Micromachining Technologies for Miniaturized Communication Devices

Transcription

1 Micromachining Technologies for Miniaturized Communication Devices Clark T.-C. Nguyen Center for Integrated Sensors and Circuits Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor, Michigan Tel: (734) , FAX: (734)

2 Outline Background: Target Application power reduction via high-q filtering Micromechanical Resonators Micromechanical Filters frequency extension interconnect series resistance Metal Technologies for RF MEMS Circuits/MEMS Integration Conclusions

3 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

4 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] Frequency [MHz] Single-Chip Board-Level Implementation Version Off-chip high-q mechanical components present bottlenecks to miniaturization replace them with µmechanical versions

5 MEMS-Replaceable Transceiver Components Antenna1 Antenna2 Antenna Switch Bandpass Filter (Ceramic) TFR or µmech. resonator µmech. switch T/R Switch µmech. switch Off-Chip Passive Elements RF LNA VCO Xstal Tank Mixer µmech. on-chip + tunable inductor capacitor Image Reject IF Filter Filter (SAW) (Ceramic) TFR or resonator µmech. Channel Select PLL µmech. resonator 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 µmech. resonator 90 o Transmit PLL µmech. resonator Receiver 90 o VCO Xstal Tank DAC DAC ADC ADC I Q I Q on-chip inductor + µmech. µmech. or resonator tunable capacitor

6 Interfering s From a Nearby Transmitter Desired Information Low Noise Amplifier (LNA) (Perfectly Linear) An Ideal Receiver Image Reject Filter Antenna inf +2 inf + inf Noise Filter Ideal Local Oscillator inf Mixer To Baseband Electronics (No Phase Noise) IF IF

7 Interfering s From a Nearby Transmitter Desired Information Low Noise Amplifier (LNA) (Perfectly Linear) An Ideal Receiver Image Reject Filter Antenna inf +2 inf + inf Noise Filter Ideal Local Oscillator inf Mixer To Baseband Electronics (No Phase Noise) IF IF

8 Interfering s From a Nearby Transmitter Desired Information Low Noise Amplifier (LNA) (Perfectly Linear) An Ideal Receiver Image Reject Filter Antenna inf +2 inf + inf Noise Filter Ideal Local Oscillator inf Mixer To Baseband Electronics (No Phase Noise) IF IF Low IF frequency allows cheaper, low-power baseband electronics

9 Interfering s From a Nearby Transmitter Desired Information Low Noise Amplifier (LNA) (Perfectly Linear) An Ideal Receiver Image Reject Filter Antenna inf +2 inf + inf Noise Filter Ideal Local Oscillator inf Mixer IF Filter To Baseband Electronics (No Phase Noise) IF IF

10 Impact of Distortion and Noise on Receivers Interfering s From a Nearby Transmitter Desired Information Image Reject Filter Antenna Low Noise Amplifier (LNA) inf inf +2 inf + Noise Filter inf Mixer To Baseband Electronics IF IF

11 Impact of Distortion and Noise on Receivers Interfering s From a Nearby Transmitter Desired Information Image Reject Filter Antenna Low Noise Amplifier (LNA) inf inf +2 inf + Noise Filter LNA Distortion IM 3 interference must lower the LNA efficiency to linearize it 3rd Order Intermodulation Component Generated by Amplifier Distortion inf Mixer To Baseband Electronics IF IF

12 Impact of Distortion and Noise on Receivers Interfering s From a Nearby Transmitter Desired Information Image Reject Filter Antenna Low Noise Amplifier (LNA) inf inf +2 inf + Noise Filter LNA Distortion IM 3 interference must lower the LNA efficiency to linearize it 3rd Order Intermodulation Component Generated by Amplifier Distortion Local Oscillator With Phase Noise inf Mixer To Baseband Electronics IF IF

