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T h e c u r v e s w e r e o b t a i n e d b y the r e l a x a t i o n m e t h ods, since a n a l y t i c c o m p u t a t i o n is m a d e difficult
b y the lack of a p p r o p r i a t e m a t h e m a t i c a l tables.
Acknowledgments
T h e a u t h o r s w i s h to t h a n k L. E. Q. W a l k e r a n d
D. W. G. B a l l e n t y n e for p r o p o s i n g t h e p r o b l e m a n d
for m a n y h e l p f u l discussions, Miss M. A. M i l l i d g e
for c o m p u t i n g some of t h e c u r v e s , also Miss E.
K o w s z y n i s for s u p p l y i n g e x p e r i m e n t a l d a t a for Fig.
5, a n d t h e Chief of R e s e a r c h of t h e M a r c o n i ' s W i r e less T e l e g r a p h C o m p a n y for t h e p e r m i s s i o n to p u b lish.
M a n u s c r i p t received A p r i l 22, 1957.
A n y discussion of this paper will appear in a Discussion Section to be published in the J u n e 1958
JOURNAL.
N o v e m b e r 1957
SOCIETY
REFERENCES
1. C. Kasper, Trans. Am. Electrochem. Soc., 77, 353
(1940).
2. R. O. Hull, "Control of P l a t i n g Baths b y P l a t i n g
Cells," "Metal F i n i s h i n g " Guidebook-Directory,
ISth a n n u a l ed., p. 367 (1949).
3. R. G i l m o n t a n d R. F. Walton, This Journal, 103, 549
(1956).
4. W. R. Smythe, "Static and D y n a m i c Electricity,"
McGraw-Hill Book Co., Inc., p. 233 (1950).
5. A. Cayley, "Elliptic Functions," G. Bell & Sons, London or Deighton, Bell & Co., Cambridge, p. 360
(1895).
6. F. Bowman, Proc. London Math. Soc., 39, 211 (1939).
7. E. J a h n k e a n d F. Emde, "Tables of F u n c t i o n s with
F o r m u l a e and Curves," Dover Publications, New
York (1945).
8. P. F. B y r d a n d M. D. F r i e d m a n , "Handbook of Elliptic Integrals for Engineers and Physicists,"
Springer Verlag-Berlin, New York (1954).
9. A. E. H. Love, Am. J. Math., 2, 158 (1889).
Transistor-Grade Silicon
I. The Preparation of Ultrapure Silicon Tetraiodide
Bernard Rubin, Guy H. Moates, and Joseph R. Weiner
Radiochemistry Section, Components and Techniques Laboratory, Electronics Research Directorate,
Air Force Cambridge Research Center, Air Research and Development Command, Bedford, Massachusetts
ABSTRACT
A stepwise method of p r e p a r i n g and p u r i f y i n g SiI, has been found involving the direct combination of the elements, recrystallization of the product,
followed b y s u b l i m a t i o n and zone purification steps. The values of the
segregation coefficients of several i m p u r i t y elements have been determined,
and it is shown that u n d e r ideal conditions some of these elements can be
removed to concentrations of less t h a n one p a r t i m p u r i t y per billion parts of
SiI, in sixty passes for a 50 cm length charge.
T h e r e q u i r e m e n t s for t r a n s i s t o r s a n d o t h e r s e m i c o n d u c t o r devices t h a t o p e r a t e at t e m p e r a t u r e s
h i g h e r t h a n those at w h i c h G e is effective h a v e
stimulated considerable research in the preparation
of " t r a n s i s t o r - g r a d e silicon." This t e r m r e f e r s to a
Si m a t r i x i n w h i c h t h e i m p u r i t y levels a r e at c o n c e n t r a t i o n s of one p a r t i n o n e h u n d r e d m i l l i o n a n d
p r e f e r a b l y as l o w as o n e p a r t p e r o n e h u n d r e d b i l l i o n of St. B e c a u s e t h e sources of this m a t e r i a l i n t h e
U n i t e d S t a t e s a r e few, the A i r F o r c e has i n i t i a t e d a
r e s e a r c h p r o g r a m i n Si c h e m i s t r y i n o r d e r to p r o vide a l t e r n a t e m e t h o d s of r e f i n e m e n t .
Most Si i n this c o u n t r y is m a d e a c c o r d i n g to t h e
reaction
2C -~ SiO~-->2CO ~- Si
in electric arc furnaces. A typical productI has the
following spectrographic analysis shown in Table I
( i ) . Semiconductor devices made of Si with levels
of impurities as shown in Table I would be of little
practicality and transistor devices with acceptors,
donors, and lifetime-killersat such high concentrations would not function. Two approaches are available for removing these impurities, a metallurgical
1 Obtained
from
Coleman
and Bell Company,
Norwood,
Ohio.
