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Sensors and

Actuat ors A,

41-42 (1994) 167-173

167

An

integrated multi-element

array transducer for ultrasound imaging

J.V. Haffiele*,

N R. Scales, A.D. Armitage, P.J. Hrcks, Q.X Chenb and P.A Payne

Deparbnent of EZectncal Engtneemg Electronrcs, and bLkparhnent of In.vtnanentatlon & Analytuzal Scrence,

UM IST, PO Box 88,

M anchester M60 1QD (UK )

Abstract

Much progress has been made towards mtegratmg the electromc ctrcmtry associated with either hnear or phased

ultrasonic array scannmg mto the hand-held case of the transducer The number of wires reqmred to connect

the transducer back to the display system has been dramatically reduced and the path length between transducer

elements and the driver cncults kept to a few nulhmetres To a&eve this new construction, polymer transducer

arrays have been fabricated and the pulse-control cucmtry has been integrated onto custom-destgned slhcon

chips

1. Introduction

High-resolutron focused beams of ultrasonic energy

are used for rmagmg purposes, for example, by the

medical profession and m non-destructwe testmg Hrgh-

resolution beam focusing 1s achieved by pulsmg the

elements of an ultrasonic array transducer m some

predetermmed manner. This may take the form of a

phased array, which allows for an electromcally steered

beam (sector scannmg), or a hnear array, which typrcally

would allow groups of elements to be pulsed successively

along the array, or even a combmatron of the two

Conventronal multi-element ultrasound transducer ar-

rays are connected by long multr-wrre cables to the

systems electronics and display unit This can cause

numerous problems, mcludmg Interference and reflec-

tions along the cables The net effect of these 1s to

make the system signal-to-notse ratio poorer These

effects are magmfied if the system is designed to operate

at higher than conventional ultrasound frequencies (e g ,

20 MHz) [l]

2. Polymers as nltrasonic transducers

It 1s extremely drfficult and expensrve to construct

high-frequency high-resolution ultrasonic arrays using

conventronal ceramrc prezoelectnc matenals, and vtr-

tually Impossible when the transducers are to operate

above 10 MHz centre frequency Most of the problems

encountered can be solved by employmg ptezoelectnc

*Author to whom correspondence should be addressed

polymer materials This 1s due to the unique charac-

tensttcs of these polymers, including ready avatlabthty

m thm-film form, rangmg m thtckness from under 10

pm to several hundred mrcrometres, flex&&y, me-

chanical robustness, and chemrcal stabity, very high

internal acoustrc and drelectnc losses, malung them

mtnnstcally wide band with very low inter-element

interference or cross-talk, both acoustically and elec-

trically

Polyvmyldenefluonde (PVDF) by itself, and parttc-

ularly wrth certain co-polymers, can be made to exhrbrt

prezoelectnc properties [2] That is, its shape may be

altered by applytng an electric field and, conversely,

deformations of shape cause electrical charge on its

surface A film of PVDF may thus be used as both a

generator and recetver of sound waves Using thin films,

the frequency of the sound produced may extend into

tens of MHz [3] It IS very easy wrth these matenals

to produce arrays of transducers using conventronal

photohthographlc techniques Alummrum IS deposited

on either side of a film of PVDF and is then patterned

by selecttvely etching on one side to form array elements

The other side is patterned into a ground plane, which

covers the active transducer area so that transducer

elements are formed wherever the two patterns overlap

The rear side of the array 1s tilled with a backmg

matenal of appropriate acoustic impedance (typrcally

epoxy resin) to optnmze the acoustic performance of

the transducer Fabncatron of arrays, pre-focused along

one axis, is achieved by surtably deformmg the array

before back-filling (Fig 1)

