How Can We Lower the Power Consumption of Gas Sensors?
There is a growing demand for low-power, high-density gas sensor arrays that can overcome problems relative to high power consumption. Low power consumption is a prerequisite for any type of sensor system to operate at optimum efficiency. Focused on fabrication-friendly microelectromechanical systems (MEMS) and other areas of sensor technology, MEMS and Nanotechnology for Gas Sensors explores the distinct advantages of using MEMS in low power consumption, and provides extensive coverage of the MEMS/nanotechnology platform for gas sensor applications.
This book outlines the microfabrication technology needed to fabricate a gas sensor on a MEMS platform. It discusses semiconductors, graphene, nanocrystalline ZnO-based microfabricated sensors, and nanostructures for volatile organic compounds. It also includes performance parameters for the state of the art of sensors, and the applications of MEMS and nanotechnology in different areas relevant to the sensor domain.
In addition, the book includes:
An introduction to MEMS for MEMS materials, and a historical background of MEMS
A concept for cleanroom technology
The substrate materials used for MEMS
Two types of deposition techniques, including chemical vapour deposition (CVD)
The properties and types of photoresists, and the photolithographic processes
Different micromachining techniques for the gas sensor platform, and bulk and surface micromachining
The design issues of a microheater for MEMS-based sensors
The synthesis technique of a nanocrystalline metal oxide layer
A detailed review about graphene; its different deposition techniques; and its important electronic, electrical, and mechanical properties with its application as a gas sensor
Low-cost, low-temperature synthesis techniques
An explanation of volatile organic compound (VOC) detection and how relative humidity affects the sensing parameters
MEMS and Nanotechnology for Gas Sensors provides a broad overview of current, emerging, and possible future MEMS applications. MEMS technology can be applied in the automotive, consumer, industrial, and biotechnology domains.

