CHAPTER 4 - APPLICATION TO SILICON PRODUCTION Thes is by

AEROSOL REACTOR FOR SILICON PRODUCTION

4.1 I n t r o d u c t i on

P r o d u c t i o n o f s i I i c o n a e r o s o l by gas phase r e a c t i o n i s a prom i s ! ng a l t e r n a t l v e t o t h e conventional methods f o r p r o d u c i ng p o l y c r y s t a l I l

ne

s i l i c o n f o r p h o t o v o l t a i c and e l e c t r o n i c applications. A t present most of t h e hlgh p u r i t y s i l i c o n i s produced by e p i t a x i a l r e a c t o r s I n t h e form of f i l ms, o r i n be l I j a r o r t u b u I a r reac-l-ors i n bu

l

k f srm. These a r e ba.5-ch operation r e a c t o r s w i t h hlgh energy and labor consumptton. Consequen

B :

y t h e c o s t o f production i s h l g h ( 1 1, Recent1 y Union Carbide (2) and i

he

J e t Propulsion Laboratory (12) have developed a

conOInusus

f l o w r e a c t o r

3n

w h i c h s i l i c o n a e r o s o l 1s o b t a i n e d f r o m s t l a n e g a s b y t h e r m a i decmpositlon:

( 1 1 SIH4

--->

S i

+

2H2

The basic concept of t h e c o n t 1 nuous f l ow r e a c t o r I s shown I n F l g, 2.

I n t h e Union C a r b i d e process, s f l a n e is i n t r o d u c e d af; t h e t o p o f t h e r e a c t o r and h e a t e d by r e c i r c u l a t i n g hydrogen. The s f l a n e t h u s produced nucleates due t o i t s low vapor pressure, and dorms a very f i n e powder of submlcron p a r t i c l e s . Thls process has t h e p o t e n f l a l o f being a continuous operation, and t h e energy consumption 1s lower than conventional methods b y a f a c t o r o f t e n o r more.

Unfortunate

I

y, t h e subm i c r o n s t l icon aerosol t h a t I s generated f n t h e f l o w r e a c t o r I s very d i f f i c u l t t o separate from t h e gas flow and t o handle

i n subsequent process1 ng. Moreover, t h e large s u r f ace area of t h e smal l s i l i c o n p a r t i c l e s makes t h e c o l lected powder susceptible t o contamlnatlon.

I f l a r g e r p a r t i c l e s c o u l d be produced I n t h e a e r o s o l r e a c t o r , t h e separation, handling and contamination problems could be g r e a t l y reduced;

and t h e process could then become a t r u l y continuous one.

As we have seen i n previous chapters, p a r t i c l e formation and growth from s vapor a r e J n i t J a1

l

y c o n t r o l led by: (

I )

homogenous nucleation, and, (ii) heterogenous condensation o r r e a c t i o n on p a r t f c l e surfaces. The JPL and Union Carbide r e a c t o r s were operated a t s u f f i c i e n t l y hJgh temperatures t o e n s u r e c o m p l e t e p y r o l y s i s o f t h e s i l a n e f e e d gas w i t h i n a l i m i t e d r e s i d e n c e ti me (about 5 seconds). The h fgh r a t e o f g e n e r a t i o n o f condenslble products o,f r e a c t i o n r e s u l t s i n a high nucleation r a t e and t h e fo?-.mat i o n o f a i a r g e number o f v e r y smal

l

p a r t i c l e s . Even a f t e r condensation o f t h e remaining vapor on these n u c l e i these p a r t i c l e s remain subm i c r o n I n size. Much o f t h e p r e v i o u s work on t h e f r e e space r e a c t o r has been devoted t o I ncreasi ng t h e s i z e o f t h e s l I Icon p a r t i c l e s . These e f f o r t s t o I n c r e a s e p a r t i c l e s i z e have focussed on p a r t i c l e g r o w t h by c o a g u l a t i o n ( L e v i n (171, P r a t u r i , e t al. (151, Lay and l y a (2)).

