The orientation-dependent etching method of slicing was investigated to see if it could be used to form more slices from high quality silicon crystals than can be achieved by mechanical slicing methods. Orientation-dependent slicing uses preferential etching down narrow slots in a silicon slab on form slices. The method has several possible advantages including high slicing yield (sq m/kg), plane parallel, thin slices, ready for processing and the chance of high throughput and low costs. There are limitations in the need for simple crystals, and in restricted depth of slicing

Iles, P. A.

Soclof, S. I.

English

NASA Technical Reports Server

W/\FERING INSIGHT PROVIDED RY THE ODE METHOD 
S.I. Soclof and P.A. Iles 
Applied -Solar Energy Corporation 
City of hdustry, California 91746 
Orientat iondependentetching (ODE) can be used t o  sire silicon. The method 
has several possible advantages including high slicing yield (m k g ) ,  plane paraliel, thin 
slices, ready for processing and the  chance of high throughput and bur costs. There 
a r e  limitatiorls in the  need for simple crystals, and in restricted depth of slicing. 
Analysis of the  overall properties of t h e  ODE method has added insight into t h e  
requirements of a successfill waferin? method. 
BACKGROUND 
Or ientat iondepcndent (OD) slicing uses preferential etching dow 1 narrow slots 
in a silicon slab on forrn slices. This method of slicir,g was investigated to see if i t s  
advantages could be used t o  form ,.lore slices from h ~ g h  quality silicon crystals than 
can be achieved by mechanical slicing methods. In particular. an a t t ract ive  feature  
was the  possibility of forming thin slices (-5Oum), ready for cell  processing; present 
methods for etching thin slices from already sliced silicon involve large losses of 
silicon. OD slicing has some limitations. which restr ict  the  cell  designs av- We. 
Attempts to overcome these ;imita'ions have led t o  study of differer t m e t k  for 
processing the  dices into cells and arrays. 
OR IENTATIQN-DEPENDEFT SLICING 
OD etching has been used t o  form solar cell structures, including vertical 
multijunction cells (11, etched groove cells (2), and polkadot  cells (3). However, OD 
slicing requires formation of cansiderably deeper grooves. 
Figure 1 shows a typical slicing sequence. (1 10) orienrated slabs a re  cu t  from 
<I 1 l >  orientated crystals. The slab thickness determ incs the  eventual sl ice width, 
but there is no requirement for es t reme  accuracy in cutting the  slabs. The slabs are 
zhemically polished, a r d  coated with SiU and/or Si3N4 wFlich a c t  as masking layers 
during the  OD etching. Usins a s ~ o t - ~ a $ t e r n  mask and optical photolithography, a 
close-spaced, fine slot pahtern is opened ir the  masking layers. An OD etchant  (T ?ically 30M KOH at 85 C) is used to etch down the  slots; slices a re  formed when 
the  slots a r e  etctbcd thrcuqh. Figure 2 shows partial slicing with widespread slots 
- 45O~tm deep. 
P r e v i ~ u s  u ;: (4) showed tha t  for slot5 in the  11 1 direction and (1 10) faces, 
e tch  ratios (downward t o  side~vays) as high as 400:1 could be achieved, and some 
reports (4,5) have quoted value as high 1s 600:l.0 These high ratios, which form dee , 
narrow slots require very accuta tc  alignment (0.1 of the  slct  direction with t h e  (1 11 ! 
https://ntrs.nasa.gov/search.jsp?R=19820015797 2020-03-21T09:04:28+00:00Z
planes which are proenc ' icular  to t h e  ( 1  10) planes. Tbesc e t c h  r a t i c s  a r e  f a r  II? excess 
of t h e  d i f ference  in bond eensi t ies  for  t h e  different  c r y s t a l l o e r ~ o h i c  ~ l a ~ e s .  s.nc' a 
t en t a t ive  explanation (6) involves preferential  oxidation cf ( I  1 1 )  pl.?nes ir. t h e  CD 
e tchants .  Accura te  a l i p m e n t  h a s  heen achieve4 by extensicn of t h e  fan-etch method 
(4) and once  t h e  fan-etch has  shown t h e  c c r r e c t  <1!1> c'irectiocs of a t v y k a l  s lab 
from one  ingot, subsequent s labs can  be aliqned with the  slot-mask usinr precise 
mechanical adjustment .  