0924-4247/94/$07 00 Q 1994 Elsewer Sequoia All rtghts resewed

SSDI 0924-4247 93)00647-M


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169

16 clocks, derwed from a smgle source but separated

by one smteenth of the clock penod from each other,

are used To achieve a 1 11s me resolution each of

the clocks has a penod of 16 ns (62 5 MHz) separated

by 1 ns from each other. In th= way high resolution

is obtamed usmg standard CMOS technology In order

to set a pulse delay by this method the most sigmficant

bits of a 19&t delay value are loaded mto a 15-bit

counter of which there IS one per pulse channel The

appropriate counter clock IS selected by the four least

sign&ant bits, by means of a X-to-1 clock multiplexer,

to grve the fine resolution Care must be exercised m

the lay-out of the clock multiplexer, as the propagation

delays from any mput to the output must be identical

A strmg of delay elements is used to produce the

16 phased clocks (Pig 4) The delay element 1s smply

two mverters m senes The speed of the zero-to-one

transition of the first mverter 1s determmed by the

extra pMOS pull-up transistor controlled by V, Thus

the propagation delay of a falling edge from IN to

OUT can be set by the voltage on V, An external

reference clock supphes pulses to the delay hne As

these progress down the delay hne they are stretched

due to the dtierence m propagation tunes for negative

and positwe edges Delay control cmxutry that 1s only

sensitwe to negative-going edges adlusts the control

voltage V, so that the negative edge of the zeroth clock

lures up with the negative edge of the surteenth clock

If these are the same, then the delay for 16 elements

IS exactly the clock period and the delay for each

element 1s exactly one snrteenth of the clock penod

Two control signals are responsible for establishing

V, and are provided by the fhp-flop arrangement of

Fig 5 &, IS high for the per& from the negatwe

edge on the eighth clock (CLK8) to a negative edge

on the stieenth clock, (CLK16) Ct,, IS high from the

negatnre edge on the eighth clock to a negatnre edge

on the zeroth clock, (CLKO) If the associated penod

Fig 4 Delay-lme schematic diagram and mdwdual delay element

Ftg 5 Fhp-flop arcmtry for generatmg delay control signals

VDD

iiiE+-JA@

Fig 6 Pump-up/pump-down delay-lockmg nrcmtry

&+< fs,,, then the pulses along the delay hne need

further stretchmg and hence V, needs to ~LX Thus

when Cts16

1s low and the mverse of Ct ,,,, (Go), I S

low, the 1 pF capacitor of Fig 6 is charged through

the two pMOS transistors P1 and PZ The potential on

this capacitor 1s the control voltage V, Conversely If

3,16> h3 0,

the pulses on the delay lme are overstretched

and V, must be lowered Thus when both ct8,16and

Cr,,0 are high, the capacitor is dscharged through

nMOSFETS N1 and NZ (Fig 6) The additional transistor

pair Ps and N, allows the control voltage V, to be reset

to zero for mnumum delay

3 2

Tesmg t he delay he

Delay-lme test results are shown m Fig. 7 They were

obtained usmg a Hewlett Packard 8180 data generator,

Hewlett Packard 8132 data analyser and a Stanford

Research DG535,5 ps resolution pulse generator This

equipment was controlled via an IEEE 488 bus, by

computer programs wntten 111 he c programmmg

language Brretly the output pulse from the DG135 was


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170

Delay Lme 5V25 C

18

16 -

l&u1square ir

14

M eamred D ata Pomt s

j/s

12

-

, F

- 10

2

L 8-

% 6-

? f

k f -i

E

d

4

f f, -I

2-

0 I

-2 .