13 Impact of Distortion and Noise on Receivers Interfering s From a Nearby Transmitter Desired Information Image Reject Filter Antenna Low Noise Amplifier (LNA) 3rd Order Intermodulation Component Generated by Amplifier Distortion inf inf +2 inf + Noise Filter LNA Distortion IM 3 interference must lower the LNA efficiency to linearize it Interference From Tail of Phase Noise Spectrum Local Oscillator With Phase Noise inf Mixer To Baseband Electronics IF IF

14 Impact of Distortion and Noise on Receivers Interfering s From a Nearby Transmitter Desired Information Image Reject Filter Antenna Low Noise Amplifier (LNA) 3rd Order Intermodulation Component Generated by Amplifier Distortion inf inf +2 inf + Noise Filter LNA Distortion IM 3 interference must lower the LNA efficiency to linearize it Interference From Tail of Phase Noise Spectrum Local Oscillator With Phase Noise inf Mixer IF Filter To Baseband Electronics IF IF For Phase Noise need Consumption and Q

15 Savings Via High-Q Filtering Interfering s From a Nearby Transmitter Desired Information Image Reject Filter Antenna Low Noise Amplifier (LNA) inf inf +2 inf + Noise Filter Local Oscillator With Phase Noise inf Mixer To Baseband Electronics IF IF

16 Savings Via High-Q Filtering Interfering s From a Nearby Transmitter Desired Information Low Noise Amplifier (LNA) Image Reject Filter (very high Q) inf inf +2 inf + Noise Filter (very high Q) Antenna Local Oscillator With Phase Noise inf Mixer To Baseband Electronics IF IF

17 Savings Via High-Q Filtering Interfering s From a Nearby Transmitter Desired Information Low Noise Amplifier (LNA) Image Reject Filter (very high Q) inf inf +2 inf + Noise Filter (very high Q) Antenna Local Oscillator With Phase Noise inf Mixer To Baseband Electronics IF IF

18 Savings Via High-Q Filtering Interfering s From a Nearby Transmitter Desired Information Low Noise Amplifier (LNA) 3rd Order Intermodulation Component Generated by Amplifier Distortion Greatly Attenuated Image Reject Filter (very high Q) inf inf +2 inf + Noise Filter (very high Q) Antenna LNA Distortion no longer a problem no need to sacrifice efficiency for linearity Local Oscillator With Phase Noise inf Mixer To Baseband Electronics IF IF

19 Savings Via High-Q Filtering 3rd Order Intermodulation Component Generated by Amplifier Distortion Greatly Attenuated Interfering s From a Nearby Transmitter Desired Information Low Noise Amplifier (LNA) Local Oscillator With Phase Noise Image Reject Filter (very high Q) inf inf +2 inf + Noise Filter (very high Q) inf Antenna LNA Distortion no longer a problem no need to sacrifice efficiency for linearity Mixer Interference From Tail of Phase Noise Spectrum no longer a factor IF Filter IF To Baseband Electronics IF Phase Noise much less of a problem (for receive)

20 Selective Low Loss Filters: Need High-Q Resonator Tank Coupler Resonator Tank Typical LC implementation: Coupler Resonator Tank i o ---- v i General BPF Implementation R x1 C x1 L x1 R x2 C x2 L x2 R x3 C x3 L x3 ο C 12 C 23 In resonator-based filters: high tank Q low insertion loss At right: a 0.3% bandwidth 70 MHz (simulated) heavy insertion loss for resonator Q < 5,000 Transmission [db] Increasing Insertion Loss Tank Q = 10,000 Tank Q = 5,000 Tank Q = 2,000 Tank Q = 1, Frequency [MHz]

21 Deficiencies in Macroscopic High-Q Filters Example: semi-monolithic crystal filters acoustically coupled thickness-shear mode resonators extremely high Q s ~ 10,000 or higher Quartz Electrodes Acoustic Coupling W e Crystal Filter Circuit: R Q1 W e g w Thickness- Shear Resonator1 Thickness- Shear Resonator2 v s C o1 C o2 R Q2 C par Monolithic Two-Resonator Filter Problems: not tunable over a large large size (cm s) precludes the use of a filter bank

