Contamination in furnaces for fabricating wafers, and cleaning procedure

Heald, D. L.

English

NASA Technical Reports Server

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- FURNACE-TUBE CONTAMINATION 
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- NASA G r a n t  NGR 0 5 - 0 1 0 - 0 4 1  
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~ e p t .  of E l e c t r i c a l  ~ n g i n e e r i n g  
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U n i v .  of C a l i f . ,  Santa  B a r b a r a  
https://ntrs.nasa.gov/search.jsp?R=19700033071 2020-03-11T22:59:25+00:00Z
FURNACE-TUBE CONTAMINATION 
C R  IC, ~ 3 6  
NASA Grant NGR 05-010-041 
Department of E l e c t r i c a l  ~ n g i n e e r i n ~ "  
Sol id-Sta te  Laboratory 
Univers i ty  of C a l i f o r n i a ,  Santa  Barbara 
A s  a r e s u l t  of improper use ,  t h e  furnaces  i n  t h e  Sol id-  
S t a t e  Laboratory became d r a s t i c a l l y  contaminated. People had 
neglec ted  t o  use  gloves i n  handl ing glassware,  c l ean ing  of w a f e r s  
had n o t  been thorough, wafers wi th  p h o t o r e s i s t  had been i n s e r t e d  
i n t o  t h e  furnaces ,  - and a b o t t l e  of welding-grade N2 had a c c i -  
d e n t a l l y  been connected t o  t h e  "house" n i t rogen  l i n e .  This  
r e p o r t  covers  t h e  e f f o r t  t o  i d e n t i f y  and e l imina te  t h e  contamina- 
t ion .  
I n  o rde r  t o  check t h e  progress  of furnace c leaning ,  d i o d e s  
were f a b r i c a t e d  and t h e  diode r e v e r s e  c h a r a c t e r i s t i c s  were e x a m -  
ined. S ince  t h e  d iodes  were probed and charac te r i zed  p r i o r  t o  
m e t a l l i z a t i o n ,  t h e  measured c h a r a c t e r i s t i c s  inc lude  t h e  e f f e c t  
o f a t h e  p o i n t  c o n t a c t s  formed by t h e  probe-s i l icon  i n t e r f a c e ,  
A p o i n t  c o n t a c t  with n-type m a t e r i a l  w i l l  be forward b i a s e d  
i f  a p o s i t i v e  p o t e n t i a l  i s  app l i ed  t o  t h e  probe with r e s p e c t  t o  
t h e  semiconductor. For p-type m a t e r i a l  t h e  c o n t a c t  w i l l  be 
forward b iased  i f  t h e  probe i s  negat ive  with r e s p e c t  t o  t h e  s e m i -  
conductor.  These b i a s  condi t ions  a r e  i l l u s t r a t e d  i n  Fig.  1. 
* 
Most of t h e  work repor ted  i n  t h i s  document was performed by Mr. 
David E ,  Heald, NASA Tra inee ,  Department of E l e c t r i c a l  ~ n ~ i n e e r i n g ,  
Unive r s i ty  of C a l i f o r n i a ,  Santa  Barbara. 
In an attempt to calibrate the effect of the probe-silicon 
contacts, two probes were applied to a p-type silicon wafer and 
to an n-type wafer. Wafers of both high and low resistivity 
were used for each type. The resulting curves are shown in 
Figs, 2 through 5. If the point contacts had been identical, 
the curves would have been symmetrical and similar to that expected 
for two diodes back to back. The reverse breakdown voltage for 
low-resistivity material (0.05-O.1Q-cm) is relatively small and 
that for high-resistivity material (1-2 Q-cm) is relatively large. 
The characteristic observed while probing a p-n junction 
diode is a result of three junctions in series. Masking the for- 
ward characteristics of the silicon p-n junction are two reverse- 
biased point contacts. For reverse bias across the silicon p-n 
junction the point contacts are forward biased and therefore do 
not materially affect the characteristics. Thus the reverse 
characteristics presented in the following photographs are 
essentially the reverse characteristics of the silicon p-n junc- 
tions. 
As an interesting tangent, Figs. 6 and 7 show the character- 
istics of a nf-n (ohmic) junction. The characteristics were 
measured by means of probing prior to metallization. The shapes 
of the characteristics are fixed almost entirely by the nature of 
the probe-silicon point contact. 
