JP2004291027A

JP2004291027A - Method for making silicon casting mold

Info

Publication number: JP2004291027A
Application number: JP2003087351A
Authority: JP
Inventors: Hiroshi Matsui; 宏史松居; Muneyoshi Yamatani; 宗義山谷
Original assignee: Kyocera Corp
Current assignee: Kyocera Corp
Priority date: 2003-03-27
Filing date: 2003-03-27
Publication date: 2004-10-21
Anticipated expiration: 2023-03-27
Also published as: JP4025671B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a silicon casting mold by which the oxidized consumption is reduced in the case of using graphite as a mold material and the increase of oxygen concentration is suppressed and such problems that the mold releasing is exfoliated and mixed into molten silicon and the mold can not reuse by sticking the releasing agent to the mold, in the case of pouring the molten silicon into the mold, in the case of solidifying thereafter or in the case of melting silicon raw material charged into the mold, are solved. <P>SOLUTION: In the method for making the silicon casting mold in which the releasing agent film is formed on its inner surface, the releasing agent film contains silicon nitride 201 and amorphous fine silica 202 obtained by heating treatment by jetting silicon tetrachloride into high temperature flame of hydrogen gas and oxygen gas, and the fine silica 202 is mixed with the above silicon nitride 201 to be 10 to 90 wt.% in the mixture. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明はシリコン鋳造用鋳型の製造方法に関し、特に太陽電池用シリコン基板の製造に好適に用いることができるシリコン鋳造用鋳型の製造方法に関する。
【０００２】
【従来の技術および発明が解決しようとする課題】
従来から太陽電池を形成するための半導体基板の一種として多結晶シリコンが用いられている。このような多結晶シリコンは高温で加熱溶融させたシリコン融液を鋳型内に注湯して凝固させることによって形成したり、シリコン原料を鋳型内に入れて溶解した後に凝固させることによって形成している。
【０００３】
このような鋳型としては、通常、分割可能な黒鉛製鋳型の内表面に離型材を塗布したものが用いられ、離型材としては窒化珪素（Ｓｉ_３Ｎ_４）粉末、炭化珪素（ＳｉＣ）粉末、シリカ（ＳｉＯ_２）粉末などが用いられる。一般に、窒化珪素、炭化珪素、酸化珪素等の粉末を適当なバインダーと溶剤とから構成される溶液中に混合して攪拌してスラリーとし、これを鋳型内壁に塗布若しくはスプレー等の手段でコーティングすることが公知の技術として知られている（例えば非特許文献１参照）。
【０００４】
しかしながら、上記に開示されている窒化珪素のみから形成される離型材皮膜は皮膜自体の強度が脆弱であり、また黒鉛製鋳型との付着性が弱いので、シリコン融液を注湯する際に離型材皮膜が破損して鋳型にシリコン融液が接触して浸入し、シリコンが鋳型に固着して取り外せなくなるという問題があった。また、シリコンと黒鉛の熱膨張係数の違いによって冷却時にシリコンインゴットに欠けが発生するという問題があった。また、シリコン融液と窒化珪素との反応が活性なため、微細な窒化珪素が融液内に混入して窒化珪素系の析出物を生成させるという問題があった。
【０００５】
また、シリカ（ＳｉＯ_２）を黒鉛製鋳型の内表面に塗布してシリコンを鋳造することも提案されている（例えば特許文献１参照）。シリカを離型材として用いる場合、離型材皮膜が強固になってシリコン融液の鋳型への浸入は防止できるが、シリカは黒鉛と付着性が良いため、またシリカはシリコンの鋳塊とも付着性がよいため、シリカが鋳型に付着して鋳型の再使用が困難になったり、鋳型が離型材を介してシリコンの鋳塊に固着して脱型するときにシリコンの鋳塊の一部に欠けが発生するという問題があった。
【０００６】
上記問題を解決するために、鋳型表面からシリカ層／窒化珪素とシリカの混合層／窒化珪素層となる多層皮膜を形成することも提案されている（例えば特許文献２参照）。このように、多層構造化して機能分離すれば、シリコン鋳塊と離型材との剥離性および鋳型と離型材との固着性の両方を兼ね備えた鋳型を製造し得るが、皮膜形成のための時間的または経済的ロスを生じる。
【０００７】
そこで、上記問題を解決するために、窒化珪素とシリカを特定比率で混合した単独層を形成することも提案されている（例えば特許文献３参照）。具体的には、窒化珪素とシリカを２８：７２〜７５：２５の重量比率で混合した離型材を黒鉛鋳型に塗布して乾燥して形成するものであり、シリカ成分がガラス化して窒化珪素粉体を固着させる効果を引き出せる。
