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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Wireless Communications
P
P
P
ω
Digital
Information
ω
ω
LNA
Mixer
Antenna
Transmitter
PA
DAC
P
Xstal
Osc.
P
P
ω
VCO
ω
Mixer
ω
P
P
ω
ω
LNA
Mixer
Baseband
Electronics
LNA
Antenna
Receiver
VCO
Xstal
Osc.
VCO
want small BW ‡ better channel selectivity
• Several selection (filtering) stages needed in super-heterodyne
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Miniaturization of Transceivers
need high-Q ‡ small BW with low loss
Antenna
LNA
Mixer
LNA
Mixer
Baseband
Electronics
VCO
Transistor
Electronics
RF Filter
(ceramic)
Receiver
Block
Diagram
VCO
Xstal
Osc.
IF Filter
(Xstal)
IF Filter
(SAW)
• High-Q functionality
required by oscillators
and filters cannot be
realized using standard
IC components ‡ use
off-chip mechanical
components
• SAW, ceramic, and
Board-Level Implementation
C. T.-C. Nguyen
crystal resonators pose
bottlenecks against
ultimate miniaturization
Univ. of Michigan
MEMS for Signal Processing
Why High-Q?: Phase Noise in Oscillators
• Superposed noise from oscillator components causes
frequency instability leading to phase or frequency noise
Frequency-Selective
Tank Element i----o
Ideal Sinusoid: v o ( t ) = V o sin ( 2πf o t )
vi
ωο
t ⇔
ω
fo=1/To
f
To
vo
Sustaining
Amplifier
Real Sinusoid:
v o ( t ) = ( V o + ε ( t ) ) sin [ 2πf o t + θ ( t ) ]
t ⇔
fo
f
Zero-Crossing Point
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
An Ideal Receiver
Interfering Signal
From a
Nearby Transmitter
RF Filter
Signal
Power
Desired
Information
Signal
Signal
Power
Signal
Power
ωinf
Ideal
Local Oscillator
IF Filter
ω
To Baseband
Electronics
Mixer
ωIF ω
ωIF
C. T.-C. Nguyen
ω
Univ. of Michigan
MEMS for Signal Processing
Impact of Phase Noise on Transceivers
• Local oscillator phase noise can mask desired information
signals after mixing
Interfering Signal
From a
Nearby Transmitter
RF Filter
Signal
Power
Desired
Information
Signal
Interference From
Tail of Phase
Noise Spectrum
Signal
Power
ωinf
Signal Local Oscillator
Power With Phase Noise
ω
To
IF Filter
Mixer
ωIF ω
ωIF
ω
• Typical phase noise spec: -100dBc/Hz @ 10kHz carrier offset
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Achieving High Oscillator Stability
Frequency-Selective
Tank Element
Low-Q
vo
-4
Δ f = 400 kHz
-6
-10
-20
Δ f = 4 kHz
-30
-40
9.50
60
10.00
10.50
11.00
Freq. [MHz]
Δθ = 40 o
0
-60
9.00
80
Phase [deg]
9.00
Phase [deg]
(e.g. crystal
oscillators)
0
Amplitude [dB]
-2
High-Q
0o+Δθ
(e.g. LC or ring
oscillators)
0
Amplitude [dB]
0o−Δθ
40
0
9.50
10.00
10.50
11.00
Freq. [MHz]
Δθ = 40 o
-40
-80
• High tank Q ‡ high frequency stability
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Phase Noise in Specific Oscillators
Low-Q Oscillators
High-Q Oscillators
• Tank Q ~ tens of thousands
• Example: crystal oscillator
• Tank Q ~ less than 30
• Example: LC or ring osc.
