Integrated Circuits - American Chemical Society

9

Chemical and Physical Processing of Ion-Implanted Integrated Circuits

Joseph C. Plunkett

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: October 2, 1985 | doi: 10.1021/bk-1985-0290.ch009

Department of Electrical Engineering, California State University, Fresno, CA 93740

A brief overview of the fundamentals of the chemical and physical processing of ion-implanted integrated c i r c u i t s i s presented. Although not intended as a thorough review paper i n the field, a modest list of references are provided to which the reader may refer for more in-depth discussions of the topics covered. As well as an overview of the p r i n c i p l e s of ion-implantation, p r o f i l e shaping as a means of improving device performance is discussed. Typical applications of ion-implantation i n s i l i c o n and gallium arsenide devices are also covered. F i n a l l y , some basic clean-up processes for laboratory ion-implantation processing are provided. T e c h n o l o g i e s f o r t h e c h e m i c a l and p h y s i c a l p r o c e s s i n g o f i n t e g r a t e d c i r c u i t s have advanced a t a v e r y r a p i d pace i n r e c e n t y e a r s . The s t r i n g e n t r e q u i r e m e n t s o f improved e l e c t r i c a l performance and f u r t h e r m i c r o m i n i a t u r i z a t i o n o f i n t e g r a t e d c i r c u i t s have f o r c e d a d vancement o f the s t a t e - o f - t h e - a r t i n f a b r i c a t i o n t e c h n o l o g y . With the evolvement o f sub-micron t e c h n o l o g y and the e x t e n s i o n o f monol i t h i c i n t e g r a t e d c i r c u i t t e c h n o l o g y t o the microwave L S I r a n g e , i o n i m p l a n t a t i o n i s p l a y i n g an i n c r e a s i n g l y i m p o r t a n t r o l e i n t h e f a b r i c a t i o n p r o c e s s e s o f s o l i d s t a t e c i r c u i t s and d e v i c e s . I o n i m p l a n t a t i o n has been the s u b j e c t o f numerous r e v i e w a r t i c l e s and b i b l i o g r a p h i e s i n recent years ( 1 - 2 0 ) . The d i s c u s s i o n s i n t h i s paper p r o v i d e fundamental and o v e r v i e w m a t e r i a l s u i t a b l e f o r the new r e s e a r c h e r i n the f i e l d o f i n t e g r a t e d c i r c u i t p r o c e s s i n g , a s w e l l a s p r o v i d e s the u s e r o f i o n i m p l a n t a t i o n w i t h some r e l e v a n t d e s i g n i n f o r m a t i o n and some r e c e n t a p p l i c a t i o n s t o some new d e v i c e s and m a t e r i a l s . I n i t s most fundamental form, i o n i m p l a n t a t i o n i s a p r o c e s s by w h i c h e n e r g e t i c , c h a r g e d - p a r t i d e s o r i m p u r i t y atoms can be i n t r o duced i n t o a t a r g e t o r s u b s t r a t e m a t e r i a l . O f t e n , t h e s e p a r t i c l e s w h i c h a r e t o be i m p l a n t e d a r e p o s i t i v e i o n s ( s i n g l y o r m u l t i p l y i o n i z e d ) w h i c h come from a s u i t a b l e source. A f t e r the f o r m a t i o n o f the p o s i t i v e i o n s , they a r e a c c e l e r a t e d b y s t a t i c e l e c t r i c f i e l d s , a r e f o c u s e d o r formed i n t o a beam, and a r e passed t h r o u g h a mass a n a l y z e r . The mass a n a l y z e r i s used t o ensure t h a t the beam i s pure. 0097-6156/ 85/0290-0127$ 10.00/ 0 §> 1985 A m e r i c a n C h e m i c a l Society

Stroeve; Integrated Circuits: Chemical and Physical Processing ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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C H E M I C A L A N D P H Y S I C A L PROCESSING O F INTEGRATED CIRCUITS

