New method for grain size characterization of a multi-crystalline silicon ingot

Transcription

1 New method for gran ze characterzaton of a mult-crytallne lcon ngot Maxme Forter, Erwann Fourmond, Jean-Mare Lebrun, Roland Enhau, Jed Kraem, Mutapha Lemt To cte th veron: Maxme Forter, Erwann Fourmond, Jean-Mare Lebrun, Roland Enhau, Jed Kraem, et al.. New method for gran ze characterzaton of a mult-crytallne lcon ngot. 24th European Photovoltac Specalt Conference, Sep 2009, Hamburg, Germany. pp.4, <hal > HAL Id: hal Submtted on 3 Jan 2011 HAL a mult-dcplnary open acce archve for the depot and demnaton of centfc reearch document, whether they are publhed or not. The document may come from teachng and reearch nttuton n France or abroad, or from publc or prvate reearch center. L archve ouverte plurdcplnare HAL, et detnée au dépôt et à la dffuon de document centfque de nveau recherche, publé ou non, émanant de établement d enegnement et de recherche frança ou étranger, de laboratore publc ou prvé.

2 NEW METHOD FOR GRAIN SIZE CHARACTERIZATION OF A MULTI-CRYSTALLINE SILICON INGOT M. Forter 1, E. Fourmond 1 *, J.M. Lebrun 1, R. Enhau 2, J. Kraem 2, M. Lemt 1 1: INSA de LYON, INL, 7 av. J. Capelle, VILLEURBANNE CEDEX 2: APOLLON SOLAR, 23 rue Claudu Collonge, 69002, Lyon, France *Correpondng Author: Tel: , Fax: , E-mal: erwann.fourmond@na-lyon.fr ABSTRACT: In order to etmate the qualty of the crytallaton proce of mult-crytallne Slcon ngot, we have developed a new optcal method for the determnaton of gran ze on a-cut wafer that were taken from dfferent heght poton of the ngot. Th fat method baed on mage analy and allow n a mple way to get numercal data whch repreent the crytallographc qualty of lcon wafer. The obtaned data were ued to calculate a gran ze dtrbuton over the analyed wafer area a well a an average gran ze by ung two dfferent algorthm. It wa thu poble to repreent the average gran ze a a functon of ngot heght, whch ndcate ngot regon of lower crytallne qualty, for example regon of equaxed growth of very mall gran ( grt ). Th technque wa ued for the characteraton of dfferent mult-crytallne Slcon ngot, ung Slcon feedtock of dfferent qualty (electronc, upgraded metallurgcal or olar grade), and alo to montor the mpact of modfcaton of the crytallaton proce parameter on the gran ze dtrbuton. It wa found that th method able to gve a frt qualtatve mpreon of the mult-crytallne ngot and wafer whch n the end are ued to produce olar cell. Keyword: crytallzaton, lcon, gran. 1 INTRODUCTION Mult-crytallne Slcon (mc-s) the materal wth the larget market hare n photovoltac cell and module producton. Mc-S uually produced n form of quare haped ngot that are crytallzed by drectonal oldfcaton of molten Slcon feedtock. The obtaned ngot are cut nto brck and wafer, whch are then ued for the proceng of olar cell. The qualty of mc-slcon wafer n term of electrcal performance depend on both, the purty of the Slcon feedtock,.e. the concentraton of remanng mpurte, and the crytallne qualty of the ngot. The latter trongly related to the crytallzaton proce telf; however the mpurty content n the Slcon can be of a certan nfluence. Th work focue on crytal defect that reult from the crytallaton proce telf, whch of relatve mportance on the degradaton of the electrcal performance of the materal, epecally when thee defect become hghly recombnant due to decoraton wth mpurte. We have developed a mple optcal method to quantfy the crytallzaton qualty of mc-s wafer by an mage analy routne and the treatment of numercal data. Smlar method already ext n the feld of metallurgy, but are ued to meaure mcrometrc gran and requre optcal and/or econdary electron mcrocopy [1], or even ue the electron backcatterng dffracton (EBSD) technque [2]. Our am to meaure the gran ze over the entre urface of a wafer, typcally mm² n dmenon, wth a mple and fat method. We preent here ome typcal reult obtaned wth our method on a drectonal crytallaton proce, pecally developed by Cybertar and Apollonolar. 2 CRYSTALLIZATION PROCESS PRESENTATION Three dfferent qualte of lcon have been crytallzed n th tudy: Electronc grade (EG), conventonal olar grade (SoG) and upgraded metallurgcal grade lcon (UMG). The crytallzaton proce ha been carred out n a furnace developed by Cybertar and Apollonolar, whch ha already been preented [3,4] and hown n Fgure 1. The proce mlar to a drectonal oldfcaton, Brdgman-type. However the furnace ued n th tudy comport two major mprovement, whch are brefly preented n the followng. Heatng ytem Heatng and Coolng ytem Thermally anotropc crucble Inducton col Graphte uceptor Iolaton Graphte uceptor Inducton col Fgure 1: Schematc vew of the confguraton of the Cybertar/AS crytallaton furnace. Th furnace feature two ndependently controlled nductve heatng element, and a quartz crucble wth a tranparent bottom plate. Th confguraton reult n a preferred heat tranfer by nfrared radaton through the crucble bottom durng the crytallzaton proce. The thermal confguraton of both, furnace and crucble allow thu hgh temperature gradent at the old/lqud nterface to be etablhed and controlled durng the oldfcaton tep. The purpoe to obtan a perfectly planar and table oldfcaton front durng crytallzaton, from bottom to top. Both condton are eental for an effcent egregaton of mpurte durng crytal growth, whch epecally mportant for the crytallzaton of lower qualty Slcon uch a upgraded metallurgcal Slcon. The control of the temperature gradent permt alo a better control of the germnaton proce, and hence an mprovement n the gran ze on the mc-s 1286

