JP3797643B2 - Crystal production equipment - Google Patents

Description

るつぼが下方向に移動した場合、ヒーター以外の温度は変化しないとして式４の両辺を全微分する。
【００３２】

Ｆ_a-1＋Ｆ_a-2＋Ｆ_a-3＝１かつＦ_b-1＋Ｆ_b-2＋Ｆ_b-5＝１、Ｓ_a＝Ｓ_b、Ｓ_a・Ｆ_a-1＝Ｓ₁・Ｆ_1-a等を代入する。その際、実際の形状はるつぼとヒーターの距離がヒーターの半径に比べて十分小さく、またＦ_a-1とＦ_a-2がＦ_a-3やＦ_a-4に比べて十分小さいので、ΔＦ_a-1＝ΔＦ_a-2＝０として、次に示す式５が得られる。
（ΔＪ_a＋ΔＪ_b＝［（Ｊ₃−Ｊ_a）ΔＦ_a-3＋（Ｊ₄−Ｊ_a）・ΔＦ_a-4）］（式５）
また、面Ａに関しては面３（断熱材の表面）を見込む立体角が減った分だけ、面４（るつぼの側面）を見込む立体角が増加するので、ΔＦ_a-3＝−ΔＦ_a-4と仮定することにより、次に示す式６が求まる。
【００３３】
（ΔＪ_a＋ΔＪ_b）＝（Ｊ₄−Ｊ₃）・ΔＦ_a-4 （式６）
ここで、Ｊ_i＝ε_i・Ｅ_i＋（１−ε_i）・Ｇ_iであり、第２項は周囲の面から入射したエネルギーを反射する項である。反射された光線が再び周囲の面を照射するために、ＪとＧを求めるには周囲の詳細な形状を求める必要があり複雑になってしまう。幸い射出率ε_iはカーボンで概ね０．８であるので、式を簡単化するためにε_i＝１とする。更に、ヒーターの温度は両面とも等しくＴ₀とすると、次に示す式７が得られる。
【００３４】
８・Ｔ₀ ³・ΔＴ＝（Ｔ₄ ⁴−Ｔ₃ ⁴）・ΔＦ_a-4
ΔＦ_a-4＝（８・Ｔ₀ ³／（Ｔ₄ ⁴−Ｔ₃ ⁴））・ΔＴ （式７）
但し、Ｔ₃，Ｔ₄は各々面３（るつぼ下の断熱材）と面４（るつぼの側壁）の温度とする。
【００３５】
Ｔ₄＞Ｔ₃なので、式７はるつぼとヒーターとの間の形態係数Ｆ_a-4が増加すればヒーターの温度Ｔが上昇することを示している。Ｔ₀＝Ｔ₄＝１７００Ｋ、Ｔ₃＝１５００Ｋとし、温度変化の許容量ΔＴを５Ｋとすると、
ΔＦ_a-4＜０．０６ （式８）
となる。したがって、るつぼが下降してもヒーターの温度を一定に保つ条件は、ヒーターとるつぼとの間の形態係数が０．０６以下しか変化しないるつぼ構造ということになる。
【００３６】
次に、図１３を参考しながら、るつぼとヒーターの半径から、実際のるつぼ構造を求める。図１３に示す様に、ヒーターの一部αより下方にＬだけ長いるつぼを考える。ヒーターの一部αより上方のるつぼの大きさは十分長いとして、るつぼが移動しても上方のるつぼの形態係数の変化は無いものとする。るつぼの半径ｒ_c、ヒーターの半径ｒ_hとすると、ヒーターの一部αとるつぼの形態係数Ｆ_aは、以下の式で表わせる。
【００３７】
Ｆ_a（Ｘ，Ｙ）＝Ｆ_a'（Ｘ，Ｙ）＋Ｆ_a'（∞，Ｙ） （式９）
但し、Ｘ、Ｙ、Ｆ_a'（Ｘ，Ｙ）は以下に示す通りである。
【００３８】

である。図１５は、ヒーターの一部αの内側の面ａとるつぼ全体との間の形態係数Ｆ_aを示すグラフである。るつぼの半径とヒーターの半径との比Ｙが１．２０の場合、例えば、各々の半径が１００ｍｍと１２０ｍｍの構造の形態係数を示すものである。横軸にＬとるつぼの半径ｒ_cの比Ｘを、縦軸に形態係数Ｆ_aをとった。
【００３９】
るつぼが十分下降した（Ｌ＝∞）場合の形態係数として図１５のＸ＝１．０の形態係数０．８３を使用し、更に、式８からるつぼの下降開始時で、Ｆ_a（Ｘ，１．２０）＝０．８３−０．０６＝０．７７以上の１を選べば、温度変化ΔＴを５Ｋ以下にすることが可能となる。図１５から、Ｆ_a＝０．７７のＸ（＝Ｌ／ｒ_c）は０．３なので、よって、Ｌ＝０．３・ｒ_c＝３０ｍｍが求められる。
【００４０】
同様に、種々のヒーターの半径に対して、ヒーターの温度変化ΔＴを５Ｋ，１０Ｋ，１５Ｋ以下にすることができるＬを求めたものを図１６のグラフに実線で示す。これらのグラフを１次近似する。
【００４１】
Ｘ＝const（ΔＴ）・（Ｙ−１）
さらに、const（ΔＴ）を複数のΔＴに対して求めて、最小２乗近似し式１０を使ってＸ，Ｙを書き直すとヒーターの温度変化をΔＴ以下にする条件式１２が得られる。
【００４２】
Ｌ≧ｒ_c・（Ｃ₂・ΔＴ²＋Ｃ₁・ΔＴ＋Ｃ₀）・（ｒ_h／ｒ_c−１）（式１２）
但し、Ｃ₂＝０．００４、Ｃ₁＝−０．１６６、Ｃ₀＝２．２２である。
【００４３】
式１２を使用して、得られるＸの値を図１６中に、×印で示した。式１２は、るつぼとヒーターの半径（それぞれｒ_c、ｒ_h）と許容される温度変動値ΔＴから、固液界面より下方のるつぼの大きさの最低値を決定するのものである。温度の変動値ΔＴは、できた結晶の性質の均一性から決定される。
【００４４】
【発明の実施の形態】
以下では、本発明に係る結晶作製装置の実施の形態について詳述する。なお、第１〜７の実施の形態には装置の基本構成例を示す。
【００４５】
（第１の実施の形態）
本発明の第１の実施の形態を図１に示す。図１は本発明に係る結晶作製装置の模式的な断面図であり、炉内が第１の領域（高温領域）と第２の領域（低温領域）に分けられ、それぞれ独立したヒーターによって温度が制御がされている炉において、本発明を適応した一例を示す断面図である。
【００４６】
図１において、１ａと１ｂはそれぞれは第１と第２のヒーター、２ａと２ｂはそれぞれ第１と第２の熱電対、３は本るつぼであり、本るつぼの底が結晶成長の開始点となる。４は本るつぼ内に入れられた原料であるＣａＦ₂、５ａと５ｂはそれぞれ第１と第２の電源、６ａと６ｂはそれぞれ第１と第２の温度測定器、７は電源制御系である。８はるつぼ支持棒、９はるつぼ下断熱材、１０は補助るつぼ、１１は補助原料であるＣａＦ₂、１４はチャンバー、１５は断熱材である。
【００４７】
チャンバー１４は、その内部が減圧できる真空チャンバーであり、ステンレス等の金属で作製されており、不図示の真空ポンプに接続されチャンバー内は１０^-5Ｔｏｒｒから１０^-6Ｔｏｒｒの真空度に減圧される。
【００４８】
断熱材１５は、真空チャンバー１４の内壁に取り付けられ、チャンバー内の熱が外部に逃げないようにするために用いた。
【００４９】
ヒーター１ａと１ｂは、カーボン等で構成され、それぞれ５ａと５ｂの電源を通して通電することで発熱する。そして、発生した熱はヒーター内に配置されたるつぼ３に輻射によって供給される。
【００５０】
るつぼ３は、作製する結晶の原料４と反応しないカーボン等の物質からなっており、るつぼ３内には原料４であるＣａＦ₂が入っている。そして、るつぼ３の下には、補助原料９が入っている補助るつぼ１０が配置されている。補助原料１１はＣａＦ₂であるが、るつぼ３と補助るつぼ１０の間は隔壁で完全に仕切られているため、補助原料１１が本るつぼ３の種結晶となることはない。したがって、本るつぼ３の最下部が本るつぼ３における結晶の成長開始点となる。さらに、補助るつぼ１０の下には、るつぼを支える支持棒８が取り付けられている。支持棒８は不図示の昇降機構に取り付けられ、この昇降機構を駆動することによってるつぼ３は上下動される。
【００５１】
熱電対２ａは、本るつぼ３の低部付近の温度が測定されるように、ヒーター１ａに取り付けらている。熱電対２ｂは、本るつぼ３の底と補助るつぼ１０の底の間もしくは補助るつぼ１０の底部付近の温度が測定されるように、ヒーター１ｂに取付けられている。