There is a significant interest in lowering the power consumption of semicon-ductor metal oxide-based gas sensors. Metal oxide gas sensors consuming low voltage and low power can be easily produced by combining microma-chining and thin film technologies. The increasing demand for faster and more honest analysis has evolved the development of microelectromechani-cal systems(MEMS) for gas sensing and biosensing. MEMS are desirable for use in low-power investigative designs for their ability to employ and analyze small volumes.
The MEMS micro heater is one of the key components of a chemical gas sensor. For semiconductor gas sensors, a uniform micro heater temperature is a necessary requirement as it often enhances the operation of the sen-s or. The micro heater that lies on top of the membrane should be maintained at a uniform temperature for maximum sensitivity. A uniform temperature implies minimization of the heater hot spot, which is a crucial requirement for heater reliability. Thus, the temperature uniformity depends mainly on the membrane materials and on the geometry of the micro heater
Low power consumption is a prerequisite for any type of sensor system to operate with acceptable battery lifetime. Power consumption of a MEMS-based gas sensor is found to depend mainly on thermal losses. It has been investigated that to achieve avery substantial reduction in power consump-tion, the dimension of the device has to be reduced, obviously in a cost-effective way. Micromachining of the silicon substrate is the only approach to obtain the desired geometries of the micromechanical structures.
Nanocrystalline oxide materials with high surface-to-volume ratios are gaining interest in the area of gas sensing because of their enhanced reaction possibilities between the adsorbed oxygen and target gases. The versatil-ity of the sensing material affects their electrical, optical, and sensing appli-cations. Among them, ZnO, owing to its unique features and advantages,attracted the attention of researchers worldwide to study its application in chemical sensors. Some of the unique features of ZnO include the availabil-ity of both n-and p-type materials, high electron mobility, wideband gap, compatibility with standard CMOS technology, and adequate lattice match-ing with Si and SiO, substrates. Graphene is another wonder material as it can sense anatomic level presence of toxic gases that would not be possible with metal oxides.
This book consists of 12 chapters. Section I consists of Chapters 1 through 6. The objective is to give a general concept about the microfabrication tech-nology needed to fabricate a gas sensoron a MEMS platform.
Chapter 1 provides an introduction of MEMS for MEMS materials. A brief cleanroom concept is also given. The historical background of MEMS is also presented in this chapter
Chapter 2 discusses the substrate materials used for MEMS. Contaminants affecting the substrate material are discussed. Some common insulating lay-ers and their properties are elaborated.
Chapter 3 discusses the two types of depostion techniques, including the most popular one, chemical vapour deposition(CVD) . Different types of CVD techniques a represented to provide a clear understanding to the reader. The methods of wire bonding are also discussed
Chapter 4 illustrates the properties of photoresists, the types of photore-sists and the photolithographic processes. Some advanced photolithographic techniques are also discussed in detail.
Chapter 5 deals with different micromachining techniques for the gas sen-sor platform, and bulk and surface micromachining. A comparison is made based on their requirements. Avery specific etch stop technique is also dis-cussed in this chapter.
Chapter 6 discusses the design issues of a micro heater for MEMS-based sensors. In this chapter, a complete investigation is presented, including an electrothermal design of a micro heater on a micromachined silicon platform, particularly applicable for relatively low-temperature(150℃-300℃) gas sensor applications. Micro heater design issues along with their geometry are also presented. The modified structure along with the performance parame-ters, e. g. temperature distribution across the membrane and the temperature variation with power consumption, is critically discussed. As this book is the amalgamation of MEMS with gas sensor, it is better to start with Chapter 7in Section II.
Chapter 7 provides the synthesis technique of a nanocrystalline metal oxide layer which is to be deposited over the heater element. In this chapter, vari-ous characterizations have been furnished to identify the nano dimensional nature and some native defects of the thin film. Structural characterizations like XRD, FE SEM and EDX confirming the crystal structure and morphol-ogy and determining the crystallite size are discussed lucidly. The impu-rity perspective has been analyzed by FTIR spectroscopy. An idea of Raman spectroscopy is also presented. Factors affecting gas sensing such as grain size, porosity of the material, nano size effect and methane sensing mecha-nism are discussed thoroughly.
The investigation of graphene and its properties is currently a hot topic in physics, materials science and nanoscale science. Graphene is the stron-gest material ever measured, a replacement for silicon and the most conduc-tive material ever discovered. It consists of a single layer of carbon atoms linked in a honeycomb-like arrangement. They be have well enough for use in nanoelectronics. Among the various potential applications of graphene, it can bean excellent sensor due to its 2D structure. Having a 2D structure, the entire volume is exposed to target gases, making it very efficient to detect adsorbed molecules. Gaseous molecules can not be readily adsorbed on to graphene surfaces as graphene has no dangling bonds on its surface, so inherently graphene is insensitive. The sensitivity of graphene can be enhanced dramatically by adding some functional groups on to it, i. e. coating the film with polymers. The unsatisfied bond in the polymer layer acts as a concentrator which absorbs gaseous molecules. This absorption of molecules introduces a local change in the electrical resistance of the graphene layer, making it a good sensor.
Chapter 8 gives a detailed review about graphene; its different deposition techniques; and its important electronic, electrical and mechanical proper-ties with its application a sagas sensor. Detection, alarm, and subsequent monitoring of poisonous and combustible gases(like H, , CH, ) for applica-tion in domestic as well as industrial environment using a very-small-size, low-cost and low-power gas sensor is highly desirable.
Chapter 9 explores the different device structures that are possible in this fabrication process and the irrespective advantages. Then, zinc oxide is described in brief. Some low-cost, low-temperature synthesis techniques are introduced in the last section of the chapter.
Chapter 10 is concerned with volatile organic compound(VOC) detec-tion. A few VOCs are mentioned. How relative humidity affects the sensing parameters is discussed
In Chapter 11, different interface systems are discussed. A brief idea about smart sensors is given.
Finally, Chapter 12 presents the applications of MEMS and nanotechnol-ogy in different areas relevant to the sensor domain.

