Baris Bayram, Edward Hæggström,

A. Sanli Ergun, Goksen G. Yaralioglu, and

Butrus T. Khuri-Yakub

Ginzton Laboratory, Stanford University, CA

2003 IEEE Ultrasonics Symposium, Honolulu, Hawaii

‡This work is funded by Office of Naval Research

Khuri-Yakub Group, Ginzton Lab, Stanford University

DYNAMIC ANALYSIS OF

CMUTs IN DIFFERENT

REGIMES OF OPERATION

Khuri-Yakub Group, Ginzton Lab, Stanford University

Outline

Motivation

FEM model of a single CMUT cell

Dynamic FEM analysis

Results

Conclusion

Khuri-Yakub Group, Ginzton Lab, Stanford University

Motivation

Goal: Low voltage, high frequency CMUTs

with more output power & less nonlinearity

Method: Investigate different operation

regimes for CMUTs:

– Conventional (no contact)

– Collapsed (always in contact)

– Collapse-snapback (intermittent contact)

Khuri-Yakub Group, Ginzton Lab, Stanford University

Different Operation Regimes

Khuri-Yakub Group, Ginzton Lab, Stanford University

FEM model of a single CMUT cell

Axisymmetric 2-D model

Electrical Properties

Structural Properties

Fluid-structure Interface

Acoustic Wave Equation

Exact Absorbing Boundary

(Grote et al)

Basic Properties

Collapse: 80 V

Snapback: 50 V

Resonance: 5 MHz

Khuri-Yakub Group, Ginzton Lab, Stanford University

Dynamic FEM Analysis

Commercially available software (ANSYS 7.1)

Transient Analysis– Coupled electrical & structural analysis

– Fluid medium

– Contact capability

– Exact absorbing boundary (Grote et al)

Large Signal Characterization– Pulse and sinusoidal (AC) excitation

– Displacement and pressure output

– Nonlinear distortion: 2nd harmonic

Khuri-Yakub Group, Ginzton Lab, Stanford University

Results: Pulse Excitation

VBIAS=70V, VPULSE=+5V, tPULSE=20ns

Conventional Collapsed

Displacement (p-p) (Å) 39 70

Resonance freq. (MHz) 3.84 8.75

0 0.2 0.4 0.6 0.8 1-0.08

-0.078

-0.076

-0.074

0 1 2 3 4 5 6 7-0.024

-0.022

-0.02

Time (s)

Avera

ge D

isp

lacem

en

t (

m)

Conventional

Collapsed

0 1 2 3 4 5 6-0.05

0

0.05

Time (s)

Pre

ssu

re (

MP

a)

0 0.2 0.4 0.6 0.8 1-0.4

-0.2

0

0.2

0.4

Conventional

Collapsed

Khuri-Yakub Group, Ginzton Lab, Stanford University

Results: Pulse Excitation

Collapsed

operation

VBIAS=65 V

Pulse excitation:

VPULSE=15V

tPULSE=20ns

Displacement (p-p):

200 Å, 13 Å/V

Pressure (p-p):

2000 kPa, 133 kPa/V

Khuri-Yakub Group, Ginzton Lab, Stanford University

Results: Bias Voltage

VPULSE=+5V, tPULSE=20ns

Collapsed operation:

- Higher resonance frequency : contact radius effect

- Larger displacement & output pressure

55 60 65 70 75 8020

30

40

50

60

70

Bias Voltage(V)

Dis

pla

cem

en

t (p

-p)

(A)

CollapsedConventional

55 60 65 70 75 800

2

4

6

8

10

Bias Voltage(V)

Reso

nan

ce F

req

uen

cy (

MH

z)

CollapsedConventional

Khuri-Yakub Group, Ginzton Lab, Stanford University

Results: AC Excitation

Collapsed

operation

VBIAS=70 V

AC excitation:

fEXC=1 MHz

VP-P=30V

2nd harmonic:

-23 dB

Displacement (p-p):

200 Å, 7 Å/V

Pressure (p-p):

36 kPa, 1.2 kPa/V

Khuri-Yakub Group, Ginzton Lab, Stanford University

Results: AC Amplitude

VBIAS=65 V, fEXC=1 MHz

Collapsed operation: MORE LINEAR RESPONSE

10 15 20 25 30-28

-26

-24

-22

-20

-18

-16

-14

AC Amplitude (p-p) (V)

2n

d o

rder

harm

on

ic level (d

B)

ConventionalCollapsed

10 15 20 25 3080

100

120

140

160

180

200

220

240

AC Amplitude (p-p) (V)

Avera

ge D

isp

lacem

en

t (p

-p)

(A)

ConventionalCollapsed

Khuri-Yakub Group, Ginzton Lab, Stanford University

Results: Collapse-snapback

Collapse-snapback

operation

VBIAS=70 V

AC excitation:

fEXC=1 MHz

VP-P=40 V

2nd harmonic:

-18 dB

3rd harmonic:

-10 dB

Displacement (p-p):

1200 Å, 30 Å/V

Pressure (p-p):

3600 kPa, 90 kPa/V

Khuri-Yakub Group, Ginzton Lab, Stanford University

Results: Collapse-snapback

Collapse-snapback

operation

VBIAS=65 V

AC excitation:

fEXC=5 MHz

VP-P=60 V

Displacement (p-p):

1600 Å, 26 Å/V

Pressure (p-p):

9500 kPa, 158 kPa/V

Khuri-Yakub Group, Ginzton Lab, Stanford University

Comparison of Operation Regimes

Performance Conventional Collapsed Collapse-

Snapback

VBIAS High Low Low

VTOTAL <VCOLLAPSE >VSNAPBACK >VCOLLAPSE

<VSNAPBACK

Output

Power

Low Medium High

Linearity Good Better Worse

Khuri-Yakub Group, Ginzton Lab, Stanford University

Investigated different regimes of operation using transient analysis in FEM

• Collapsed operation:

– More linear

– Low voltage bias

– Higher frequency

• Collapse-snapback operation:

–Higher output pressure

Future work: 3D analysis of CMUT arrays

Conclusion