a n d a p h y s i c o c h e m i c a l . T h e f o r m e r i n v o l v e s t h e zone
r e f i n i n g t e c h n i q u e o r i g i n a t e d b y P f a n n (2). B y this
Table I. Spectrographicanalysisof reagentSi
Impurity element
A1
As
B
Ca
Co
Cr
Cu
Fe
Ga
In
K
Li
Mg
Mn
Na
Ni
P
Ta
Ti
T1
V
Zr
C o n c . i n p a r t s of i m p u r i t y
p e r m i l l i o n p a r t s of Si
6900
150
60
7100
7
250
300
6700
<10
3
<10
2.5
120
350
18
80
80
140
1300
5
60
250
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Vol. 104, No. 11
TRANSISTOR-GRADE
m e t h o d , s o m e e l e m e n t s , e.g., A1, m a y b e r e m o v e d
e f f e c t i v e l y f r o m St. H o w e v e r , B, w i t h a s e g r e g a t i o n
coefficient of a b o u t 0.9 (3) c a n n o t b e s e p a r a t e d effic i e n t l y f r o m t h e m a t r i x . I n a d d i t i o n , b e c a u s e of t h e
h i g h m e l t i n g p o i n t of Si ( 1 4 2 0 ~
contamination
by leaching from the container material presents
problems. For these reasons, the physicochemical
a p p r o a c h w h i c h i n v o l v e s t h e s y n t h e s i s of a c o n v e n i e n t c o m p o u n d of St, its p u r i f i c a t i o n to t h e d e s i r e d l e v e l s of p u r i t y t h a t a r e r e q u i r e d u l t i m a t e l y
f o r t r a n s i s t o r - g r a d e silicon a n d its d e c o m p o s i t i o n
into e l e m e n t a l St, h a s b e e n c h o s e n b y this l a b o r a t o r y . I n o r d e r t h a t t h i s a p p r o a c h b e effective, t h e r e
w e r e c e r t a i n p r e r e q u i s i t e s . T h e c o m p o u n d h a d to
b e e i t h e r e a s i l y s y n t h e s i z e d or a v a i l a b l e , i t h a d to
b e c a p a b l e of b e i n g p u r i f i e d to t h e e x t e n t r e q u i r e d
b y t h e final Si specification, a n d it h a d to b e c a p a b l e
of b e i n g d e c o m p o s e d r e l a t i v e l y e a s i l y i n t o e l e m e n t a l Si w h i c h w o u l d n o t b e c o n t a m i n a t e d i n t h e
d e c o m p o s i t i o n process. Of t h e v a r i e t y of Si c o m p o u n d s t h a t s u g g e s t e d t h e m s e l v e s , i.e., t h e silanes,
s i l o x a n e s , silicates, a n d h a l i d e s , t h e l a t t e r s e e m e d
to b e b e s t s u i t e d to fulfill t h e a b o v e r e q u i r e m e n t s .
T h e y a r e s t a b l e s u b s t a n c e s , r e l a t i v e l y e a s y to s y n thesize and handle, and are potentially decomposa b l e or r e d u c i b l e to p o l y c r y s t a l l i n e o r s i n g l e - c r y s t a l
Si a n d t h e h a l o g e n o r h y d r o g e n h a l i d e . Of t h e f o u r
u n m i x e d t e t r a h a l i d e s , S i F , is t h e m o s t s t a b l e a n d
d e c o m p o s e s o n l y u n d e r e x t r e m e s of t e m p e r a t u r e
a n d p r e s s u r e . F u r t h e r m o r e , it is a g a s u n d e r n o r m a l
c o n d i t i o n s a n d , as such, is difficult to h a n d l e a n d
p u r i f y . S i B r , a n d SiCL a r e b o t h l i q u i d s p o s s e s s i n g
t h e a d v a n t a g e s of t h i s s t a t e w i t h r e s p e c t to p u r i f i c a tion, b u t b o t h a r e m o r e difficult to d e c o m p o s e t h a n
the tetraiodide. Thermodynamic data indicate that
t h e l a t t e r d e c o m p o s e s at a b o u t 1500~ at one a t m o s phere and can be reduced with hydrogen at about
600~ u n d e r t h e s e c o n d i t i o n s . A t 1000~
the diss o c i a t i o n e q u i l i b r i u m c o n s t a n t (Kp) w a s c a l c u l a t e d
to b e 1.86 x 10 -~ f r o m t h e f r e e e n e r g y d a t a . A s s u m i n g a p a r t i a l p r e s s u r e of 3 m m of S i L in t h e r e a c t i o n
c h a m b e r , t h e n t h e l i m i t i n g p a r t i a l p r e s s u r e of I~ is
0.236 ram. B y l o w e r i n g t h e p a r t i a l p r e s s u r e of L
b e l o w this v a l u e , d e c o m p o s i t i o n of t h e SiI, occurs.