-4

0 2 4 6 8 10 12 14 16

Delay Tap

FIN 7 Delay along delay he as a function of delay element

Any one of 16 delayed clocks can be selected to clock pro-

grammable down counters

connected to the D mput of an on-chip fip-flop The

DG135 was tnggered by the delay-hne clock The delay

between the tngger and the mput to the flip-flop could

thus be vaned untd a change of &p-flop output was

observed Thus was done with each of the delay-line

taps connected to the flip-flop clock input by means

of an on-chip multiplexer The phase-&lung mechamsm

should, of course, make the IC self-compensating with

respect to temperature and power-supply vanatlons,

provided that the delay-lme input clock remains stable

All tests indicate that over all conceivable ranges of

interest this 1s indeed the case

4. The

integrated piemelectric transducer array

The mam concern 1s to interface the array mth the

on-board ASIC chips m a reliable and smple way The

present design is based on a rear filhng method, and

alumma substrates with screen-printed gold patterns

have been used to achieve the reqmred mterfacmg with

on-board electronics The mam part of the array trans-

ducer consists of the array stub and the bonding pad

extension, as shown m Fig 8 The array stub comprises

an alumma substrate, the plezoelectrrc polymer film

and the acoustic backing For an mltlal feaslbdlty study,

a 32-element array was deslgned and fabncated Each

array element 1s 0 2 mm wide and 7 mm long v&h an

inter-element gap of 005 mm The alumma substrate

has 32 bonding pads on two of the four sides, and two

bondmg pad extensions were used for easy connection

to the electronics The piezoelectnc polymer film is

glued to the top surface of the substrate and electrode

patterns are produced usmg standard photohthographlc

techmques

One of the cntlcal areas of construetlon 1s connecting

electrode patterns on the film to the electrode pads

on the alumina substrate This was done by using sdver-

loaded conductwe pamt The ground electrode was

formed by vacuum coatmg the other side of the plezo-

electnc film through the central opening of the ahunma

substrate (Fig 8) Conductive epoxy or pamt was used

to attach an electrIca lead to this ground electrode

A square tube was glued to the lower side of the

alumma substrate and epoxy-based backmg matenals

havmg acoustic Impedances close to that of the polymer

were filled mto the hollow thus formed

Azimuthal focusing effects are achieved by pressing

the front face of the stub structure against a suitable

cyhndrlcal surface durmg the curmg of the epoxy-based

backmg matenals, and different focal lengths can be

obtained by changing the cylinder diameter The two

otymw

jum unth

FIN

8

Transducer assembly before fIxmg pulse-dnvmg clrcmtry Also shown 1s the substrate to wblcb the polymer film IS attached


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bondmg pad extension boards extend the bonding pads

on the alumma substrate from 0 5 mmX 0 5 mm to

15 mm x 15 mm, makmg direct soldering to the pads

possible They also make It possible to evaluate the

array element usmg an independent pulser and receiver,

without relymg upon the on-board electronic clrcultry

Smce the mam part of this 32-element array assembly

IS only about 12

mmX

12 mm

X

10 mm, there IS plenty

of room for the on-board electronic cucuitry and the

whole integrated array can be encapsulated m a hand-

held case

7

4

1 hhatunzamn

of pul se-generat i ng ucurl ry

Work has also been successful m mmlatunzmg the

high-voltage pulse generator components ZETEX

FMMT 415 surface mount avalanche transistors provide

very fast high-voltage pulses whilst occupymg


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172

5. Array performance measurements

51 Unlformrty

The untfonmty test was carried out m a water tank

A reflected echo from a plane steel target is obtamed

from each element, usmg the same pulser and receiver

Because of the high preclslon of the electrode pattern

made with pbotohthographlc techniques, the Nnformrty

between the array elements IS excellent The responses

from different elements are vntually rdentlcal

52 Cross-talk

There are two forms of interference between elements

m an array transducer. the electrrcal and the acoustic

cross-talk Electrical cross-talk arrses from the dlelectnc

couplmg between elements and the acoustic cross-talk

1s caused by the propagation of acoustic waves along

the surface of the array For transducer arrays made

from ceramics, cross-talk between elements 1s a major

problem due to the high drelectnc constant and low

internal loss of these matenals For transducer arrays

made from ptezoelectnc polymer matenals, these prob-

lems are largely non-exrstent The drelectrrc constant

of PVDF 1s only about

1

of that of lead xrrconate

titanate (PZT) and the internal mecbaNca1 loss IS about

25 times larger The low dielectnc permtttrvrty reduces

the electrical cross-talk and the high mechanical loss

reduces the acoustic couphng As a result, the cross-

talk between elements for a polymer array IS very small

Tlus greatly slmphfies the construction of polymer trans-

ducer arrays and there 1s vutually no design constramt

on the dlmensrons of the array elements from the

mechanical point of view

Expements have been carned out to measure the

cross-talk between adJacent array elements A 15 MI-Ix

tone burst wrth 10 V peak-to-peak value was used to

drove one element of the array transducer and the

output from SIX adlacent elements was monitored by

a Ngh-input-mpedance oscdloscope Typical cross-talk

1s observed to be m the 100 mV range As there 1s

no time delay between mdlvldual signals, we can con-

clude that mechanical cross-talk IS too small to be

measured

53 P e-echo measurementr

These were carried out using a pulser and receiver

uNt The trme-domain waveforms were drgttrxed by a

Tektronix digital oscilloscope (model 7854), and then

sent to a computer for frequency spectrum analysis

Figure 10 shows a typical example of the measurement

results It can be clearly seen that the transducer gwes

a very short pulse (80 nS) and wide bandwidth

-*ml

0

250 500

(4

Tvncns

16 32 48 6i

@)