The a r e a  yielci, i.e. t h e  s l ice a r e a  Der s tar t inq mass of sil$on. car. he hiril if t i &  
e t c h  rat ios (2200:I) car! !x maintaine::. F i ~ u r e  3 shows t h e  rnL /k~fb ta inab l?  !cr tS>.c 
s l ic i .1~  depths. Also. on this f i r u r e  a r e  indicated t h e  r a c c e  cf rr-->:r ?.vZikble f r c r  
presect o r  projected mechanical slicinq rnethcSs imult i ~ l e  v *  i res  c r  sau.s! a?.< shee t  
pro~vth.  F i rv re  4 shcws how t h e  e t ch  r t t i c  i ~ c r e a s e c  a s  t he  rrisalirrnme:.t a r z l e  
decreases.  The e t c h  r a t e  do\;w t h e  s lc t s  (ir. t h e  < 1 10 > c ' i rect ic~! is fa. riv s l r ~  
( - 1 u r r f ~ i ~ . ) .  birt c'espite this  slow rzte.  a I s r ~ e  rcr rhor  of  s lo ts  CAP 5e f ~ r r r e c !  
sirni~ltaneouslv by e t c h i t y  several slabs ~t cnce.  [!sine I Cllrir wic'e $lots. s ~ a c c i '  6Oum 
apar t .  !60 sl ic~s?5Ourr  thick a r e  obtained for  each  c e ~ t i m e t e r  cf s lab width. 3 c s  t h e  
a r e a l  cu tput  rcn- Irrin' can  be hi lb .  To fern? ~ I i c e s  ~ l r w  tbick r w u i r e s  e t c 5 n r  for  
-1000 minutes. This c l zces  severe  recrttirements on  q u z l i t ~  of t h e  rraskincr 1s:fcrs. 
M a u s e  the  mecl~anlcz l  f c r ces  cr! t h e  slices a r e  sn-all (nope i r  s!ici;.?. n?air!lv 
f rom the  e t chan t  motion cr  b y i r c c m  ;>ressure! i t  is o o s i h l e  to form very thi3 slices. 
Slices 5Oum thick have bee r  t he  t a r r e t .  Stlt t h i ~ n e r  s l ices (down to lum)  k v e  been 
formed. Typical s lo t  t ~ i d t h s  a r e  ! P i w .  and for  200:l e t c h  ratio, a sl ice I m q  \vide v~i l l  
irvolve a kerf bss ~ 2 0 u r r .  n . e  slice faces a r e  res t r ic ted  to b e  (I! 1) plane?. qivine 
chance  of qmd ~ r a l l e l i s r n  for  all  s l ices fcrmed. had expected t h e  s l ice  f aces  to 
b e  very  f h t  and they a r e  f l a t  ; c' ?arzllel. tlc\\vever, tt?e prolonqed e x m s u r e  to t h e  
e t chan t s  forms e t c h  f icures  or. t5e sl ice f aces  (see Fiqure 5),  pr.3bablv arisinq f rom 
inherent bulk imperfect ions ir: t h e  s ta r t ing  s i n ~ l e  crvstal .  
The OD sl ices a r e  formec! i r  siliccr! which has  never experienced anv mechanical 
?rocessinq. This is of interest .  kcagse i t  is believec' t ha t  even a f t e r  excessive e tchinq  
to remove mechanical vscrk c'arnaoe. t h e  e f f ec t s  of this c 'amaae cm Pever be 
c o m p k t e l v  erasee.  
'?'e have successfullv PF sl iced s i l i c c ~  mcT 5ave  feu* suitable combinations of 
slab ~ e p a r a t i o n ,  masking layers. alirrnec' s lot  formation ant' e t c h a n t  ccnc'itions. 
'3wever. vte have also i l e ~  tifiec! some  ~ r a c t i c s l  l i n i t a t  icns 2nd they a r e  c'i:cussec' 
next. 
Limitations in P!? Slicinc 
?ere a;-e several intrinsic limitations. incluc'ine the  nee< for  s t a r t i y  single 
crystals ,  suitably crientec'. t he  forn3aticn of  very  accuratelv aliqned slots,  and  l imited 
s l ice  v id th  (limitation en s lo t  .'er?th obtainable!. Y'e have zlso found some pract ical  
l imitat  ions as follows. 
The thin s l ices formec! n?ust b e  supportec' durinc slicinq to prevent  breakage  as 
slicirlq ?roceeds, and t o  pre\.!nt the  neec' to hanele manv t5In slices separatelv.  Some 
support can be provided by maskinc the  back sur face  of t he  slab? al thoueh i t  is 
difficult for  this  thill r n a s k i n ~  membrane t o  act as x l e  s u p F r t .  -Several o the r  methods 
have been ~ s e d  t o  give add~ t iona l  suppcrt.  These methods include use of a heavily 
doped P+ layer, a few rn icrome ters thick formed at t h e  back sur face  to supplement t h e  
19 
masking layers. These P +  layers (for concentrations >6r10 ~ m ' ~ )  a r e  at tacked very 
slowly by the  etchants, so that  t h t v  act as  a self-stopping membrane at t h e  bottom of 
t h e  slots. Such a stopping layer c?n also help t o  allow complete slot etchinq when t h e  
e tch  r a t e  may differ in different slots. W e  have also studied t h e  use of top surface 
support methods. Ry mask design, support s t ru ts  can be left at intervals across t h e  
slot pattern. It is also possible to change the  mask design to  provide many mask 
bridges across each slot. There is some conflict involved in the  need for slice support 
during formation, and the  need for easy removal later  in the  process, and this will be 
discussed in the  fie8.; section. 
The slot wid+5. slice thickness. and slot depth (slice width) values used were 
-IOum, -50um, and -1000um, respectively. V1e found several factors which s a v e  
variable etching in the  deep slots. Recause of capillary effects, i t  was observed thz t  
the  e tchant  near the  bottom of the  slots could become depleted, thus slowinq t h e  
etching rate. This e tch depletion could also a f fec t  t h e  oxidation ra te  of the  (1 11) 
surfaces; if the  slot faces a r e  not protected, sideways etching can proceed by ledge 
exposore. In addition stresses at the  top surrrlce mask-silicon interface, or severe 
crystallographic defects on the  etchinp: slot faces, could also lead to sidways erosion. 
W e  did not find c l e a r c u t  connection between edge dislocations on t h e  top slab surface 
o r  on the slice faces, and the  occurence of e x c e s s i ~ ?  slot wall etching. No mat ter  
what the  reason, this enhanced sidewavs etching formed very thin slices on parts of 
the  slab, and often while the  slots were etching deep, the  slot pattern was "washed- 
out". Compounding these sideways etching problems was the  formation of limitink 
surfaces near the  Oottorn of the  slots. These limiting surfaces were  the  family of (1 11) 
faces  which are not at  right angles to  the  !I  10) surface. (Figures 5,6.) These limiting 
surfaces slowed the e tch  rate. either requiring very long e t c h  t imes t o  complete t h e  
slots. with greater chance of sideways etching, or they hindered methods developed to 
separate the  completed slices. 
We did form many slices -1000-1250um thick. but the  slicing was incomplete 
across the  slab. These etching problems have slowed development of the  slicing 
method. To avoid processinp, of many separate thin slizes, we considered use of 
"matrix processinq" wberein comp!ete cells cculd be formed on the  supported slices, 
before separation and use with spectral concentration (7). 
COM%!L?EF!TS OR' OP SL1CINC 
It  is instructive t o  use the experience of the  ODE slicing method. t o  add insight 
into the  waferinq requirements needed to  meet  the  cor: goals of the  rrOE solar cell 
programs. 
Slicing is needed for qrown or cast  ingots of silicon. Present trends in these 
ingot technologies involve combination of reasonably pure start ing silicon, growth t o  
provide large .r4ins (>mm size), and for reduced casts, growth of large ingots +I00 kg 
per growth sequence for cont.inuous Czochralski or FZ m ~ ; : n ) d ~ ,  +50kg for cas t  ingots. 
