First International Conference and School on Nanoscale/Molecular Mechanics: Maui, HI; May 2002 School Lecture/Tutorial on Micromechanical Signal Processors for Low-Power Communications Instructor: Clark T.-C. Nguyen Center for Integrated Wireless Microsystems Dept. of Electrical Engineering and Computer Science University of Michigan Ann Arbor, Michigan 48109-2122 http://www.eecs.umich.edu/~ctnguyen
Outline Miniaturization of Transceivers the need for high-q High-Q Micromechanical Resonators Micromechanical Circuits micromechanical filters micromechanical mixer-filters micromechanical switches Using MEMS in Comm. Receivers direct replacement of passives trade Q (or selectivity) for power MEMS-based receiver architecture Research Issues Conclusions
Frequency Division Multiplexed Communications Information is transmitted in specific frequency channels within specific bands Transmitted Power Band GSM Band Adj. Band DCS1800 Band Frequency
Frequency Division Multiplexed Communications Information is transmitted in specific frequency channels within specific bands Transmitted Power Band Filter GSM Band Adj. Band DCS1800 Band Frequency
Frequency Division Multiplexed Communications Information is transmitted in specific frequency channels within specific bands Transmitted Power Band Filter GSM Band Adj. Band DCS1800 Band Need: high frequency selectivity need high-q Frequency
Need for High-Q: Selective Low-Loss Filters In resonator-based filters: high tank Q low insertion loss At right: a 0.3% bandwidth filter @ 70 MHz (simulated) heavy insertion loss for resonator Q < 5,000
Attaining High-Q Problem: IC s cannot achieve Q s in the thousands transistors consume too much power to get Q on-chip spiral inductors Q s no higher than ~10 off-chip inductors Q s in the range of 100 s Observation: vibrating mechanical resonances Q > 1,000 Example: quartz crystal resonators (e.g., in wristwatches) extremely high Q s ~ 10,000 or higher (Q ~ 10 6 possible) mechanically vibrates at a distinct frequency in a thickness-shear mode
Miniaturization of Transceivers High-Q functionality required by oscillators and filters cannot be realized using standard IC components use off-chip mechanical components SAW, ceramic, and crystal resonators pose bottlenecks against ultimate miniaturization
So Many Passive Components! The total area on a printed circuit board for a wireless phone is often dominated by passive components passives pose a bottleneck on the ultimate miniaturization of transceivers Transistor Transistor Chips Chips Quartz Quartz Crystal Crystal Inductors Inductors Capacitors Capacitors Resistors Resistors IF IF Filter Filter (SAW) (SAW) RF RF Filter Filter (ceramic) (ceramic) IF IF Filter Filter (SAW) (SAW)
Surface Micromachining Fabrication steps compatible with planar IC processing
Post-CMOS Circuits+μMechanics Integration Completely monolithic, low phase noise, high-q oscillator (effectively, an integrated crystal oscillator) [Nguyen, Howe] Oscilloscope Output Waveform To allow the use of >600 o C processing temperatures, tungsten (instead of aluminum) is used for metallization
Target Application: Integrated Transceivers Off-chip high-q mechanical components present bottlenecks to miniaturization replace them with μmechanical versions
Micromechanical Resonators
Vertically-Driven Micromechanical Resonator To date, most used design to achieve VHF frequencies Smaller mass higher frequency range and lower series R x
HF μmechanical CC-Beam Resonator Surface-micromachined, POCl 3 -doped polycrystalline silicon Extracted Q = 8,000 (vacuum) Freq. and Q influenced by dc-bias and anchor effects
Anchor Dissipation in Fixed-Fixed Beams f o Q
92 MHz Free-Free Beam μresonator Free-free beam μmechanical resonator with non-intrusive supports reduce anchor dissipation higher Q
92 MHz Free-Free Beam μresonator Free-free beam μmechanical resonator with non-intrusive supports reduce anchor dissipation higher Q
70MHz Nano-Scale Bulk Si Resonators
Scaling-Induced Performance Limitations Mass Loading Noise Contaminant Molecules [J. R. Vig, 1999] Temperature Fluctuation Noise Photons f = o 1 2π k m mass ~10-13 kg volume ~10-15 m 3 Differences in rates of adsorption and desorption of contaminant molecules mass fluctuations frequency fluctuations Absorption/emission of photons temperature fluctuations frequency fluctuations Problem: If dimensions too small phase noise significant! The smaller the resonator smaller the power handling
