JP2004067448A

JP2004067448A - Graphite crucible for preparing silicon single crystal and its manufacturing process

Info

Publication number: JP2004067448A
Application number: JP2002229125A
Authority: JP
Inventors: Shoji Kamiya; 神谷　庄司; Fumito Morikawa; 森川　文人; Hidehiko Usuha; 薄葉　秀彦
Original assignee: SHIN NIPPON TECHNO CARBON KK
Current assignee: SHIN NIPPON TECHNO CARBON KK
Priority date: 2002-08-06
Filing date: 2002-08-06
Publication date: 2004-03-04

Abstract

<P>PROBLEM TO BE SOLVED: To provide a long-lasting graphite crucible which shows excellent durability and inhibits deformation and damage that occur in CZ method operation. <P>SOLUTION: The graphite crucible for preparing a silicon single crystal is formed from a graphite crucible material showing a three-point bending strength of ≥40MPa, an average pore size measured by mercury penetration of 3.5-7.5μm, a nitrogen-gas permeability at an ordinary temperature of 1.0-2.5 permeability (centidarcys), a plastic yield stress at 1,680°C of ≤4MPa and a warpage of ≤0.4mm accompanying silicon carbide formation at the crucible surface. In the process for manufacturing the graphite crucible, the material is molded through rubber press method (CIP, cold isostatic pressing method), baked at 700-1,100°C and subsequently graphitized at ≥2,800°C without performing pitch impregnation/ recarbonization treatment. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、シリコン単結晶引き上げ装置において長時間安定的に使用可能な黒鉛ルツボおよびその製造方法に関する。
【０００２】
【従来の技術】
チョコラルスキー法（以下、ＣＺ法と称する）と呼ばれる回転引き上げ法によるシリコン単結晶引き上げ装置では、原料シリコンを溶解する一体形状の石英ルツボとこれを収納して外側から保持する分割形状の黒鉛ルツボ（縦方向に通常２分割もしくは３分割形状）とが二重構造で使用されている。ＣＺ法の操業時には原料シリコンの溶解部で約１５００℃の高温で操業されるため、石英ルツボの熱変形によって両ルツボの内外面が密着する状態となる。この際、石英ルツボと黒鉛ルツボは高温状態で接触し、以下のような反応により黒鉛ルツボ表面のＳｉＣ化が進行していくと考えられている。
ＳｉＯ_２＋Ｃ　　　　→ＳｉＯ＋ＣＯ　　　　　　　　　（１）
ＳｉＯ　＋２Ｃ　　　→ＳｉＣ＋ＣＯ　　　　　　　　　　（２）
ＳｉＣ　＋２ＳｉＯ_２　→３ＳｉＯ＋ＣＯ　　　　　　　　（３）
【０００３】
反応式（１）で発生したＳｉＯガスが黒鉛組織内部に拡散し、反応式（２）により黒鉛をＳｉＣに転化し、黒鉛ルツボの内側表面からルツボ内部に徐々にＳｉＣ化が進行していく。更に、生成したＳｉＣは石英ルツボ（ＳｉＯ_２）と反応式（３）で反応し、ＳｉＯとＣＯに分解する。（３）式で生成したＳｉＯは、反応式（２）により黒鉛をＳｉＣ化させていく。その結果、黒鉛ルツボの内側表層にＳｉＣ層が形成されＣＺ法の操業回数を重ねる毎に厚くなるとともに、黒鉛ルツボは減肉消耗していく。黒鉛ルツボの内側表層に形成されたＳｉＣ層は、黒鉛ルツボが減肉消耗により継続使用不可能となる段階には、数ｍｍの厚みまで到達する。また、黒鉛はＳｉＣ化によって体積膨張が起こるため、ルツボ内側表層のＳｉＣ化進行とともに黒鉛ルツボには引っ張り方向のストレスが内存されていくことになる。
【０００４】
