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COMMENTS ON THE MECHANISMS OF LASER ANNEALING
D. Turnbull
To cite this version:
D. Turnbull. COMMENTS ON THE MECHANISMS OF LASER ANNEALING. Journal de Physique
Colloques, 1980, 41 (C4), pp.C4-109-C4-110. �10.1051/jphyscol:1980419�. �jpa-00219934�
JOURNAL DE PHYSIQUE Cofloque C4, supplPment au n O 5 , Tome 41, mai 1980, page C4-109
COMMENTS ON THE MECHANISMS OF LASER ANNEALING
0. T u r n b u l l
D i v i s i o n o f Applied S c i e n c e s Harvard U n i v e r s i t y , Cambridge, Em 01 23 8 , U . S.A
.
The phenomenology of t h e k i n e t i c s of c r y s - should p e r s i s t i n p l a n a r form d u r i n g ref- t a l growth i n t o molten o r g l a s s y o v e r l a y s on gr.owth. We e x p e c t t h a t c r y s t a l s grown i n c r y s t a l s was surveyed r e c e n t l y by Spaepen t h i s way would have lower i m p e r f e c t i o n con- and t h e a u t h o r / l / . When t h e m e l t i s a pure t e n t t h a n t h o s e grown under c o n d i t i o n s such m e t a l , growth proceeds w i t h no a p p a r e n t t h e r - t h a t shape i n s t a b i l i t i e s developed, w i t h a t - ma1 a c t i v a t i o n and, a t o n l y a few d e g r e e s of t e n d i n g c e l l u l a r o r d e n d r i t i c growth m d e s .
i n t e r f a c i a l undercooling, a t speeds of seve- I n r a p i d regrowth t h e r e s i d e n c e time T
r r a l m e t e r s / s . I n s h a r p c o n t r a s t , growth i n t o of t h e i n t e r f a c e w i t h i n a n atom spacing of c o v a l e n t m e l t s o r g l a s s e s r e q u i r e s high a c t i - a p a r t i c u l a r p o s i t i o n w i l l be very s h o r t . v a t i o n energy and, a t r e l a t i v e l y high under- For example, a t a growth r a t e of 1 0 meters/s.
c o o l i n g , t h e r a t e s a r e , t y p i c a l l y , o n l y a T % 2 0 picoseconds. Under t h e s e c o n d i t i o n s r
few imanometers/s. Spaepen /1,2/ showed t h a t a n impurity atom would be trapped i n t h e i n t e t r a h e d r a l l y c o o r d i n a t e d systems t h i s s o l i d .by t h e moving i n t e r f a c e u n l e s s i t s growth can b e e f f e c t e d by t h e following sequ- mean speed i n t h e l i q u i d exceeded a v a l u e
%
ence of s t e p s : t h e r m a l l y a c t i v a t e d r u p t u r e corresponding t o a d i f f u s i v i t y > 2 x of a c o v a l e n t bond a t t h e i n t e r f a c e ; migra- cm2s-l.
t i o n of t h e r e s u l t a n t dangling bonds, w i t h The i s s u e h a s been r a i s e d of whether t h e a t t e n d a n t c r y s t a l l i z a t i o n , along t h e i n t e r f a - s h o r t d u r a t i o n of t h e l a s e r p u l s e p r o v i d e s c i a 1 l e d g e ; a n n i h i l a t i o n of t h e dangling time enough f o r t h e energized o v e r l a y t o bonds by r e c a n b i n a t i o n . By t h i s mechanism a r e a c h t h e s t a b l e o r undercooled s t a t e of s i n g l e a c t i v a t i o n can l e a d t o t h e c r y s t a l l i - normal molten s i l i c o n . I n p a r t i c u l a r , J . z a t i o n of s e v e r a l molecular u n i t s . Such a Van Vechten / 4 / proposes t h a t t h e p u l s i n g p r o c e s s may be c a t a l y z e d o r i n h i b i t e d by d i f - period i s g e n e r a l l y t o o s h o r t f o r thermal f e r e n t i m p u r i t i e s , even i n t r a c e amounts. e q u i l i b r a t i o n of t h e charge c a r r i e r s , c r e a -
I n t h e l a s e r annealing of d i s o r d e r e d over- t e d by i r r a d i a t i o n , w i t h t h e S i i o n s . Also, l a y s on s i l i c o n c r y s t a l s it i s t h e regime of some time would be r e q u i r e d f o r t h e S i i o n s v e r y r a p i d c r y s t a l l i z a t i o n which has become whether o r n o t t h e y a r e i n a h o t plasma,
a major i s s u e i n t h i s conference. According t o r e l a x t o t h e i r s t a b l e o r m e t a s t a b l e equi- t o t h e thermodynamic c a l c u l a t i o n s of Bagley l i b r i u m arrangement. A c t u a l l y , t h e q u a l i t a - and Chen, /3/, a s w e l l a s t h o s e of Spaepen t i v e f e a t u r e s of t h e r a p i d regrowth, o u t l i - and t h e a u t h o r , / I / , a f i l m of amorphous si- ned above, should be observed provided t h e l i c o n could, when t h e h e a t i n g time i s t o o energized o v e r l a y t r a n s f o m s t o any m e t a l l i c s h o r t f o r s o l i d s t a t e regrowth, transform t o l i q u i d , whether o r n o t it i s t h e one formed t h e undercooled m e t a l l i c m e l t upon reaching i n e q u i l i b r i u m m e l t i n g .
