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RF MEMS in Wireless
Architectures
Clark T.-C. Nguyen
Dept. of Electrical Engineering & Computer Science
University of Michigan
Ann Arbor, Michigan 48105-2122
(presently at DARPA/MTO)
DAC’05, June 13-17, 2005
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Outline
• Introduction: Miniaturization of Transceivers
need for high-Q
merged transistor/MEMS process
• MEMS Components for RF Front Ends
micromechanical RF switches
tunable micromechanical C’s & L’s
vibrating micromechanical resonators
• LSI Micromechanical Circuits
• Conclusions
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Motivation: Miniaturization of RF Front Ends
Problem: high-Q
passives pose a
bottleneck against
miniaturization
RF Filter
(ceramic)
RF Filter
(SAW)
Wireless
Phone
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Inductors
Capacitors
Resistors
Transistor
Chips
IF Filter
(SAW)
Quartz
Crystal
Surface Micromachining
• Fabrication steps compatible with planar IC processing
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Single-Chip Ckt/MEMS Integration
• Completely monolithic, low phase noise, high-Q oscillator
(effectively, an integrated crystal oscillator)
Oscilloscope
Output
Waveform
[Nguyen, Howe 1993]
• To allow the use of >600oC processing temperatures,
tungsten (instead of aluminum) is used for metallization
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Benefit of MEMS: Size Reduction
RF Filter
(ceramic)
RF Filter
(SAW)
Inductors
Capacitors
Resistors
Quartz
Crystal
Transistor
Chips
IF Filter
(SAW)
MEMS
Technology
Solution: replace
passives with
MEMS devices
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Single-Chip
Transceiver
MEMS Replaceable Components
• Next generation handsets need multi-band reconfigurability
 even larger number of high-Q components needed
• Micromachined versions of off-chip components, including
vibrating resonators, switches, capacitors, and inductors,
could maintain or shrink the size of future wireless phones
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Micromechanical Switches
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
mMechanical RF Switch Uses
Switchable LC
Bandpass Filter
Again, switch in
elements to program
center frequency
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Switch in capacitors
to program the filter
center frequency
Micromechanical Switch
• Operate the micromechanical beam in an up/down binary
fashion
Input
Output
Electrode
Dielectric
[C. Goldsmith, 1995]
• Performance: I.L.~0.1dB, IIP3 ~ 66dBm (extremely linear)
• Issues: switching voltage ~ 50V, switching time: 1-5ms
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
RF MEMS Switch (Radant)
100 mm
Gate
• Metal cantilever DC switch
 3-terminal device
 Pt contact interface
 high R silicon substrate
 electrostatic actuation
Vactuate ~ 90V
• Package: wafer-to-wafer
glass frit bonded cap
 low cost
 env. protection
• Reliability (gov’t tested):
Drain
Beam
[Radant]
Contact Detail
 >1 T mechanical cycles
 >100 B cycles 100mW RF cold switch
• Reliability (Radant tested):
 >2.5 B cycles 2W RF cold switched
 >100 B cycles 0.5W RF cold switched
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Source
Packaged Device
Phased Array Antenna
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Medium-Q Resonators
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Medium-Q Resonator Needs
Problem: Switch loss
compromises filter loss
Switchable LC
Bandpass Filter
• Medium-Q best achieved via tunable
micromachined capacitors and inductors
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Medium-Q Resonator Needs
Eliminates switch
loss  better
insertion loss
Tunable LC
Bandpass Filter
• Medium-Q best achieved via tunable
micromachined capacitors and inductors
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Mechanically tunable LC tank
with higher Q than
conventional on-chip tanks
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
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Larger Capacitive Tuning Range
• Use comb-transducers to actuate
multiple plate capacitors
Vtune
• Left: lateral comb[Yao 1999]
[Rockwell]
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
•
capacitor in deep
RIE’ed silicon
Nearly 250% tuning
range with ~100V of
actuation input
Suspended, Stacked Spiral Inductor
• Strategies for maximizing Q:
 15mm-thick, electroplated Cu windings  reduces series R
 suspended above the substrate  reduces substrate loss
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Out-of-Plane Micromachined Inductor
• Molybdenum-chromium metal
solenoids perpendicular to the plane
of the substrate
 reduced substrate loss  high Q
• Assembled out-of-plane via curling
stresses, then locked into place
• Record Q’s: ~70 on glass, ~40 on
20W-cm silicon (85 w/ Cu underside)
Stress Curled
Metal
Design/Performance:
D=600mm, t=1mm
On Glass Substrate:
L = 8nH, Q = 70 @ 1GHz
On 20W-cm Silicon:
L = 6 nH, Q = 40 @ 1GHz
(Q ~ 85 w/ Cu underside)
D
[Chua,Locking
Hilton Head’02]
Mechanism
[PARC]
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Solenoid
Inductor
High-Q Resonators
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
High-Q Resonator Needs
Would like
Q’s >2,000
Would like
Q’s >5,000
Would like
Q’s >10,000
Best if Q >300
• High-Q best achieved via vibrating
micromechanical resonators
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Best when
highest Q used
Thin-Film Bulk Acoustic Resonator
(FBAR)
• Piezoelectric membrane sandwiched by metal electrodes
 extensional mode vibration: 1.8 to 7 GHz, Q ~500-1,500
 dimensions on the order of 200mm for 1.6 GHz
 link individual FBAR’s together in ladders to make filters
h
Agilent FBAR
• Limitations:
freq ~ thickness
 Q ~ 500-1,500, TCf ~ 18-35 ppm/oC
 difficult to achieve several different freqs. on a single-chip
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Basic Concept: Scaling Guitar Strings
mMechanical Resonator
Vib. Amplitude
Guitar String
Low Q
High Q
110 Hz
Freq.
