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
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
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)
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
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
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
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]
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
HF Spring-Coupled Micromechanical Filter
High-Order μmechanical Filter
Nonlinear Micromechanical Circuits
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
[Wong, Nguyen 1998] MEMS for Wireless Communications Micromechanical Mixer-Filter
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
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
Suspended, Stacked Spiral Inductor Strategies for maximizing Q: 15μm-thick, electroplated Cu windings reduces series R suspended above the substrate reduces substrate loss
MEMS-Based Receiver Architectures
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 Extremely high-q insertion loss no longer a problem Pre-Select Pre-Select Filter Filter not not needed needed LNA LNA not not needed needed
MEMS-Based Receiver Front-End Single Single High-Order High-Order μmechanical μmechanical RF RF Image-Reject Image-Reject Filter Filter @ 1.8 1.8 GHz GHz No No LNA LNA Power Power Reduction Reduction Problem: RF local oscillator synthesizer (w/ PLL and pre-scaler) is a power hog!
Single Single High-Order High-Order μmechanical μmechanical RF RF Image-Reject Image-Reject Filter Filter @ 1.8 1.8 GHz GHz MEMS for Wireless Communications MEMS-Based Receiver Front-End No No LNA LNA Power Power Reduction Reduction Solution: Solution: μmechanical μmechanicalif IF Channel-Selecting Channel-Selecting Mixer- Mixer- Filter Filter Bank Bank @ 70 70 MHz; MHz; One One Mixler MixlerPer Per Channel Channel No No longer longer need need freq. freq. tunable tunable LO LO
Single Single High-Order High-Order μmechanical μmechanical RF RF Image-Reject Image-Reject Filter Filter @ 1.8 1.8 GHz GHz MEMS for Wireless Communications MEMS-Based Receiver Front-End No No LNA LNA Power Power Reduction Reduction Solution: Solution: μmechanical μmechanicalif IF Channel-Selecting Channel-Selecting Mixer- Mixer- Filter Filter Bank Bank @ 70 70 MHz; MHz; One One Mixler MixlerPer Per Channel Channel Single-Frequency Single-Frequency μmechanical μmechanical RF RF Local Local Oscillator Oscillator @ 1.73GHz 1.73GHz No No Tuning Tuning Very Very Low Low Power Power Size Size Reduction Reduction
Conclusions MEMS for Wireless Communications 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
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)