HIGHLY CONTROLLED DIFFUSION OF ION IMPLANTED ARSENIC BY MULTIPLE SCAN ELECTRON BEAM HEATING

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HIGHLY CONTROLLED DIFFUSION OF ION IMPLANTED ARSENIC BY MULTIPLE SCAN

ELECTRON BEAM HEATING

D. Godfrey, R. Mcmahon, H. Ahmed, M. Dowsett

To cite this version:

D. Godfrey, R. Mcmahon, H. Ahmed, M. Dowsett. HIGHLY CONTROLLED DIFFUSION OF

ION IMPLANTED ARSENIC BY MULTIPLE SCAN ELECTRON BEAM HEATING. Journal de

Physique Colloques, 1983, 44 (C5), pp.C5-229-C5-233. �10.1051/jphyscol:1983536�. �jpa-00223122�

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JOURNAL DE PHYSIQUE

Colioque C5, suppl6ment au nD1O, Tome 44, octobre 1783 page C5-229

HIGHLY CONTROLLED D I F F U S I O N OF I O N IMPLANTED ARSENIC BY M U L T I P L E SCAN ELECTRON BEAM HEATING

D . J . Godfrey, R.A. ~ c ~ a h o n + , H . ~hmed' and M . Dowsett ++

GEC Research Laboratories, Hirst Research Centre, WembZey HA9 7PP, U.K.

+Cambridge University, Microcircuit Engineering Laboratory, Cambridge, U. K.

+ + ~ e ~ a r t m e n t of Physics, C i t y of London Po Zy technic, London, U. K.

Resume

-

Le c h a u f f a g e p a r f a i s c e a u e l e c t r o n i q u e 1 balayage mu1 t i p l e p e u t - 6 t r e employe pour r e c u i r e l e s degdts d ' i m p l a n t a t i o n d ' a r s e n i c en doses @ l e v e e s dans l e s i l i c i u m sans i n t r o d u i r e de d i f f u s i o n i m p o r t a n t e . Ce r a p p o r t c o u v r e l ' a p - p l i c a t i o n d e c e t t e t e c h n i q u e I l a d i f f u s i o n d ' a r s e n i c c o n t r B l e e de t r e s pres, q u i e s t adoptee pour l e s elements 1 emetteurs b i p o l a i r e s . I 1 s ' e s t a v e r e pos- s i b l e de m o d e l i s e r l a d i f f u s i o n en a d o p t a n t une methode s i m i l a i r e 1 c e l l e em- p l o y e e p o u r l e r e c u i t c l a s s i q u e dans l e s fours. T o u t e f o i s , dans l a p r a t i q u e , il se p r o d u i t une e v a p o r a t i o n s e n s i b l e du dopeur 2. moins q u ' u n e couche d6po- see de recouvrement s o i t employ@e. Le r a p p o r t demontre que l a t e c h n i q u e p e u t a s s u r e r une d i f f u s i o n c o n t r b l e e de t r i ' s pri's (+5%) pour des j o n c t i o n s de 0,2 microns de p r o f o n d e u r 1 des temperatures de t F a i t e m e n t @ g a l e s ?I 1100°C e n v i - r o n . On y r e t r o u v e , p a r a i l l e u r s , un p r o f i l i n i t i a l emetteur-base b i p o l a i r e p r o d u i t p a r c e t t e technique.

