A Critical Analysis of the Thermoacoustic Microscope

Abstract

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...

Ion Acoustic Microscopy

“…According to theoretical estimates based solely on thermal wavelength, it should not be possible to record changes in surface thermal properties on a depth scale less than about 10 microns for this frequency, while the implant depth is only about 0.1 microns. However, consideration of the volume into which the energy is deposited and from which the thermal and acoustic waves propagate into the solid is the other part of resolution in thermal microscopies so that our observations are in good agreement with current theory [22].…”

Contrast Mechanisms in the Thermoacoustic Microscope

“…These equations and their solut·ions have been described elsewhere [10,11]. These solutions were obtained by separating the coupled equations into equations for their uncoupled normal modes.…”

“…we have evaluated the effects of the presence of a free surface and have calculated [10] the relative efficiencies of tour distinct processes for imaging of a discontinuity such as a grain boundary. It is assumed that the source is an electron or photon beam for which the momentum contribution to the waves is small compared to the energy contribution.…”

Deep Thermoacoustic Imaging Using Scanning Electron Acoustic Microscopy

“…This work strongly supports the Jackson -Amer thermal bending model [3] for low frequency (up to approximately 100kHz in small specimens) acoustic detection in the solid as compared to the theory of Opsal and Rosencwaig [4] which does not incorporate total specimen response to the thermal source. This problem has more recently been theoretically investigated by Favro [5,6], who has developed a more general theory encompasing both the Opsal-Rosencwaig short wavelength limit and the Jackson-Amer long wavelength limit. Favro's theory also accounts for features in the image shown.…”