3 ubtrate. 3 OPTICAL CHARACTERIZATION OF THE WAFER After oldfcaton, the ngot cut nto wafer. The characteraton method preented here cont of the dgtzaton of the wafer urface by an optcal canner. The obtaned mage (Fgure 2-a) then treated wth a ucceon of functon ung the ImageJ publc doman and open ource oftware [5]. 3.1 Method The ImageJ treatment enable u to get a bnary pcture (Fgure 2-b), wth the gran area beng black and the gran boundare beng whte, whch allow to determne the urface of each gran by countng the number of pxel wthn each gran. Image are hown before and after ImageJ treatment n Fgure 2. It mportant for the meaurement that the urface of the wafer clean. To guaranty a prece meaurement, t advantageou to work wth anotropcally textured wafer, obtaned wth KOH chemcal etchng for ntance. Th enure a trong optcal contrat between the dfferent gran, and thu allow better recognton of gran boundare. The obtaned data are then treated wth a preadheet to calculate the average gran ze and the dtrbuton of dfferent gran ze over the wafer urface. way. Frt, we can repreent the cumulatve percentage of the number of gran for each ze. For example, n the red curve of Fgure 3, we can read that 50% of the gran of the analyzed wafer are maller than 0,015 cm². However, th repreentaton doe not gve any nformaton about the proporton of the wafer urface whch covered by gran maller than cm². A econd depcton how the cumulatve percentage of the occupaton of the wafer urface by gran ze. The blue curve on the Fgure 3 tell u that le than 10% of the wafer urface occuped by gran that are maller than cm². Th econd repreentaton eem to be more relevant to qualfy the tructural qualty of a wafer, a the electrcal properte of the olar cell wll depend on the proporton of the urface occuped by mall gran. We can note that th gran ze dtrbuton follow generally an exponental law. Fgure 3: 2 dfferent type of repreentaton for the dtrbuton of gran ze over the wafer urface. (b) Fgure 2: Scanned mage of a typcal mc-s mm² wafer before (a) and after (b) the numercal mage proceng. (a) We can then repreent the reult n two dfferent A calculaton of the average gran ze can be done for the two dfferent type of repreentaton. The frt one repreent the average of the gran ze (Eq. 1) dependng on the number of gran of each ze: n n N = Eq. 11 where N the total number of gran, n the number of gran wth the urface S. The econd repreent the ze of the gran (Eq. 2) by whch the wafer urface n average occuped. = Eq. 2 where S the total urface of the wafer. In the followng the gran ze average wll be condered a equal to. 3.2 Lmtaton of the method Fgure 4 gve an example of a canned wafer and the related proceed mage for a wafer from a top part of an ngot, contanng mall equaxed gran. Thee gran ( grt ) preent a very hgh rato of the gran boundary urface to the gran urface, whch make them dffcult to quantfy wth accuracy, nce the gran boundare are very cloe to each other and the gran urface extremely mall. Fgure 5 how the correpondng dtrbuton curve of the two wafer preented n Fgure 2 and 4. In the cae of the wafer wth mall equaxed gran, more S n 1287

