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SCANNING ELECTRON ACOUSTIC MICROSCOPY WITH SUBNANOSECOND TIME RESOLUTION
L. Balk, N. Kultscher
To cite this version:
L. Balk, N. Kultscher. SCANNING ELECTRON ACOUSTIC MICROSCOPY WITH SUB-
NANOSECOND TIME RESOLUTION. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-873-
C2-876. �10.1051/jphyscol:19842200�. �jpa-00223877�
Colloque C2, s u p p l i m e n t a u n02, Tome 45, f 6 v r i e r 1984 page C2-873
SCANNING ELECTRON ACOUSTIC MICROSCOPY
WITH
SUBNANOSECOND TIME RESOLUTIONL . J . Balk and N. Kultscher
Universitdt &isburg, Fackgebiet Werkstoffe der EZektrotecknik, Konunandantenstrasse 6 0 , 0-4200 &isburg, F .R. G.
Resume
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En microscopic 6 l e c t r o n i q u e a c o u s t i q u e p a r balayage l e s i g n a l acous- t i q u e de l ' e c h a n t i l l o n p e u t G t r e a n a l y s e completement p a r des mesures r e s o - l u e s en temps. L ' u t i l i s a t i o n d ' u n "boxcar" permet de r e a l i s e r des m i c r o g r a - p h i e s e l e c t r o n i q u e s a c o u s t i q u e s avec un dephasage d e t e r m i n e p a r r a p p o r t aux i m p l u s i o n s du f a i s c e a u p r i m a i r e e l e c t r o n i q u e . En p r i n c i p e ces r n i c r o g r a p h i e s peuvent G t r e a p p l i q u e e s aux analyses en p r o f o n d e u r de f a q o n non d e s t r u c t i v e . On m o n t r e des exemples d ' a p p l i c a t i o n aux semiconducteurs e t aux d i s p o s i t i f s . A b s t r a c t-
I n scanning e l e c t r o n a c o u s t i c microscopy t h e l o c a l a c o u s t i c response o f t h e specimen can be a n a l y s e d by t i m e r e s o l v e d experiments. Use o f boxcar i n t e g r a t i o n t e c h n i q u e s enables t h e p r o d u c t i o n o f e l e c t r o n a c o u s t i c images w i t h f i x e d t i m e d e l a y s w i t h r e s p e c t t o t h e p r i m a r y e l e c t r o n beam p u l s e . I n p r i n c i p l e , t h e s e micrographs s h o u l d a1 low a n o n - d e s t r u c t i v e d e p t h p r o f i 1 i n g . Examples a r e shown f o r semiconducting m a t e r i a l s and d e v i c e s .INTRODUCTION
Scanning a c o u s t i c microscopy has become a powerful t o o l f o r imaging specimen f e a t u r e s , which a r e combined w i t h t h e mechanisms o f sound g e n e r a t i o n , a t t e n u a t i o n and propaga- t i o n w i t h i n t h e s o l i d . The f r e q u e n c i e s o f o p e r a t i o n a r e g e n e r a l l y i n t h e u l t r a s o n i c r e g i o n and cover t y p i c a l l y s e v e r a l kHz up t o G i g a h e r t z . Commonly r e p o r t e d i n l i t e r a - t u r e a r e t h r e e techniques,which m a i n l y d i f f e r i n t h e source f o r g e n e r a t i n g t h e sound.
These a r e p u r e s c a n n i n g a c o u s t i c microscopy u s i n g a focused sound wave f o r generation, scanning p h o t o a c o u s t i c microscopy u t i l i z i n g a l a s e r beam, and f i n a l l y t h e scanning e l e c t r o n a c o u s t i c microscopy (SEAM) which i m p l i e s a focused e l e c t r o n beam f o r gener- a t i n g t h e u l t r a s o n i c wave.The a m p l i f i c a t i o n and a n a l y s i s o f t h e d e t e c t e d sound s i g n a l can be done by d i f f e r e n t means. I n SEAM u n t i l now o n l y RF a m p l i f i c a t i o n /1/ o r -as a more s e n s i t i v e technique - phase s e n s i t i v e l o c k - i n a m p l i f i c a t i o n /2/ have been used f o r p r o c e s s i n g t h e a c o u s t i c s i g n a l e m i t t e d by t h e specimen, whereas w i t h a c o u s t i c microscopy sampling t e c h n i q u e s a r e r e p o r t e d more o f t e n , u s i n g e i t h e r sampling o s c i l l o - scopes o r - a t more c r i t i c a l s i g n a l c o n d i t i o n s
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s i g n a l averagers / 3 / , which w i l l be denoted as boxcar i n t e g r a t o r s i n t h e following.Compared t o l o c k - i n a m p l i f i c a t i o n t h e use o f boxcar i n t e g r a t o r s w i t h SEAM i s advantageous, as i t n o t o n l y a l l o w s t o achieve v e r y h i g h temporal r e s o l u t i o n s and t h u s t o use extremely h i g h f r e q u e n c i e s f o r produc- t i o n o f e l e c t r o n a c o u s t i c (EA) micrographs, b u t a l s o enables d e t e r m i n a t i o n o f t h e a- c o u s t i c waveform as a r e p l y t o a s i n g l e e l e c t r o n beam p u l s e o f v e r y s h o r t ( l e s s t h a n a nanosecond ) d u r a t i o n / 4 / .EXPERIMENTAL