Signal
Power
Signal
Power
ω
Crystal
Tank
ω
• Adv.: extremely stable
• Disadv.: large size: requires
off-chip component
• Adv.: high frequency, large
tuning range (good VCO’s),
small size
• Disadv.: poor stability
• Need: high-Q in small size
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Why High-Q? Insertion Loss in Filters
Resonator
Tank
Coupler
Resonator
Tank
Coupler
General BPF
Implementation
Resonator
Tank
Typical LC implementation:
Rx1 Cx1 Lx1
Rx2 Cx2 Lx2
C12
Rx3 Cx3 Lx3
C23
0
-5
-10
Tank Q = 10,000
Increasing
Insertion
Loss
Tank Q = 5,000
Tank Q = 2,000
• 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]
-15
Tank Q = 1,000
-20
-25
-30
-35
-40
-45
-50
-55
-60
69.60
69.80
70.00
70.20
70.40
Frequency [MHz]
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
MEMS-Replaceable Transceiver Components
Image Reject IF Filter
Filter
(SAW)
(Ceramic)
μmech.
TFR or resonator
Antenna1 Antenna2
Antenna
Switch
μmech.
switch
T/R
Switch
Receiver
IF Mixer
ADC I
IF LNA
RF LNA
VCO
Xstal
Tank
Bandpass
Filter
(Ceramic) μmech.
switch
TFR
or
μmech.
resonator
Off-Chip
Passive
Elements
μmech.
resonator
90o
Mixer
AGC
Channel
Select PLL
ADC Q
IF PLL
μmech.
resonator
μmech.
on-chip + tunable
inductor capacitor
DAC
I
DAC
Q
90o
Power Amplifier
Transmitter
Modulator
VCO
Transmit PLL
μmech.
resonator
Xstal
Tank
on-chip
inductor
+
μmech.
μmech. or tunable
resonator capacitor
• A large number of off-chip high-Q components replaceable
with μmachined versions; e.g., using μmachined resonators,
switches, capacitors, and inductors
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Synthesizer Oscillators Within Transceivers
Image Reject IF Filter
Filter
(SAW)
(Ceramic)
μmech.
TFR or resonator
Antenna1 Antenna2
Antenna
Switch
μmech.
switch
T/R
Switch
Receiver
IF Mixer
ADC I
IF LNA
RF LNA
VCO
Xstal
Tank
Bandpass
Filter
(Ceramic) μmech.
switch
TFR
or
μmech.
resonator
Off-Chip
Passive
Elements
μmech.
resonator
90o
Mixer
AGC
Channel
Select PLL
ADC Q
IF PLL
μmech.
resonator
DAC
I
DAC
Q
90o
Power Amplifier
Transmitter
μmech.
on-chip + tunable
inductor capacitor
Modulator
VCO
Transmit PLL
μmech.
resonator
Xstal
Tank
on-chip
inductor
+
μmech.
μmech. or tunable
resonator capacitor
• Synthesizers indicated in yellow
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Local Oscillator Synthesizer
Phase
Detector
Crystal
Oscillator
θXstal
10MHz
• High-Q
• Not tunable
LNA
Loop
θe Filter
θVCO÷N
800MHz
(for N=80)
VCO
• Medium Q
• Tunable
÷N
Before
Lock
Signal
Power
ω
Synthesizer
Output
After
Lock
Signal
Power
Signal
Power
• Phase locking cleans up the
VCO output spectrum
• Result: sufficiently stable,
ω
ωn
ω
tunable local oscillator
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Voltage-Controlled Oscillators (VCOs)
Off-Chip Implementation
On-Chip Implementation
On-chip
Spiral
Inductor
Grounded
Transmission
Line Inductor
C2
Leff
C1
Varactor
Diode
–1 / 2
C
C
1
2
ω o = ⎛ L eff ------------------⎞
⎝
C 1 + C 2⎠
• Off-chip inductor ‡ Q~100’s