156 MHz Radial Contour-Mode Disk μmechanical Resonator Below: Balanced radial-mode disk polysilicon μmechanical resonator (34 μm diameter) μmechanical Disk Resonator Metal Electrode R Design/Performance: R=17μm, t=2μm d=1,000å, V P =35V f o =156.23MHz, Q=9,400 Metal Electrode Anchor f o =156MHz Q=9,400 [Clark, Hsu, Nguyen IEDM 00]
Desired Filter Characteristics Small shape factor generally preferred
Micromechanical Circuits
Micromechanical Circuits MEMS for Wireless Communications A single mechanical beam can t really do much on its own But use many mechanical beams attached together in a circuit, and attain a more complex, more useful function Input Force F i Output Displacement x o F i x o t Key Design Property: High Q t
Desired Filter Characteristics Small shape factor generally preferred
MEMS for Wireless Communications Micromechanical Filter Circuit
Ideal Spring-Coupled μmechanical Filter Symmetric Mode Anti-Symmetric Mode BW ~ k s12 k r c r 1 m r1 k s 12 m r2 c r 2 k r 1 k r 2
MEMS for Wireless Communications Micromechanical Filter Circuit
HF Spring-Coupled Micromechanical Filter
MEMS for Wireless Communications High-Order μmechanical Filter
Electromechanical Mixing MEMS for Wireless Communications ω o =ω IF Electrical Signal Input Filter Response ω IF ω LO ω RF ω Mechanical Signal Input ω IF ω LO ω RF ω
Micromechanical Mixer-Filter [Wong, Nguyen IEDM 98]
Micromechanical Switch MEMS for Wireless Communications Operate the micromechanical beam in an up/down binary fashion [C. Goldsmith, 1995] Performance: I.L.~0.1dB, IIP3 ~ 66dBm (extremely linear) Issues: switching voltage ~ 20V, switching time: 1-5μs
MEMS-Based Receivers
MEMS-Based Receiver Architecture Most Direct Approach: replace off-chip components (in orange) with μmechanical versions (in green) L 1 ~2dB 1 ~2dB L 3 ~6dB 3 ~6dB L 5 ~12dB 5 ~12dB NF NF = 8.8dB 8.8dB Higher Q L 1 ~0.3dB 1 ~0.3dB L 3 ~0.5dB 3 ~0.5dB L 5 ~1dB 5 ~1dB Replace with MEMS Antenna Antenna Diversity Diversity for for resilience resilience against against fading fading Obvious Benefit: substantial size reduction NF NF = 2.8dB 2.8dB
MEMS-Based Receiver Front-End Received Power Desired Signal Conventional RF Filter Q res ~400 ω LO ω RF Frequency
MEMS-Based Receiver Front-End Received Power Desired Signal Conventional RF Filter Q res ~400 Out-of-Band Interferers Removed ω LO ω RF Frequency
MEMS-Based Receiver Front-End Received Power Desired Signal μmechanical Q res ~400 RF Filter Q res ~10,000 ω LO ω RF Frequency
MEMS-Based Receiver Front-End Provides Provides robustness robustness against against jammers jammers and and extends extends battery battery lifetime lifetime Received Power Desired Signal μmechanical Q res ~400 RF Filter Q res ~10,000 All Interferers Removed ω LO ω RF Frequency
MEMS-Based Receiver Front-End Provides Provides robustness robustness against against jammers jammers and and extends extends battery battery lifetime lifetime Reduces Reduces loss loss and and removes removes power power consumption consumption by by active active devices devices Eliminates Eliminates active active phase-locking phase-locking ckt. ckt. power power consumption consumption Miniaturization Miniaturization
MEMS-Based Receiver Front-End Low Loss Eliminate the RF LNA? If possible, could enhance robustness substantially reduce RF front-end power [Nguyen, Top. Mtg. on Si IC s in RF Systems 2001]
Research Issues
Research Issues: Frequency Extension Needed diameters to achieve UHF fundamental-mode resonance frequencies using a 2μm-thick disk resonator
38.8 khz CVD Polydiamond Folded- Beam μmechanical Resonator In situ-doped polydiamond deposited via a microwave plasma reactor (methane and diborane reactants) at 540 o C 80% higher resonance frequency than polysilicon version Design/Performance: L b =160μm, W b =2μm, h=2μm, d=2μm, V P =25V, f o =38.8kHz Q=19,500 f o =38.8kHz Q=19,500 [Wang, Butler, Nguyen, MEMS 02]
