Transducers with Screen Printed Matching Layers

R.F. Vogel, A s s i s t a n t Professor

E l e c t r i c a l and Computer Engineer ing Department U n i v e r s i t y o f Iowa, Iowa City, Iowa 52242

Abs t rac t

The b e n e f i t s and p o s s i b i l i t i e s o f con t i nuous ly v a r y i n g acous t i c impedance matching has been discussed i n a p rev ious paper [l]. Near ly con t inuous ly v a r y i n g impedance can be r e a l i z e d w i t h screen p r i n t e d l a y e r s . Screen p r i n t a b l e m a t e r i a l s w i t h t h e e n t i r e range o f impedances needed f o r matching PZT t o water have now been i d e n t i f i e d and tes ted . New t e s t methods w i l l be expla ined. Resu l t s fo r t h e new m a t e r i a l s w i l l be g iven. A t ransducer w i t h p r i n t e d matching l a y e r s has been const ructed. The method o f c o n s t r u c t i o n and v a r i o u s measured r e s u l t s w i l l be given. Techniques f o r us ing t h i s match ing method on severa l t ypes o f t ransducers, i n c l u d i n g th i ckness , a r rays , backed unbacked, and annular types, w i 11 be d i scussed .

I n t r o d u c t i o n

The development o f acous t i c impedance matching techniques has produced t ransducers w i t h as many as t h r e e match ing l a y e r s [2], and a v a r i e t y o f a n a l y t i c a l techniques [3,4,5] t o o p t i m i z e l a y e r s and e l e c t r i c a l match ing networks t o c e r t a i n performance c r i t e r i a . An optimum impedance versus d i s t a n c e cu rve was g i ven i n [l] t o match a t ransducer w i t h a c o n t i n u o u s l y v a r y i n g a c o u s t i c impedance.

A n e a r l y cont inuous impedance matching media can be made w i t h screen p r i n t e d l a y e r s . Measurements on a number o f screen p r i n t a b l e m a t e r i a l s was made and repo r ted i n [l]. Resu l t s o f a d d i t i o n a l measurements w i l l be g i ven i n t h i s r e p o r t . Advantages o f u s i n g screen p r i n t a b l e m a t e r i a l s f o r match ing l a y e r s a r e 1. A n e a r l y cont inuous impedance matching cu rve i s poss ib le , 2. The m a t e r i a l s have good adherence, and 3. Dev ia t i ons f rom i d e a l va lues i n t h e f i r s t l a y e r s can be c o r r e c t e d f o r by s l i g h t adjustments i n l a t t e r l aye rs . As w i l l be shown l a t e r , t h i s ad justment can be made on t h e b a s i s o f e l e c t r i c a l measurements made a f t e r each p r i n t e d l a y e r .

The methods o f measurement o f a c o u s t i c p r o p e r t i e s o f screen p r i n t a b l e m a t e r i a l s have been p r e v i o u s l y r e p o r t [1,6]. The comparison o f t h e r e s u l t s i n [l] and [7], where poss ib le , show good c o r r e l a t i o n between b u l k p r o p e r t i e s and p r o p e r t i e s o f comparable p r i n t e d m a t e r i a l s .

704 - 1985 ULTRASONICS SYMPOSIUM

Sample P repara t i on and T e s t i n g

Sample p r e p a r a t i o n and t e s t i n g procedures are s i m i l a r t o those desc r ibed i n [l] and [ 6 ] . The I n a t e r i a l s t o be t e s t e d a r e p r i n t e d on to p o l i s h e d alumina subs t ra tes and cured accord ing t o t h e manufacturers recommendations. The backs ide i s metal p l a t e d f o r e l e c t r i c a l con tac t t o t h e t ransducer . A PZT t ransducer i s then l a i d on t h e subs t ra tes and echo ineasurement a re made. The d i f f e r e n c e i n t i m e between t h e r e t u r n echo f o r a l a y e r on a s u b s t r a t e and a b lank s u b s t r a t e combined w i t h th i ckness g i ves v e l o c i t y i n fo rma t ion . Acoust ic impedance i s obta ined from t h e p roduc t o f t h i s va lue o f v e l o c i t y and d e n s i t y , determined from volume and weight measurements.

