Wafer slicing using internal diameter (ID) saw, multiblade slurry (MBS) saw and multiwire slurry (MWS) saw techniques were evaluated. Wafer parameters such as bow, taper, and roughness which may not be important factors for solar cell fabrication, were considerably better for ID saw than those of the MBS and MWS saw. Analysis of add-on slicing cost indicated that machine productivity seems to be a major limiting factor for ID saw, while expendible material costs are a major factor for both MBS and MWS saw. Slicing experience indicated that the most important factors controling final wafer cost are: (1) silicon cost (wafer thickness + kerf loss); (2) add-on slicing cost, and (3) mechanical yield. There is a very strong interaction between these parameters, suggesting a necessity of optimization of these parameters

Yoo, H. I.

English

NASA Technical Reports Server

FIELD EXPERIENCE WITH VARIOUS SLICING METHODS 
, . 
H.I. YO0 
Appl ied  Solar Energy Corporati on 
C i t y  of Industry,  Ca l i f o rn ia  91746 
ABSTRACT 
S l i c i n g  methods used are i n te rna l  diameter ( ID)  saw, mul t i -b lade s l u r r y  
(MBS) saw and mu1 t i - w i  r e  s l u r r y  (MWS) saw. S l  i c i n g  parameters in l fuenc ing  
f i na l  wafer cost are reviewed based on f i e l d  experience and i n te rac t i on  be- 
tween the parameters are discussed. 
Substrate preparat ion i n  sheet form i s  a f i r s t  step i n  so la r  c e l l  fab- 
r i ca t i on .  Sheets f o r  s i l i c o n  so la r  c e l l s  are o f ten  preoared from ingots 
s l i c e d  by mechanical means. This s l  i c i n q  steo r e s u l t s  i n  loss  o f  s i l i c o n  
( c a l l e d  kerf  loss) ,  and t h i s  loss adds considerably t o  the ove ra l l  cos t  be- 
cause already much expense has accrued i n  forming the ingots.  A number o f  
d i f fe ren t  techniques f o r  s l i c i n g  s i l i c o n  have beer t r i e d  and some have been 
l i m i t e d  t o  production use. Methods t r i e d  include: 
- In te rna l  o r  outer diameter (I.D. o r  O.D.) wheel saw. 
- Mu1 t i b l a d e  saw, using s lu r r y ,  o r  diamond par : ic les p la ted  t o  the 
blade. 
- Spark discharge w i th  wires o r  blades. 
- Pulsed l ase r  discharge. 
- Electro-chemi cal removal w i t h  current  (e tch-cu t t ing)  
- U l t ra -h igh  pressure (100,000 PSI) va te r  j e t .  
Among these techniques, the I .D. saw i s  the most extensively  used i n  
industry  and i s  a wel l  developed method f o r  prenaring l a rge  area sheets from 
s i  1 icon ingots f o r  so la r  c e l l s .  Typical shortcomings o f  o ther  techniques 
inc lude excessive taper, unpredictable work damage, low mechanical y i e l d ,  
and lack o f  machine p roduc t i v i t y  (mainly because o f  slow c u t t i n g  r a t e ) .  The 
ob jec t ive  o f  t h i s  paper i s  t o  i d e n t i f y  s l  i c i n a  oarameters in f luenc ing  wafer- 
i n g  cost  o f  s i  1 icon ingots f o r  so la r  sheet mater ia ls .  S l i c i n g  method used 
were I .D. saw, mu1 t i - b lade  s l u r r y  saw [ tv9S)  and mu1 t i - w i r e  s l u r r y  saw (MWS) 
w i t h  an emphasis on I .D. saw 
2.0 SLICINGTESTS 
S l  i c i n g  condi t ions used for  both I. D. and MBS saw were chosen based 
on f i e l d  experience a t  ASEC, i n  such a way t h a t  reasonably high wafer y i e l d  
( - 90%) can be obtained reproducibly.  MWS s l i c i n g  was ca r r i ed  out  a t  
Yusunaga Engineering Co. , LTD. and s l i c i n g  condi t ions were chosen t o  provide 
https://ntrs.nasa.gov/search.jsp?R=19820015789 2020-03-21T17:01:01+00:00Z
re1 i d h l e  operation. 
S1 ic ing- Co11ditions 
.-.- - ..-- - .---- --. . 