黒鉛ルツボの寿命判定は、通常ルツボの減肉状態を見て判断される。しかし、時として黒鉛ルツボの減肉があまり進まない段階で、黒鉛ルツボの内側表層のＳｉＣ化にともなうストレスにより、黒鉛ルツボの変形や破損が発生する。このような変形や破損は、黒鉛ルツボの寿命を短くし、ＣＺ法における大きな問題となっている。また、近年シリコンウェハーは８インチ以上の大口径品が主流となり、それに伴ってＣＺ法で使用される黒鉛ルツボも大型化されており、ＳｉＣ化による破損の危険度が高まる傾向にある。
【０００５】
黒鉛ルツボのＳｉＣ化ストレスを抑えることを目的として、黒鉛のＳｉＣ化反応を抑制するために黒鉛の気孔径あるいはガス透過度を低減した黒鉛ルツボ（特開昭５８−１５６５９５号，特開昭６３−８５０８６号，特開平０７−１８７８７８号）が提案されている。
また、ルツボの材質を従来の微粉焼結型の黒鉛から炭素繊維強化炭素材（以下Ｃ／Ｃ材と略記）に変更したＣ／Ｃ材ルツボ（特開平１０−２１８６９７号，特開平１０−１０１４７１号）の提案もなされている。
【０００６】
【発明が解決しようとする課題】
しかしながら、本発明者らの検討結果によれば、気孔径あるいはガス透過度を低減させた黒鉛ルツボは、黒鉛ルツボの内側表層に形成されるＳｉＣ層の厚みを低減させる効果はあるものの、黒鉛の緻密化を伴い塑性降伏応力の増大を招くため、黒鉛ルツボ破損の発生頻度は従来の黒鉛ルツボとほとんど差がなく、寿命延長効果に乏しいことが判明した。また、Ｃ／Ｃ材ルツボは従来の微粉焼結型黒鉛ルツボよりも変形や破損防止効果は認められるものの、Ｃ／Ｃ材が微粉焼結型黒鉛材よりも高価なため、コストパフォーマンスの点で需要家の満足を得るものではない。したがって、かかる現状から本発明の目的は、従来の微粉焼結型黒鉛ルツボについて、ＣＺ法操業で発生する変形や破損を防止した耐久性に優れた長寿命の黒鉛ルツボとその製造方法を提供することにある。
【０００７】
【課題を解決するための手段】
本発明者は、上記課題を解決するために検討の結果、黒鉛ルツボの塑性降伏応力の大きさがＣＺ法における黒鉛ルツボの破損と関連していることを見出して本発明を完成した。
即ち、本発明のシリコン単結晶製造用黒鉛ルツボは、３点曲げ強度で４０ＭＰａ以上、水銀圧入法により測定される平均気孔径が３．５〜７．５μｍで、且つ常温における窒素ガスのガス透過率で１．０〜２．５ｐｅｒｍｅａｂｉｌｉｔｙ（ｃｅｎｔｉｄａｒｃｙｓ）であって、１６８０℃における塑性降伏応力が４ＭＰａ以下であり、且つ下記（Ｂ）による表面炭化珪素化に伴う反りが０．４ｍｍ以下である黒鉛ルツボ素材で形成されていることを特徴とする。
【０００８】
（Ｂ）：１６８０℃で、５０ｍｍφ×３ｍｍ（長さ）の高純度黒鉛試験片上に、５０ｍｍφ×５ｍｍ（長さ）の石英ガラスを重ねて、荷重５０ｇｆ相当の荷重付加下で１６時間接触状態で表面炭化珪素化を行なった後における試験片の反り（ｍｍ）を求める。
【０００９】
また本発明のシリコン単結晶製造用黒鉛ルツボの製造方法は、室温〜５００℃間の平均熱膨張係数が４．５〜５．４×１０^−６／℃であるコークス粉砕物１００重量部に対し、バインダー５０〜７０重量部を配合し、バインダーの溶融温度以上で混練して揮発分率を調整後に再粉砕したものを原料として、ラバープレス法（ＣＩＰ成型法）により成形後に７００〜１１００℃で焼成し、次いでピッチ含浸・再炭化処理をすることなく２８００℃以上で黒鉛化することを特徴とする。
【００１０】
【発明の実施の形態】
以下、本発明の実施の形態を具体的に説明する。
前記した反応式（２）で説明したように、黒鉛素材がＳｉＣに転化すると体積膨張を伴う。したがって、黒鉛ルツボの内側表面からＳｉＣ化が進行していくとＳｉＣ化に伴う体積膨張のため、ＳｉＣ化層側に圧縮方向の歪みを、ＳｉＣ層と接する黒鉛側に引っ張り方向の歪みを生じる。歪みが小さい場合、黒鉛は弾性体の挙動を示し発生応力は歪みの大きさに比例して増大する。しかし、歪みが大きくなり発生応力が黒鉛の塑性降伏応力を越えると、歪みの増加に対する応力の増加度合いは弾性変形時に比べ急激に小さくなる。
【００１１】
歪みによって黒鉛に発生する応力は、ＳｉＣ化層が厚くなるほど黒鉛ルツボ内部へと移るとともにＳｉＣ化に伴う歪みが大きくなるため、図１（イ）（ロ）（ハ）に示すようなイメージでより大きな残留応力が黒鉛に掛かることになる。その結果、黒鉛ルツボをＣＺ法で繰り返し使用していくと通例の繰り返し使用中（例えば３ヶ月間で５０回繰り返し使用）に破損を生じてしまうことがある。こうした黒鉛ルツボの破損は、微妙な振動でもシリコン結晶の品質問題に影響がでてしまうＣＺ法では最悪の事態である。本発明者は、この黒鉛ルツボの塑性降伏応力の大きさがＣＺ法における黒鉛ルツボの破損と関連していることを見出し本発明に至ったものである。
【００１２】
即ち、本発明では、１６８０℃におけるこの黒鉛ルツボの塑性降伏応力の大きさが４ＭＰａ以下でなければならない。１６８０℃における塑性降伏応力が４ＭＰａ以下であるような黒鉛からなる黒鉛ルツボは、ＳｉＣ化進行に伴う歪みによって黒鉛ルツボに発生する応力が大幅に低減し、ＣＺ法における黒鉛ルツボの破損頻度が大幅に低減する。１６８０℃における塑性降伏応力が４ＭＰａを越えると、ＳｉＣ化進行時の塑性降伏が起こりにくくなり歪みに対する発生応力が大きくなるため、黒鉛ルツボの破損が生じやすくなる。ＣＺ法における黒鉛ルツボ破損防止をより確実にするためには、好ましくは１６８０℃の塑性降伏応力が３．７ＭＰａ以下であることが望ましい。
