EE C245 - ME C218 Introduction to MEMS Design Fall 2003 · EE C245 – ME C218 Fall 2003 Lecture 27...
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1 EE C245 – ME C218 Fall 2003 Lecture 27 EE C245 - ME C218 Introduction to MEMS Design Fall 2003 Roger Howe and Thara Srinivasan Lecture 27 Micromechanical Resonators II 2 EE C245 – ME C218 Fall 2003 Lecture 27 Today’s Lecture • MEMS resonators for telecommunications: motivation • Electrical feedthrough and its suppression Shielding techniques Electromechanical amplitude modulation (EAM) technique Reducing the motional resistance • Scaling resonators for high frequencies (10 MHz to > 1 GHz) Mass reduction NEM resonators Stiffness increase bulk acoustic modes • Fabrication technologies for integrating MEMS resonators with CMOS Integration by batch transfer Monolithic processes • Reading/reference list: see Lecture 26
Transcript of EE C245 - ME C218 Introduction to MEMS Design Fall 2003 · EE C245 – ME C218 Fall 2003 Lecture 27...
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EE C245 – ME C218 Fall 2003 Lecture 27
EE C245 - ME C218Introduction to MEMS Design
Fall 2003
Roger Howe and Thara SrinivasanLecture 27
Micromechanical Resonators II
2EE C245 – ME C218 Fall 2003 Lecture 27
Today’s Lecture
• MEMS resonators for telecommunications: motivation
• Electrical feedthrough and its suppressionØ Shielding techniquesØ Electromechanical amplitude modulation (EAM) techniqueØ Reducing the motional resistance
• Scaling resonators for high frequencies (10 MHz to > 1 GHz)Ø Mass reduction à NEM resonatorsØ Stiffness increase à bulk acoustic modes
• Fabrication technologies for integrating MEMS resonators with CMOSØ Integration by batch transfer Ø Monolithic processes
• Reading/reference list: see Lecture 26
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3EE C245 – ME C218 Fall 2003 Lecture 27
Motivation for BSAC-BWRC RF MEMS Research (2001 – Present)
• 100 X power reduction over CMOS sensor node transceivers (to 1-5 nJ bit)Ø Power ≤ 100 µW average à indoor solar or ambient
mechanical energy scavenging
• Prof. Jan Rabaey, BWRCØ FBARs + 0.13 µm CMOS + chip-on-board à
demo new transceiver concepts
• Profs. Roger Howe, Tsu-Jae King, Roya Maboudian, Al PisanoØ Poly-SiGe and Poly-SiC MEMS technologies for arrays of MEMS
resonators at frequencies circa 1 GHz.
Integrated Microwatt Transceiver,DARPA MTO, NMASP Program
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“Analog OFDM” Subsampling Transceiver using Nanomechanical (NM) Filters
fclock
Rejects non-linear LNA components Shapes LNA thermal noise Selects System Frequency Bands
Prefilter: micro-machined LC passive
RF Filter (Low Q)
A
D LNA
NM Filter
NM Filter
NM Filter
Need a range of bandpass filter frequencies
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Electrical Feedthrough
+
-vd(t)
≈ is(t)+ - R
f
Probe Tip
vo(t)
is (t) + if (t)
What is the origin of the feedthroughcurrent if(t)?