For some m a t e r i a l s , t h e PZT t ransducer was p laced d i r e c t l y on t h e p r i n t e d l a y e r w i t h a .0002" aluminum f o i l i n between t o serve as an e l e c t r i c a l con tac t . The r e s u l t s o f t h i s method were used t o c o r r e l a t e w i t h t h e p r e v i o u s l y desc r ibed i n d i r e c t method.

Other t e s t s o f some m a t e r i a l s were used t o p r o v i d e f u r t h e r c o r r e l a t i o n o f r e s u l t s . A f t e r t e s t s o f v e l o c i t y were made by t h e methods descr ibed, t h e p r i n t e d l a y e r s were lapped down and t h e measurements made again. I n some cases, t h i s was done severa l t imes t o c o n f i r m v e l o c i t y c a l c u l a t i o n s . Another method o f t e s t i n g was t o p r i n t m a t e r i a l s d i r e c t l y on t o PZT t ransducers. A f t e r cu r ing , t h e PZT had t o be r e p o l a r i z e d and again, echo measurements were made. Resu l t s o f t h i s method o f t e s t i n g were no t s u b s t a n t i a l l y d i f f e r e n t t han those o f t h e o t h e r methods.

A t a b l e o f r e s u l t s o f acous t i c p r o p e r t i e s o f va r ious screen p r i n t a b l e m a t e r i a l s was g iven i n [l]. A d d i t i o n a l r e s u l t s , a long w i t h some o f t h e former r e s u l t s a p p r o p r i a t e f o r t ransducer c o n s t r u c t i o n , a r e g i ven i n Table 1.

Transducer Cons t ruc t i on

Using m a t e r i a l s l i s t e d i n Table 1, severa l t h i c k n e s s t ransducers were cons t ruc ted t o v e r i f y t h e m a t e r i a l p r o p e r t i e s and t h e t h e o r e t i c a l match ing c a l c u l a t i o n s . The goal was t o make t ransducers matched t o t h e a c o u s t i c impedance o f water w i t h a cen te r f requency near 3 MHz and a

0090-560718510000-0704 $1.00 0 1985 IEEE

Table 1. P r o p e r t i e s o f Var ious Thick F i lms P r i n t e d

Acoust ic Imp danc

(10 g kg/m 9 sec) Za Th 'ckness

(lo-' meters) De Y . 9 i t y , y e l o c i t y , V (10 kg/cm ) (10 meters/sec) Ma te r i a1

G1 assy D i e l e c t r i c DuPont 9841 DuPont 9841

139 216

2.9 2.9

4.96 4.50

14.4 13.0

244 274 256 314 179

1.1 2.6 4.7 5.4 5.1

3.16 2.11 1.99 1.82 1.48

3.5 5.6 9.3 9.9 7.6

10% Tungsten 20% Tungsten 22% Tungsten 30% Tungsten

.5 !MI)

.5 m)

.5 m)

.5 m) Eoo-Tek 360

20% Tungsten (.5 m) 30% Tungsten

305 288

4.1 5.0

1.97 1.56

8.1 7.8

Dental R e s t o r a t i v e Compounds [9] ESPE VISIO-DISPERS 345

380 372

1.5 1.4 2.0

3.07 3.30 4.81

4.6 4.7 9.6

3M S i l u x Un ive rsa l Paste 3M P-30 EXP Lo t 5

bandwidth as wide as poss ib le . I n choosing t h e match ing l a y e r s , t h e impedance curves o f [l] were used as a guide. However, t h e i d e a l cont inuous match ing cu rve cou ld no t be cons t ruc ted f o r severa l reason. F i r s t , a1 1 t h e m a t e r i a1 candidates f o r match ing a r e cured a t e leva ted temperatures and t h e i r thermal expansion c o e f f i c i e n t s a re cons ide rab ly d i f f e r e n t f rom t h a t o f t h e p i e z o e l e c t r i c , PZT. Th is means t h a t s t resses a r e induced when t h e u n i t i s cooled, caus ing bending o f t h e p la te . To keep t h e p l a t e s f l a t , m a t e r i a l was p r i n t e d on b o t h s ides. The second problem was t h e i n s u f f i c i e n t s e l e c t i o n o f m a t e r i a l s w i t k t h e Ppedance range o f 10 t o 15 megarayls (10 kg/m sec). Choosing t h e f i r s t l a y e r t o be 9841 d i e l e c t r i c compound and l i m i t i n g t h e impedance o f t h e second l a y e r t o something no g r e a t e r than 10 megarayls, p u t s i n t e r e s t i n g r e s t r i c t i o n s on t h e succeeding l aye rs . Wi th these c o n s i d e r a t i o n s i n mind, t ransducers w i t h impedance l a y e r s as shown i n F igu res 1 t o 3 were cons t ruc ted by p r i n t i n g l a y e r s on .020" t h i c k by .75" d iameter PZT-5A m a t e r i a l w i t h f i r e d - o n s i l v e r e l e c t r o d e s [ l o ] .