【０００８】
しかしながら、この技術によるとシリカの一部が窒化珪素に置換しているものの、離型材中のシリカと黒鉛が部分的に接触しているため、シリコンの融点付近で黒鉛の酸化が促進して鋳型の基材の損傷を早めることになる。特にシリカの比率が高いほどこの問題は顕著に表われる。
【０００９】
また、離型材最表面層にシリカ粒子が過剰に存在するので、シリコン融液と混合層中のシリカとの接触が盛んになって融液中の酸素濃度が増加する。融液中の酸素はウエハーにした後のデバイス工程における種々の処理工程において析出物として顕在化して品質に影響を及ぼすことから、酸素濃度を適度に最適化する必要があるが、シリコン融液と長時間に渡って接触する離型材層に多量のシリカが存在することは本来望ましくはない。
【００１０】
また、従来離型材に使用するシリカは平均粒径が２０μｍ程度のものであった（例えば特許文献３参照）。このようなシリカは石英ガラスを粉砕し、分級することによって得ることができる。しかし、このようなシリカを使用すると窒化珪素粒子間の空隙にシリカ紛が細密に充填されないので、鋳型加熱中に溶融したシリカが効率よく窒化珪素粒子間の空隙に入りこむ効率が軽減される。これにより鋳型材強度が弱くなるという問題がある。この問題を回避するには、上記シリカを更に粉砕し微細化するという方法がある。しかしこの方法によれば、コンタミが多く混入しウエハーにした後のデバイス工程において品質に悪影響を及ぼすという問題があった。
【００１１】
本発明は、このような従来技術の問題点に鑑みてなされたものであり、鋳型内にシリコン融液を注湯する際、その後に凝固する際、或いは鋳型に入れたシリコン原料を溶解する際に、離型材が剥離してシリコン融液中に混入したり、離型材が鋳型に付着して鋳型が再使用できなくなったり、鋳型材を黒鉛にした場合の酸化消耗を抑えたり、鋳塊中の酸素濃度の増加を抑えたシリコン鋳造用鋳型を提供することを目的とする。
【００１２】
〔特許文献１〕
特開２００２−２９２４４９号公報
〔特許文献２〕
特開平７−２０６４１９号公報
〔特許文献３〕
特開平９−１７５８０９号公報
〔非特許文献１〕
１５ｔｈＰｈｏｔｏｖｏｌｔａｉｃＳｐｅｃｉａｌｉｓｔＣｏｎｆ．（１９８１），Ｐ５７６〜Ｐ５８０，“ＡＮＥＷＤＩＲＥＣＴＩＯＮＡＬＳＯＬＩＤＩＦＩＣＡＴＩＯＮＴＥＣＨＮＩＱＵＥＦＯＲＰＯＬＹＣＲＹＳＴＡＬＩＮＥＳＯＬＡＲＧＲＡＤＥＳＩＬＩＣＯＮ”
【００１３】
【課題を解決するための手段】
上記目的を達成するために、請求項１に係るシリコン鋳造用鋳型の製造方法では、鋳型内表面に離型材皮膜を形成するシリコン鋳造用鋳型の製造方法において、前記離型材皮膜は窒化珪素と、水素ガスと酸素ガスとの高温火炎中に四塩化珪素を噴射して加熱処理して得られる非晶質微細シリカとを含有してなり、この微細シリカを前記窒化珪素との総量の１０〜９０重量％となるように混合してなることを特徴とする。
【００１４】
上記シリコン鋳造用鋳型の製造方法では、前記微細シリカを前記窒化珪素との総量の１０〜２０重量％となるように混合してなることが望ましい。
【００１５】
また、請求項３に係るシリコン鋳造用鋳型の製造方法では、鋳型内表面に離型材皮膜を形成するシリコン鋳造用鋳型の製造方法において、前記離型材被膜はシリコンジイミドの熱分解法によって得られる粉末を酸化処理して表面に非晶質シリカ層を形成した窒化珪素と、水素ガスと酸素ガスとの高温火炎中に四塩化珪素を噴射して加熱処理して得られる非晶質微細シリカとを含有してなり、この微細シリカを前記窒化珪素との総量の５〜９０重量％となるように混合してなることを特徴とする。
【００１６】
また、上記シリコン鋳造用鋳型の製造方法では、前記微細シリカを前記窒化珪素との総量の５〜２０重量％となるように混合してなることが望ましい。
【００１７】
【発明の実施の形態】
以下、本発明の実施形態を添付図面に基づき詳細に説明する。
【００１８】
図１は本発明の製造方法で得られるシリコン鋳造用鋳型を示す図であり、鋳型１と離型材２から構成される。
【００１９】
（鋳型１）
例えば黒鉛などから成り、一つの底部材１０１と四つの側部材１０２とを組み合わせた分割と組み立てが可能な分割型鋳型などで構成される。黒鉛以外にもシリカで形成することも可能である。この場合には底と側面を一体で成形した鋳型にする場合が多い。底部材１０１と側部材１０２は、ボルト（不図示）や、底部材１０１と側部材１０２が嵌合する枠部材（不図示）で固定することによって分割可能に組み立てられる。また、鋳型部材への離型材スラリーの接着性、定着性を向上させるために、底部材１０１および側部材１０２の少なくとも一面には凹凸加工を施すことが好ましい。
【００２０】
（離型材２）
本発明で使用する離型材２は、窒化珪素２０１と微細シリカ２０２を有機バインダー水溶液で攪拌混合したスラリーとして使用する。
【００２１】
＜窒化珪素２０１＞
窒化珪素はシリコンジイミドの熱分解法で得られる球状粉体を用いる。この窒化珪素は粒径が０．１〜１．０μｍ程度の球状粉体である。
【００２２】
更にこの粉末を電気炉で酸化雰囲気下、７００℃〜１２００℃で加熱処理を施し、表面に非晶質シリカ層２０１ａを形成することがより好ましい。このように、シリコンジイミドの熱分解法で得られる窒化珪素の球状粉体を酸化雰囲気下、７００℃〜１２００℃で加熱処理すると、表面に１〜１００ｎｍ程度の厚みを有する非晶質シリカ層２０１ａが形成される。
【００２３】
酸化改質した窒化珪素粒子２０１表面のＳｉ−ＯＨ（シラノール基）と、微細シリカ粒子２０２表面のＳｉ−ＯＨ（シラノール基）間でＳｉ−Ｏ−Ｓｉ（シロキサン結合）が生じるので、窒化珪素２０１同士の密着性が大幅に改善され、離型材皮膜２が強固なものになる。その結果、鋳型１内にシリコン融液を注湯する際、或いはその後に凝固する際に、離型材皮膜２が剥離したり欠落してシリコン融液内に混入したり、微細な窒化珪素２０１がシリコン融液内に巻き込まれることを防げる。
【００２４】
なお、例えば金属珪素の直接窒化法で得られる窒化珪素は、直接窒化反応時に生成する粗大粒子の未粉砕粒子を多く含み、粒度分布の幅が広いため、加熱焼成後の凝集粒子及び／又は融着粒子の粒径制御が困難となるので好ましくない。
【００２５】
＜微細シリカ２０２＞
