Microelectromechanical Devices for Wireless Communications

Microelectromechanical Devices for Wireless Communications Clark T.-C. Nguyen Center for Integrated Sensors and Circuits Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor, Michigan 48109-2122 Tel: (313)764-1220, FAX: (313)763-9324 email: ctnguyen@eecs.umich.edu

Outline Background: Target Application the need for high-q Local Oscillator Synthesizer micromachined Tunable C s micromachined L s Vibrating Mechanical Resonators bulk acoustic resonators micromechanical resonators mm-wave Applications Conclusions

Wireless Communications P P P Digital Information ω DAC ω Xstal Osc. LNA Mixer VCO PA Antenna ω P Transmitter P P P P ω ω ω ω ω Baseband Electronics Mixer LNA Mixer LNA Antenna Receiver VCO Xstal Osc. VCO want small BW better channel selectivity Several selection (filtering) stages needed in super-heterodyne

Antenna Miniaturization of Transceivers LNA Mixer VCO need high-q small BW with low loss LNA Mixer VCO Baseband Electronics Receiver Block Diagram RF Filter (ceramic) Transistor Electronics Xstal Osc. IF Filter (SAW) IF Filter (Xstal) Board-Level Implementation 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

Why High-Q?: Phase Noise in Oscillators Superposed noise from oscillator components causes frequency instability leading to phase or frequency noise Frequency-Selective i Tank Element o ---- v i ω ο ω Ideal Sinusoid: v o () t = V o sin( 2πf o t) t f o =1/T o f T o Sustaining Amplifier v o Real Sinusoid: v o () t = ( V o + ε() t ) sin[ 2πf o t + θ() t ] t f o f Zero-Crossing Point

An Ideal Receiver Signal Power Interfering Signal From a Nearby Transmitter RF Filter Desired Information Signal Signal Power IF Filter Signal Power Ideal Local Oscillator ω inf ω Mixer ω IF ω To Baseband Electronics ω IF ω

Impact of Phase Noise on Transceivers Local oscillator phase noise can mask desired information signals after mixing Signal Power Interfering Signal From a Nearby Transmitter RF Filter Desired Information Signal Signal Power Interference From Tail of Phase Noise Spectrum Signal Power Local Oscillator With Phase Noise ω inf ω Mixer ω IF ω To IF Filter ω IF ω Typical phase noise spec: -100dBc/Hz @ 10kHz carrier offset

Achieving High Oscillator Stability Frequency-Selective Tank Element 0 o Δθ v o Low-Q 0 o +Δθ High-Q Amplitude [db] 0-2 -4 Δ f = 400 khz (e.g. LC or ring oscillators) Amplitude [db] 0-10 -20-30 (e.g. crystal oscillators) Δ f = 4 khz -6-40 60 9.00 9.50 10.00 10.50 11.00 Freq. [MHz] 80 9.00 9.50 10.00 10.50 11.00 Freq. [MHz] Phase [deg] 0 Δθ = 40 o Phase [deg] 40 0-40 Δθ = 40 o -60-80 High tank Q high frequency stability

Phase Noise in Specific Oscillators High-Q Oscillators Tank Q ~ tens of thousands Example: crystal oscillator Low-Q Oscillators Tank Q ~ less than 30 Example: LC or ring osc. Signal Power Crystal Tank Signal Power ω Adv.: extremely stable Disadv.: large size: requires off-chip component ω Adv.: high frequency, large tuning range (good VCO s), small size Disadv.: poor stability Need: high-q in small size

Why High-Q? Insertion Loss in Filters Resonator Tank Coupler Resonator Tank Typical LC implementation: Coupler Resonator Tank General BPF Implementation R x1 C x1 L x1 R x2 C x2 L x2 R x3 C x3 L x3 C 12 C 23 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 Transmission [db] 0-5 -10-15 -20-25 -30-35 -40-45 -50-55 -60 Increasing Insertion Loss Tank Q = 10,000 Tank Q = 5,000 Tank Q = 2,000 Tank Q = 1,000 69.60 69.80 70.00 70.20 70.40 Frequency [MHz]

MEMS-Replaceable Transceiver Components Antenna1 Antenna2 Antenna Switch Bandpass Filter (Ceramic) TFR or resonator switch T/R Switch switch Off-Chip Passive Elements RF LNA VCO Xstal Tank Mixer on-chip + tunable inductor capacitor Image Reject IF Filter Filter (SAW) (Ceramic) TFR or resonator Channel Select PLL resonator Power Amplifier Transmitter IF LNA IF PLL Modulator IF Mixer A large number of off-chip high-q components replaceable with μmachined versions; e.g., using μmachined resonators, switches, capacitors, and inductors AGC resonator 90 o Transmit PLL resonator Receiver 90 o VCO Xstal Tank DAC DAC ADC ADC I Q I Q on-chip inductor + or resonator tunable capacitor