P a r t i c l e growth by coagulation was examined by applying t h e sectional model developed by Gelbard, e t al. (13) t o s i m u l a t e p a r t i c l e g r o w t h i n a t y p i c a l f l o w r e a c t o r . Fig. 1 2 shows t h e c a l c u l a t e d e v o l u t i o n o f p a r t i c l e s l z e d i s t r i b u t i o n i n normal o p e r a t i o n o f t h e c o n t i n u o u s f l o w p y r o l yzer developed a t t h e J e t Propu I s i on Laboratory. The mean d l ameter of t h e p r o d u c t a.9. t h e end of a b o u t 4 seconds r e s i d e n c e t I me i s a b o u t 0.2 micron. It I s apparent t h a t p a r t i c l e growth by coagulation i s t o o slow t o grow p a r t i c l e s o u t o f t h e submicron range.

Figure 12.

Sectional

_model calculations of

aerosol evolution in

t h e JPL reactor.

P a r t i c I e growth must, therefore, be ach ieved by vapor deposit ion on t h e particles. However, given t h e large number concentrations produced I n t h e i n i t i a l b u r s t o f nucleation, growth sign1 f i c a n t l y beyond t h e subm icron range I s n o t p r a c t i c a l . Large aerosol p a r t i c l e s can be grown I f t h e number c o n c e n t r a t i o n can be reduced by s e v e r a l o r d e r s o f magnitude and m a i n t a i n e d a t t h l s low l e v e l t h r o u g h o u t t h i s g r o w t h process. T h i s requires t h a t additional nucleation be prevented.

From t h e t h e o r y developed i n t h e p r e v i o u s chapters, t h e method t o p r e v e n t n u c l e a t i o n i s t o keep t h e t o t a l c l e a r a n c e v o l ume f r a c t i o n , 52

,

greater than unity. I n general, t h i s condition can be met by doing any o f t h e f o l lowing :

I ) Maintain aerosol concentration a t a s u f f i c i e n t l y high level, O 1 ) Keep t h e vapor pressure low, o r

1 1 I ) I ncrease t h e temperature.

F o r t h e s i l a n e system, t h e a i m i s t o d e c r e a s e t h e number c o n c e n t r a t i o n o f t h e aerosol as much as p o s s l b l e I n o r d e r t o maximize p a r t i c l e size. Vapor concentration can be kept low by I i m i t l n g t h e r a t e o f s i l a n e decomposition. T h i s would t h e n d i s q u a l i f y t h e t h i r d option, s i n c e s i l a n e undergoes a t h e r m a l decomposition, and t h e r a t e o f decomposition increases w i t h temperature.

A possible means of producing large p a r t i c l e s i n t h e s i l i c o n aerosol r e a c t o r I s t o s t a r t w i t h a smal I number of seed p a r t i c l e s and t o control t h e temperature of t h e r e a c t i o n a t a lower level than t h e present systems, thereby

l

l m l ti ng t h e r a t e o f

s

1 lane decomposi t Ion and a I lowing t h e seed aerosol t o grow. An understand1 ng of t h e k i n e t i c s o f

s

i lane decomposi t i o n

fs, therefore, very important f o r c o n t r o l o f t h e system.

4 K l n e t l c s o f Silane Decomposition

The k l n e t i c s o f t h e t h e r m a l d e c o m p o s i t i o n o f s i l a n e (SiH4) a r e n o t y e t f u l l y understood. P u r n e l l and Walsh ( 3 ) made a p l o k e r i n g s t u d y (1966) I n which p y r o l y s i s o f s i l a n e was c a r r i e d o u t a t 650°K t o 700°K I n a s t a t l c system. The p r o d u c t s were seen t o be hydrogen, d i s i l a n e (S12H6) and a s o i l d p r o d u c t w1.P.h 4he composition S i n . The hydrogen t o s t l icon r a t i o x had t h e value 2 when t h e s o l i d product was f i r s t formed, b u t decreased w i t h r e a c t i o n time, and u l t i m a t e l y pure s i l i c o n i s obtained.