微細シリカとしては、水素ガスと酸素ガスとの高温火炎中に四塩化珪素を噴射して加熱処理して得られる非晶質球状シリカ微粉末（所謂、フュームドシリカ微粉末、或いはフューズドシリカ）を用いる。このような水素ガスと酸素ガスとの高温火炎中に四塩化珪素を噴射して加熱処理して得られる非晶質球状シリカ微粉末は粒径０．０１〜０．１μｍ程度の微粉末である。
【００２６】
このように鋳型１の内表面に形成する離型材層２として、水素ガスと酸素ガスとの高温火炎中に四塩化珪素を噴射して加熱処理して得られる非晶質微細シリカ２０２と窒化珪素２０１の混合スラリーを用いるので、図２に示すように粗大窒化珪素２０１の周囲に微細シリカ２０１が取り巻き、窒化珪素粒子２０１同士が強固に結合する効果を誘発する。
【００２７】
なお、珪酸ソーダ（Ｎａ_２Ｏ・ｎＳｉＯ_２）水溶液の加水分解法やイオン交換法で得られる非晶質球状シリカ微粉末はアルカリ金属不純物を多く含み、シリコン鋳塊汚染の原因となるので好ましくない。
【００２８】
また、離型材皮膜中の微細シリカの重量比率は窒化珪素との総量の１０〜９０重量％の範囲で用いるが、もっとも好適には微細シリカを１０〜２０重量％添加するのが望ましい。微細シリカのこの重量比率が９０重量％よりも大きくなると、離型材皮膜が鋳型１に付着して剥がれなくなり、鋳型１の再使用が困難になる。また、鋳型１が離型材を介してシリコンの鋳塊に付着し、シリコンの鋳塊から鋳型を剥離するときに、シリコンの一部に欠けが発生する。また、微細シリカの重量比率が１０％より小さくなると、窒化珪素と微細シリカの固着効果が低減し、離型材のシリコン融液への混入が多発するので好ましくない。
【００２９】
また、窒化珪素としてその表面を酸化改質した窒化珪素を用いる場合は、離型材皮膜中の微細シリカの重量比率は窒化珪素との総量の５〜９０重量％の範囲で用い、もっとも好適には微細シリカを５〜２０重量％添加するのが望ましい。窒化珪素としてその表面を酸化改質した窒化珪素を用いる場合は、使用する微細シリコンの量を少なくできる。微細シリカのこの重量比率が９０重量％よりも大きくなると、離型材皮膜が鋳型１に付着して剥がれなくなり、鋳型１の再使用が困難になる。また、鋳型１が離型材を介してシリコンの鋳塊に付着し、シリコンの鋳塊から鋳型を剥離するときに、シリコンの一部に欠けが発生する。また、この場合の微細シリカの重量比率が５％より小さくなると、窒化珪素と微細シリカの固着効果が低減し、離型材のシリコン融液への混入が多発するので好ましくない。
【００３０】
微細シリカを微細シリカを１０〜２０重量％、窒化珪素としてその表面を酸化改質した窒化珪素を用いる場合は５〜２０重量％の低比率で使用すると、微細シリカが鋳型に付着して鋳型の再使用が困難になったり、鋳型が離型材を介してシリコンの鋳塊に付着して脱型するときにシリコンの鋳塊の一部に欠けが発生するという問題を防げると同時に、溶解シリコンとの反応が軽減されることによってシリコン融液中の酸素濃度が低下するので、太陽電池素子化したデバイスの光電特性に悪影響を及ぼすことが軽減される。
【００３１】
また、離型材皮膜中の微細シリカとして、水素ガスと酸素ガスとによって形成される高温火炎中に四塩化珪素を噴射して加熱処理して得られる非晶質球状微細シリカを使用するので、微細シリカの添加比率が５〜２０重量％と少なくても高揺変性を付与し、鋳型への離型材皮膜形成が容易になる。
【００３２】
ところで、特開２００１−１９８６４８号公報に、珪酸ソーダのイオン交換法で得られる微細シリカ（所謂コロイダルシリカ）と窒化ケイ素粉末を混合して得られた混合素地と微細溶融シリカ砂のスタッコ層からなる離型材皮膜を形成する例が開示されているが、この発明は内部残留応力の少ないシリコンインゴットを製作すべく湿式法によって得られる微細球状シリカを用いることを考案した発明であり、本発明のように、離型材皮膜を強化する目的で四塩化珪素から気相法によって得られる微細球状シリカを用いるのとは異なる。
【００３３】
本発明によるシリカを用いることにより従来問題であった窒化珪素粉体間の固着力不足を有効に回避することができる。また水素ガスと酸素ガスとの高温火炎中に四塩化珪素を噴射して加熱処理して得られる非晶質微細シリカを使用していることから、従来の石英ガラスを粉砕して得るシリカと異なりコンタミの問題が発生することはない。
【００３４】
（離型材皮膜形成）
本発明による離型材皮膜の形成には、刷毛、箆で鋳型材に塗布してホットプレート上で乾燥させる方法を用いることが好ましいが、例えばスプレーなどを用いて鋳型内表面に塗布して乾燥するなどして形成する方法、また例えば加熱板やシリコンラバーダイアフラムを備えたラミネート装置を用いて加熱圧着させる方法も可能である。
【００３５】
以上のように、本発明による離型材皮膜を形成すると、（１）窒化珪素系異物混入の抑制（２）光電特性の向上（３）経済性に優れたシリコンインゴット鋳造鋳型を提供できる。
【００３６】
なお、本発明に係るシリコンインゴット鋳造用鋳型は以上の実施形態に何等制限されるものではなく、その要旨の範囲内で種々変更することができる。
【００３７】
【実施例】
（実施例１）
シリコンジイミドの熱分解法によって得られた平均粒子径が０．５μｍの窒化珪素と四塩化珪素の水素・酸素燃焼法で得られた平均粒径が０．０５μｍの非晶質シリカ微粉末を表１に示す重量比で秤量し、８％のポリビニルアルコール水溶液で攪拌混合して得られた離型材スラリーを黒鉛製鋳型の内表面に箆で塗布して離型材皮膜を形成した。この鋳型を８．０Ｔｏｒｒに減圧したアルゴン雰囲気中に置き、黒鉛ヒータを使って１０００℃に加熱した状態で鋳型内にシリコン融液６８ｋｇを注湯して７時間かけて徐々に凝固させた。冷却後固化したシリコンの鋳塊を鋳型から取り出してスライスしてシリコン基板とし、鋳型からの離型材皮膜の剥離およびシリコンインゴット中の析出物の有無を調べた。また、比較として従来の平均粒径２０μｍの石英ガラスを粉砕して分級して得られた微細シリカと平均粒径０．５μｍの窒化珪素を重量比１０：９０で秤量し、８％のポリビニルアルコール水溶液で攪拌混合して得られた離型材スラリーを用いた場合についても同様に調べた。その結果を表１に示す。
【００３８】
【表１】

【００３９】