【００１３】
なお、１６８０℃における黒鉛の塑性降伏応力の測定方法は、例えば下記に示した（Ａ）によって求められる。
（Ａ）：黒鉛の塑性降伏応力測定方法
１６８０℃のアルゴン雰囲気下で、１ｍｍ（厚）×８ｍｍ（幅）×１５０ｍｍ（長さ）の黒鉛材試験片１を図２の（ａ）に示すように１２０ｍｍの二支点間で支え４点曲げ試験に準じて中央部へ最大荷重３０ＭＰａ相当（１８０ｇｆ）の荷重２をかける。その時の応力分布は（ｂ）のようになり、支点から最大荷重までは応力は直線的に増加する。（ａ）の状態を保ったまま１６８０℃のアルゴン雰囲気下で、１６時間保持してやると試験片は、（ｃ）に示すように端部は直線ａに、中央部は曲線状ｂに変形する。ここで直線部分ａでは弾性体の挙動を示し、曲線状部分ｂでは塑性変形が発生していることから、支点部と直線部分ａと曲線部分ｂの境界との長さｃを求めてやれば（ｂ）で示す相似の応力分布の関係から境界部の応力（１６８０℃における塑性降伏応力）σｒが求められる。
【００１４】
さらに本発明のシリコン単結晶製造用黒鉛ルツボを形成する黒鉛ルツボ素材としては、３点曲げ強度で４０ＭＰａ以上、水銀圧入法で測定される平均気孔径が３．５〜７．５μｍで、常温における窒素ガスのガス透過率が１．０〜２．５ｐｅｒｍｅａｂｉｌｉｔｙ（ｃｅｎｔｉｄａｒｃｙｓ）であることが必要である。３点曲げ強度が４０ＭＰａ未満の場合、１６８０℃における塑性降伏応力が４ＭＰａ以下であったとしても黒鉛ルツボの絶対強度不足のため、ＣＺ法における黒鉛ルツボの破損を生じやすくなる。
【００１５】
また水銀圧入法で測定される平均気孔径が３．５μｍ未満では、１６８０℃における塑性降伏応力を４ＭＰａ以下に抑えるのが難しく、７．５μｍを超えると後述する（Ｂ）法で求めた黒鉛の反りを０．４ｍｍ以下に抑えることが困難となる。常温での窒素ガスのガス透過率も１．０ｐｅｒｍｅａｂｉｌｉｔｙ未満では１６８０℃における塑性降伏応力を４ＭＰａ以下に抑えるのが難しく、２．５ｐｅｒｍｅａｂｉｌｉｔｙを超えれば（Ｂ）法で求めた黒鉛の反りを０．４ｍｍ以下に抑えることが困難となる。
【００１６】
なお、ガス透過率は、ＡＳＴＭ　Ｃ−５７７‐６８に従って測定されるものである。
また３点曲げ強度試験方法はＪＩＳ　Ｒ７２１２に準じて求められる。（試験片寸法１０×１０×６０ｍｍ、温度条件：常温、支点間寸法４０ｍｍ、中央部に破損する迄荷重を加えていき、破損した時の荷重から求める）
本発明の効果をより確実にするためには、３点曲げ強度で４５ＭＰａ以上、水銀圧入法により測定される平均気孔径が４．０〜６．５μｍで、常温における窒素ガスのガス透過率で１．２〜２．２ｐｅｒｍｅａｂｉｌｉｔｙ（又はｃｅｎｔｉｄａｒｃｙｓ）であることが望ましい。
【００１７】
さらに本発明のシリコン単結晶製造用黒鉛ルツボを形成する黒鉛ルツボ素材としては、（Ｂ）法による表面炭化珪素化に伴う反りが０．４ｍｍ以下である黒鉛ルツボ素材であることが必要である。
ここで黒鉛ルツボ内側表面のＳｉＣ化による体積膨張のため、黒鉛ルツボはＣＺ法で使用回数を重ねるほど外側に開く方向に変形していく。その結果、黒鉛ルツボの内側に載置されるべき石英ルツボをセットすることが困難となる。そのため、こうした黒鉛ルツボの変形が生じた場合には、通例の繰り返し使用（例えば３ヶ月間で５０回繰り返し使用）よりも前に、新品の黒鉛ルツボと交換という事態になってしまう。
【００１８】
本発明者らは、ＣＺ法による黒鉛ルツボの変形が、下記（Ｂ）によって求められる黒鉛の反りの大きさから定量的に類推できることを見いだした。
（Ｂ）：黒鉛の反りの測定方法
１６８０℃、アルゴン雰囲気下で、灰分量２０ｐｐｍ以下の高純度黒鉛試験片３（５０ｍｍφ×３ｍｍ（長））に図３の（ａ）に示す要領で石英ガラス４（５０ｍｍφ×５ｍｍ（長））を荷重５（５０ｇｆ）の付加状態で１６時間接触させた後、（ｂ）に示す黒鉛試験片３の反りｄを求める。
【００１９】
即ち、本発明者が見いだした上記（Ｂ）で測定した反りが０．４ｍｍ以下であるような黒鉛からなる黒鉛ルツボは、ＣＺ法における変形度合いが小さく、黒鉛ルツボの減肉による寿命到達以前に継続使用が不可能となるような変形を起こしにくくなる。これに対し、上記（Ｂ）による反りが０．４ｍｍを越えるような黒鉛では、ＣＺ法における黒鉛ルツボの変形が大きくなり、減肉寿命以前の段階で黒鉛ルツボの変形により継続使用不可となる確率が大幅に大きくなる。黒鉛ルツボの減肉寿命前の変形防止をより確実にするためには、黒鉛ルツボとして使用される黒鉛のＢ法による反りが０．３５ｍｍ以下であることが望ましい。
【００２０】
次に本発明の黒鉛ルツボ（素材）の製造方法について説明する。
本発明における黒鉛ルツボの素材は、微粉状のコークスとタールピッチ等のバインダー質を、バインダー質を分割投入することなくコークスと同時に混練機に投入し、バインダー質の溶融温度以上で混練後冷却し、再度微粉砕した２次粉を水中でのラバープレスで圧縮成型後、焼成し、特にピッチ含浸・再炭化工程を経ずにそのまま黒鉛化処理することで製造することができる。
【００２１】
以下、具体的な製造条件等を説明する。
先ず原料コークスは、室温〜５００℃間の平均熱膨張係数が４．５〜５．４×１０^−６／℃のコークスを微粉砕する。この時の微粉砕コークス粉の粒度は平均粒径１５±３μｍで、最大粒径が２００μｍ以下が好ましい。
