- Email: [email protected]
Preparation of silicon cross-sectional specimens for TEM observation
106
A bstracts o f 7h e XI V Congress o f ttt e Italian SocieO' o f Electron Micn~scopy
T h e r e c o r d i n g m e d i u m is s i l i c o n w a f e r , w h i l e the i n f o r m a t i o n is s t o r e d as c u b e s or p a r a l l e l e p i p e d a w i t h a s q u a r e b a s e of different metals selectively deposited on the s i l i c o n s u r f a c e u s i n g the m i c r o f a b r i c a t i o n t e c h n i q u e s a n d p r o c e s s e s alr e a d y d e v e l o p e d in the f i e l d of v e r y l a r g e s c a l e i n t e g r a t i o n (VLSI) t e c h n o l o g y . T h e r e a d o u t s y s t e m is a s c a n n i n g mic r o s c o p e w i t h p e r f o r m a n c e s w h i c h are o p t i m i z e d for the d e t e c t i o n of b a c k s c a t t e r e d e l e c t r o n s w h i c h are u s e d for information retrieval. The v a r i a t i o n of the b a c k - s c a t t e r i n g coefficients with a t o m i c n u m b e r s a l l o w s the r e a d o u t s y s t e m to d i s t i n g u i s h d i f f e r e n t m e t a l s , and to r e a l i z e a b a s e for the r e p r e s e n t a t i o n of s y m b o l s (numbers, c h a r a c t e r s . . . . ) e q u a l to the n u m b e r n of d i s t i n g u i s h a b l e metals. In this way, o n c e the m i n i m u m e l e m e n t a r y a r e a a c h i e v a b l e w i t h the m i c r o f a b r i c a t i o n p r o c e s s has b e e n e s t a b l i s h e d , the s i l i c o n a r e a r e q u i r e d on the c h i p to r e p r e s e n t a c e r t a i n n u m b e r o f s y m b o l s is c o n s i d e r a b l y r e d u c e d . For e x a m p l e , by u s i n g six e l e m e n t s a n d t h r e e e l e m e n t a r y c e l l s it is p o s s i b l e to rep r e s e n t 216 s y m b o l s (63 ) w h e r e a s w i t h two e l e m e n t s ( b i n a r y r e p r e s e n t a t i o n ) and s e v e n c e l l s 128 (27 ) s y m b o l s are r e p r e sented. T h e d a t a r e a d o u t on the w h o l e s u r f a c e of Si w a f e r is p e r f o r m e d b y c o u p l i n g the electromagnetic t r a n s l a t i o n of the b e a m a n d the m e c h a n i c a l m o v e m e n t of the s t a g e u s i n g the t e c h n i q u e s a l r e a d y d e v e l o p e d in the e l e c t r o l i t h o g r a p h i c systems. A c a r e f u l a n a l y s i s of a v a i l a b l e d a t a on the p e r f o r m a n c e s of e l e c t r o n e m i s s i o n s o u r c e s a n d of b a c k - s c a t t e r e d electron d e t e c t o r s s h o w e d t h a t a s y s t e m w i t h n=6 a n d d a t a r a t e h i g h e r t h a n 1 M b y t e s / s is c o m p a t i b l e w i t h the p r e s e n t t e c h n o l o g y . T h e i n f o r m a t i o n d e n s i t y d e p e n d s on the p e r f o r m a n c e of the m i c r o f a b r i c a t i o n apparatus. W i t h a r e s o l u t i o n of 0.5 nm a s y s t e m w i t h n=6 p r o v i d e s a d a t a d e n s i t y of 1.33 x 10 8 b y t e s / c m 2. This data dens i t y is h i g h e r t h a n t h a t a c h i e v a b l e w i t h o p t i c a l r e c o r d i n g s y s t e m s a n d c a n be inc r e a s e d w i t h the d e v e l o p m e n t of m i c r o f a b r i c a t i o n p r o c e s s e s r e q u i r e d by V L S I .
PREPARATION OF SILICON CROSS-SECTIONAL SPECIMENS FOR TEM OBSERVATIONS
A. A.
Garu_lli, M. Armigliato
Vanzi*,
M. G i a n n i n i * *
i.