A d e t a l l e d a n a l y s l s by P u r n e l l and Walsh o f t h e I n i t i a l p a r t o f +he r e a c t i o n dur-lng w h i c h 0 i s 3 p e r c e n t d e c o m p o s l t i on o c c u r s suggested a unfmolecular decomposltion w i t h a temperature and pressure dependent r a t e constant. Two mechan t sms

were

p o r t u l ated. On t h e basis of thermodynam fc canslder-%t9nns8

Purnel

f and Walsh favored t h e f o l l o w i n g mechanism:

It has been since confirmed (5,6) t h a t t h i s mechanism i s operative f o r t h e I n i t i a l p a r t o f t h e r e a c t i o n . The A r r h e n i us p a r a m e t e r s o f t h e s e r e a c t i o n s a r e shown 1 n Tab I e 4. These p a r a m e t e r s a r e t a k e n from a more r e c e n t s t u d y by R i n g

et

at. ( 9 ) .

The l a t e r p a r t o f t h e r e a c t i o n mechanism is n o t we1 l bnderstood.

D u r i n g t h i s stage, t h e d e c o m p o s l t l o n o f d i s i l a n e and t r l s l l a n e a r e

TABLE 4.

Arrhenlus Parameters f o r Silane Decomposition

React l on loglO A EaCt(MJ/kmole) Ref.

a ) The parameters a r e f o r t h e high pressure 1 i m i t (k, 1. The a c t u a l v a l u e o f kl depends on pressure and temperature. The

301

lowing c h a r t

glves

3he kaPio k t / k a :

Pressure I Temperature (

%)

btorr) 1 650 71 0 1050 1

expected t o become I mportant. The f o l low I ng mechanisms have been proposed f o r these decompssltlon reactlons.

S13H8

--->

u SlH 4

+

SiH3SlH

The k i n e t i c parameters o f these reactions are also shown i n Table 4.

Above 580aC, *he goiyhydrides o f s l l i c o n tend t o decompose t o s l l i c o n and hydrogen 871, O f p e r + ? c u l a r I n t e r e s t i n t h e I a t e r stages o f s l 1 ane p y r o f y s Y s 1s f h @

soffd

p r o d u c t (SIH,), where 0

<

2. It has been sugges%ed thaB 15) <-his could be a mixture of (SIH2In, (SIH), and sllfcon.

P u i = n s $ t and Walsi7 %keorl+ed t h a t t h e s o l I d p r o d u c t I s formed by t h e successive insertism o f SOH2 i n t h e lower silanes which then d i f f u s e s t o a surface, ThJs would Involve t h e following mechanism:

AP l a r g e v a l ues sf

n

t h e above t w o f o r m u l a e become exper 1 mental l y

W

f ng and OrNeal (99 have proposed t h a t , dur l n g t h e l a t e r stages o f t h e r e a c t l on, hetel-sgenous mechan 1 sms become important and r e a c t i o n occurs a t t h e w a l l surfaces of t h e r e a c t i o n vessel. They proposed t h e f o l l o w i n g additional reactlons:

(9)

SiH2

--->

Hal I-SiH2 ( 1

0)

SiH2-Wal I

--->

Wal I-StH2*

( 1 1 ) Wal I-SIH2*

+

SiH4

---->

Wal I-SIH3

+

^SiH3*

(12) SiH3*

+

SIH4

--->

SI2H6

+

(13) H*

+

SiHg

--->

2

9 SIH 3 (14) SIH3*

+ SfH5B --->

$ 1 ₂

H

₆

The r e a c t i o n s (9-11) l n l t i a t e SfH3* r a d l c a l formation, whfch then r e a c t s w i t h a d d i t i o n a l s i l a n e , T h i s may explain f h e observed I n c r e a s e i n r e a c t i o n r a t e i n t h e f i n a l s t a g s o f t h e pyrolysis.