[Bannon 1996]
Vibrating “A”
String (110 Hz)
Stiffness
Guitar
Freq. Equation:
1 kr
fo 
2 mr
Freq.
Mass
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
fo=8.5MHz
Qvac =8,000
Qair ~50
Performance:
Lr=40.8mm
mr ~ 10-13 kg
Wr=8mm, hr=2mm
d=1000Å, VP=5V
Press.=70mTorr
3CC 3l/4 Bridged mMechanical Filter
Performance:
fo=9MHz, BW=20kHz, PBW=0.2%
I.L.=2.79dB, Stop. Rej.=51dB
20dB S.F.=1.95, 40dB S.F.=6.45
VP
In
Out
Transmission [dB]
0
-10
-20
Pin=-40dBm
Sharper
roll-off
-30
Loss Pole
-40
-50
[S.-S. Li, Nguyen, FCS’05]
-60
8.7
8.9
9.1
Frequency [MHz]
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Design:
Lr=40mm
Wr=6.5mm
hr=2mm
Lc=3.5mm
Lb=1.6mm
VP=10.47V
P=-25dBm
9.3
RQi=RQo=12kW
Radial-Contour Mode Disk Resonator
Input
Supporting
Electrode
Stem
Output
Electrode
Q ~10,000
Disk
io
v
i
R
io
vi
VP
C(t)
Frequency:
Young’s Modulus
Stiffness
1
fo 
2
kr

mr
VP
E 1

 R
Density
Mass
(e.g., mr =
10-13
kg)
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
wo
w
Note: If VP = 0V
 device off
dC
io  VP
dt
Smaller mass  higher freq.
range and lower series Rx
1.51-GHz, Q=11,555 Nanocrystalline
Diamond Disk mMechanical Resonator
• Impedance-mismatched stem for reduced anchor dissipation
• Operated in the 2nd radial-contour mode
• Q ~11,555 (vacuum); Q ~10,100 (air)
Design/Performance:
• Below: 20 mm diameter disk
R=10mm, t=2.2mm, d=800Å, V =7V
P
Polysilicon
Electrode
CVD Diamond
mMechanical Disk
Resonator
R
Ground
Plane
-84
Transmission [dB]
Polysilicon Stem
(Impedance Mismatched
to Diamond Disk)
fo=1.51 GHz (2nd mode), Q=11,555
-86
-88
fo = 1.51 GHz
Q = 11,555 (vac)
Q = 10,100 (air)
-90
-92
-94
Q = 10,100 (air)
-96
-98
-100
1507.4 1507.6 1507.8
1508
1508.2
Frequency [MHz]
[Wang, Butler, Nguyen MEMS’04]
Need for Q’s > 10,000
The higher the
Q of the PreSelect Filter 
the simpler the
demodulation
electronics
Pre-Select
Filter in the
GHz Range
Antenna
Wireless
Phone
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Presently use
resonators with
Q’s ~ 400
If can have
resonator
Q’s > 10,000
1.5-GHz Polydiamond Disk
Demodulation Electronics
Need for Q’s > 10,000
The higher the
Q of the PreSelect Filter 
the simpler the
demodulation
electronics
Presently use
resonators with
Q’s ~ 400
Pre-Select
Filter in the
GHz Range
If can have
resonator
Q’s > 10,000
1.5-GHz Polydiamond Disk
Demodulation Electronics
Antenna
Non-Coherent FSK Detector?
(Simple, Low Frequency, Low Power)
Wireless
Phone
Front-End RF
Channel Selection
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Substantial Savings in
Cost and Battery Power
RF Channel-Select Filter Bank
Switch filters
on/off via
application
and removal
of dc-bias VP,
controlled by
a decoder
Freq.
1 2 3 4 5 6 7 n
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Freq.
Transmission
RF Channels
Transmission
Transmission
Bank of UHF
mmechanical
filters
Freq.
Conclusions
• Integrated micromechanical technologies possess high-Q and
low loss characteristics capable of greatly enhancing the
performance of wireless communications
• MEMS or NEMS offer the same scaling advantages that IC
technology offers (e.g., speed, low power, complexity, cost),
but they do so for domains beyond electronics:
Size
resonant frequency (faster speed)
actuation force (lower power)
# mechanical elements (higher complexity)
integration level (lower cost)
• Time to turn our focus towards mechanical circuit design and
mechanical integration
 maximize, rather than minimize, use of high-Q components
 e.g., RF channelizer  paradigm-shift in wireless design
 CAD tools to automatically generate mmechanical circuits
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Appendix
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Vibrating RF MEMS Wish List
• Micro-scale wafer-level fabrication
1800 MHz
1900 MHz
 need >10,000 parts per wafer (for
433 MHz
200 MHz
cost reasons)
 would like >1,000 parts per die
(for performance reasons)
70 MHz
 need wafer-level packaging
• Single-chip integrated circuit or
system capability
900 MHz
 discrete parts not interesting
Frequencies should be
 must allow many different
determined by lateral
frequencies on a single-chip
dimensions (e.g., by layout)
 need on-chip connectivity
 integration w/ transistors desired
Best if systems can be
 need real time reconfigurability
reconfigured w/o the need
for RF MEMS switches
• Q’s >10,000 at RF would allow a
revolution in wireless capability
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Micromechanical Filter Circuit
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Benefits of MEMS: High On-Chip Q
Q <10 too
small
Single-Chip
Realization
Planar Spiral
Inductor
Raised Inductor
Q ~30-70
Wireless
Phone
C. T.-C. Nguyen, “RF MEMS in Wireless Architectures,” DAC’05, 6/(13-17)/05
Vibrating Resonator
1.5-GHz, Q~12,000
Vibrating Resonator
72-MHz, Q~146,000