Poly0 probe pad
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Electrical Equivalent Circuit: Ideal Case
+
vd
Lx Cx Rx
Co Cint
structure node - -
+
is
drive Co
Rint
Cint
Rint
sense
Cf if
* Assumes that structure node (poly0 “ground plane” layer) and conducting layer underneath poly0 interconnect to probe padare shorted together
Interconnect (poly0)Interconnect (poly0) Core comb-drive resonator
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7EE C245 – ME C218 Fall 2003 Lecture 27
Obvious Feedthrough Paths
Cf
* Direct probe-probe feedthrough can be 30 fF for closely spacedprobes: suppress by using coaxial probes
* If chip is packaged, then there are direct feedthrough paths betweenbond wires or between pins on the package
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Substrate Feedthrough PathsCase 1. Substrate is grounded on back of chip (through probe station chuck)
+
-vd
resonatorif
silicon substrateRs
Cp Cp
neglect lateralresistances
Circuit model is an approximation using lumped elements
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9EE C245 – ME C218 Fall 2003 Lecture 27
Substrate Feedthrough Analysis
+
-Vd Rs
jωCp
1jωCp
1
If
dps
psf V
CRj
CRI
)2(1
22
ω
ω
+
−=
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Substrate Feedthrough Example
Substrate resistance Rs = 1 kΩ
Probe-pad capacitance Cp = 1 pF
|If /Vd|
ω101 102 103 104 105 106 107
ωRsCp << 1 for ω < 108 rad/s
dpsf VCRI 22ω−≈
10-5
10-7
10-9
10-11
10-13
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100 kHz Comb-Drive Resonator in Air
Drive voltage vd(t) = 5 V cos(ωt)
Feedthroughcurrent if(t) = -(2π x 105 )2 (10-19)(5) cos(ωt)= -197 nA cos(ωt)
Typical motional current is(t) = 4 nA cos(ωt) << if(t)
How can the feedthroughcurrent be reduced?
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Feedthrough ReductionCase 2. Substrate is grounded on the top of the chip (through a metal
interconnect to ground)
+
-
vd
resonator
if
silicon substrateRs
Cp Cp
optional heavily doped layer
Top contacts to ground greatly minimize equivalent Rs àminimize feedthrough current
RlatRlat
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Electromechanical Amplitude Modulation (EAM)
Clark Nguyen, Ph.D., EECSDept., UC Berkeley, 1994
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EAM Circuit Implementation
Clark Nguyen, Ph.D., EECSDept., UC Berkeley, 1994
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Spectrum of EAM Sense Current
Clark Nguyen, Ph.D., EECSDept., UC Berkeley, 1994
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Demodulation of EAM Sense Current
ω
ω
ωClark Nguyen, Ph.D., EECSDept., UC Berkeley, 1994
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Reducing the Motional Resistance
21 ηQkm
Rx =
Increase Q, increase electromechanical coupling coefficient η2
2
∂∂
=xC
VPη
xC
∂∂
is higher for || plate capacitors à shrink gap g
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Increasing the Resonant Frequency
mk
=ω →
option 1. mass à zero
Michael Roukes, Caltech
Motivation: NEM resonator as a sensitive probe of phonontransitions near absolute zero.
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Increasing the Resonant Frequencyoption 2. spring rate à ∞
Clark Nguyen, Michigan
Motivation: keep mass as large as possible in order to improve precision of fab, power handling
IEEE IEDM 2000.
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1.14 GHz Poly-Si Disk Resonator
Transducers ’03, Boston
* Note Q in vacuum and in air is the same: little energy loss to ambient; however,energy loss through anchor (“stem ”) is significant
* EAM-like technique is used to extract the motional current.
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Reducing the Motional Resistance
M. Demirci (Michigan),Transducers ’03, Boston
Mechanical coupling leads to a degenerate mode à lower Rx.
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Silicon-on-Insulator Platform Transfer
Clark Nguyen,Univ. of Michigan,Transducers 01.
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SEM of Platform Prior to Transfer
Clark Nguyen, Univ. of Michigan, Transducers 01.
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Transferred Platform on RF Circuit
Clark Nguyen, Univ. of Michigan, Transducers ‘01.
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Quality Factor Degradation
Clark Nguyen, Univ. of Michigan, Transducers ‘01.
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Resonators in the Metal Stack
J. Lund (IBM Research), Hilton Head 2002.
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Poly-SiGe Integrated RF MEMS
RESDrive Electrode Sense Electrode
Microshell Encapsulation(anchors not shown)
DC Bias toResonator
Shielded Interconnectto Drive Electrode Shielded
Vertical SignalPath to Gate ofInput Transistor
5-level metalfoundry CMOS
Poly-SiGe RFMEMS technology
R. T. Howe, T.-J. King, and A. P. Pisano, DARPA MTO NMASP Projects.
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Why are MEMS Peripheral to the Mainstream of the Industry?
• Answer: because they ARE peripherals
• What is the core of the semiconductor industry?• Computing (microprocessors, memory)
Ø Communications (wired and wireless)
• MEMS RF resonators, RF switches, and RF passivesmay be the breakthrough devices that will finallyget MEMS into the core.
