View
1
Download
0
Category
Preview:
Citation preview
11
Texas Christian University Department of Engineering Ed Kolesar
Introduction toMicroeletromechanical Systems
(MEMS)Lecture 12 Topics
• MEMS for Wireless CommunicationComponents for Wireless CommunicationMechanical/Electrical Systems
Mechanical Resonatorso Quality Factor
OscillatorsVoltage-Tunable CapacitorsMicromachined InductorsFiltersSwitchesAntennas
Texas Christian University Department of Engineering Ed Kolesar
MEMS Overview
Micromachining: lithography, deposition, etching, …
Processes & Foundries
Devices & Structures
Methodology
History & Market
Introduction &
Background
22
Texas Christian University Department of Engineering Ed Kolesar
MEMS for Wireless Communication
Why microfabrication?• Process integration• Mass fabrication• Beyond cell phones: combine sensing, actuation,
computation, and communication → intelligent sensor/actuator network (“smart dust”)
Why mechanical components?• High-frequency oscillators (GHz range and more)• Stability w.r.t. temperature, aging• Low loss oscillators (high quality factor)• Tunable oscillators (voltage controlled)
Texas Christian University Department of Engineering Ed Kolesar
Components for Wireless MEMS• Antennas• Amplifiers
• SwitchesResistively coupledCapacitively coupled
• Resonators• Filters• Oscillators
It is difficult to build integrated high quality electronic resonators for high-frequency applications (»1MHz)
→ Use discrete components, or→ Use corresponding micromechanical devices instead
33
Texas Christian University Department of Engineering Ed Kolesar
Mechanical / Electrical Systems
Vi
L
C Vo
R
mK
mbm
ssFxsH
KbmFtKxtxbtxmxF
++==
=++
2
1
)(
:functionTransfer stiffness damping, mass,
)()()(nt displaceme :Output force external :Input
&&&
LCLRLC
i
o
iC
o
i
ssVVsH
CRLVtqtqRtqL
VV
12
1
1
)(
:functionTransfer capacit. resist., induct.,
)()()(
voltage :Output voltage :Input
++==
=++ &&&
x
F
b
Km
Texas Christian University Department of Engineering Ed Kolesar
Mechanical / Electrical Systems
Vi
L
C VoR
LCRC
LC
i
o
iCRCL
o
i
ssVVsH
CRLVtqtqtqL
VV
112
1
1
)(
:functionTransfer ecapacitanc ,resistance ,inductance
)()()(
voltage :Output voltage :Input
++==
=++ &&&
Alternative circuit:
44
Texas Christian University Department of Engineering Ed Kolesar
Resonators
• Analogy between mechanical and electrical system:Mass m - inductivity LSpring K - capacitance CDamping b - resistance R (depending where R is placed in circuit)
• Solution to 2nd order differential equation:
factorquality
system electrical system, mechanical
frequency natural 2
)(
100
00
20
2
20
0
Q
ωω
πfω
sssH
LCmK
Q
==
=
++=
ωωω
Texas Christian University Department of Engineering Ed Kolesar
Mechanical Resonator
• Frequency and phase shift under damping:
• Energy dissipation:
shift phase 4
-14
1-1
timedamping
)cos()(
2
0220
01
12
ϕ
ωτω
ωω
τ
ϕωτ
Kmb
bm
tAetxt
==
=
+= −
τt
eEtE −= 0)(
55
Texas Christian University Department of Engineering Ed Kolesar
Quality Factor
• How fast does energy dissipate?• What is the maximum amplitude for a given frequency?
Definition: Quality factor (Q factor)Ratio of stored energy
and lost energy:
Mechanical system:
Similar for electric systems: (a)
(b)
bKm
bmQ == 0ω
CL
RRLQ 1
0 ==ω
τωτππ 022 ==∆
=TE
EQ
LCRRCQ == 0ω
Texas Christian University Department of Engineering Ed Kolesar
Quality Factor
• How fast does energy dissipate?