I f surface r e a c t l o n s a r e important, Ohe fa-matlion o f t h e aerosol may become important t o t h e k i n e t i c s o f t h e system, sf nce It would provide a surface f o r t h e reactan-ts, No ~ i u d y o f -&Ria

2spec.t

o f t h e s l l a n e pyrolysis h a s been made t o date. I t

Is

p o s s l b l e

-thdt

surface reactfons on aerosols could e x p l a i n t h e d i f f e r e n c e i n s l l a n e dscornpssltlon r a t e s t h a t have been observed by d i f f e r e n t authors

In

t h e

later

stages o f t h e reaction.

From t h e above d l s c u s s i ~ n it i s o b v l o u s t h a t t h e f i n a l s t a g e s o f s i l a n e p y r o l y s i s a r e u n c e r t a i n , Mosf of QRs dynamic processes I n t h e a e r o s o l r e a c t o r o c c u r b e f o r e a s i gn 1 f I can9 amount o f condensat i o n has occurred. If n u c l e a t i o n can be c o n t r o l led during t h l s period, c o n t r o l I n t h e l a t e r stages f o l l o w s q u i t e easlly. Surface r e a c t i o n s w l l l enhance t h e p a r t i c l e growth r a t e and minimize t h e r i s k o f f u r t h e r nucleation. The important r e a c t i o n is, t h e r e f o r e , t h e i n i t i a l s l l a n e decompositJon reaction. The r a t e o f sf lane consumption I s

The r a t e c o n s t a n t kl depends on t e m p e r a t u r e as we1 I as pressure; and a t atmospheric pressure,

thls

constant can be approximated by:

kl= (0.96

-

8 , 7 3 5 ~ 1 0 ~ ~ ~ ) exp(35.69

-

^30024/T) ^recW1

i n t h e temperature range 650% Po 1050°K.

4.3

Cl

earance Vol issne -;.>a* 611icon ka-osol

To determine t h e

cleix-~rtce

volume o f t h e aerosol i n t h l s system, it i s requ 1 r e d t o deferat i rib Y he-f'her fhe condens1 ng species I s a pol y s f lane, S i ,HX, o r s 1 l l con, mnd -:hather s u r f a c e r%actOons a r e l m p o r t a n t I n t h e process. Studies o*f FI

lsns pyrolysis

have n o t addressed t h i s Issue. I n t h e absence o f be-Her t ri.f~f-ndd?~on, %

t

w 1 l

I

be assumed t h a t

sl l

I con i s t h e condens t ng spec!

e s , .

-;;it;cta s ? 8

?con

;;I

E

i Rave muck l o w e r vapor p r e s s u r e than a condens9 r.ig p s i ys l i ane and sur-.sac@ ?e;ic=fions w

I

I l promote aerosol growth, it i s obvious 'tha"rhe worst case I s being considered.

With t h e above asstsmption, t h e parameters f o r aerosol evol u t i o n i n t h e f low r e a e f

or

a r e shown f

n

Tab

l

e 5, These v a l ues a r e f o r a system

where a smal l amount 1% t o 10%) sf s l lane I n n l t r o g e n I s pyrolyzed. For parameters w h ic h vary w 13% %emperaf use, t h e val ue a t 800°K has been taken.

The vapor p r e s s u r e e x y r s s s l o n has been o b t a l nec! by e x t r a p o l a t io n o f t h e vapor pressure curve shown

tn

Fig. 13.

The a s s u m p t i o n s made

In

o b t a l n l n g t h e p a r a m e t e r s a r e somewhat extreme. It I s I mportan% So n o t e t h a t , f r r e s p e c t I v e o f t h e va

l

ues o f physical p r o p e r t i e s of t h i s system, t h e me%hod f o r producing large aerosol p a r t l c l e s w i l l rema t n t h e same. There m u s t be a smal

l

amount o f seed aeroso I, and a c o n t r o l led, s B ow r e a c t io n r a t e t o promote seed .growth and prevent n u c l e a t l on.