22 Parallel Bank of Switchable Filters Rather than cover the band by tuning, cover with a bank of switchable filters Filter On Antenna

23 Parallel Bank of Switchable Filters Rather than cover the band by tuning, cover with a bank of switchable filters Filter On Antenna

24 Parallel Bank of Switchable Filters Rather than cover the band by tuning, cover with a bank of switchable filters Filter On Antenna

25 Parallel Bank of Switchable Filters Rather than cover the band by tuning, cover with a bank of switchable filters Filter On Antenna Problem: macroscopic high-q filters are too big Requirement: tiny filters micromechanical high-q filters present a good solution

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

27 Outline Background: Target Application power reduction via high-q filtering Micromechanical Resonators Micromechanical Filters frequency extension interconnect series resistance Metal Technologies for RF MEMS Circuits/MEMS Integration Conclusions

28 Surface Micromachining 2 um 2 um o Sacrifical Oxide (450 C) Polycrystalline Silicon o ( C) Silicon Nitride o (835 C) Oxide Silicon Substrate Hydroflouric Acid

29 Surface Micromachining 2 um 2 um o Sacrifical Oxide (450 C) Polycrystalline Silicon o ( C) Silicon Nitride o (835 C) Oxide Silicon Substrate Hydroflouric Acid

30 Surface Micromachining 2 um 2 um o Sacrifical Oxide (450 C) Polycrystalline Silicon o ( C) Silicon Nitride o (835 C) Oxide Silicon Substrate Hydroflouric Acid 2 um Oxide Free standing Polycrstalline Silicon Beam Silicon Substrate Fabrication steps compatible with planar IC processing

31 Comb-Transduced Folded-Beam µresonator Micromachined from in situ phosphorous-doped polysilicon Anchor Comb Transducers TC fo = 10 ppm/ o C f 6 x 10 f o 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 Frequency [Hz]

32 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) Smaller mass higher frequency range and lower series R x E = Youngs Modulus ρ=density

33 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] Press.=70mTorr V P =10V, v i =3mV w r Frequency [MHz]

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

35 High-Frequency µmechanical Filters Input Electrode Resonators 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 o x y R x2 L x2 L C x1 o1 C R x1 x1 C Cx2 12 C o2

36 Two Uncoupled Resonators c r1 Ideal Spring Coupled Filter X F d Resonator Stiffness Coupler Stiffness k r1 c r2 F m r1 m r2 X F d o o BW Normalized Coupling Coefficient = f o k sij k ij k r X F d o1 k r2 Massless Spring o c r1 k r1 m r1 k s12 m r2 c r2 k r2 Spring Coupled Resonators

37 High-Frequency µmechanical Filters Input Electrode Resonators 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 o x y R x2 L x2 L C x1 o1 C R x1 x1 C Cx2 12 C o2

38 First Mode Shape

39 Second Mode Shape

40 Third Mode Shape

41 v i V f Input Electrode R Q1 C P1 Termination Resistance L r1 Anchor V P R x Resonator Electrode L 12 CP2 W r2 Output Electrode R Q2 v o h d = gap Need to minimize R Q to minimize noise want: d = small, V P =large, A o =large Electrode-to-Resonator Resonator Q Gap R Q = km Q res d Q res V2 P ( Co d) q i Q fltr V2 P Ao 2 Electrode-to-Resonator Static Capacitance Filter Q Resonator dc-bias Voltage Electrode-to-Resonator Overlap Area

42 Small Electrode-to-Resonator Gaps Anchor Polysilicon Micromechanical Resonator d Electrode Silicon Nitride 2 µm Isolation Oxide Silicon Substrate For a 2µm-thick, 70 MHz, 0.1% bandwidth filter, with V P =6V: Termination R Q Resonator R x Gap d 2,000 Ω 516 Ω 250 Å 500 Ω 140 Ω 180 Å 100 Ω 28 Ω 120 Å