An experimental set-up f o r SEAM i s i n t r o d u c e d i n t h i s paper, which a l l o w s t h e analysis o f t h e u l t r a s o n i c s i g n a l w i t h subnanosecond t i m e r e s o l u t i o n . I n f i g . 1 t h e d e t e c t i o n system i s shown /4/, f i g . 2 i s a schematic diagram o f t h e experimental arrangement.The g e n e r a t i o n o f sound waves can be done e i t h e r p e r i o d i c a l l y by a square wave g e n e r a t o r o r by s i n g l e p u l s e s d e l i v e r e d from a p u l s e g e n e r a t o r v i a an electron beam chopper. The d e t e c t i o n o f t h e sound i s done by clamping t h e t r a n s d u c e r element d i r e c t l y t o t h e bot- tom s u r f a c e o f t h e sample, t h e r e b y any specimen p r e p a r a t i o n can be o m i t t e d . T r i g g e r i n g o f t h e boxcar i n t e g r a t o r has t o be done v i a a frequency d i v i d e r i n t h e case o f p e r i o d -
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842200
C2-874 JOURNAL DE PHYSIQUE
i c a l chopping (square o r s i n e wave),as t h e boxcar i n t e g r a t o r (model PAR 162) o n l y ac- cepts t r i g g e r r a t e s up t o some MHz. F o r p u l s e d e x c i t a t i o n w i t h lower r e p e t i t i o n r a t e s t h i s i s n o t necessary. F o r low s i g n a l c o n d i t i o n s a I n s - s a m p l i n g head w i t h h i g h i n p u t impedance can be used; m o s t l y a 75ps-sampling head w i t h 50 Ohm impedance has been ap- p l i e d , which g i v e s a t i m e r e s o l u t i o n o f about 200ps t a k i n g i n t o account j i t t e r o f t h e experimental arrangement and minimum slew r a t e o f t h e electron beam p u l s e . To adapt t o d i f f e r e n t s i g n a l c o n d i t i o n s a p r e a m p l i f y i n g system i s i n s t a l l e d , which can be arranged t o g i v e v a r i o u s a m p l i f i c a t i o n s by cascading low n o i s e p r e a m p l i f i e r s w i t h a maximum g a i n o f 86dB.
APPLICATIONS
Fig.3 shows an a p p l i c a t i o n o f t h e technique d e s c r i b e d above t o t h e i n v e s t i g a t i o n of s e l e c t i v e l y d i f f u s e d 1nP.Within t h e w h i t e r e g i o n s o f t h e secondary and b a c k s c a t t e r e d (SE+RE) micrograph Zn has been d i f f u s e d down t o an approximate depth o f 4 um. I n t h e t i m e r e s o l v e d EA micrograph t h e doped r e g i o n s show a s t r o n g s i g n a l which 4s due t o the f a c t t h a t f o r h i g h c a r r i e r c o n c e n t r a t i o n s a l a r g e e l e c t r o n a c o u s t i c c o u p l i n g occurs i n t h e m a t e r i a l y i e l d i n g a h i g h sound amplitude. F o r f i g . 3 b t h e t i m e d e l a y has been s e t t o g i v e a s i g n a l maximum. Besides o f b r i g h t n e s s v a r i a t i o n between doped and undoped materia1,a s t r o n g d i r e c t i o n dependent b l a c k and w h i t e c o n t r a s t can be n o t i c e d a t t h e
l o c a t i o n o f t h e p n - j u n c t i o n s l o o k i n g l i k e a t o p o g r a p h i c s t r u c t u r e . T h i s i s due t o t h e f a c t t h a t t h e space charge w i t h i n t h e p n - j u n c t i o n couples d i r e c t l y w i t h t h e e l e c t r i c f i e l d a s s o c i a t e d w i t h t h e sound wave due t o b o t h t h e d.c. f i e l d o f t h e p n - j u n c t i o n and t h e p i e z o e l e c t r i c p r o p e r t i e s o f InP. T h i s e f f e c t c o u l d be proven b o t h by s i n g l e p u l s e o r p e r i o d i c a l e x c i t a t i o n and by p r o d u c i n g EA micrographs w i t h l o c k - i n a m p l i f i - c a t i o n /4/. Due t o t h i s e f f e c t t h e s t r u c t u r e s ( d o t s w i t h b l a c k and w h i t e r i m s ) o f t h e EA image, n o t v i s i b l e i n t h e SE+RE image, can be c o r r e l a t e d t o l o c a t i o n s i n which no doping occured and i n which t h e m a t e r i a l i s s t i l l n-conducting. F i g . 4 i s a sequence o f t i m e r e s o l v e d EA images o f a s i m i l a r specimen w i t h d i f f e r e n t t i m e delays . w i t h r e - spect t o t h e t r i g g e r p u l s e o f t h e g e n e r a t o r . Although t h e t i m e d i f f e r e n c e s between the images a r e below I n s , t h e images c l e a r l y d i f f e r . C o n t r a s t f o r t h i s example i s inversed due t o an a d d i t i o n a l i n v e r t i n g a m p l i f i e r compared t o f i g . 3 .
