SINGLE-STEP MULTIPLE-LAYERS WAFER SLICING FROM MACROPOROUS SILICON
M. Ga ín1, D. He nández1, T. T i ono 2, D. Ca dado 1, and R. Alcubilla1,2*.
1 G up de ece ca en Mic o i Nano ecnologies, Depa amen d’Enginye ia Elec ònica
Uni e si a Poli ècnica de Ca alunya, c/Jo di Gi ona 1—3, Mòdul C4, 08034 Ba celona, Spain.
2 Cen e de Rece ca en Nanoenginye ia, Uni e si a Poli ècnica de Ca alunya
c/ Pascual i Vilà 15, 08028, Ba celona, Spain.
* a[email p o ec ed]
ABSTRACT: The e is a ising in e es , om bo h pho o ol aics and mic oelec onics indus y, in wa e hickness
educ ion. Du ing he las decade, i has been s eadily educed om 350 µm o 180 µm, bu bene i s a e o eseen
o hicknesses well below hese alues. The cu en sawing echnology, howe e , su e s om la ge ke losses
and u he educ ions a e inc easingly di icul . Se e al echnologies ha e eme ged aiming o p oduce hin Si
oils om a wa e , such as laye ans e , induced clea ing, o po e eo ganiza ion. These me hods p oduce a
single laye by s ep. In his wo k we epo on a me hod able o p oduce many c ys alline laye s om a single
silicon wa e and in a single ab ica ion s ep.
Keywo ds: C ys allisa ion, Silicon-Films, Thin Film Sola Cell, Wa e ing
1 INTRODUCTION
O e he pas decade, comme cial silicon wa e s ha e
educed hei hickness om 350 µm down o 180 µm,
bu cu en oadmaps al eady claim o e en hinne
subs a es in bo h pho o ol aic and mic oelec onics
indus ies. Up o now he p og essi e educ ion in wa e
hickness has been achie ed by imp o emen s in he
sawing p ocess. This echnique, howe e , is eaching a
poin whe e u he educ ions a e no possible wi hou
punishing yield and/o ke losses (i.e. cu losses). In
o de o o e come he limi a ions o sawing, a ew
echnologies ha e eme ged ollowing di e en
app oaches: induced clea ing [1,2,3], laye ans e [4,5],
and mac opo e eo ganiza ion [6, 7, 8]. These echniques
p oduce a single silicon laye pe p ocess and pe
subs a e. Al hough he subs a e can be ecycled o
p oduce mo e han one laye , his c ea es a bo leneck o
cos -e ec i e mass p oduc ion. I would be desi able,
he e o e, o ha e a me hod able o p oduce mo e han
one laye pe subs a e and pe p ocess; ideally as many
laye s as physically possible in he subs a e.
Ve y ecen ly we epo ed on a me hod [9] ha can
p oduce mul iple ee-s anding c ys alline-silicon laye s
om a single silicon wa e and in a single s ep. This
me hod is based on he eo ganiza ion o po ous
s uc u es bu , unlike in he s anda d echnique, ha uses
an a ay o shallow s aigh po es, we decided o c ea e
ex emely deep po es wi h a ia ions in po e diame e
wi h he aim o c ea ing many laye s a once. To ab ica e
hese po es we used elec ochemical e ching echniques
[10], on which we had al eady many yea s o
accumula ed know-how in ou g oup [11,12], allowing o
c ea e o de ed a ays o po es wi h ex eme aspec a ios
(e en wa e - h ough po es) and wi h a p ecise con ol
o e he po es’ diame e in-dep h. Wi h his echnique,
and ollowing an in ui i e app oach, we c ea ed o de ed
po ous s uc u es wi h al e na ing sec ions wi h low po e
diame e and high po e diame e (see Fig. 1) and
annealed hem in A a mosphe e a 1200 ºC. A e he
eo ganiza ion, low diame e egions p oduced
c ys alline silicon ilms while high diame e egions
p omo ed o spacing laye s, demons a ing ha he
o ma ion o many simul aneous laye s is easible, and
ha laye hickness can be con olled h ough he ini ial
po e p o ile. We call he esul ing mul ilaye s uc u e
“silicon mille euille” by analogy wi h he amous ench
pas y.