Transducers 1 and 2 a r e designed f o r near optimum bandwidth w i t h i n t h e c o n s t r a i n t s ment ioned above. These were cons t ruc ted as f o l l o w s . The 9841 d i e l e c t r i c m a t e r i a l was screen p r i n t e d on to b o t h s ides o f a .020" PZT 5A d i s k . The l a y e r s were cured i n a b e l t furnace w i t h a peak temperature o f 775°C. A f t e r c o o l i n g t h e PZT was repo la r i zed . Epoxy l a y e r s were then p r i n t e d and cured a t 90°C. Transducer 3 was designed f o r near optimum bandwidth us ing o n l y t h e f i l l e d epoxies o f Table 1. I n o t h e r words no h i g h temperature ( > l O O " C ) m a t e r i a l s . Th i s t ransducer d i d no t have t o be repo led a f t e r t h e p r i n t i n g and c u r i n g o f t h e match ing l a y e r s .

3.5

I ' 4

GLLSS rl 10

0

5

3 13 - 20 30 40 56

" l rn rn l

F i g u r e 1. Acoust ic Impedance Layers f o r Transducer 1.

34 5

P I T

- 2C .30 40 50 M 70 10 .90 1

x l m m l

F i g u r e 2. Acoust ic Impedance Layers f o r Transducer 2.

1985 ULTRASONICS SYMPOSIUM - 705

10 20 30 40

N

z 0

0 - L?-

z

I Imml

F i g u r e 3. Acoust ic Impedance Layers f o r Transducer 3.

L 1

R e s u l t s o f Transducer Ana lys i s and Tests

Several t ypes of a n a l y t i c r e s u l t s were generated t o compare w i t h t h e measurements made on t h e completed and p a r t i a1 l y compl e t e d dev ices. I n p u t impedance verses frequency was c a l c u l a t e d as i n [ l ] f o r t h e completed t ransducers, and i n one case, t h i s was done f o r t h e t ransducer a t va r ious stages o f complet ion. I n b o t h t h e a n a l y s i s and measured r e s u l t s o f t h i s t ype , t h e e f f e c t o f each added l a y e r can be seen. I n most cases, t h e a n a l y s i s was done f o r b o t h water l o a d and a i r load. Th is same t ype o f curve was generated from measured da ta w i t h t h e a i d o f a network analyzer (HP3577A). Losses were i n t roduced by m o d i f y i n g t h e l o a d i n g c o n d i t i o n s t o s imu la te loss i n l a y e r s .

F i g u r e 4 shows t h e magnitude o f t h e i n p u t impedance m u l t i p l i e d by angular f requency, named " i n p u t impedance f u n c t i o n " i n t h e f i g u r e l a b e l s , f o r t h e t ransducer o f F i g u r e 1 i n water. The v e r t i c a l axes on these graphs a r e normal ized t o t h e i n v e r s e o f t h e clamped capaci tance o f t h e t ransducer . Cal c u l a ted and measured r e s u l t s a r e shown. F i g u r e 5 i s a s i m i l a r s e t o f curves f o r t h e t ransducer o f F i g u r e 2, and F i g u r e 6 corresponds t o F i g u r e 3. F igu res 7, 8, and 9 a r e t h e same t ype o f curves f o r t ransducer number 2 taken a t i n t e r m e d i a t e s teps o f cons t ruc t i on . F i g u r e 7 i s t h e r e s u l t w i t h j u s t t h e d i e l e c t r i c l a y e r on e i t h e r s ide, F i g u r e 8 w i t h t h e 20% f i l l e d epoxy l a y e r added, F i g u r e 9 w i t h t h e 5% f i l l e d epoxy l a y e r added. F i g u r e 5 i s f o r t h e completed t ransducer .