S i n c e it is a solid u n d e r n o r m a l c o n d i t i o n s w i t h a
relatively low boiling point (290~
it can be purified n o t o n l y b y t h e u s u a l t e c h n i q u e s of r e c r y s t a l l i z a t i o n a n d d i s t i l l a t i o n , b u t it l e n d s i t s e l f to zone
p u r i f i c a t i o n . S i n c e SiI, offers a d i f f e r e n t m a t r i x t h a n
Si to i m p u r i t i e s , it n e e d n o t b e e x p e c t e d t h a t t h e
s e g r e g a t i o n coefficients of t h e s e i m p u r i t i e s b e t h e
s a m e in b o t h m a t r i c e s . If t h e i m p u r i t i e s t h a t a r e n o t
r e m o v e d in t h e c u r r e n t t e c h n i q u e of z o n e p u r i f i c a t i o n of Si c a n b e r e m o v e d in Sil~, t h e n a c o m b i n a t i o n of t h e t w o z o n e p u r i f i c a t i o n s c o u l d l e a d to
t r a n s i s t o r - g r a d e St. I t s e e m e d f o r m o s t i n t e n t s a n d
p u r p o s e s t h a t SiI~ o f f e r e d t h e g r e a t e s t p o t e n t i a l i t y
as a n i n t e r m e d i a t e in t h e s y n t h e s i s of t r a n s i s t o r g r a d e St.
A l i t e r a t u r e s u r v e y i n d i c a t e d t h a t h i g h p u r i t y Si
has been prepared by methods utilizing the tetrac h l o r i d e , t h e t e t r a b r o m i d e , a n d t h e t e t r a i o d i d e of
St. T h e t e t r a c h l o r i d e is r e d u c e d in a q u a r t z a p p a r a t u s a t a b o u t 1000~ w i t h Z n v a p o r as t h e r e ductant (4).
657
SILICON
S a n g s t e r (5) h a s r e d u c e d t h e p u r i f i e d t e t r a b r o m i d e w i t h H~. L i t t o n (6) i n v e s t i g a t e d t h e t h e r m a l
d e c o m p o s i t i o n of f r a c t i o n a l l y d i s t i l l e d t e t r a i o d i d e .
M o r e recentlY, T h e u e r e r (7) r e d u c e d SIC1, w i t h H~.
Experimental
Preparation of silicon tetraiodide.--SiI, w a s p r e p a r e d b y t h e d i r e c t r e a c t i o n of I~ w i t h S i in a h o r i zontal reaction chamber (8). The iodine boiler,
h e a t e d b y a m a n t l e to a b o u t 115~
w a s a 500 m l
r o u n d - b o t t o m e d P y r e x flask s e a l e d a t t h e t o p a n d
e q u i p p e d w i t h a 28/15 b a l l j o i n t a t r i g h t a n g l e s to
t h e n e c k . A side a r m w a s p r e s e n t to p e r m i t t h e e n t r y of t h e d r i e d f l u s h i n g gas, a r g o n . T h e r e a g e n t
was Baker and Adamson resublimed iodine. The
reaction chamber was a 30-mm inner diameter Vycor f u r n a c e t u b e , 70 c m long, w i t h 28/15 s o c k e t
j o i n t s a t e a c h end. T h e c o n n e c t i o n of P y r e x to V y c o r
p e r m i t t e d e a s y r e m o v a l of o n e j o i n t f r o m t h e o t h e r
o w i n g to t h e d i f f e r e n c e in coefficients of e x p a n s i o n .
The furnace tube was heated by two electric multiple unit furnaces made by Hevi-Duty Electric Company, Milwaukee, Wisconsin, and the temperature
monitored by using a chromel-alumel thermocouple
a d j a c e n t to t h e f u r n a c e t u b e . T h e SiI, r e c e i v e r w a s a
5 0 0 - m l r o u n d - b o t t o m e d P y r e x flask s i m i l a r to t h e
iodine boiler.
I n a t y p i c a l r u n , r e a g e n t Si w a s g r o u n d i n t o p a r t i c l e s a n d t h e n c o l l e c t e d b e t w e e n No. 4 a n d No. 10
s i e v e s to p e r m i t as close p a c k i n g of t h e p a r t i c l e s as
w a s p r a c t i c a b l e in t h e f u r n a c e t u b e w i t h o u t c h a n n e l i n g or b a c k p r e s s u r e s of L. T h e c h a r g e d I~ flask,
reaction chamber, and receiver were connected usi n g a m i n i m u m of D o w C o m i n g S i l i c o n e g r e a s e a t
the joints and a mercury pressure release valve was
i n s e r t e d in p a r a l l e l w i t h t h e s y s t e m . T h e a r g o n w a s
f l u s h e d t h r o u g h t h e s y s t e m a t a flow r a t e of a b o u t
524 m l / m i n a n d t h e t e m p e r a t u r e of t h e S i w a s
r a i s e d to 810~
All exposed connecting sections
between boiler, chamber, and receiver were maint a i n e d a t s u i t a b l e t e m p e r a t u r e s b y m e a n s of h e a t i n g
tapes. When temperature equilibrium was attained,
t h e I~ w a s h e a t e d to 115~ a n d c a r r i e d i n t o c o n t a c t
w i t h t h e St. T h e p r o d u c t as it e n t e r e d t h e r e c e i v e r
w a s a w h i t e m i s t and, a f t e r c o n d e n s a t i o n , a p i n k i s h
w h i t e to r e d solid. T h e c o l o r a t i o n w a s p r o b a b l y d u e
to u n r e a c t e d Is o r s o m e Si~I, f r o m t h e r e a c t i o n :
Si ~- SiL ~ Si~L
Gravimetric analysis. Calculated for SiI,: St, 5.2%;
I, 94.8%. Found: St, 5.6%; I, 91.0%. Emission spectrographic analysis of the crude SiL gave the impurities as listed in Table II.