Research Issues: Frequency Extension Needed diameters to achieve UHF fundamental-mode resonance frequencies using a 2μm-thick disk resonator Problem: geometry not the only consideration; other important factors include: impedance vs. linearity/power handling manufacturing issues: trimming, vacuum encapsulation, MEMS/transistor integration thermal and aging stability
Research Issue: Termination Resistance Need to minimize R Q for impedance matching want: V P = large A o = large d = small
Small Electrode-to-Resonator Gaps
Design Issue: Process Tolerances Process variations can lead to distortion in the filter passband
μmechanical Filter Passband Correction [Wang, Nguyen, 1999] Problems: too many interconnect leads, Δf small at VHF Need: a permanent frequency trimming technique
Research Issue: Frequency Trimming For banks of filters or resonators need automated trimming on a massive scale, preferably voltage-activated Localized Annealing: current through structure heats it like a filament extremely fast thermal time constants allow for ultra-rapid annealing 16 ppm f o shift per anneal pulse [Wang, Wong, Hsu, Nguyen Transucers 97]
Research Issue: Thermal Stability [Wang, Yu, Nguyen 2000] Need temperature compensation or control methods
Vacuum Encapsulation Below: localized heated bonding to seal a vacuum cap over a released micromechanical resonator Schematic of the Bonding Encapsulation Procedure Broken Glass Cap V anneal Glass Cap Microcavity Q 3000 2750 µheater and Aluminum Solder 2500 2250 80 weeks at 25 mtorr 2000 0 2 4 6 8 10 12 [Cheng, Hsu, Lin, Nguyen, Najafi MEMS 2000] Weeks
Thermal Stability Comparison 1.7ppm/ o C Poly-Si μresonator -17ppm/ o C Thermal stability of poly-si micromechanical resonator is 10X worse than the worst case of AT-cut quartz crystal
Geometric-Stress Compensation Use a temperature dependent mechanical stiffness to null frequency shifts due to Young s modulus thermal dep. [Hsu and Nguyen, IEDM 00] [W.-T. Hsu, et al., IEDM 00] Problems: stress relaxation compromised design flexibility [Hsu, Nguyen IEDM 2000]
Electrical Spring Constant Displacement-dependent E fields generate motional force in quadrature with the input force electrical stiffness Effective electrical stiffness subtracts from mechanical stiffness, causing frequency shift DC-Bias Gap Spacing
Operation of Stiffness Compensation To implement stiffness compensation: add a top electrode of a material with a larger thermal expansion coefficient than resonator material design such that the top electrode-to-resonator gap spacing increases with increasing temperature T increase gap increase freq. increase counteract freq. decrease caused by Young's modulus [Hsu, Nguyen MEMS 02] V p V c TC f of Uncompensated CC-beam ( ) Compensation via electrical stiffness (+)
SEM of 10MHz Stiffness-Compensated Resonator with with Slitted Top Electrode Top Electrode Driving Anchors Electrode [Hsu, Nguyen MEMS 02] Anchors Anchors Anchors Slits Anchors Anchors Resonator Beam
1500 1000 500 0-500 -1000-1500 MEMS for Wireless Communications Measured Δf/f vs. Temperature for Electrical Stiffness-Compensated μresonators Design/Performance: f o =10MHz, Q=4,000 V P =8V, h e =4μm d o =1000Å, h=2μm W r =8μm, L r =40μm [Hsu, Nguyen MEMS 02] Δf/f [ppm] 0.24ppm/ o C 300 320 340 360 380 Temperature [K] V P -V C Slits help to release the stress generated by lateral thermal expansion linear TC f curves 0.24ppm/ o C!!! 16V 14V 12V 10V 9V 8V 7V 6V 4V 2V 0V CC
Summary of μresonator TC f s 1.7ppm/ o C Elect.-Stiffness Compensation 0.24ppm/ o C Poly-Si μresonator Geom.-Stress Compensation 2.5ppm/ o C With more accurate V C, it may be possible to completely null the TC f using electrical stiffness compensation
Conclusions Via enhanced selectivity on a massive scale, micromechanical circuits using high-q elements have the potential for shifting communication transceiver design paradigms, greatly enhancing their capabilities Advantages of Micromechanical Circuits: orders of magnitude smaller size than present mechanical resonator devices better performance than other single-chip solutions potentially large reduction in power consumption alternative transceiver architectures that maximize the use of high-q, frequency selective devices for improved performance For more information: http://www.eecs.umich.edu/~ctnguyen
