EFFECTS QF VARYING OXYGEN PARTIAL PRESSURE ON MOLTEN ILIISON--CER~.M IC SUBSTRATE .wr~umiowi

Work Performed. Under Contra& No. NA&7-10&95J4"15

U.S. Department of Energy

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DOE/JPL/955415-2 Distribution Category UC-63b

EFFECTS OF VARYING OXYGEN PARTIAL PRESSURE ON

MOLTEN S I L I C O N - CERAMIC SUBSTRATE INTERACTIONS

F I N A L REPORT

by P . D a r r e l l O w n b y , H a r o l d V. R o m e r o and M i c h e l W: B a r s o u m C e r a m i c E n g i n e e r i n g D e p a r t m e n t

U n i v e r s i t y of M i s s o u r i - R o l l a , . R o l l a , M i s s o u r i 6 5 4 0 1

A p r i l 1 9 8 0

T h e J P L L o w - C o s t S i l i c o n Solar A r r a y P r o j e c t i s funded b y DOE and f o r m s p a r t of t h e DOE P h o t o v o l t a i c C o n v e r s i o n P r o g r a r i ~ t o i n i t i a t e a m a j o r e f f o r t t o w a r d t h e d e v e l o p m e n t of l o w - c o s t solar ar rays .

Prepared U n d e r C o n t r a c t No. 9 5 5 4 1 5 fo r

J E T PROPULSION LABORATORY CALIFORNIA I N S T I T U T E OF TECHNOLOGY

P a s a d e n a , C a l i f o r n i a 9 1 1 0 3

ABSTRACT

The o b j e c t i v e of t h e Un ive r s i t y of Missour i - Rol la

program under t h e Je t Propuls ion Laboratory Low Cost S i l i c o n

S o l a r Array c o n t r a c t i s t o i n v e s t i g a t e t h e i n t e r a c t i o n of

mol ten s i l i c o n wi th v a r i o u s d i e and c o n t a i n e r cand ida t e

m a t e r i a l s under vary ing oxygen p a r t i a l p r e s s u r e s . This has

been done by making s i l i c o n sessile drop c o n t a c t ang le

measurements on t h e c a n d i d a t e i i~a te r i a l s Lo determine t h e

deg ree t o which s i l i c o n w e t s t h e s e subs tances , and subse-

q u e n t l y s e c t i o n i n g t h e p o s t - s e s s i l e d rop experiment samples

and t a k i n g photomicrographs of t h e silicon-substrate.inter-

f a c e t o observe t h e deg ree of s u r f a c e d i s s o l u t i o n and

deg rada t ion .

Seve ra l d i f f e r e n t m a t e r i a l s supp l i ed by J P L have been

i n v e s t i g a t e d i n t h i s manner, i . e . , ho t p re s sed s i l i c o n n i -

t r i d e (from bo th Kawecki Berylco, Inc . ( K B I ) and AVCO) ,

CNTD s i l i c o n n i t r i d e coa ted on h o t p ressed s i l i c o n n i t r i d e

(Chemetal-Eagle P i c h e r ) , CVD s i l i c o n c a r b i d e coa ted on

g r a p h i t e ( U l t r a c a r b o n ) , and Q S i a l o n ( B a t t e l l e ) . Of t h e s e

m a t e r i a l s , s i l i c o n d i d n o t fonn a t r u e s e s s i l e d rop on t h e

U l t r aca rbon S i c on g r a p h i t e due t o i n f i l t r a t i o n of t h e

s i l i c o n through t h e S i c c o a t i n g o r on t h e R S i a l o n due t o

t h e format ion of a more-or-less r i g i d c o a t i n g on t h e l i q u i d

s i l i c o n . The lowes t s e s s i l e d rop c o n t a c t a n y l e ( i .e . , t h e

most we t t i ng ) was obtained on t h e CNTn coa ted S i 3 N 4 w i t h a

v a l u e of 42O. The AVCO and CNTD coa ted m a t e r i a l s showed t h e

leas t amount of i n t e r a c t i o n w i t h molten s i l i c o n .

The importance of oxygen p a r t i a l p r e s s u r e on t h e i n t e r -

a c t i o n of molten s i l i c o n wi th r e f r a c t o r y m a t e r i a l s hav ing '

been es tabl ished: t h e oxygen c o n c e n t r a t i o n s i n t h e EFG

s i l i c o n r ibbon fu rnace a t Mobil T ~ C O S o l a r Energy Corp.,

Waltham, Massachuset ts , and i n t h e J P L s i l i c o n sessile drop

fu rnace a t Pasadena, C a l i f o r n i a , w e r e measured us ing t h e

p o r t a b l e t h o r i a - y t t r i a s o l i d s o l u t i o n e l e c t r o l y t e oxygen

senso r cons t ruc t ed a t UMR f o r t h i s purpose. Oxygen p a r t i a l

p r e s s u r e s of and lo-* atm. w e r e ob t a ined f o r t h e Mobil

Tyco and JPL f a c i l i t i e s , r e s p e c t i v e l y . The measurements

made a t t h e s e two f a c i l i t i e s are be l i eved t o r e p r e s e n t non-

equ i l i b r ium cond i t i ons .

* Eagle-Picher' F i f t h Q u a r t e r l y Report t o J P L on c o n t r a c t #DOE/JPL-954877-78/3.

iii

TABLE OF CONTENTS

Page

Abstract ................................................ ii

Table of Contents ....................................... i v

List of Figures ......................................... v

Introduction ............................................ 1

Sessile Drop ~xperiments ................................ 2

Silicon Nitride .................................... 5

Silicon Carbide Coated Graphite .................... 8

Sialon ............................................. 13

Surface Energies ........................................ 20

Devitrification of Fused Silica ......................... 21

Oxygen Partial Pressure Measurements at Other Facilities .: ...................................... 22

Mobil TYCO ......................................... 22

JPL ............................................. 23 Resulbs and Discussion ............................. 23

8 . .