Another a p p l i c a t i o n o f t i m e r e s o l v e d SEAM i s shown i n f i g . 5 a + b . Here t h e EA micrograph o f a s i l i c o n i n t e g r a t e d c i r c u i t r e v e a l s r e g i o n s o f h i g h doping c o n c e n t r a t i o n s as b r i g h t areas, t h e s i g n a l i n t h e s e r e g i o n s b e i n g i n f l u e n c e d by o v e r l y i n g conduction l i n e s . Fig.5c+d shows an e n l a r g e d s e c t i o n o f one o f these r e g i o n s . I n f i g . 6 a sequence o f EA micrographs w i t h d i f f e r e n t t i m e delays i s given. One can see t h a t f o r t h e e a r l y image o n l y a t o p o g r a p h i c a l c o n t r a s t o f t h e conduction l i n e s comes up because o f an a l t e r e d b a c k s c a t t e r i n g c o e f f i c i e n t . Then two doped r e g i o n s become v i s i b l e ( a square and a l i n e ) , which b o t h a r e beneath a m e t a l l i z a t i o n . F o r l o n g e r delays a t these r e - gions t h e d i r e c t i o n a l p n - j u n c t i o n c o n t r a s t , a l r e a d y r e p o r t e d f o r t h e InP sample, be- comes more pronounced. F i n a l l y these areas a r e a m p l i f i e d i n " o v e r l o a d " , whereas now o t h e r doped areas can be seen by r e v e a l i n g t h e p n - j u n c t i o n s .
DISCUSSION
From t h e examples o f f i g . 4 and f i g . 6 t h e r e i s evidence f o r t h e p o s s i b i l i t y o f d e p t h p r o f i l i n g . The t i m e d i f f e r e n c e s o f t h e micrographs, however, a r e much t o o l a r g e t o be e x p l a i n e d by a s i m p l e model i m p l y i n g m a i n l y v e l o c i t y o f sound and d e p t h o f sound gen- e r a t i o n . I t must be assumed here t h a t processes l i k e h i g h e x c i t a t i o n o f excess c a r - r i e r s , c a r r i e r l i f e t i m e s and s c r e e n i n g e f f e c t s dominate t h e t i m e b e h a v i o u r o f t h e EA s i g n a l f o r v a r i o u s depths w i t h i n t h e sound g e n e r a t i o n volume. By means o f specimens
age of s e l e c t i v e l y d i f - fused InP with periodic e x c i t a t i o n ( 8MHz square wave ) a t a primary e l e c - t r o n energy of 5keV:
time delay of 75ps-sampl ing head a t signal maximum
F i g . 4
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Time resolved e l e c t r o n a c o u s t i c micrographs of InP a t various time delays with r e s p e c t t o t h e t r i g g e r of t h e pulse generator( pulsed e x c i t a t i o n , 75ps-sampling head, 30keV ) : a ) 21.7ns ; b ) 19.4ns ; c ) 18.5ns ;d ) 1 7 . l n s ; e ) 16.6ns ; f ) 15.7ns ; 200pm g) 15.3ns ; h ) 14.8ns ; i ) 13.9ns
.
C2-876 JOURNAL DE PHYSIQUE
o f known p r o p e r t i e s t h e s e p o s s i b l e i n f l u e n c e s s h a l l be c a l i b r a t e d i n o r d e r t o determine, whether and how p r e c i s e l y a d e p t h i n f o r m a - t i o n can be g a i n e d i n case o f semiconducting m a t e r i a l . ACKNOWLEDGEMENT
The a u t h o r s l i k e t o t h a n k P r o f . Dr. E. Kubalek f o r h i s encouragement.
REFERENCES
/l/G.S. CARGILL 111, Scanned Image Microscopy
(1980) by E.A. Ash (ed.) Academic Press, 319 /2/A. ROSENCWAIG, Scanned
Image Microscopy (1980) 291
/3/R.L. HOLLIS, R. HAMMER, Scanned Image Microscopy (1980) 155
/4/L. J. BALK, N. KULTSCHER, 1nst.Phys.Conf .Ser.(1983) t o be p u b l i s h e d
r c u i t
F i g . 6