Conclusions Compelling parallels between MEMS and integrated transistor signal processor technologies: Before 1960: discrete transistor circuits wired on boards with limited functionality After IC s: VLSI CPU s and memory circuits have revolutionized the way things are done Today: discrete mechanical circuits coupled by welded wires with limited functionality With VLSI Micromechanical Signal Processors: functions never before possible now realizable via a combination of transistor and mechanical circuits? a functional and system architectural revolution reminiscent of the IC revolution? potential for true revolution? but there is much work yet to be done
Acknowledgments Numerous authors referenced throughout Former and present graduate students, especially Kun Wang, Frank Bannon III, and Ark-Chew Wong, who are largely responsible for the micromechanical filter work, and Wan-Thai Hsu and Mustafa Demirci, who are largely responsible for the resonator work My government funding sources: mainly DARPA and an NSF Engineering Research Center
MEMS for Wireless Communications Selected Readings [1], Transceiver front-end architectures using vibrating micromechanical signal processors (invited), Dig. of Papers, Topical Meeting on Silicon Monolithic Integrated Circuits in RF Systems, Sept. 12-14, 2001, pp. 23-32. [2], L. P.B. Katehi, and G. M. Rebeiz, Micromachined devices for wireless communications (invited), Proc. IEEE, vol. 86, no. 8, pp. 1756-1768, Aug. 1998. [3], Frequency-selective MEMS for miniaturized lowpower communication devices (invited), IEEE Trans. Microwave Theory Tech., vol. 47, no. 8, pp. 1486-1503, Aug. 1999. [4] F. D. Bannon III, J. R. Clark, and, High frequency micromechanical filters, IEEE J. Solid-State Circuits, vol. 35, no. 4, pp. 512-526, April 2000. [5] K. Wang and, High-order medium frequency micromechanical electronic filters, IEEE/ ASME J. Microelectromech. Syst., vol. 8, no. 4, pp. 534-557, Dec. 1999. [6] K. Wang, A.-C. Wong, and, VHF free-free beam high-q micromechanical resonators, IEEE/ASME J. Microelectromech. Syst., vol. 9, no. 3, pp. 347-360, Sept. 2000.
Selected Readings (cont.) MEMS for Wireless Communications [7] J. R. Clark, W.-T. Hsu, and, High-Q VHF micromechanical contour-mode disk resonators, Technical Digest, IEEE Int. Electron Devices Meeting, San Francisco, California, Dec. 11-13, 2000, pp. 399-402. [8] J. R. Vig and Y. Kim, Noise in microelectromechanical system resonators, IEEE Trans. Utrason. Ferroelec. Freq. Contr., vol. 46, no. 6, pp. 1558-1565, Nov. 1999. [9] A. N. Cleland and M. L. Roukes, Fabrication of high frequency nanometer scale mechanical resonators from bulk Si crystals, Appl. Phys. Lett., 69 (18), pp. 2653-2655, Oct. 28, 1996. [10] M. L. Roukes, Nanoelectromechanical systems, Tech. Digest, 2000 Solid-State Sensor and Actuator Workshop, Hilton Head Island, South Carolina, June 4-8, 2000, pp. 367-376. [11] A.-C. Wong, H. Ding, and, Micromechanical mixer+filters, Technical Digest, IEEE International Electron Devices Meeting, San Francisco, California, Dec. 6-9, 1998, pp. 471-474. [12] W.-T. Hsu, J. R. Clark, and, Mechanically temperature compensated flexural-mode micromechanical resonators, Technical Digest, IEEE Int. Electron Devices Meeting, San Francisco, California, Dec. 11-13, 2000, pp. 493-496.
Selected Readings (cont.) MEMS for Wireless Communications [13] W.-T. Hsu and, Stiffness-compensated temperature-insensitive micromechanical resonators, Tech. Digest, 2002 IEEE Int. Micro Electro Mechanical Systems Conf., Las Vegas, Nevada, Jan. 20-24, 2002, pp. 731-734. [14] K. Wang, A.-C. Wong, W.-T. Hsu, and, Frequencytrimming and Q-factor enhancement of micromechanical resonators via localized filament annealing, Dig. of Technical Papers, 1997 International Conference on Solid-State Sensors and Actuators, Chicago, Illinois, June 16-19, 1997, pp. 109-112. [15] Y.-T. Cheng, W.-T. Hsu, L. Lin,, and K. Najafi, Vacuum packaging using localized aluminum/silicon-to-glass bonding, Tech. Digest, 14th Int. IEEE Micro Electro Mechanical Systems Conference, Interlaken, Switzerland, Jan. 21-25, 2001, pp. 18-21. [16] and R. T. Howe, An integrated CMOS micromechanical resonator high-q oscillator, IEEE J. Solid-State Circuits, vol. 34, no. 4, pp. 440-445, April 1999.