表１の従来例と条件Ｎｏ．３の試料とを比較すると、窒化珪素と微細シリカの重量比は９０：１０で同じであるが、従来例ではシリコインゴット中の析出物がみられたのに対し、本発明による四塩化珪素の水素・酸素燃焼法で得られた平均粒径が０．０５μｍの非晶質シリカ微粉末を使用した条件Ｎｏ．３では鋳型からの離型材の剥離やシリコンインゴット中の析出物は見られなかった。
【００４０】
また、表１から明らかなように、条件３〜７の鋳型を用いた場合、離型材皮膜の破損や鋳型からの剥離、シリコンインゴット中への混入、さらにはシリコンインゴットのスライス歩留を低下させる析出物の発生を改善または有効に抑制することができる。したがって、離型材皮膜中の微細シリカの重量比率は１０〜９０重量％に設定しなければならない。
【００４１】
（実施例２）
シリコンジイミドの熱分解法によって得られた平均粒子径が０．５μｍの窒化珪素を酸化雰囲気下でバッチ式電気炉（酸化炉）で１０００℃で加熱処理した窒化珪素と、四塩化珪素の水素・酸素燃焼法で得られた平均粒径が０．０５μｍの非晶質シリカ微粉末とを表２に示す重量比で秤量し、８％のポリビニルアルコール水溶液で攪拌混合して得られた離型材スラリーを黒鉛製鋳型プレートの内表面に箆で塗布して離型材皮膜を形成した。この鋳型を８．０Ｔｏｒｒに減圧したアルゴン雰囲気中に置き、黒鉛ヒータを使って１０００℃に加熱した状態で鋳型内にシリコン融液６８ｋｇを注湯して７時間かけて徐々に凝固させた。冷却後固化したシリコンの鋳塊を鋳型から取り出してスライスしてシリコン基板とし、鋳型からの離型材皮膜の剥離やシリコンインゴット中への離型材皮膜の混入の有無、およびシリコンインゴット中の析出物の有無を調べた。また、酸化処理しないシリコンジイミドの熱分解法によって得られた平均粒子径が０．５μｍの窒化珪素と、四塩化珪素の水素・酸素燃焼法で得られた平均粒径が０．０５μｍの非晶質シリカ微粉末とを９５：５で秤量し、８％のポリビニルアルコール水溶液で攪拌混合して得られた離型材スラリーを用いた場合についても同様に調べた。その結果を表２に示す。
【００４２】
【表２】

【００４３】
表１の比較例と条件Ｎｏ．２を比較すると、窒化珪素と微細シリカの重量比は９５：５で同じであるが、従来例ではシリコインゴット中の析出物がみられたのに対し、本発明により表面に非晶質微細シリカ層を形成した窒化珪素を使用した条件Ｎｏ．２では鋳型からの離型材の剥離やシリコンインゴット中の析出物は見られなかった。
【００４４】
表２から明らかなように、条件２〜７の鋳型を用いた場合、離型材皮膜の破損や鋳型からの剥離、シリコンインゴット中への混入、さらにはシリコンインゴットのスライス歩留を低下させる析出物の発生を改善または有効に抑制することができる。したがって、離型材皮膜中の微細シリカの重量比率は５〜９０重量％に設定したほうがよい。
【００４５】
さらに、表１と表２の結果を比較すると、表１に示すように、窒化珪素の表面に非晶質シリカ層を形成しなかった場合には、微細シリカの重量比が１０％以上必要であったのに対し、表２に示すように窒化珪素の表面に非晶質シリカ層を形成した場合には、微細シリカの重量比が５％以上あればよく、使用できる重量比の幅が広くなった。すなわち、使用する微細シリカの重量比を小さくすることが可能になった。
【００４６】
【発明の効果】
以上詳細に説明したように、本発明に係るシリコン鋳造用鋳型の製造方法では、離型材皮膜が窒化珪素と、水素ガスと酸素ガスとの高温火炎中に四塩化珪素を噴射して加熱処理して得られる非晶質微細シリカとを含有してなり、この微細シリカを前記窒化珪素との総量の１０〜９０重量％となるように混合してなることから、粗大窒化珪素の周囲に微細シリカが有効的に取り巻き、窒化珪素粒子同士が強固に結合するため、鋳型からの離型材の剥離や、シリコインゴット中への析出といった問題を有効に抑制することができる。また、離型材皮膜中の微細シリカが水素ガスと酸素ガスとによって形成される高温火炎中に四塩化珪素を噴射して加熱処理して得られる非晶質球状微細シリカを使用するので、微細シリカの添加比率が少なくとも高揺変性を付与し鋳型への離型材皮膜の形成が容易になる。
【００４７】
また、請求項３に係るシリコン鋳造用鋳型の製造方法によれば、離型材被膜がシリコンジイミドの熱分解法によって得られる粉末を酸化処理して表面に非晶質シリカ層を形成した窒化珪素と、水素ガスと酸素ガスとの高温火炎中に四塩化珪素を噴射して加熱処理して得られる非晶質微細シリカとを含有してなり、この微細シリカを前記窒化珪素との総量の５〜９０重量％となるように混合してなることから、酸化改質した窒化珪素粒子表面のＳｉ−ＯＨ（シラノール基）と微細シリカ粒子表面のＳｉ−ＯＨ（シラノール基）との間でＳｉ−Ｏ−Ｓｉ（シロキサン結合）が生じ、窒化珪素同士の密着性が大幅に改善され、離型材皮膜が強固なものになる。その結果、鋳型内にシリコン融液を注湯する際、或いはその後に凝固する際に、離型材皮膜が剥離して欠落してシリコン溶湯内に混入したり、微細な窒化珪素がシリコン溶湯内に巻き込まれることを防げる。
【００４８】
また、前記微細シリカを１０〜２０重量％、窒化珪素としてその表面を酸化改質した窒化珪素を用いる場合は５〜２０重量％の低比率で使用すると、微細シリカが鋳型に付着して鋳型の再使用が困難になったり、鋳型が離型材を介してシリコンの鋳塊に付着して脱型するときにシリコンの鋳塊の一部に欠けが発生するという問題を防げると同時に、溶解シリコンとの反応が軽減されることによりシリコン融液中の酸素濃度が低下するので、太陽電池素子化したデバイスの光電特性に悪影響を及ぼすことを軽減できる。
【図面の簡単な説明】
【図１】本発明の製造方法で得られるシリコン鋳造用鋳型を示す図である。
【図２】本発明の離型材層を模試的に示す図である。
【符号の説明】
１鋳型
１０１底部材
１０２側部材
２離型材
２０１窒化珪素
２０１ａ窒化珪素表面に形成されたシリカ層
２０２微細シリカ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a silicon casting mold, and more particularly to a method for manufacturing a silicon casting mold that can be suitably used for manufacturing a silicon substrate for a solar cell.