The t ransmiss ion from e l e c t r i c a l t o acous t i ca l power i s cha rac t r i z e d by s c a t t e r i n g

acous t i ca l t o e l e c t r i c a l power. F i g u r e 10 shows t h e o r e t i c a l and measured va lues f o r IS311 w i t h o u t e l e c t r i c a l matching. F i g u r e 11 i s t h e same w i t h an e l e c t r i c a l match ing c i r c u i t shown i n t h e f i g u r e . The da ta f o r t hese f i g u r e s was taken w i t h a c a l i b r a t e d hydrophone p laced 1 cm from t h e face o f t h e t ransducer .

parameter S31, where IS311 5 ' I S t h e r a t i o o f

0.0 1.2 2.4 3.6 4.8 6.0 FREOUENCY (MHZI

F i g u r e 4. I n p u t Impedance Func t ion * f o r Transducer 1 i n Water. Ca lcu la ted : s o l i d l i n e , measured: dots .

F i g u r e 5. I npu t Impedance Funct ion* f o r Transducer 2 i n Water. Calcu lated: s o l i d l i n e , measured: dots .

706 - 1985 ULTRASONICS SYMPOSIUM

0

a

I I 1 I 0.0 1.2 2.4 3.6 4.8 6.0

FREQUENCY [MHZI

F i g u r e 6. I n p u t Impedance Funct ion* for Transducer 3 i n Water. Ca lcu la ted : s o l i d l i n e , measured: dots .

9 N

z 0 E". 0" z 3 t w 0

0 w a z

HL" =Is II z

z ? a-

H

r(

? 0

I I I I I .o 1.2 2 - 4 3.6 4. 8 6.0

FREQUENCY (MHZI

F i g u r e 7. I n p u t Impedance Funct ion* f o r Transducer 2 i n Water w i t h D i e l e c t r i c Layers Only. Ca lcu la ted : s o l i d l i n e , measured: do ts .

"1

9 0

I I 1 I 1 0.0 1.2 2.4 3.6 4.8 6.0

FREOUENCY (MHZI

F i g u r e 8. I n p u t Imepdance Funct ion* f o r Transducer 2 i n Water w i t h D i e l e c t r i c Layers and One Epoxy Layer. Calcu lated: s o l i d l i n e , measured: do ts .

? N

z 0

0 - z 3 t W 0

0 W II E

i-m =Jd a z

s".

z ? a -

c(

r(

? 0

- .

7 . . .

1 I 1 I I

2.4 3.6 4.8 6.0 ,o 1.2

FREQUENCY (MHZ)

F i g u r e 9. I npu t Impedance Funct ion* f o r Transducer 2 i n Water w i t h D i e l e c t r i c Layers i n Two Epoxy Layers. Calcu lated: s o l i d l i n e , measured: dots .

*The Inpu t Impedance Func t ion i n these graphs i s a c t u a l l y t h e impedance o f t h e t ransducer m u l t i p l i e d by t h e susceptance o f i t ' s clamped capaci tance.

1985 ULTRASONICS SYMPOSIUM - 707

9 - OD 0

.--I

M v)

L L ? o o

n b.:

W 3

- 0 z c3 (r

cu 0'

9 0 .

F i r s t , t h e thermal expansion d i f f e r e n c e i n P Z T and match ing l a y e r m a t e r i a l s can l e a d t o bending o f t h e t ransducer , e s p e c i a l l y i f t h e matching l a y e r m a t e r i a l s need t o be cured a t h i g h temperature. The s o l u t i o n t o t h i s problem i s t o use no h i g h temperature cured m a t e r i a l s o r , more p r a c t i c a l l y f o r our work, use a combinat ion o f m a t e r i a l s on b o t h s ides o f t h e PZT which r e s u l t i n equal s t resses b u i l t i n on bo th s ides . Th is was done on t ransducers 1 and 2 and r e s u l t e d i n no measurable c u r v a t u r e i n t h e f i n i s h e d t ransducers.