Preface xiii

Authors xvii

Section I Fabrication Procedure

1. Introduction 3

      1.1 Cleanroom Technology 3

      1.2 Microelectromechanical System 6

      1.2.1 History 7

      1.2.2 Definitions and Classifications 8

      1.2.3 Market and Application 9

      1.2.4 Materials for MEMS 10

      1.3 Significance of MEMS 14

      References 15

2. Substrate for MEMS 17

      2.1 Introduction 17

      2.2 Silicon: The Base 18

      2.2.1 Silicon as a Semiconductor 18

      2.2.2 Surface Contamination 19

      2.2.2.1 Contaminants on Silicon Wafers 39

      2.2.3 Cleaning and Etching 22

      2.2.3.1 Cleaning 22

      2.2.3.2 Etching 22

      2.3 Dielectrics 22

      2.3.1 Silicon Dioxide (SiO2) 27

      2.3.2 Silicon Nitride (Si3N4) 28

      2.3.3 Low-Temperature Oxidation 29

      2.3.4 Oxide Properties 29

      References 30

3. Deposition 31

      3.1 Physical Vapour Deposition 31

      3.1.1 Vacuum Technology for MEMS 32

      3.1.2 e-Beam Evaporation 34

      3.1.3 Thermal Evaporation 34

      3.1.4 Sputtering 36

      3.1.5 Molecular Beam Epitaxy 37

      3.2 Chemical Vapour Deposition 38

      3.2.1 Atmospheric Pressure CVD (APCVD) 38

      3.2.2 Plasma CVD 39

      3.2.3 MOCVD 40

      3.3 Metallization 42

      3.3.1 Different Types of Metallization 42

      3.3.2 Methods 44

      3.3.2.1 Filament Evaporation 45

      3.3.2.2 Electron Beam Evaporation 45

      3.3.2.3 Induction Evaporation 45

      3.3.2.4 Sputtering 45

      3.3.3 Wire Bonding 45

References 47

4. Photolithography: Pattern Transfer 49

      4.1 Introduction 49

      4.2 Photoresist for Structuring 50

      4.3 Some Important Properties of Photoresist 50

      4.3.1 Sensitivity 50

      4.3.2 Adhesion 51

      4.3.3 Etch Resistance 51

      4.3.4 Bubble Formation 51

      4.4 Types of Photoresists: Negative and Positive Photoresists 52

      4.4.1 Negative Photoresists 52

      4.4.2 Positive Photoresists 53

4.5 Designing of Mask Layout 53

      4.6 Photolithography Process 54

      4.7 Application of Photoresist and Pre bake 54

      4.8 Alignment, Exposure, and Pattern Formation 55

      4.9 PR Developer and Post bake 56

      4.10 Stripping (Photoresist Removal) 56

      4.11 Some Advanced Lithographic Techniques 57

      4.11.1 Electron Beam Lithography 57

      4.11.2 Ion Beam Lithography 58

      4.11.3 X-Ray Lithography 59

      4.11.4 Phase-Shift Lithography 60

5. Structuring MEMS: Micromachining 63

      5.1 Introduction 63

      5.2 Bulk Micromachining 64

      5.2.1 Wet Etching 65

      5.2.1.1 Isotropic and Anisotropic: Empirical Observations 66

      5.2.1.2 Convex and Concave Corner Compensations 69

      5.2.2 Dry Etching 72

      5.3 Surface Micromachining 72

      5.3.1 Processes 72

      5.3.2 Hurdles 74

      5.3.3 Lift-Off versus Etch Back 74

      5.3.3.1 Lift-Off 74

      5.3.3.2 Etch Back 75

      5.4 Etch-Stop Technique 76

      5.4.1 Boron Etch Stop 77

      5.4.2 Electrochemical Etch Stop 78

      5.4.3 Photo-Assisted Electrochemical Etch Stop (for n-Type Silicon) 80

      5.4.4 Etch Stop at Thin Films: Silicon on Insulator 81

      5.5 High-Aspect-Ratio Micromachining 82

      5.5.1 LIGA 83

      5.5.2 Laser Micromachining 84

      References 85

6. Micro heaters for Gas Sensor 87