A comparison with Table I indicates that three
elements have increased in concentration: Na, B,
a n d V. A n a n a l y s i s of P y r e x glass s h o w e d t h a t N a
w a s p r e s e n t to t h e e x t e n t of a b o u t 8.5% a n d B
a b o u t 10.8%. I t w a s e v i d e n t t h a t l e a c h i n g of t h e s e
t w o i m p u r i t i e s f r o m t h e glass t o o k p l a c e u n d e r t h e
c o n d i t i o n s of t h e S i L s y n t h e s i s . T h e r e w a s also t h e
p o s s i b i l i t y of l e a c h i n g f r o m t h e V y c o r . F o r t h i s r e a son, a n a l l - q u a r t z a p p a r a t u s w i l l b e s u b s t i t u t e d f o r
P y r e x in t h e s y n t h e s i s step. T h e d a i l y p r o d u c t i o n
r a t e is a b o u t 450 g of S i L a n d t h e a p p a r a t u s m a y b e
scaled up for larger yields if necessary.
There is
about,: ~( 95% c o n v e r s i o n to SiI~ b a s e d o n I~.
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658
JOURNAL
OF THE ELECTROCHEMICAL
Table II. Emissionspectrographic analyses
Impurity
element
C r u d e SiIa
Ag
A1
As
Au
B
Be
Bi
Ca
Cd
Co
Cr
Cu
Fe
Ga
Hg
In
K
Li
Mg
Mn
Mo
Na
Ni
P
Pb
Sb
Sn
Ti
V
Zn
Zr
1.0
28.0
<1
N.D. *
16.0
N.D.
N.D.
N.D.
<1
N.D.
1.5
12.0
55.0
<0.1
<1
<0.1
N.D.
N.D.
1.6
4.5
N.D.
2.0
1.0
<5
<0.5
<1
<0.5
21.0
12
N.D.
1.2
N o v e m b e r 1957
Table Ill. Sensitivities oF impurities by emission
spectrographic analysis
Cone. of i m p u r i t y i n p a r t s of
i m p u r i t y p e r m i l l i o n p a r t s SiI4
i R e c r y s t a l l i z e d SiI4
Sublimed Slit
0.1
6.5
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
0.3
0.5
N.D.
N.D.
0.5
N.D.
N.D.
0.4
2.0
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
16.5
N.D.
N.D.
5.0
SOCIETY
N.D.
0.2
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
2.7
0.6
N.D.
N.D.
N.D.
N.D.
N.D.
0.1
0.1
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
1.3
N.D.
N.D.
N.D.
9 Nat detected.
Analysis of silicon tetraiodide.--Analyses of SiL
were carried out ~ after hydrolyzing a sample with
c o n d u c t i v i t y w a t e r i n P t c r u c i b l e s a n d h e a t i n g to
500~ u n t i l t h e r e a c t i o n w a s complete. C o m m e r cially available mixed internal standards were then
a d d e d to t h e silica. T h i n - w a l l e d g r a p h i t e electrodes
w e r e filled w i t h a d e q u a t e silica a n d i g n i t e d i n a
He a t m o s p h e r e for 10 sec at 5 amp. A n o t h e r silica
s a m p l e w a s i g n i t e d i n air for 50 sec at 10 a m p a n d
b u r n e d to c o m p l e t i o n . T h o s e e l e m e n t s w h o s e l i n e s
w e r e m a s k e d b y SiO, SiO~, a n d CN b a n d s i n t h e
atmospheric r u n were read from the spectrum obt a i n e d i n t h e He r u n . T h e s e n s i t i v i t i e s of some of
the i m p u r i t y e l e m e n t s i n SiO2 are g i v e n i n T a b l e
III.
Crystallization.--Of t h e v a r i o u s p u r i f i c a t i o n t e c h n i q u e s t h a t are a v a i l a b l e for solids, zone p u r i f i c a t i o n has t h e p o t e n t i a l i t y , if t h e s e g r e g a t i o n coefficients a r e f a v o r a b l e , of y i e l d i n g a m a t r i x i n w h i c h
t h e d e s i r e d c o n c e n t r a t i o n s of i m p u r i t i e s c a n b e a t t a i n e d p r a c t i c a b l y . A l t h o u g h successful z o n e p u r i f i c a t i o n is n o t d e p e n d e n t o n a t t a i n i n g low c o n c e n t r a t i o n s of i m p u r i t y i n i t i a l l y , t h e u l t i m a t e c o n c e n t r a t i o n c a n b e m i n i m i z e d if t h e i n i t i a l c o n c e n t r a t i o n s
a r e low. F o r this r e a s o n , o t h e r m e t h o d s of p r e l i m i n a r y p u r i f i c a t i o n h a v e b e e n i n v o k e d . T h e first of
these w a s c r y s t a l l i z a t i o n of t h e c r u d e SiI, f r o m
t o l u e n e . T h e s o l u b i l i t y of SiL i n t o l u e n e was f o u n d
to b e 10.3% b y w e i g h t a t l l 0 ~ a n d 3.2% a t 20~
i n d i c a t i n g a b o u t a n 82 % r e c o v e r y of SiI, i n a s i n g l e
step c r y s t a l l i z a t i o n .