[0002]
2. Description of the Related Art
Conventionally, polycrystalline silicon has been used as one type of semiconductor substrate for forming a solar cell. Such polycrystalline silicon is formed by pouring a silicon melt melted by heating at a high temperature into a mold and solidifying it, or by forming a silicon raw material in a mold and dissolving and then solidifying. I have.
[0003]
As such a mold, a mold obtained by coating a mold release material on the inner surface of a splittable graphite mold is usually used. As the mold release material, silicon nitride (Si ₃ N ₄ ) powder, silicon carbide (SiC) powder, Silica (SiO ₂ ) powder or the like is used. Generally, a powder of silicon nitride, silicon carbide, silicon oxide, or the like is mixed in a solution composed of an appropriate binder and a solvent and stirred to form a slurry, which is applied to the inner wall of a mold by coating or spraying. This is known as a known technique (for example, see Non-Patent Document 1).
[0004]
However, the release material film formed of only silicon nitride disclosed above has weak strength of the film itself and weak adhesion to a graphite mold, so that when the silicon melt is poured, it is released. There has been a problem that the mold material film is damaged and the silicon melt comes into contact with the mold and penetrates, and the silicon adheres to the mold and cannot be removed. In addition, there is a problem that the silicon ingot is chipped during cooling due to a difference in thermal expansion coefficient between silicon and graphite. Further, since the reaction between the silicon melt and the silicon nitride is active, there is a problem that fine silicon nitride is mixed into the melt to generate silicon nitride-based precipitates.
[0005]
It has also been proposed to apply silica (SiO ₂ ) to the inner surface of a graphite mold to cast silicon (for example, see Patent Document 1). When silica is used as a release material, the release material film becomes strong and can prevent infiltration of the silicon melt into the mold.However, since silica has good adhesion to graphite, silica also has adhesion to silicon ingots. Due to the good properties, silica adheres to the mold, making it difficult to reuse the mold, or a part of the silicon ingot is chipped when the mold adheres to the silicon ingot through the release material and is released from the mold. There was a problem that occurred.
[0006]
In order to solve the above problem, it has been proposed to form a multilayer film of a silica layer / a mixed layer of silicon nitride and silica / a silicon nitride layer from a mold surface (for example, see Patent Document 2). As described above, if the functions are separated by forming a multilayer structure, a mold having both the releasability of the silicon ingot and the release material and the fixability of the mold and the release material can be manufactured. Results in loss of security or economics.
[0007]
In order to solve the above problem, it has been proposed to form a single layer in which silicon nitride and silica are mixed at a specific ratio (for example, see Patent Document 3). Specifically, a mold release material in which silicon nitride and silica are mixed at a weight ratio of 28:72 to 75:25 is applied to a graphite mold and dried to form the silica component. It can bring out the effect of fixing the body.
[0008]
However, according to this technique, although part of the silica is replaced by silicon nitride, the silica in the release material and the graphite are partially in contact with each other. Will accelerate the damage to the substrate. In particular, the higher the ratio of silica, the more pronounced the problem.
[0009]
Further, since the silica particles are excessively present in the outermost layer of the release material, the contact between the silicon melt and the silica in the mixed layer becomes active and the oxygen concentration in the melt increases. Oxygen in the melt is manifested as precipitates in various processing steps in the device process after the wafer is formed and affects the quality, so it is necessary to optimize the oxygen concentration appropriately. It is inherently undesirable that a large amount of silica is present in the release material layer that is in contact for a long time.
[0010]
Further, the silica conventionally used for the release material has an average particle size of about 20 μm (for example, see Patent Document 3). Such silica can be obtained by crushing and classifying quartz glass. However, when such silica is used, the gap between the silicon nitride particles is not finely filled with the silica powder, so that the efficiency with which the silica melted during the heating of the mold efficiently enters the gap between the silicon nitride particles is reduced. This causes a problem that the strength of the mold material is reduced. In order to avoid this problem, there is a method in which the silica is further pulverized and refined. However, according to this method, there is a problem that a large amount of contaminants are mixed in and adversely affect the quality in a device process after forming a wafer.
[0011]
The present invention has been made in view of such problems of the related art, and is used when pouring a silicon melt into a mold, when solidifying thereafter, or when dissolving a silicon raw material put in a mold. In addition, the mold release material peels and mixes into the silicon melt, the mold release material adheres to the mold, making it impossible to reuse the mold, suppresses oxidative consumption when the mold material is made of graphite, It is an object of the present invention to provide a silicon casting mold in which an increase in oxygen concentration is suppressed.