【００５２】
ヒーター１ａは、電源５ａから電力が供給される。また、熱電対２ａの電圧信号は測定器６ａによって温度信号に変換され制御系７に送られる。そして、制御系７は電源５ａを制御し、ヒーター１ａへの投入電力を制御する役割を担っている。同様に、ヒーター１ｂは、電源５ｂによって電力を供給される。また、熱電対２ｂの電圧信号は測定器６ｂによって温度信号に変換され制御系７に送られる。そして、制御系７は電源５ｂを制御し、ヒーター１ｂへの投入電力を制御する。
【００５３】
以下では、図１に示した構成からなる装置の動作について、結晶の作製手順に従って説明する。
【００５４】
（１）初め熱電対２ａの温度Ｔａが結晶の融点の上、即ちＣａＦ₂の融点１３６０℃より高い温度になるように、ヒーター１ａへの入力が制御される。熱電対２ｂの温度Ｔｂが結晶の融点の下、即ちＣａＦ₂の融点１３６０℃より低い設定になるように、ヒーター１ｂへの入力が制御される。
【００５５】
（２）ヒーターからの熱を受けて本るつぼ３は暖められ、やがて本るつぼ３内の原料４はＣａＦ₂の融点に達し溶けはじめ、ついには本るつぼ３内の原料４は完全に融解する。
【００５６】
（３）さらに、補助るつぼ１０内の補助原料１１のＣａＦ₂の一部も融解する。この状態で、十分時間が経過し融液が安定するまで放置される。補助るつぼ１０の最下部はＣａＦ₂の融点より低い温度に設定されているために、補助るつぼ１０内の補助原料１１のＣａＦ₂は時間が十分経過しても補助るつぼ１０の底まで完全に融解することはない。
【００５７】
（４）融液が安定したら、徐々にるつぼを引き下げはじめる。
その際、補助るつぼ１０の内部の補助原料１１は完全に融けていないので、結晶成長開始点が複数できる可能性があり、単結晶のできる可能性は低い。また、結晶成長の開始点である本るつぼ３の下部と補助るつぼ１０の上部は仕切られているので、補助るつぼ１０の結晶性が本るつぼ３の結晶性に影響を及ぼすことはない。
【００５８】
次に、温度変動に関して考察する。固液界面付近である位置Ａのヒーターの温度は、本るつぼ３が引き下げられてもほとんど変動しない。何故ならば、位置Ａのヒーター１ａは本るつぼ３の移動開始以前から本るつぼ３と面しており、本るつぼ３が引き下げられても、本るつぼ３と位置Ａのヒーター１ａとの位置関係は変わらないからである。しかし、初め本るつぼ３が内側になかった位置Ｃのヒーター１ｂの温度は、本るつぼ３が下降するにしたがって急激に上昇する。しかし、位置Ｃの温度は融点より十分低く位置も固液界面から十分離れているので、位置Ｃの温度変動が、固液界面付近の位置Ａにあるヒーター１ａの温度に影響を及ぼす可能性は小さい。
【００５９】
また、上述したように固液界面付近の温度は本るつぼ３の位置に依存しないので、固液界面の上下に設けられた熱電対２ａと２ｂの出力の変動は、結晶作製装置の環境の変化によるものと考えてよい。従って、熱電対２ａと２ｂの出力が一定になるように、制御系７によって電源５ａと５ｂを制御し、ヒーター１ａと１ｂへの投入電力を制御することによって、さらに本るつぼ３の温度を一定に保てるので、結晶作製装置の環境が変化しても安定した結晶の成長が可能となる。
【００６０】
ゆえに、本発明に係る装置では、るつぼが移動しても固液界面付近の位置および温度勾配は変動しない、言い換えれば、るつぼの移動に伴い、るつぼに入れた原料の固液界面付近を、ヒーター側から見た立体角が変わらないので、安定した結晶の作製が可能となる。さらに、熱電対の出力を一定に保つことにより結晶作製装置の環境の影響をも排除することが可能となる。
【００６１】
（第２の実施の形態）
図２は、本発明の第２の実施の形態に係る結晶作製装置の要部、すなわち図１における本るつぼ３、補助るつぼ１０及びこれらに付属する部分のみを示した模式的な断面図である。図２の装置は、成長させる結晶の方位を制御するために、本るつぼの下部に種結晶を入れるための本るつぼの直径より細い円筒形の部分を設けた点が図１の装置と異なっている。他の点は、図１の装置と同様とした。
【００６２】
図２の装置では、本るつぼ３の下部に本るつぼ３の直径より細い円筒形の部分２０を設ける。そして、円筒形の部分２０の周囲、すなわち本るつぼ３の下部に、補助るつぼ１０を取り付け、補助るつぼ１０の内部には、補助原料１１を入れる。
【００６３】
本るつぼ３において結晶を成長する際に用いる種結晶は、所望の方位の結晶が成長できるように円筒形の部分２０内に入れる。そして、円筒形の部分２０の上部の温度は原料４の融点より高く、円筒形の部分２０の下部の温度は原料４の融点より低くなるように、ヒーター（不図示）のパワーを制御系（不図示）で調整する。このような温度制御によって、円筒形の部分２０の内部の種結晶は上部が溶け下部が結晶の状態となる。
【００６４】
このような状態に種結晶をしてから、第１の実施の形態と同様に、本るつぼ３を引き下げることによって結晶の成長が開始される。本るつぼ３における固液界面に対応する温度領域は補助るつぼ１０の内部にあるので、本るつぼ３が移動しても固液界面付近の位置および温度勾配は変動しない。さらに、円筒形の部分２０の内部の種結晶の下部は溶けていないので、種結晶と融液の界面を成長の開始点として結晶が成長する。従って、図２の装置では、所望の方位の結晶が安定して得られる。
【００６５】
上述した実施の形態では、補助るつぼ１０内にいれる補助原料１１としてＣａＦ₂を用いたが、補助原料１１は、必ずしもＣａＦ₂である必要は無く、熱容量（Ｃ_p）と密度（ρ）との積及び熱伝導率（λ）がＣａＦ₂と同等の材料であればよい。
【００６６】
従って、補助原料１１としては、表１に示すようなＡｌ₂Ｏ₃やＭｇＯなどのセラミックが使用できる。
【００６７】
【表１】

【００６８】
さらに、本るつぼ３の引き下げ速度が十分小さい場合には、熱容量（Ｃ_p）と密度（ρ）との積のみＣａＦ₂と同等な物質を、補助原料１１として用いても構わない。
【００６９】
（第３の実施の形態）
図３は、本発明の第３の実施の形態に係る結晶作製装置の要部、すなわち図１における本るつぼ３、補助るつぼ１０及びこれらに付属する部分のみを示した模式的な断面図である。図３の装置は、原料４としてＣａＦ₂の融点より高温の融点をもつ材料を補助材料１１として使用した点が図１の装置と異なっている。他の点は、図１の装置と同様とした。
【００７０】
この場合には、本るつぼ３の位置に関係無くヒーターの熱流出量を等しくするために、図３に示す様にヒーター（不図示）と向かい合う面は本るつぼ３の輻射率と同じ材料で覆う必要があるが、下部は補助原料１１が剥きだしでもよい。
【００７１】
（第４の実施の形態）
図４は、本発明の第４の実施の形態に係る結晶作製装置の要部、すなわち図１における本るつぼ３、補助るつぼ１０及びこれらに付属する部分のみを示した模式的な断面図である。図４の装置は、補助原料１１の形状の簡単化を図った点が図１の装置と異なっている。他の点は、図１の装置と同様とした。
【００７２】
図４に示す様に、本るつぼ３の底を平面（図４）にすることによって、本るつぼ３及び補助るつぼ１０の作製が容易な結晶作製装置が得られる。このように、本発明は、本るつぼ３の形状に依存せず様々な形状のるつぼに適応できる技術である。
【００７３】
（第５の実施の形態）