2 T o b e p u b l i s h e d b y t h e M e t a l H y d r i d e s Co., Journal o$ A n a l y t i cal Chemistry.
Impurity elements
Ag
A1
As
B
Be
Ca
Cd
Cr
Cu
Fe
Hg
Mg
Mn
p
Sb
Ti
Zn
Zr
S e n s i t i v i t y i n p a r t s of i m p u r i t y
p e r m i l l i o n p a r t s SiO2
0.25
0.65
15
0.50
0.25
0.50
1.0
2.5
0.25
0.65
5.0
0.25
0.75
25
3.O
1.0
10
5
I n practice, the c r u d e SiI, w a s t r a n s f e r r e d i n t h e
sealed r e c e i v e r to a d r y box, a n d sufficient f r a c t i o n a l l y distilled t o l u e n e , d r i e d over Na, w a s a d d e d to
m a k e a 10% solution. T h e s o l u t i o n w a s h e a t e d to
the b o i l i n g point, a n d t h e r e s u l t i n g s o l u t i o n w a s
cooled s l o w l y to 0~
T h e l i q u o r was d e c a n t e d , a n d
the r e m a i n d e r r e m o v e d u n d e r v a c u u m at a b o u t
70~
A s p e c t r o g r a p h i c a n a l y s i s of t h e c r y s t a l l i z e d
p r o d u c t is g i v e n i n T a b l e II.
A c o m p a r i s o n of t h e c r y s t a l l i z e d a n d c r u d e p r o d u c t i n d i c a t e s t h a t t h e r e is a c o n s i d e r a b l e d e c r e a s e
i n t h e o v e r - a l l i m p u r i t y c o n c e n t r a t i o n s of m o s t of
t h e e l e m e n t s . It c a n be p o s t u l a t e d t h a t t h e i m p u r i t y
e l e m e n t s are i n a m o l e c u l a r f o r m t h a t is t o l u e n e soluble, and, i n all p r o b a b i l i t y , this f o r m is t h e
iodide. It is p o s s i b l e t h a t t h e efficiency of t h e c r y s t a l l i z a t i o n step is the r e s u l t , i n p a r t , of s m a l l
a m o u n t s of silica i n t r o d u c e d b y t h e u n a v o i d a b l e
h y d r o l y s i s of the SiI, d u r i n g h a n d l i n g . F u r t h e r m o r e ,
since the e x t r a c t i o n is a r e l a t i v e l y low t e m p e r a t u r e
step, t h e r e is l i t t l e or no occasion for l e a c h i n g as is
i n d i c a t e d b y the low c o n c e n t r a t i o n s of such c o n s t i t u e n t s of glass as B a n d Fe. G r a v i m e t r i c a n a l y t i c a l m e t h o d s o n the c r y s t a l l i z e d SiI~ y i e l d e d t h e
f o l l o w i n g a n a l y s i s : St, 5.4%; I, 91.9%. T h e s e v a l u e s
are closer to t h e o r e t i c a l t h a n the c r u d e SiI,, a n d i n d i c a t e t h a t some of the I~ or h i g h e r h o m o l o g u e s h a v e
b e e n r e m o v e d b y t h e c r y s t a l l i z a t i o n step. T h e a p p e a r a n c e of t h e m o r e l i g h t l y colored m a t e r i a l s u p p o r t s this a s s u m p t i o n .