• Tunable Varactor Diode
Capacitor ‡ Q~60
Silicon
Diode
Junction
Capacitor
• 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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
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
Al Top
Plate
Anchor
Lp
Al
Plate
Vtune
force
+
d
-
4 μm
Oxide
Al
Al Ground Plane
Al Suspension
Al Layer Under Suspension
Top View
Cross-Section
[Young, Boser 1996]
• Challenges: microphonics, tuning range truncated by pull-in
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Fabricated Voltage-Tunable High-Q Capacitor
• Surface micromachined in sputtered aluminum
Four Capacitors
in Parallel
200 μm
[Young, Boser 1996]
• Ctot=2.2pF; 16% tuning range for ΔVtune=5.5V; Q~60
• Challenge: contact and support line resistance ‡ degrades Q
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Spiral Inductor Deficiencies
Co
Ls
Circuit Metal
Interconnect
Csub
Thermal Oxide
pwell
Silicon Substrate
Rs
Cox
Cox
Rsub
Rsub
Csub
ωo Ls
Q ≈ ------------Rs
• Series Rs ‡ degrades Q
— solns: increase L per unit length; use thicker metal
• Parasitic Co, Cox, Csub and Rsub ‡ self-resonance, degrades Q
— soln: isolate from substrate
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Spiral Inductor With Magnetic Core
• NiFe core; electroplated Cu windings
Dcoil
Cu Windings
Polyimide
Wwind
[Von Arx, Najafi Trans’97]
hwind
NiFe Core
Ws
Silicon Substrate
Design/Performance:
Wwind=50μm, hwind=10μm
10 turns
Ltot=2.7μH
Q=6.6 @ 4 MHz
• Challenge: limited magnetic core response bandwidth
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Isolated Spiral Inductor
• Electroplated gold windings on a substrate-isolating platform
Electroplated
Coil
Dielectric
Membrane
Wwind
[Ziaie, Kocaman,
Najafi Trans’97]
hwind
p+
Air or
Vacuum
p+
Glass Substrate
Design/Performance:
Wwind=25μm, hwind=5μm
2 turns
Ltot=115nH
Q=22 @ 275 MHz
fself-res>750 MHz
• Challenge: self-resonance frequency can still limit bandwidth
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Three-Dimensional Coil Inductor
• Electroplated copper winds achieved using maskless, 3-D,
direct-write laser lithography to pattern resist mold
• 3-D structure ‡
Insulating
minimizes substrate
Core
coupling and eddy
current loss
• Thick copper ‡
reduces series R
Copper
Winds
500 μm
Substrate
Performance:
Wwind=50μm
hwind=5μm
for 1 turn:
Ltot=4.8nH
Q=30 @ 1 GHz
[Young, Boser
IEDM’97]
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
LC-Tank Transceiver Components
Image Reject IF Filter
Filter
(SAW)
(Ceramic)
μmech.
TFR or resonator
Antenna1 Antenna2
Antenna
Switch
μmech.
switch
T/R
Switch
Receiver
IF Mixer
ADC I
IF LNA
RF LNA
VCO
Xstal
Tank
Bandpass
Filter
(Ceramic) μmech.
switch
TFR
or
μmech.
resonator
Off-Chip
Passive
Elements
μmech.
resonator
90o
Mixer
AGC
Channel
Select PLL
ADC Q
IF PLL
μmech.
resonator
μmech.
on-chip + tunable
inductor capacitor
DAC
I
DAC
Q
90o
Power Amplifier
Transmitter
Modulator
VCO
Transmit PLL
μmech.
resonator
Xstal
Tank
on-chip
inductor
+
μmech.