H. R. Pettit and G. R. Booker, Proceedinms 25th Anniversary Meeting, EMAG (1971), p. 290.
2.
D. Cherns, G. R. Anstis, J. L. Hutchison and J. C. H. Spence, Phil. Mag. A 46 (1982) 849.
and
CNR Istituto L A M E L , V i a C a s t a g n o l i i, 40126 Bologna; *TELETTRA S.p.A., S. G i o v a n n i in P e r s i c e t o (BO); **Istit u t o di F i s i c a , Universita di B o l o g n a -
A m a j o r l i m i t a t i o n of e l e c t r o n microscopical investigations carried out on p l a n e s n e c i m e n s is t h a t o n l y a p r o j e c t e d i m a g e of t h e i r t r i - d i m e n s i o n a l s t r u c t u r e can be o b t a i n e d . Although from diffraction contrast analysis some i n - d e p t h i n f o r m a t i o n can be g a t h e r e d , m a n y i m p o r t a n t f e a t u r e s are lost, such as the s t r u c t u r e of a f i l m / s u b s t r a t e i n t e r f a c e , the v e r t i c a l g r a i n g r o w t h of a polycrystalline s a m p l e , the l o c a l i z a t i o n of a b u r i e d l a y e r of l a t t i c e defects. T h i s is p a r t i c u l a r l y i n t e r e s t i n g in the s t u d y of d e f e c t s induced into s i l i c o n b y the t e c h n o l o g i c a l p r o c e s s e s a n d e x p l a i n s the c o n s i d e r a b l e e f f o r t m a d e in s e v e r a l l a b o r a t o r i e s s i n c e the p i o n e e r i n g w o r k b y P e t t i t a n d B o o k e r ] to obtain thin cross-sectional specimens s u i t a b l e for i00 k V T E M o b s e r v a t i o n s . D e s p i t e the i n c r e a s i n g n u m b e r of b e a u t i f u l m i c r o g r a n h s r e p o r t e d in l i t e r a t u r e (see, e.g., the h i g h r e s o l u t i o n s t u d y of a s i l i c o n / s i l i c i d e interface d o n e by C h e r n s e t al.2) , a g o o d k n o w l e d g e of the e x p e r i m e n t a l p r o c e d u r e s for preparing cross-sections is n o t so q u i c k ly a t t a i n a b l e b y an u n e x p e r i e n c e d m i c r o s c o p i s t , as in the case of the m o r e u s u a l c h e m i c a l or e l e c t r o p o l i s h i n g thinning methods. We w i l l d e s c r i b e the t e c h n i q u e e m p l o y e d in o u r I n s t i t u t e to o b t a i n c r o s s - s e c t i o n s of s i l i c o n m i c r o c i r c u i t s a n d T i N / S i s y s t e m s for e l e c tronic and photovoltaic applications. The f i r s t s t a g e c o n s i s t s , as u s u a l , of m e c h a n i c a l l y p r e p a r i n g s e c t i o n s less t h a n 30 ~m thick. To this e n d two n i e c e s of s a m p l e are g l u e d t o g e t h e r (with U H U or D E V C O N r e s i n s ) a n d a p i e c e of s c r a p s i l i c o n is g l u e d o n e i t h e r s i d e as a s u p p o r t . This " s a n d w i c h " is cut i n t o s l i c e s (800 ~m thick) to get v e r t i cal s e c t i o n s of the s a m p l e a n d t h e n i n t o d i s k s (3 m m d i a m e t e r ) to be m e c h a n i c a l l y thinned. The s u b s e q u e n t e t c h i n g is obt a i n e d b y A r ions, h a v i n g e n e r g y of 6 keV, c u r r e n t of 5 0 - 7 0 ~m a n d an a n g l e of inc i d e n c e of 20o; t h e s e v a l u e s are t h e n s o m e w h a t r e d u c e d for the f i n a l p o l i s h . The c h o i c e of the v a l u e s of the p a r a meters, which have a relevant influence on the f i n a l r e s u l t , w i l l be r e v i e w e d and critically discussed.