コークスに添加し混練するタールピッチ等のバインダー質の性状としては、軟化温度１００〜１１０℃でトルエン不溶分２９％以上、且つキノリン不溶分８〜１３％のものが望ましい。またバインダー質の添加率としては、コークス１００重量部に対して５０〜７０重量部の範囲内が望ましい。
コークスとバインダーの混合物は、バインダーの溶融温度以上で混練して揮発分率が９００℃×７分の測定で８．５〜９．５％に調整する。
【００２２】
次いで、混練調整物は再粉砕（２次粉砕）するが、この時の平均粒径は「１次粉砕粒径〜１次粉砕粒径＋１０μｍ」で最大粒子径が５００μｍが望ましい。
かかる再粉砕したものを原料としてラバーに充填し水中で均等に圧縮成形（ＣＩＰ成型）するが、この時のＣＩＰ成型圧力が０．５〜１．０ｔ／ｃｍ^２（４．９×１０^７〜９．８×１０^７Ｎ／ｍ^２）程度とする。ＣＩＰ成型後に、ラバーより取り出してから、焼成温度７００〜１１００℃で焼成し、次いで通常実施されているようなピッチ含浸・再炭化処理をすることなく、そのまま２８００℃以上で黒鉛化することで黒鉛素材が得られる。
【００２３】
黒鉛素材としては、３点曲げ強度が４０ＭＰａ以上、水銀圧入法により測定される平均気孔径が３．５〜７．５μｍで、且つ常温における窒素ガスのガス透過率で１．０〜２．５ｐｅｒｍｅａｂｉｌｉｔｙ（ｃｅｎｔｉｄａｒｃｙｓ）であって、前記した（Ａ）による黒鉛の塑性降伏応力が４ＭＰａ以下であり、且つ（Ｂ）による表面炭化珪素化に伴う反りが０．４ｍｍ以下の物性値を有するものが、本発明の黒鉛ルツボとして使用される。かかる物性値を有する黒鉛素材を、適宜所定の形状、寸法からなるシリコン単結晶製造用黒鉛ルツボとして切削加工することで黒鉛ルツボが得られる。
【００２４】
【作用】
本発明によるＣＺ法用の黒鉛ルツボは、特に１６８０℃における（Ａ）による塑性降伏応力が４ＭＰａ以下であるため、ＣＺ法の操業回数を重ね黒鉛ルツボ内側表面のＳｉＣ化に伴う歪みが大きくなっても塑性降伏が容易に起こり、黒鉛ルツボに発生する引っ張り応力は小さくなる。その上、３点曲げ強度で４０ＭＰａ以上の強度を有していることから、黒鉛ルツボの減肉寿命前での破損頻度は大幅に低減する。更に、（Ｂ）で測定される１６８０℃での表面炭化珪素化に伴う反りが０．４ｍｍ以下であるため、ＣＺ法の操業回数を重ねることにより生じる黒鉛ルツボの変形度合いも小さくなる。その結果、本発明によるＣＺ黒鉛ルツボは、減肉寿命前の変形や破損発生が大幅に低減し、耐久性に優れたものとなる。
【００２５】
【実施例】
実施例１〜３、比較例１〜５
原料コークスとバインダーをそれぞれ調整し溶融混練後に２次粉砕してから常法によりＣＩＰ成形し、焼成、黒鉛化して得られた黒鉛素材から切り出した試験片の特性値を第１表に実施例と比較例毎に示す。但し比較例２，４，５は焼成後にピッチ含浸・再炭化処理を実施して製造したものであるのに対して、実施例１〜３及び比較例１．３は焼成後にピッチ含浸・再炭化処理を実施しないで得られた物である。
【００２６】
【表１】

【００２７】
第１表に示した実施例１〜３及び比較例１〜５の黒鉛（高純度処理品：灰分１０ｐｐｍ未満）と同一の素材を切削加工してそれぞれ１８インチＣＺ法用の黒鉛ルツボを製作し、シリコン単結晶の引き上げ装置で実機試験を行った。黒鉛ルツボの耐用回数と寿命原因を第２表に示す。第２表の結果から、本実施例１〜３の黒鉛ルツボはいずれも、５０回の繰り返し使用（減肉寿命）前に変形及び破損が発生せず、耐用回数として２０％以上の向上がみられた。これに対し、比較例１〜５の黒鉛ルツボは、いずれもルツボ減肉寿命に到達する前に変形もしくは破損が発生してしまった。
【００２８】
【表２】

【００２９】
【発明の効果】
以上説明した通り、本発明の黒鉛ルツボを用いることによりＣＺ法と呼ばれる回転引き上げ法によるシリコン単結晶引き上げ装置において、黒鉛ルツボ減肉消耗寿命前の破損や変形が抑制され、耐久性に優れた黒鉛ルツボを提供することができる。これにより、シリコン単結晶（あるいは太陽電池用シリコン多結晶）の引き上げを長時間安定して行うことができ、工業上有益な効果がもたらされる。
【００３０】
【図面の簡単な説明】
【図１】（イ）（ロ）（ハ）は黒鉛に発生するＳｉＣ化層の厚みの進行に伴う歪みのイメージ説明図である。
【図２】（ａ）は黒鉛の塑性降伏応力の測定方法（Ａ）における測定要領を示す概略図であり、（ｂ）は黒鉛の塑性降伏による応力分布図であり、（ｃ）は黒鉛試験片の変形状態を示す説明図である。
【図３】（ａ）は黒鉛の反りの測定方法（Ｂ）における測定要領を示す概略図であり、（ｂ）は黒鉛試験片の反り状態を示す説明図である。
【符号の説明】
１　黒鉛材試験片
２　荷重
ａ　直線部分
ｂ　曲線状部分
ｃ　直線部分ａと曲線部分ｂの境界との長さ
３　反り測定用の黒鉛試験片
４　石英ガラス
５　荷重
ｄ　反り[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a graphite crucible that can be used stably for a long time in a silicon single crystal pulling apparatus, and a method for manufacturing the same.