• What is the maximum amplitude for a given frequency?At resonance, amplitude is Q times the DC response
l)(mechanica 0 b
mQ== τ
ωτ
ω (1/s)
Gain (dB)
0
ω0
66
Texas Christian University Department of Engineering Ed Kolesar
Summary: Mechanical/Electrical Resonator
Mechanical resonator:
Torsional resonator:
Electrical resonator:
) small(for frequency natural
stiffness damping, mass, 0)()()(
0 bmK
KbmtKxtxbtxm
=
=++
ω
&&&
Ik
kbItktbtI
=
=++
0frequency natural
stiffness damping, inertia, ofmoment 0)()()(
ω
θθθ &&&
LC
CRL
tqC
tqRtqL
1frequency natural
ecapacitanc ,resistance y,inductivit
0)(1)()(
0 =
=++
ω
&&&
Texas Christian University Department of Engineering Ed Kolesar
Wireless Communication
Frequency Spectrum
2015105
RF
VLFLF
MFHF
VHFUHF
SHFEHF
IR
visible
UV X-rays
10n Hz
cm µmwavelength
77
Texas Christian University Department of Engineering Ed Kolesar
Wireless Communication Components
[Nguyen et al., 1998]
Texas Christian University Department of Engineering Ed Kolesar
Oscillators for Wireless Communication
• Current Technology:Quartz crystalSurface acoustic wave (SAW)Discrete elements (variable capacitors and inductors)
• Advantages:High quality factor: > 10,000Extremely high stability against thermal variations and agingExtremely selective filtering (small channel bandwidth)
• Problem: requires incompatible materials (e.g.,GaAs) → assembly necessary
88
Texas Christian University Department of Engineering Ed Kolesar
Oscillators for Wireless Communication
• Goals:High Q oscillatorsIntegrated VCO’s (voltage controlled oscillators)Compatibility with micromachining processesLow cost
• Challenges:Q is proportional to L (inductivity) or m (mass)Tunable devicesMEMS devices with sufficiently high C and LNoise from temperature changes, vibrations, agingInductor losses: eddy currents in substrate
Texas Christian University Department of Engineering Ed Kolesar
Oscillator Stability
[Nguyen et al., 1998]
99
Texas Christian University Department of Engineering Ed Kolesar
Voltage-Tunable Capacitors
[Young and Boser 1996]200µm
Texas Christian University Department of Engineering Ed Kolesar
Voltage-Tunable Capacitors
[Young and Boser 1996]
4 tunable parallel capacitors 2.04pF … 2.35pF3V tuning voltage, Q=62 at 1GHz
1010
Texas Christian University Department of Engineering Ed Kolesar
Micromachined Inductors
[Najafi et al., 1997]
NiFe core under planar metal spiral
2.7 µH inductanceQ=6.6 at 4MHz
Spiral inductor on substrate-isolating platform/membrane
1.2 nH inductanceQ=60-80 at 40GHz
Texas Christian University Department of Engineering Ed Kolesar
Micromachined Inductors
[Young and Boser 1997]
Electroplated 3D coil inductors with 4nH, Q=30 at 1GHz
1111
Texas Christian University Department of Engineering Ed Kolesar
Thin-Film Bulk Acoustic Resonators
[Lakin, Kline and McCarron 1995]
Thin-film bulk-acoustic mode piezoelectric resonator (FBAR)
Q > 100f0 = 1.5 - 7.5 GHz
Thin-film resonator on substrateAcoustic isolation by strategic
selection of separating layers
Texas Christian University Department of Engineering Ed Kolesar
Comb-Drive Resonator
Vp DC biasVd drive signal at ωd
I0 output current ∝ VpdC/dt
feedback via transimpedance amplifier
Vc carrier signal at ωc
output signal:frequency spectrum includes ωd , ωc , but also ωc±ωd
basis for frequency transfer (heterodyning)
[Figure: Maluf 2000]
1212
Texas Christian University Department of Engineering Ed Kolesar
Bandpass Filters
Intuition:• Two resonance modes• Loosely coupled
resonators: resonance modes are close and effectively form bandpass
• Additional coupled resonators widen frequency bandpass
[Figure: Maluf, 2000]
Texas Christian University Department of Engineering Ed Kolesar
Resonators and Filters
[Nguyen et al., 1998]
1313
Texas Christian University Department of Engineering Ed Kolesar
MEMS Medium Frequency Filters
[Nguyen et al., 1998]
Texas Christian University Department of Engineering Ed Kolesar
Cantilever Beam Switches
Issues:• Maxium current when ON, max. voltage when OFF• Impedance for DC to GHz frequency range• Speed, stiction, lifespan, contact deterioration
[de los Santos 1999]
1414
Texas Christian University Department of Engineering Ed Kolesar
Mercury Switches
• Side-driven mercury switch [Saffer et al. 1996]• Curved electrodes achieve small gap size (large el.stat. force)
over larger actuation range• Bumpers prevent electrode contact / short• Mercury drop (selectively deposited only on Au patches)• Originally fabricated in MUMPs
[S. Saffer et al., 1996]
Texas Christian University Department of Engineering Ed Kolesar
Capacitive Switches
• Metal membrane• Electrostatic actuation• Avoid stiction with
dielectric layer • Capacitive, not resistive
coupling • No DC component,
which is ok for microwave frequencies
[Goldsmith et al., 1996]
1515
Texas Christian University Department of Engineering Ed Kolesar
Antennas
[Gauthier, Courtay and Rebeiz 1997]
Recommended