      6.1 Introduction 87

      6.2 Need of Micro heater 88

      6.3 Types of Micro heater 89

      6.3.1 Closed-Membrane Type 90

      6.3.2 Suspended-Membrane Hotplates 91

      6.4 Micro heater Design Issues 93

      6.5 Heater Material Selection 94

      6.6 Heater Geometry Selection 95

      6.7 Function of Interdigitated Electrode 98

      6.8 Software Used 99

      6.8.1 Temperature Distribution 100

      6.8.2 Mechanical Stability 101

      6.8.3 Thermal Response Time 102

      6.9 Heating Power Consumption 103

      6.10 Fabrication of Micro heater 106

      6.11 Microheater Array 108

      References 110

Section II Sensor Applications

7. Semiconductors as Gas Sensors 119

      7.1 Introduction 119

      7.2 Development of Semiconductor Sensors 120

      7.2.1 Fundamentals of Semiconductor Sensors 121

      7.2.2 Classification of Semiconductor Sensors 121

      7.2.3 Different Structures of Semiconductor Gas Sensors 122

      7.2.3.1 Resistive-Type Metal Oxide-Based Gas Sensors 122

      7.2.3.2 Schottky-Type Metal Oxide-Based Gas Sensor 122

      7.2.3.3 Metal Oxide Homojunction Gas Sensor124

      7.2.3.4 Metal Oxide Heterojunction Gas Sensor 124

      7.2.3.5 Mixed Metal Oxide Gas Sensors 124

      7.2.3.6 MEMS Gas Sensors 125

      7.3 What Is a Nanosensor? 126

      7.3.1 Thin Film Sensors 127

      7.3.2 Thick-Film Sensors 127

      7.3.2.1 Thick-Film Materials 128

      7.4 Solid-State Chemical Sensors 128

      7.4.1 Metal Oxide Semiconductors 129

      7.4.2 Nanocrystalline Metal Oxide Semiconductors 130

      7.4.3 Adsorption of Oxygen: Analyses 130

      7.4.4 Reaction between Gas (e.g.CH) and Oxygen 132

      7.4.5 Role of Catalyst on Gas Sensing Mechanism 134

      7.4.6 Thick-and Thin-Film Fabrication Process 135

      7.4.6.1 Bulk Growth 135

      7.4.6.2 Substrate Growth 137

      7.4.6.3 Chemical Vapour Deposition 137

      7.4.6.4 Sputtering 137

      7.4.6.5 Chemical Route 138

      7.4.7 Sensor Characterizations 138

      7.4.7.1 X-Ray Diffraction 138

      7.4.7.2 Determination of Crystal Size 139

      7.4.7.3 Field Emission Scanning Electron Microscopy 139

      7.4.7.4 Transmission Electron Microscopy 140

      7.4.7.5 Photoluminescence Spectroscopy 140

      7.4.7.6 Fourier Transform Infrared Spectroscopy 141

      7.4.7.7 Qualitative Analysis 141

      7.4.7.8 UV/VIS Spectroscopy 142

      7.4.7.9 Raman Spectroscopy 142

      7.4.8 Sensor Reliability Issues 143

      References 143

8. Sensing with Graphene 147

      8.1 Introduction 147

      8.2 Properties of Graphene 150

      8.2.1 Electronic Property 151

      8.2.2 Mechanical Properties 152

      8.2.3 Optical Properties 153

      8.2.4 Electronic Transport 154

      8.2.5 Anomalous Quantum Hall Effect 154

      8.2.6 Magnetic Properties 155

      8.2.7 Thermal Properties 156

      8.3 Characterization Techniques 156

      8.3.1 Optical Microscopy.157

      8.3.2 Field Emission Scanning Electron Microscopy157

      8.3.3 Atomic Force Microscopy 158

      8.3.4 Diffraction Imaging Electron Microscopy 158

      8.3.5 Transmission Electron Microscopy 160

      8.3.6 Raman Scattering 160

      8.4 Synthesis of Single-Layer Graphene/Few-Layer Graphene 161

      8.4.1 Micromechanical Exfoliation 161

      8.4.2 Chemical Exfoliation 163