The beam i s o f t e n f u r t h e r m a n i p u l a t e d by s l i t s o r quadrupole anal y z e r s p r i o r t o i t s s t r i k i n g t h e t a r g e t . As a p p l i e d t o t h e d o p i n g of m a t e r i a l s , t h e i o n beam must u s u a l l y be made t o form a r a s t e r scan o f t h e t a r g e t . The i o n beam then bombards t h e t a r g e t s u b s t r a t e m a t e r i a l by a c a r e f u l l y c o n t r o l l e d and q u a n t i f i e d p r o c e s s whereupon most o f t h e i o n s e n t e r t h e s u b s t r a t e m a t e r i a l . Upon t h e i n t r o d u c t i o n of t h e i o n s i n t o t h e t a r g e t m a t e r i a l , t h e m a t e r i a l a c q u i r e s new e l e c t r i c a l o r chemical properties. A b r i e f o v e r v i e w o f some o f t h e advantages and d i s a d v a n t a g e s o f i o n i m p l a n t a t i o n p r o c e s s e s i n t h e d o p i n g o f s o l i d s t a t e semiconductor m a t e r i a l s i s p r e s e n t e d below. (1) V a r i e t y o f s o u r c e s — f o r most semiconductor a p p l i c a t i o n s t h e i o n s w h i c h a r e i n t r o d u c e d must be e l e c t r i c a l l y a c t i v e . T h i s means t h a t f o r i m p u r i t y doping o f a s u b s t r a t e m a t e r i a l t h e i o n s must occupy s u b s t i t u t i o n a l s i t e s i n the l a t t i c e structure. (2) V a r i e t y o f s u b s t r a t e m a t e r i a l — a l t h o u g h v i r t u a l l y any materi a l c a n be i m p l a n t e d , one must choose a m a t e r i a l f o r i m p u r i t y d o p i n g i n s o l i d s t a t e s e m i c o n d u c t o r s w h i c h c a n be e l e c t r i c a l l y a c t i v a t e d . (3) I m p u r i t y c o n c e n t r a t i o n p r o f i l e s c a n be shaped t o c e r t a i n s p e c i f i c a t i o n s . One i m p o r t a n t a s p e c t o f i o n i m p l a n t a t i o n i n t o semiconductors, i n contrast t o the d i f f u s i o n process, i s that the number o f i m p l a n t e d atoms c a n be p r e c i s e l y c o n t r o l l e d by t h e e x t e r n a l system, r a t h e r than by t h e p h y s i c a l parameters o f t h e t a r g e t material. (4) O r d i n a r y p h o t o l i t h o g r a p h y o r m e c h a n i c a l masking i s used f o r impurity p o s i t i o n i n g i n the substrate. (5) I o n i m p l a n t a t i o n i s a l o w temperature p r o c e s s w h i c h w i l l n o t u s u a l l y d i s t u r b e a r l i e r i m p u r i t y d i s t r i b u t i o n s w h i c h may have been p l a c e d i n t h e m a t e r i a l . An a n n e a l c y c l e must, however, be c o n s i d e r e d i n o r d e r t o e l e c t r i c a l l y a c t i v a t e t h e dopant atoms. (6) I m p l a n t a t i o n i s n o t s o l u b i l i t y l i m i t e d — o f t e n however, d u r ing the process of e l e c t r i c a l a c t i v a t i o n a c o n d i t i o n of e q u i l i b r i u m may be reached and p r e c i p i t a t i o n o f t h e e x c e s s i m p u r i t i e s may o c c u r . (7) E l e c t r i c a l a c t i v a t i o n c a n be a c h i e v e d a t t e m p e r a t u r e s l e s s than t h a t r e q u i r e d f o r d i f f u s i o n i n c e r t a i n m a t e r i a l s , a l t h o u g h g a l l i u m arsenide r e q u i r e s a r a t h e r high temperature. I n s i l i c o n , a n o m i n a l temperature o f 600 t o 800 degrees C i s used. (8) The p r o c e s s o f i o n i m p l a n t a t i o n l e n d s i t s e l f t o a u t o m a t i c c o n t r o l . W i t h t h e advent o f m i c r o p r o c e s s o r s and m i c r o c o m p u t e r s , such p r o c e s s c o n t r o l a s p r o f i l e s h a p i n g c a n be programmed i n t o t h e c o n t r o l and made a r o u t i n e and p r e c i s e p r o c e s s . Ion D i s t r i b u t i o n and P e n e t r a t i o n Ion i m p l a n t a t i o n i s n o t an e q u i l i b r i u m p r o c e s s , a l t h o u g h i n t h e p r o c e s s o f e l e c t r i c a l a c t i v a t i o n , e q u i l i b r i u m may be a c h i e v e d . The e n e r g e t i c i o n s l o s e t h e i r energy t o t h e h o s t l a t t i c e o f t e n c r e a t i n g damage t o t h e b u l k m a t e r i a l . An a n n e a l i n g p r o c e s s i s used t o remove most o f t h e damage and t o e l e c t r i c a l l y a c t i v a t e t h e i o n s . The l o s s o f energy t o t h e s u b s t r a t e m a t e r i a l i s brought about by two p r o c e s s e s . One i s by e x c i t a t i o n and i o n i z a t i o n o f e l e c t r o n s . The o t h e r i s by e l a s t i c c o l l i s i o n s w i t h n u c l e i . The r e s u l t s o f v i o l e n t c o l l i s i o n s may be t h e d i s p l a c e m e n t o f atoms i n t h e h o s t l a t t i c e a l o n g the i o n p a t h s e t t i n g up a c h a i n r e a c t i o n as l o n g a s t h e k i n e t i c