Summary and Conclusions ................................. 27

References .............................................. 30

LIST OF FIGURES

F igu re

1

Caption

Schematic dlagram showing how a s i l i c o n cube i s t i l t e a us ing a s i l i c o n c h i p s o a l l f a c e s of t h e cube and t h e e n t i r e s u b s t r a t e s u r f a c e a r e exposed t o t h e f lowing b u f f e r qas p r i o r t o mel t ing .

Contac t ang le v s . t i m e and p02 f o r s i l i c o n ses- s i l e d rops on s i l i c o n n i t r i d e : ( a ) Kawecki- Berylco ( K E I ) h o t p re s sed , ( b ) KbI h o t p re s sed wi th no prernelt ho ld , ( c ) AVCO h o t p re s sed , ( D ) CNTD coa ted .

Contac t a n g l e v s . t ime f o r K B I ho t p ressed s i l i - con n i t r i d e : (a ) With no pre-melt hold t ime, p02=1~-19-8. atm., ( b ) With 3 hours pre-melt hold t i m e , p02=10-190 5 atm.

Photomicrographs of K B I h o t p re s sed s i l i c o n - s i l i c o n n i t r i d e i n t e r f a c e a f t e r 8 hours a t 1 4 3 0 ~ ~ : ( a ) With no pre-melt hold t i m e , p02=10-19*8 atm., ( b ) With 3 hours pre-melt hold t i m e , p02=10-19.5 atm.

Photomicrograph of s i l i c o n - AVCO h o t p re s sed s i l i c o n n i t r i d e i n t e r f a c e a f t e r 8 hours a t 1 4 3 0 ~ ~ w i t h a p02=10-19*9 atm.

I n s i t u photographs a t 1 4 3 0 ~ ~ of s i l i c o n sessile drop experiment on po l i shed Ul t racarbon s i l i c o n c a r b i d e coa ted g r a p h i t e wi th p02=10'19* atm. : (a) 1 hour a f t e r s i l i c o n m e l t , ( b ) 1 hour, 45

minutes a f t e r s i l i c o n m e l t .

Photomicrograph of p o s t - s e s s i l e d rop s e c t i o n of po l i shed Ul t racarbon s i l i c o n c a r b i d e coa ted g r a p h i t e a f t e r 1 3/4 hour experiment a t p~2=10-19*9 atm. showing i n f i l t r a t i o n of s i l i c o n below S i c coa t ing .

I n s i t u photographs a t 1 4 3 0 ~ ~ of s i l i c o n sessile d r o p experiment on unpol ished Ul t racarbon s i l i c o n c a r b i d e coa ted g r a p h i t e wi th p02=10-20* atm. a f t e r : ( a ) 4 min., ( b ) 6 min.,, ( c ) 8 mln., ( d ) 1 hour.

Photomicrograph of p o s t - s e s s i l e drop s e c t i o n of unpol ished Ul t racarbon s i l i c o n c a r b i d e coa ted g r a p h i t e a f t e r 1 hour a t p~2=10-20 .2 atm. showing i n f i l t r a t i o n of s i l i c o n below S i c coa t ing : ( a )

,Under o r i g i n a l p o s i t i o n of t h e s i l i c o n , (b) a t t h e edge of t h e sample.

LIST OF FIGURES

F igu re Capt ion

1 0 I n s i t u photographs of s i l i c o n sessile drop . ' expe r imen t s on R S i a l o n 25 minutes a f t e r reach ing

me l t i ng tempera ture f o r sample he ld j u s t below me l t i ng t e m ~ e r a t u r e f o r : ( a ) 9 hours , ( b ) 5 hours , (c) 1 .5 hours.

11 . I n s i t u photographs of s i l i c o n sessile drop exper iment on R S i a l o n a f t e r coo l ing below mel t ing

, , . . p o i n t .

1 2 Scanning E l e c t r o n Microscope photomicrograph of s i l i c o n s u r f a c e a f t e r sessile drop experiment on R S i a lon .

1 3 Non-dispersive x-ray a n a l y s i s of s i l i c o n sanp le of F ig . 12 showing: ( a ) S i l i c o n i n d a r k e r a r e a s , ( b ) Calcium i n l i g h t e r a r e a s .

14 Graph of Log p02 of e q u i l i b r a t e d g a s a t 1273K v e r s u s p a r t s 02 unreac ted a t 1700K. Based on 100 p a r t s oxygen impur i ty pe r m i l l i o n p a r t s purge gas.

I . . . _ . . .. \

INTRODUCTION

The o b j e c t i v e of t h e Low Cost S i l i c o n S o l a r Array (LSSA)

program of t h e Jet Propuls ion Laboratory ( J P L ) i s t o pro-

duce inexpens ive s i l i c o n p h o t o v o l t a i c c e l l s . An impor tan t

c o n s i d e r a t i o n i n ach iev ing t h i s o b j e c t i v e i s t h e t y p e s of

m a t e r i a l s t o be used i n forming t h e s i l i c o n i n t o t h e re-

q u i r e d t h i n s h e e t s , i .e. , how t h e s e m a t e r i a l s w i l l a f f e c t

and be a f f e c t e d by t h e s i l i c o n i n t h e molten state. The

U n i v e r s i t y of Missouri-Rolla (UMR) has under taken t o make

s i l i c o n s e s s i l e d rop measurements on r e f r a c t o r y materials

supp l i ed by J P L t o determine t h e i r c o m p a t i b i l i t y wi th molten

s i l i c o n , and t h e e x t e n t t o which molten s i l i c o n w i l l degrade

t h e r e f r a c t o r y - molten s i l i c o n i n t e r f a c e , a s w e l l a s t h e

e f f e c t of oxygen p a r t i a l p r e s s u r e on t h e molten s i l i c o n - r e f r a c t o r y i n t e r a c t i o n .

