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MEMS for Wireless Communications
RF MEMS for Low-Power Communications
Clark T.-C. Nguyen
Center for Wireless Integrated MicrosystemsDept. of Electrical Engineering and Computer Science
University of MichiganAnn 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 filtersmicromechanical mixer-filtersmicromechanical switchmicromechanical C’s & L’s
• Using MEMS in Comm. Receiversdirect replacement of passivestrade Q (or selectivity) for powerMEMS-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
TransistorChips
TransistorChips
QuartzCrystalQuartzCrystal
IF Filter(SAW)
IF Filter(SAW)
InductorsCapacitorsResistors
InductorsCapacitorsResistors
IF Filter(SAW)
IF Filter(SAW)
RF Filter(ceramic)RF Filter(ceramic)
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+μMechanics Integration• Completely monolithic, low phase noise, high-Q oscillator
(effectively, an integrated crystal oscillator) [Nguyen, Howe]
• To allow the use of >600oC processing temperatures, tungsten (instead of aluminum) is used for metallization
OscilloscopeOutput
Waveform
MEMS for Wireless Communications
Target Application: Integrated Transceivers
• Off-chip high-Q mechanical components present bottlenecks to miniaturization replace them with μmechanical 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 Rx
MEMS for Wireless Communications
HF μMechanical CC-Beam Resonator• Surface-micromachined, POCl3-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 μResonator• Free-free beam μmechanical resonator with non-intrusive
supports reduce anchor dissipation higher Q
MEMS for Wireless Communications
92 MHz Free-Free Beam μResonator• Free-free beam μmechanical resonator with non-intrusive
supports reduce anchor dissipation higher Q
MEMS for Wireless Communications
156 MHz Radial Contour-Mode Disk μMechanical Resonator
• Below: Balanced radial-mode disk polysilicon μmechanicalresonator (34 μm diameter)
μmechanical DiskResonator
MetalElectrode
MetalElectrode
R
Anchor
Design/Performance:R=17μm, t=2μm
d=1,000Å, VP=35Vfo=156.23MHz, Q=9,400
[Clark, Hsu, Nguyen IEDM’00]
fo=156MHzQ=9,400
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
InputForce
Fi
OutputDisplacement
xo
t
xo
t
Fi
Key Design Property: High Q
MEMS for Wireless Communications
HF Spring-Coupled Micromechanical Filter
MEMS for Wireless Communications
High-Order μMechanical Filter
MEMS for Wireless Communications
Nonlinear Micromechanical Circuits
MEMS for Wireless Communications
ElectricalSignal Input
MechanicalSignal Input
ωRFωLO
ωRFωLOωIF
ω
ω
FilterResponse
Electromechanical Mixing
ωIF
ωo=ωIF
MEMS for Wireless Communications
Micromechanical Mixer-Filter
[Wong, Nguyen 1998]
MEMS for Wireless Communications
Micromechanical Switch
[C. Goldsmith, 1995]
• Operate the micromechanical beam in an up/down binary fashion
• Performance: I.L.~0.1dB, IIP3 ~ 66dBm (extremely linear)• Issues: switching voltage ~ 20V, switching time: 1-5μs
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:
15μm-thick, electroplated Cu windings reduces series Rsuspended 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 μmechanical versions (in green)
• Obvious Benefit: substantial size reduction
Replace with MEMSL1~0.3dBL1~0.3dB
L1~2dBL1~2dB
NF = 8.8dBNF = 8.8dB
NF = 2.8dBNF = 2.8dB
L3~6dBL3~6dB L5~12dBL5~12dB
L3~0.5dBL3~0.5dB L5~1dBL5~1dB
Antenna Diversity for
resilience against fading
Antenna Diversity for
resilience against fading
Higher Q
MEMS for Wireless Communications
MEMS-Based Receiver Front-End• Extremely high-Q insertion loss no longer a problem
LNA not neededLNA not needed
Pre-Select Filter not needed
Pre-Select Filter not needed
MEMS for Wireless Communications
MEMS-Based Receiver Front-EndSingle High-Order μMechanical RF
Image-Reject Filter @ 1.8 GHz
Single High-Order μMechanical RF
Image-Reject Filter @ 1.8 GHz
No LNA Power Reduction
No LNA Power Reduction
• Problem: RF local oscillator synthesizer (w/ PLL and pre-scaler) is a power hog!
MEMS for Wireless Communications
MEMS-Based Receiver Front-EndSingle High-Order μMechanical RF
Image-Reject Filter @ 1.8 GHz
Single High-Order μMechanical RF
Image-Reject Filter @ 1.8 GHz
No LNA Power Reduction
No LNA Power Reduction
Solution: μMechanical IF Channel-Selecting Mixer-
Filter Bank @ 70 MHz; One Mixler Per Channel
Solution: μMechanical IF Channel-Selecting Mixer-
Filter Bank @ 70 MHz; One Mixler Per Channel
No longer need freq. tunable LONo longer need freq. tunable LO
MEMS for Wireless Communications
MEMS-Based Receiver Front-EndSingle High-Order μMechanical RF
Image-Reject Filter @ 1.8 GHz
Single High-Order μMechanical RF
Image-Reject Filter @ 1.8 GHz
No LNA Power Reduction
No LNA Power Reduction
Solution: μMechanical IF Channel-Selecting Mixer-
Filter Bank @ 70 MHz; One Mixler Per Channel
Solution: μMechanical IF Channel-Selecting Mixer-
Filter Bank @ 70 MHz; One Mixler Per Channel
Single-Frequency μMechanical RF Local Oscillator @ 1.73GHzNo Tuning Very Low Power
Single-Frequency μMechanical RF Local Oscillator @ 1.73GHzNo Tuning Very Low Power
SizeReduction
SizeReduction
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 devicesbetter performance than other single-chip solutionspotentially large reduction in power consumptionalternative 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)