      8.4.3 Epitaxial Growth on Silicon Carbide (SiC) 164

      8.4.4 Chemical Vapour Deposition 165

      8.5 Graphene Oxide 169

      8.6 Potential Application 171

      8.6.1 Graphene Sensors 171

      8.7 Summary 174

      References 175

9. Nanocrystalline ZnO-Based Microfabricated Chemical Sensor 185

      9.1 Introduction 185

      9.2 Device Structure: Vertical and Horizontal 186

      9.3 Comparison of Vertical and Horizontal Structure 187

      9.4 Metal-Insulator-Metal Structure 187

      9.5 Nanocrystalline ZnO as Sensing Material 188

      9.6 Sensing Layer Deposition by Chemical Route 189

      9.6.1 Sol-Gel Method for Synthesis of ZnO Thin Films 189

      9.6.2 Chemical Bath Deposition Technique 190

      9.6.2.1 Growth Mechanism 191

      9.6.3 Chemical Deposition Technique 191

      References 192

10. Nanostructures for Volatile Organic Compound Detection 193

      10.1 Introduction 193

      10.2 Volatile Organic Compounds 193

      10.3 Different Nanostructures 195

      10.3.1 Fabrication Processes 195

      10.3.2 Characterization Processes 196

      10.4 Sensing Mechanism 196

      10.5 Measurement Technique 197

      10.6 Effect of Relative Humidity on VOC Detection 198

      References 199

11.Sensor Interfaces 201

      11.1 Signal Processing 201

      11.2 Smart Sensors 202

      11.2.1 System Components 205

      11.3 Interface Systems 206

      References 207

12. MEMS-and Nanotechnology-Enabled Sensor Applications 209

      12.1 MEMS and Nanotechnology 209

      12.2 Automotive Applications: An Elaborated Study 210

      12.2.1 Safety 210

      12.2.2 Vehicle Diagnostics/Monitoring 211

      12.2.3 Engine/Drive Train 211

      12.2.4 Comfort, Convenience and Security 212

      12.3 Home Appliances 212

      12.4 Aerospace 213

      12.4.1 Turbulence Control 213

      12.5 Environmental Monitoring 213

      12.6 Process Engineering 214

      12.7 Medical Diagnostic 215

      References 216

Index 217

Chandan Kumar Sarkar is professor of electronics and telecommunications engineering at Jadavpur University, Calcutta, India. He received his BSc (Hons)and MSc in physics from Aligarh Muslim University, Aligarh, India, in 1975 and earned his PhD from Calcutta University in 1979 and DPhil from the University of Oxford, Oxford, United Kingdom, in 1984. He has been teaching for more than 30 years. PA\In 1980, Dr. Sarkar received the British Royal Commission Fellowship to work at the University of Oxford. He worked as a visiting scientist at the Max Planck Laboratory in Stuttgart, Germany, as well as at Linkoping University in Sweden. Dr. Sarkar also taught in the Department of Physics at the University of Oxford and was a distinguished lecturer of the IEEE EDS. PA\Sarkar has served as a senior member of the IEEE and was chair of the IEEE EDS chapter, Calcutta Section, India. He served as fellow of the IETE, fellow of IE (India), fellow of WBAST, and member of the Institute of Physics, United Kingdom. PA\Dr. Sarkar's research interests include semiconductor materials and devices, VLSI devices, and nanotechnology. He has published and presented more than 300 research papers in international journals and conferences and also mentored 21 PhD students.

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