A b s t r a c t

-

M u l t i p l e scan e l e c t r o n beam h e a t i n g my be used t o anneal h i g h dose a r s e n i c i m p l a n t a t i o n damage i n s i l i c o n . T h i s paper covers t h e e x t e n s i o n o f t h i s t e c h n i q u e t o h i g h l y c o n t r o l l e d a r s e n i c d i f f u s i o n w i t h a p p l i c a t i o n t o b i p o l a r e m i t t e r s t r u c t u r e s . It has proved p o s s i b l e t o model t h e d i f f u s i o n u s i n g a s i m i l a r approach t o t h a t used f o r c o n v e n t i o n a l furnace a n n e a l i n g . However, i n p r a c t i c e s i g n i f i c a n t dopant e v a p o r a t i o n i s found u n l e s s a d e p o s i t e d capping l a y e r i s used. It i s shown t h a t t h e t e c h n i q u e i s a b l e t o p r o v i d e h i g h l y c o n t r o l l e d d i f f u s i o n (+_5%) f o r j u n c t i o n depths o f 0.2 pm a t p r o c e s s i n g t e m p e r a t u r e s o f -1100°C. C a r r i e r concent r a t i o n p r o f i l e s from s p r e a d i n g r e s i s t a n c e data and a t o m i c concent r a t i o n p r o f i l e s measured by SIMS f o r b i p o l a r e m i t t e r - b a s e s t r u c t u r e s a r e i n c l u d e d .

1 LIMITATIONS OF CONVENTIONAL FURNACING

F o r a number o f s i l i c o n d e v i c e a p p l i c a t i o n s , t h e r e i s a requirement f o r r e g i o n s which a r e h e a v i l y doped and have s t e e p l y graded j u n c t i o n s w i t h t h e u n d e r l y i n g m a t e r i a l . The f o r m a t i o n o f t h e e m i t t e r r e g i o n i n b i p o l a r npn t r a n s i s t o r s i s an I m p o r t a n t example o f t h i s s i t u a t i o n and i s t h e p a r t i c u l a r case which i s s t u d i e d i n t h i s paper. C o n v e n t i o n a l l y , a n n+ e m i t t e r i s produced by h i g h dose a r s e n i c

i m p l a n t a t i o n ( t y p i c a l l y 40 keV 1 x 1016 cm-2) f o l l o w e d by a h i g h t e m p e r a t u r e f u r n a c e heat t r e a t m e n t (950°C 30 m i n u t e s ) . The d i f f u s i o n o f a r s e n i c i s known t o be s t r o n g l y concent r a t i o n dependent [I] r e s u l t i n g i n enhanced d i f f u s i o n a t h i g h concent r a t i o n s . F i g u r e 1 shows a c a l c u l a t e d a t o m i c dopant p r o f i l e u s i n g a model which i s d e s c r i b e d l a t e r . The p r o f i l e a f t e r 950°C 30 m i n u t e s heat t r e a t m e n t o f a 40 keV 1 x 1016 a r s e n i c i m p l a n t i l l u s t r a t e s t h e v e r y a b r u p t j u n c t i o n which r e s u l t s f r o m t h e s e d i f f u s i o n e f f e c t s . A l s o shown i n t h e f i g u r e i s t h e expected a r s e n i c s o l i d s o l u b i l i t y l i m i t a t t h i s temperature. It can be seen t h a t a l a r g e amount o f a r s e n i c i s e l e c t r i c a l l y i n a c t i v e , which m y r e s u l t i n u n d e s i r a b l e c r y s t a l l i n e d e f e c t s i n t h e e m i t t e r region. A l t e r n a t i v e l y , r e d u c i n g t h e a r s e n i c dose produces an e m i t t e r i n

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983536

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C5-230 JOURNAL DE PHYSIQUE

which a1 1 t h e a r s e n i c 1s a c t i v e . However, t h l s r e s t r i c t s t h e d e s ~ r e d concentration enhanced d i f f u s i o n (see c a l c u l a t e d p r o f i l e f o r 2 x 1015 cm-2 4 0 keV i m p l a n t i n F i g u r e 1) l e s s e n i n g t h e s l o p e o f t h e a r s e n i c p r o f i l e edge. F o r c o n v e n t i o n a l f u r n a c e a n n e a l i n g , process temperatures s i g n i f i c a n t l y g r e a t e r t h a n 950°C a r e g e n e r a l l y not a c c e p t a b l e f o r t h i s a p p l i c a t i o n a s t h e l e v e l o f d i f f u s i o n i s l i k e l y t o produce t o o deep a n e m i t t e r / b a s e j u n c t i o n . Hence t h e r e r e m i ns a compromi se between dopant a c t i v a t i o n and j u n c t i o n g r a d i e n t .