MBS s l i c i ~ i g  tes ts  were condlicted using a Norton 686 wafering llmchine 
( s m e  as Varian 686). A pre-assembled blade package from Varian was loaded 
i t 1  the blade head and a1 iqned and tensioned. NOTE; O i f f i c u l  t y  i n  a1 ignment 
and tensioning, espec ia l l y  i n  tensioning. forced ASEC t o  stop usiny p i n  type 
blade packages which are cheaper t h d ~  pt'e-assen~bled blade packages. De ta i l  - 
ed s l i c i n g  condit ions are  given i n  Tdble 1. 
A MWS s l  i c i n g  tes t  was perfor!aed d t  Yasunagd Engineering Co., Ltd.,  use- 
ing t h e i r  Yo-100 wafering ~liachine. Deta i led s l i c i n g  in format ion i s  given i n  
Table 2. 
1.3. s l  i c i n g  was ca r r i ed  out using wafering n~achines from S i l i c o n  Tech- 
nology Corporation; Model STC-16 for 3" ingots. Table 3 shows s l i c i n g  con- 
d i t i r n s  used i n  the t e s t .  
Co~l~par i son of  Wafer Pararnettzt-s 
--- -- - ---- - .- -.-----..- - - - 
The par-~nleters olrta ined fro111 the wafers o f  three d i f f e r e n t  s 1 i c i  ny 
types, MBS saw, MWS saw, and 1 .D. saw, were compared f o r  the evaluat ion o f  
the n~echariical qua1 i t y  o f  the s l i c e d  wafers. A f t e r  the wafers were demount- 
ed, degreased and cleaned. thickness, bow and roughness (RMS) were measured. 
Their  dverage values, standard deviat ions, and ranges were obtained. Thick- 
ness was n~e~sured  a t  seven po in ts  on each s l i c e  using a d i a l  gauge (Mitutoyo, 
Model DGS-E), one a t  the center and s i x  a t  po in ts  120 degrees apart ,  and an 
average o f  these seven points  ddta represented a thickness o f  a s ing le  wafer. 
Bow i s  n~easured b.y supporting a wafer on three po in ts  120 degrees apart  i n  
the periphery. The center pos i t i on  o f  the s l i c e  r e l a t i v e  t o  the three 
points  i s  def ined as bow. Bow was measured by a Brown & Sharp bow gauqe. 
Taper was determined by tak iny the d i f fe rence between the r~~axirnum and min i -  
nlum s l i c e  thickness measured. Surface roughness (RMS) was measured ill par- 
a1 l e l  t o  the c u t t i n g  d i rec t ion ,  using a Metro-surf  (Model 181. Ai r t tSonics,  
I l l i t t o i s )  
Cornpari son o f  the measured pdranleters fo r  d i  f f e r e n t  s l  i c  iny  types i s  
given i n  Figure 1. Thickness var ia t ion ,  from wafer t o  wafer and w i t h i n  a 
s ing le  wafer, o f  the MBS wafer were higher than those o f  the I . D .  saw and 
MWS saw. Bow and roughness (RMS) a lso  indicated t h a t  the MRS saw wafers 
showed dbout a fac to r  o f  two higher values than those w i th  the I . D .  saw 
wafers. I n  yeneral , co~ l~pa r i  son o f  the parameters ind icated tha t  the wafers 
s l i c e d  w i t h  the I.D. saw and MWS saw had much s n ~ l l e r  values and var ia t ions ,  
than those w i th  the MBS saw. Wafers s l i c e d  by the I .D. saw ( c u t  a t  o r  below 
2 1PM of cu t  r a t e )  showed s l i g h t l y  b e t t e r  nwchanical q u a l i t y  than those w i t h  
MWS saw. 
Add-On S l i c i n g  Cost 
.------ - --- -- 
Input  data f o r  SAMICS were obtained from the s l i c i n g  experiments per- 
formed and the costs were estimated based on SAMICS Workbook (September, 
1977). Cost assessment on wire saw s l i c i n g  was obtained from the i n f o n a -  
t i o n  suppl ied by the l i ~ n u f a c t u r e r  who d i d  a  s l i c i n g  t e s t .  