図５は、本発明の第５の実施の形態に係る結晶作製装置の要部、すなわち図１における本るつぼ３、補助るつぼ１０及びこれらに付属する部分のみを示した模式的な断面図である。図５の装置は、本るつぼ３を複数の領域に分かれているディスク型るつぼとした点が図１の装置と異なっている。他の点は、図１の装置と同様とした。図５において、１２はダミーるつぼであり、この領域で成長した結晶が、上の領域の結晶の種となる構造である。実施例１と同様にダミーるつぼ１２の下部に補助るつぼ１０が取り付けられ、補助るつぼ１０の上部と底部との間に補助原料１１の融点がくるようにヒーターへの投入電力が調整されている。このような構成のるつぼにも、本発明は十分に適用できる。
【００７４】
（第６の実施の形態）
図６は、本発明の第６の実施の形態に係る結晶作製装置の要部、すなわち図１における本るつぼ３、補助るつぼ１０及びこれらに付属する部分のみを示した模式的な断面図である。図６の装置は、本るつぼ３を複数の領域に分かれているディスク型るつぼとし、さらに本るつぼ３で成長させる結晶の方位を制御するために、ダミーるつぼの代わりに本るつぼ３の底に円筒形の部分２０を取り付けた点が図１の装置と異なっている。他の点は、図１の装置と同様とした。
【００７５】
図６の装置では、所望の方位の結晶が成長できるように円筒形の部分２０内に種結晶を入れる。ここで、円筒形の部分２０の上部の温度が融点より高く、円筒形の部分２０の下部の温度が融点より低くなるように、ヒーター（不図示）のパワーを制御して用いる。
【００７６】
（第７の実施の形態）
図７は、本発明の第７の実施の形態に係る結晶作製装置の模式的な断面図である。図７の装置は、ヒーターが分割されておらず一つの構造体である点が図１の装置と異なっている。他の点は、図１の装置と同様とした。
【００７７】
図７の装置では、ヒーター１ａの最も低い位置より、補助るつぼ１０の底が低くなるようにヒーター１ａとるつぼ３及び補助るつぼ１０との相対的な位置関係を設定した。この状態において、ヒーター１ａの最低部の温度が原料４の融点より低く、かつ、本るつぼ３の最低部の温度が原料４の融点より高くなるように、ヒーターの投入電力を制御する。このような構成により、るつぼ３が引き下げられても、ヒーター１ａの全ての点は本るつぼ３の引き下げ前から、本るつぼ３もしくは補助るつぼ１０と面しているために、ヒーター１ａの温度分布は変化しない。よって、上述した本発明の作用が得られる。
【００７８】
（第８の実施の形態）
図１７は、本発明の第８の実施の形態に係る結晶作製装置の模式的な断面図である。図１７の装置は、補助るつぼの代わりに、るつぼ３の底が厚い即ち結晶の成長点であるＣとるつぼ３の底との距離を大きく取ることで、固液界面近辺の温度変化を小さくした点が図１の装置と異なっている。他の点は、図１の装置と同様とした。
【００７９】
図１７の装置では、るつぼ３の外周の半径をｒ_c、ヒーターの内側の半径をｒ_hとし、るつぼ３の大きさと設置位置を決める。温度が融点であるヒーターの位置をＡ、位置Ａとるつぼの底の高さの差をＬとする。るつぼ３が下降しても位置Ａの温度上昇ΔＴが５Ｋ以下の変動にするには、式１２からＬを求めればよい。例えば、ｒ_c＝１００ｍｍ、ｒ_h＝１２０ｍｍの場合、ｒ_h／ｒ_c＝１．２となるので、
Ｌ≧０．３・１００ｍｍ＝３０ｍｍ
となる。
【００８０】
よって、るつぼの底の厚さは３０ｍｍ以上例えば４０ｍｍとして、位置Ａとるつぼ３の底の距離Ｌが３０ｍｍとなるようにるつぼ３の位置とヒーターの温度が調節され、原料をいれたるつぼ３が設置されて原料が溶かされる。結晶成長の開始点Ｃは位置Ａより高い位置にあるので、結晶成長の開始点でも原料は溶ける。るつぼ内の原料が十分溶かされ安定した後、るつぼが徐々に引き下げられる。そして、融点の位置Ａに結晶成長の開始点Ｃが達したとき、結晶成長が開始される。るつぼが下降してきても、位置Ａの温度変動は５Ｋ程度しか変動しないので、固液界面の位置は変化せず安定した結晶成長が可能となる。
【００８１】
【発明の効果】
以上説明したように、本発明に係る結晶作製装置は、るつぼの移動に伴い、該るつぼに入れた原料の固液界面付近を、ヒーター側から見た立体角が変わらないので、るつぼの移動開始前後で、ヒーターとるつぼに入れた原料の固液界面との位置関係を所定の位置関係に保持することができる。
【００８２】
従って、るつぼに入れた原料の固液界面の位置と、固液界面に対して垂直方向の温度勾配とを一定に保つことが可能な結晶作製装置が得られる。
【００８３】
上記装置を用いることにより、屈折率の均質性に優れた大口径の単結晶光学材を安定して製造することが可能となる。
【図面の簡単な説明】
【図１】第１の実施の形態に係る結晶作製装置の模式的な断面図である。
【図２】第２の実施の形態に係る結晶作製装置の要部を示した模式的な断面図である。
【図３】第３の実施の形態に係る結晶作製装置の要部を示した模式的な断面図である。
【図４】第４の実施の形態に係る結晶作製装置の要部を示した模式的な断面図である。
【図５】第５の実施の形態に係る結晶作製装置の要部を示した模式的な断面図である。
【図６】第６の実施の形態に係る結晶作製装置の要部を示した模式的な断面図である。
【図７】第７の実施の形態に係る結晶作製装置の模式的な断面図である。
【図８】従来の結晶作製装置の一例を示す模式的な断面図である。
【図９】図８の装置において、原料が溶融状態になったときのヒーターの温度分布を示すグラフである。
【図１０】従来の結晶作製装置の他の一例を示す模式的な断面図である。
【図１１】図１０の装置において、るつぼが上方にあった場合の温度分布（実線ａ）と、るつぼが下降してきた場合の温度分布（実線ｂ）とを示すグラフである。
【図１２】図１０の装置において、るつぼの移動に伴う、位置Ａ及びＢのヒーターの温度変化を示すグラフである。
【図１３】るつぼとヒーターとの位置関係を示す模式図である。
【図１４】るつぼ及びヒーターからなるチャンバの模式的な断面図であり、ヒーターと周囲の壁との熱収支を説明する図面である。
【図１５】るつぼの構造に対する形態係数を示すグラフである。
【図１６】炉の構造とヒーターの温度変化との関係を示すグラフである。
【図１７】第８の実施の形態に係る結晶作製装置の模式的な断面図である。
【符号の説明】
１ａ、１ｂ ヒーター、
２ａ、２ｂ 熱電対、
３ 本るつぼ、
４ 原料、
５ａ、５ｂ 電源、
６ａ、６ｂ 温度測定器、
７ 電源制御系、
８ つぼ支持棒、
９ るつぼ下断熱材、
１０ 補助るつぼ、
１１ 補助原料、
１２ ダミーるつぼ、
１４ チャンバー、
１５ 断熱材、
２０ 円筒形の部分。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a crystal manufacturing apparatus. More specifically, the present invention relates to a crystal manufacturing apparatus capable of keeping a position of a solid-liquid interface of a raw material put in a crucible and a temperature gradient in a direction perpendicular to the solid-liquid interface constant.