Fmqnmcy MHz

Fig

10 (a) Puke-echo response from a smgle array element

and (b) a frequency spectrum sbowmg a ccntre frequency of 32

MHz, bandwxith (3 dB) 24l MHz, pulse duration 180 us

6. Conclusions

New polymer transducer array desrgns have been

produced and an interconnect technology allowing

ASICs to be connected to the polymer transducers has

been devrsed However, further work on the long-term

rehabdrty of tbrs approach is required Work 1s well

advanced towards this objective Indlvldual components

have been fabncated and tested and come up to spec-

ification in every respect Most importantly, a genenc

techrnque has been developed, and successfully dem-

onstrated, whrch wrll enable both transrmt and recerve

NulbchaMel ASICs to be reahxed Current work is

focused on deslgnmg recerve ASICs

The authors acknowledge with gratitude the Science

and Engmeenng Research Counnl of the UK for

provrdmg funding for tbts prolect, and EUROCHIP

for making commerctal rmcro-fabncatton facthtles avall-

able


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PA Payne, Medxal and mdustnal apphcatxms of lngh res-

olutlon ultrasound,l P tys, E SCL Idnon 28 (1985) 465-473

H Kaw;u, The p~emelectnc~ty of polyvmyldene fluonde, Ipn

I Appi Fhys, 8 (1969) 975-976

PA Payne and Q X Chen, m C Brook and P D Hanstead

(eds ), Rdabrllty WI Non-Lksmetwe Testmg NDT-88.

Per-

gamon, Oxford, 1989, Ch 10, pp 319-330

Y Aral and T Oshug~, TMC - a CMOS time to dlgtal

converter VLSI, IEEE Thm.s Nuclear SCL NS-36 (1989)

528-531

A Rothermel and F Dellova, Analog phase measurmg cmxut

for dlgtal CMOS-IW, m ESSCZRC 98 Copenhagen Den-

monk Sept 23-25 1992 pp 331-334

Biographies

John V Hat fi eld

ecensed his B SC degree m physics

from the Umverslty of Leeds m 1973 and the M SC

degree from the Umverslty of Manchester Institute of

Science and Technology (UMIST), Manchester, UK,

m 1984 He was awarded a Ph D by the same university

m 1988 for his researches into posItion-sensitive particle

detectors Currently he IS a senior lecturer m the

Department of Electrical Engmeenng and Electronics

at UMIST and technical director of Integrated Sensors

Ltd Ha research mterests are m the area of mtegrated

sensors and transducers

Nlgel R Scales

received a B SC degree m physics

from the Umversity of Manchester m 198.5 and an

M SC m VLSI systems engmeermg from UMIST m

1991 He ts currently workmg as a research a-ate

173

m the Department of Electrical Engmeenng and Elec-

tronics at UMIST, where he 1s also completmg work

on a Ph D thesis

An w D Armtage w as awarded a B Eng m elec-

tronic engmeermg by Manchester Polytechnic m 1991

and an M SC m VLSI systems engmeermg by UMIST

1111992 He 1s currently a research student m the

Department of Electrical Engmeermg and Electronics

at UMIST, where he IS reading for a Ph D m the area

of integrated ultrasonic transducers

Peter J H s was awarded a Ph D by the Umverslty

of Manchester m 1973 and Joined the Department of

Electrical Engmeermg and Electronics at UMIST as

a lecturer m 1978, where he is currently professor of

Mlcroelectromc Orcult Design I-hs major research

interests are largely m the area of integrated sensors

and he IS chauman of Integrated Sensors Ltd

QX

Chen

received his B Eng from the Department

of Scientific Instruments, ZheJlang University, Hang-

zhou, China m 1982 He was awarded a Ph D m 1989

after studymg m the Department of Instrumentation

and Anabtical Science at UMIST, where he 1scurrently

a research associate HIS current fields of interest are

ultrasonic sensors and mmlature Imaging arrays for

medical applications

Pet erA Payne eceived his Ph D from the Umversrty

of Wales m 1972 He 1s currently professor of Instru-

mentation and chairman of the Department of Instru-

mentation and Analytical Science, UMIST His fields

of interest are ultrasonic and acoustic sensors and

systems for medical and mdustrlal applications

cmos driver hatfield

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