Synthesizer Oscillators Within Transceivers Antenna1 Antenna2 Antenna Switch Bandpass Filter (Ceramic) TFR or resonator switch T/R Switch switch Off-Chip Passive Elements RF LNA VCO Xstal Tank Mixer on-chip + tunable inductor capacitor Image Reject IF Filter Filter (SAW) (Ceramic) TFR or resonator Channel Select PLL resonator Power Amplifier Transmitter Synthesizers indicated in yellow AGC IF LNA IF PLL resonator Modulator IF Mixer 90 o Transmit PLL resonator Receiver 90 o VCO Xstal Tank DAC DAC ADC ADC I Q I Q on-chip inductor + or resonator tunable capacitor

Local Oscillator Synthesizer Crystal Oscillator 10MHz Signal Power θ Xstal Phase Detector High-Q Not tunable θ e θ VCO N Loop Filter LNA N 800MHz (for N=80) Before Lock VCO After Lock Synthesizer Output Medium Q Tunable Phase locking cleans up the VCO output spectrum Result: sufficiently stable, tunable local oscillator ω Signal Power ω Signal Power ω n ω

Voltage-Controlled Oscillators (VCOs) Off-Chip Implementation Grounded Transmission Line Inductor On-Chip Implementation On-chip Spiral Inductor L eff C 2 C 1 Varactor Diode Silicon Diode Junction Capacitor ω C o L 1 C 2 eff ----------------- 12 / = C 1 + C 2 Off-chip inductor Q~100 s Tunable Varactor Diode Capacitor Q~60 Spiral (shown) or bond-wire inductor Q: 3 to 10 Tunable reverse-biased diode capacitor high series R Problem: capacitor lacks sufficient Q and tuning range

Voltage-Tunable High-Q Capacitor Micromachined, movable plate-to-plate capacitors Tuning range exceeding that of on-chip diode capacitors and on par with off-chip varactor diode capacitors Anchor Al Top Plate Al Suspension Al Layer Under Suspension Top View d V tune + - 4 μm Oxide Al Plate L p force Al Ground Plane Cross-Section Al [Young, Boser 1996] Challenges: microphonics, tuning range truncated by pull-in

Fabricated Voltage-Tunable High-Q Capacitor Surface micromachined in sputtered aluminum Four Capacitors in Parallel 200 μm [Young, Boser 1996] C tot =2.2pF; 16% tuning range for ΔV tune =5.5V; Q~60 Challenge: contact and support line resistance degrades Q

Spiral Inductor Deficiencies C o L s R s C ox C ox Circuit Metal Interconnect C sub R sub Rsub C sub pwell Thermal Oxide Silicon Substrate Q ω o L s ------------- R s Series R s degrades Q solns: increase L per unit length; use thicker metal Parasitic C o, C ox, C sub and R sub self-resonance, degrades Q soln: isolate from substrate

Spiral Inductor With Magnetic Core NiFe core; electroplated Cu windings D coil Cu Windings h wind Polyimide W wind [Von Arx, Najafi Trans 97] NiFe Core Silicon Substrate W s Design/Performance: W wind =50μm, h wind =10μm 10 turns L tot =2.7μH Q=6.6 @ 4 MHz Challenge: limited magnetic core response bandwidth

Isolated Spiral Inductor Electroplated gold windings on a substrate-isolating platform h wind p+ Electroplated Coil Air or Vacuum Dielectric Membrane W wind p+ [Ziaie, Kocaman, Najafi Trans 97] Glass Substrate Design/Performance: W wind =25μm, h wind =5μm 2 turns L tot =115nH Q=22 @ 275 MHz f self-res >750 MHz Challenge: self-resonance frequency can still limit bandwidth

Three-Dimensional Coil Inductor Electroplated copper winds achieved using maskless, 3-D, direct-write laser lithography to pattern resist mold Copper Winds Insulating Core 500 μm Substrate 3-D structure minimizes substrate coupling and eddy current loss Thick copper reduces series R [Young, Boser IEDM 97] Performance: W wind =50μm h wind =5μm for 1 turn: L tot =4.8nH Q=30 @ 1 GHz