[0002]
[Prior art]
In recent years, as the integration of semiconductors progresses, semiconductor printing apparatuses used in the semiconductor manufacturing process are required to have higher resolution. In order to meet this requirement, semiconductor baking apparatuses have come to use excimer lasers with short wavelengths such as KrF lines (248 nm) and ArF lines (193 nm) as light sources. Along with this, since the amorphous optical material used conventionally cannot transmit 193 nm light, fluorite (CaF) can be used as an alternative to the amorphous optical material.₂) Came to be expected. In addition, in order to achieve a high resolving power, a single crystal having a large diameter is also required for fluorite as an optical material.
[0003]
Conventionally, single crystal optical materials such as fluorite have been manufactured by the crucible descent method (Bridgeman method). As a typical crystal production apparatus, for example, an apparatus described in US Pat. No. 2,214,976 (FIG. 8) can be cited.
[0004]
In the apparatus having the configuration shown in FIG. 8, two upper and lower heaters 1a and 1b are attached, and these are controlled independently. And the thermocouple 2a was attached to the upper part of the heater 1a, and the input of the heater was controlled so that the temperature became constant. With such a configuration, a constant temperature can be maintained even if the environment (for example, temperature or voltage) of the crystal manufacturing apparatus changes. The apparatus shown in FIG. 8 includes a chamber 14, a heat insulating material 15 attached to the inner wall of the chamber, and graphite heaters 1 a and 1 b disposed inside the chamber 14, and the crucible support rod 8 that penetrates the chamber 14. Is installed to support the crucible.
[0005]
Hereinafter, a method for manufacturing a crystal using the apparatus of FIG. 8 will be described.
[0006]
First, CaF which is raw material 4 of fluorite₂Is put in the crucible 3 and installed in a place surrounded by the heater 1a. Next, the raw material 4 is brought into a molten state by applying heat from the heaters 1 a and 2 a to the crucible 3 to a temperature exceeding the melting point of the raw material 4 (about 1360 ° C. in fluorite).
[0007]
A curve a shown in FIG. 9 shows a temperature distribution of the heater when the raw material 4 is in a molten state. In FIG. 9, the vertical axis represents the position in the chamber, and the horizontal axis represents the heater temperature. From FIG. 9, it can be seen that the apparatus of FIG. 8 has a structure in which the temperature rapidly decreases at the lower end α of the heater 1a. Therefore, in the apparatus shown in FIG. 8, the input powers of the heaters 1a and 2a are adjusted so that the freezing point of the crystal comes near that point and has an appropriate temperature gradient.
[0008]
In the apparatus of FIG. 8, the inside of the chamber 14 is 10 by a vacuum pump (not shown).^-FiveTorr to 10^-6A vacuum of about Torr is maintained. Crystal growth is performed by lowering the crucible 3 at a low speed of about several mm / hour (sometimes accompanied by rotation). The crucible 3 gradually escapes from the space surrounded by the heater 1 a and is cooled from below the crucible 3. Along with this, the crystallization of the raw material 4 in a molten state starts from the bottom of the low temperature, and is crystallized when the solid-liquid interface that is the growth point of the crystal that is the boundary between the solid and the liquid reaches the top of the melt. The conversion ends.
[0009]
In the above-described apparatus, the size of the crystal to be produced is not so large. For example, when the thickness of the crystal is about several tens of millimeters, the temperature distribution of the heater hardly changes even if the crucible containing the raw material moves downward. So there was no problem.
[0010]
However, if the diameter of the crucible is increased to obtain a large-diameter crystal, the heat capacity of the crucible and the raw material contained therein becomes larger than the heat capacity of the heater. For this reason, it was found that when the crucible descends, the temperature distribution of the heater changes depending on the position of the crucible even if the input power to the heater is constant.
[0011]
In addition, since the crystal growth rate depends on the vertical temperature gradient (dT / dy) near the solid-liquid interface, the position of the solid-liquid interface and the crystal growth rate change depending on the position of the crucible. It became clear that stable crystal growth could not be expected.
[0012]
Furthermore, even if the temperature at a specific position is measured with a thermocouple to control the input power of the heater, the factor of whether the temperature changes due to the movement of the crucible or the environment (temperature and voltage) of the crystal manufacturing equipment is separated. I couldn't. Therefore, it is difficult to apply feedback to the input power to the heater. Rather, keeping the input power to the heater constant irrespective of the measured temperature has resulted in a stable temperature.
[0013]
For example, in the apparatus shown in FIG. 10, when the crucible is above, the temperature distribution indicated by the solid line a in FIG. 11 is obtained. However, it was found that when the crucible descends, the temperature distribution changes to the solid line b in FIG. 11 and the solid-liquid interface that is the crystal growth point moves. In addition, even if the heater power is controlled so that the temperature measured by the thermocouple 2a is constant, the temperature distribution changes, so it is clear that the position of the solid-liquid interface cannot be kept constant. It became. Even if the temperature near the solid-liquid interface is measured and the input power is controlled so that the temperature does not change, it is difficult to keep the temperature gradient (dT / dy) in the vertical direction constant.
[0014]
[Problems to be solved by the invention]
An object of the present invention is to provide a crystal production apparatus capable of keeping a position of a solid-liquid interface of a raw material placed in a crucible and a temperature gradient in a direction perpendicular to the solid-liquid interface constant.
[0015]
[Means for Solving the Problems]
The crystal production apparatus of the present invention comprises:In an apparatus for producing a crystal by providing a crucible containing a raw material and a heater arranged so as to surround the crucible, and cooling the raw material from below the crucible by lowering the crucible with respect to the heater. The crucible length L below the temperature corresponding to the solid-liquid interface of the raw material placed in the crucible satisfies the following formula.
L≧r _c ・ (C ₂ ・ ΔT ² + C ₁ ・ ΔT + C ₀ ) ・ (R _h / R _c -1)
(However, C ₂ = 0.004, C ₁ = -0.166, C ₀ = 2.22 and r _c Crucible radius, r _h Is the heater radius and ΔT is the allowable temperature change of the heater)
[0016]
According to the above configuration, since the positional relationship with the solid-liquid interface of the raw material placed in the heater crucible does not change before and after the start of the movement of the crucible, the position of the solid-liquid interface of the raw material placed in the crucible and the solid-liquid interface On the other hand, the temperature gradient in the vertical direction can be kept constant. As a result, a crystal production apparatus suitable for producing a large-diameter single crystal optical material excellent in refractive index homogeneity can be obtained.