As previously stated,the degree of furnace contamination 
is measured in a qualitative manner by the amount of reverse leak- 
age current in a diode fabricated by use of the furnace. As an 
a r b i t r a r y  s t andard ,  leakage c u r r e n t s  were measured a t  a r e v e r s e  
b i a s  of about twenty v o l t s .  An a r b i t r a r y  diode geometry was used 
a s  a s tandard .  Diodes were judged t o  be acceptable  i f  t h e  leakage 
c u r r e n t  without  i l l u m i n a t i o n  was 0.5 na o r  l e s s  and i n  normal 
room i l l u m i n a t i o n  was 0.5 pa o r  l e s s .  The cross-sec t ion  a r e a  of 
t h e  s tandard  diode was about 1 x 1 0 - ~  sq.  cm. 
P r i o r  t o  t h e  f a b r i c a t i o n  of t h e  diodes, a l l  furnace t u b e s  and 
glassware were ( a )  r insed  i n  t r i c h l o r o e t h y l e n e ,  acetone,  and D I  
water ,  (b) washed i n  concentrated HF ( 4 8 % )  d i l u t e d  wi th  water  (1:l) 
f o r  f i v e  t o  t e n  minutes,  and ( c )  given a f i n a l  DI-water r i n s e .  The 
glassware and tubes  were then d r i e d  a s  quickly  a s  poss ib le .  
During t h e  f i r s t  p a r t  of t h e  p r o j e c t ,  wafer c l ean ing  a f t e r  
p h o t o r e s i s t  was l i m i t e d  t o  5-100 s t r i p p e r  ( f r e s h ) ,  methyl a l c o h o l  
r i n s e ,  D I  water  r i n s e ,  ten-second d i p  i n  10 : l  HF followed by another  
r i n s e  i n  D I  water ,  and blow dry  i n  N2. (Using buffered  HF f o r  t h e  
d i p  l e f t  a r e s i d u e  on t h e  wafer.)  Af te r  t h e  above procedure was 
c a r r i e d  o u t ,  dark  s p o t s  remained on t h e  wafer. I n  an e f f o r t  t o  
e l i m i n a t e  t h e  spo t s raqua  r e g i a  was t r i e d  wi th  l i t t l e  e f f e c t  on t h e  
appearance of t h e  wafer; however, t h e  r e s u l t i n g  diode c h a r a c t e r i s t i c s  
were no t i ceab ly  worse! 
L a t e  i n  t h e  p r o j e c t  t h e  fol lowing c leaning  procedure adapted  
from F. Cocca, e t  a l ,  was used. 
J-100 s t r i p p e r  ( f r e s h )  
Methyl a l coho l  r i n s e  
DE r i n s e  
Blow dry  i n  N 2  
Boi l  i n  t r i c h l o r o e t h y l e n e  - 5 minutes 
U l t r a s o n i c a l l y  c l ean  i n  t r i c h l o r o e t h y l e n e  - 
2 minutes 
Boi l  i n  acetone - 5 minutes 
U l t r a s o n i c a l l y  c l ean  i n  acetone - 2 minutes 
D I  r i n s e  
B o i l  i n  conc. HN03 - 15 minutes 
D I  r i n s e  
1 0 : l  H F  d i p  us ing  p l a s t i c  o r  Teflon tweezers 
(no t  Teflon-coated metal  tweezers) 
-
D I  r i n s e  
Blow d r y  i n  N2 
The foregoing procedure r e s u l t e d  i n  a  cleaner-looking w a f e r  
and i n  good d iode  c h a r a c t e r i s t i c s .  
A t  t h e  beginning of t h e  p r o j e c t ,  t h e  f i v e  furnaces  w i t h  
suspected contamination were s h u t  down and t h e  tubes  removed. The 
fu rnaces  involved i n  t h i s  procedure were those  f o r  BC13 p r e d e p o ~ i -  
t i o n ,  boron d r i v e ,  phosphorous d r i v e ,  P O C l  p redepos i t ion ,  and  3 
ox ida t ion .  The boron and phosphorous d r i v e  tubes  and t h e  BC13 
p redepos i t ion  tube  were replaced  wi th  new tubes.  The POC13 p r e -  
d e p o s i t i o n  and o x i d a t i o n  tubes were cleaned and reused. 