In he ollowing we epo on he silicon mille euille
p ocess and ou cu en achie emen s, and co ela e he
esul s wi h po e e olu ion simula ions. F om he
calcula ions we es ima e he ange o a ainable
hicknesses conside ing he cu en po e p o ile shape.
2 EXPERIMENTAL
We p oduce po es by elec ochemical dissolu ion o
n- ype silicon in HF solu ion unde back-side
illumina ion [11]. A squa e a ay (2x2 µm) o in e ed
py amids is i s c ea ed on he wa e su ace o de ine
whe e po es will nuclea e. The elec ochemical
dissolu ion o silicon in HF consumes holes. Since holes
a e mino i y ca ie s in n- ype silicon, he eac ion is
con olled by he supply o holes by pho o-gene a ion in
he wa e ’s backside. Pho ogene a ed holes di use
owa ds he su ace allowing silicon dissolu ion a he
po e ips and p omo ing a s able po e g ow h. The
po osi y o he s uc u e (i.e. diame e o po es) is
p opo ional o he ins an aneous cu en lowing h ough
he sys em, which is in u n con olled by he in ensi y o
he back-side illumina ion. By p ecisely con olling he
cu en lowing du ing he e ch, we can c ea e po es wi h
a de ined po e diame e p o ile in dep h. Po ous silicon
echnology has he ad an ages o allowing ull wa e
su ace p ocessing, p o iding a good con ol o e he
po e p o ile in dep h, and pe mi ing o p oduce po es as
deep as he wa e hickness.
Using he abo e p ocess, we ha e ab ica ed samples
wi h po es whose diame e al e na e in-dep h be ween
na ow and wide diame e egions. The s uc u al
pa ame e s o he p o ile a e: he na ow po e diame e ,
dm, wide po e diame e , dw, leng h o na ow egions, Ln,
leng h o wide egions, Lw, modula ion leng h, L=Ln+Lw,
and he numbe o pe iods (i.e. inal laye s) N.
28 h Eu opean Pho o ol aic Sola Ene gy Con e ence and Exhibi ion
933
Figu e 1: Schema ic o he p ocess. (a) Po e nuclea ion
laye . (b) Elec ochemical e ching o po es. (c—e)
Du ing he annealing, po es collapse in o bubbles which
collapse wi h coplana bubbles o ming a s ack o laye s.
( ) SEM image o a ypical ini ial modula ed s uc u e.
(g) SEM image o a ypical inal mul ilaye s uc u e wi h
8+1 laye s.
These s uc u es ha e been annealed in a s anda d
ho izon al u nace in A gon ambien a 1200ºC o wo
hou s, enabling he po e mo phology e olu ion by
su ace di usion. Du ing his p ocess, po es collapse
o ming bubbles i s , which la e coalesce la e ally
o ming emp y laye s sepa a ing he di e en inal Si
oils. Fig. 1 shows schema ically he p ocess, along wi h
a SEM image o he ini ial modula ed s uc u e and a
inal mul ilaye s uc u e.
Finally, hese laye s can be de ached one-by-one
om he sample by ex olia ion. As an example, igu e 2
shows se e al laye s ex olia ed using s anda d o ice
adhesi e ape. The i s six laye s we e ex olia ed om
he same sample. No ice ha , al hough lexible, laye s
can b eak easily, especially i oid laye s ha e de ec s.
The las wo laye s co espond o wo highe quali y
laye s de ached om a di e en sample.
