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28 Understanding Smart SensorsSacrificiallayerSubstrateIsolationlayerStructurallayerSubstrateMicromechanicalstructureSubstrate(a)Layer 3Layer 2Layer 1(b)Figure 2.6 Surface micromachining: (a) process steps and (b) example of actual 3-layerstructure.
Micromachining 294-inch (100-mm) wafer that is 0.015 inch (0.38 mm) thick [15]. A siliconwafer serves as the substrate for the mechanical structure. The trampolineshapedmiddle layer is suspended by four support arms. That movable structureis the seismic mass for the accelerometer. The upper and lower polysilicon layersare fixed plates for the differential capacitor. The PSG is sacrificially etchedby an isotropic etch, such as hydrofluoric acid (HF).Because of the small spacing (≈2 mm) possible with surface micromachining,new issues arise that affect both the sensor design and the manufacturingprocess. Squeeze-film damping, stiction, and particulate control must beaddressed in each new design. The next three sections describe those areas andsome approaches being used to deal with them.2.4.1 Squeeze-Film DampingThe movement of structures separated by only a few microns can be greatlyaffected by the actual spacing and ambient (gas or vacuum) between the structures.That effect is known as squeeze-film damping. Squeeze-film dampingcan be significant in bulk micromachined capacitive structures, in which closerspacing is needed to achieve higher capacitance values. It is inherent in surfacemicromachining, in which spacing is only a few microns. For a particularstructure, the gas that separates the layers has a viscous damping constantthat increases with the inverse cube of the spacing [16]. Incorporating holesin the surface micromachined structure allows the damping to be tuned fordesired characteristics. Holes also provide distributed access for the etchantto reduce etching time and the possibility of overetching portions of thestructure.2.4.2 StictionStiction (static friction) is a phenomenon that occurs in surface micromachiningresulting from capillary (van der Waals) forces generated during the wetetching of the sacrificial layers [17]. Under certain fabrication conditions, themicrostructures can collapse and permanently adhere to the underlying substrate.The failure is catastrophic and must be prevented to achieve highprocessyield and a reliable design. Preventing the top structure from contactingthe bottom structure requires minimizing the forces acting on the device whenthe liquid is removed, or minimizing the attractive forces between the structuresif they contact each other. Techniques used to prevent stiction depend onthe manufacturer, the product design, and the process flow [17]. Many othersolutions for stiction are being developed.
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Getting Sensor Information Into the
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5Using MCUs/DSPs to Increase Sensor
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Using MCUs/DSPs to Increase Sensor
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7Control TechniquesA machine is int
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Control Techniques 151Table 7.1Type
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Control Techniques 153otherwise tak
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Control Techniques 155consisting of
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Control Techniques 157Supplied byap
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Control Techniques 159InputX1Synaps
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Control Techniques 1617.6 Adaptive
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Control Techniques 163ObserverHB−
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Control Techniques 165linear-vector
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Control Techniques 1677.7.2 Combine
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Control Techniques 169Domain-type s
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Control Techniques 171[19] De Silva
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8Transceivers, Transponders, andTel
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Transceivers, Transponders, and Tel
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9MEMS Beyond Sensors“And these ot
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MEMS Beyond Sensors 203would not ex
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MEMS Beyond Sensors 205PyrexSilicon
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MEMS Beyond Sensors 207(a)IFluxIIMa
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MEMS Beyond Sensors 209InletThermop
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MEMS Beyond Sensors 211Figure 9.8 A
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MEMS Beyond Sensors 2139.3.2 Microo
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MEMS Beyond Sensors 215Over-range p
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MEMS Beyond Sensors 217+ 10°− 10
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MEMS Beyond Sensors 219achieved wit
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MEMS Beyond Sensors 221Methods for
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MEMS Beyond Sensors 223Figure 9.19
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MEMS Beyond Sensors 225[21] Tortone
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10Packaging, Testing, and Reliabili
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Packaging, Testing, and Reliability
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11Mechatronics and Sensing SystemsP
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12Standards for Smart SensingThe la
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Standards for Smart Sensing 275Netw
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Standards for Smart Sensing 277NCAP
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Standards for Smart Sensing 287Tabl
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Standards for Smart Sensing 2911 0
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Standards for Smart Sensing 293obje
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Standards for Smart Sensing 295Mult
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13The Implications of Smart SensorS
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14The Next Phase of Sensing Systems
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The Next Phase of Sensing Systems 3
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List of Acronyms and Abbreviations3
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List of Acronyms and Abbreviations
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Glossaryablationvaporization of mat
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Glossary 353buscalibrationconnectio
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Glossary 355end point straight line
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Glossary 357data transmission and p
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Glossary 359multiplexer device that
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Glossary 361repeatability the maxim
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Glossary 363spread spectrum techniq
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Glossary 365transducer a device con
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Selected BibliographyBooks and Jour
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Selected Bibliography 369Northeaste
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Selected Bibliography 371Miscellane
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About the AuthorRandy Frank is a te
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Index4-to 20-mA signal transmitter,
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Index 377definitions, 119-20introdu
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Index 379HVAC, 318-19MCU with integ
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Index 381Linearization, 108Linear p
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Index 383in chemical measurements,
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Index 385Ratiometricity, 56Reactive
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Index 387IC fabrication, 19micromec
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Index 389Transducer-independent int
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