LC-Tank Transceiver Components Antenna1 Antenna2 Antenna Switch Bandpass Filter (Ceramic) TFR or resonator switch T/R Switch switch Off-Chip Passive Elements RF LNA VCO Xstal Tank Mixer on-chip + tunable inductor capacitor Image Reject IF Filter Filter (SAW) (Ceramic) TFR or resonator Channel Select PLL resonator Power Amplifier Transmitter IF LNA IF PLL Modulator IF Mixer Yellow: replaceable LC tanks (low to medium Q required) Red: very high-q tanks required (Q > 1,000) AGC resonator 90 o Transmit PLL resonator Receiver 90 o VCO Xstal Tank DAC DAC ADC ADC I Q I Q on-chip inductor + or resonator tunable capacitor

Thin-Film Bulk Acoustic Resonator Membrane-supported FBAR resonator Dimensions on the order of 200 μm for 1.6 GHz resonators Top Electrode Piezoelectric Film p+ Layer Etched Via Interface Bottom-Side Electrode Substrate [Krishnaswamy et al. 1991] Link together in ladder networks to make filters

Solidly Mounted Resonator More robust than membrane-supported resonators Substrate acoustically isolated via impedance transformation using quarterwavelength-thick layers Electrodes Piezoelectric Film Substrate Acoustic Impedance Transforming Layers [Lakin et al. 1995]

Comb-Transduced Folded-Beam μresonator Micromachined from in situ phosphorous-doped polysilicon Anchor Comb Transducers 0-200 TC fo = 10 ppm/ o C Δ f 6 x 10 f o -400-600 -800 300 320 340 360 TEMPERATURE [K] Movable Shuttle Folded-Beam Suspension At right: Q = 50,000 measured at 20 mtorr pressure (Q = 27 at atmospheric pressure) Problems: large mass limited to low frequencies; low coupling v o Magnitude [db] v i -5-10 -15-20 -25-30 18820 18822 18824 18826 18828 18830 Frequency [Hz]

Vertically-Driven Micromechanical Resonator Resonator Beam L W i o h Electrode v i d V P C(t) i o i o V P x f o z f y 1 k f o = ----- --- = 1.03 E 2π m --- ----- h ρ L 2 (e.g. m=10-13 kg) E = Youngs Modulus ρ=density Smaller mass higher frequency range and lower series R x

Fabricated HF μmechanical Resonator Surface-micromachined, POCl 3 -doped polycrystalline silicon Anchor Resonator Electrodes L r =40.8 μm, w r =8 μm, L r 20 μm h=2 μm, d=0.1μm Extracted Q = 8,000 Freq. influenced by dc-bias and anchor effects d Transmission [db] 0-5 -10-15 -20-25 Press.=70mTorr V P =10V, v i =3mV w r 8.48 8.49 8.50 8.51 8.52 8.53 Frequency [MHz]

Desired Filter Characteristics Insertion Loss 0 Attenuation [db] Ripple Ultimate Attenuation 3dB bandwidth 20dB bandwidth 3dB 20dB Frequency [Hz] 20 db-down Bandwidth 20 db-down Shape Factor = ----------------------------------------------------------------- 3 db-down Bandwidth Small shape factor is preferred better selectivity

Ideal Spring Coupled Filter D1 k1 D2 F M1 ω o Two Uncoupled Resonators ω o1 M2 ω o k2 Massless Spring ω o D 1 k 1 M 1 k 12 M2 D 2 k 2 Spring Coupled Resonators

High-Frequency μmechanical Filters Input Electrode Resonators (each @ ω o ) Coupling Spring Anchor Output Electrode v i V P R Q -C 12 -C 12 x v o R Q v o ----- v i z ω o x y ω R x2 L x2 L C x1 o1 C R x1 x1 C Cx2 12 C o2

HF Spring-Coupled Micromechanical Filter w r Electrode Electrode Coupling Coupling Spring Spring L 12 L r Resonators Transmission [db] 0-5 -10-15 -20-25 -30-35 -40 7.81 MHz Electrodes -45-50 20 μm Anchor Anchor 7.76 7.80 7.84 Frequency [MHz] 7.88 2-Resonator HF (4th Order) [Bannon, Clark, Nguyen 1996] Performance f o =7.81MHz, BW=15kHz Rej.=35dB, I.L.<2dB

Attaining Better Performance Use more resonators to attain higher order Filter Order = 2 x (# of resonators) Transmission [db] -5-10 -15-20 -25-30 -35-40 -45-50 453 454 455 456 457 Frequency [khz] One-Resonator (second-order) Two-Resonator (fourth-order) Three-Resonator (sixth-order) Higher order sharper roll-off better stopband rejection