3 MODELLING
3.1 P o ile e olu ion
The spa ial and ime e olu ion o he po es’ su ace
du ing he annealing can be desc ibed using he
mac oscopic linea heo y o su ace di usion. In his
model, a oms di use om high cu a u e egions owa d
lowe cu a u e ones. In eg a ion o su ace di usion
leads o he well-known Mullin’s equa ion [13]
HB sn 
, (1)
whe e n is he no mal eloci y o he e ol ing su ace,
∆s is he su ace Laplace ope a o and H=½(k1+k2) is he
mean cu a u e o he su ace, de ined as he a e age o
Figu e 2: Consecu i e ilms ex olia ed om a silicon
mille euille. No ice ha hey a e agile.
he wo p incipal cu a u es, k1 and k2. The pa ame e B
accoun o he ma e ial and empe a u e T h ough
B=DsγνΩ2/(kbT), wi h Ds he di usion cons an , γ he
su ace ension, ν he a omic su ace densi y, Ω he
a omic olume and kb he Bol zmann cons an .
In o de o model he e ec o po e modula ion on i s
e olu ion h ough he annealing, we ha e e ol ed
equa ion 1 in ime by ini e di e ences o a single
cylind ical po e wi h a de ined diame e p o ile in dep h.
Ins ead o compu ing he po e e olu ion in e ms o a 3D
iangula mesh e olu ion, we exploi ed he cylind ical
symme y o he sys em and calcula ed jus he e olu ion
o he e ical c oss-sec ion C o he po e [14]. A any
poin o he su ace, he i s p incipal cu a u e is
de ined by he local cu a u e o C a ha poin :
2/322
1)''(
''''''
zx
xzzx CC
CCCC
k


(2)
whe e he pa ial de i a i es o he componen s o C a e
wi h espec o he a c leng h s. The second p incipal
cu a u e, on he con a y, is calcula ed om he adius
o he po e a he poin , , applying he Meusnie ’s
heo em
)cos(
1
2

k
, (3)
wi h θ he angle be ween he su ace no mal a P and he
plane pe pendicula o he po e axis. A he bo om o a
po e and a he bo om and op o a apped bubble,
equa ion (3) becomes unde e mined. In he icini y o
hese poin s we ake k2=k1.
3.2 Calcula ion o laye hickness
Wi h he cylind ical app oach we can accu a ely
desc ibe he e olu ion o a single po e, bu canno
desc ibe he la e al coalescence o mul iple bubbles (i.e.
emp y laye o ma ion) due o he lack o cylind ical
symme y o such sys em. None heless, and p o ided ha
he po es collapse be o e bubble s a o coalesce, we can
s ill de e mine he hickness and spacing o he inal
laye s based on olume conse a ion p inciples. A po e,
ei he smoo h o pe iodically modula ed, will e ol e in o
a se ies o bubbles o adius b spaced pe iodically wi h a
pe iodic dis ance λ. I po es a e a anged in a squa e
a ay wi h pi ch a<2 b, bubbles will coalesce in o oids.
Conside ing ha he o al olume o he coplana bubbles
and he inal emp y laye mus be main ained, he inal
spacing laye hickness will be
(d)
(e)
( )
(a)
(b)
(c)
28 h Eu opean Pho o ol aic Sola Ene gy Con e ence and Exhibi ion
934
Figu e 3: SEM images, clea ed side, o samples wi h
di e en a e age hickness namely: (a) 5.5 µm (solid),
(b) 6.5 µm (solid), (c) 7.4 µm (one ow o bubbles) and
(d) 15 µm a e age hickness ( wo ows o bubbles).
2
3
3
4
a
wb
oid


, (4)
and he hickness o he silicon oils
oid oil ww 

. (5)
4 RESULTS & DISCUSSION
Aiming o explo e he possibili ies and e sa ili y o
his echnique, we ha e ab ica ed samples wi h di e en
numbe o laye s and di e en low po osi y leng hs Ln.
The ypical po e diame e s a e dn=0.4—0.8 µm and
dw=1.5—1.7 µm. The leng h Lw is choosen a ound 5 um,
which leads o eliable emp y laye o ma ion.
