MEMS for Wireless Communications RF MEMS for Low-Power Communications Clark T.-C. Nguyen Center for Wireless Integrated Microsystems Dept. of Electrical Engineering and Computer Science University of Michigan Ann Arbor, Michigan 48109-2122 http://www.eecs.umich.edu/~ctnguyen
MEMS for Wireless Communications Outline • Miniaturization of Transceivers � the need for high- Q • High- Q Micromechanical Resonators • Micromechanical Circuits � micromechanical filters � micromechanical mixer-filters � micromechanical switch � micromechanical C ’s & L ’s • Using MEMS in Comm. Receivers � direct replacement of passives � trade Q (or selectivity) for power � MEMS-based receiver architecture • Conclusions
MEMS for Wireless Communications 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
MEMS for Wireless Communications 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 Quartz Quartz RF Filter RF Filter Crystal Crystal (ceramic) Inductors (ceramic) Inductors IF Filter Capacitors Transistor IF Filter Capacitors Transistor (SAW) Resistors Chips (SAW) Resistors Chips IF Filter IF Filter (SAW) (SAW)
MEMS for Wireless Communications 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
MEMS for Wireless Communications Surface Micromachining • Fabrication steps compatible with planar IC processing
MEMS for Wireless Communications Post-CMOS Circuits+ μ Mechanics 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
MEMS for Wireless Communications Target Application : Integrated Transceivers • Off-chip high- Q mechanical components present bottlenecks to miniaturization � replace them with μ mechanical versions
MEMS for Wireless Communications Micromechanical Resonators
MEMS for Wireless Communications Vertically-Driven Micromechanical Resonator • To date, most used design to achieve VHF frequencies • Smaller mass � higher frequency range and lower series R x
MEMS for Wireless Communications HF μ Mechanical 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
MEMS for Wireless Communications 92 MHz Free-Free Beam μ Resonator • Free-free beam μ mechanical resonator with non-intrusive supports � reduce anchor dissipation � higher Q
MEMS for Wireless Communications 92 MHz Free-Free Beam μ Resonator • Free-free beam μ mechanical resonator with non-intrusive supports � reduce anchor dissipation � higher Q
MEMS for Wireless Communications 156 MHz Radial Contour-Mode Disk μ Mechanical Resonator • Below : Balanced radial-mode disk polysilicon μ mechanical resonator (34 μ m diameter) Design/Performance : μ mechanical Disk R =17 μ m, t =2 μ m Resonator d =1,000Å, V P =35V Metal f o =156.23MHz, Q =9,400 Electrode R Metal Electrode f o =156MHz Q =9,400 Anchor [Clark, Hsu, Nguyen IEDM’00]
MEMS for Wireless Communications Micromechanical Circuits • 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 x o F i Key Design Property : High Q t t
MEMS for Wireless Communications HF Spring-Coupled Micromechanical Filter
MEMS for Wireless Communications High-Order μ Mechanical Filter
MEMS for Wireless Communications Nonlinear Micromechanical Circuits
MEMS for Wireless Communications Electromechanical Mixing ω o = ω IF ω IF Electrical Filter Signal Input Response ω LO ω RF ω Mechanical Signal Input ω IF ω LO ω RF ω
MEMS for Wireless Communications Micromechanical Mixer-Filter [Wong, Nguyen 1998]
MEMS for Wireless Communications Micromechanical Switch • 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: 1-5 μ s
MEMS for Wireless Communications Phased Array Antenna
MEMS for Wireless Communications Voltage-Tunable High- Q Capacitor • Micromachined, movable, aluminum plate-to-plate capacitors • Tuning range exceeding that of on-chip diode capacitors and on par with off-chip varactor diode capacitors • Challenges : microphonics, tuning range truncated by pull-in
MEMS for Wireless Communications Suspended, Stacked Spiral Inductor • Strategies for maximizing Q : � 15 μ m-thick, electroplated Cu windings � reduces series R � suspended above the substrate � reduces substrate loss
MEMS for Wireless Communications MEMS-Based Receiver Architectures
MEMS for Wireless Communications MEMS-Based Receiver Architecture • Most Direct Approach : replace off-chip components (in orange) with μ mechanical versions (in green) L 1 ~2dB L 3 ~6dB L 5 ~12dB L 1 ~2dB L 3 ~6dB L 5 ~12dB NF = 8.8dB NF = 8.8dB Replace with MEMS Higher Q � L 1 ~0.3dB L 3 ~0.5dB L 5 ~1dB L 1 ~0.3dB L 3 ~0.5dB L 5 ~1dB Antenna Antenna Diversity for Diversity for resilience resilience NF = 2.8dB NF = 2.8dB against fading against fading • Obvious Benefit : substantial size reduction
MEMS for Wireless Communications MEMS-Based Receiver Front-End • Extremely high- Q � insertion loss no longer a problem Pre-Select Filter Pre-Select Filter not needed not needed LNA not needed LNA not needed
MEMS for Wireless Communications MEMS-Based Receiver Front-End Single High-Order Single High-Order No LNA � Power μ Mechanical RF No LNA � Power μ Mechanical RF Reduction Image-Reject Filter Reduction Image-Reject Filter @ 1.8 GHz @ 1.8 GHz • Problem : RF local oscillator synthesizer (w/ PLL and pre-scaler) is a power hog!
MEMS for Wireless Communications MEMS-Based Receiver Front-End Solution : μ Mechanical IF Single High-Order Solution : μ Mechanical IF Single High-Order No LNA � Power Channel-Selecting Mixer- μ Mechanical RF No LNA � Power Channel-Selecting Mixer- μ Mechanical RF Reduction Filter Bank @ 70 MHz; Image-Reject Filter Reduction Filter Bank @ 70 MHz; Image-Reject Filter One Mixler Per Channel @ 1.8 GHz One Mixler Per Channel @ 1.8 GHz No longer need No longer need freq. tunable LO freq. tunable LO
MEMS for Wireless Communications MEMS-Based Receiver Front-End Solution : μ Mechanical IF Single High-Order Solution : μ Mechanical IF Single High-Order No LNA � Power Channel-Selecting Mixer- μ Mechanical RF No LNA � Power Channel-Selecting Mixer- μ Mechanical RF Reduction Filter Bank @ 70 MHz; Image-Reject Filter Reduction Filter Bank @ 70 MHz; Image-Reject Filter One Mixler Per Channel @ 1.8 GHz One Mixler Per Channel @ 1.8 GHz Size Size Single-Frequency μ Mechanical Reduction Single-Frequency μ Mechanical Reduction RF Local Oscillator @ 1.73GHz RF Local Oscillator @ 1.73GHz No Tuning � Very Low Power No Tuning � Very Low Power
MEMS for Wireless Communications 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 off-chip passive 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 … but there is much work yet to be done …
MEMS for Wireless Communications Acknowledgments • 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 John Clark, who are largely responsible for the resonator work • My government funding sources: mainly DARPA and an NSF Engineering Research Center on Wireless Integrated Microsystems (WIMS)
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