High-Order μmechanical Filter Balanced Electrodes Frequency Tuning Electrode Coupling Springs 74μm Coupling Beam Resonators Balanced Electrodes Transmission [db] 0-10 -20-30 -40-50 3-Resonator MF (6th Order) f o =360kHz,BW=450Hz Rej.=48dB,I.L.=0.8dB [Wang, Nguyen 1997] 360 khz 10μm -60 358 359 360 361 362 Frequency [khz]

Drive Resonator Improved Three-Resonator Filter Comb-Transducer Coupling Springs Ratioed Folded Beam Sense Resonator 3-Resonator MF (6th Order, 1/5- Velocity Coupled) f o =340kHz BW=403Hz %BW=0.09% Stop.R.=64 db I.L.<0.6 db 340 khz Anchor 20μm 32μm Coupling Beam L sij =95μm Folding Truss Transmission [db] 0-10 -20-30 -40-50 -60 337 338 339 340 341 342 343 Frequency [khz]

Merged Circuits+μStructures Technology Modular technologies minimize product development effort Module 1: circuit process (planar IC technology) Module 2: micromachining process (planar technology) Challenge: retain conventional metallization minimize micromachining processing temperatures Circuit Metal Interconnect Circuit Polysilicon Release Etch Barrier (e.g., PECVD nitride) Mechanics Interconnect (e.g., polysilicon, nickel, etc.) Micromechanical Resonator (e.g., polysilicon, nickel, etc.) Thermal Oxide pwell Silicon Substrate

CMOS Micromechanical Resonator Oscillator Completely monolithic, low phase noise, high-q oscillator (effectively, an integrated crystal oscillator) [Nguyen, Howe] R amp Sustaining Amplifier v i (Input) Comb-Transducer Shuttle Mass x y i o Folded-Beam Suspension 300 μm Anchors + V P

Oscillator Circuit Schematic Sustaining Amplifier 3-Port μmechanical Resonator Output Amplifier V DD M 4 V ST V GC M 2 M 6 V OC M 8 M 10 M 13 M 14 M 18 M 3A M 15 M 16 v C M o c1 3 C c2 M 12 M 5 M 1 M + 7 M 9 V P - M 11 M 17 M 19 V SS MOS resistors used to implement transresistance function Oscillation amplitude controllable via resonator dc-bias V P or via feedback control of V GC Dc-bias voltage V P also affords limited frequency tuning

Oscillator System Level Schematic Transresistance Amplifier 3-Port Micromechanical Resonator Output Buffer i i i i R amp Ro v osc v o R i i fb V P Series resonant architecture using a low input R i transresistance sustaining amplifier to minimize Q-loading Three-port micromechanical resonator

Extending the Frequency Range To obtain even higher frequency: Shrink beam dimensions Must shrink gap d dimensions, as well Resonator Beam h Electrode d 100 MHz: L r =11.8 μm, w r =8 μm, h=2 μm, d=0.1μm Anchor The useful frequency range will, however, depend on other factors: quality factor soln: material and design research noise limitations soln: transducer design power limitations soln: transducer design fabrication tolerances (absolute and matching) L W 1 f o = ----- 2π k ----- r m r

Steerable, Directional Radiation Pattern For Each Antenna: Antenna Amplifier Phased-Array Antennas Time-Delay Phase Shifter Control Inputs Input Array of Antennas Several Switches Within, Each Contributing Insertion Loss Properly phased radiation patterns from each antenna are combined to produce a maximum in a desired direction (spatial filtering) If switches are lossy + amplifiers (one for each antenna) must be used to recover losses

Micromechanical Switch Electroplated membrane, electrostatically actuated Vias Switch Up Dielectric Input Post Post Output Input Vias Recessed Electrode Top Membrane Removed V switch Post Post Output Recessed Electrode Performance: I.L.~0.2 db Switch Down Input Output @ 20GHz C off ~35fF Recessed Electrode f merit ~2,000GHz [Goldsmith et al. MTT-S 95] Required V switch =30-50V, switching time: t switch =10-100ns Post

Low-Voltage Micromechanical Switch Two structural layers of electroplated gold Top Electrode Ground Plane Conductor Suspended Structure Required V switch =3-10V Ground Plane [Pacheco, Katehi, Nguyen MTT-S 98]

MEMS vs. SAW Comparison MEMS Resonator Resonator Beam SAW Resonator Quartz Substrate Interdigital Transducers 1 cm Electrode 5 μm 1000X Magnification Anchor Silicon Die 1 cm 1 cm MEMS offers the same or better high-q frequency selectivity with orders of magnitude smaller size