Add-on s l i c i n g  cos t  o f  th ree  s l i c i n g  types i s  shown i n  Table 4 .  MBS 
saw su f fe red  froi i i  d i r e c t  ~ i a t e r i a l  cost ,  i n  which the  blade package and s l u r r y  
(P.C. o i l  and abras ive)  fo,rn~ a  ,.lajot- n o r t i o n  o f  the  cos t .  D i r e c t  ma te r i a l  
cos t  fonns a  niajor p o r t i o n  o f  MI,IS s l i c i n g ,  which collies from expensive w i r e  
and s l u r r y .  Analys is  o f  1.0. saw shows r e l a t i v e l y  un i fonn  d i s t r i b u t i o n  i n  
cos t  between equipment, d i r e c t  l a b o r  and d i r e c t  ma te r i a l .  H igh equipment 
cos t  i s  l i ~ a i n l y  due t o  low wafer ! t tuoduc t i v i t y  per  d o l l a r s  invested f o r  I.D. 
saw. 
3.0 SENSITIVE SLICING PARAMETERS INFLUENCING WAFER ---- COST 
S l  i c i n g  experience showed t h a t  the  niost in iportant f a c t o r s  c o n t r o l  1  i ng 
f i n a l  wafer cos t  a re  s i l i c o n  cos t  (wafer th ickness + k e r f  l o s s ) ,  add-on s l i c -  
i n q  cos t ,  and f i n a l l y  ~i iechanical  y i e l d .  Wafer cos t  can be w r i t t e n  i n  s imple 
ex/,!-ession. 
Where, W :  Wafer Cost 
M: Mate r ia l  Cost ( S i l i c o n )  
S:  Add-on S l i c i n a  Cost 
Y :  Y i e l d  
and, M = f (T + K )  
T: Wafer Thickness 
K:  Ker f  Loss 
Most in ipor tant ly ,  there i s  a very s t rong  i n t e r a c t i o n  between these 
parameters, i .e., an e f f o r t  t o  reduce s i l i c o n  cos t  by decreasing e i t h e r  wafer 
th ickness o r  k e r f  loss,  r e s u l t s  i n  increase o f  add-on s l i c i n g  c o s t  and reduc- 
t i o n  i n  wafer y i e l d .  
S l i c i n g  paraineters f o r  both r lRS and I . D .  saw i n f l uenc ing  these th ree  
parameters are g iven i n  Table 5 f o r  ~ l ~ t e r i a l  ( s i l i c o n )  cost ,  Table 6 f o r  add- 
on s l i c i n g  cost .  and Table 7 f o r  y i e l d .  The t ab les  show t h a t  the re  i s  a  very  
s t rong  i n t e r a c t i o n  between the parameters; i . e . ,  an e f f o r t  t o  reduce s i l i c o n  
cos t  by reducing e i t h e r  wafer th ickness o r  k e r f  loss,  r e s u l t s  i n  increase o f  
add-on s l i c i n g  cos t  and reduc t ion  i n  wafer y i e l d ,  suggesting a  necess i t y  o f  
op t i r i i i za t ion  between these parameters. Th is  procedure i s  i l l u s t r a t e d  i n  
F igure 2, i n  which s i l i c o n  cos t  (f1) and add-on s l i c i n g  cos t  ( S )  a re  shown as 
a f unc t i on  of wafer th ickness and k e r f  l o ss .  F i n a l  wafer c o s t  i s  an a d d i t i o n  
o f  M and S, and cross liiark ( X )  i nd i ca tes  ~ninirnurn wafer  cos t .  NOTE: Y i e l d  i s  
considered i n  the  f i gu re .  
4 .0  CONCLUSION 
--.-- --- - -  
Wafer parameters such as bow, taper,  and roughness which liiay no t  be 
in iportant f a c t o r s  f o r  s o l a r  c e l l  f a b r i c a t i o n ,  were cons iderab ly  b e t t e r  fo r  
I .D. saw than those o f  the  MBS and h1WS saw. 
Analysis o f  add-on s l i c i n g  cost  ind icated t h a t  machine p roduc t i v i t y  
seems t o  be a major l i m i t i n g  factor  f o r  I .D. saw, wh i le  expendi b l e  mater ia l  
costs are a major f a c t o r  f o r  both tlBS and MWS saw. 