9.

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energy i s a v a i l a b l e . A l t h o u g h some o f the damage t o the m a t e r i a l s e t s up an amorphous s t a t e , a n n e a l i n g can r e t u r n t h e l a t t i c e t o i t s c r y s t a l l i n e form. C e r t a i n c r i t i c a l a l i g n m e n t s o f t h e i o n beam can cause c h a n n e l i n g o r g u i d i n g o f the i o n s by the c r y s t a l l a t t i c e . T h i s can cause deep anomolous p e n e t r a t i o n of the s u b s t r a t e i f they a r e c r i t i c a l l y a l i g n e d a l o n g t h e axes o f the t a r g e t c r y s t a l . In p r a c t i c e , t h e t a r g e t i s u s u a l l y d e l i b e r a t e l y m i s a l i g n e d w i t h r e s p e c t t o t h e major axes o f t h e c r y s t a l . T h i s m i s a l i g n m e n t a n g l e i s t y p i c a l l y 7 t o 10 degrees o f f - a x i s . T h i s quenches t h e c h a n n e l i n g e f f e c t and s i m u l a t e s an amorphous t a r g e t . T h e o r e t i c a l Range D e t e r m i n a t i o n . The LSS t h e o r y (21) i s o f t e n used to c a l c u l a t e t h e t h e o r e t i c a l range, R, and t o t a l s t r a g g l e AR. The s t r a g g l e r e p r e s e n t s the s t a t i s t i c a l f l u c t u a t i o n o f t h e t o t a l range. I t i s assumed t h a t an amorphous t a r g e t i s used and t h a t a G a u s s i a n d i s t r i b u t i o n i s c r e a t e d . F i g u r e 1 i l l u s t r a t e s the d e p t h d i s t r i b u t i o n o f i m p l a n t e d atoms i n an amorphous t a r g e t . I n t h e f i r s t c u r v e , t h e i n c i d e n t i o n mass i s l e s s than the s u b s t r a t e atomic mass. I n the second c u r v e , the i o n mass i s g r e a t e r than the h o s t m a t e r i a l atomic mass. As can be seen, the s t r a g g l e i s g r e a t e r f o r the f i r s t case. For p r a c t i c a l s o l i d s t a t e d e v i c e d o p i n g , t h e mean p e r p e n d i c u l a r d e p t h o f p e n e t r a t i o n , R-, and t h e a s s o c i a t e d s t r a g g l e , ARp, a r e the i m p o r t a n t parameters. For c r i t i c a l masking c o n t r o l , however, t r a n s v e r s e s t r a g g l e , AR , can be i m p o r t a n t . F i g u r e 2 shows the r e f e r e n c e c o o r d i n a t e s and n o m e n c l a t u r e of t h i s geometry. T h e o r e t i c a l c a l c u l a t i o n s o f the p r o j e c t e d range and s t r a g g l e f o r v a r i o u s dopants and s u b s t r a t e s have been c a l c u l a t e d and shown i n g r a p h i c a l form ( 4 , 21-26). Some common dopant i o n s f o r s i l i c o n and g a l l i u m a r s e n i d e a r e shown i n F i g u r e s 3, 4, 5, and 6. t