Target Application: Integrated Transceivers Antenna LNA Mixer VCO LNA Mixer VCO Baseband Electronics Receiver Block Diagram RF Filter (ceramic) Transistor Electronics Xstal Osc. IF Filter (SAW) IF Filter (Xstal) Micromechanical Electrode Filter Resonator Coupling Spring MEMS Resonators Anchor Transmission [db] 0-5 -10-15 -20-25 -30-35 7.30 7.40 7.50 7.60 7.70 Frequency [MHz] Single-Chip Board-Level Implementation Version Off-chip high-q mechanical components present bottlenecks to miniaturization replace them with μmechanical versions

Switchable, Tunable Micromechanical Filters Input Resonator Freq. Pulling Electrode Input Electrode Anchor Coupling Spring Output Electrode Output Resonator Freq. Pulling Electrode v i v o V iδf V oδf V iδf V oδf V switch Res. frequency vs. Vp [Lr=60um, d=1000a] v i v o 4800000 4750000 fr (Hz) 4700000 4650000 4600000 4550000 4500000 4450000 4400000 fr(measured value) fr ( Fitting Value, with Alpha = 0.31) 0 5 10 15 20 25 Vp (v) Δf o =7% V switch v i = input voltage v o = output voltage V iδf, V oδf = freq. pulling voltages V switch = bias and on/off switch voltage

Micromechanical RF Pre-Selector Use a massively parallel array of tunable, switchable filters) Antenna Micromechanical Switches Within Switchable Matching Network Filter 1 Filter 2 or replace with sub-sampling A/D converter LNA Reference Oscillator Mixer VCO substantial power savings Baseband Electronics Micromechanical Resonators Within Filter n Frequency and Switch Control Electronics Mode Parallel Bank of Tunable/ Switchable Micromechanical Filters

Micromechanical Mixer+Filters Add a carrier to the input resonator bias voltage to achieve a mixer+filter function Information Input Input Resonator Freq. Pulling Electrode R Q1 n-type Resonator Input Electrode Anchor p-type Coupling Spring n-type Resonator Output Electrode Output Resonator Freq. Pulling Electrode v i V 1Δf C P1 v c Carrier V 2Δf V Input P1 V switch CP2 R Q2 v o v i = information input v c = carrier input v o = output V iδf, V oδf = freq. pulling voltages V switch = bias and on/off switch voltage

Design Issue: Process Tolerances Frequency Tuning Electrodes Output Comb 1st Resonator Transducers 3rd Resonator Coupling Beams Anchors L 12 0.5%Mass Deviation in Central Resonator Input Comb Transducers D 1 2nd Resonator m 1 m 2 m 3 k 1 k 2 k 12 D k 23 2 k 3 D 3 Transmission [db] -10-20 -30-40 -50-60 0.1%Mass Deviation in Central resonator Ideal Process variations can lead to distortion in the filter passband -70-80 298 299 300 301 302 Frequency [khz]

70 MHz Three-Resonator Filter Coupling Spring L S = 18 μm W = 8 μm μmechanical Resonator L = 12.4 μm

70 MHz Nano-Scale Bulk Si Resonators B = 7 Tesla Induced EMF (μv) 12 8 4 12 8 4 B 2 0 0 2 4 6 8 B (T) T = 4.2 K Magnetically driven [Cleland and Roukes, 1996] 0 0 Tesla 70.68 70.72 70.76 Frequency (MHz)

Conclusions High-Q functionality required in communication transceivers presents a major bottleneck against ultimate miniaturization Micromechanical L s and tunable C s offer improved Q performance over on-chip alternatives and can be applied advantageously to VCO s and tuning/matching networks With Q s in the thousands, μmechanical resonators can serve well as miniaturized high-q on-chip tanks for use in reference oscillators and in IF and RF filters Micromechanical switches offer lower insertion loss than diode counterparts with less power dissipation and are very attractive for phased-array antenna applications Micromechanical advantages: orders of magnitude smaller size better performance than other single-chip solutions potentially large reduction in power consumption alternative transceiver architectures for improved performance

Acknowledgments B. Boser, D. Young (UC Berkeley): tunable C s and L s B. Ziaie, J. Von Arx, K. Najafi (Univ. of Michigan): advanced inductors S. Krishnaswamy (Northrop Grumman), K. Lakin (TFR): bulk acoustic resonators C. Goldsmith (Raytheon TI Systems): micromechanical switches L. Katehi, G. Rebeiz (Univ. of Michigan): micromechanical switches, antennas, and mm-wave filters Former and present graduate students, especially Frank Bannon III and Kun Wang, who are largely responsible for the micromechanical filter work My funding sources: DARPA, NASA/JPL, NSF, ARO MURI, and various industrial partners