4 than 50% of the urface covered wth gran maller than 0.1 cm². how a dependence of the gran ze on the feedtock qualty,.e. the total mpurty concentraton. We have meaured gran ze average n a range from le than 0.1 cm² to 0.5 cm² for the UMG-S, from 0.3 cm² to 1.6 cm² for the SoG-S and from 0.5 cm² to 2.1 cm² for the EG-S. Th can be explaned by the fact that mpurte act a nucleaton te for the formaton of new gran durng crytallzaton. (a) Fgure 6: Evoluton of the average gran ze wthn the heght of the ngot, dependng on the Slcon feedtock qualty. (b) Fgure 4: Scanned mage of a mc-s wafer wth large area of equaxed mall gran, before (a) and after (b) the numercal mage treatment. We can alo notce an evoluton of the gran ze dependng on the vertcal poton of the wafer n the ngot. Gran ze are maller n the bottom of the ngot becaue of the random germnaton whch occur at the begnnng of crytallzaton, due to a hgh coolng rate. The gran ze then ncreae wth the heght toward the mddle of the ngot and fnally decreae toward the top. Th can be explaned by dfferent effect: (a) egregaton of mpurte n the upper ngot regon, (b) accumulaton of non metal complexe lke SC and/or SN n th regon, (c) a lght decreae of the vertcal temperature gradent at the end of the crytallzaton due to the preence of an nulatng ma of lcon oldfed Comparon between lfetme value and gran ze Fgure 7 preent the average gran urface and the related average effectve lfetme value, for wafer comng from an UMG-S ngot. The effectve lfetme ha been meaured by the QSSPCD technque, ung odneethanol for the wafer urface pavaton Fgure 5: Cumulatve urface occupaton of dfferent gran ze for wafer wth tandard gran ze and zone of mall equaxed gran. 4 RESULTS 4.1 Influence of the lcon feedtock Gran ze meaurement have been carred out on wafer from dfferent heght poton of 3 dfferent ngot, crytallzed wth upgraded metallurgcal grade lcon (UMG-S), olar grade lcon (SG-S) and electronc grade lcon (EG-S) feedtock, by applyng the ame crytallzaton condton. The obtaned reult (Fgure 6) Average gran ze (cm²) Gran ze Effectve Lfetme 0% 20% 40% 60% 80% 100% % of the ngot heght Fgure 7: Comparon between average gran ze and effectve lfetme value (UMG-S). Th UMG ngot contan a relatvely large quantty of mpurte, and exhbt maller gran, compared to ngot from SoG Slcon. A mall gran ze could have a trong mpact on the lfetme value, f the gran dmenon are of the ame order of magntude or maller than the dffuon length of the mnorty carrer (typcally Effectve lfetme (µ) 1288

5 100 µm). We can thu notce that the lfetme value follow the ncreae of gran ze throughout the top of the ngot. 4.2 Influence of the temperature profle durng crytallaton On Fgure 8 compared the average gran ze for 2 UMG-S ngot, wth dfferent temperature profle appled durng the oldfcaton tep. The ngot A preent very mall gran ze, and alo exhbt ome grt n the centre of the wafer, tartng n the mddle of the ngot. In uch area of the wafer the average gran ze reman maller than 0.1 cm². A typcal wafer from th regon preented n Fgure 4. Snce the grt area of the wafer located n the centre of the wafer, the average gran ze doe not ncreae wth the ngot heght, and reman very low. On ngot B the temperature profle ha been mproved. The average gran ze hgher than for ngot A, and no more grt vble. The average gran ze thu ncreae wth the heght of the ngot. gran average ze (cm²) 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 0% 20% 40% 60% 80% 100% % of ngot heght Ingot A Ingot B Fgure 8: Influence of the temperature profle durng crytallaton: comparon of wafer gran ze for a tandard (A) and an optmed profle (B). 4.3 Influence of other crytallzaton parameter Fgure 9 how the average gran ze dtrbuton a a functon of the ngot heght for two ngot, both crytallzed ung upgraded metallurgcal grade Slcon. The only dfference between the two ngot A and B a modfcaton concernng the geometry of the Argon njecton ytem nde the crytallaton chamber. The Argon njecton toward the urface of the lqud Slcon ha an nfluence on the carbon content nde the Slcon, nce t allow evacuatng both SO and CO 2 that have formed nde the crytallaton chamber due to the preence of graphte and quartz element nde the furnace. Ingot B wa crytalled wth an optmed njecton geometry and flow rate and how a larger average gran ze dtrbuton, epecally n the mddle ecton of the ngot. Th tend to prove that the optmed Argon njecton ha been effcent enough to evacuate C from the melt and thu lmt the grt formaton. Fgure 9: Average gran ze dtrbuton comparon between the tandard proce (ngot A) and a modfed proce (ngot B). 4 CONCLUSION We have developed a mple method of optcal analy to quantfy the crytallne qualty of lcon wafer n the early tage of producton. Th method gve acce to numercal reult, whch can be ued n the optmaton of a drectonal oldfcaton proce of lcon. We have appled th method to dfferent ngot, and ued t to tudy the nfluence of the S-feedtock qualty and the thermal parameter of a new type of furnace. The reult obtaned have led to the mprovement of the temperature and tme profle ued durng the crytallaton tep a well a to the geometry of ome furnace element. 5 REFERENCES [1] B. Roebuck & E.G. Bennett: The metallographc meaurement of WC gran ze. NPL Meaurement Good Practce Gude No. 22, Augut [2] K.P. Mngard et al.: Comparon of EBSD and conventonal method of gran ze meaurement of hardmetal. Int. J. of Refractory Metal and Hard Materal, 27 (2), 2009, pp [3] F. Lalde et al.: Innovatve Crytallaton Proce and Furnace for Solar Grade Slcon, Proceedng of the 22nd European PVSEC, Mlano 2007, pp [4] J. Kraem et al.: Innovatve Crytallaton of Mult- Crytallne Slcon Ingot from Dfferent Type of Slcon Feedtock. Proceedng of the 23rd European PVSEC, Valenca 2008, pp [5] W. Raband: ImageJ, Image proceng and Analy n Java. On-lne: