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RF MEMS Capacitors and Variable Capacitors – the Future of Wireless Communication Before We Begin… Amro M. Elshurafa May 2013 RF MEMS Capacitors and Variable Capacitors – The Future of Wireless Communication Before we begin… You are free to use this presentation as you see fit. Any publications, in the form of slides, conference papers, journal articles, technical reports, or otherwise, in which these slides will be used (in their original or modified format) should cite one or more of the following references: [+] Amro M. Elshurafa et al., "Low voltage puzzle-like fractal MEMS variable capacitor suppressing pull- in," IET/IEEE Micro & Nano Letters, Vol. 7, No. 9, pp. 965-969, 2012. [+] Amro M. Elshurafa et al., "Differential RF MEMS Interwoven Capacitor Immune to Residual Stress Warping," IET/IEEE Micro & Nano Letters, Vol. 7, No. 7, pp. 658-661, 2012. [+] Amro M. Elshurafa et al., "Two-Layer RF MEMS Fractal Capacitors in PolyMUMPS for S-Band Applications," IET/IEEE Micro & Nano Letters, Vol. 7, No. 5, pp. 419-421, 2012. [+] Amro M. Elshurafa et al., "A Low Voltage RF MEMS Variable Capacitor with a Linear C-V Response," IET/IEEE Electronics Letters, Vol. 48, No. 7, pp. 392-393, 2012. [+] Amro M. Elshurafa et al., "RF MEMS Fractal Capacitors with High Self Resonant Frequencies," IEEE JMEMS, Vol. 21, No. 1, pp. 10-12, 2012. [+] Amro M. Elshurafa et al., "MEMS Variable Capacitance Devices Utilizing the Substrate: I. Novel Devices with Customizable Tuning Range," Journal of Micromechanics and Microengineering, Vol. 20, No. 4, 045027 (8pp), 2010. [+] Amro M. Elshurafa et al., "Effects of Non-uniform Nanoscale Deflections on Capacitance in RF MEMS Parallel Plate Variable Capacitors," Journal of Micromechanics and Microengineering, Vol. 18, No. 4, 040512 (11pp), 2008. [+] Amro M. Elshurafa et al., "Finite Element Modeling of Low Stress Suspension Structures and Applications in RF MEMS Parallel Plate Variable Capacitors,“ IEEE Transactions of Microwave Theory and Techniques, Vol. 54, No. 5, pp. 2211-2219, 2006. 2 Before we begin… This presentation was prepared in May 2013. Publications, research, and results reported afterwards will not be reflected herein. About the author: Amro Elshurafa obtained his PhD in 2008 in electrical engineering with a focus on RF MEMS variable capacitors. Amro is a registered professional engineer (PEng) and is a senior member of the IEEE. He can be contacted via email at [email protected]. 3 Agenda What is MEMS and RF MEMS RF MEMS Capacitors RF MEMS Variable Capacitors Simulation Measurements State of the Art 4 What is MEMS? • MEMS abbreviates Micro Electro Mechanical Systems • Integrating sensors and actuators possessing dimensions ranging from 1mm to 1μm and relying on electrical, mechanical, optical, chemical, etc, phenomena. • You can hear NEMS, (similar to RF, it is no longer radio frequencies; microelectronics also is nano now!). • Hair diameter thickness is ~100μm. • Strong emphasis on fabrication. • If coupled with IC, the sky is the limit. 5 Most Famous Applications • Printer Ink Jet Nozzles • Inertial sensors – Accelerometers (1D acceleration meter) – Gyroscope (rotation rate meter) – Applications in: air bags, Wii, game arcades, iPhones, Samsung phones, satellites, missiles, etc. • Biomedical medication dispensers (microfluidics). • RF switch array in mobile phones (2012). 6 Most Famous Companies • AD • HP • TI • Siemens • Bosch • Xerox • GE • STMicroelectronics • Qualcomm • Cavendish • WiSpry • Omron • Raytheon • Intel (classified) • This list is different than pure MEMS design, fab, and fabless companies. 7 Fabrication – Briefly Deposit and pattern Deposit and pattern structural layer sacrificial layer Some Substrate 1 2 3 Etch away sacrificial layer to get free standing structures Repeat steps 2 and 3 again 5 4 8 Foundry – Standard vs. Non-standard • The PolyMUMPS process from MEMSCAP, NC, USA. • Has been in business since 1992. • Very reliable and robust. • Used by hundreds of groups throughout the world in countless applications. • What about CMOS? Can we have that? 9 Foundry – Standard vs. Non-standard 10 Why RF MEMS? Inductors Capacitors IC’s Resistors It is the high-Q passive components that are hindering miniaturization! Slide by Dr. Clark Nguyen at University of California at Berkeley. 11 Why RF MEMS? • Ceramic filters: • Made of piezoelectric ceramics • Frequency is adjusted by thickness and size of the ceramic element • Typical dimensions are: ~20mm × ~10mm × ~5mm • Extremely dimensions sensitive: a ±0.1mm dimension tolerance yields a frequency accuracy of ±220MHz → Expensive • No further opportunities for further miniaturization 12 MEMS Benefit: General Less power consumption Discrete Better performance ICs electronics High volume fabrication Less power consumption Off-chip Better performance MEMS passives and filters High volume fabrication 13 What Can MEMS Offer? High Q filters: fo = 8.5MHz Qvac = 8,000 Qair ~ 50 Lr = 40μm F. Bannon, J. Clark, and C. Nguyen, “High Frequency Microelectromechanical IF Filters," IEEE International Electron Device Meetings, pp. 773-776, 1996. 