Work done by UMR f o r Eagle-Picher I n d u s t r i e s under sub-

c o n t r a c t t o J P L f o r t h e LSSA p r o j e c t demonstrated t h e impor-

t a n c e of ve ry low amounts of oxygen on t h e molten s i l i c o n - r e f r a c t o r y i n t e r a c t i o n . ' The c a p a b i l i t y of r o u t i n e l y

measuring oxygen p a r t i a l p r e s s u r e s w e l l below t h e equ i l i b r ium

p a r t i a l p r e s s u r e f o r t h e format ion of S i02 (about 2 x 10 -19

atm. a t 1700K) has been achieved us ing a t h o r i a - y t t r i a s o l i d

s o l u t i o n e l e c t r o l y t e oxygen c e l l . The ce l l w a s c o n s t r u c t e d

s o t h a t , i n a d d i t i o n t o beinq, used i n t h e UMR l a b o r a t o r y f o r ..

measuring oxygen p a r t i a l p r e s s u r e s i n sessile drop exper iments ,

it cou ld be t r a n s p o r t e d t o o t h e r c o n t r a c t o r s ' f a c i l i t i e s

where s i l i c o n s h e e t s and r i b b o n s are produced and t h e oxygen

c o n c e n t r a t i o n i n t h e i r s i l i c o n s h e e t and r i bbon f u r n a c e s

measured. Such measurements w e r e c a r r i e d o u t i n t h e JPL EFG

s i l i c o n r i b b o n f u r n a c e a t t h e Mobil Tyco f a c i l i t y i n Waltham,

Massachuse t t s , and a t t h e J P L s i l i c o n sessile d r o p f u r n a c e

i n Pasadena, C a l i f o r n i a .

S e s s i l e d r o p exper iments were c a r r i e d o u t a t UMR on a

v a r i e t y o f c a n d i d a t e d i e and c o n t a i n e r m a t e r i a l s , i n c l u d i n g

h o t p r e s s e d s i l i c o n n i t r i d e , CVD coa t ed s i l i c o n n i t r i d e ,

S i a l o n , and CVD coa t ed S i c on g r a p h i t e . These exper iments

c o n s i s t e d of i n s i t u measurements of t h e c o n t a c t a n g l e

between t h e l i q u i d s i l i c o n and t h e s u b s t r a t e and subsequen t

examina t ion o f t h e s i l i c o n - s u b s t r a t e i n t e r f a ~ e ~ t o de t e rmine

t h e d e g r e e o f i n t e r a c t i o n .

SESSILE DROP EXPERIMENTS

S i l i c o n sessile d r o p exper iments were conducted u s i n g a

molybdenum w i r e wound t u b e f u r n a c e c a p a b l e of r e a c h i n g tempera-

t u r e s up t o 2000K. The m a t e r i a l t o be t e s t e d was p l aced on

a n a lumina "Dl' t u b e , t h e n a 7 mm cube o f s i l i c o n was p l aced

on t h e sample. I t was shown t h a t it is d e s i r a b l e t h a t t h e

s i l i c o n cube n o t be i n complete c o n t a c t w i t h t h e sample,

because i f it i s , adsorbed oxygen under t h e s i l i c o n cube i s

n o t p r o p e r l y exposed t o and e q u i l i b r a t e d w i t h t h e b u f f e r gas

and a n oxygen promoted h i g h deg ree of a t t a c k of t h e sample

by t h e s i l i c o n occu r s . I n o r d e r t h a t a l l f a c e s o f t h e s i l i c o n

cube and t h e sample s u r f a c e be e q u i l i b r a t e d w i t h t h e f lowing

buffer gas, a small chip of silicon was used to tilt the

cube (Fig. 1).

The buffer gas used to maintain the required low oxygen

partial pressure in the sessile drop furnace was a H2/H20

4 6 mixture having a ratio of between 10 /1 and 10 /1 and a flow

rate of about 700 cc/min. The hydrogen supply was from

standard cylinders, routed first through a liquid nitrogen

bath trap to condense out water, thu,s increasing the H2/H20

ratio and lowering the oxygen partial pressure, and then

through the oxygen cell to determine the precise oxygen partial

pressure, which was typically between 10 -I8 and 10 -21 atm.

All samples, with the exception of the ultracarbon Sic

coated graphite which will be discussed! in a later paragraph,

were polished to a uniform degree prior to the sessile drop

experiments to eliminate the .surface roughness variable,

permitting.comparison of the surface attack on each sample

after sessile drop experiments had been carried out.

Heat-up time for the system after insertion of the

sample was about 4 hours, so some effects were observed

herein which were not seen by other experimenters whose

systems come to temperature much more quickly. Information

was desired about the molted silicon-refractory interaction

over a long enough period to be considered characteristic

of actual silicon sheet and ribbon production times, so the

silicon sessile drop was usually maintained at 1700K for I

8 hours.

* Equivalent to a linear flow rate of -1 cm/sec. to assure equilibrium between sample and buffer.

Figure 1: Schematic diagram showing how a silicon cube is tilted using a silicon chip sc all faces of the cube and the entire substrate surface.are exposed' to the flowing buffer gas prior to melting. , ' .

S i l i c o n N i t r i d e

' . Contact a n g l e s were measured a s a f u n c t i o n .of time,

and , t h e r e s u l t s a r e p l o t t e d on t h e graph i n F igu re 2 .