I n  o rde r  t o  i s o l a t e  t h e  contamination,groups of d iodes  w e r e  
f a b r i c a t e d  by t h e  use of d i f f e r e n t  combinations of fu rnaces .  The  
procedure w i l l  be descr ibed  i n  t h e  fol lowing paragraphs.  
Two s e t s  of d iodes  were f a b r i c a t e d  us ing  t h e  o x i d a t i o n  f u r -  
nace f o r  ox ide  growth and t h e  POCl p r e d e p o s i t i o n  f u r n a c e  f o r  t h e  3  
p r e d e p o s i t i o n  and d r i v e .  One set  was g e t t e r e d .  The r e s u l t s  a r e  
shown i n  F igs .  8  through 10. The g e t t e r i n g  improved t h e  charac-  
ter is t ics  s l i g h t l y .  F i g u r e  1 0  shows one of t h e  wors t  dev ices .  A t  
t h i s  p o i n t  it was n o t  known i f  t h e  contaminat ion was from t h e  N2 
l i n e  or t h e  o l d  f u r n a c e  tubes .  
Next, t h r e e  more s e t s  of d i o d e s  w e r e  made u s i n g  t h e  same sub- 
s t r a t e  wafer and t h e  same POC13 p r e d e p o s i t i o n  f u r n a c e  f o r  predepo- 
s i t i o n  and d r i v e .  Two groups had t h e  ox ide  grown i n  t h e  o x i d a t i o n  
fu rnace ,  one w i t h  wet O2  (F ig .  11) and one wi th  d r y  O2  (Fig.  1 2 ) .  
The ox ide  f o r  t h e  t h i r d  group was grown i n  t h e  boron d r i v e  fu rance ,  
which was expected t o  be  c l e a n  (Fig .  1 3 ) .  S ince  a11  t h r e e  groups 
had channel  c u r r e n t s f i t  was concluded t h a t  t h e  POC13 p r e d e p o s i t i o n  
f u r n a c e  was contaminated.  
The c l e a n l i n e s s  of  t h e  N 2  l i n e  was nex t  i n v e s t i g a t e d  by com- 
p a r i n g  d iodes  from t h e  B C 1  p r e d e p o s i t i o n  f u r n a c e  wi th  a set  from 3  
a new and c l e a n  B B r  p r e d e p o s i t i o n  fu rnace .  Oxides w e r e  grown i n  3  
t h e  boron d r i v e  f u r n a c e  (new t u b e  and no N ~ ) .  A s e p a r a t e  N 2  b o t t l e  
was connected t o  t h e  BC13 f u r n a c e  (new t u b e ) .  The B B r 3  f u rnace  
a l s o  had a new tube  b u t  p r e v i o u s l y  had "house" n i t r o g e n  f lowing 
th rough  it. The r e s u l t s  a r e  shown i n  F igs .  1 4  and 15  and were 
2 
n e a r l y  i d e n t i c a l .  According t o  Grove , F i g .  6.28, a  one-sided s t e p  
j u n c t i o n  wi th  a  s u b s t r a t e  r e s i s t i v i t y  of 1.0 $2-cm o r  5 x 1 0 ' ~  impuri- 
ties/crn3 should have a  breakdown of about  1 0 0  v o l t s .  Consider ing 
j u n c t i o n  cu rva tu re ,  F ig .  6.31, t h e  breakdown should be  between 
50 and 7 0  v o l t s  f o r  a j u n c t i o n  d e p t h  of 2-3 p e  (After  p r e d e p o s i t i o n  
f o r  B B r 3 ,  V / I % 2 ;  f o r  BC13, V/I%8; p r e d e p o s i t i o n  was fo l lowed by 
one-hour o x i d a t i o n  i n  d r y  O2  a t  1150°C.) I f  t h e  j u n c t i o n  i s  n o t  
one - s ided , the  breakdown w i l l  be s l i g h t l y  h igher .  S i n c e  t h e  observed 
breakdown i s  i n  t h e  expec ted  r ange ,  it was f e l t  t h a t  t h e  "house" 
n i t r o g e n  l i n e  was c l e a n .  