F i g u r e 1 C a l c u l a t e d p r o f i l e s comparing a 0.2 pm e m i t t e r formed by f u r n a c e a n n e a l i n g and r a p i d i s o t h e r m a l a n n e a l i n g

For h i g h speed d e v i c e o p e r a t i o n , it i s advantageous t o reduce t h e e m i t t e r l b a s e j u n c t i o n d e p t h t o a minimum w h i l s t m a i n t a i n i n g a h i g h l e v e l o f c o n t r o l o v e r t h e a b s o l u t e v a l u e o f t h e depth ( l e s s t h a n lr5% v a r i a t i o n ) . C a l c u l a t i o n s show t h a t a k 3 m i n u t e c o n t r o l on t h e f u r n a c e heat t r e a t m e n t a t 950°C i s r e q u i r e d t o a c h i e v e a +5%

c o n t r o l on a 0.2 pm e m i t t e r / b a s e j u n c t i o n depth f o r a 40 keV 1 x 1016 a r s e n i c i m p l a n t . Due t o t h e l a r g e t h e r m a l mass o f q u a r t z ware and wafers i n furnace h e a t i n g , it i s u n l i k e l y t h a t such p r e c i s e c o n t r o l w i l l be r e a d i l y a c h i e v a b l e .

These r e s t r i c t i o n s my be overcome by u s i n g m u l t i p l e scan e l e c t r o n beam h e a t i n g a t t e m p e r a t u r e s up t o 1300°C f o r p e r i o d s up t o 30 seconds. F o r example, t h e c a l c u l a t e d p r o f i l e f o r a 4 sec 1200°C heat t r e a t m e n t o f a 1 x 1016 cm-2 40 keV a r s e n i c i m p l a n t i s shown i n F i g u r e 1. It can be seen t h a t a h i g h l e v e l o f a c t i v a t i o n should be o b t a i n e d w h i l s t m a i n t a i n i n g a s t e e p l y graded j u n c t i o n . It i s p o s s i b l e t o c o n t r o l t h e t i m e f o r which t h e e l e c t r o n beam i s i n c i d e n t on t h e s i l i c o n w a f e r t o w i t h i n 0.1 seconds. The peak t e m p e r a t u r e reached d u r i n g i r r a d i a t i o n w i l l be o b t a i n e d when t h e power i n p u t i s equal t o t h e r a d i a t i o n power l o s s . T h i s l o s s i s p r o p o r t i o n a l t o ( t e m p e r a t u r e ) 4 and so i n c r e a s e s i n t h e peak t e m p e r a t u r e w i l l r e q u i r e s i g n i f i c a n t i n c r e a s e s i n t h e i n p u t e l e c t r o n beam power. Hence, m u l t i p l e scan e l e c t r o n beam h e a t i n g o f f e r s t h e p o s s i b i l i t y o f we1 1 c o n t r o l l e d , s h o r t d u r a t i o n h i g h t e m p e r a t u r e processing.