The CNTD Si3N4 coa ted on h o t p ressed s i l i c o n n i t r i d e

( supp l i ed by Chemetal - E a g l e P iche r ) was used t o v e r i f y . .

r e p r o d u c i b i l i t y s i n c e t h i s m a t e r i a l had undergone ' s i m i l a r

t e s t i n g i n p rev ious work f o r J P L a t UMR. Both c o n t a c t a n g l e

d a t a (Fig. 2 ) and t h e c h a r a c t e r of t h e p o s t sessile drop

s i l i c o n - CNTD Si3N4 c o a t i n g i n t e r f a c e w e r e i n e x c e l l e n t

agreement w i t h t h e prev ious work.

The c o n t a c t a n g l e d a t a f o r the-AVCO h o t pressed s i l i c o n . .

n i t r i d e a g r e e d f avorab ly wi th t h e d a t a on CNTD coa ted Si jN4,

b u t the .KBI h o t p ressed s i l i c o n n i t r i d e gave c o n t a c t . angles

t h a t decreased much more r a p i d l y , t h i ' s being i n d i c a t i v e of . . . .

much i n t e r a c t i o n between t h e molten s i l ' i c o n and t h e K B I . .

s u b s t r a t e . , ' , . ' I

Contact a n g l e vs . t i m e g raphs f o r two s i l i c o n . s e s s i l e , : . .

. . . . d r o p exper iments on i d e n t i c a l K B I h o t p ressed Si3N4 samples . . ,,

are shown i n F igu re 3 t o demonstra te t h e importance of

a l lowing t h e system t o come t o e q u i l i b r i u m be fo re me l t ing

t h e s i l i c o n . I n t h e f i r s t run , t h e system was t aken : to . .

1700K d i r e c t l y , and t h e s i l i c o n melted wi thout a pre-melt

hold t i m e . The sessile drop t h a t was 'formed had a c o n t a c t I . . .. .

. .

a n g l e w e i l above go0, which decreased t o 46O a f t e r 8 hours.

I n t h e ' n e x t run, t h e system was he ld a t 16.50K f o r 3 .hours , '

t h e n r a i s e d p a s t t h e s i l i c o n mel t ing temperature . his 'time ' ,

t h e i n i t i a l c o n t a c t a n g l e w a s w e l l below go0. ' ~ h o t o m i c r o -

graphs of t h e i n t e z f a c e between t h e K B I . ' s i l i c o n n i t r i d e a n d . ,

Figure 2: Contact angle vs. time and p02 for silicon sessile drops on silicon nitride: (a) Kawecki-Berylco (KBI) hot pressed, (b) KBI hot pressed with no premelt hold, (c) AVCO hot pressed, (D) CNTD coated.

Figure 3: Contact angle vs. time-for KBI hot pressed silicon - nitride: (a) With no pre-melt hold time, p02=10-19- 8 atm., (b) With 3 hours pre-melt hold time, p02=10'19*5 atm.

I I I I I 1 I I I I 2 3 4 5 6 7 8 4

TIME (hours)

. .

: the silicon (Fig. 4) show that the silicon attack is more . .

. . . . . : pronounced in the sample that did not experience a pre-melt

hold time (Fig. 4a) than in the case where the sample was

held for 3 hours prior to melting (Fig. 4b). I

' . The AVCO hot pressed silicon nitride sample (Fig. 5)

does not show any evidence of the infiltration of silicon

below the silicon-silicon nitride interface, although there

is about the same degree of irregularity of the interface as

compared to the KBI material. Of the hot pressed materials

without coating, the AVCO material exhibited properties

' ' '(both contact angle and resistance to silicon attack) that

were comparable to the CNTD material and superior (i.e.,

. . ' lower, more stable molten silicon contact angle and less

.. . interaction) to. any other hot pressed SijN4 tested.

Silicon Carbide Coated Grawhite

When a silicon sessile drop experiment was made on a

polished sample of silicon carbide coated graphite supplied

,'by Ultracarbon (Fig. 6), the silicon did not melt into a

smooth drop, but showed facets, such as would occur if the . ,

' , faces of the silicon cube had merely spread out with the

. molten silicon underneath. There was no charlye in this con- . .

. . figuration for nearly 1 3/4 hours, when it was observed that

the drop had completely disappeared. After cooling and sec-

tioning, it could be seen that the silicon had completely

penetrated the coating and was absorbed into the graphite

substrate (Fig. 7). To eliminate the possibility that the

silicon carbide coating had been too thin after polishing

Figure 4 : Photori~icr~graph of RBI hot pressed silicon n i t r i d e - s i l i c o n interface a f t e r 8 hours a t 1430OC.

(a) With no pre-melt hold time, p02 = 10 -19.8

(b) With 3 hours pre-melt hold time, p02 = 10 -19.9

Figure 5: Photomicrograph of s i l i c o n - AVCO hot presued s i l i con nitride intcrfaae after 8 hours a t 14300C with a PO, = 10-19-9 atm.

Fiqure 6: In s i t u photographs a t 1430°c of s i l i c o n sessile drop experiment on polished Ultracarbon s i l i c o n carbide coated gra h i t e with p02 = 10-?9*9 atme

(a) 1 hour a f t er s i l i c o n melt.

(b) 1 hour, 45 m i n u t e @ @ft.ev adlqann malt.

Figure 7: Photomicrograph of post-sessile drop section of polished Ultracarbon silicon carbide after 1 3/4 hour experiment at p02 = 10 showing infiltration of silicon below SIC caating,

to be impervious to the liquid silicon, another experiment

using an unpolished sample was tried. This time, a stable

drop was never established, but the silicon was absorbed

into the substrate as rapidly as it melted (Fig. 8). Photo-

micrographs of the sectioned sample, Figure 9, show that the

Sic coating has many pores through which the silicon could

flow, and the silicon has spread out uniformly through the

surface of the graphite underneath the silicon carbide coating.