[0017]
The apparatus according to the present invention was born from the following research and development by the present inventors.
Using the conventional apparatus shown in FIG. 10, the temperature change of the heaters at positions A and B accompanying the movement of the crucible was examined.
[0018]
FIG. 12 is a graph showing the temperature change, where the horizontal axis represents the moving distance of the crucible and the vertical axis represents the temperature. The following points became clear from FIG.
[0019]
(1) Temperature T of the heater at position A lower than the bottom of the crucible_AIt was found that the temperature rapidly increased as the crucible moved. Before the movement of the crucible, the heater at position A did not face the side of the crucible, and the heater exchanged heat with the bottom of the crucible and the heat insulating material to reach thermal equilibrium. However, when the crucible moves downward and the crucible descends inside the heater at position A, the heater temperature T_AWas found to rise. This was considered because the temperature of the crucible was higher than the temperature of the heat insulating material and the temperature of the heater exchanging heat with the crucible increased.
[0020]
(2) On the other hand, the temperature T of the heater at position B higher than the bottom of the crucible_BFrom the beginning of the crucible movement, it was found that the temperature change with the movement of the crucible was small because the heater crucible was facing.
[0021]
From the results of (1) and (2) above, in order to prevent the movement of the solid-liquid interface accompanying the temperature change, the method of controlling the temperature of the heater so that the solid-liquid interface comes near the position B is considered desirable. . However, according to this method, the temperature change in the vicinity of the solid-liquid interface is reduced, but since the lowermost part of the crucible is below the solid-liquid interface, the lowermost part of the crucible cannot serve as a starting point for crystal growth. Accordingly, it has been found that in order to set the lowest part of the crucible as the starting point of crystal growth, it is necessary to control the temperature of the heater so that at least the bottom temperature is higher than the melting point.
[0022]
Therefore, the present inventor has studied a configuration in which the positional relationship between the heater and the crucible does not change before and after the crucible starts moving so that the temperature of the heater does not change with the movement of the crucible. Based on this idea, the present inventor has devised the apparatus of the present invention described above. For example, in the apparatus of FIG. 10, an auxiliary crucible is installed under the crucible, and the heater at position A faces the auxiliary crucible before the crucible is moved.
[0023]
Furthermore, the results of studying the structure of the auxiliary crucible are described below.
Referring to FIGS. 13 and 14, a crucible structure is obtained in which the temperature of the heater does not change even if the crucible moves.
[0024]
FIG. 13 is a schematic diagram showing the positional relationship between the crucible and the heater. A part α of the heater in the vicinity of the position of the solid-liquid interface shows a state at a height L from the bottom of the crucible. If the crucible is infinitely large upward and L is also infinitely large, even if the crucible moves downward and reaches the position of the broken line, the positional relationship of the crucible that exchanges heat with part of the heater α changes. do not do. In that case, even if the crucible moves, the temperature of the heater does not change.
[0025]
However, with the increase in L, there are drawbacks such as the furnace becoming too large and heat becoming difficult to escape. Furthermore, since it is necessary to sufficiently dissolve the raw material in the crucible conventionally, the position of the heater that has reached the temperature corresponding to the solid-liquid interface is lower than the bottom of the crucible (L <0) or L is considerably small ( L≈0) state.
[0026]
The basic structure of the present invention is to obtain a condition in which the crucible moves practically downward and the positional relationship of the crucible for exchanging heat with a part α of the heater does not change. First, consider the exchange of energy between part α of the heater and the surrounding surface.
[0027]
FIG. 14 is a schematic cross-sectional view of a chamber composed of a crucible and a heater, and is a view for explaining a heat balance between the heater and a surrounding wall. In FIG. 14, a part α (inner surface A, outer surface B) of the heater is a hatched surface, that is, surface 1 (ceiling) and surface 2 (chamber bottom), surface 3 (insulation material under the crucible). ), Surface 4 (crucible side wall), surface 5 (chamber inner wall) and heat exchange by radiation. Furthermore, the heat exchange is performed between the part α of the heater and the other part of the heater, but the heat exchange amount with the faces 1 to 5 is small, and the change in the heat exchange amount due to the movement of the crucible is small. Ignore it because it is small. In fact, the distance between the crucible and the heater is small compared to the radius of the heater, and is a relation that is hardly expected from one part of the heater to the other, and this is a reasonable assumption. Assuming that thermal equilibrium is reached, the following equation holds:
[0028]
Q = Q₁+ Q₂+ Q_Three+ Q_Four+ Q_Five          (Formula 1)
Here, Q is a calorific value per unit time of a part α of the heater, and Q_i(I = 1 to 5) is the amount of net heat exchange between surface A or surface B and surface i,
Q_i= (J_a・ S_a・ F_ai-J_i・ S_i・ F_ia)
+ (J_b・ S_b・ F_bi-J_i・ S_i・ F_ib)
It can be expressed as However, S_iIs the area of surface i, F_iaIs the ratio of the energy to reach the surface a with the energy that generates the surface i with the form factor, F_aiIndicates the proportion of energy that reaches the surface i with the energy emitted from the surface a.
[0029]
In addition, between the area S and the form factor F
S_i・ F_ia= S_a・ F_ai               (Formula 2)
There is a relationship. J_iIs the emissivity, and is the total radiation energy emitted from the surface i per unit time and unit area. Further, E, G, and ε have the following relationship.
[0030]
J_i= Ε_i・ E_i+ (1 · ε_i) G_i        (Formula 3)
Where ε_iIs the injection rate, E_iIs the black body injection amount E_i= Σ · T_i ^FourGiven in. σ is the Stefan-Boltzmann constant, T_iIs the absolute temperature of surface i. Furthermore, G_iIs the amount of incident light, and is the total radiation energy incident on the surface i per unit time and unit area. F_a-5= F_5-a= F_b-4= F_4-b= F_b-3= F_3-bSince = 0, the formula 1 is as follows.
[0031]

When the crucible moves downward, both sides of Equation 4 are fully differentiated assuming that the temperature other than the heater does not change.
[0032]

F_a-1+ F_a-2+ F_a-3= 1 and F_b-1+ F_b-2+ F_b-5= 1, S_a= S_b, S_a・ F_a-1= S₁・ F_1-aAnd so on. At that time, the actual shape is such that the distance between the crucible and the heater is sufficiently smaller than the radius of the heater, and F_a-1And F_a-2Is F_a-3Or F_a-4ΔF because it is sufficiently small compared to_a-1= ΔF_a-2Assuming = 0, the following formula 5 is obtained.
(ΔJ_a+ ΔJ_b= [(J_Three-J_a) ΔF_a-3+ (J_Four-J_a) ・ ΔF_a-4]] (Formula 5)
In addition, with respect to the surface A, the solid angle for which the surface 4 (side surface of the crucible) is expected to increase by the amount of the solid angle for which the surface 3 (surface of the heat insulating material) is expected to decrease._a-3= -ΔF_a-4As a result, Equation 6 shown below is obtained.
[0033]
(ΔJ_a+ ΔJ_b) = (J_Four-J_Three) ・ ΔF_a-4      (Formula 6)
Where J_i= Ε_i・ E_i+ (1-ε_i) ・ G_iThe second term is a term that reflects the energy incident from the surrounding surface. Since the reflected light beam irradiates the surrounding surface again, it is necessary to obtain the detailed shape of the surrounding area to obtain J and G, which is complicated. Fortunately, the injection rate ε_iIs approximately 0.8 for carbon, so to simplify the equation ε_i= 1. Furthermore, the temperature of the heater is the same on both sides.₀Then, the following Expression 7 is obtained.
