EE C245 - ME C218 Introduction to MEMS Design Fall 2003 · EE C245 – ME C218 Fall 2003 Lecture 27 Reducing the Motional Resistance M. Demirci (Michigan), Transducers ’03, Boston

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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

4EE C245 – ME C218 Fall 2003 Lecture 27

“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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5EE C245 – ME C218 Fall 2003 Lecture 27

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

6EE C245 – ME C218 Fall 2003 Lecture 27

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

8EE C245 – ME C218 Fall 2003 Lecture 27

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

+

−=

10EE C245 – ME C218 Fall 2003 Lecture 27

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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11EE C245 – ME C218 Fall 2003 Lecture 27

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?

12EE C245 – ME C218 Fall 2003 Lecture 27

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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13EE C245 – ME C218 Fall 2003 Lecture 27

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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15EE C245 – ME C218 Fall 2003 Lecture 27

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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17EE C245 – ME C218 Fall 2003 Lecture 27

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

18EE C245 – ME C218 Fall 2003 Lecture 27

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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19EE C245 – ME C218 Fall 2003 Lecture 27

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.

20EE C245 – ME C218 Fall 2003 Lecture 27

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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21EE C245 – ME C218 Fall 2003 Lecture 27

Reducing the Motional Resistance

M. Demirci (Michigan),Transducers ’03, Boston

Mechanical coupling leads to a degenerate mode à lower Rx.

22EE C245 – ME C218 Fall 2003 Lecture 27

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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27EE C245 – ME C218 Fall 2003 Lecture 27

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.

28EE C245 – ME C218 Fall 2003 Lecture 27

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.