[0002]
[Prior art]
In a silicon single crystal pulling apparatus using a rotary pulling method called the Czochralski method (hereinafter, referred to as CZ method), an integrated quartz crucible for melting raw silicon and a divided graphite crucible for accommodating the silicon crucible and holding it from the outside are provided. (Usually divided into two or three in the vertical direction) is used in a double structure. During the operation of the CZ method, since the operation is performed at a high temperature of about 1500 ° C. in the melting portion of the raw silicon, the inner and outer surfaces of both crucibles are brought into close contact due to thermal deformation of the quartz crucible. At this time, it is considered that the quartz crucible and the graphite crucible come into contact with each other in a high temperature state, and the surface of the graphite crucible is converted into SiC by the following reaction.
SiO ₂ + C → SiO + CO (1)
SiO + 2C → SiC + CO (2)
SiC + 2SiO ₂ → 3SiO + CO (3)
[0003]
The SiO gas generated by the reaction formula (1) diffuses into the graphite structure, converts the graphite to SiC by the reaction formula (2), and the SiC gradually progresses from the inner surface of the graphite crucible into the crucible. Further, the generated SiC reacts with the quartz crucible (SiO ₂ ) according to the reaction formula (3), and is decomposed into SiO and CO. The SiO generated by the formula (3) converts graphite into SiC according to the reaction formula (2). As a result, the SiC layer is formed on the inner surface layer of the graphite crucible and becomes thicker as the number of operations of the CZ method increases, and the thickness of the graphite crucible is reduced and consumed. The SiC layer formed on the inner surface layer of the graphite crucible reaches a thickness of several mm at the stage where the graphite crucible cannot be used continuously due to wall thinning consumption. Further, since graphite undergoes volume expansion due to SiC conversion, a tensile stress is inherently contained in the graphite crucible as the formation of SiC on the inner surface layer of the crucible progresses.
[0004]
The determination of the life of a graphite crucible is usually made by checking the reduced thickness of the crucible. However, at a stage where the thickness reduction of the graphite crucible does not proceed very much, deformation or breakage of the graphite crucible occurs due to stress accompanying the SiC conversion of the inner surface layer of the graphite crucible. Such deformation or breakage shortens the life of the graphite crucible and is a major problem in the CZ method. In recent years, silicon wafers having a large diameter of 8 inches or more have become mainstream, and accordingly, graphite crucibles used in the CZ method have been increased in size, and the risk of breakage due to SiC tends to increase.
[0005]
Graphite crucibles having a reduced pore diameter or gas permeability to suppress the SiC conversion reaction of graphite for the purpose of suppressing the SiC conversion stress of graphite crucibles (JP-A-58-156595, JP-A-63-163) No. 85086, JP-A-07-187878).
Further, a C / C crucible in which the material of the crucible is changed from conventional fine powder sintered type graphite to carbon fiber reinforced carbon material (hereinafter abbreviated as C / C material) (Japanese Patent Laid-Open Nos. 10-21897 and 10-101471). No.) has also been proposed.
[0006]
[Problems to be solved by the invention]
However, according to the study results of the present inventors, graphite crucibles with reduced pore diameter or gas permeability have the effect of reducing the thickness of the SiC layer formed on the inner surface layer of graphite crucibles, but have the effect of reducing the thickness of graphite. Since the plastic yield stress increases with densification, the occurrence frequency of graphite crucible breakage is almost the same as that of the conventional graphite crucible, and it has been found that the effect of extending the life is poor. In addition, although the C / C crucible is more effective in preventing deformation and breakage than the conventional fine-powder sintered graphite crucible, the C / C material is more expensive than the fine-powder sintered graphite crucible. It does not get customer satisfaction. Accordingly, an object of the present invention is to provide a long-life graphite crucible with excellent durability, which prevents deformation and breakage occurring in the CZ method operation, and a method for manufacturing the same, with respect to the conventional fine powder sintered type graphite crucible. It is in.
[0007]
[Means for Solving the Problems]
The present inventor has studied to solve the above problems, and as a result, has found that the magnitude of the plastic yield stress of the graphite crucible is related to the breakage of the graphite crucible in the CZ method, and completed the present invention.
That is, the graphite crucible for producing a silicon single crystal of the present invention has a three-point bending strength of 40 MPa or more, an average pore diameter measured by a mercury intrusion method of 3.5 to 7.5 μm, and a gas permeation of nitrogen gas at room temperature. A graphite crucible having a plastic yield stress at 1680 ° C. of 4 MPa or less, and a warp due to surface silicon carbide conversion according to the following (B) of 0.4 mm or less, having a ratio of 1.0 to 2.5 permeability (centidarcys). It is characterized by being formed of a material.
[0008]
(B): At 1680 ° C., a 50 mmφ × 5 mm (length) quartz glass layer was superimposed on a 50 mmφ × 3 mm (length) high-purity graphite test piece, and in a contact state for 16 hours under a load equivalent to a load of 50 gf. The warpage (mm) of the test piece after performing surface siliconization is determined.
[0009]
Further, the method for producing a graphite crucible for producing a silicon single crystal according to the present invention is based on 100 parts by weight of a coke pulverized product having an average coefficient of thermal expansion between room temperature and 500 ° C. of 4.5-5.4 × 10 ⁻⁶ / ° C. 50 to 70 parts by weight of a binder, kneaded at a temperature equal to or higher than the melting temperature of the binder, adjusted to a volatile content, and re-ground to obtain a raw material. It is characterized in that it is calcined and then graphitized at 2800 ° C. or more without performing pitch impregnation and recarbonization.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be specifically described.
As described in the above reaction formula (2), when the graphite material is converted into SiC, the graphite material is accompanied by volume expansion. Therefore, as the SiC formation proceeds from the inner surface of the graphite crucible, volume expansion accompanying the SiC formation causes strain in the compression direction on the SiC layer and strain in the tensile direction on the graphite side in contact with the SiC layer. When the strain is small, graphite exhibits the behavior of an elastic body, and the generated stress increases in proportion to the magnitude of the strain. However, when the strain increases and the generated stress exceeds the plastic yield stress of graphite, the degree of increase of the stress with respect to the increase of the strain becomes sharply smaller than at the time of elastic deformation.
[0011]
The stress generated in the graphite due to the strain moves into the graphite crucible as the thickness of the SiC layer increases, and the strain accompanying the SiC formation increases. Therefore, the stress as shown in FIGS. A large residual stress will be applied to the graphite. As a result, if the graphite crucible is repeatedly used by the CZ method, breakage may occur during normal repeated use (for example, repeated use of 50 times in three months). Such breakage of the graphite crucible is the worst case in the CZ method in which even a slight vibration affects the quality problem of the silicon crystal. The present inventors have found that the magnitude of the plastic yield stress of the graphite crucible is related to the breakage of the graphite crucible in the CZ method, and have reached the present invention.
[0012]
That is, in the present invention, the magnitude of the plastic yield stress of this graphite crucible at 1680 ° C. must be 4 MPa or less. Graphite crucibles made of graphite having a plastic yield stress at 1680 ° C. of 4 MPa or less greatly reduce the stress generated in the graphite crucible due to the strain accompanying the progress of SiC formation, and the frequency of breakage of the graphite crucible in the CZ method is greatly increased. Reduce. If the plastic yield stress at 1680 ° C. exceeds 4 MPa, the plastic yield during SiC formation is less likely to occur, and the stress generated due to strain increases, so that the graphite crucible is likely to be damaged. In order to more reliably prevent the graphite crucible from being damaged in the CZ method, it is preferable that the plastic yield stress at 1680 ° C. is 3.7 MPa or less.
[0013]
The method for measuring the plastic yield stress of graphite at 1680 ° C. can be determined, for example, by (A) shown below.
(A): Method for measuring plastic yield stress of graphite A graphite material test piece 1 of 1 mm (thickness) x 8 mm (width) x 150 mm (length) is shown in Fig. 2 (a) in an argon atmosphere at 1680 ° C. Then, a load 2 equivalent to a maximum load of 30 MPa (180 gf) is applied to the center according to a four-point bending test supported between two supporting points of 120 mm. The stress distribution at that time is as shown in (b), and the stress increases linearly from the fulcrum to the maximum load. When the test piece is held for 16 hours in an argon atmosphere at 1680 ° C. while maintaining the state of (a), the test piece is deformed into a straight line a at the end and into a curved line b at the center as shown in (c). Here, since the linear portion a shows the behavior of the elastic body and the curved portion b has plastic deformation, the length c between the fulcrum portion and the boundary between the straight portion a and the curved portion b can be calculated. The stress at the boundary (plastic yield stress at 1680 ° C.) σr is determined from the relationship between the similar stress distributions shown in FIG.
[0014]
Furthermore, the graphite crucible material forming the graphite crucible for producing a silicon single crystal of the present invention has a three-point bending strength of 40 MPa or more, an average pore diameter measured by a mercury intrusion method of 3.5 to 7.5 μm, and at room temperature. It is necessary that the gas permeability of the nitrogen gas is 1.0 to 2.5 permeability (centidarcys). When the three-point bending strength is less than 40 MPa, even if the plastic yield stress at 1680 ° C. is 4 MPa or less, the graphite crucible is liable to breakage in the CZ method due to insufficient absolute strength of the graphite crucible.
[0015]
If the average pore diameter measured by the mercury intrusion method is less than 3.5 μm, it is difficult to suppress the plastic yield stress at 1680 ° C. to 4 MPa or less, and if it exceeds 7.5 μm, the graphite yield determined by the method (B) described later will be used. It becomes difficult to suppress the warp to 0.4 mm or less. If the gas permeability of nitrogen gas at room temperature is less than 1.0 permeability, it is difficult to suppress the plastic yield stress at 1680 ° C. to 4 MPa or less. If it exceeds 2.5 permeability, the warpage of graphite obtained by the method (B) is 0.4 mm. It will be difficult to keep it below.
[0016]
The gas permeability is measured according to ASTM C-577-68.
The three-point bending strength test method is determined according to JIS R7212. (Specimen size 10 × 10 × 60 mm, temperature condition: normal temperature, distance between fulcrums 40 mm, load is applied until the center is broken, and it is obtained from the load at the time of breakage.)
In order to further ensure the effect of the present invention, the three-point bending strength is 45 MPa or more, the average pore diameter measured by a mercury intrusion method is 4.0 to 6.5 μm, and the gas permeability of nitrogen gas at normal temperature is It is desirable to be 1.2 to 2.2 permeability (or centidarcys).
[0017]
Further, the graphite crucible material forming the graphite crucible for producing a silicon single crystal of the present invention needs to be a graphite crucible material having a warp of 0.4 mm or less due to surface siliconization by the method (B).
Here, the graphite crucible is deformed in a direction to open outward as the number of times of use is increased by the CZ method due to volume expansion due to SiC conversion of the inner surface of the graphite crucible. As a result, it becomes difficult to set a quartz crucible to be placed inside the graphite crucible. Therefore, when such deformation of the graphite crucible occurs, the graphite crucible may be replaced with a new graphite crucible before the usual repeated use (for example, repeated use of 50 times in three months).
[0018]
The present inventors have found that the deformation of the graphite crucible by the CZ method can be quantitatively inferred from the magnitude of the warpage of graphite determined by the following (B).
(B): Method for measuring the warpage of graphite Quartz glass was applied to a high-purity graphite test piece 3 (50 mmφ × 3 mm (long)) having an ash content of 20 ppm or less at 1680 ° C. in an argon atmosphere in the manner shown in FIG. 4 (50 mmφ × 5 mm (long)) was brought into contact with the load 5 (50 gf) for 16 hours, and the warp d of the graphite test piece 3 shown in FIG.
[0019]
That is, the graphite crucible made of graphite whose warpage measured in the above (B) is 0.4 mm or less found by the inventor of the present invention has a small degree of deformation in the CZ method, and before the life of the graphite crucible is reached due to the thinning of the graphite crucible. Deformation that makes continuous use impossible is less likely to occur. On the other hand, in the case of graphite having a warpage of more than 0.4 mm due to the above (B), the deformation of the graphite crucible in the CZ method becomes large, and the probability that the graphite crucible becomes unusable before deformation due to the deformation of the graphite crucible before the wall thinning life. Is greatly increased. In order to more reliably prevent deformation of the graphite crucible before the life of the reduced thickness, it is desirable that the warp of graphite used as the graphite crucible by the B method is 0.35 mm or less.
[0020]
Next, a method for manufacturing the graphite crucible (raw material) of the present invention will be described.
The material of the graphite crucible in the present invention, a binder material such as fine-powder coke and tar pitch, is charged into a kneader at the same time as coke without dividing and charging the binder material, and then kneaded at a melting temperature of the binder material and then cooled. Alternatively, the secondary powder can be manufactured by compression-molding the secondary pulverized powder in a rubber press in water, firing, and graphitizing as it is without passing through a pitch impregnation / recarbonization step.
[0021]
Hereinafter, specific manufacturing conditions and the like will be described.
First, as raw coke, coke having an average thermal expansion coefficient between room temperature and 500 ° C. of 4.5 to 5.4 × 10 ⁻⁶ / ° C. is finely pulverized. At this time, the particle size of the finely ground coke powder is 15 ± 3 μm in average particle size, and the maximum particle size is preferably 200 μm or less.
The properties of the binder such as tar pitch to be added and kneaded to coke are desirably those having a softening temperature of 100 to 110 ° C and a toluene-insoluble content of 29% or more and a quinoline-insoluble content of 8 to 13%. The addition rate of the binder is preferably in the range of 50 to 70 parts by weight with respect to 100 parts by weight of coke.
The mixture of coke and the binder is kneaded at a temperature equal to or higher than the melting temperature of the binder, and the volatile content is adjusted to 8.5 to 9.5% by measurement at 900 ° C. for 7 minutes.
[0022]
Next, the kneaded mixture is re-pulverized (secondary pulverization), and the average particle size at this time is preferably “primary pulverization particle size to primary pulverization particle size + 10 μm” and the maximum particle size is preferably 500 μm.
The re-ground material is filled in rubber as a raw material, and is uniformly compression-molded in water (CIP molding). The CIP molding pressure at this time is 0.5 to 1.0 t / cm ² (4.9 × 10 ⁷ to 9.8 × 10 ⁷ N / m ² ). After CIP molding, it is taken out of the rubber, fired at a firing temperature of 700 to 1100 ° C., and then graphitized at 2800 ° C. or higher without being subjected to pitch impregnation and recarbonization as is usually performed. The material is obtained.