2 MULTIPLE SCAN ELECTRON BEAM INDUCED DIFFUSION

It has been shown [2] t h a t t h e t e m p e r a t u r e - t i m e c y c l i n g caused by m u l t i p l e scan e l e c t r o n beam h e a t i n g can be a d e q u a t e l y model l e d by c o n s i d e r i n g a t h e r m a l l y i s o l a t e d sample w i t h t h e o n l y heat l o s s mechanism b e i n g r a d i a t i o n . A computer model has been developed which uses such t e m p e r a t u r e - t i m e c y c l i n g t o c a l c u l a t e t h e d o p i n g p r o f i l e a f t e r e l e c t r o n beam i r r a d i a t i o n . The model c o n s i s t s o f an e x p l i c i t f i n i t e d i f f e r e n c e s o l u t i o n t o t h e concent r a t i o n dependent d i f f u s i o n e q u a t i o n f o r a s i n g l e dopant ( e i t h e r a r s e n i c , boron o r phosphorus). The model a l l o w s f o r surface e v a p o r a t i o n o f dopant by c o n s i d e r i n g a t e m p e r a t u r e and dopant dependent surface t r a n s p o r t c o e f f i c i e n t ( s i m i l a r t o t h e method adopted by SUPREM [3]). F o r e l e c t r o n

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beam a n n e a l i n g , it i s assumed t h a t any dopant p a s s i n g t h r o u g h t h e s u r f a c e 1s l o s t . The more s t a n d a r d b l o c k i n g s u r f a c e boundary c o n d i t i o n i s a l s o a v a i l a b l e i n t h e _--

programme a n a T T K 6 e used-for conventiona l f u r n a c e heat t r e a t m e n t s o r e l e c t r o n beam h e a t i n g where no dopant l o s s i s expected.

A 40 keV 1 x 1016 c m - z a r s e n i c i m p l a n t was chosen f o r i n i t i a l e x p e r i m e n t a l s t u d i e s . F i g u r e 2 shows t h e j u n c t i o n depth as measured by t h e bevel and s t a i n t e c h n i q u e a f t e r e l e c t r o n beam heat t r e a t m e n t s o f v a r i o u s power d e n s i t i e s up t o 40 ~ c m - 2 ( c a l c u l a t e d a s y m p t o t i c t e m p e r a t u r e 1360°C) f o r t i m e s up t o 100 seconds. Also shown i n t h e f i g u r e ( s o l i d l i n e s ) a r e t h e j u n c t i o n depths p r e d i c t e d by t h e d i f f u s i o n model d e s c r i b e d e a r l i e r i n c l u d i n g t h e e f f e c t o f dopant e v a p o r a t i o n . The agreement between t h e c a l c u l a t e d and measured j u n c t i o n depths i s encouraging. T h i s ill u s t r a t e s t h a t e l e c t r o n beam h e a t i n g may be m o d e l l e d s a t i s f a c t o r i l y by t h e same approach t h a t i s adopted i n convent i ona 1 f u r n a c e annea 1 i ng.

40 krV 1 xl0" cm+ imphnt

The e f f e c t on t h e e l e c t r i c a l sheet r e s i s t a n c e o f dopant l o s s d u r i n g e l e c t r o n beam h e a t i n g i s shown i n F i g u r e 3. F o r a n n e a l i n g w i t h o u t dopant l o s s , t h e sheet r e s i s t a n c e would be expected t o f a l l r a p i d l y as t h e i m p l a n t e d l a y e r i s regrown f o l l o w e d by a more gradual decrease as dopant d i f f u s i o n occurs. However, f o r each o f t h e power d e n s i t i e s shown i n F i g u r e 3, a minimum i s found i n t h e sheet r e s i s t a n c e a n d subsequent h e a t i n g produces s i g n i f i c a n t dopant l o s s w i t h a r e s u l t i n g r i s e i n t h e sheet r e s i s t a n c e .

o

5-

-;; ^0.4

-

a 2 0 . 3 -

Y O

g 0.2

.- _.

. a r=

Z 0.1

8or 4 0 keV 1 x 1 0 ' ~ cm-' As implant - experimmtol

F i g u r e 2 Measured j u n c t i o n d e p t h

- of a 40 keV

1 x 1016 an-z AS

i m p l a n t a f t e r

A m u l t i p l e scan

- Z o w ~ m - ~ e l e c t r o n beam

a nnea 1 i ng. The

- s o l i d l i n e s a r e

c a l c u l a t e d

F i g u e 3 Measured

e l e c t r i c a l sheet r e s i s t a n c e a f t e r uncapped

e l e c t r o n beam a n n e a l s

0 1 2 3 4 5 6 7 8 9 1 0 j u n c t i o n depths.

