Advanced RF MEMS Edited by STEPAN LUCYSZYN Imperial College London n CAMBRIDGE UNIVERSITY PRESS
Contents List of contributors Preface List of abbreviations page xiv xvii xx Introduction 1 1.1 Introduction 1 1.1.1 Defining terms 1 1.1.2 Enabling technology roadmap 2 1.2 Fabrication technologies 4 1.3 Electromechanical actuation 5 1.4 Generic RF MEMS components 8 1.4.1 Switches 8 1.4.2 Variable capacitors 10 1.4.3 Antennas 11 1.5 Circuits and subsystems 15 1.6 Conclusions 18 References 19 Electromechanical modelling of electrostatic actuators 23 2.1 Introduction 23 2.2 Energy methods and the equilibrium/momentum equation 24 2.3 Static equilibrium and stability 26 2.3.1 Actuators with one degree of freedom 26 2.3.2 Actuators with several degrees of freedom 27 2.3.3 Distributed systems 29 2.3.4 Numerical methods 30 2.4 Dynamic response of electrostatic actuators 32 2.4.1 Dynamic pull-in of a one-dof system 32 2.4.2 Dynamic pull-in of the clamped-clamped beam actuator 35 2.4.3 Switching time of electrostatic actuators 37 2.5 Conclusions 38 References 38
viii Contents Switches and their fabrication technologies 41 3.1 3.2 3.3 3.4 ; Introduction Substrate materials and fabrication technologies Actuation principles 3.3.1 Capacitive switches with electrostatic actuation 3.3.2 Ohmic switches with electrostatic actuation 3.3.3 Switches with piezoelectric actuation 3.3.4 Switches with electrothermal and electromagnetic actuation Switch building blocks 3.4.1 Metallisation 3.4.2 Capacitive switch dielectrics 3.4.3 Ohmic switch contacts 3.4.4 Sacrificial layers 3.4.5 Moveable structures References Niche switch technologies 4.1 4.2 4.3 4.4 Introduction Latching switches 4.2.1 Magnetically actuated bistable switches 4.2.2 Electrothermally actuated bistable switches 4.2.3 Electrostatic bistable switch 4.2.4 Mechanically latching switches Multiway switches 4.3.1 SPDT switches 4.3.2 SP4T switches 4.3.3 SP6T switches 4.3.4 SP8T switches 4.3.5 SP9T switches and SP48T modules 4.3.6 DPDT switches 4.3.7 Switch matrices High-power switches 4.4.1 Additional hold electrodes 4.4.2 Beam cross section 4.4.3 Switching arrays 4.4.4 Contact force 4.4.5 Materials 4.4.6 Non-beam architectures 4.4.7 Unconventional 3D power switch 4.4.8 RF power measurements References 41 42 43 45 51 55 56 58 58 60 61 63 65 67 73 73 73 74 75 77 79 83 84 88 88 88 90 92 94 95 95 95 96 97 98 98 100 102 105
Contents Reliability 109 5.1 Introduction 109 5.1.1 Terminology 110 5.1.2 Failure- and application-driven reliability methodology 111 5.2 Failure mechanisms in RF MEMS 112 5.2.1 Creep/stress relaxation 112 5.2.2 Temperature- and stress-induced plastic deformation 115 5.2.3 Temperature-induced elastic deformation 117 5.2.4 Fatigue 119 5.2.5 Suction 120 5.2.5.1 Introduction 120 5.2.5.2 Microwelding 120 5.2.5.3 Dielectric charging 121 5.2.6 Electromigration 127 5.2.7 Self-actuation 127 5.2.7.1 Lorentz forces 127 5.2.7.2 Self-Biasing due to RF power 128 5.2.7.3 Self-actuation due to drop or shock 129 5.2.8 Fly-catching effect 130 5.2.9 Outgassing and adsorption 130 5.2.10 Mechanical and acoustic coupling 132 5.3 Conclusions 132 5.4 Acknowledgments 132 References 133 Dielectric Charging 140 6.1 Introduction 140 6.2 Dielectric charging in MEMS 140 6.2.1 Capacitance-voltage characteristic 141 6.2.2 Switch-ON and switch-off capacitance transients 146 6.2.3 Shift in pull-in and pull-out voltages 151 6.2.4 Lifetime tests, material and environmental effects 159 6.2.5 Dielectric charging in MIM capacitors 163 6.2.6 Kelvin probe force microscopy 169 6.3 Dielectric polarisation 172 6.3.1 Dipolar polarisation 174 6.3.2 Space-charge polarisation 176 6.3.3 Interfacial polarisation 177 6.4 Charge injection mechanisms 178 6.4.1 Trap-assisted tunnelling 179 6.4.2 Poole-Frenkel process 180 6.4.3 Schottky injection 181 6.5 Conclusions 181 References 182
x Contents Stress and thermal characterisation 188 7.1 Introduction 188 7.2 Theoretical background 188 7.2.1 Stress tensor 188 7.2.2 Strain tensor 190 7.2.3 Thermal transfer 192 7.2.4 Origin of stress and temperature 193 7.3 Stress and temperature effects 194 7.3.1 Temperature dependence of resonance frequency in BAW resonators 194 7.3.2 Thermoelastic deformations in tuneable capacitors 196 7.4 Stress and temperature measurement 197 7.4.1 X-ray diffraction for stress determination 198 7.4.2 Infrared thermography for temperature determination 200 7.4.3 Other methods 203 7.5 Conclusions 203 References 203 High-power handling 205 8.1 Introduction 205 8.2 RF power handling related phenomena 206 8.2.1 Electromigration 206 8.2.2 Self-biasing and RF latching 208 8.2.3 Electromagnetic-induced thermoelectromechanical effects 214 8.2.3.1 Experimental investigation of self-heating due to RF power 214 8.2.3.2 Numerical investigation of self-heating due to RF power 221 8.3 Overview of power handling RF MEMS components 224 8.3.1 Capacitive switches and varactors 224 8.3.2 Ohmic contact switches 226 8.4 Conclusions 227 References 229 Packaging 232 9.1 Introduction 232 9.2 Zero-level packaging 233 9.2.1 Design considerations and performance 235 9.2.2 Technologies and materials for the zero-level package 248 9.2.2.1 Thin-film encapsulation 248 9.2.2.2 Chip capping 251 9.3 Package integration 258 9.3.1 Multiple MEMS in a single package 258