[0012]
[Patent Document 1]
JP-A-2002-292449 [Patent Document 2]
JP-A-7-206419 [Patent Document 3]
JP-A-9-175809 [Non-Patent Document 1]
15th Photovoltaic Specialist Conf. (1981), P576-P580, "A NEW DIRECTION SOLIDIFICATION TECHNIQUE FOR POLYCRYSTALLINE SOLAR GRADE SILICON"
[0013]
[Means for Solving the Problems]
In order to achieve the above object, in the method for manufacturing a silicon casting mold according to claim 1, in the method for manufacturing a silicon casting mold that forms a release material film on the inner surface of the mold, the release material film includes silicon nitride; It contains amorphous fine silica obtained by injecting silicon tetrachloride into a high-temperature flame of hydrogen gas and oxygen gas and performing heat treatment, and this fine silica is contained in an amount of 10 to 90% of the total amount of the silicon nitride. % By weight.
[0014]
In the method for manufacturing a silicon casting mold, it is preferable that the fine silica is mixed so as to be 10 to 20% by weight of the total amount with the silicon nitride.
[0015]
Further, in the method for manufacturing a silicon casting mold according to claim 3, in the method for manufacturing a silicon casting mold that forms a release material film on an inner surface of the mold, the release material film is a powder obtained by a pyrolysis method of silicon diimide. Silicon oxide having an amorphous silica layer formed on the surface by oxidizing the silicon nitride, and amorphous fine silica obtained by heating and injecting silicon tetrachloride into a high-temperature flame of hydrogen gas and oxygen gas. The fine silica is mixed with the silicon nitride so as to be 5 to 90% by weight of the total amount.
[0016]
In the method of manufacturing a casting mold for silicon, it is preferable that the fine silica is mixed so as to be 5 to 20% by weight of the total amount with the silicon nitride.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[0018]
FIG. 1 is a diagram showing a silicon casting mold obtained by the production method of the present invention, and is composed of a mold 1 and a release material 2.
[0019]
(Mold 1)
For example, it is made of graphite or the like, and is made of a split mold that can be divided and assembled by combining one bottom member 101 and four side members 102. In addition to graphite, it can be formed of silica. In this case, a mold in which the bottom and side surfaces are integrally formed is often used. The bottom member 101 and the side member 102 are assembled in a dividable manner by fixing with a bolt (not shown) or a frame member (not shown) in which the bottom member 101 and the side member 102 fit. Further, in order to improve the adhesiveness and fixability of the release material slurry to the mold member, it is preferable that at least one surface of the bottom member 101 and the side member 102 is subjected to uneven processing.
[0020]
(Release material 2)
The release material 2 used in the present invention is used as a slurry obtained by stirring and mixing silicon nitride 201 and fine silica 202 with an organic binder aqueous solution.
[0021]
<Silicon nitride 201>
As the silicon nitride, a spherical powder obtained by a thermal decomposition method of silicon diimide is used. This silicon nitride is a spherical powder having a particle size of about 0.1 to 1.0 μm.
[0022]
Further, it is more preferable that the powder is subjected to a heat treatment at 700 ° C. to 1200 ° C. in an oxidizing atmosphere in an electric furnace to form an amorphous silica layer 201a on the surface. As described above, when the spherical powder of silicon nitride obtained by the thermal decomposition method of silicon diimide is heat-treated at 700 ° C. to 1200 ° C. in an oxidizing atmosphere, the amorphous silica layer 201a having a thickness of about 1 to 100 nm on the surface is obtained. Is formed.
[0023]
Since Si—O—Si (siloxane bond) is generated between the Si—OH (silanol group) on the surface of the oxidized and modified silicon nitride particles 201 and the Si—OH (silanol group) on the surface of the fine silica particles 202, the silicon nitride 201 Adhesion between them is greatly improved, and the release material film 2 becomes strong. As a result, when the silicon melt is poured into the mold 1 or when the silicon melt is subsequently solidified, the release material film 2 is peeled or missing and mixed into the silicon melt, or the fine silicon nitride 201 is removed. It can be prevented from getting caught in the silicon melt.
[0024]
Note that, for example, silicon nitride obtained by direct nitridation of metallic silicon contains many uncrushed coarse particles generated during the direct nitridation reaction, and has a wide particle size distribution. It is not preferable because it becomes difficult to control the particle size of the deposited particles.
[0025]
<Fine silica 202>
As fine silica, amorphous spherical silica fine powder (so-called fumed silica fine powder or fused silica) obtained by injecting silicon tetrachloride into a high-temperature flame of hydrogen gas and oxygen gas and performing heat treatment is used. Is used. The amorphous spherical silica fine powder obtained by injecting silicon tetrachloride into such a high-temperature flame of hydrogen gas and oxygen gas and performing heat treatment is a fine powder having a particle size of about 0.01 to 0.1 μm. .
[0026]
As the mold release material layer 2 formed on the inner surface of the mold 1 as described above, amorphous fine silica 202 and silicon nitride obtained by injecting silicon tetrachloride into a high-temperature flame of hydrogen gas and oxygen gas and performing heat treatment are used. Since the mixed slurry of the silicon nitride 201 is used, the fine silica 201 surrounds the coarse silicon nitride 201 as shown in FIG. 2 to induce an effect that the silicon nitride particles 201 are strongly bonded to each other.
[0027]
In addition, the amorphous spherical silica fine powder obtained by the hydrolysis method or the ion exchange method of an aqueous solution of sodium silicate (Na ₂ O · nSiO ₂ ) contains a large amount of alkali metal impurities and is undesirable because it causes silicon ingot contamination. .
[0028]
The weight ratio of the fine silica in the release material coating is used in the range of 10 to 90% by weight of the total amount with silicon nitride, but it is most preferable to add 10 to 20% by weight of the fine silica. When the weight ratio of the fine silica is more than 90% by weight, the release material film adheres to the mold 1 and does not peel off, making it difficult to reuse the mold 1. In addition, when the mold 1 adheres to the silicon ingot via the mold release material, and the mold is separated from the silicon ingot, a part of silicon is chipped. On the other hand, if the weight ratio of the fine silica is less than 10%, the effect of fixing the silicon nitride and the fine silica is reduced, and the release material is frequently mixed into the silicon melt, which is not preferable.
