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NONLINEAR SCANNING ELECTRON ACOUSTIC MICROSCOPY
L. Balk, N. Kultscher
To cite this version:
L. Balk, N. Kultscher. NONLINEAR SCANNING ELECTRON ACOUSTIC MICROSCOPY. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-869-C2-872. �10.1051/jphyscol:19842199�. �jpa-00223875�
JOURNAL DE PHYSIQUE
Colloque C2, supplCment au n02, Tome 45, fCvrier 1984 page C2-869
N O N L I N E A R S C A N N I N G ELECTRON A C O U S T I C MICROSCOPY
L.J. Balk and N. Kultscher
Universittlt Duisburg, Fachgebiet Werkstoffe der Elektrotechnik, Konunandantenstrasse 6 0 , 0-4100 Duisburg, F.B.G.
Resume - La microscopie e l e c t r o n i q u e acoustique par balayage n o n - l i n e a i r e e s t -- une technique s p e c i a l e de l a microscopie acoustique, qui u t i l i s e l e s ampli-
tudes e t l e s phases des harmoniques, p a r t i c u l i e r e m e n t l e second harmonique de l ' o n d e acoustique provenant d ' u n f a i s c e a u e l e c t r o n i q u e module s u r une c e r t a i n e frequence de base. Comme ces harmoniques s o n t determines par l e couplage non- l i n e a i r e e n t r e l e son e t l e s o l i d e , i l s d s t e c t e n t d ' u n e maniere t r e s s e n s i b l e l e s inhomog&nPit@s du matsriau avec une grande r@solut:$on s p a t i a l e .
Abstract - Nonlinear scanning e l e c t r o n a c o u s t i c microscopy i s a s p e c i a l tech- nique of a c o u s t i c microscopy which uses amplitudes and phases of higher harmon- i c s , e s p e c i a l l y t h e second harmonic, of t h e sound wave o r i g i n a t e d by an elec- tron beam modulated a t a c e r t a i n ground frequency. As t h e s e harmonics a r e deter- mined by t h e nonlinear coupling between sound and t h e s o l i d , they reveal very s e n s i t i v e l y material inhomogeneities with high s p a t i a l r e s o l u t i o n .
INTRODUCTION
Scanning e l e c t r o n a c o u s t i c microscopy (SEAM) i s a recently developed technique /1-4/, which allows determination of microscopic v a r i a t i o n s of material parameters associ- a t e d with sound generation and propagation within t h e examined material. Until now SEAM has been used only i n t h e l i n e a r mode. This means: the e l e c t r o n beam i s modu- l a t e d by a s i n e or square wave generator via a chopping device a t a c e r t a i n frequency, t h e sound wave i s detected by a transducer mounted t o the bottom of the specimen, the signal i s amplified by means of phase-sensitive lock-in amplification a t t h e same fre- quency. Thus only l i n e a r coupling mechanisms between sound and specimen a r e used f o r t h e e l e c t r o n a c o u s t i c (EA) image formation.
NONLINEAR ELECTRON ACOUSTICS
As can be shown by consideration of primary e l e c t r o n beam parameters and material p r o p e r t i e s , nonlinear i n t e r a c t i o n may occur due t o several reasons: non-validity of Hooke's law because of t h e large amplitudes within t h e generation volume and thus an a n e l a s t i c behaviour, nonlinear coupling of sound because of p i e z o e l e c t r i c i t y or space charges, f i n a l l y nonlinear i n t e r a c t i o n between f r e e c a r r i e r s and sound in a semicon- ductor material /5/. These n o n l i n e a r i t i e s lead t o deformation of the o r i g i n a l wave and t o generation of harmonic waves, e s p e c i a l l y t h e second harmonic. Amplitudes and phase s h i f t s of these harmonics a r e strongly r e l a t e d t o material parameters. In t h i s paper e s p e c i a l l y nonlinear i n t e r a c t i o n s a r e used f o r t h e production of micrographs.
By simultaneous measurement of amplitudes, phases and phase d i f f e r e n c e of ground wave and the chosen harmonic, a large amount of datas on t h e s o l i d can be gained. The real- i z a t i o n of nonlinear SEAM has been c a r r i e d out f o r a frequency range from several kHz up t o 5OMHz and t o t h e use of second and fourth harmonic /6/, though applications of t h i s paper a r e r e s t r i c t e d t o a chopping frequency of lOOkHz and t o the second harmon- i c . Fig.1 shows t h e principal experimental arrangement.
APPLICATIONS
Fig.2 and f i g . 3 a r e examples of EA i n v e s t i g a t i o n s of an InP s i n g l e c r y s t a l s u b s t r a t e with Zinc doped regions. Zinc has been diffused i n t h e b r i g h t regions of t h e secondary Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842199
C2-870 J O U R N A L D E PHYSIQUE
and backscattered (SE+RE) electron image of fig.2a. Whereas t h i s c o n t r a s t has been only in the 1%-order, the EA micrographs show very good c o n t r a s t . The l i n e a r EA amplitude image ( A ( f ) ) of fig.2b only gives a uniform d i s t r i - bution determining t h e doped regions. The sec- ond harmonic EA amplitude image ( A ( 2 f ) ) of f i g . 2 ~ i s more d e t a i l e d and shows inhomogenei- t i e s , preferably a t t h e edges of the s t r u c - t u r e s and a t the narrow gates, which might be due t o v a r i a t i o n of t h e diffusion depth ; achieved l o c a l l y , a s the primary e l e c t r o n pen- Fig. 1- Experimental set-up e t r a t i o n depth c o r r e l a t e s q u i t e accurately
with the average d i f f u s i o n depth of t h i s sample.