[0034]
8.T₀ ^ThreeΔT = (T_Four ^Four-T_Three ^Four) ・ ΔF_a-4
ΔF_a-4= (8 ・ T₀ ^Three/ (T_Four ^Four-T_Three ^Four)) ・ ΔT (Formula 7)
However, T_Three, T_FourAre the temperatures of surface 3 (insulating material under the crucible) and surface 4 (side wall of the crucible), respectively.
[0035]
T_Four> T_ThreeTherefore, Equation 7 is a form factor F between the crucible and the heater._a-4It shows that the temperature T of the heater rises if the value increases. T₀= T_Four= 1700K, T_Three= 1500K, and the allowable temperature change ΔT is 5K,
ΔF_a-4<0.06 (Formula 8)
It becomes. Accordingly, the condition for keeping the heater temperature constant even when the crucible is lowered is a crucible structure in which the shape factor between the heater and the crucible changes only by 0.06 or less.
[0036]
Next, referring to FIG. 13, the actual crucible structure is obtained from the radius of the crucible and the heater. As shown in FIG. 13, consider a crucible having a length L below a part α of the heater. It is assumed that the size of the crucible above the part α of the heater is sufficiently long and that the shape factor of the upper crucible does not change even if the crucible moves. Crucible radius r_c, Heater radius r_hThen, form factor F of crucible part α of heater_aCan be expressed by the following equation.
[0037]
F_a(X, Y) = F_a'(X, Y) + F_a'(∞, Y) (Formula 9)
However, X, Y, F_a'(X, Y) is as follows.
[0038]

It is. FIG. 15 shows a form factor F between the inner surface a of the heater part α and the entire crucible._aIt is a graph which shows. When the ratio Y between the radius of the crucible and the radius of the heater is 1.20, for example, it indicates the form factor of a structure in which each radius is 100 mm and 120 mm. Radius r of L crucible on the horizontal axis_cThe ratio X of_aI took.
[0039]
As the form factor when the crucible is sufficiently lowered (L = ∞), the form factor 0.83 of X = 1.0 in FIG. 15 is used._aIf 1 of (X, 1.20) = 0.83−0.06 = 0.77 is selected, the temperature change ΔT can be made 5K or less. From FIG._a= 0.77 X (= L / r_c) Is 0.3, so L = 0.3 · r_c= 30 mm is required.
[0040]
Similarly, the solid line in the graph of FIG. 16 shows L obtained for various heater radii so that the temperature change ΔT of the heater can be 5K, 10K, or 15K or less. These graphs are approximated by first order.
[0041]
X = const (ΔT) · (Y−1)
Furthermore, when const (ΔT) is obtained for a plurality of ΔTs, and approximated to the least squares and X and Y are rewritten using Equation 10, Conditional Equation 12 that makes the heater temperature change equal to or less than ΔT is obtained.
[0042]
L ≧ r_c・ (C₂・ ΔT²+ C₁・ ΔT + C₀) ・ (R_h/ R_c-1) (Formula 12)
However, C₂= 0.004, C₁= -0.166, C₀= 2.22.
[0043]
Using Equation 12, the value of X obtained is shown in FIG. Equation 12 shows the radius of the crucible and the heater (r_c, R_h) And the allowable temperature fluctuation value ΔT, the minimum value of the size of the crucible below the solid-liquid interface is determined. The temperature fluctuation value ΔT is determined from the uniformity of the properties of the resulting crystal.
[0044]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the crystal manufacturing apparatus according to the present invention will be described in detail.The first to seventh embodiments show examples of the basic configuration of the apparatus.
[0045]
(First embodiment)
A first embodiment of the present invention is shown in FIG. FIG. 1 is a schematic cross-sectional view of a crystal production apparatus according to the present invention, in which a furnace is divided into a first region (high temperature region) and a second region (low temperature region), and the temperature is controlled by independent heaters. It is sectional drawing which shows an example which applied this invention in the furnace currently controlled.
[0046]
In FIG. 1, 1a and 1b are first and second heaters, 2a and 2b are first and second thermocouples, respectively, 3 is a main crucible, and the bottom of the main crucible is the starting point of crystal growth. Become. 4 is CaF which is the raw material put in this crucible₂5a and 5b are first and second power sources, 6a and 6b are first and second temperature measuring devices, and 7 is a power control system. 8 is a crucible support rod, 9 is a crucible lower heat insulating material, 10 is an auxiliary crucible, and 11 is an auxiliary material, CaF.₂, 14 is a chamber, and 15 is a heat insulating material.
[0047]
The chamber 14 is a vacuum chamber whose inside can be depressurized, and is made of metal such as stainless steel. The chamber 14 is connected to a vacuum pump (not shown) and has 10 chambers.^-Five10 from Torr^-6The pressure is reduced to a vacuum level of Torr.
[0048]
The heat insulating material 15 was attached to the inner wall of the vacuum chamber 14 and used to prevent the heat in the chamber from escaping to the outside.
[0049]
The heaters 1a and 1b are made of carbon or the like, and generate heat when energized through the power sources 5a and 5b, respectively. The generated heat is supplied by radiation to the crucible 3 arranged in the heater.
[0050]
The crucible 3 is made of a material such as carbon that does not react with the crystal raw material 4 to be produced.₂Is included. Under the crucible 3, an auxiliary crucible 10 containing the auxiliary raw material 9 is arranged. Auxiliary material 11 is CaF₂However, since the crucible 3 and the auxiliary crucible 10 are completely partitioned by a partition wall, the auxiliary raw material 11 does not become a seed crystal of the main crucible 3. Therefore, the lowest part of the main crucible 3 is a crystal growth start point in the main crucible 3. Further, a support bar 8 that supports the crucible is attached below the auxiliary crucible 10. The support bar 8 is attached to an elevating mechanism (not shown), and the crucible 3 is moved up and down by driving the elevating mechanism.
[0051]
The thermocouple 2a is attached to the heater 1a so that the temperature near the lower part of the crucible 3 is measured. The thermocouple 2b is attached to the heater 1b so that the temperature between the bottom of the main crucible 3 and the bottom of the auxiliary crucible 10 or near the bottom of the auxiliary crucible 10 is measured.
[0052]
The heater 1a is supplied with electric power from the power source 5a. The voltage signal of the thermocouple 2a is converted into a temperature signal by the measuring instrument 6a and sent to the control system 7. The control system 7 controls the power source 5a and plays a role of controlling the input power to the heater 1a. Similarly, the heater 1b is supplied with power by the power source 5b. The voltage signal of the thermocouple 2b is converted into a temperature signal by the measuring instrument 6b and sent to the control system 7. The control system 7 controls the power source 5b and controls the input power to the heater 1b.
[0053]
In the following, the operation of the apparatus having the configuration shown in FIG. 1 will be described in accordance with the crystal production procedure.
[0054]
(1) Initially, the temperature Ta of the thermocouple 2a is above the melting point of the crystal, that is, CaF.₂The input to the heater 1a is controlled to be higher than the melting point of 1360 ° C. The temperature Tb of the thermocouple 2b is below the melting point of the crystal, that is, CaF₂The input to the heater 1b is controlled to be lower than the melting point of 1360 ° C.
[0055]
(2) The main crucible 3 is warmed by receiving heat from the heater, and eventually the raw material 4 in the main crucible 3 is CaF.₂At this point, the raw material 4 in the crucible 3 is completely melted.
[0056]
(3) Further, CaF of auxiliary raw material 11 in auxiliary crucible 10₂A part of it melts. In this state, it is left until a sufficient time has passed and the melt is stabilized. The bottom of the auxiliary crucible 10 is CaF₂Since the temperature is lower than the melting point of the auxiliary crucible 10, the auxiliary raw material 11 CaF in the auxiliary crucible 10₂Does not melt completely to the bottom of the auxiliary crucible 10 even after sufficient time has passed.