One o he main goals we pu sued in ou se o
expe imen s was o demons a e ha many laye s can be
c ea ed wi h his echnique. As we inc ease he numbe o
laye s, po es become deepe e ealing changes in he
elec ochemical e ching condi ions wi h dep h. O e all,
his c ea es de ia ions in he po e p o ile ha leads, in he
bes case, o incomple e solid o space laye s o ma ion
(see bo om spaces in igu e 1(g)) o , in he wo s case, o
an uns able po e g ow h uining he s uc u e. In
summa y, we needed o ollow an i e a i e p ocess o
e ining he e ching condi ions in o de o ob ain he
desi ed p o ile a high dep hs. Up o now we ha e been
able o p oduce 9+1 mille euille s uc u es. The +1 laye
co esponds o an ex a hin laye (1—2 um) ha o ms a
he op o he s uc u e. No ice ha we s a ou po e
p o iles wi h a highly po ous band ha should c ea e a
space laye , bu a hin laye appea s on op o his space
due o su ace closing du ing he annealing. As a ma e
o ac , we do no o esee any s ong limi a ion in he
maximum numbe o laye s, and a g ea e numbe should
be possible by u he e ining he e ching a g ea e
dep hs.
Ano he in e es ing ea u e o he silicon mille euille
echnique is ha he hickness o he p oduced silicon
oils is de ined no only by he po e a angemen
pe iodici y, bu by he pa icula po e p o ile, i.e. he po e
diame e s and leng hs (dn, dw, Ln and Lw) o he di e en
po e p o ile sec ions, ha can be independen ly
con olled. Fu he mo e, he laye hicknesses achie ed
a e conside ably hicke han wha can be ob ained wi h
he s anda d (single-laye ) po e eo ganiza ion echnique.
Figu e 4: E olu ion o a single po e o he case o
dmin=0.7 µm and dmax=0.8 µm. The es o pa ame e s a e
iden ical.
Figu e 3 shows he clea ed side o di e en mul ilaye
s uc u es ob ained o di e en leng hs Ln leading o
di e en silicon oil hicknesses. Fo sho Ln alues
a ound 4µm, ( ig 3.a and b) usually solid laye s a ound 6
µm hick o m, whe eas bubbles a e apped o laye s
sligh ly hicke ( ig 3.c). Fo e y hick laye s, a long
apped oid is le , ha p omo e o wo o mo e sphe ical
oids i he annealing is la ge enough ( ig 3.d).
I is well known ha smoo h cylinde s o adius
become uns able unde small pe u ba ions o pe iod
la ge han 2π (Rayleigh c i e ion) due o capilla y
ins abili ies [15, 16]. Ou p o iles, howe e , we en’
designed elying on Rayleigh ins abili y c i e ion, bu on
nonlinea po e pinch o due o a s ong modula ion o
he po e diame e . Since ou po e p o iles a e composed
o s aigh po e sec ions, i u ns ou ha e e y sec ion is
subjec ed o capilla y ins abili ies and sphe oidiza ion.
This explains he appea ance o bubbles as he leng h o
he na ow-po e sec ion inc eases. Fu he mo e, since
sphe oidiza ion in a ini e cylinde p og esses om he
endings [17], la ge apped ai cylinde s will u n in o
wo o mo e bubbles i he annealing p ocess is long
enough.
In o de o ge a mo e p ecise unde s anding o he
phenomenon, we ha e simula ed he p o ile e olu ion o
a single modula ed po e. In pa icula , we a e in e es ed
in ep oducing he collapsing and bubble o ma ion in he
na ow po e sec ions and we ha e ound ha he
o ma ion o a apped bubble is e y sensi i e o he
exac diame e dn. Mo e speci ically, we ha e simula ed
he collapsing sequence o a po e p o ile wi h Ln=4 µm,
Lw=5 µm, dw=1.6 µm, and dn alues anging om 0.6 o
0.9 µm. Figu e 4 shows he po e e olu ion o wo
pa icula cases: dn = 0.7 um and 0.8 µm. Fo dn alues
below 0.75 um, oughly, he su ace e olu ion a he
sudden diame e changes igge s he pinching o he
po e, apping an emp y space ha apidly becomes
sphe ical. Fo dn alues abo e 0.75, a sudden beha io
change occu s. Ins ead o a as po e pinch-o on he
edges, he na ow po e sec ion ends o smoo h slowly
becoming ins able and pinching he po e o in he cen e
egion, hus lea ing no apped oid. This beha io
esembles, in ac , a s anda d Rayleigh ins abili y
igge ed by a small pe u ba ion. Fu he mo e, as dn is
inc eased, he mo e he low pe u ba ion case is
app oached. Rega ding he pinch-o ime (see ig. 5) i
inc eases exponen ially wi h dn, bu he cu e makes a
bump a dn = 0.75 µm, signaling he change o beha io .