Fig.2d i s a phase d i f f e r e n c e image between ground wave and second harmonic showing up many inhomogeneities even in t h e undoped a r e a s . As the second harmonic image depends on a high signal s i t u a t i o n , i t i s very s u i t a b l e f o r high s p a t i a l r e s o l u t i o n , f u r t h e r i t reveals more d e t a i l s on t h e material. This can be seen in t h e EA images of f i g . 3 taken within sections of the doped regions. Fig.3a i s a 30keV micrograph showing white l i n e s which can be associated with c r y s t a l d i s l o c a t i o n s . When lowering t h e primary energy t o 5keV, the signal i s mainly generated i n t h e surface near region. Thenthe EA image ( f i g . 3 b ) shows pitch l i k e s t r u c t u r e s , from which a s p a t i a l resolution of l e s s than 0.2 um can be deduced. In f i g . 4 a second harmonic EA micrograph i s shown f o r a uniform14 diffused InP surface. Besides of many other e f f e c t s , i t shows v a r i a t i o n s of the EA amplitude due t o flow channels caused by the l i q u i d phase process. The black c i r c u l a r area i s etched down t o the undoped material as a reference.
A s t r i k i n g application of nonlinear SEAM has been t h e examination of solar graded poly- c r y s t a l l i n e s i l i c o n , which has not obtained any kind of specimen preparation.In s p i t e of a rough surface,imaging of grain boundaries has been possible with high s e n s i t i v i t y and s p a t i a l resolution. Fig.5 shows some remarkable r e s u l t s ( f o r more d e t a i l s see 141).
Fig.5a-c give a low magnification overlook.Whereas in t h e SE+RE image only surface topography can be seen, both amplitude and phase EA images of the secondharmonic yield the p o l y c r y s t a l l i n e s t r u c t u r e . Fig.5d+e demonstrate t h e high s p a t i a l resolution possi- ble with the A(2f)-mode. The imaged area i s a grain boundary, showing a black contrast, surrounded by an about 20 um wide bright region on both s i d e s . These regions corre- l a t e t o so-called denuded zones of decreased oxygen and carbon concentration. The cor- 1 responding phase image of fig.5f shows a rapid signal v a r i a t i o n a t the boundary i t - s e l f allowing a precise determination of the boundary l o c a t i o n . When comparing l i n e a r and nonlinear modes f o r t h i s a p p l i c a t i o n , e s p e c i a l l y f o r twin boundaries, a s i g n i f i - cant c o n t r a s t difference occurs. Whereas in the second harmonic image boundary and denuded zones show up c l e a r l y ( f i g . 5 h ) , the l i n e a r EA image of the same section gives only a change in t h e amplitude from one grain t o the other ( f i g . 5 i ) . In choosing the primary electron beam energy one has t o be careful i n those cases,in which t h e bound- ary i s inclined with a small angle t o t h e specimen surface. Then the electron beam can reach t h e surrounding bright areas a t various depths corresponding t o the energy d i s s i p a t i o n of the electrons. As a r e s u l t an i n t e g r a t i o n of the bright signal occurs, which seems t o broaden the denuded zone, a s shown i n fig.5k-m f o r an example, which yielded an overall width of the wide region of 20 um a t a primary energy of 5keV.
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ACKNOWLEDGEMENT The authors 1 i ke t o thank Prof. E.Kubalek f o r he1 pful dicussions.
REFERENCES
/ I / G.S. CARGILL 111, Physics Today Oct. (1981) 27
/2/ A. ROSENCWAIG, Scanned Image Microscopy (1980) Academic Press 291
/3/ D.G. DAVIES, J . Scanninq Electron Microscopy (1983) SEM Inc. ,USA,to be pub1 ished /4/ L. J . BALK, N . KULTSCHER; Bei tr. ~1ektronenm;krosko~: Direktabb.Oberf1. BEDO-16(1983)
t o be oublished
/5/ see e.g.: P. DAS, M . K . ROY, R.T. WEBSTER, K. VARAHRAMYAN, Ultrasonics Symposium Proceedings Sept. (1979) 278
/6/ L.J. BALK, N . KULTSCHER, I n s t . Phys. Conf. Ser. (1983) t o be published
- -Fig. 2 - EA images of Zn-di ffused re- gions i n InP f o r a primary e l e c t r o n energy of 3OkeV
F i a . 3 - ComDarison of non- rimary e l e c t r o n energies : ) 30keV; b ) 5keV
a ) EA -5,um A(2f) b ) EA -2,um
P F i s . 4 - EA micro-
C2-872 JOURNAL DE PHYSIQUE
a ) SE+?E tA(21) c ) EA phase g ( 2 f )
-LO,unr h ) EA A ( 2 f j i ) EA l i n e a r A j f )
F i g . 5 - ~ l e c t r o n a c o u s t i c images o f s o l a r grade p o l y c r y s t a l l i n e s i l i c o n