[0029]
When silicon nitride whose surface is oxidized and modified is used as silicon nitride, the weight ratio of the fine silica in the release material coating is used in the range of 5 to 90% by weight of the total amount with silicon nitride, and most preferably. It is desirable to add 5 to 20% by weight of fine silica. When silicon nitride whose surface is oxidized and modified is used as silicon nitride, the amount of fine silicon used can be reduced. When the weight ratio of the fine silica is more than 90% by weight, the release material film adheres to the mold 1 and does not peel off, making it difficult to reuse the mold 1. In addition, when the mold 1 adheres to the silicon ingot via the mold release material, and the mold is separated from the silicon ingot, a part of silicon is chipped. If the weight ratio of the fine silica is less than 5% in this case, the effect of fixing the silicon nitride and the fine silica is reduced, and the release material is frequently mixed into the silicon melt, which is not preferable.
[0030]
When the fine silica is used in a low ratio of 10 to 20% by weight of fine silica and 5 to 20% by weight in the case of using silicon nitride whose surface is oxidized and modified as silicon nitride, the fine silica adheres to the mold and At the same time, it is possible to prevent the problem that re-use becomes difficult or the mold is attached to the silicon ingot through the mold release material and the silicon ingot is chipped when the mold is released. Is reduced, the oxygen concentration in the silicon melt is reduced, so that adverse effects on the photoelectric characteristics of the device formed into a solar cell element are reduced.
[0031]
Further, as the fine silica in the release material film, amorphous spherical fine silica obtained by injecting silicon tetrachloride into a high-temperature flame formed by hydrogen gas and oxygen gas and performing heat treatment is used. Even if the addition ratio of silica is as small as 5 to 20% by weight, high thixotropic properties are imparted, and a release material film can be easily formed on a mold.
[0032]
Incidentally, Japanese Patent Application Laid-Open No. 2001-198648 discloses that a mixed body obtained by mixing fine silica (so-called colloidal silica) obtained by an ion exchange method with sodium silicate and silicon nitride powder, and a stucco layer of fine fused silica sand. Although an example of forming a release material film is disclosed, the present invention is an invention devised to use fine spherical silica obtained by a wet method in order to manufacture a silicon ingot having a small internal residual stress, as in the present invention. In addition, it is different from using fine spherical silica obtained from silicon tetrachloride by a gas phase method for the purpose of strengthening the release material film.
[0033]
By using the silica according to the present invention, it is possible to effectively avoid the problem of insufficient bonding force between silicon nitride powders, which has been a problem in the past. Also, since amorphous fine silica obtained by injecting silicon tetrachloride into a high-temperature flame of hydrogen gas and oxygen gas and performing heat treatment is used, it differs from silica obtained by grinding conventional quartz glass. No contamination problems occur.
[0034]
(Formation of release material film)
For the formation of the release material film according to the present invention, it is preferable to use a method in which the material is applied to the mold material with a brush or a spatula and dried on a hot plate. It is also possible to use a method in which the heat-press bonding is performed by using a laminating apparatus provided with a heating plate or a silicon rubber diaphragm.
[0035]
As described above, when the release material film according to the present invention is formed, it is possible to provide a silicon ingot casting mold excellent in (1) suppression of contamination of silicon nitride based foreign substances, (2) improvement of photoelectric characteristics, and (3) economical efficiency.
[0036]
In addition, the mold for casting a silicon ingot according to the present invention is not limited to the above embodiment at all, and can be variously changed within the scope of the gist.
[0037]
【Example】
(Example 1)
Amorphous silica fine powder having an average particle diameter of 0.5 μm obtained by the hydrogen-oxygen combustion method of silicon nitride and silicon tetrachloride having an average particle diameter of 0.5 μm obtained by the pyrolysis method of silicon diimide is shown. A release material slurry obtained by weighing at a weight ratio shown in No. 1 and stirring and mixing with an 8% aqueous polyvinyl alcohol solution was applied to the inner surface of a graphite mold with a spatula to form a release material film. This mold was placed in an argon atmosphere depressurized to 8.0 Torr, and while being heated to 1000 ° C. using a graphite heater, 68 kg of a silicon melt was poured into the mold and gradually solidified over 7 hours. After cooling, the solidified ingot of silicon was taken out of the mold and sliced into a silicon substrate, and the release of the release material film from the mold and the presence of precipitates in the silicon ingot were examined. For comparison, fine silica obtained by pulverizing and classifying conventional quartz glass having an average particle size of 20 μm and silicon nitride having an average particle size of 0.5 μm were weighed at a weight ratio of 10:90, and 8% polyvinyl alcohol was weighed. In the same manner, a case where a release material slurry obtained by stirring and mixing with an aqueous solution was used was used. Table 1 shows the results.
[0038]
[Table 1]

[0039]
Table 1 shows the conventional example and the condition No. In comparison with the sample No. 3, the weight ratio of silicon nitride to fine silica was the same at 90:10, but in the conventional example, precipitates in the silicon coin were observed, whereas the silicon tetrachloride according to the present invention showed a precipitate. Condition No. 1 using an amorphous silica fine powder having an average particle diameter of 0.05 μm obtained by a hydrogen / oxygen combustion method. In No. 3, peeling of the release material from the mold and precipitation in the silicon ingot were not observed.
[0040]
Further, as is apparent from Table 1, when the mold under the conditions 3 to 7 is used, the release material film is damaged or peeled off from the mold, mixed into the silicon ingot, and further, the slice yield of the silicon ingot is reduced. The generation of precipitates can be improved or effectively suppressed. Therefore, the weight ratio of the fine silica in the release material film must be set to 10 to 90% by weight.