28 h Eu opean Pho o ol aic Sola Ene gy Con e ence and Exhibi ion
935
Figu e 5: Dependence o he po e pinch-o ime, du ing
annealing, on he po e diame e in he na ow po e
sec ion.
We ha e calcula ed he maximum laye hickness ha
can be achie ed wi hou bubbles o di e en Ln alues
conside ing a=2 µm, Lw=5 µm and dw=1.6 µm. The
na ow diame e was allowed o be he minimum alue
no leading o bubble apping a e e y poin . To
de e mine he inal laye hickness we ha e simula ed he
collapse o a single po e, measu ed he olumes and
spacing be ween he sphe ical oids, and hen applied
equa ions (4) and (5). Resul s a e shown in igu e 6 o
Ln alues up o 6.5 µm. Beyond 6.5 µm, he modula ion
is so shallow ha i akes e y long o g ow ins able and
he po es sphe oidizes jus as a s aigh po e. As a ule o
humb, po es can’ be modula ed a a pe iodici y sho e
han he in-plane pe iodici y a; he e o e, he minimum Ln
conside ed was a=2.0 µm.
As can be seen in he igu e 6, he inal hickness can
be adjus ed by changing Ln and dn, al hough he ela ion
is no p opo ional peaking a Ln=6.0 wi h a maximum
hickness alue o 6.9 µm. This is no su p ising since, as
we inc ease Ln, we a e also inc easing dn o a oid bubble
o ma ion, educing he o al amoun o Si le in he
s uc u e, educing also he inal laye hicknesses and
inc easing he oid spaces be ween hem, e en hough he
pe iodic dis ance be ween laye s would s ay he same. I
is wo h no icing ha ig. 6 does no ep esen he
absolu e maximum hicknesses ha can be achie ed wi h
he mille euille echnique. He e we ha e kep cons an Lw
and dw, ha ha e a main e ec on he space o ma ion,
bu also on he o al silicon a ailable in he po ous
s uc u e be o e annealing and, hus, on he inal laye
hicknesses. E en mo e impo an , calcula ions ha e been
pe o med o a ec angula -like po e modula ion in
dep h, mimicking ou ea ly expe imen s. A di e en
p o ile modula ion, o ins ance ollowing a simple
iangula o sinusoidal shape (easily a ainable wi h
mac opo ous silicon echnology) should help o imp o e
he inal laye s hicknesses. As a ma e o ac , we
en isage ha wi h an op imal po e p o ile i could be
possible o achie e laye s up o 10 µm o a=2 µm.
5 SUMMARY
Many c ys alline silicon laye s ha e been ab ica ed
simul aneously by po e eo ganiza ion du ing annealing
a 1200 ºC in A gon ambien . We call his s uc u e
silicon mille euille. The numbe o laye s and hei
hicknesses we e con olled by adjus ing he po e p o ile
Figu e 6: Es ima ion o he maximum laye hickness
wi hou bubbles, as a unc ion o he na ow po e leng h.
The na ow po e diame e , in blue, was adjus ed o he
minimum a oiding bubbles.
in-dep h. Depending on he exac p o ile, laye s can ap
oids. Calcula ions show ha laye s up o 6.9
mic ome e s can be p oduced wi hou bubbles using a
ec angula po e p o ile. Thicke laye s could be a ained
by u he op imizing he p o ile shape. Laye s o en hs
o mic ome e s can be p oduced i apped oids a e
allowed.
This wo k has been pa ially unded by TEC2008-
02520 and he Ne wo k o Excellence “Nanopho onics
o Ene gy.”
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