S l i c i n g  experience ind ica ted  t h a t  the most important fac tors  con t ro l -  
i ng  f i n a l  wafer cost  are 1) s i l i c o n  cost  (wafer thickness + k e r f  loss) ,  2)  
add-on s l i c i n g  cost,and 3) mechanical y i e l d .  There i s  a very strong i n t e r -  
ac t i on  between these parameters, suggesting a necessity o f  op t im iza t ion  o f  
these parameters. 
ACKNOWLEDGEMENT 
The work described was suoported by the Low Cost Solar Array Pro jec t  
o f  the J e t  Propulsion Laboratory by aqreement between NASA and DOE. 
FIGURE ? 
PARAMETERS OF WAFERS SLICED BY 
THREE DIFFERENT SLICING METHODS 
.---- I( O I L  1:i-m) 
-.-- \ \ A r n . c N )  
'. 
e 
I 
. 
\ 
, 
. 
b '. 
' 
. 
:
V. - 
I 
. - \ 
' .* ---- 
- - -  
- - 
J 
FIGURE 2 
ILLUSTRATION OF FINDING 
OPTIMUM WAFtR COST 
TABLE 1 
--
MBS SAW S L I C I N G  CONDITIONS 
TABLE 2 
MWS SAW SL IC ING CONDITIONS 
B L M t  PACUSf 
N u h e r  of Bladcs 
Spacer Th~ckness. m (mr ls )  
Blade Thickness. m (mils) 
Blade Uidth. r.. , ~ m h l  
l b r a s ~ v e  (400.51C). ng ( I b )  
S u s w ~ s ~ a n  O i l  (P.C O i l ) .  l i t e r  ( g a l l o n )  
M1. + q l l ~ t e r  i lb /gd I lon)  
Load on Blade. qramlblade 
B l d h  Seed.  cm/su. 
Y e d r  Ratlo 
p@FLIr!l!L~ iu!fLR) 
c m 2 / ~ c h l n e l ~ o u r  
cn? f~ lade l t+our  
TABLE 3 
I .D.SAW S L I C I N G  CONDITIONS 
2 30 
0 451 (18) 
0 20. ( 8 )  
6.35 ('a) 
5 4 (12) 
6 : (1.8) 
0 79 (6.1) 
I 0 0  
57 
-. - 
1.005 
4.33 
L.D.. (inch) 
0 D.. cm ( lnzh)  
Corn T h i c k n r r .  m ( m l l ~ )  
Diamond T h i c k n r s ,  m (sr115) 
Blade Rotdt lon. R.P 
Blade Return 5lwed. cntlmln i lnc h/n:lnj 
Blade Stmkr .  i n 1  Onch) 
 la& i l r r b :  ~ n ~ l .  A f t r r  N w k r  c j f  \ I  Ice5 
(00iANl 
Flrw Rdte. . i , l . ln  
Ulx  Rdt lo.  Udtpr - I L ~  
ru! H*~I,. I n ~ t ~ , ' U l n u l e  
\ I t ,  lncl ~ y ~ l .  , M l n ~ ~ l e U r t , ~ t  
Prtdln t l ~ l t ~ , w d I , ~ l  1 .  % W L  Ud1 1hII I I  '!(I 
TABLE 4 
ANALYSIS OF ADD-ON S L I C I N G  COST 
(SAMICS, 1977 DOLLARS) OF THREE 
INCH CZ INGOT. (PAPEhTHESIS NUM- 
15.24 (6) 
02.23( 16-518) 
0.10 ( 4 )  
0 28 0.30111-12) 
1 .1M 
78 1 5 )  
H I 3 ( i  21 
i o  
1 .'o 
Ill1 1 
I 
( . I  
hIII1 
BERS I N  U N I T  OF $/m2). 
u !Ri 
Rol le r  P ~ t c h .  ml ( m l l ~ )  
n l m t e r  o f  U ~ r e .  nm ( ~ 1 1 5 )  
~u&,+r o f  UI~PI Under ( u t t l n q  
M a n  l l n l t  Yzlqht,  q:cml*rre 
Total u l m  Tenrlon. !ll 
Breaklnq i . , lnt  o f  Ylre.  Kc1 
YIIY Feed Rate. m'mln 
R ~ c ~ p r o c a t l o n  f Mire. r v c l ~ ~ m ~ n .  