I I I I 1 .o 1.2 2.4 3.6 4.8 6.0

FREQUENCY [ MHZ I F i g u r e 10. Square Root o f Energy Conversion,

IS311 f o r Transducer 2 Unmatched E l e c t r i c a l l y . Ca lcu la ted : s o l i d l i n e , measured: do ts .

F i g u r e 11. Square Root o f Energy Conversion, IS311 f o r Transducer 2 Matched E l e c t r i c a l l y . Ca lcu la ted : s o l i d 1 ine , measured : do ts .

D iscuss ion

Cons t ruc t i ng t ransducers a c o u s t i c a l l y matched t o water i s g r e a t l y s i m p l i f i e d and improved by screen p r i n t i n g t h e match ing l a y e r s . Reasons f o r t h i s were l i s t e d i n t h e i n t r o d u c t i o n . Some o f t h e new problems which a r i s e as a r e s u l t o f t h i c k f i l m methods w i l l be d e t a i l e d here, and some s o l u t i o n s w i l l be g iven.

Another r e s u l t o f h i g h temperature c u r i n g i s t h e d e p o l a r i z a t i o n o f t h e PZT. Th is s imp ly means t h a t i t needs t o be r e p o l a r i z e d a f t e r process ing. E l e c t r i c a l con tac ts f o r r e p o l a r i z i n g and t e s t i n g a r e made by l e a v i n g smal l wedges o f conductor a t t h e edge o f t h e conductor p l a t i n g on t h e P Z T d i s k s .

C o m p a t i b i l i t y o f screen p r i n t e d l a y e r s i s a c o n s i d e r a t i o n which has been repea ted ly addressed by t h i c k f i l m h y b r i d c i r c u i t manufacturers . Much i s known about us ing t h i c k f i l m s i n combinat ions (see re fe rences i n [l]). There a re unanswered ques t i ons about us ing comp le te l y d i f f e r e n t t ypes o f i n k s toge the r . Except f o r t h e thermal expansion problem mentioned e a r l i e r , we have had no o t h e r problems w i t h combinat ions prov ided one s imp le r u l e i s f o l l owed : The cu re temperature o f succeeding l a y e r s must be equal t o o r l e s s than t h a t o f t h e preceding l a y e r s . Many m a t e r i a l s which would make good candidates f o r -match ing 1 ayers may have adherence problems, b u t screen p r i n t a b l e i n k s a r e g e n e r a l l y f o rmu la ted f o r good adherence above a l l e l se . Using screen p r i n t a b l e i n k s and f o l l o w i n g t h e s imple r u l e s t a t e d above, adherence has n o t been a problem.

The p rog ress ion o f curves shown i n F igu res 7, 8, 9, and 5 i l l u s t r a t e how t h e e l e c t r i c a l i n p u t impedance changes as matching l a y e r s a r e added. F i g u r e 5 i s t h e f i n i s h e d t ransducer i n water and shows a n e a r l y i d e a l cu rve f o r a t r a n s d u c t i o n f requency range from 2.4 MHz t o 4.1 MHz. I n p u t impedance i s ob ta ined from these curves by d i v i d i n g t h e v e r t i c a l axes va lues by angular f requency and m u l t i p l y i n g by t h e susceptance o f t h e clamped capaci tance. Doing t h i s t o t h e cu rve i n F i g u r e 5 y i e l d s a n e a r l y cons tan t va lue f o r i n p u t impedance over t h a t f requency range. Knowing what the p rog ress ion o f curves should l o o k l i k e d u r i n g c o n s t r u c t i o n a l l o w s f o r t h e c o r r e c t i o n o f l a y e r parameters a f t e r each measurement t o i n s u r e t h a t a maximum bandwidth o f t r a n s d u c t i o n i s achieved.

Th is method o f match ing can be used j u s t as w e l l on o t h e r types o f t ransducers, such as a r r a y s and annular t ransducers. Arrays cou ld have t h e match ing l a y e r s screen p r i n t e d on t h e s u r f a c e b e f o r e t h e i n d i v i d u a l e lements a r e separated or, i f t h e geometry were n o t t o o sma l l , t h e l a y e r s cou ld even be p r i n t e d a f t e r t h e elements a r e separated. Annular t ransducers o f t h e t h i c k n e s s t y p e cou ld be t r e a t e d t h e same way. Annular t ransducers o f t h e su r face t y p e [ll] can be matched w i t h p r i n t e d l a y e r s s ince t h e

708 - 1985 ULTRASONICS SYMPOSIUM

su r face mot ion which couples t o t h e water i s normal t o t h e sur face. The i d e a l acous t i c impedance con tou r o f t h e l a y e r s may be somewhat d i f f e r e n t t han f o r t h i c k n e s s types. Thickness t ransducers w i t h back ing m a t e r i a l s can be handled j u s t as e a s i l y except t h a t t h e r e q u i r e d impedance contours a r e q u i t e d i f f e r e n t .