A bstracts o f 7h e XI V Congress o f ttt e Italian SocieO' o f Electron Micn~scopy
T h e r e c o r d i n g m e d i u m is s i l i c o n w a f e r , w h i l e the i n f o r m a t i o n is s t o r e d as c u b e s or p a r a l l e l e p i p e d a w i t h a s q u a r e b a s e of different metals selectively deposited on the s i l i c o n s u r f a c e u s i n g the m i c r o f a b r i c a t i o n t e c h n i q u e s a n d p r o c e s s e s alr e a d y d e v e l o p e d in the f i e l d of v e r y l a r g e s c a l e i n t e g r a t i o n (VLSI) t e c h n o l o g y . T h e r e a d o u t s y s t e m is a s c a n n i n g mic r o s c o p e w i t h p e r f o r m a n c e s w h i c h are o p t i m i z e d for the d e t e c t i o n of b a c k s c a t t e r e d e l e c t r o n s w h i c h are u s e d for information retrieval. The v a r i a t i o n of the b a c k - s c a t t e r i n g coefficients with a t o m i c n u m b e r s a l l o w s the r e a d o u t s y s t e m to d i s t i n g u i s h d i f f e r e n t m e t a l s , and to r e a l i z e a b a s e for the r e p r e s e n t a t i o n of s y m b o l s (numbers, c h a r a c t e r s . . . . ) e q u a l to the n u m b e r n of d i s t i n g u i s h a b l e metals. In this way, o n c e the m i n i m u m e l e m e n t a r y a r e a a c h i e v a b l e w i t h the m i c r o f a b r i c a t i o n p r o c e s s has b e e n e s t a b l i s h e d , the s i l i c o n a r e a r e q u i r e d on the c h i p to r e p r e s e n t a c e r t a i n n u m b e r o f s y m b o l s is c o n s i d e r a b l y r e d u c e d . For e x a m p l e , by u s i n g six e l e m e n t s a n d t h r e e e l e m e n t a r y c e l l s it is p o s s i b l e to rep r e s e n t 216 s y m b o l s (63 ) w h e r e a s w i t h two e l e m e n t s ( b i n a r y r e p r e s e n t a t i o n ) and s e v e n c e l l s 128 (27 ) s y m b o l s are r e p r e sented. T h e d a t a r e a d o u t on the w h o l e s u r f a c e of Si w a f e r is p e r f o r m e d b y c o u p l i n g the electromagnetic t r a n s l a t i o n of the b e a m a n d the m e c h a n i c a l m o v e m e n t of the s t a g e u s i n g the t e c h n i q u e s a l r e a d y d e v e l o p e d in the e l e c t r o l i t h o g r a p h i c systems. A c a r e f u l a n a l y s i s of a v a i l a b l e d a t a on the p e r f o r m a n c e s of e l e c t r o n e m i s s i o n s o u r c e s a n d of b a c k - s c a t t e r e d electron d e t e c t o r s s h o w e d t h a t a s y s t e m w i t h n=6 a n d d a t a r a t e h i g h e r t h a n 1 M b y t e s / s is c o m p a t i b l e w i t h the p r e s e n t t e c h n o l o g y . T h e i n f o r m a t i o n d e n s i t y d e p e n d s on the p e r f o r m a n c e of the m i c r o f a b r i c a t i o n apparatus. W i t h a r e s o l u t i o n of 0.5 nm a s y s t e m w i t h n=6 p r o v i d e s a d a t a d e n s i t y of 1.33 x 10 8 b y t e s / c m 2. This data dens i t y is h i g h e r t h a n t h a t a c h i e v a b l e w i t h o p t i c a l r e c o r d i n g s y s t e m s a n d c a n be inc r e a s e d w i t h the d e v e l o p m e n t of m i c r o f a b r i c a t i o n p r o c e s s e s r e q u i r e d by V L S I .
PREPARATION OF SILICON CROSS-SECTIONAL SPECIMENS FOR TEM OBSERVATIONS
A. A.
Garu_lli, M. Armigliato
Vanzi*,
M. G i a n n i n i * *
i.