~ i r n e ' ~ (re$ I

Secondary i o n mass spectoscopy (SIMS) p r o f i l e s have been used t o measure t h e chemical c o n c e n t r a t i o n o f a r s e n i c a f t e r e l e c t r o n beam h e a t i n g . F i g u r e 4 shows SIMS p r o f i l e s f o r e l e c t r o n beam h e a t i n g a t 40 ~ c m - 2 fo r v a r i o u s t i m e s . It can be. seen t h a t t h e i n t e g r a t e d area under t h e p r o f i l e s i s g r e a t l y reduced f r o m t h e a s - i m p l a n t e d dose w i t h i n c r e a s i n g h e a t i n g t i m e . A l s o , t h e near s u r f a c e r e g i o n (0.1 pm) shows a r e d u c t i o n i n a r s e n i c c o n c e n t r a t i o n . B o t h t h e s e e f f e c t s m y be seen i n F i g u r e 5 where t h e computer model f o r dopant d i f f u s i o n has been used t o s i m u l a t e t h e same h e a t i n g c o n d i t i o n s . The agreement between t h e SIMS and c a l c u l a t e d p r o f i l e s i s good.

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C5-232 JOURNAL DE PHY SlQUE

40keV 1 x 1016 cm-' As 4OW uncapped anneal

- -

^.*. ^SIMS

Depth -microns

4 SIMS a t o m i c p r o f i l e s a f t e r upcapped e l e c t r o n beam annea 1 s showing t h e e f f e c t o f s u r f a c e e v a p o r a t i o n

40kev 1 x 1016 cm-' AS 40w cm-' uncapped anneal calculated

. ⁵sec 7 sec 10 sec 1 5 sec

. 1 .2 .3 . 4 . 5 Depth -microns

F i g u r e 5 C a l c u l a t e d a t o m i c p r o f i l e s f o r t h e same a n n e a l i n g c o n d i t i o n s used i n F i g u r e 4

An 0.5 pm d e p o s i t e d o x i d e l a y e r has been used t o cap t h e a r s e n i c i m p l a n t e d l a y e r s . No minimum i n sheet r e s i s t a n c e was o b t a i n e d a f t e r a n n e a l i n g under s i m i l a r c o n d i t i o n s t o t h o s e g i v e n i n F i g u r e 3. F i g u r e 6 shows SIMS p r o f i l e s produced from such samples and t h e r e i s no evidence o f a r s e n i c l o s s .

A m j o r a i m o f t h i s present work was t h e p r o d u c t i o n o f h i g h l y c o n t r o l l e d dopant d i f f u s i o n w i t h p a r t i c u l a r r e f e r e n c e t o a r s e n i c e m i t t e r r e g i o n s f o r b i p o l a r t r a n s i s t o r s . F o r comparison w i t h f u r n a c e a n n e a l i n g , a requirement f o r a 0.2 pm j u n c t i o n d e p t h w i t h c o n t r o l o f + 5 % m s considered. F r o m s i m i l a r c a l c u l a t i o n t o t h o s e i l l u s t r a t e d i n F i g u r e 2, it can be shown t h a t t h e d e s i r e d l e v e l o f c o n t r o l my be met w i t h a +9 second c o n t r o l on t h e i r r a d i a t i o n t i m e f o r a 20 Wcm-2 power d e n s i t y . Although t h i s may be a c h i e v e d w i t h o u t d i f f i c u l t y , it i s o f course, necessary t o c o n t r o l t h e power d e n s i t y a c c u r a t e l y . Assuming t h a t power d e n s i t i e s between 15 Wcm-2 and 25 ~ c m - 2 f o r 60 second heat t r e a t m e n t s a r e used, it can be e s t i m a t e d t h a t a +2% c o n t r o l on t h e beam power d e n s i t y i s needed t o m a i n t a i n a +5%