T
5
0.8 Tm, where T i s t h e e q u i l i b r i u m mel- The Van Vechten s t a t e , c o o l S i i o n s i n a mt i n g p o i n t of S i . Upon i n t e r r u p t i o n of t h e h o t plasma, i s thought t o r e s u l t f r a t h e h e a t i n g t h e u n d e r l y i n g c r y s t a l would regrow v a n i s h i n g of t h e s h e a r modulus of t h e s o l i d e p i t a x i a l l y i n t o t h i s m e l t i n t h e r a p i d mode a t some c r i t i c a l degree of i o n i z a t i o n . Exne- c h a r a c t e r i s t i c o f g r o w t h i n l i q u i d m e t a l s . r i e n c e shows t h a t pure m e t a l s normally m e l t Since t h e flow of h e a t i n t h i s p r o c e s s i s a t temperatures f a r l o b ~ e r than Y?ose
p r i m a r i l y through t h e c r y s t a l t h e i n t e r f a c e a t which s h e a r i n s t a b i l i t y would develop.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980419
C4-110 JOURNAL DE PHYSIQUE
We e x p e c t , t h e r e f o r e , t h a t t h e Van Vechten phase, once formed, would q u i c k l y t r a n s f o r m toward t h e more s t a b l e l i q u i d c o n f i g u r a t i o n s which would b e p r e f e r r e d by t h e c o o l i o n s . Indeed, it i s r e c o g n i z e d t h a t , t o a c c o u n t f o r i m p u r i t y r e d i s t r i b u t i o n d u r i n g r e g r o w t h , t h e a t o m i c m o b i l i t y i n t h i s phase must b e o f t h e o r d e r of t h a t i n p u r e l i q u i d m e t a l s . Thus, a p e r i o d of one nanosecond, i n which each i o n would have undergone a b o u t
l o 4
c o l - l i s i o n s and a d i f f u s i v e d i s p l a c e m e n t a v e r a - g i n g a b o u t 1 0 atom d i a m e t e r s , should b e ade- q u a t e t o l a r g e l y e f f e c t t h e t r a n s i t i o n from t h e u n s t a b l e s o l i d t o t h e l i q u i d s t a t e . I t a p p e a r s t h a t , a s f o r o t h e r l i q u i d m e t a l s and t e t r a h e d r a l random networks, a t o p o l o g i c a l r e c o n s t r u c t i o n of t h e s h o r t r a n g e o r d e r would be e s s e n t i a l f o r c r y s t a l l i z a t i o n . Such a r e c o n s t r u c t i o n would b e e f f e c t e d by e p i t a - x i a l regrowth. I t i s h a r d l y p l a u s i b l e t h a t it c o u l d o c c u r , w i t h g e n e r a t i o n o f a good c r y s t a l i n r e g i s t r y w i t h t h e s u b s t r a t e , by any homogeneous p r o c e s s . I f t h e f o r e g o i n g c o n s i d e r a t i o n s a r e c o r r e c t , t h e c o n c l u s i o n emerges t h a t t h e m o r p h o l o g i c a l e v o l u t i o n i n l a s e r a n n e a l i n g w i l l be q u a l i t a t i v e l y t h e s a n e whether it o c c u r s v i a t h e Van Vechten o r t h e normal l i q u i d p h a s e .I t i s p o s s i b l e t h a t l a s e r p u l s i n g w i l l produce a m e t a l l i q u i d which i s l e s s s t a b l e t h a n normal m o l t e n s i l i c o n , even w i t h t h e c h a r g e c a r r i e r s and S i i o n s i n t h e r m a l e q u i - l i