It appears that the graphite would have to be saturated with

silicon, forming a dense Sic layer before this material would

be impervious to further liquid silicon penetration. Siltec

Corporation is successfully using this type of Ultracarbon

material in a non-liquid silicon environment transfer tube

application. With rapid heat-up, JPL has obtained short time

silicon sessile drop data on this material.

Sialon 1 -'

As has been mentioned above, it was considered important

in the silicon sessile drop experiments to allow all surfaces

to come to equilibrium with the low oxygen partial pressure

in the flowing hydrogen buffer gas. The experiments were

carried out with the sample just below the melting temperature

of silicon from one to three hours so equilibrium could be

reached prior to melting. This same procedure was applied to

the experiments on f2 Sialon (supplied by Battelle) with even

longer equilibration times. Surprisingly, the silicon cube

never malted (Fig, 10a) even though the temperature was raised

Figure 8: In s i t u photographs a t 1 4 3 0 ~ ~ of s i l i c o n sessile drop experiment on unpolished Ultracarbon s i l i c o n carbide coated graphite with p01 = 10-20*2 atm.

(a) 4 mine (b) 6 mine

( c ) 8 mine (d) 1 hour.

Figure 9: Photomicrographs of post-sessile drop section of unpolished Ultracarbon silicon carbide coated graphite after 1 hour at p02=10-20.2 atm. showing infiltration of silicon below Sic coating.

(a) Under original position or cne silicon.

(b) At the edge of the sample.

(a) 9 hours, p02

= 10-19.7 am, (b) 5 hours

- 10-2002 atm. PO2 -

Fiyure 10 ; In s i t u phom- graphs of sili- con s e s s i l e drop experiments on n Sialon 25 min. after reaching melting tempera- ture for sam@le held just below melting tempera- ture for:

(c) 1.5 hours po2 = 1 0 ~ 2 0 . ~ atm.

t o w e l l above t h e s i l i c o n melt ing temperature. I n subse-

quent experiments (Fig. lob, lOc), t h e l eng th of t i m e t h e

system was allowed t o remain j u s t below t h e melt ing tempera-

t u r e was reduced u n t i l it was poss ib le t o achieve an apparent

sessile drop with a con tac t angle g r e a t e r than go0. However,

i n every case when t h e system was again cooled below t h e

melt ing po in t , l i q u i d s i l i c o n was observed t o protrude from

t h e cool ing drop a s i f breaking through a sk in (Fig. l l), even

i n t h e case where it appeared t h a t a c l ean s i l i c o n sessile

drop had formed. When t h e s i l i c o n was examined wi th a scanning

e l ec t ron microscope, it was found t h a t t h e r e was indeed a sk in

over p a r t s of t h e s i l i c o n (Fig. 12) having a high concent ra t ion

of calcium (Fig. 1 3 ) . The su r face of t h e R S ia lon a l s o had a

white powdery coat ing , x-ray a n a l y s i s of which showed a s t ruc -

t u r e q u i t e d i f f e r e n t from t h e o r i g i n a l Sialon. There i s ample

evidence i n t h e alumina l i t e r a t u r e (2-10) t h a t calcium presen t

i n only very small amounts a s an impuri ty can migrate t o

g r a i n boundaries and su r faces t o q u i t e high concent ra t ions ,

which appears t o be t h e case i n these experiments. This i s

one example of an e f f e c t which would n o t be observable i f a

r ap id heat-up r a t e w e r e used. I t i s l i k e l y , however, t h a t

a sk in over t h e s i l i co ,n having a high concent ra t ion of c a l -

cium would be p r e sen t even though it appeared t h a t a c l ean

sessile drop formed o n . t h e R S ia lon when heated rapid ly . The

"contac t angle" i n t h i s case would no t r epresen t t h a t of a

c l ean s i l i c o n drop i n equilibrium.

Figure 11: In s i t u photograph of s i l i c o n sessile drop experiment on Q Sialon a f t e r cooling below melting point.

19

171 . :f , '1, i4...4. 1 . 1 r6. *.3

94440, iii86, -* <2, 1K.,44644(4 K ": 6,1"#je -I.1, »1,1,9*9,4 11 5 i

r<:' ,

/ 1/2 . I4 ... -05 4

..1-4..

r + ,':, %,4 **

03%4 14

0.-. e 43 , '., 3

/1... Al<,Pi,8.7/...3,-. ,- -1 ,/9101.7 6/-(<    < :*A a , MIW////// 

Figure 12: Scanning Electron Microscope phtomicrographof silicon surface after sessile drop experi-ment on 0 Sialon.

4 ..'

..

j. :'.f=«.:46 =»..IIi. Y ..,41541;*e:-4*w4'.'ri .r. s..-9.£0-44-*--96-Si Fe Cu Si Ca Ti Fe Cu Zn

Figure 13: Non-dispersive x-ray analysis of siliconsample o f Fig. 12 showing:

(a) Silicon in darker areas, (b) Calcium in lighter areas.