The c o n d i t i o n  o f  t h e  o x i d a t i o n  f u r n a c e  w a s  nex t  de te rmined .  
F i g u r e s  17 through 21  show t h e  c h a r a c t e r i s t i c s  of j u n c t i o n s  made 
on t h e  same s u b s t r a t e  wafer  and u s i n g  t h e  same BC13 p r e d e p o s i t i o n  
cyc l e .  F igu re  17 r e p r e s e n t s  t h e  c o n t r o l  group us ing  t h e  boron 
d r i v e  f u r n a c e  f o r  o x i d e  growth. Oxides on t h e  o t h e r  two sets were 
grown i n  t h e  o x i d a t i o n  f u r n a c e  w i t h  wet 02 .  One group ( F i g s .  20 
and 2 1 )  w a s  p laced  i n  t h e  POC13 p r e d e p o s i t i o n  f u r n a c e  and g e t t e r e d  
i n  t h e  contaminated tube .  These could n o t  be d i s t i n g u i s h e d  e l e c -  
t r i c a l l y  from t h e i r  coun te r  group which had n o t  been g e t t e r e d  
(Figs .  18 and 1 9 ) .  I n  bo th  of t h e s e  groups,about  h a l f  o f  t h e  d iodes  
- 
had " s o f t "  c h a r a c t e r i s t i c s  and about  h a l f  had "hard"  c h a r a c t e r i s -  
t i c s .  The contamina t ion  e v i d e n t l y  r e s u l t s  from u s e  o f  t h e  ox ida -  
t i o n  f u r n a c e .  
t h i s  p o i n t  i n  t h e  p r o  j e c t t  t h e  boron d r i v e ,  and 
f u r n a c e s  were known t o  be  c l e a n ,  and t h e  POCl and o x i d a t i o n  f u r -  3 
naces  known t o  be  contaminated.  The phosphorous d r i v e  f u r n a c e  w a s  
expec ted  t o  be  c l e a n ,  b u t  had n o t ' b e e n  t e s t e d .  
During t h e  fo l lowing  two weeks, new tubes  w e r e  p l aced  i n  t h e  
POCl p r e d e p o s i t i o n  and o x i d a t i o n  f u r n a c e s  and good d i o d e s  were  3 
f a b r i c a t e d  us ing  t h e  POC13 p r e d e p o s i t i o n  fu rnace .  U s e  of t h e  oxida-  
t i o n  furnace  r e s u l t e d  i n  poor diodes.  By t h e  end of t h e  fol lowing 
month,both BC13 and POC13 predepos i t ion  furnaces  were g iv ing  poor 
r e s u l t s  again.  
I t  was found t h a t  jungl ing on t h e  P O C l  p redepos i t ion  fu rnace  3 
allowed p ressure  t o  b u i l d  up and f o r c e  l i q u i d  POC13 i n t o  t h e  va lves  
and f low meters.  This  was f e l t  t o  be r e spons ib le  f o r  t h e  degraded 
r e s u l t s  when us ing  t h e  P O C l  p redepos i t ion  furnace.  Solenoid va lves  3 
were e l iminated  and t h e  jungl ing s i m p l i f i e d  and c o r r e c t .  A leaky 
va lve  was probably r e s p o n s i b l e  f o r  t h e  poor r e s u l t s  from t h e  BC13 
furnace .  Addi t ional  checks on t h e  B B r 3  and t h e  two d r i v e  fu rnaces  
i n d i c a t e d  t h a t  they  were s t i l l  c l ean .  
I n  an a t tempt  t o  e l imina te  t h e  contamination i n  t h e  ox ide  f u r -  
nace, a  new tube  was i n s t a l l e d .  F igure  2 2  shows t h e  r e s u l t s  from a  
BBr3 p redepos i t ion  and oxide grown wi th  wet 0  i n  t h e  o x i d a t i o n  2 
furnace.  Since t h e  o x i d a t i o n  furnace  was running a t  950°C, t h e  
junct ions  remained shallow. The r e s u l t i n g  sharp cu rva tu re  of  t h e  
junct ion  probably i s  respons ib le  f o r  t h e  low breakdown v o l t a g e .  