[0057]
(4) When the melt is stable, gradually start lowering the crucible.
At this time, since the auxiliary raw material 11 inside the auxiliary crucible 10 is not completely melted, there is a possibility that a plurality of crystal growth start points can be formed, and a possibility that a single crystal can be formed is low. Further, since the lower part of the main crucible 3 and the upper part of the auxiliary crucible 10 which are the starting points of crystal growth are partitioned, the crystallinity of the auxiliary crucible 10 does not affect the crystallinity of the main crucible 3.
[0058]
Next, the temperature variation will be considered. The temperature of the heater at position A near the solid-liquid interface hardly fluctuates even when the crucible 3 is pulled down. This is because the heater 1a at the position A faces the main crucible 3 before the movement of the main crucible 3 starts, and even if the main crucible 3 is pulled down, the positional relationship between the main crucible 3 and the heater 1a at the position A is Because it doesn't change. However, the temperature of the heater 1b at the position C where the main crucible 3 was not initially inside increases rapidly as the main crucible 3 descends. However, since the temperature at the position C is sufficiently lower than the melting point and the position is also sufficiently away from the solid-liquid interface, the temperature fluctuation at the position C may affect the temperature of the heater 1a at the position A near the solid-liquid interface. small.
[0059]
Further, as described above, since the temperature near the solid-liquid interface does not depend on the position of the crucible 3, the fluctuations in the outputs of the thermocouples 2a and 2b provided above and below the solid-liquid interface are caused by changes in the environment of the crystal production apparatus. It can be considered that. Accordingly, the power supply 5a and 5b are controlled by the control system 7 so that the outputs of the thermocouples 2a and 2b are constant, and the power supplied to the heaters 1a and 1b is controlled to further keep the temperature of the main crucible 3 constant. Therefore, stable crystal growth is possible even if the environment of the crystal production apparatus changes.
[0060]
Therefore, in the apparatus according to the present invention, even if the crucible moves, the position and temperature gradient in the vicinity of the solid-liquid interface do not change.In other words, as the crucible moves, the vicinity of the solid-liquid interface of the raw material placed in the crucible is heated. Since the solid angle seen from the side does not change, a stable crystal can be produced. Furthermore, it is possible to eliminate the influence of the environment of the crystal production apparatus by keeping the output of the thermocouple constant.
[0061]
(Second Embodiment)
FIG. 2 is a schematic cross-sectional view showing only the main part of the crystal manufacturing apparatus according to the second embodiment of the present invention, that is, only the main crucible 3, the auxiliary crucible 10 and the parts attached thereto. . The apparatus shown in FIG. 2 differs from the apparatus shown in FIG. 1 in that a cylindrical portion thinner than the diameter of the main crucible for containing a seed crystal is provided at the bottom of the main crucible in order to control the orientation of the crystal to be grown. Yes. The other points were the same as in the apparatus of FIG.
[0062]
In the apparatus of FIG. 2, a cylindrical portion 20 that is thinner than the diameter of the main crucible 3 is provided at the bottom of the main crucible 3. Then, an auxiliary crucible 10 is attached around the cylindrical portion 20, that is, at the lower part of the main crucible 3, and the auxiliary raw material 11 is placed inside the auxiliary crucible 10.
[0063]
A seed crystal used for growing a crystal in the crucible 3 is placed in the cylindrical portion 20 so that a crystal having a desired orientation can be grown. Then, the power of the heater (not shown) is controlled so that the temperature of the upper part of the cylindrical part 20 is higher than the melting point of the raw material 4 and the temperature of the lower part of the cylindrical part 20 is lower than the melting point of the raw material 4. Adjust with (not shown). By such temperature control, the upper portion of the seed crystal inside the cylindrical portion 20 is melted and the lower portion is in a crystalline state.
[0064]
After the seed crystal is formed in such a state, the growth of the crystal is started by pulling down the crucible 3 as in the first embodiment. Since the temperature region corresponding to the solid-liquid interface in the main crucible 3 is inside the auxiliary crucible 10, the position and temperature gradient in the vicinity of the solid-liquid interface do not change even if the main crucible 3 moves. Furthermore, since the lower part of the seed crystal inside the cylindrical portion 20 is not melted, the crystal grows with the interface between the seed crystal and the melt as the starting point of growth. Therefore, in the apparatus of FIG. 2, a crystal having a desired orientation can be stably obtained.
[0065]
In the above-described embodiment, CaF is used as the auxiliary raw material 11 placed in the auxiliary crucible 10.₂However, the auxiliary material 11 is not necessarily CaF.₂The heat capacity (C_p) And density (ρ) and thermal conductivity (λ) is CaF₂As long as the material is the same as that.
[0066]
Therefore, as the auxiliary material 11, Al as shown in Table 1 is used.₂O_ThreeCeramics such as MgO can be used.
[0067]
[Table 1]

[0068]
Further, when the pulling speed of the main crucible 3 is sufficiently small, the heat capacity (C_p) And density (ρ) only CaF₂A substance equivalent to the above may be used as the auxiliary material 11.
[0069]
(Third embodiment)
FIG. 3 is a schematic cross-sectional view showing only the main part of the crystal manufacturing apparatus according to the third embodiment of the present invention, that is, only the main crucible 3 and the auxiliary crucible 10 in FIG. . The apparatus of FIG. 3 uses CaF as the raw material 4₂1 is different from the apparatus shown in FIG. 1 in that a material having a melting point higher than the melting point is used as the auxiliary material 11. The other points were the same as in the apparatus of FIG.
[0070]
In this case, the surface facing the heater (not shown) is covered with the same material as the radiation rate of the main crucible 3 as shown in FIG. 3 in order to equalize the heat outflow amount of the heater regardless of the position of the main crucible 3. Although it is necessary, the auxiliary material 11 may be exposed at the bottom.
[0071]
(Fourth embodiment)
FIG. 4 is a schematic cross-sectional view showing only the main part of the crystal manufacturing apparatus according to the fourth embodiment of the present invention, that is, only the main crucible 3, the auxiliary crucible 10 and the parts attached thereto. . The apparatus in FIG. 4 is different from the apparatus in FIG. 1 in that the shape of the auxiliary raw material 11 is simplified. The other points were the same as in the apparatus of FIG.
[0072]
As shown in FIG. 4, by making the bottom of the main crucible 3 flat (FIG. 4), a crystal manufacturing apparatus in which the main crucible 3 and the auxiliary crucible 10 can be easily manufactured can be obtained. Thus, the present invention is a technique that can be applied to crucibles of various shapes without depending on the shape of the crucible 3.
[0073]
(Fifth embodiment)
FIG. 5 is a schematic cross-sectional view showing only the main part of the crystal manufacturing apparatus according to the fifth embodiment of the present invention, that is, only the main crucible 3 and the auxiliary crucible 10 in FIG. . The apparatus shown in FIG. 5 differs from the apparatus shown in FIG. 1 in that the main crucible 3 is a disc-type crucible divided into a plurality of regions. The other points were the same as in the apparatus of FIG. In FIG. 5, reference numeral 12 denotes a dummy crucible having a structure in which a crystal grown in this region becomes a seed of crystals in the upper region. The auxiliary crucible 10 is attached to the lower part of the dummy crucible 12 as in the first embodiment, and the input power to the heater is adjusted so that the melting point of the auxiliary raw material 11 is between the upper part and the bottom part of the auxiliary crucible 10. The present invention can be sufficiently applied to a crucible having such a configuration.