Sublimation.--Fractional d i s t i l l a t i o n of t h e r e c r y s t a l l i z e d SiI, w a s n e x t c o n s i d e r e d as a possible
p u r i f i c a t i o n t e c h n i q u e . It w a s o b s e r v e d t h a t s u b l i m a t i o n o c c u r r e d w i t h use of a v a c u u m f r a c t i o n a l
d i s t i l l a t i o n system. S u b l i m a t i o n h a d also b e e n u s e d
as a t r a n s f e r p r o c e d u r e (6) for SiL a n d it w a s d e cided to u t i l i z e i t as the second step i n t h e p u r i f i c a tion. T h e s u b l i m a t i o n a p p a r a t u s consisted of a 30c m l o n g P y r e x c y l i n d e r 10 cm i n d i a m e t e r w i t h
1 0 - r a m g r o u n d flanges o n each end. A r o u n d t h e
cylinder were wrapped two heating tapes individually controlled by Powerstats. A dome with a
4 - r a m stopcock w a s c o n n e c t e d to o n e e n d of the
c y l i n d e r , and, o n t h e o t h e r end, a second d o m e
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VoL 104, No. 11
TRANSISTOR-GRADE
e q u i p p e d w i t h a 3 2 - c m l o n g cold finger. T h e l a t t e r
d o m e w a s c h a r g e d in a d r y b o x w i t h e n o u g h c r y s t a l l i z e d S i L to fill it. T h e a p p a r a t u s w a s a s s e m b l e d
w i t h t h e e n d s h e l d t e m p o r a r i l y in p o s i t i o n b y
c l a m p s ; a p a r t i a l v a c u u m w a s a p p l i e d to c o m p l e t e
t h e seals. T h e c h a r g e d a s s e m b l y w a s t h e n r e m o v e d
f r o m t h e d r y box, c o n n e c t e d to a v a c u u m s y s t e m ,
a n d t h e c h a r g e d e n d w a s h e a t e d b y a m a n t l e to
a b o u t 100~ W h e n t h e s u b l i m a t e a p p e a r e d as l a r g e
w h i t e c r y s t a l s on t h e a r e a a d j a c e n t to t h e h e a t e d
charge, the sublimation was repeated by heating
t h e first t a p e , t h e n t h e s e c o n d u n t i l u l t i m a t e l y t h e
c r y s t a l s a p p e a r e d on t h e d o m e . T h e a p p a r a t u s w a s
d i s a s s e m b l e d a n d t h e p r o d u c t s c r a p e d off w i t h L u cite rods. A n e m i s s i o n s p e c t r o g r a p h i c a n a l y s i s of t h e
s u b l i m e d p r o d u c t is g i v e n in T a b l e II. I t is o b v i o u s
f r o m c o m p a r i s o n of t h e r e s u l t s in T a b l e II t h a t c o n s i d e r a b l e p u r i f i c a t i o n is effected b y s u b l i m a t i o n of
t h e c r y s t a l l i z e d p r o d u c t , p a r t i c u l a r l y i n t h e cases of
A1, Mn, Ti, a n d Mg. T h e C u c o n c e n t r a t i o n is s e e n to
i n c r e a s e f r o m 0.3 to 2.7 p p m a n d t h i s is d u e to C u
i m p u r i t i e s f o u n d in t h e P t c r u c i b l e s u s e d in t h e h y d r o l y s i s of t h e SiI, p r i o r to s p e c t r o g r a p h i c a n a l y s i s .
T h e s o u r c e of c o n t a m i n a t i o n h a s b e e n e l i m i n a t e d b y
s u b s t i t u t i n g q u a r t z for Pt. C o n c e n t r a t i o n s of a l l
t h e i m p u r i t i e s d e t e c t e d a r e n o w in or b e l o w t h e p a r t
per million range. Further purification may be
effected b y t h e z o n e - m e l t i n g p r o c e s s in t h e c a s e of
i m p u r i t i e s w i t h f a v o r a b l e s e g r e g a t i o n coefficients.
Zone purification.---Determination of effective
s e g r e g a t i o n coefficients, K , , , u n d e r a g i v e n set of
e x p e r i m e n t a l c o n d i t i o n s w a s u n d e r t a k e n . I t is e m p h a s i z e d t h a t t h e v a l u e s r e p o r t e d a r e t h o s e of e f f e c t i v e coefficients, a n d n o t e q u i l i b r i u m v a l u e s . B e c a u s e m o r e s u b t l e v a r i a b l e s s u c h as c r y s t a l o r i e n t a tion, c o n v e c t i o n , e f f e c t i v e b o u n d a r y l a y e r , s i l i c a
formation, and supercooling were not controlled,
t h e r e e x i s t s s o m e d i s c r e p a n c y in t h e v a l u e s r e ported.
Silicon tetraiodide recrystallized from toluene
w a s e n c a p s u l a t e d in s e a l e d P y r e x a m p o u l e s , d e n s i fled, a n d l e v e l e d b y r e p e a t e d v e r t i c a l r e v e r s e p a s s age of a s i n g l e z o n e ( 2 ) . A single m o l t e n zone 2 - c m
l o n g w a s p a s s e d t h r o u g h t h e 2 0 - c m l e n g t h of l e v e l e d
charge. The zone-melted charge was then segm e n t e d i n t o t e n e q u a l sections, e a c h s e c t i o n h y d r o l y z e d to silica in c o n d u c t i v i t y w a t e r , a n d f i n a l l y
e a c h s e c t i o n w a s a n a l y z e d s p e c t r o g r a p h i c a l l y as d e scribed above.
P r o f i l e s c o n s t r u c t e d f r o m t h e r e s u l t s of t h e s e
a n a l y s e s d e p i c t e d c o n c e n t r a t i o n s , C~, of i m p u r i t i e s
r e t a i n e d in t h e solid a f t e r p a s s a g e of o n e m o l t e n
zone. T h e i n i t i a l c o n c e n t r a t i o n , Co, w a s t a k e n as t h e
a v e r a g e c o n c e n t r a t i o n of t h e t e n s e g m e n t s . This
a v e r a g e v a l u e is v a l i d b e c a u s e t h e c h a r g e w a s
l e v e l e d to a s s u r e a n i n v a r i a n c e of i m p u r i t y c o n c e n t r a t i o n s a l o n g t h e l o n g i t u d i n a l axis. B y d e f i n i t i o n
C, = KC~
(I)
w h e r e C, is t h e c o n c e n t r a t i o n of a n i m p u r i t y in t h e
l i q u i d . A t t h e p o i n t of t h e first f r e e z i n g (i.e., a t
X = 0, if X is t h e d i s t a n c e a l o n g t h e c h a r g e ) , C~ =
Co. T h e r e f o r e ,
K = C./C. a t X = 0
(II)
659
SILICON
i
/, -T-3
~'
FJ - T - 3 ~
O -T-54
X - T gl~Ol~ll IC~IUCURVE
Foe.