Clearly these large ingots should be processed a s  large slices, and failure to meet  this 
requirement . , s  the m a j ~ r  disadvantage of the  OPE method. As the  cost  of t h e  
starting s i l i ~ ~ n  and the costs of growth are  decreased, kerf losses can be 
accommodated, although the  cost of g e ~ e r a t i n g  -50% scrap silicon will always be a 
heavy price to pay. Mos* casting methods gi1.e polycrystalline silicon, and ODE cannot 
be used in these cases: the mechanical methods have no similar limitations. 
Present  dav techncloqy (C-.cchralski c rvs ta ls  s l i c e ~ '  hv Ir s a v d  strews t h a t  t h e  
s l i c i n ~  throughput is ar? ear lv  !xxtieneck in the  vphole ce l l  ?rocessino sequer?ce, and  
already much space  a n t  upkeen is r e a u i r e ~  for  t h e  manv IF p a c h i n e s  r?eeded. 
Ayumine, a \ ~ o r k i n ~ ~ c ! s r  of ?O hcurs. presw.t 1" rrac'lines car! c u t  of' t-*afers a t  -7 
m ;day; at 20.7 m /kc yield, this means z_tkc/rclchine/c '~v.  For t h e  s a m e  working 
day, present Czochralski grower c a n  qene ra t e  a t  least  2Okq per day  and assuming 
-50% kerf loss. recyirinq p o r e  than ? slicinc; machines for  each  crysta! qrower. For 
all slicing methods. t he  slicing vield depends onlv on t h e  sum of t h e  (slice + kerf! 
thickness 87ip;ure 7). It is c l ea r  t k z t  t h e  vielc! rises rapidiv as this sum d e c r e a e s :  also 
thz t  for  hiah vie15 i t  is i ~ p o r t a r t  t o  rel l rce tc sl ice thic!.r?ess. as vrcll as the  kerf 
losses. To make such reductions. it is w c e s s a r v  t o  reduce tbe  thickness of t h e  slicinq 
means, and t o  also redvce r a t e  cf slicinc. This lea& t o  metho<s for simultaneous 
formation cf manv sl ices at once  to  v a i n t a i ~  a reasonable tbrounbp:~t. In this respect.  
t h e  ODE slicing m e t h o t  can  he reearded as t h e  u l t imate  ir simultaneous slicine, in 
t h a t  -= 100 sl ices can  be tormec! F r  cent imeter  of silicon. 2nd manv cen t ime te r s  can  
be simultaneouslv etched.  Slicinr to ~ r o d u c e  n:c!ucerJ kerf loss also tends tc ~ o v i d e  
sl ices with less work damage. This has  been Ct-monstratec' \vith c'amaqe c'eptks 3Sum 
for  ID sawing, 2 0 u m  fo r  ba B saw ine, an& - I  5um for hW' saw in? method: aeain t h e  
OPE method is t h e  limitine case. v.ith no   amaze proeuced. 
Estimates of t h e  oract ical  limit5 fcr  t h e  various s!Icin~: r e t b c $ s  s!!ovv tha t  t h e  
s l ice  and kerf thicknesses fall  cff relativelv slo\tllv (v.ith issociatec' increase In t h e  
slicing yield) as the  number cf simultaneous c u t s  is increased. TFe results of t hese  
e s t ima tes  a r e  qiven in Table !. Fxtxr ience  v~ltir t h e  CI'E rnethc+s shcu.s t h a t  2s tsn.ore 
sin?ultaneous c u t s  a r e  msde. rec'ucec' space is rytr irec '  for t h e  y u i ~ ~ e n t :  if t h e  
throuo,hput is s i m i F  - t o  t 5a t  c f  a n  IT' machine. a s i n i l a r  number of machines wil! still  
be needed. Also. .vith increasec' number of simultaneous c u t s  t o  ensure e f f ec t ive  
slicing, t h e  complexity Fav rise. 3rd this addeci corrplevity !or t h e  need for frequent  
maintenance)  may add unwantc? cos t  increments  t o  t h e  slicinc process. \I1!len -verv 
high yields a r e  obtained !resultin@ in tbin slices). there  may he t h e  need for  suppcrt  of 
t h e  slices. to avoid severe  hrezkace. .91so, t o  ensure lower overs.11 costs. i t  ,s 
important  t h a t  t h e  s l ices forrrlee should not b e  so thin t h a t  ex t r a  c a r e  in o r o c ~ s s i n q  is 
required. 