However, t h e  s o f t  breakdown does i n d i c a t e  contamination. In  an 
e f f o r t  t o  e l i m i n a t e  t h e  contamination, a l l  glassware was rec leaned  
and t h e  solenoid v a l v e s  bypassed wi th  Teflon tubing.  The r e s u l t  of 
t h i s  process  i s  shown i n  Fig.  2 3 .  The leakage c u r r e n t  a t  20-vol t s  
r e v e r s e  b i a s  was 60 nanoamps. While t h i s  l e v e l  of r e v e r s e  l eakage  
c u r r e n t  i s  c ~ n s i d e r e d  t o  be s a t i s f a c t o r y ,  some improvement i s  
p o s s i b l e  by t h e  use  of h i g h e r - r e s i s t i v i t y  water f o r  t h e  wet-O2 
ox ida t ion .  
In summary, great care and large blocks of time are required 
to isolate and eliminate contamination of the furnaces in the 
diffusion room. As outlined in the foregoing paragraphs, the 
fabrication of diodes by use of various combinations of furnaces 
may be used to isolate the contamination. 
REFERENCES 
1. F. Cocca, R. Cohen, S. Simonne, "A Simple Method of Preparing 
'Sodium-Free' Thermally Grown Silicon Dioxide on Silicon," 
Proc. IEEE - 55, p. 2193, December 1967. 
2. A. S. Grove, Physics and Technology of Semiconductor Devices, 
John Wiley, 1967. 
Figure  1 - (a) Left-hand probe-s i l icon  junct ion  i s  r e v e r s e  
biased.  Right-hand p r  be - s i l i con  junct ion  i s  9 
r e v e r s e  biased.  The p -n junc t ion  i s  forward 
biased.  
(b) Left-hand probe-s i l icon  junct ion  i s  forward 
biased.  Right-hand prybe-s i l icon  junct ion i s  
forward b iased .  The p -n junct ion  i s  r e v e r s e  
biased.  
Fig. 2 Probes con tac t ing  h i g h - r e s i s t i v i t y  
p r eg ion  (1-2  Cl - cm) 
V e r t i c a l :  1 ma/div. 
Hor izonta l :  5  v/div. 
Fig, 3 Probes con tac t ing  l o w - r e s i s t i v i t y  
p+ r eg ion  (0.05 - 0.1  R - cm) 
V e r t i c a l :  0.05 ma/div. 
Hor izonta l :  0 . 2  v/div.  
Fig.  5 P r o b e s  con tac t ing  l o w - r e s i s t i v i t y  
n' r e g i o n  ( 0 . 0 5  - 0 . 1  fl - cm) 
Fig .  4 P r o b e s  c o n t a c t i n g  h i g h - r e s i s t i v i t y  
n region (1-2 R - em) 
V e r t i c a l :  0 . 0 5  ma/div. 
H o r i z o n t a l :  5.0 v/div. 
V e r t i c a l :  0 . 0 5  ma/div. 
H o r i z o n t a l :  0 . 1  v /div.  
+ F i g ,  6 n -n junc t ion  (Oxide grown i n  boron 
d r i v e  fu rnace )  Leakage c u r r e n t ,  40na. @20v. 
V e r t i c a l :  0 . 0 2  ma/div. 
Hor izonta l :  2 v/div.  (fwd. ) 
Horizonta l :  1 0  v/div. ( r ev . )  
-I- 
Fig. 7 n  -n junc t ion  (Oxide grown i n  boron d r i v e  
fu rnace )  
V e r t i c a l :  0 . 0 2  ma/div. 
Hor izonta l :  1 . 0  v /d iv .  (fwd.) 
Hor izonta l :  L O .  v /d iv .  ( r e v . )  
+ Fig. 8 n -p (Phosphorous predeposition furnace, 
oxide grown in oxidation furnace.) 
Vertical: 0.1 ma/div. 
Horizontal: 20 v/div. (fwd. & rev.) 
+ 
Fig- 9 n -p Gettered junction (Otherwise, same 
as Fig. 8) 
Vertical: 0.05 ma/div. 
Horizontal: 20 v/div. (fwd. & rev.) 
grown i n  ox ida t ion  fu rnace )  
V e r t i c a l :  0 . 5  ma/div. 
