S t a t e U n i v e r s i t y , U e t r o i t , M I 482U2 Abstract A one-dimensional a n a l y s i s o f t h e inechani sms by which a c o u s t i c waves a r e generated i n t h e thermoacoustic microscope i s presented. Thermal waves and a c o u s t i c waves a r e t r e a t e d as eigen-modes o f t h e system, so t h a t t h e r e i s no a c o u s t i c wave generated by thermal waves i n t h e bulk. It i s then shown t h a t t h e r e a r e f o u r channels by which t h e i n c i d e n t energy can be converted i n t o a c o u s t i c energy which can produce an image. One i s p u r e l y acoustic, and t h e others a l l i n v o l v e mode-conversion o f thermal waves t o a c o u s t i c waves. An important consequence i s t h a t thermoacoustic microscope images can r e f l e c t d i f f e r e n t combinations o f p h y s i c a l parameters than do images made w i t h microscopes based on pure thermal wave s c a t t e r i n g . s c a t t e r i n g mechanisms t o e x h i b i t r e s o l u t i o n which i s much b e t t e r than e i t h e r t h e thermal o r a c o u s t i c wave l e n g t h . We comment on t h e a b i l i t y o f 1. I n t r o d u c t i o n The development o f t h e new f i e l d o f thermal wave imaging has r e s u l t e d i n t h e i n v e n t i o n o f several new types o f Inicroscopes which a r e capable o f imaging near sub-surface f e a t u r e s o f opaque s o l i d s . microscope which uses a modulated p a r t i c l e beam ( e l e c t r o n s , photons o r i o n s ) as a heat source on one sample surface, and a p i e z o e l e c t r i c transducer attached t o t h e sample as an a c o u s t i c de5ector.6 The modulation frequency i s t y p i c a l l y 10 t o 10 Hz so t h e s i g n a l i s i n t h e u l t r a s o n i c range. When t h e e l e c t r o n beatn i n t h i s device i s scanned i n a r a s t e r over t h e sample surface, w i t h t h e l o c k -i n -d e t e c t e d transducer s i g n a l being simultaneously scanned over t h e screen of a video monitor, an image o f surface and near-surface f e a t u r e s appears on t h e screen. Depending on t h e n a t u r e o f t h e sample, these f e a t u r e s could i n c l u d e such t h i n g s as metal1 u r g i c a l g r a i n boundaries, implanted regions i n semiconductors, o r cracks i n the m a t e r i a l . However, i n order t o make f u l l use o f t h e microscope as a q u a n t i t a t i v e NDE t o o l , one must be a b l e t o i n t e r p r e t t h e imayes i n terms o f t h e physical p r o p e r t i e s o f t h e f e a t u r e s being imaged. For example, when one sees a g r a i n boundary whose image i s dark on one s i d e and l i g h t on t h e other, what i s t h e p h y s i c a l p r o p e r t y which i s responsible f o r t h e c o n t r a s t ? Is i t a change i n thermal Oye o f these i s t h e thermoacoustic The a p p l i c a t i o n s t o NUE are obvious. c o n d u c t i v i t y , a change i n e l a s t i c constants, a change i n thermal e x p a n s i v i t y , o r some o t h e r p r o p e r t y which i s bei...
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