

Glass

(b)

Fixed electrode

Fixed electrode

Seismic massSi

1.5 Integrated Microsystems b 11


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p r es su r e- s en s it i ve r e si s to r s ( p ie z or e si s to r s) i n te g ra t e d o n a m i cr o ma c hi n ed s i li c on d i a-

p h ra g m. M i cr o ma c h in e d a c ce l er o me t er i s y e t a n ot h er d e vi c e w hi c h h a s r e ce i v ed c o ns i d-

e r a b le a t t e nt i o n f r o m t h e a e r o s p a c e, a u t o mo b i l e , a n d b i o m ed i c a l i n d u s t r ie s . F i g u r e 1 . 1 0 ( b )

s h o w s a s c h e m a t i c c r o s s - s e c t i o n a l v i e w o f o n e s u c h d e v i c e . T h e s e i s m i c m a s s r e s p o n d s t o

a cc el e ra ti on a nd d efl ec t s, t hu s b ri ng in g a bo ut a c ha ng e i n t he c ap ac i ta nc e b et we en t he m as s

a nd t he fi xe d e le ct r od es . T he c ha ng e i n c ap ac i ta nc e i s a m ea su re o f t he d is pl ac em en t,w hi c h i n t u rn d e pe n ds u p on t h e a c ce l e ra t io n .

1.5.3 Microactuators

O v e r t h e l a s t f e w y e a r s , m i c r o m a c h i n e d a c t

Micro and Smart Systems

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