TABLE

5 .

Parameter Values f o r

Sllison

Aerosol Growth

*-.--C---CI-I---C---.----e-".----"---"---m

I Parameter I Val ue I I Parameter I Val ue I

I bI,. . / , .

W ¹ ^{" :} ^{. . '} ^{1 ,} I " ' , , , , 1

$I), I/ !.: ..

--w

, '

'

^I " , I I ; I , ; , : 4 ' ' : ! i : j l I , i [ ; ~ : , : t i \ , , 1 1 1 *- , : , % , , ' , , , . , L - ~ , , ~ . $ - ~ , ~

f i - . : : :

Figure 13.

Vapor

pressure curve for sllicon.

(Source: Yaws, et

a l . ( 1 6 ) )

The d l mensl onless clearance volume o f sf l Icon aerosol was cal cul ated on t h e bas i s o f t h e p a r a m e t e r s I n T a b l e 5 and t h e r e s u I t s a r e p l o t t e d I n Fig. 14. The behaviour of p3 as a f u n c t i o n o f Knudsen number I s s l m l l l a r t o t h a t o f t h e H20 aerosol system. Following t h e r e s u l t s o f t h e clearance volume a n a l y s i s 1 n chapters 2 and 3, we can c o r r e l a t e t h e p l o t s o f p 3 by t h e a n a i y t i c a l expression given below:

p 3

=

5 . 9 5 ~ 1 0 ~ ( 1 + 3 . 1 ~ n ) ( ~ / ~ ~ ) 600'~

<

11 OO'K

4.4 Controlled Rate S l l l c o n Aerosol Reactor

The k i n e t i c s o f s i l a n e decomposition can now be integrated i n t o t h e model o f s i m u l t a n e o u s n u c l e a t i o n and p a r t i c l e g r o w t h t o s i m u l a t e t h e prevlous s i l i c o n f r e e space r e a c t o r s and t o i d e n t i f y operating c o n d l t i o n s w h i c h would a l l o w g r o w t h o f l a r g e p a r t i c l e s . The f l r s - t case t o be considered i s t h e f l o w r e a c t o r operated a t high temperature (high r e a c t i o n r a t e ) w i t h no seed aerosol. When 20% s i l a n e I s pyrolyzed I n t h i s system a t 875 OK, n u c l e a t I o n o c c u r s a l most i n s t a n t 1 y, p r o d u c l ng 1.17~1 017 /m3 p a r t i c l e s b e f o r e n u c l e a t i o n I s quenched. The n u c l e a t e d p a r t i c l e s t h e n grow by d e p ~ s i t i o n o f vapor. As shown i n Fig. 17, t h e p a r t i c l e s grow t o a s i z e o f a b o u t 0.12 micron. I n fact, s I m u l a t J o n s c a r r i e d o u t f o r condItlons under which s u b s t a n t i a l nucleatton occurs always g l v e a product between 0.05 and 0.3 m i c r o n 1 n slze, as observed i n t h e f I ow r e a c t o r s a t 1 Union Carbide and t h e J e t Propulsion t,aboratory (2, 17).