K=O.I
}
TE = $ cM/w
d .20~
Fig. 1. Concentration profiles of Mn impurity species in a
Sih matrix.
B y p l o t t i n g the ratio C,/Co vs. X / l (l is the zone
length) i t was possible to determine K e , f r o m the
intersection of the profile w i t h the y - a x i s . Fig. I is
a series of experimental profiles for the M n species
in t h e silicon t e t r a i o d i d e m a t r i x . A l s o d e p i c t e d is
t h e c a l c u l a t e d c u r v e f o r a s o l u t e h a v i n g a K e q u a l to
0.1.
T h e s e profiles a r e t y p i c a l of t h o s e s p e c i e s w h i c h
d i s p l a y s e g r e g a t i o n in a c c o r d a n c e w i t h K e , of a p p r o x i m a t e l y 0.1. T h e p r e p o n d e r a n c e of i m p u r i t y
s o l u t e in t h e e n d of t h e c h a r g e t o w a r d w h i c h t h e
zones t r a v e l e d is e v i d e n c e t h a t t h e s e g r e g a t i o n c o efficient is less t h a n u n i t y . T h e s h a r p d r o p s in c o n c e n t r a t i o n a t t h e m i n i m a o b s e r v e d i n t h e n e x t to
l a s t zone a r e d u e to t h e g r o w t h of t h e l a s t zone a t
t h e c o n c l u s i o n of t h e pass. I t w a s n e c e s s a r y to s t o p
t h e t r a v e l of t h e zone b e f o r e i t p r o c e e d e d i n t o a n
u n l e v e l e d p o r t i o n of t h e c h a r g e ; a t t h i s t i m e t h e
final zone g r e w b a c k i n t o a l a r g e p a r t of t h e p r e c e d i n g zone, a n d t h e n s o l i d i f i e d b y n o r m a l f r e e z i n g
f r o m b o t h ends.
Although only a single molten zone was passed
t h r o u g h t h e c h a r g e , t h e c o n c e n t r a t i o n s i n t h e first
h a l f of t h e c h a r g e to b e m e l t e d w e r e b e l o w t h e l i m its of s p e c t r o g r a p h i c d e t e c t a b i l i t y for a b o u t h a l f of
t h e i m p u r i t i e s s t u d i e d . T h e r e f o r e , it w a s n e c e s s a r y
to r e s o r t to a m e t h o d of m a t h e m a t i c a l e x t r a p o l a t i o n in o r d e r to e x t e n d t h e p r o f i l e to t h e y - a x i s . T h e
expression
C.=K~.
C o + K.----~--
was e m p l o y e d . H e r e C. is t h e s o l u t e c o n c e n t r a t i o n
f r o z e n o u t i n a n y g i v e n zone ( e x c e p t t h e l a s t ) , C,_,
is t h e s o l u t e c o n c e n t r a t i o n f r o z e n o u t in t h e p r i o r
zone, a n d Co is t h e i n i t i a l i n v a r i a n t c o n c e n t r a t i o n .
T h e e q u a t i o n is a n e x p r e s s i o n of a p p r o x i m a t i o n a n d
b e c o m e s less v a l i d as t h e l/d r a t i o ( z o n e l e n g t h :
c h a r g e l e n g t h ) i n c r e a s e s . V a l u e s of K , ~ a r e s u b ~ t i t u t e d u n t i l C~-1 a t X = 0 is e q u a l to K , ~ Co. V a l i d -
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660
JOURNAL
OF THE ELECTROCHEMICAL
i t y of t h e s e e x t r a p o l a t i o n s is b a s e d on t h e r e a s o n a b l e
d e g r e e of a g r e e m e n t b e t w e e n c o m p u t e d a n d e x p e r i m e n t a l c u r v e s f o r i m p u r i t i e s d e t e c t a b l e in a l l s e g ments.
Zones have been passed downward through the
charge with similar results, thereby eliminating the
p o s s i b i l i t y of a n y s e p a r a t i o n b y flotation. T h e r e f o r e ,
r e g a r d l e s s of t h e a n o m a l i e s in t h e profile, it m a y b e
a s s u m e d t h a t a K of a c e r t a i n v a l u e e x i s t s in o r d e r
to r e l o c a t e t h e i m p u r i t i e s as d e p i c t e d . This is so
if o n e a s s u m e s t h a t s e g r e g a t i o n is c a u s e d o n l y b y
t h e d i f f e r e n c e s in s o l u b i l i t y in t h e s o l i d a n d l i q u i d
phases.