SURFACE ENERGIES

The q u a n t i t i e s which determine t h e way i n which a l i qu id

w e t s a s o l i d , and thus t h e contact angle of a sessile drop,

a r e the sur face energies ysvr ysl, and ylv, where the sub-

s c r i p t s r e f e r t o solid-vapor, sol id- l iquid , and liquid-vapor,

respect ively . If t h e sessile drop and the subs t r a t e a r e i n

an atmosphere containing molecules such a s oxygen t h a t w i l l

adsorb unto t h e s o l i d subs t ra te , then the surface energy,

ysv, w i l l be a f fec ted , decreasing as more molecules ere

adsorbed, while t he o ther surface energies, ysl and ylv,

w i l l be r e l a t i v e l y unaffected. By in tegra t ing the Gibbs'

adsorption isotherm,

where: y = i n t e r f a c i a l f r e e energy

y 5 elienli~al potential of the adsorbed syecies

r = adsorbed species per u n i t area (coverage)

and assuming the adsorbed species t o be oxygen, so t ha t :

= kT I n p02

then combining t h i s r e s u l t with t he Young equation:

YBV = Yls + Ylv CQS 0

where O i s t h e contact angle of t h e sessile drop, we get :

Y l s cos O = - (kTr02) 1, p02 + - -

Ylv 1 171

Since yls and ylv a r e r e l a t i v e l y i n sens i t i ve t o changes i n

pOZ, and assuming TO2 v a r i e s much more.slowly than cos B

wi th changes i n pOZI then by measuring t h e c o n t a c t a n g l e s of

sessile d rops as a f u n c t i o n of oxygen p a r t i a l p r e s s u r e , t h e

v a l u e of r02, t h e number of oxygen molecules adsorbed p e r

u n i t a r e a , and ysvt t h e solid-vapor s u r f a c e energy f o r t h e

s o l i d s u b s t r a t e can be ob ta ined . This c o n s t i t u t e s a new

method f o r measuring t h e solid-vapor s u r f a c e energy.

A more d e t a i l e d d e r i v a t i o n of t h e t h e o r e t i c a l r e l a t i o n -

s h i p s between c o n t a c t ang le , oxygen p a r t i a l p r e s s u r e , s u r f a c e

coverage and e n e r g i e s can be found i n r e f e r e n c e l l a long wi th

measured v a l u e s of t h e s e q u a n t i t i e s f o r CNTD s i l i c o n c a r b i d e ,

s i l i c o n n i t r i d e , and aluminum n i t r i d e .

DEVITRIFICATION OF FUSED S I L I C A

When fused s i l i c a has been used a s a s u b s t r a t e i n s i l i c o n

s e s s i l e d rop exper iments a t UMR, d e v i t r i f i c a t i o n has taken

p l a c e i n every c a s e , e f f e c t i v e l y e l i m i n a t i n g fused s i l i c a

from f u r t h e r s e s s i l e d rop experiments. Some a t t empt was made

t o de te rmine i f t h e fu rnace atmosphere had any e f f e c t on t h e

d e v i t r i f i c a t i o n . Although it was determined t h a t t h e primary

cause of d e v i t r i f i c a t i o n was t h e slow heat-up ( 4 hours) and

cool-down ( 8 hours) c y c l e s of t h e UMR sessile drop fu rnace ,

t h e degree of d e v i t r i f i c a t i o n was a f f e c t e d by t h e atmosphere,

w i t h n e a r l y complete d e v i t r i f i c a t i o n t ak ing p l a c e i n t h e

CO/C02 atmosphere ( 6 0 p a r t s CO t o 1 p a r t C 0 2 ) and s u r f a c e

d e v i t r i f i c a t i o n on ly on t h e s i l i c a heated i n a i r and i n

helium.

OXYGEN PARTIAL PRESSURE MEASUREMENTS

AT OTHER FACILITIES

Mobil Tyco

The atmosphere i n t h e Mobil-Tyco EFG r ibbon p u l l i n g f u r -

nace c o n s i s t s of argon which f lows i n t o t h e system a t a r a t e

va ry ing from 10 l i t e r s p e r minute t o 2 l i t e r s pe r minute.

T y p i c a l l y , commercially a v a i l a b l e argon c o n t a i n s oxygen a s

a n impur i ty i n about 1 t o 1 0 0 p a r t s pe r m i l l i o n , corresponding

t o t o atm. oxygen p a r t i a l p r e s s u r e a t 1 atm. t o t a l

p r e s s u r e . The fu rnace c o n t a i n s l a r g e amounts of g r a p h i t e a s

c r u c i b l e s , h e a t e r s , d i e and d i e ho lde r , etc. , a l l of which

s e r v e t o reduce t h e amount of f r e e oxygen i n t h e system and

t h u s lower t h e p02m

P r i o r t o a c t u a l measurements of t h e oxygen p a r t i a l p res -

s u r e of t h e fu rnace atmosphere, t h e performance of t h e oxygen

ce l l was f i r s t checked by measuring t h e p02 of forming g a s

(95% N 2 , 5% HZ). The v a l u e of 10 -I9 atm. a t 1 0 0 0 ~ ~ ob ta ined

w a s c o n s i s t e n t w i t h p rev ious measurements made wi th t h e c e l l

a t UMR.

Thc argon purge gas d i d liuL l i d v t ! a specific gas e x i t

p o r t from t h e fu rnace , b u t i n s t e a d w a s al lowed t o escape

th rough t h e s i l i c o n r ibbon e x i t s l o t and v a r i o u s o t h e r

openings t o t h e atmosphere. S ince t h e r e was n o t s u f f i c i e n t

p r e s s u r e under t h e s e c o n d i t i o n s t o f o r c e t h e argon through

t h e oxygen ce l l , it was found necessary t o sample t h e fu rnace

g a s oxygen p a r t i a l p r e s s u r e by i n s e r t i n g a q u a r t z t ube through

one of t h e openings t o t h e fu rnace and drawing g a s i n t o t h e

oxygen sensor w i th a mechanical pump hooked t o t h e oxygen

c e l l exhaust l i n e . S ince t h e ce l l had n o t i n i t i a l l y been

designed t o o p e r a t e under a nega t ive p re s su re , t h e r e e x i s t e d

a p o s s i b i l i t y t h a t oxygen may have been drawn i n t o t h e ce l l

( i n smal l q u a n t i t i e s ) from t h e a i r . The s e a l s have subse-

quen t ly been improved and checked under nega t ive p r e s s u r e s

comparable t o t hose e x i s t i n g i n t h e above measurements.