Contents xi 9.3.2 Integrated RF MEMS 259 9.3.2.1 System-on-chip versus system-in-package 259 9.3.2.2 RF MEMS planar system-in-package 260 9.3.2.3 RF MEMS 3D system-in-package 261 9.4 Conclusions 264 References 264 10 Impedance tuners and tuneable filters 271 10.1 Introduction 271 10.2 Impedance tuners 272 10.2.1 Stub-tuner design 272 10.2.1.1 Stub topologies 272 10.2.1.2 Tuning methods 273 10.2.2 Stub-based impedance tuners and matching networks 276 10.2.3 Distributed impedance tuners and matching networks 280 10.2.3.1 Slug tuners 281 10.2.3.2 DMTL tuners 282 10.2.3.3 DGS-based tuners 282 10.2.3.4 Other switchable tuner architectures 285 10.2.4 Discussion on impedance tuners and matching networks 285 10.3 Tuneable filters 285 10.3.1 Tuning technologies for filters 285 10.3.2 RF MEMS-based filters 287 10.3.2.1 Reconfigurable filter design basics 288 10.3.2.2 Analogue versus digital tuning 292 10.3.2.3 Discussion on future challenges for filter designers 297 10.4 Conclusions 301 References 301 11 Phase shifters and tuneable delay lines 307 11.1 Introduction 307 11.1.1 Definition and applications 307 11.1.2 Technologies 308 11.1.3 Specifications 309 11.2 Reflection-type phase shifters 310 11.2.1 Theoretical analysis 310 11.2.1.1 Capacitive load 311 11.2.1.2 Series LC load 312 11.2.2 Directional couplers 314 11.2.3 Analogue implementations 315 11.2.4 Digital implementations 316 11.2.5 Phase variation and bandwidth 316 11.2.6 State of the art 317 11.3 Phase shifters based on LC networks 317
xii Contents 11.3.1 Low/high-pass Networks 318 11.3.2 All-pass networks 319 11.3.3 State of the art 322 11.4 Loaded-line phase shifters 323 11.4.1 Theoretical analysis 323 11.4.2 Practical implementation and state of the art 326 11.5 Switched delay-line phase shifters 327 11.5.1 Principle and design issues 327 11.5.2 State of the art 328 11.6 Distributed loaded-line phase shifters 329 11.6.1 Theoretical analysis 329 11.6.2 Practical implementation 331 11.6.3 State of the art 333 11.7 General issues 333 11.7.1 Power handling 333 11.7.2 Noise and linearity 335 11.8 Conclusions 336 References 337 12 Reconfigurable architectures 343 12.1 Introduction 343 12.2 Reconfigurable radios 344 12.2.1 General architectures 344 12.2.2 Handset front-ends 344 12.2.3 Base stations 347 12.2.4 Multiband and tracking receivers 348 12.3 Reconfigurable antennas 348 12.3.1 Beam-steering antennas 348 12.3.1.1 Phased-array antennas 349 12.3.1.2 Reflectarray antennas 350 12.3.1.3 Lenses, grids and frequency-selective surfaces 351 12.3.1.4 Switched-diversity antenna arrays 351 12.3.2 Handset antennas 352 12.4 Measurement applications 353 12.4.1 Multifunctional RFOW probes 353 12.5 Conclusions 355 References 356 13 Industry roadmap for RF MEMS 359 13.1 Introduction 359 13.2 Roadmap of RF MEMS components 360 13.2.1 RF MEMS switches 360 13.2.1.1 Current state of the art and potential improvements 361 13.2.1.2 RF MEMS switches versus alternative technologies 361
Contents xiii 13.2.1.3 Switch applications roadmap 363 13.2.2 Tuneable capacitors 365 13.2.2.1 Current state of the art and potential for improvements 365 13.2.2.2 Alternative technologies 366 13.2.2.3 Capacitor applications roadmap 367 13.2.3 Emerging RF MEMS and RF NEMS components 369 13.2.3.1 Tuneable inductors 369 13.2.3.2 CNT-based RF NEMS 370 13.3 Applications roadmap 371 13.3.1 RF MEMS for mobile and wireless systems 371 13.3.1.1 Mobile handsets 371 13.3.1.2 Base stations 378 13.3.1.3 Wireless interconnections 383 13.3.2 RF MEMS for road transport applications 386 13.3.2.1 Automotive radar 386 13.3.2.2 Roofantennas 390 13.3.3 RF MEMS for aeronautics 393 13.3.4 RF MEMS for satellites 395 13.3.4.1 Potential applications of RF MEMS in satellites 395 13.3.4.2 Roadmap ofrf MEMS for satellites 397 13.4 Implementation of the roadmap 398 13.4.1 Modelling and design 398 13.4.2 Materials and processes 400 13.4.3 Heterogeneous integration, assembly and packaging 400 13.4.4 Testing, characterisation and reliability 400 13.5 Conclusions 401 References 402 Author biographies 404 Index 409