A f l o w r e a c t o r f o r t h e g r o w t h o f l a r g e s i l ic o n particles by s l l a n e decomposi t i o n r e q u i r e s a much slower r e a c t i o n r a t e than was achleved f n t h e f r e e space reactors. Because o f t h e extreme r e a c t I v l t y o f s l lane gas and f o r reasons o f c o n v e r s i o n e f f i c i e n c y , t h e amount o f $ 1 l a n e f n t h e

l o - ' 1 o0

T i m e . s e c s

Figure 15. Partlcle growth In a conventional free

space

reactor

gases l e a v l n g t h e r e a c t o r should be k e p t v e r y small. Thus, It I s desirable t o accelerate t h e r e a c t l o n t o ensure complete conversion w i t h i n t h e r e a c t o r . To do t h i s , t h e r e a c t o r wal I t e m p e r a t u r e may be increased along I t s length. I n general, t h e temperature prof 1 l e I n t h e reactor may be w r i t t e n as a function of a x l a l posltlon, z, lee.,

The p o i n t

2-0

I s chosen where t h e s l l a n e r e a c t l o n I s n e g l f g l b l t ; ; i n t h e present simulations t h i s was taken t o be a t a temperature T,=775 ^OK. The end p o i n t f o r simulations I s a t tIme t'-tf, when t h e aerosol flow reaches t h e p o i n t z=L i n t h e reactor. The temperature a t t h i s goln-k

Is

Tf.

I t can be shown t h a t t h e t e m p e r a t u r e p r o f i l e I n f he r e a c l - o r as a function o f t i m e 1s given by

where U i i s t h e v o l ume f l o w r a t e a t z=0 and d

Is

t h e d i a m e t e r of t h e reactor tube.

The temperature p r o f i l e s chosen f o r t h e reactor were of t h e form:

where

The above equation can be s u b s t i t u t e d I n

Eq.

(113) t o find:

1.e. t h e t e m p e r a t u r e p r o f 1 l e i s l i n e a r I n t i m e and p a r a b o l i c a l o n g t h e l e n g t h o f t h e r e a c t o r . The t o t a l r e s 1 dence t i me i n t h e r e a c t o r i s t h e n glven by

The t e m p e r a t u r e p r o f I l e chosen I s by no means t h e optimum f o r t h e r e a c t o r , b u t was s e l e c t e d f o r t h e s i m p l i c i t y w i t h which it may be implemented I n t h e experiments and simulations, A s e t o f parameters f o r which experiments were c a r r i e d o u t I s shown I n Table 6.

Figures 16-21 show t h e r e s u l t s o f a s i m u l a t i o n o f a f l o w r e a c t o r i n w h i c h 1

$

t o 2% s i l ane i s r e a c t e d i n presence o f seed aerosol, The

t e m p e r a t u r e p r o f 1 l e f o r t h i s c a I c u l a t l o n I s g i v e n by t h e p a r a m e t e r s 1 n Table 6. The seed aerosol s i z e d i s t r i b u t i o n f o r t h i s s i m u l a t i o n

Is

shown I n Flg. 16. This s i z e d l s t r l b u t i o n I s obtained from experimental data and w l l l be discussed later. The seed I s a submicron aerosol very s l m l l l a r t o t h e product obtalned I n t h e JPL and Union Carbide reactors; b u t t h e number concentration has been reduced by a very l a r g e factor. Flg. 17 shows t h e

TABLE

6.

Temperature Parameters f o r S i l i c o n Reactor

I

ParmePer I Val

ue I I

Parameter

I

Val

ue

-w---nmamo-U-o---

I I

I

I I

k

1 0.37

m I

I _f 1 0 . 9 8 s e c I

I I

I I I

.-

i 1 1 775 ^{O K}

I I

_U₁

I ~~~~~~~5

^m3/sec ^I

I

I I I

i Tr I llOO°K

I I

d 1 0.0095111

I

I I I

I I

I %

¹ ^2.7424 I

I

I 1 0.3104m/sec

I

*rna<e,rjj. .bl - ^ _ _ - a l e -_-----------s---_---------------------------------.---

R a d a . pm

Figure 16. SOze

dlstrlbution

of seed

aerosol

(

T=775

O K )

Total number

concentrat 1 on-1.02~1

0 ' ' 4 2

O.? 2 0.4 0-6 0.8

Dalam dokumen APPLICATION TO SILICON PRODUCTION Thes is by (Halaman 76-94)