Concl us ions

Screen p r i n t a b l e m a t e r i a l s s u i t a b l e f o r use as a c o u s t i c match ing l a y e r s have been i d e n t i f i e d . A t ransducer has been cons t ruc ted us ing screen p r i n t e d matching l a y e r s . P r o p e r t i e s o f t h e t ransducer a t d i f f e r e n t stages o f c o n s t r u c t i o n have been shown. The p o s s i b i l i t y o f making " i n course" c o r r e c t i o n s has been demonstrated and t h e f i n a l t ransducers have been shown t o agree w i t h t h e theory. Matching e l e c t r i c a l c i r c u i t s have been used w i t h these t ransducers t o make e f f i c i e n t , wide band t ransducers.

Acknowledgement

T h i s work was supported by t h e Nat iona l I n s t i t u t e o f General Medical Sciences o f t h e U.S. Department o f Hea l th and Human Services, Grant Number 5R01 GM31671-03.

References

1. Vogel, R.F., "Acoust ic P r o p e r t i e s o f Thick F i lms f o r Use as Matching Layers," Proc. IEEE U l t r a s o n i c Symposium, p. 988, 1984.

2. Souquet, J. , Defranould, P. and Desbois, J., "Design o f Low-Loss Wide-Band U1 t r a s o n i c Transducers f o r Noninvasive Medical App l i ca t i on , " I E E E Trans. Sonics and U l t r a s o n i c s , Vol. SU-26, No. 2, pp. 75-81, March 1979.

3. S e l f r i d g e , A.R., Baer, R., Khuri-Yakub, B.T. and Kino, G.S., "Computer-Optimized Design o f Quarter-Wave Acoust ic Matching and E l e c t r i c a l Matching Networks f o r Acoust ic Transducers ," IEEE U l t r a s o n i c Symposium, p. 644, 1981.

4. Szabo, T.L. , " P r i n c i p l e s o f Nonresonant Transducer Design ," IEEE U1 t r a s o n i c Symposium, p. 804, 1984.

5. Hayward, G. and Jacko, M.N., " A L a t t i c e Model f o r t h e D i s c r e t e Time S imu la t i on o f a M u l t i - Layered P i e z o e l e c t r i c Transducer System," IEEE U l t r a s o n i c Symposium, p. 888, 1984.

6. Vogel, R.F., Ke l l ey , S.D., and K i r k , D.M., "Acoust ic Impedance P r o p e r t i e s o f Thick Film," I n t e r n a t i o n a l J . Hyb. Microelect . , Vol. 8, No. 1, 1985.

7. Se1 f r i d g e , A.R., "Approximate M a t e r i a l P r o o e r t i e s i n I s o t r o D i c Mater ia ls. ." IEEE _. Trans. Serv ices and U i t r a s o n i c s , Vol. S U X No. 3, pp. 381-394, May 1985.

8. Screen p r i n t a b l e epoxies donated by Frank W. Kulesza o f Epoxy Technology, Inc., B i l l e r i c a , Mass.

9. The den ta l r e s t o r a t i v e compounds a r e u l t r a v i o l e t l i g h t cu re r e s i n s w i t h g lassy f i l l i n g . The 3M, P-30 compound i s 75% f i l l e d , S i l u x i s 36% f i l l e d , and V i s i o - d i s p e r s i s 66% f i l l e d .

10. The PZT-5A d i s c s were obta ined f rom V e r n i t r o n .

11. Vogel, R.F. and P o l l a r i , G.M., "Annular Sur face Wave t ransducers f o r Medical Imaging," Acous t i ca l Imaging, Vol . 13. Plenum Press, New York, 1983 9 P. 381-

1985 ULTRASONICS SYMPOSIUM - 709