Another v i s i t t o t h e Mobil Tyco f a c i l i t y i s planned t o v e r i f y

t h e i n i t i a l measurements.

J P L - The oxygen ce l l was t r a n s p o r t e d t o t h e J P L f a c i l i t y a t

Pasadena, C a l i f o r n i a , where t h e oxygen p a r t i a l p r e s s u r e of

t h e atmosphere i n t h e i r s i l i c o n sessile d rop fu rnace was

measured. The g a s used i n t h i s furnace was helium from

s tandard c y l i n d e r s , which t y p i c a l l y have oxygen impur i ty

l e v e l s of about l o m 5 atm. p02 a t one atmosphere t o t a l p res -

su re . The flow r a t e s used v a r i e d between 16 and 22 SCFH, o r

from 5 t o 1 0 R/min. I n t h i s c a s e , t h e fu rnace was sea l ed s o

t h e i n t a k e and exhaus t g a s could be sampled a t t h e r e q u i r e d

flow r a t e wi thout t h e u s e of a pumping system. There was

g r a p h i t e p r e s e n t i n t h e fu rnace , a s i n t h e c a s e of t h e Mobil

Tyco furnace , which would reduce t h e oxygen c o n t e n t of t h e

atmosphere by forming CO and C 0 2 .

R e s u l t s and Discussion

When an i n e r t c a r r i e r g a s , such a s argon o r helium, .

having an oxygen impur i ty l e v e l of between 1 ppm and 100 ppm

(corresponding t o oxygen p a r t i a l p re s su re between l o m 6 atm.

and lo-* atm.), comes in contact with graphite at 1700K, the

following reaction occurs: 2C + O2 + 2CO. If the gas flow

is sufficiently slow and all the gas comes in intimate con-

tact with 1700K graphite such that an equilibrium is

established, all but a very small fraction of the oxygen

would combine with carbon to form CO, resulting in a p02 of

9.5 x 10 -17 atm. in the exit gases at 1700K. However, if the

flow rate were too high and/or the gas did not all contact

170UK graphite, then equilibrium would not be established,

and significant amounts of oxygen would remain unreacted,

resulting in a higher p02 and a lower pCO.

This non-equilibrium behavior is in qualitative agree-

ment with the CO measurements made on the Mobil Tyco furnace

gas by the Kitagawa cell method simultaneously with the oxy-

gen partial pressure measurements. With a high (10 k/min.)

argon purge rate, the CO concentration was less than 30 ppm,

but this increased to over 100 gpm when the purge ra tc was

dropped to 2 R/min. as would be expected nearer equilibrium

conditions.

The oxygen partial pressure of the purge gas after

gassing through the furnace was measured at 1273K by the

oxygen cell to be 10 -lZol a t m . for the ~ ~ b i l Tycn nyrtem

and 10 -13.4 am. for the JPL system, indicating that in

both cases, roughly half of the original oxygen impurity

was converted to CO.

The figure of one-half can be rationalized by considering . . ..

the following scenario: Consider one million units of "inert"

- g a s con ta in ing 100 u n i t s O2 as an impur i ty e n t e r i n g a molten si l i-

con fu rnace a t 1700K. (1) U s e x p a r t s O2 t o form 2x p a r t s CO,

l e a v i n g 100-x p a r t s O2 unreac ted . Th i s non-equi l ib ra ted mix ture

of g a s e s c o n s i s t i n g of n e a r l y one m i l l i o n p a r t s i n e r t g a s , 2x

p a r t s CO, and 100-x p a r t s oxygen, t hen l e a v e s t h e s i l i c o n fu rnace

and e n t e r s t h e oxygen cel l where it e q u i l i b r a t e s a t 1273K. I f x

i s less than 50, t hen (2 ) 2x p a r t s CO combine w i t h x p a r t s O2 t o

form 2x p a r t s C02 and l e a v i n g 100-2x p a r t s O2 s t i l l unreac ted .

This c o n s t i t u t e s a p02 of (100-2x )x10-~ a t m . I f x i s g r e a t e r

t han 50, t h e n ( 3 ) most of t h e O2 l e a v i n g t h e s i l i c o n fu rnace ,

t h a t i s , 100-x p a r t s O2 combine wi th 2x-6 p a r t s CO ( l e a v i n g 6

p a r t s unreac ted) t o form 2x-6 p a r t s C02 accord ing t o t h e r e a c t i o n :

For t h i s t o ba lance , t hen 2x-6=2(100-x), o r 6=4x-200, and t h e

r e s u l t i n g CO/C02 r a t i o is :

The amount of oxygen used i n t h e f i r s t r e a c t i o n , x , can be found

from t h e p02 measured i n t h e oxygen ce l l by so. lving t h e mass

a c t i o n equa t ion f o r x; us ing t h e above CO/C02 r a t i o :

PC02 2 pO2 = (-

AG PCO

) exp - RT

The r e s u l t s of p l o t t i n g Log p02 vs . 100-x i s shown on F igu re

1 4 , where it can be seen t h a t i f h a l f t h e impur i ty O2 r e a c t s

Figure 1 4 : Graph of Log p02 of e q u i l i b r a t e d gas a t 1273K ver sus p a r t s 02 unreacted a t 1700K. Rased on 100 p a r t s oxygen impurity per m i l l i o n p a r t s purge gas.

Ports 0, Unreacted (ppm, or X I O - ~ to get POe in silicon furnace 1

with carbon in the silicon furnace, the oxygen partial pres-

sures in the range between l ~ - ~ and 10 -13 atm. should be.

measured with the oxygen cell.