μmech. or tunable
resonator capacitor
• Yellow: replaceable LC tanks (low to medium Q required)
• Red: very high-Q tanks required (Q > 1,000)
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
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]
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Comb-Transduced Folded-Beam μResonator
• Micromachined from in situ phosphorous-doped polysilicon
Anchor
0
Comb Transducers
TCfo = −10 ppm/oC
Δf
6
x 10
fo
-200
-400
-600
-800
300
320
340
360
TEMPERATURE [K]
20 mTorr pressure
• (Q = 27 at atmospheric pressure)
• Problems: large mass ‡ limited
to low frequencies; low coupling
C. T.-C. Nguyen
[dB]
vi
• At right: Q = 50,000 measured at
-5
vo
Folded-Beam
Suspension
Magnitude
Movable
Shuttle
-10
-15
-20
-25
-30
18820
18822
18824
18826
18828
18830
Frequency [Hz]
Univ. of Michigan
MEMS for Signal Processing
Vertically-Driven Micromechanical Resonator
Resonator Beam
io
L
W
h
d
io
fo
C(t)
Electrode
vi
io
VP
f
x
VP
z
y
1 k
Eh
f o = ------ ---- = 1.03 ---- -----2
ρL
2π m
(e.g. m=10-13kg)
E = Youngs Modulus
ρ=density
• Smaller mass ‡ higher frequency range and lower series Rx
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Fabricated HF μMechanical Resonator
• Surface-micromachined, POCl3-doped polycrystalline silicon
Anchor
Resonator
Lr
wr
d
20 μm
Lr=40.8 μm, wr=8 μm,
h=2 μm, d=0.1μm
• Extracted Q = 8,000
• Freq. influenced by dc-bias
and anchor effects
C. T.-C. Nguyen
Transmission [dB]
Electrodes
0
-5
Press.=70mTorr
VP=10V, vi=3mV
-10
-15
-20
-25
8.48
8.49
8.50
8.51
8.52
8.53
Frequency [MHz]
Univ. of Michigan
MEMS for Signal Processing
Desired Filter Characteristics
Insertion Loss
Attenuation [dB]
0
Ripple
3dB bandwidth
3dB
20dB
Ultimate
Attenuation
20dB bandwidth
Frequency [Hz]
dB-down Bandwidth
• 20 dB-down Shape Factor = 20
-----------------------------------------------------------------3 dB-down Bandwidth
• Small shape factor is preferred ‡ better selectivity
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Ideal Spring Coupled Filter
D1
ωo
M1
k1
Two Uncoupled
Resonators
D2
ωo
M2
k2
D1
k12
M1
k1
C. T.-C. Nguyen
ωo1
F
Massless
Spring
D2
M2
ωo
Spring
Coupled
Resonators
k2
Univ. of Michigan
MEMS for Signal Processing
High-Frequency μMechanical Filters
Input
Electrode
Resonators
(each @ ωo)
Coupling
Spring
Anchor
vo
----vi
RQ
x
vi
Output
Electrode
vo
RQ
ωo
ω
x
VP
y
-C12 -C12
Co1
Rx1
Lx1 C
x1
C. T.-C. Nguyen
Rx2 C Lx2
x2
C12
z
Co2
Univ. of Michigan
MEMS for Signal Processing
HF Spring-Coupled Micromechanical Filter
Electrode
Electrode
Coupling
Coupling
Spring
Spring
Resonator
Resonators
Lr
0
-5
Transmission [dB]
wr
L12
20
20 μm
μm
Electrodes
Electrodes
2-Resonator HF
(4th Order)
[Bannon, Clark,
Nguyen 1996]
C. T.-C. Nguyen
7.81 MHz
-10
-15
-20
-25
-30
-35
-40
-45
-50
Anchor
Anchor
7.76
7.80
7.84
7.88
Frequency [MHz]
Performance
fo=7.81MHz, BW=15kHz
Rej.=35dB, I.L.<2dB
Univ. of Michigan
MEMS for Signal Processing
Attaining Better Performance
• Use more resonators to attain higher order
• Filter Order = 2 x (# of resonators)
Transmission [dB]
-5
-10
One-Resonator
(second-order)
-15
-20
-25
Two-Resonator
(fourth-order)
-30
-35
-40
Three-Resonator
(sixth-order)
-45
-50
453
454
455
456
457
Frequency [kHz]
• Higher order ‡ sharper roll-off ‡ better stopband rejection
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
High-Order μMechanical Filter
Coupling
Springs
Balanced
Electrodes
Balanced
Electrodes
3-Resonator MF
(6th Order)
fo=360kHz,BW=450Hz
Rej.=48dB,I.L.=0.8dB
[Wang, Nguyen 1997]
Resonators
74μm
Coupling Beam
0
Transmission [dB]
Frequency Tuning
Electrode
360 kHz
-10
-20
-30
-40
-50
-60
10μm
C. T.-C. Nguyen
358
359
360
361
362
Frequency [kHz]
Univ. of Michigan