b r i u m . The t r a n s i t i o n t i m e from such a s t a t e t o t h e normal one s h o u l d be s h o r t , b u t it would be d i f f i c u l t t o compute. The thermo- dynamic d r i v i n g f r e e e n e r g y f o r i m p u r i t y tra:auing i n r e g r o w t h w i l l i n c r e a s e w i t h t h e i n s t a b i l i t y o f t h e l i q u i d o v e r l a y r e l a t i v e t o t h e normal m e l t . The q u e s t i o n a r i s e s : c a n l a r g e e x c e s s e s o f c e r t a i n i m p u r i t i e s over t h e e q u i l i b r i u m s o l u b i l i t i e s b e t r a p p e d , a s r e p o r t e d i n t h i s c o n f e r e n c e by VJhite e t a l . /5/ and Hoonhout and S a r i s , / 6 / u n l e s s t h e m e l t s i n t o which r e g r o w t h o c c u r s a r e much l e s s s t a b l e t h a n t h e normal one ? The thermodynamic c o n d i t i o n s f o r s o l u t e t r a p p i n g have been d e r i v e d by Baker and Cahn /7/ and a r e s e t f o r t h i n t h e i r e q u a t i o n s ( l g ) ,
(21) and (23) .From t h e s e e q u a t i o n s I e s t i m a - t e t h a t a t t h e h i g h e s t t r a p p e d c o n c e n t r a - t i o n s , a b o u t 1 atom% i n t h e e x p e r i m e n t s r e f - f e r e d t o , t h e d r i v i n g Cree e n e r g y s h o u l d be
a d e q u a t e t o s u s t a i n r a p i d r e g r o w t h i n t o t h e normal l i q u i d . Thus, w h i l e t h e m e l t e d l a y e r may b e u n s t a b l e s u c h i n s t a b i l i t y seems n o t
t o b e r e q u i r e d t o a c c o u n t f o r tHe r e p o r t e d l e v e l s o f s o l u t e t r a p p i n g .
Acknow1edqement.- T h i s r e s e a r c h was sup- p o r t e d by t h e N a t i o n a l S c i e n c e Foundation under t h e M a t e r i a l s Research G r a n t No. DHR
R e f e r e n c e s
/1/ Spaepen F . and T u r n b u l l , D . "Laser-So- l i d I n t e r a c t i o n s and Laser P r o c e s - s i n q " , A.I.P. Conf
.
P r o c e e d i n q s ,(A., 1 n s t . o f P h y s i c s , NY)
50
11979) pp.73-83./2/ Spaepen, F. Acta Met.26, (1978) 1167.
/3/ Bagley, B.G. and Chen, H.S. "Laser- S o l i d I n t e r a c t i o n s and L a s e r P r o c e s - s i n g " , A . 1 .P. Conf
.
P r o c e e d i n g s50,
pp. 97-1 01, (Am.Inst. of P h y s i c s , NY 50 (1979) pp.97-101.
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/ 4 / Van Vechten, J., P r o c e e d i n g s of t h i s Conference.
/5/ Wilson, S.R., White, C.3J., Young, F .PI.
J r . and Appleton, B.R., s u b m i t t e d t o Phys .Rev . L e t t and P r o c e e d i n g s of t h i s Conference
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/ 6 / Hoonhou"L,D. and S a r i s , F.I.J., P h y s . L e t t . A . , ( i n p r e s s ) and P r o c e e d i n g s of t h i s Conference
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/7/ Baker, J.C., and Cahn, J . W . , " S o l i d i f i - f i c e t i o n " , (ed..by T . J . Huguel and G . P. B o l l i n g )
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Am. Soc. H e t a l s , ? l e t a l s? a r k , Ohio (1571)
