Single-Step multiple-layers wafer slicing from macroporous silicon

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.” [1] A. B ailo e e al., P oc. o he 25 h Eu opean Pho o ol aic Sola Ene gy Con ., (2010) 1613. [2] S. C. Bae , Pa en No. US 2009/0056513 A1 (2009) [3] F. D oss, e al. Appl. Phys. A-Ma e 89, 149 (2007). [4] J.H. Pe e mann, D. Zielke, J. Schmid , e al. P og. Pho o ol .: Res. Appl. 20 (2012) 1. [5] R. B. Be gmann, C. Be ge, T. J. Rinke, J. Schmid , and J. H. We ne , Sol. Ene g. Ma . Sol. C. 74, 213 (2002). [6] I. Mizushima, T. Sa o, S. Taniguchi, and Y. Tsunashima Appl. Phys. Le . 77 (2000) 3290. [7] V. Depauw, I. Go don, G. Beauca ne e al. J. Appl. Phys. 106 (2009) 033516. [8] V. Depauw, Y. Qiu, K. Van Nieuwenhuysen e al. P og. Pho o ol : Res. Appl. 19 (2011) 844. [9] D. He nández, T. T i ono , M. Ga ín, e al. Appl. Phys. Le . 102 (2013) 172102. [10] V. Lehmann, “Elec ochemis y o Silicon. Ins umen a ion, Science, Ma e ials and Applica ions” (Wiley-VCH, Weinheim, Ge many, 2002). [11] T. T i ono , L. F. Ma sal, A. Rod iguez, e al. Phys. S a us Solidi C 2 (2005) 3104. [12] T. T i ono , M. Ga ín, A. Rod iguez, e al. Phys. S a us Solidi A 204 (2007) 3237. [13] W. W. Mullins, J. Appl. Phys. 28 (1957) 333. [14] F. A. Nichols, and W. W. Mullins, J. Appl. Phys. 36 (1965) 1826. [15] M. S. McCallum, P. W. Voo hees, M. J. Miksis, e al. J. Appl. Phys. 79 (1996) 7604. [16] H. Wong, M. J. Miksis, P. W. Voo hees, and S. H. Da is, Sc ip a Ma e ialia 39 (1998) 55. [17] T. Sa o, K. Mi su ake, I. Mizushima, and Y. Tsunashima, Jpn. J. Appl. Phys. 39 (2000) 5033. 28 h Eu opean Pho o ol aic Sola Ene gy Con e ence and Exhibi ion 936