T h e o r e t i c a l Impurity P r o f i l e C a l c u l a t i o n s . Using information prov i d e d by t h e p r e c e d i n g c u r v e s , a t h e o r e t i c a l a s - i m p l a n t e d concent r a t i o n p r o f i l e can be c a l c u l a t e d i n terms o f the p r e c e d i n g p a r ameters (from Stone and P l u n k e t t (26) and r e f e r e n c e s t h e r e i n ) . X -

n(x,y) (2tt) x

3/2

exp

— AR

p

-[-

e x

— 2 AR

P

1/2 (2)

t

Rn

_ ARp

(D "

2

M

t

where t h e c o o r d i n a t e s a r e shown i n F i g u r e 2. n i s t h e i o n c o n c e n t r a t i o n f o r js i o n s / u n i t s u r f a c e p e r p e n d i c u l a r to t h e t a r g e t s u r f a c e i n t h e x d i r e c t i o n . For t y p i c a l a p p l i c a t i o n s , the AR parameter can be e f f e c t i v e l y e l i m i n a t e d by t h e beam scan w h i c h c r e a t e s an o v e r l a p o f the i m p l a n t e d i o n s over t h e t r a n s v e r s e s t r a g g l i n g range. Of c o u r s e , the l a t e r a l s t r a g g l i n g e f f e c t cannot be e l i m i n a t e d a t the mask edges. t



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C H E M I C A L A N D P H Y S I C A L PROCESSING OF INTEGRATED CIRCUITS

F i g u r e 1. The d e p t h d i s t r i b u t i o n o f i m p l a n t e d atoms i n an amorphous t a r g e t .

INCIDENT

TARGET

ION BEAM

SURFACE

/\ N

/ !

PROJECTED RANGE

Rp

4

STANDARD

PEAK OF

DEVIATION

CONCENTRATION

OF PROJECTED RANGE

PROFILE

AR

t

TRANSVERSE STRAGGLE

F i g u r e 2. R e f e r e n c e c o o r d i n a t e s f o r i o n i m p l a n t a t i o n parameters.



PLUNKETT

Ion-Implanted

Integrated

Circuits

ENERGY ( k e V )

Figure 3. Theoretical calculations of the projected range of B, P, As, and Sb i n s i l i c o n . (Adapted with permission from Reference 27, copyright 1983, John Wiley and Sons).

Figure 4. Theoretical calculations of projected straggle for B, P, As, and Sb i n s i l i c o n . (Adapted with permission from Reference 27, copyright 1983, John Wiley and Sons).



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C H E M I C A L A N D PHYSICAL PROCESSING O F INTEGRATED CIRCUITS

10000 ENERGY

(keV)

F i g u r e 5. T h e o r e t i c a l c a l c u l a t i o n s o f t h e p r o j e c t e d range o f Be, Se, Cd, and Te i n GaAs. (Adapted w i t h p e r m i s s i o n from R e f e r e n c e 27, c o p y r i g h t 1983, John W i l e y and S o n s ) .

F i g u r e 6. T h e o r e t i c a l c a l c u l a t i o n s o f t h e p r o j e c t e d s t r a g g l e o f Be, Se, Cd, and Te i n GaAs. (Adapted w i t h p e r m i s s i o n from R e f e r e n c e 27, c o p y r i g h t 1983, John W i l e y and Sons).


9.

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Circuits

I n t h e p r i m a r y a r e a o f t h e scan, t h e p r o f i l e e x p r e s s i o n becomes: (x - R )

thus

2

D

(2)

n(x) = N exp max r

2 AR

n

where i s the_peak o f t h e c o n c e n t r a t i o n p r o f i l e o c c u r r i n g at the point x = R : p

0.4

N

(3)


Integrated Circuits - American Chemical Society

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