c o g t r o l on t h e j u n c t i o n depth. It i s a n t i c i p a t e d t h a t t h i s l e v e l o f c o n t r o l s h o u l d be r e a d i l y a c h i e v a b l e w i t h m u l t i p l e scan e l e c t r o n beam annealing. F i g u r e 7 shows a s p r e a d i n g r e s i s t a n c e c a r r i e r p r o f i l e o f t e s t b i p o l a r s t r u c t u r e (40 keV 1 x 1016 a r s e n i c and 25 keV 1 x 1014 cm-2 boron) annealed f o r 50 sec a t 20 ~ c m - 2 . It can be seen t h a t a n e m i t t e r j u n c t i o n d e p t h o f 0.2 pm and a base w i d t h o f 0.1 pm a r e obtained. A1 so, t h e e m i t t e r sheet r e s i s t a n c e o f 2lbl/square i l l u s t r a t e s t h e h i g h l e v e l o f a c t i v a t i o n which has been achieved.

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I O k e V 1 x l ~ ' ~ c r n - ~ R, 1 0 w r n - ~ capped anneal

' emltter ,unction depth 0 . 2 ~

'. base wldth 0.1".

=mrtrer sheet resLstance 2 1 n m base sheet resxstance 150kIlllI

Oepth -microns Oepth -microns

F i g u r e 6 SIMS a t o m i c p r o f i l e s a f t e r F i g u r e 7 Spreading r e s i s t a n c e c a r r i e r capped (0.5 pm) d e p o s i t e d p r o f i l e o f a t e s t b i p o l a r o x i d e ) e l e c t r o n beam a n n e a l s ) s t r u c t u r e a f t e r 20 ~ c m - 2 50 sec

capped e l e c t r o n beam annea 1

3 CONCLUSIONS

It has been shown t h a t m u l t i p l e scan e l e c t r o n beam h e a t i n g may be used t o produce h i g h l y c o n t r o l l e d dopant d i f f u s i o n o f a r s e n i c i n s i l i c o n . However, s i g n i f i c a n t dopant l o s s by e v a p o r a t i o n a t t h e sample s u r f a c e my o c c u r u n l e s s a capping l a y e r i s used. The s u i t a b i l i t y o f e l e c t r o n beam h e a t i n g t o p r o v i d e a c o n t r o l o f +5% on a j u n c t i o n depth o f 0.2 pm has been demonstrated. The requirement o f +2% c o n t r o l i n t h e power d e n s i t y i s a t t a i n a b l e by c a r e f u l use o f e x i s t i n g e x p e r i m e n t a l equipment.

To produce b i p o l a r s t r u c t u r e s , it i s necessary t o anneal b o t h a r s e n i c and boron.

I n i t i a l experiments, u s i n g a n o x i d e cap, show t h a t s a t i s f a c t o r y e m i t t e r - b a s e p r o f i l e s m y be o b t a i n e d by t h i s t e c h n i q u e . F u r t h e r work w i l l i n c l u d e d e t a i l e d s t u d y o f t h e c o u p l e d a r s e n i c - b o r o n d i f f u s i o n and t h e i n c l u s i o n o f m u l t i p l e scan e l e c t r o n beam h e a t i n g t o complete b i p o l a r t r a n s i s t o r s .

REFERENCES

1 D Shaw., Physica S t a t u s S o l i d i b 72, 11 (1978)

2 D J Gbdfrey, A E Adams, R A McMahon and H Ahmed., 1 s t I n t e r n a t i o n a l Symposium on VLSI ECS 162nd Meeting, October 1982, D e t r o i t

3 D A A n t o n i a d i s and R W Dutton., IEE Trans E l e c t r o n Device Vol ED-26, 490, (1979)