MEMS for Signal Processing
Improved Three-Resonator Filter
Coupling
Springs
Drive Resonator
Comb-Transducer
Sense Resonator
3-Resonator MF
(6th Order, 1/5Velocity Coupled)
fo=340kHz
BW=403Hz
%BW=0.09%
Stop.R.=64 dB
I.L.<0.6 dB
Ratioed Folded Beam
340 kHz
20μm
Coupling Beam
Lsij=95μm
32μm
Folding Truss
Transmission [dB]
Anchor
0
-10
-20
-30
-40
-50
-60
337 338 339 340 341 342 343
Frequency [kHz]
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
CMOS Micromechanical Resonator Oscillator
• Completely monolithic, low phase noise, high-Q oscillator
(effectively, an integrated crystal oscillator) [Nguyen, Howe]
y
Sustaining
Amplifier
Ramp
vi
io
x
(Input)
Comb-Transducer
Shuttle
Mass
Folded-Beam
Suspension
300 μm
+
Anchors
VP
−
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Oscillator Circuit Schematic
Sustaining Amplifier 3-Port μmechanical Resonator
Output Amplifier
VDD
M4
VST VGC
M2
M13
M6
M8
VOC
M12
M3A
Cc1
M3
M5
M18
M14
M10
M15 M16
Cc2
M1
M7
+
VP
-
M9
M11
vo
M19
M17
VSS
• MOS resistors used to implement transresistance function
• Oscillation amplitude controllable via resonator dc-bias VP
or via feedback control of VGC
• Dc-bias voltage VP also affords limited frequency tuning
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Oscillator System Level Schematic
Transresistance
Amplifier
3-Port Micromechanical
Resonator
Output
Buffer
ii
iiRamp
Ro
vosc
vo
Ri
ifb
VP
• Series resonant architecture using a low input Ri
transresistance sustaining amplifier to minimize Q-loading
• Three-port micromechanical resonator
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Extending the Frequency Range
• To obtain even higher frequency:
— Shrink beam dimensions
— Must shrink gap d dimensions, as well
Resonator
Beam
h
Electrode
L
Anchor
W
d
1 kr
f o = ------ -----2π m r
100 MHz: Lr=11.8 μm, wr=8 μm, h=2 μm, d=0.1μm
• 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)
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Phased-Array Antennas
• Properly phased
Array of
Antennas
Steerable,
Directional
Radiation
Pattern
• If switches are
For Each Antenna:
Antenna
Time-Delay
Phase Shifter
Input
Amplifier
Control Inputs
C. T.-C. Nguyen
radiation
patterns from
each antenna
are combined to
produce a
maximum in a
desired direction
(spatial filtering)
lossy +
amplifiers (one
for each
antenna) must
be used to
recover losses
Several Switches Within,
Each Contributing Insertion Loss
Univ. of Michigan
MEMS for Signal Processing
Micromechanical Switch
• Electroplated membrane, electrostatically actuated
Vias
Post
Input
Dielectric
Post
Switch Up
Recessed Electrode
Vias
Input
Top
Membrane
Removed
Output
Vswitch
Recessed
Electrode
Switch Down
Post
Input
Post
Post
Performance:
I.L.~0.2 dB
@ 20GHz
Coff~35fF
fmerit~2,000GHz
Output
Output
Recessed Electrode
[Goldsmith et al. MTT-S’95]
• Required Vswitch=30-50V, switching time: tswitch=10-100ns
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Low-Voltage Micromechanical Switch
• Two structural layers of electroplated gold
Top
Electrode
Ground
Plane
Conductor
Ground
Plane
Suspended
Structure
[Pacheco,
Katehi,
Nguyen
MTT-S’98]
• Required Vswitch=3-10V
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
MEMS vs. SAW Comparison
Resonator Beam
MEMS
Resonator
SAW Resonator
Quartz
Substrate
Anchor
Electrode
Interdigital
Transducers
5 μm
1000X
Magnification
Silicon
Die
1 cm
1 cm
1 cm
• MEMS offers the same or better high-Q frequency selectivity
with orders of magnitude smaller size
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Target Application: Integrated Transceivers
Antenna
LNA
Mixer
LNA
Receiver
Block
Diagram
Mixer
Baseband
Electronics
Transistor
Electronics
RF Filter
(ceramic)
VCO
Xstal
Osc.