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

44 Attaining Better Performance Use more resonators to attain higher order Filter Order = 2 x (# of resonators) Transmission [db] One-Resonator (second-order) Two-Resonator (fourth-order) Three-Resonator (sixth-order) Frequency [khz] Higher order sharper roll-off better stopband rejection

45 Drive Resonator Comb-Transducer High-Order µmechanical Filter 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 [Wang, Nguyen 1997] 340 khz Anchor 20µm 32µm Coupling Beam L sij =95µm Folding Truss Transmission [db] Frequency [khz]

46 µmechanical Filter Passband Correction Frequency Tuning Electrodes Coupling Beam Transmission[dB] Comb Transducer Folded Beam Resonator As-Fabricated Voltage-Tuned Properly Terminated Frequency[kHz] Transmission [db] Frequency [khz] Problem: large DC voltages needed for frequency tuning Need: a permanent pre-trim technique Transmission [db] Frequency [khz]

47 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 i f V o f V i f V o f V switch Res. frequency vs. Vp [Lr=60um, d=1000a] v i v o fr (Hz) fr(measured value) fr ( Fitting Value, with Alpha = 0.31) Vp (v) f o =7% V switch v i = input voltage v o = output voltage V i f, V o f = freq. pulling voltages V switch = bias and on/off switch voltage

48 Parallel Bank of Switchable Filters Rather than cover the band by tuning, cover with a bank of switchable filters Filter On Antenna

49 Parallel Bank of Switchable Filters Rather than cover the band by tuning, cover with a bank of switchable filters Filter On Antenna

50 Parallel Bank of Switchable Filters Rather than cover the band by tuning, cover with a bank of switchable filters Filter On Antenna

51 Micromechanical RF Pre-Selector Use a massively parallel array of tunable, switchable filters tiny size of µmechanical filters allows this) 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

52 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 thermal stability soln: design, compensation, control noise limitations soln: transducer design power limitations soln: transducer design fabrication tolerances (absolute and matching) L W 1 f o = π k r m r

53 54.2 MHz µmechanical Resonator Fabricated in implant-doped polycrystalline silicon with interconnect chrome/gold metallized Metallized Electrodes W r µresonator Beam L r Anchors h Transmission [db] MHz Q meas =840 (with R P ) Q extract =9,000 Design/Performance: -70 L r =16µm, W r =8µm -75 h=2µm, d=300å V P =35V, v i =200mV Frequency [MHz] f o ~54.2MHz (extracted value) 10mTorr [Wong, Nguyen 1998]

54 Degradation of Interconnects Thin-gap resonators require lengthy HF release etches degrades polysilicon interconnect quality/conductivity Interconnect Polysilicon Removed Interconnect Polysilicon Attacked (Roughened)

55 Result: High Interconnect Series Resistance Problem: high R S degrades the Q of the system, making measurement and intrinsic Q determination difficult Electrode µmechanical Resonator C v T v S R S V P R BIAS R T Measured Q: R x Q meas = Q res R x + R par C P(fd) If R x << R par Q meas << Q res C o R BIAS R P2 ~100Ω R S R P1 R x C x L x R P2 C v S R P1 ~100Ω µmechanical Resonator R x ~20Ω R T v T Q meas = Q res 10

56 54.2 MHz µmechanical Resonator Fabricated in implant-doped polycrystalline silicon with interconnect chrome/gold metallized Metallized Electrodes W r µresonator Beam L r Anchors h Transmission [db] MHz Q meas =840 (with R P ) Q extract =9,000 Design/Performance: -70 L r =16µm, W r =8µm -75 h=2µm, d=300å V P =35V, v i =200mV Frequency [MHz] f o ~54.2MHz (extracted value) 10mTorr [Wong, Nguyen 1998]

57 VHF Spring-Coupled Micromechanical Filter Coupling Spring Resonators 34.5 MHz 0-5 Transmission [db] Electrodes Anchors Frequency [MHz] 2-Resonator HF (4th Order) [Wong, Ding, Nguyen 1998] Performance f o =34.5MHz, BW=460kHz (1.3%) Rej.=25dB, I.L.<6dB

