rf mems for low-power - eecs at uc berkeleyctnguyen/... · mems for wireless communications...

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


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


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

TransistorChips

TransistorChips

QuartzCrystalQuartzCrystal

IF Filter(SAW)

IF Filter(SAW)

InductorsCapacitorsResistors

InductorsCapacitorsResistors

IF Filter(SAW)

IF Filter(SAW)

RF Filter(ceramic)RF Filter(ceramic)


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]

• To allow the use of >600oC processing temperatures, tungsten (instead of aluminum) is used for metallization

OscilloscopeOutput

Waveform


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 Rx


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


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 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


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


HF Spring-Coupled Micromechanical Filter


High-Order μMechanical Filter


Nonlinear Micromechanical Circuits


ElectricalSignal Input

MechanicalSignal Input

ωRFωLO

ωRFωLOωIF

ω

ω

FilterResponse

Electromechanical Mixing

ωIF

ωo=ωIF


Micromechanical Mixer-Filter

[Wong, Nguyen 1998]


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


Phased Array Antenna


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 Rsuspended 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)

• 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-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-Based Receiver Front-EndSingle High-Order μMechanical RF

Image-Reject Filter @ 1.8 GHz

Single High-Order μMechanical RF


No LNA Power Reduction


• Problem: RF local oscillator synthesizer (w/ PLL and pre-scaler) is a power hog!








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












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


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 …


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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