The QPE process hae  sever7l c t h e r  fea tures  \vhich %\.ere favors.ble to la rqe  s c a l t  
use. These incluc'ec! t h e  need fcr  ofilv m ~ d e r a t e l y  c o r n ~ l e v  methcc's (immersion in a 
0 
solution k l o w  !OO C )  an easily maintained condition (v.ater bath), and modest 
equipment needs ( laree containers  an? exhaust fans). Also. t he re  \.&?ere tulo o t h e r  
possible f ea tu re s  cf interest.  ?be O r  e tchine  Crocess qenera tes  hvdrcqen. and i t  is 
possible t ha t  in a larqe scale ~ r o c e s s .  tbis b v d r q e n  could be ccllectec'. an<' used as 
fuel. Also. t h e  e ? c k c I  silicon is lef t  in t 3e  etchine solution. 2nd sboulc' be re2.sonablv 
easy  and economical t o  recover. 
In conclusion. vre have f q u ~ c '  tb.t  study of t h e  OPF slicing method has  focussed 
a t ten t ion  on the  overr l l  p r m e r t  ies requirec' cf  a n  ef fec t ive  slicinq method. In i t s  
present  s t a t e  of develooment. CPE slicinc is an example of a meth& which has many 
of the  a t t r ac t ive  fea tures  required. anc' vet  cannot  be rerarc'ed as a solution t o  m e e t  
t h e  slicitl;: qoals of t h e  PCE low cost s ~ l i c c n  cell oroqrams. 
ACK NOW LEDCEMENTS 
The authors thank Selma Coulson for help in developing new processes. Financial 
support was provided by SOLAR ENERGY RESEARCH INSTITUTE, under Contract No. 
XS-9010-1. 
REFERENCES 
I. R.K. Smelzer, D.L. Kendall, and G.L. Vamell, Proceedings of the 10th IEEE 
Photovoltaic Specialists Conference, 1973, p. 194. 
2. R.L. Frank and J.L. Goodrich, Proceedings of 14th IEEE Photovoltaic 
Specialists Conference, 1980, p.423. 
3. R.N. Hall and T.J. Soltys, ibid, p.550. 
'I. D.L. Kendall, Appl. Phys. Letl., Vo1.26, 1975, p.195. 
5. K.E. Bean, IEEE Trans. F.D-25, 1978, p.1185 
6.  K.L. Kendall, Ann.Rev. Mater. Sci., Vo1.9, 1979, p.373. 
7. P.A. lles and S.I. Soclof, Conference record of 15th Photovoltaic 
Specialists Conference, 198 1 ( to  be published). 
FIGURE 1 
0.0. SLICING SEQUENCE 
FIGURE 2 
PARTIAL SLICING (SLUTS 0.5mm DEEP) 
I P E A L  YIELD l$/kgl VFSSUS ETCC !UTIf l  G'lR F I X E D  SLICE 'HICKNESS iS(Unl WSK LPERTUQf I llhml 
riml T Y O  V L L E S  OF ~ L D I  DEPTHln.1. 0.%l 
FIGURE 3 
FIGURE 4 
ETCH RATIn  !!ISALIGNMENT ANGLE 
(KOH -85'~) 
FIGURE 5 
(111) FACE ETCH FEATURES 
FIGURE 6 
L I M I T I K G  LEDGES ON (111) FACE (AT 
BOUNDARY BErAEEN OD ETCHED AND CLERVED SECTIONS) 
TABLE 1 
COClPARlSOW OF VARIOUS SLICING M E T H O E  
CAN ADJUST X .  Y. OR Z TO G l V E  10-25nnLlmin.  
I RECENTLY 145cm2,min. for  EFG. 


Wafering insight provided by the ODE method

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