[0041]
(Example 2)
Silicon nitride obtained by subjecting silicon nitride having an average particle diameter of 0.5 μm obtained by a thermal decomposition method of silicon diimide to heat treatment at 1000 ° C. in a batch type electric furnace (oxidizing furnace) in an oxidizing atmosphere; A release material slurry obtained by weighing amorphous silica fine powder having an average particle size of 0.05 μm obtained by the oxyfuel combustion method at a weight ratio shown in Table 2 and stirring and mixing with an 8% aqueous polyvinyl alcohol solution. Was applied to the inner surface of a graphite mold plate with a spatula to form a release material film. This mold was placed in an argon atmosphere depressurized to 8.0 Torr, and while being heated to 1000 ° C. using a graphite heater, 68 kg of a silicon melt was poured into the mold and gradually solidified over 7 hours. After cooling, the solidified silicon ingot is removed from the mold and sliced into a silicon substrate.The release of the release material film from the mold, the presence of the release material film in the silicon ingot, and the precipitation in the silicon ingot are investigated. The presence or absence was checked. In addition, silicon nitride having an average particle diameter of 0.5 μm obtained by a thermal decomposition method of silicon diimide not oxidized and amorphous silicon having an average particle diameter of 0.05 μm obtained by a hydrogen-oxygen combustion method of silicon tetrachloride. The fine silica powder was weighed at a ratio of 95: 5, and a release material slurry obtained by stirring and mixing with an 8% aqueous polyvinyl alcohol solution was used. Table 2 shows the results.
[0042]
[Table 2]

[0043]
Comparative example of Table 1 and condition No. Comparison of No. 2 shows that the weight ratio between silicon nitride and fine silica is 95: 5, which is the same. However, in the conventional example, precipitates in silico coins were observed. Condition No. using silicon nitride having a layer formed thereon. In No. 2, no peeling of the release material from the mold and no precipitate in the silicon ingot were observed.
[0044]
As is clear from Table 2, when the molds under the conditions 2 to 7 are used, breakage of the release material film, peeling from the mold, mixing into the silicon ingot, and further, precipitates that lower the slice yield of the silicon ingot. Can be improved or effectively suppressed. Therefore, the weight ratio of the fine silica in the release material film is preferably set to 5 to 90% by weight.
[0045]
Further, comparing the results in Tables 1 and 2, as shown in Table 1, when no amorphous silica layer was formed on the surface of silicon nitride, the weight ratio of fine silica was required to be 10% or more. On the other hand, when an amorphous silica layer was formed on the surface of silicon nitride as shown in Table 2, the weight ratio of the fine silica only needed to be 5% or more, and the range of usable weight ratio was wide. became. That is, the weight ratio of the fine silica to be used can be reduced.
[0046]
【The invention's effect】
As described in detail above, in the method for manufacturing a silicon casting mold according to the present invention, the release material film is subjected to heat treatment by injecting silicon tetrachloride into a high-temperature flame of silicon nitride, hydrogen gas and oxygen gas. And fine amorphous silica obtained by mixing the fine silica with the silicon nitride so as to be 10 to 90% by weight of the total amount of the silicon nitride. And the silicon nitride particles are firmly bonded to each other, so that problems such as peeling of the release material from the mold and precipitation in the silicon coin can be effectively suppressed. In addition, since amorphous silica fine silica obtained by injecting silicon tetrachloride into a high-temperature flame formed by hydrogen gas and oxygen gas and heat-treating the fine silica in the release material coating is used, fine silica is used. At least gives high thixotropic properties and facilitates formation of a release material film on a mold.
[0047]
Further, according to the method for manufacturing a silicon casting mold according to claim 3, a release material film is formed by oxidizing a powder obtained by a pyrolysis method of silicon diimide and forming a silicon nitride layer on the surface of the silicon nitride. Containing amorphous fine silica obtained by injecting silicon tetrachloride into a high-temperature flame of hydrogen gas and oxygen gas and performing heat treatment, wherein the fine silica has a total amount of 5 to 5 with the silicon nitride. Since it is mixed so as to be 90% by weight, Si—O (silanol group) on the surface of the oxidized and modified silicon nitride particles and Si—OH (silanol group) on the surface of the fine silica particles have a Si—O -Si (siloxane bond) is generated, the adhesion between silicon nitrides is greatly improved, and the release material film becomes strong. As a result, when pouring the silicon melt into the mold, or when subsequently solidifying, the release material film is peeled off and dropped and mixed into the silicon melt, or fine silicon nitride is introduced into the silicon melt. Prevents getting caught.
[0048]
When the fine silica is used in a low ratio of 10 to 20% by weight and silicon nitride whose surface is oxidized and modified as silicon nitride, the fine silica adheres to the mold when used in a low ratio of 5 to 20% by weight. At the same time, it is possible to prevent the problem that re-use becomes difficult or the mold is attached to the silicon ingot through the mold release material and the silicon ingot is chipped when the mold is released. Is reduced, the oxygen concentration in the silicon melt is reduced, so that adverse effects on the photoelectric characteristics of the device formed into a solar cell element can be reduced.
[Brief description of the drawings]
FIG. 1 is a view showing a silicon casting mold obtained by a production method of the present invention.
FIG. 2 is a view schematically showing a release material layer of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Mold 101 Bottom member 102 Side member 2 Release material 201 Silicon nitride 201a Silica layer 202 formed on silicon nitride surface 202 Fine silica

Claims

In the method for manufacturing a silicon casting mold for forming a release material film on the inner surface of a mold, the release material film is subjected to heat treatment by injecting silicon tetrachloride into a high-temperature flame of silicon nitride, hydrogen gas and oxygen gas. A method for producing a silicon casting mold, characterized by containing the obtained amorphous fine silica and mixing the fine silica with the silicon nitride so as to be 10 to 90% by weight of the total amount. .

The method for producing a silicon casting mold according to claim 1, wherein the fine silica is mixed so as to be 10 to 20% by weight of the total amount with the silicon nitride.

In a method of manufacturing a mold for silicon casting, wherein a release material film is formed on an inner surface of a mold, the release material film is formed by oxidizing a powder obtained by a pyrolysis method of silicon diimide to form an amorphous silica layer on the surface. Silicon, and amorphous fine silica obtained by injecting silicon tetrachloride into a high-temperature flame of hydrogen gas and oxygen gas to perform heat treatment. A method for producing a silicon casting mold, characterized by being mixed so as to be 5 to 90% by weight.

The method for producing a silicon casting mold according to claim 3, wherein the fine silica is mixed so as to be 5 to 20% by weight of the total amount with the silicon nitride.