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DEVELOPMENT AND CHARACTERISATION OF A
POINT ULTRASONIC DETECTOR BY ACOUSTIC
MICROSCOPY
L. Germain, J. Cheeke
To cite this version:
JOURNAL D E PHYSIQUE
Colloque C10, suppl6ment
au
n012,
Tome46,
d6cembre1985
page C10-759DEVELOPMENT AND CHARACTERISATION OF A POINT ULTRASONIC DETECTOR BY
ACOUSTIC MICROSCOPY
L.
GERMAIN AND J.D.N. CHEEKEDepartement de physique, Universit6 de Sherbrooke,
Sherbrooke, Quebec,
J1K 2R1,
Canada
R6sum6 - Nous pr6sentons les r6sultats obtenus avec une nouvelle technique pour imager les vibrations d'un rdsonateur pidzodlectrique 3 l'aide d'un microscope acoustique. Cette technique a St6 utilis6e pour caractdriser la r6ponse d'un transducteur ponctuel fait
1
partir d'un grand transducteur de LiNb03 dont l'une des dlectrodes prend la forme d'un fil trzs fin. Une application2
l'dtude de l'attdnuation dans un liquide est pr6sentbe.Abstract - We show results for a new technique used to image the vibrations of a piezoelectric resonator using an acoustic microscope. This method is used to characterise the spatial response of a point POGO detector used as one electrode on a LiNb03 transducer. Possible application to attenuation measurements in liquids is demonstrated.
T!e have recently developed a net7 technique for studying the vibration characteris- tics of acoustic resonators (1). The method involves scanning the focussed ultra-- sound beam of an acoustic microscope over the surface of the resonator and using the resonator as the detecting element for the microscope, so that we essentially make a transmission image of the resonator itself. Such piezoelectric resonators
are known to have very complex mode patterns (2),(3). Depending on the crystal cut, electrode configuration and supporting arrangement we can excite longitudinal, shear and flexural vibrations giving rise to a complex standing wave pattern in the crystal which we can image directly with the acoustic microscope to visualise the positions of the nodes and anti-nodes.
An example of this technique is shown in fig. 1 for a rectangular longitudinally cut 30 MHz LiNbOg transducer. The transducer is supported in cantilever fashion and the part observed in the figure is free standing and fully electroded over both surfaces. A very regular mode pattern is observed and we have verified that the'fringe spacing decreases regularly with frequency in experiments at 30, 90, 150 and 210 ?iHz.
In the present work we have applied this technique to the development and charac- terisation of a "point" ultrasonic detector using a 30 1Mz longitudinal LiNb03 circular transducer. The ground side of the transducer is fully electroded and
C10-760
JOURNAL DE PHYSIQUE
F i g . 1: Image of t h e v i b r a t i o n modes i n a r e c t a n g u l a r LiNb03 t r a n s d u c e r a t t h i r d harmonic (90 MHz). The w i d t h of t h e t r a n s d u c e r i s 2 mm. t h e b a r e c r y s t a l s u r f a c e (non m e t a l l i s e d ) i s exposed on t h e o t h e r s i d e . An e l e c - t r o d e i s d e f i n e d by t h e p o s i t i o n and c o n t a c t s u r f a c e of a s p r i n g l o a d e d POGO con- t a c t o f a b o u t 300 microns d i a m e t e r . Images o b t a i n e d u s i n g t h e a c o u s t i c microscope t e c h n i q u e a t 30 MHz and 150 PMz a r e shown i n f i g . 2 and f i g . 3. At 30 MHz t h e d i a m e t e r of t h e POGO c a n b e d i s c e r n e d i n t h e c e n t r a l p a r t of t h e image and fol-. lowing p r e v i o u s work we a s s o c i a t e t h e i n t e n s i t y v a r i a t i o n s around t h e c e n t e r t o f l e x u r a l v i b r a t i o n s . A t h i g h e r f r e q u e n c i e s a s i n f i g . 3 t h e f l e x u r a l v i b r a t i o n s a r e l e s s e x c i t e d and we o b s e r v e o n l y t h e t h i c k n e s s v i b r a t i o n a t t h e p o s i t i o n of t h e POGO and t h e s p o t d i a m e t e r c o r r e s p o n d s r o u g h l y t o t h a t of t h e POGO. I n o r d e r t o examine t h i s more q u a n t i t a t i v e l y we have done l i n e s c a n s o v e r a d i a m e t e r of t h e c r y s t a l t o o b t a i n t h e i n t e n s i t y v a r i a t i o n of t h e d e t e c t e d p u l s e a s f u n c t i o n of p o s i t i o n ; a n example is shown i n f i g . 4 a t 90 MHz. We n o t e t h a t i n t h i s configu- r a t i o n t h e e f f e c t i s e s s e n t i a l l y r e v e r s i b l e s o t h a t t h e same image i s o b t a i n e d i f t h e t r a n s d u c e r i s used a s e i t h e r s o u r c e o r d e t e c t o r .The p o i n t d e t e c t o r h a s proved u s e f u l i n u l t r a s o n i c i n v e s t i g a t i o n s of l i q u i d s . The a c o u s t i c r a d i a t i o n p a t t e r n s a r e o f t e n s t u d i e d u s i n g m i n i a t u r i z e d hydrophones