T a b l e I V s u m m a r i z e s t h e effective s e g r e g a t i o n
coefficients of v a r i o u s i m p u r i t y s p e c i e s in a SiI,
m a t r i x . I n a s m u c h as a l l v a l u e s r e p o r t e d in T a b l e IV
a r e less t h a n u n i t y , p u r i f i c a t i o n c a n b e effected b y
m o v i n g e a c h of t h e s o l u t e i m p u r i t i e s to t h e e n d of
t h e c h a r g e t o w a r d w h i c h t h e m o l t e n zones t r a v e l .
A p p l y i n g t h e v a l u e of K~ff as d e t e r m i n e d f o r
b o r o n t r i i o d i d e (i.e., 0.4) to t h e f a m i l y of e q u a t i o n s
g i v e n b y P f a n n ( 2 ) , it c a n be s h o w n t h a t a n u l t i m a t e c o n c e n t r a t i o n r a t i o of 1 X 10 -= c a n b e a t t a i n e d
for a c h a r g e h a v i n g a d / l r a t i o of 20: 1. F u r t h e r m o r e , 70% of this c h a r g e w i l l h a v e a c o n c e n t r a t i o n
w h i c h h a s b e e n r e d u c e d to one p a r t in a b i l l i o n o r
less if t h e i n i t i a l c o n c e n t r a t i o n w a s o n e p a r t i n a
million.
T h e n u m b e r of p a s s e s r e q u i r e d to a t t a i n t h i s c o n c e n t r a t i o n is a p p r o x i m a t e d b y
K"=
(IV)
C.=o
w h e r e n is t h e n u m b e r of passes. A n efficiency f a c t o r
(9) of 53% m u s t b e a p p l i e d for a K e q u a l to 0.4.
W i t h u s e of t h i s f a c t o r i n c o n j u n c t i o n w i t h Eq. ( I V ) ,
Table IV. Effective segregation coefficients of various
impurity species in a Sil~ matrix
( C h a r g e l e n g t h , d = 20 c m ; z o n e l e n g t h , 1 = 2 c m ; z o n e t e m p e r r a t e of c r y s t a l l i z a t i o n = 5 c m / h r )
ature = 135 ~ ~ 4 ~
Keff
Impurity species
B*
A1
Na
Mg*
Cu
Fe*
Ti
Mn*
Boron triiodide%
* Values obtained by extrapolation.
t Doped samples.
M a x i m u m v a r i a b i l i t y of results.
0.16
0.70
0.10
0.16
0.64
0.15
0.91
• 0.072
+_ 0.35
(single r u n )
• 0.01
_ 0.17
• 0.08
_ 0.08
0.09 • 0.04
0.42 +__0.22
SOCIETY
November
1957
60 p a s s e s a r e r e q u i r e d to a t t a i n this c o n c e n t r a t i o n
profile.
The concentrations computed above indicate that
SiI, can b e p u r i f i e d to less t h a n o n e p a r t of i m p u r i t y
p e r b i l l i o n p a r t s of SiI, p r o v i d e d e q u i l i b r i u m c o n d i t i o n s p r e v a i l to t h e e x t e n t d e m o n s t r a t e d i n t h e s e
e x p e r i m e n t s . F u r t h e r m o r e , it m u s t b e a s s u m e d t h a t
impurities are not leached from container walls.
F i n a l l y , it s h o u l d b e m e n t i o n e d t h a t t h e p u r i t y of a
sample can be improved by: (a) decreasing the
v a l u e of t h e effective s e g r e g a t i o n coefficient ( f o r
K < 1) b y c o n t r o l l i n g c o n d i t i o n s in o r d e r to m o r e
closely approximate equilibrium, (b) controlling
t h e l/d r a t i o in o r d e r to m a k e t h e u l t i m a t e c o n c e n t r a t i o n profile m o r e f a v o r a b l e , a n d (c) c h o o s i n g a
s m a l l e r f r a c t i o n of t h e z o n e - r e f i n e d c h a r g e .
A m o r e d e t a i l e d t r e a t m e n t of t h e z o n e p u r i f i c a t i o n
of SiI, is f o r t h c o m i n g in t h e s e c o n d p a p e r of this
series.
Acknowledgments
T h e a u t h o r s t h a n k B. M a n n i n g of T e c h n i c a l O p e r a t i o n s , Inc., A r l i n g t o n , M a s s a c h u s e t t s , f o r his a d v i c e
on t h e d e s i g n o f t h e z o n e p u r i f i c a t i o n f u r n a c e ; W.
J a c k s o n a n d R. M o r r i s o n of t h e E n g i n e e r i n g D i v i sion, A i r F o r c e C a m b r i d g e R e s e a r c h C e n t e r , for t h e
m o d i f i c a t i o n s a n d c o n s t r u c t i o n of this a p p a r a t u s .
A c k n o w l e d g m e n t is also e x p r e s s e d to A. K a n t , W a t e r t o w n A r s e n a l , W a t e r t o w n , Mass., for his h e l p f u l
suggestions.
M a n u s c r i p t r e c e i v e d J u l y 16, 1956. This p a p e r was
p r e p a r e d for d e l i v e r y before the W a s h i n g t o n Meeting,
M a y 12-16, 1957.
A n y discussion of this p a p e r will a p p e a r in a Discussion Section to b e p u b l i s h e d in t h e J u n e 1958
JOURNAL.
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