SUbIMAHY & CONCLUSIONS

All sessile drop experiments were conducted at one atmos-

phere total pressure and oxygen partial pressures less than

1 x 10 -19 atmospheres. The materials investigated for potential

die and container applications in the present study fall into

two categories; Those uson which silicon melts, forming a

sessile drop with an equilibrium contact angle, and those upon

which a contact angle representing equilibrium between clean. .

liquid silicon, the substrate candidate material, and the gaseous

environment, is not obtained.

In the first category are two varieties of hot pressed'

silicon nitride (from ~awecki-~er~lco (KBI) and AVCO) and

CNTD silicon nitride coated on hot pressed silicon nitride.

The silicon - CNTD silicon nitride contact angle data obtained in this .study agree very well with previous measurements made

on the same material, and therefore provided a control for re-

producibility verification. Although the final contact angle

(after 8 hours) of molten silicon on the AVCO hot pressed

silicon nitride is higher than for the KBI material, the latter

appears to react more strongly with molton silicon, as evi-

denced by photomicrographs and the continued decrease in. contact

angle after 8 hours. . .

, .

,, .. ,:;'

Ultracarbon silicon carbide coated. graphite and Battelle 52'

Sialon fell into the category of materials upon which silicon did

not form a true sessile drop. In the former material, the Sic

coating proved to not be a completely integral surface, so molten

silicon could seep through the coating to the underlying graphite,

which is very porous and subject to impregnation by the molten

silicon. For the limited size silicon sample used in the sessile

drop experiments, all of the silicon penetrated the Sic coating,

saturating the graphite under the coating and precluding the

formation and measurement of an equilibrium contact angle on this

material.

A pseudo-sessile drop formed on the Battelle R 1 Sialon be-

cause a semi-rigid, skinlike coating having a high concentration ,

of calcium formed on the surface of the liquid silicon drop pre-

venting attainment of equilibrium. The thickness of this coating,

and thus its rigidity, depended on the length of time the s.ample

was held just beiow the silicon me'lting temperati-lre prior to the

iticlt p l u s Lilt! time after melting. For short pre-melt hold times,

the silicon appeared to form a high-contact angle drop, but sub-

sequent cooling showed evidence of the thin coating on the silicon

surface. The lJl tracarbon SiC on graphile drld the Battelle 9 ' Sialon materials are, therefore, not recommended for use as die

or cantair~er ~rlaterials tor containment of molten silicon.

Analysis of the oxygen cont-ent of the purge gases in the

!.lobil Tyco silicon ribbon pulling furnace and the JFL silicvn '

sessile drop furnace shows the presence of relatively high

quantities of oxygen - of the order of 1 to 100 parts per million - which is far higher than the Si-O2 equilibrium partial pressure of 10 -I9 atm. The fact that oxide does not

form to the extent expected under equilibrium conditions such

as those established in the UMR sessile drop experiments in-

dicates that these equilibrium conditions are not achieved in

the Mobil Tyco and JPL furnaces. Possible reasons for this

include the high purge gas flow rates, large amounts of hot

graphite which removes oxygen from the gas preferentially

near the molten silicon, and reaction rates that are slow

compared to experimental hold and thermal cycle times in

the case of the JPL sessile drop furnace.

REFERENCES

1. Eagle Picher Final Report to JPL on contract #DOE/JPL- 954877-78/3.

2. P.J. Jorgensen and J.H. Kestbrook, "Role of Solute Segregation at Grain Boundaries During Final-Stage Sintering of Alumina," J. Am. Cer. Soc., 47(7), '332-38, (1964).

3. S.S .C. Tong and J.P. Williams, "Chemical Analysis of Grain Boundary Impurities in Polycrystalline Ceramic Iflaterials by Spark Source Iflass Spectrometry, " ibid., 53 (I), 58-59, (1970) .

4. E.L . Marcus and M.E. Fine, "Grain-Boundary Segregation in MgO-Doped Al203," ibid., 55 (11) , 568-70, (1972).

5. W.C. Johnson and D.F. Stein, "Additive and Impurity Distributions at Grain Boundaries in Sintered Alum.ina," ibid., 58 (11-12) , 485-88, (1975) .

6. R.I. Tay10r~'J.P. Coad, and R.J. Brook, "Grain-Boundary Segregation in A1203, " ibid., 57 (12) , 539-40, (1974) .

7. R.I. Taylor, J.P. Coad, and A.E. Hughes, "Grain-Boundary Segregation in. PlgO-Doped A 1 2 0 3 , " ibid. , 59 (7-8) , 374-75, (1976).

8. A.W. Funkenbusch and D.W. Smith, "Influence of Calcium on the Fracture Strength of Polycrystalline Alumina," !.letall. 'I'rans., A6 (12), 2299-2301, (1975) .

9. W.C. John~on, "Mg DistriLuLio~is at G r a i n Boundaries in Sintered Alumina Containing MgA1203 Precipitates," J. Amer. Cer. Soc., 61(5-6), 234-37, (1,978).

18. U . K . Clarke, "Grain-~oundary Segregation in an MgO-Doped A1203,il J. Amer. Cer. Soc., 63, 339, (1980).

M . W . Barsoum and P.D. Ownby, "Effect of Oxygen Partial Pressure on Wetting of Sic, AlN, and Si7N4 by Si, and a Method for Calculating Surface Bnergies'lnvolved," Pro- ceedings of 7th LDL/MMRD International MaLerials Pymposium, 17th University Conference on Ceramics, University of California, Berkeley, California, July 28-August 1, 1980 and M.W. Barsoum, M. S. Thesis, University o f Missouri- Rolla, (1980).

LU.S . G O V E R N M E N T P R I N T I N G OFFICE: 1980-740-1451735