[0023]
The graphite material has a three-point bending strength of 40 MPa or more, an average pore diameter measured by a mercury intrusion method of 3.5 to 7.5 μm, and a gas permeability of 1.0 to 2.5 permeability of nitrogen gas at ordinary temperature. (Centidarcys), wherein the plastic yield stress of graphite according to (A) above is 4 MPa or less, and the warpage due to surface siliconization by (B) 0.4 mm or less has physical property values of 0.4 mm or less. Used as the graphite crucible of the invention. A graphite crucible can be obtained by appropriately cutting a graphite material having such physical properties into a graphite crucible having a predetermined shape and dimensions for producing a silicon single crystal.
[0024]
[Action]
In the graphite crucible for the CZ method according to the present invention, since the plastic yield stress due to (A) at 1680 ° C. is particularly 4 MPa or less, the number of operations of the CZ method is repeated, and the strain accompanying the SiC conversion of the inner surface of the graphite crucible becomes large. Also, plastic yielding easily occurs, and the tensile stress generated in the graphite crucible decreases. In addition, since the graphite crucible has a three-point bending strength of 40 MPa or more, the breakage frequency of the graphite crucible before the life of reducing the wall thickness is greatly reduced. Furthermore, since the warpage due to the surface silicon carbide conversion at 1680 ° C. measured in (B) is 0.4 mm or less, the degree of deformation of the graphite crucible caused by increasing the number of operations of the CZ method also decreases. As a result, the CZ graphite crucible according to the present invention has significantly reduced deformation and breakage before the reduced thickness life, and has excellent durability.
[0025]
【Example】
Examples 1-3, Comparative Examples 1-5
The raw coke and the binder were each adjusted, melt-kneaded, then secondary-crushed, then CIP-molded by a conventional method, calcined, and graphitized, and the characteristic values of the test pieces cut out from the obtained graphite material are shown in Table 1 with those of Examples. This is shown for each comparative example. However, Comparative Examples 2, 4, and 5 were manufactured by performing pitch impregnation and recarbonization after firing, whereas Examples 1-3 and Comparative Example 1.3 were pitch impregnated and recarbonized after firing. It was obtained without performing any treatment.
[0026]
[Table 1]

[0027]
The same material as the graphite (high-purity treated product: less than 10 ppm ash) of Examples 1 to 3 and Comparative Examples 1 to 5 shown in Table 1 was cut to produce graphite crucibles for the 18-inch CZ method, respectively. An actual machine test was performed using a silicon single crystal pulling apparatus. Table 2 shows the number of service life and the cause of life of the graphite crucible. From the results shown in Table 2, all of the graphite crucibles of Examples 1 to 3 did not undergo deformation or breakage before repeated use (wall thinning life) of 50 times, and the service life was improved by 20% or more. Was done. In contrast, all of the graphite crucibles of Comparative Examples 1 to 5 were deformed or damaged before reaching the crucible thinning life.
[0028]
[Table 2]

[0029]
【The invention's effect】
As described above, by using the graphite crucible of the present invention, in a silicon single crystal pulling apparatus using a rotation pulling method called a CZ method, breakage and deformation before a reduced life of the graphite crucible is reduced, and graphite having excellent durability is obtained. Crucibles can be provided. Thereby, the silicon single crystal (or silicon polycrystal for a solar cell) can be stably pulled for a long time, and an industrially advantageous effect is brought about.
[0030]
[Brief description of the drawings]
FIGS. 1 (a), 1 (b) and 1 (c) are illustrations showing the image of distortion generated in graphite as the thickness of a SiC layer progresses.
2 (a) is a schematic diagram showing a measurement procedure in a method (A) for measuring the plastic yield stress of graphite, FIG. 2 (b) is a stress distribution diagram due to plastic yield of graphite, and FIG. It is explanatory drawing which shows the deformation state of a piece.
FIG. 3 (a) is a schematic view showing a measurement procedure in a method (B) for measuring the warpage of graphite, and FIG. 3 (b) is an explanatory view showing a warped state of a graphite test piece.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Graphite test piece 2 Load a Linear part b Curved part c Length of the boundary between linear part a and curved part b 3 Graphite specimen for warpage measurement 4 Quartz glass 5 Load d Warpage

Claims

The three-point bending strength is 40 MPa or more, the average pore diameter measured by a mercury intrusion method is 3.5 to 7.5 μm, and the gas permeability of nitrogen gas at room temperature is 1.0 to 2.5 permeability (centidarcys). A silicon single crystal formed of a graphite crucible material having a plastic yield stress at 1680 ° C. of 4 MPa or less, and a warp due to surface siliconization according to the following (B) of 0.4 mm or less. Graphite crucible for manufacturing.
(B): At 1680 ° C., a 50 mmφ × 5 mm (length) quartz glass layer was superimposed on a 50 mmφ × 3 mm (length) high-purity graphite test piece, and contacted for 16 hours under a load equivalent to a load of 50 gf. It is defined as the warpage (mm) of the test piece after surface siliconization.

50 to 70 parts by weight of a binder is blended with 100 parts by weight of a coke pulverized product having an average coefficient of thermal expansion between room temperature and 500 ° C of 4.5 to 5.4 × 10 ⁻⁶ / ° C. The mixture was kneaded to adjust the volatile content and then re-ground. The resulting material was molded by a rubber press method (CIP molding method) and then fired at 700 to 1100 ° C., and then 2800 ° C. without pitch impregnation and re-carbonization. A method for producing a graphite crucible for producing a silicon single crystal, characterized by being graphitized as described above.