Hor izonta l :  1 0  v/div.  (fwd. ) 
Horizonta l :  20 v/div.  ( r ev . )  
+ Fig. 11 n -p j unc t ion  (Wet-02, o x i d a t i o n  f u r -  
nace)  
V e r t i c a l :  0,05 ma/div, 
Horizontal:  5 v/div.  (fwd. ) 
Horizontal:  2 0  v/div.  ( r ev .  ) 
+ Fig. 12 n -p junction (Dry 02,  oxidation furnace) 
(same substrate wafer and phosphorous pre- 
deposition furnace as for Fig. 11.) 
Vertical: 0.05 ma/div. 
Horizontal: 5 v/div. (fwd.) 
Horizontal: 20 v/div, (rev.) 
+ 
Fig. 13 n -p junction (Dry 02, boron drive furnace) 
(same substrate wafer and phosphorous predep- 
osition furnace as for Fig. 11. 
Vertical: 0.05 ma/div. 
Horizontal: 5 v/div. 
Vertical: 0.2 ma/div. 
Horizontal: 20 v/div. (rev. ) 
4- 
Fig. P4 p -n j u n c t i o n  (BC1 p r e d e p o s i t i o n  f u r n a c e ,  
o x i d e  grown i n  boran d r i v e  f u r n a c e )  
0.45 pa l eakage  c u r r e n t  @ 2017. 
V e r t i c a l :  0 . 05  ma/div. 
Hor i zon ta l :  2 0  v/div.  (fwd. & rev. )  
+ 
Fig. 15 p  -n j u n c t i o n  (BEr3  p r e d e p o s i t i o n  f u r n a c e ,  
o x i d e  grown i n  boron d r i v e  f u r n a c e )  0 . 4 5  
pa leakage  @ 2 0 v .  
V e r t i c a l :  0.05 ma/div. 
Hor i zon ta l :  2 0  v /div .  (fwd. & r e v . )  
+ Fig. 1 6  p -n junct ion  (BC1 p redepos i t ion  furnace ,  
same as f o r  Fig.  13)  
V e r t i c a l :  0 .05  rna/div. 
Horizontal :  20  v/div.  (fwd. & r ev . )  
+ Fig .  17  p -n junct ion  ( B C l  p redepos i t ion  furnace)  3 (oxide grown i n  boron d r i v e  furnace) 0.43 
Fa leakage @ 20v. 
V e r t i c a l :  0.05 ma/div. 
Horizontal  : 1 0  v/div.  (fwd. ) 
Horizonta l :  20  v/div.  ( r e v . )  
- - 
+ Fig. 18 p -n junction (BC1 predeposition furnace, 
Wet 02, oxidation lurnace) (same substrate 
wafer and predeposition as for Fig. 17) 
Vertical: 0.05 ma/div. 
Horizontal: 10 v/div. (fwd.) 
Horizontal: 20 v/div. (rev.) 
Fig, 19 Different junction on same wafer. Otherwise 
identical to Fig. 18 
+ 
Fig. 20 p -n junct ion  - "phosphorous g e t t e r e d "  
(Otherwise i d e n t i c a l  t o  Fig.  1 8 )  
V e r t i c a l :  0 .05  ma/div. 
Horizontal :  1 0  v/div.  (fwd. ) 
Horizontal :  2 0  v/div. ( r ev . )  
F ig .  2 1  D i f f e r e n t  junct ion  on same wafer .  O t h e r -  
wise i d e n t i c a l  t o  F i g ,  2 0 .  
+ Fig .  2 2  p -n junct ion  (oxide i n  ox ida t ion  furnace)  
leakage c u r r e n t  @ 20  v .  - l ua .  
V e r t i c a l :  0.02 ma/div. 
Horizontal :  1 0  v . /div.  (fwd. & r ev . )  
Fig. 2 3  pf-n junct ion  (solenoid va lves  of oxida t ion  
furnace by-passed) leakage c u r r e n t  @ 20v. - 
60 na. 
V e r t i c a l :  0.02 ma/div. 
Horizontal :  1 0  v . /d iv .  (fwd. & r e v . )  


Furnace-tube contamination

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