58 Outline Background: Target Application power reduction via high-q filtering Micromechanical Resonators Micromechanical Filters frequency extension interconnect series resistance Metal Technologies for RF MEMS Circuits/MEMS Integration Conclusions

59 Metal Micromechanical Resonators Very first micromechanical resonators: Nathanson et al (1967) metal structural material resonant-beam serves as gate for NMOS transistor Problems: surface charge, high TC f, frequency drift, Q~90 MOS Drain Diffusion Resonator Anchor MOS Channel MOS Source Diffusion Resonator Drive Electrode First polysilicon-based resonator combined with NMOS technology by Howe et al (1984)

60 d 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 V tune µm Oxide Al Plate L p force Al Ground Plane Al Design/Performance: C tot =2.2pF for four plates in parallel 16% tuning range for V tune =5.5V Q=60 Al Suspensions Al Top Plates [Young, Boser Hilton Head 96] 200 µm Anchors Challenges: microphonics, tuning range truncated by pull-in

61 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 1 GHz

62 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 20GHz C off ~35fF Recessed Electrode f merit ~2,000GHz [Goldsmith et al. MTT-S 95] Required V switch =15-50V, switching time: t switch = 100ns-10µs Post

63 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

64 Electroplated-Nickel Surface Micromachining Sputter/evaporate Cr/Au interconnects and pattern: Cr/Au Interconnect Thermal Oxide Silicon Substrate

65 Electroplated-Nickel Surface Micromachining Sputter/evaporate sacrificial layer, pattern anchor openings: Cr/Au Interconnect Sacrificial Aluminum Thermal Oxide Silicon Substrate

66 Electroplated-Nickel Surface Micromachining Sputter-deposit nickel seed layer: Cr/Au Interconnect Nickel Seed Layer Sacrificial Aluminum Thermal Oxide Silicon Substrate

67 Electroplated-Nickel Surface Micromachining Define photoresist mold: Cr/Au Interconnect Photoresist Mold Nickel Seed Layer Sacrificial Aluminum Thermal Oxide Silicon Substrate

68 Electroplated-Nickel Surface Micromachining Plate the nickel: Plated Nickel Sacrificial Aluminum Thermal Oxide Silicon Substrate

69 Electroplated-Nickel Surface Micromachining Remove mold and nickel seed layer (wet etch): Plated Nickel Sacrificial Aluminum Thermal Oxide Silicon Substrate

70 Electroplated-Nickel Surface Micromachining Remove aluminum (wet etch): Plated Nickel Thermal Oxide Silicon Substrate

71 Nickel µmechanical Resonator Below: surface micromachined Ni resonator electroplated in a NiSO 4 6H 2 O solution at 50 o C using an Al sacrificial layer Outer Beam Length µmechanical Nickel Resonator db Spectrum of Ni Resonator fo= Q= Inner Beam Length [Hsu, Nguyen 1998] k Hz

72 Ratioed Folded-Beam Design Induces stress variations with temperature counteract frequency changes due to Young s modulus T dependence Anchors Output Electrode DC-Bias Electrode L bo L bi Outer Beam Inner Beam Input Electrode L bi = inner folded-beam length L bo = outer folded-beam length L bi L bo

73 Frequency vs. Temperature Curve \ Polysilicon Nickel [Hsu, Nguyen 1998] 300 ppm total frequency change over a T = o C range

74 Micro-Oven Temperature Stabilization Reduce the TC fo via micro-oven control (1-2 mw for 80 o C) Temperature Sensing Resistor Heating Resistor Support Struts Substrate Edge Micro-Platform

75 Vacuum Encapsulation Needed for two main reasons: reduce gas damping in µresonators high-q minimize surface contamination high-q and stability Silicon Nitride Polysilicon Interconnect Silicon Substrate Structural Sacrificial Polysilicon Oxide Thermal Oxide Contact Pad Vacuum Silicon Nitride Sacrificial Oxide Thick PSG Thin PSG Silicon Substrate Silicon Substrate Etch Hole Sacrificial Oxide Silicon Nitride Silicon Substrate Encapsulating Shell Micromechanical Resonator [Lin, et al. 1993]