IF Filter
(Xstal)
IF Filter
(SAW)
Electrode
Micromechanical
Filter
Resonator
0
-5
Transmission [dB]
VCO
-10
-15
-20
-25
-30
Coupling
Spring
-35
7.30
Resonators
Anchor
7.40
7.50
7.60
Frequency [MHz]
MEMS
Board-Level Implementation
Single-Chip
Version
• Off-chip high-Q mechanical components present bottlenecks
to miniaturization ‡ replace them with μmechanical versions
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
7.70
Switchable, Tunable Micromechanical Filters
Input
Input
Resonator
Electrode
Freq. Pulling
Electrode
Coupling
Spring
Anchor
Output
Output
Electrode Resonator
Freq. Pulling
Electrode
vi
vo
ViΔf
VoΔf
ViΔf
VoΔf
Vswitch
vi
Res. frequency vs. Vp [Lr=60um, d=1000A]
vo
4800000
4750000
fr (Hz)
4700000
Δfo=7%
4650000
4600000
4550000
fr(Measured value)
4500000
fr ( Fitting Value, with
Alpha = 0.31)
4450000
4400000
0
5
10
15
Vp (v)
C. T.-C. Nguyen
20
25
Vswitch
vi = input voltage
vo = output voltage
ViΔf, VoΔf = freq. pulling
voltages
Vswitch = bias and on/off
switch voltage
Univ. of Michigan
MEMS for Signal Processing
Micromechanical RF Pre-Selector
• Use a massively parallel array of tunable, switchable filters)
Filter 1
or replace with
sub-sampling
A/D converter
Micromechanical
Switches Within
Antenna
LNA
Mixer
Filter 2
Switchable
Matching
Network
substantial
‡ power
savings
Baseband
Electronics
VCO
Reference
Oscillator
Filter n
Micromechanical
Resonators Within
Frequency
and
Switch
Control
Electronics
Mode
Parallel Bank of Tunable/
Switchable Micromechanical Filters
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
Micromechanical Mixer+Filters
• Add a carrier to the input resonator bias voltage to achieve a
mixer+filter function
n-type
Resonator
n-type
Input
p-type
Resonator
Resonator
Output
Output
Coupling
Electrode
Freq. Pulling
Resonator
Anchor Spring
Input
Electrode
Freq. Pulling
Electrode
Information
Electrode
RQ1
Input
vi
vo
CP1
V1Δf
vc
VP1
vi = information input
vc = carrier input
vo = output
C. T.-C. Nguyen
CP2
Carrier
Input
RQ2
V2Δf
Vswitch
ViΔf, VoΔf = freq. pulling
voltages
Vswitch = bias and on/off
switch voltage
Univ. of Michigan
MEMS for Signal Processing
Design Issue: Process Tolerances
Frequency Tuning
Electrodes
Output Comb
Transducers
1st Resonator
3rd Resonator
Coupling Beams
Anchors
L12
0.5%Mass Deviation
in Central Resonator
Input Comb
Transducers
-10
m1
k2
k12 D
2
m2
m3
k23
D3
• Process variations can lead to
distortion in the filter passband
C. T.-C. Nguyen
Ideal
-20
Transmission [dB]
k3
D1
k1
0.1%Mass Deviation
in Central resonator
2nd
Resonator
-30
-40
-50
-60
-70
-80
298
299
301
300
302
Frequency [kHz]
Univ. of Michigan
MEMS for Signal Processing
70 MHz Three-Resonator Filter
Coupling Spring
LS = 18 μm
W = 8 μm
L = 12.4 μm
μMechanical
Resonator
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
70 MHz Nano-Scale Bulk Si Resonators
B = 7 Tesla
12
12
Induced EMF (μV)
B
• Magnetically driven
[Cleland and Roukes, 1996]
C. T.-C. Nguyen
2
8
8
4
0
0
T = 4.2 K
2
4
6
8
B (T)
4
0 Tesla
0
70.68
70.72
70.76
Frequency (MHz)
Univ. of Michigan
MEMS for Signal Processing
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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing
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
C. T.-C. Nguyen
Univ. of Michigan
MEMS for Signal Processing