14 What Can MEMS Offer? High Q resonators: -84 20μm ) -86 Q = 10,100 (air) -88 (dB Polysilicon -90 Electrode -92 -94 -96 -98 CVD Diamond Transmission -100 mMechanical Disk Ground 1507.4 1507.6 1507.8 1508 1508.2 Resonator Plane Frequency (MHz) J. Wang, J. Butler, T. Feygelson, and C. Nguyen, “1.51GHz nanocrystaline Diamond Micromechanical Disk Resonator with Material Mismatched Isolating Support,” IEEE Conference on Microelectromechanical Systems, pp. 641-644, 2004. 15 What Can MEMS Offer? High Q inductors: J. Zou, J. Nickel, D. Trainor, C. Liu, and J. Schutte-Aine, “Development of Vertical Planar Coil Inductors Using Plastic Deformation Magnetic Assembly,” IEEE International Microwave Symposium, pp. 193-196, 2001. Jun-Bo Yoon, Byeong-I1 Kim, Yun-Seok Choi, and Euisik Yoon, “3-D Lithography and Metal Surface Micromachinig for RF and Microwave 16 MEMS,” IEEE Microelectromechanical Systems, pp. 673-676, 2002. Varactors: CMOS vs. MEMS CMOS Varactors MEMS Varactors (reverse biased diodes) Leakage currents exists No leakage current Typical Q is 30-40, but can reach to Q can reach up to 200 – 300 50-60 Due to continuous downscaling, the Tuning ranges are high (~5 for tuning range (C /C ) is max min varactors and ~50 for switches and decreasing – Maximum ratio is 3 at even higher) millimeter-wave range. Good at low frequency (inductor loss dominates for LC tank), but No real concern lossy at millimeter-wave range K. Kwok and J. Long, “A 23-to-29 GHz Transconductor-Tuned VCO MMIC in 0.13um CMOS,” IEEE Journal of Solid State Circuits, Vol. 42, No. 12, pp. 2878-2886, 2007. 17 Why MEMS Varactors? • Internal antenna, form factor, touch screens, etc, pose real challenges (remember the death grip in iPhone4). • Technologies changing rapidly, 2.5G, 3G, 4G: many carriers and bands. Hence, antennas, filters, power amplifiers need to tune to these bands. • Dennis Yost, CEO of Cavendish, states: Theoretical 4G limit is 80Mbps, though testing shows ~8Mbps at best. • Gabriel Rebeiz: a tunable front end is the holy grail of advanced multi-mode multi-frequency mobile devices. • Paratek shipped a tunable device to Samsung (thin-film based varactor). Interestingly, RIM bought Paratek in 2012! 18 Why MEMS Varactors • The RF filters are mostly ceramic or SAW filters, and are very bulky and expensive (off-chip). • Typical dimensions are 2cm x 1cm x 0.5 cm! 19 Why MEMS Varactors • Add MEMS filters and receive whatever you want (no size limitation). You can add many filters or a tunable filter. • Quality factors > 15,000 at 1.4GHz BW = 100kHz; better than the current BW of 35MHz found in today’s phones. 20 The Ultimate Goal is: A complete MEMS-based transceiver: 21 However… • Fabrication and integration challenges: CMOS and MEMS. • Actuation voltages for MEMS varactors: 10V ~ 40V. • Lifetime and reliability despite tests have been performed for billions of cycles in lab conditions. • Temperature stability and drift. • Modeling vs. trial and error fabrication. C. T. C. Nguyen, “Mechanical Radio,” IEEE Spectrum, December 2009. 22 Publications: Numbers 1000 RF MEMS Publications 884 882 900 862 814 826 Variable Capacitor Publications 782 800 737 750 700 600 537 500 378 400 358 349 368 324 342 313 316 298 305 300 253 197 200 157 Number of Publications 147 113 135 116 82 100 59 0 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 Year Source: Engineering Village 23 Publications – Where? 3000 2750 2627 2500 2250 2000 1750 1500 1250 1000 808 750 621 462 450 500 400 Number of Publications 299 272 240 250 189 0 United China France Japan Korea Germany Canada Italy India Belguim States Country Source: Engineering Village. 24 Agenda What is MEMS and RF MEMS RF MEMS Capacitors RF MEMS Variable Capacitors Simulation Measurements State of the Art 25 A MEMS Perspective • Typical capacitors in MEMS are of the parallel-plate type: Criterion CMOS MEMS Etching Holes No Concern Concern Residual-Stress No Concern Concern Warping Availability of Metal No Concern Concern Layers Parasitics/balanced Concern Concern capability 26 Etching Holes When a sacrificial layer is present between two structural layers, it has to be removed. One way is to do wet- etching: submerge wafer in an etcher. How much time will the etching take for the example here if we assume that the etching rate is 10μm/min? 27 Etching Holes • Now, by adding etching holes throughout the large structure, the etchant will have more opportunities to penetrate through the structure. Hence, reducing the etching time required significantly. • Also called release holes and access holes. • Concerns: affect capacitance, mechanical performance, and optical performance. • What about CMOS? 28 Metal Layer Scarcity • Self explanatory! • Most MEMS processes possess several polysilicon layers but a single metal layer obtaining high Q is difficult. • What about CMOS? • In CMOS processes, there are ~9 Faraday Technology Corporation metal layers both capacitor www.design-reuse.com terminals can be metal and hence not affect Q. 29 Substrate Parasitics: Balanced/Differential Capability • In a typical parallel-plate capacitor, the bottom-plate/substrate parasitic is larger than the top-plate/substrate parasitic.
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