76 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

77 Post-CMOS Circuits+µMechanics Integration Modular technology minimizes product development effort Module 1: circuit process (planar IC technology) Module 2: micromachining process (planar technology) Adv.: topography after circuit fabrication is much smaller Problem: limited thermal budget metal and junctions must withstand temperatures ~ 835 o C tungsten metallization used with TiSi 2 contact barriers in situ doped structural polysilicon; rapid thermal annealing Ground Plane Polysilicon Structural Polysilcion (Suspended Beams) Si N 3 4 TiSi 2 Contact Barrier Tungsten Interconnect Poly-to-Poly Capacitor Thermal SiO 2 n-substrate pwell

78 Outline Background: Target Application power reduction via high-q filtering Micromechanical Resonators Micromechanical Filters frequency extension interconnect series resistance Metal Technologies for RF MEMS Circuits/MEMS Integration Conclusions

79 71.8 MHz µmechanical Resonator Fabricated in implant-doped polycrystalline silicon with chrome/gold metallized interconnect µresonator Beam L r W r Metallized Electrodes 71.8 MHz h Anchors Design/Performance: L r =14µm, W r =6µm h=2µm, d=300å V P =28V, v i =40mV f o ~71.8MHz 10mTorr (extracted value) [Wong, Ding, Nguyen 1998] Transmission [db] Q meas =300 (with R P ) Q extract =1, Frequency [MHz]

80 71 MHz Free-Free Beam µresonator Free-free beam µmechanical resonator with non-intrusive supports reduce anchor dissipation higher Q Drive Electrode Support Beams 74 µm 14.3µm 1µm Anchor Flexural-Mode Beam 13.3µm Ground Plane and Sense Electrode Design/Performance: -70 L r =14.3µm, W r =6µm -72 h=2µm, d=1000å (extracted -74 V P =28V, v i =500mV value) -76 f o ~71MHz 10mTorr -78 [Wang, Yu, Nguyen 1998] 71 MHz Transmission [db] Measured Q=3,200 Extracted Q=5, Frequency [MHz]

81 Conclusions High-Q functionality required in communication transceivers presents a major bottleneck against ultimate miniaturization and power reduction With Q s in the thousands, µmechanical resonators can serve well as miniaturized high-q on-chip tanks for use in extremely sharp IF and RF filters Two- and three-resonator µmechanical filters in the MF, HF, and VHF ranges have been demonstrated, some with filter Q s in excess of 800, less than 1 db of insertion loss, and greater than 64 db stopband rejection Although polysilicon has so far been the structural material of choice, integrated high-q resonators may stand to benefit from metal technologies already used for tunable capacitors, inductors, and switches Micromechanical switches offer lower insertion loss than diode counterparts with less power dissipation and are very attractive for phased-array antenna applications

82 Micromechanical Processors 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 Research Issues: frequency extension to UHF and beyond stability enhancement (w/r to temperature, aging, mass loading, etc....) manufacturing aides: (automated) frequency tuning/ trimming, localized annealing, Q-enhancing procedures dynamic range optimization cost-effective integration with electronics CAD development for automatic micromechanical signal processor generation transceiver architecture exploration, harnessing the size and zero dc power consumption advantages

83 Acknowledgments B. Boser, D. Young (UC Berkeley): tunable C s and L s C. Goldsmith (Raytheon TI Systems): micromechanical switches L. Lin (Univ. of Michigan): planar processed vacuum encapsulation Former and present graduate students, especially Kun Wang and Ark-Chew Wong, who are largely responsible for the micromechanical filter work, and Wan-Thai Hsu, who is responsible for work on temperature compensated devices My funding sources: DARPA, NASA/JPL, NSF, ARO MURI, and various industrial partners