Years of U l r r .  m 
SLURRY 
Abrarlve. GC # lo rn  ( 1 6 d  I q  
Lapplnq 011. P C 011. t r l  
~ a f p r  ThlcknCrr. mn (mils) 
Kerf Yldth.  mn ( m l l r l  
S l r c l n q  T i m  nourr 
k c h d n l c r l  71eld. 1 
Y ~ e l d c d  wfcr Area. m7 
P m d u c t l v ~ t y ,  cn2 /n rch lmlhnur  
l r f  
I .',I11 
J 
0 47i18.5) 
0 . !6 (6  3 )  
163 
13 
I 7  
5 7 
K 
65 
12 
5 
3 
o ? ? ( ~ n  6) 
0 20/7 9) 
H 35 
q l  
0 72 
840 
ORlGlNAb PAGE IS 
OF POOR QUAUV 
TABLE 5 
SLICINC, PARAMETERS INFLUENCING 
WAFER THICKNESS AND KERF LOSS 
TABLE 6 
SL IC ING PARAMETERS INFLUENCING 
ADD-ON SL IC ING COST 
1.D. SW 
INGUlDIuIFTCR 
i'!rw\rr 
O_L yf 
- Tenslonl~lq 
L~L* 
- T h l r i n r \ \  of I I l d ~ ~ . ~ n t l  
Plated i l t j v  
Tenr )on i !nr 
MAI IIIN~ 
A < $  ura8 v o t  T* , t v t  8 
tlrtwr!~ H I , I ~ I ~  n f t . i  lrl,jt8t 
rSS SW 
TABLE 7 -_- 
SLICING PARAMETERS INFLUENCING 
MECHANICAL WAFER Y IELD 
"3 
JI 
Y 
E 
D 
I. 
2 
U. 
;?; 
: 
L 
lwr Ewnl 
UEAF! _rnfIrJIcss 
- Spacer Thickness 
c u r  .WJ 
IN607 DlAITTER 
-- . - - - - 
CUT L T L  
-- 
B l o c -  PACUg 
- S p d ~ e r  T h l ~ k n e r s  
- Nwnber of R ladr r  
- Al l p n ? n t  and Te~t%lonlny 
SLLIRRV 
- Ahtdr l re  \rzc 
- k n s l t y  o f  Ahrdr lve I ra  
Tusix?nslon 011 
~knnt rnt ?P+L 
- \pd'er Thrckness 
- Y ~ r l w r  of  Rlrdes 
- 411gnnmt and lens ion l l t y  
EL!IRRI 
- A I ~ r a > ~ v r  517 
- Den5ltv o f  R ~ ~ , ~ s I v c  1" 
5erpenslnn ai !  
- Travel Speed 
- l o a d  on  Blade 
BLL. WYAGE 
- Thickne%r of Blade 
- t imber o f  Blades 
- A l l y n m n t  and tenslonlnq 
I Ncor ng.q_rj 
-.- 
UAFER B F J I N G  
- H a n d l ~ n o  ( 5 l i p w r y )  
OPcwLOp's q!Lk 
- Bladp . ? l i g n m t  and Tenslonlnq 
- Special  a t t e n t l o n  of l a s t  
~mment of  c u t t l n q  
- Core I (a te r1 r l  
- nlannnd P l d t l n q  l o n d l t l o n  
- Dressvnq 
- Blade H ls to rv  
- Tens~on 11 
- Accurx  y 01  Travel  , . r twen 
blade and r n w t  
- Re la t l ve  v~hratlon :,?tween 
bladc rltqr nnd l l ~ o t  , i en te r l t tq )  
Bla& Meunt~nq (A i rynnent  dnd 
Tens~onln,])  
DISCUSSION: 
WOLF: I'll make one comment at this point, to keep the multiblade and multi- 
wire people from walking out. I have found that the best performance of a 
machine is usually obtained in a real production environment. Often the 
manufacturers of the equipment themselves don't get the best performance 
out of their equipment because they don't get the experience in running 
it. There's always an exception to these generalizations, but this is 
frequently the experience. 
This seems to be a general thing. People take time learning with a 
particular piece of equipment, and find out how to use it right. They 
make modifications on equipment frequently, to make it easier to use, to 
get better yield and so on. It's often very difficuit, therefore, to make 
exact comparisons between methods because we often don't find out exactly 
what the experience is of the people who really have it down pat and are 
running it day in and day out under all optimized conditions. So I think 
we have to, in these comparisons, be a little bit careful with how we use 
these numbers. 


Field experience with various slicing methods

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