F i g . 2: Image o f a " p o i n t " t r a n s d u c e r F i g . 3: Image of t h e same t r a n s d u c e r a s
F i g . 4 : Line s c a n o v e r t h e d i a m e t e r t h e p o i n t d e t e c t o r of f i g . 2 , a t 90 ( t h i r d harmonic). of MHz 0.0 0.5 1.0 1.5 D i s t a n c e ( m m ) F i g . 5: Echo p a t t e r n s a t 30 MHz o b t a i n e d i n e t h a n o l w i t h a s t a n - d a r d 3 mm e l e c t r o d e LiNb03 t r a n s - d u c e r and a POGO e l e c t r o d e p o i n t t r a n s d u c e r , s e p a r a t e d by 0.45 mm. ( a ) T r a n s m i s s i o n mode, s t a n d a r d t o p o i n t t r a n s d u c e r . ( b ) R e f l e x i o n mode, w i t h t h e p o i n t t r a n s d u c e r . ( c ) R e f l e x i o n mode, w i t h t h e s t a n - d a r d t r a n s d u c e r .
JOURNAL
DE
PHYSIQUE
c e n t r a l sounil f i e l d .
Fig. 5 (b) corresponds t o emission and r e c e p t i o n by t h e POGO d e t e c t o r i n r e f l e c t i o n mode. I n t h i s c a s e t h e wave f r o n t should b e approximately s p h e r i c a l i n t h e c e n t r a l zone and should remain s o a f t e r s u c c e s s i v e r e f l e c t i o n s l e a d i n g t o very s m a l l phase v a r i a t i o n s a c r o s s t h e wave f r o n t i n t h e r e g i o n of t h e d e t e c t o r . This appears t o b e t h e c a s e a s t h e decay p a t t e r n remains p e r f e c t l y e x p o n e n t i a l o u t t o t h e l a s t detec- t a b l e echo. F i n a l l y i n f i g . 5 ( c ) t h e echo p a t t e r n f o r t h e t r a n s d u c e r w i t h 3 mm c e n t r a l e l e c t r o d e s i n r e f l e c t i o n mode i s shown and we s e e t h a t t h e decay p a t t e r n
i s n o t even approximately e x p o n e n t i a l .
I n c o n c l u s i o n t h e p r e s e n t work h a s shown t h a t i t i s p o s s i b l e t o develop a "point" u l t r a s o n i c d e t e c t o r u s i n g a l a r g e t r a n s d u c e r . The a p p l i c a t i o n t o t h e s t u d y of a t t e n u a t i o n i n l i q u i d s appears t o b e extremely promising. The system could a l s o be used f o r s t u d y i n g t h e r a d i a t i o n p a t t e r n from t r a n s d u c e r s i n t o s o l i d s and l i q u i d s by performing a mechanical displacement of t h e p o i n t o v e r t h e s u r f a c e of t h e r e c e i - ving t r a n s d u c e r . A f i n a l a s p e c t t h a t i s n o t y e t determined i s t h e s m a l l e s t p o i n t t h a t can b e used i n t h i s method; under p r e s e n t c o n d i t i o n s we have o b t a i n e d s p o t s i z e s of t h e o r d e r of 100 microns. On g e n e r a l grounds we b e l i e v e t h a t t h i s l i m i t w i l l be of t h e o r d e r of e i t h e r t h e a c o u s t i c wavelength i n t h e t r a n s d u c e r o r i t s t h i c k n e s s . Work a t h i g h e r f r e q u e n c i e s and d e t e c t i o n a t much h i g h e r harmonics of a 30 MHz t r a n s d u c e r i s planned t o i n v e s t i g a t e t h i s q u e s t i o n .
Acknowledgements: We would l i k e t o thank AndrC BeausCjour f o r t e c h n i c a l a s s i s t a n c e w i t h t h e microscope imaging. This p r o j e c t was supported by a s t r a t e g i c g r a n t from t h e N a t u r a l Sciences and Engineering Research Council of Canada.
References
(1) L. Germain and J.D.N. Cheeke, t o be p u b l i s h e d .
(2) E. Hafner, IEEE Trans. on Sonics and U l t r a s o n i c s SU-21,220,1974.
( 3 ) H. Bahadur and R. Parshad i n P h y s i c a l Acoustics XV1,ed. W.P. Mason and
R.N. Thurston, Academic P r e s s , N.Y. 1982.
(4) F.M. Boler and H.A. S p e t z l e r , Rev. S c i . Instrum.,