H. R. Pettit and G. R. Booker, Proceedinms 25th Anniversary Meeting, EMAG (1971), p. 290.
2.
D. Cherns, G. R. Anstis, J. L. Hutchison and J. C. H. Spence, Phil. Mag. A 46 (1982) 849.
and
CNR Istituto L A M E L , V i a C a s t a g n o l i i, 40126 Bologna; *TELETTRA S.p.A., S. G i o v a n n i in P e r s i c e t o (BO); **Istit u t o di F i s i c a , Universita di B o l o g n a -
A m a j o r l i m i t a t i o n of e l e c t r o n microscopical investigations carried out on p l a n e s n e c i m e n s is t h a t o n l y a p r o j e c t e d i m a g e of t h e i r t r i - d i m e n s i o n a l s t r u c t u r e can be o b t a i n e d . Although from diffraction contrast analysis some i n - d e p t h i n f o r m a t i o n can be g a t h e r e d , m a n y i m p o r t a n t f e a t u r e s are lost, such as the s t r u c t u r e of a f i l m / s u b s t r a t e i n t e r f a c e , the v e r t i c a l g r a i n g r o w t h of a polycrystalline s a m p l e , the l o c a l i z a t i o n of a b u r i e d l a y e r of l a t t i c e defects. T h i s is p a r t i c u l a r l y i n t e r e s t i n g in the s t u d y of d e f e c t s induced into s i l i c o n b y the t e c h n o l o g i c a l p r o c e s s e s a n d e x p l a i n s the c o n s i d e r a b l e e f f o r t m a d e in s e v e r a l l a b o r a t o r i e s s i n c e the p i o n e e r i n g w o r k b y P e t t i t a n d B o o k e r ] to obtain thin cross-sectional specimens s u i t a b l e for i00 k V T E M o b s e r v a t i o n s . D e s p i t e the i n c r e a s i n g n u m b e r of b e a u t i f u l m i c r o g r a n h s r e p o r t e d in l i t e r a t u r e (see, e.g., the h i g h r e s o l u t i o n s t u d y of a s i l i c o n / s i l i c i d e interface d o n e by C h e r n s e t al.2) , a g o o d k n o w l e d g e of the e x p e r i m e n t a l p r o c e d u r e s for preparing cross-sections is n o t so q u i c k ly a t t a i n a b l e b y an u n e x p e r i e n c e d m i c r o s c o p i s t , as in the case of the m o r e u s u a l c h e m i c a l or e l e c t r o p o l i s h i n g thinning methods. We w i l l d e s c r i b e the t e c h n i q u e e m p l o y e d in o u r I n s t i t u t e to o b t a i n c r o s s - s e c t i o n s of s i l i c o n m i c r o c i r c u i t s a n d T i N / S i s y s t e m s for e l e c tronic and photovoltaic applications. The f i r s t s t a g e c o n s i s t s , as u s u a l , of m e c h a n i c a l l y p r e p a r i n g s e c t i o n s less t h a n 30 ~m thick. To this e n d two n i e c e s of s a m p l e are g l u e d t o g e t h e r (with U H U or D E V C O N r e s i n s ) a n d a p i e c e of s c r a p s i l i c o n is g l u e d o n e i t h e r s i d e as a s u p p o r t . This " s a n d w i c h " is cut i n t o s l i c e s (800 ~m thick) to get v e r t i cal s e c t i o n s of the s a m p l e a n d t h e n i n t o d i s k s (3 m m d i a m e t e r ) to be m e c h a n i c a l l y thinned. The s u b s e q u e n t e t c h i n g is obt a i n e d b y A r ions, h a v i n g e n e r g y of 6 keV, c u r r e n t of 5 0 - 7 0 ~m a n d an a n g l e of inc i d e n c e of 20o; t h e s e v a l u e s are t h e n s o m e w h a t r e d u c e d for the f i n a l p o l i s h . The c h o i c e of the v a l u e s of the p a r a meters, which have a relevant influence on the f i n a l r e s u l t , w i l l be r e v i e w e d and critically discussed.