[0074]
(Sixth embodiment)
FIG. 6 is a schematic cross-sectional view showing the main part of the crystal manufacturing apparatus according to the sixth embodiment of the present invention, that is, only the main crucible 3, the auxiliary crucible 10 and the parts attached to them in FIG. . The apparatus shown in FIG. 6 uses a disc-type crucible divided into a plurality of regions as the main crucible 3 and a cylinder at the bottom of the main crucible 3 instead of a dummy crucible in order to control the orientation of the crystal grown in the main crucible 3. 1 differs from the apparatus of FIG. 1 in that a shaped portion 20 is attached. The other points were the same as in the apparatus of FIG.
[0075]
In the apparatus of FIG. 6, a seed crystal is placed in a cylindrical portion 20 so that a crystal of a desired orientation can be grown. Here, the power of a heater (not shown) is controlled so that the temperature of the upper part of the cylindrical part 20 is higher than the melting point and the temperature of the lower part of the cylindrical part 20 is lower than the melting point.
[0076]
(Seventh embodiment)
FIG. 7 is a schematic cross-sectional view of a crystal manufacturing apparatus according to a seventh embodiment of the present invention. The apparatus of FIG. 7 is different from the apparatus of FIG. 1 in that the heater is not divided and is a single structure. The other points were the same as in the apparatus of FIG.
[0077]
In the apparatus of FIG. 7, the relative positional relationship between the heater 1a and the crucible 3 and the auxiliary crucible 10 is set so that the bottom of the auxiliary crucible 10 is lower than the lowest position of the heater 1a. In this state, the input power of the heater is controlled so that the temperature of the lowest part of the heater 1 a is lower than the melting point of the raw material 4 and the temperature of the lowest part of the crucible 3 is higher than the melting point of the raw material 4. With such a configuration, even if the crucible 3 is lowered, all points of the heater 1a face the main crucible 3 or the auxiliary crucible 10 before the main crucible 3 is lowered. It does not change. Therefore, the operation of the present invention described above can be obtained.
[0078]
(Eighth embodiment)
FIG. 17 is a schematic cross-sectional view of a crystal manufacturing apparatus according to an eighth embodiment of the present invention. The apparatus of FIG. 17 reduces the temperature change in the vicinity of the solid-liquid interface by increasing the distance from the bottom of the crucible 3 where the bottom of the crucible 3 is thick, that is, the crystal growth point, instead of the auxiliary crucible. This is different from the apparatus of FIG. The other points were the same as in the apparatus of FIG.
[0079]
In the apparatus of FIG. 17, the radius of the outer periphery of the crucible 3 is set to r._c, The radius inside the heater is r_hThe size and installation position of the crucible 3 are determined. Let A be the position of the heater whose temperature is the melting point, and L be the difference in the height of the bottom of the crucible at position A. In order to make the temperature rise ΔT at the position A fluctuate 5K or less even when the crucible 3 is lowered, L can be obtained from Equation 12. For example, r_c= 100 mm, r_h= 120 mm for r_h/ R_c= 1.2, so
L ≧ 0.3 ・ 100mm = 30mm
It becomes.
[0080]
Therefore, the thickness of the bottom of the crucible is 30 mm or more, for example, 40 mm, the position of the crucible 3 and the temperature of the heater are adjusted so that the distance L between the bottom of the crucible 3 at position A is 30 mm, and the crucible 3 containing the raw material is It is installed and the raw material is melted. Since the starting point C of crystal growth is higher than the position A, the raw material is melted even at the starting point of crystal growth. After the raw material in the crucible is sufficiently melted and stabilized, the crucible is gradually pulled down. Then, when the crystal growth start point C reaches the melting point A, the crystal growth is started. Even if the crucible descends, the temperature fluctuation at the position A varies only by about 5K, so that the position of the solid-liquid interface does not change and stable crystal growth is possible.
[0081]
【The invention's effect】
As described above, the crystal production apparatus according to the present invention starts moving the crucible because the solid angle seen from the heater side in the vicinity of the solid-liquid interface of the raw material put in the crucible does not change with the movement of the crucible. Before and after, the positional relationship with the solid-liquid interface of the raw material put in the heater and the crucible can be maintained in a predetermined positional relationship.
[0082]
Therefore, it is possible to obtain a crystal manufacturing apparatus capable of keeping constant the position of the solid-liquid interface of the raw material placed in the crucible and the temperature gradient in the direction perpendicular to the solid-liquid interface.
[0083]
By using the above apparatus, it becomes possible to stably manufacture a large-diameter single crystal optical material excellent in refractive index homogeneity.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a crystal manufacturing apparatus according to a first embodiment.
FIG. 2 is a schematic cross-sectional view showing a main part of a crystal manufacturing apparatus according to a second embodiment.
FIG. 3 is a schematic cross-sectional view showing a main part of a crystal manufacturing apparatus according to a third embodiment.
FIG. 4 is a schematic cross-sectional view showing a main part of a crystal production apparatus according to a fourth embodiment.
FIG. 5 is a schematic cross-sectional view showing a main part of a crystal manufacturing apparatus according to a fifth embodiment.
FIG. 6 is a schematic cross-sectional view showing a main part of a crystal manufacturing apparatus according to a sixth embodiment.
FIG. 7 is a schematic cross-sectional view of a crystal manufacturing apparatus according to a seventh embodiment.
FIG. 8 is a schematic cross-sectional view showing an example of a conventional crystal manufacturing apparatus.
9 is a graph showing the temperature distribution of the heater when the raw material is in a molten state in the apparatus of FIG.
FIG. 10 is a schematic cross-sectional view showing another example of a conventional crystal manufacturing apparatus.
11 is a graph showing a temperature distribution (solid line a) when the crucible is above and a temperature distribution (solid line b) when the crucible is lowered in the apparatus of FIG.
12 is a graph showing the temperature change of the heaters at positions A and B accompanying the movement of the crucible in the apparatus of FIG.
FIG. 13 is a schematic diagram showing a positional relationship between a crucible and a heater.
FIG. 14 is a schematic cross-sectional view of a chamber composed of a crucible and a heater, illustrating the heat balance between the heater and the surrounding wall.
FIG. 15 is a graph showing the form factor for the structure of the crucible.
FIG. 16 is a graph showing the relationship between the furnace structure and the temperature change of the heater.
FIG. 17 is a schematic cross-sectional view of a crystal manufacturing apparatus according to an eighth embodiment.
[Explanation of symbols]
1a, 1b heater,
2a, 2b thermocouple,
3 crucibles,
4 raw materials,
5a, 5b power supply,
6a, 6b Temperature measuring device,
7 Power control system,
8 vase support rods,
9 Insulation material under crucible,
10 auxiliary crucible,
11 auxiliary materials,
12 Dummy crucible,
14 chambers,
15 thermal insulation,
20 Cylindrical part.

Claims (6)

An apparatus comprising a crucible containing a raw material and a heater arranged so as to surround the crucible, and lowering the crucible with respect to the heater to cool the raw material from below the crucible to produce crystals. A crystal production apparatus, wherein the length L of the crucible below the temperature corresponding to the solid-liquid interface of the raw material put in the crucible satisfies the following formula.
L ≧ r _c · (C ₂ · ΔT ² + C ₁ · ΔT + C ₀ ) · (r _h / r _c -1)
(Where C ₂ = 0.004, C ₁ = −0.166, C ₀ = 2.22, r _c is the crucible radius, r _h is the heater radius, and ΔT is the allowable temperature change of the heater. is there)  Under the crucible containing the raw material, an auxiliary material having the same product of heat capacity and density as the raw material is installed, and the temperature of the upper part of the auxiliary material is higher than the melting point of the raw material and the temperature of the lower part of the auxiliary material. 2. The crystal manufacturing apparatus according to claim 1, further comprising means for controlling an input to the heater so that a temperature of the raw material is lower than a melting point of the raw material.  The crystal preparation apparatus according to claim 2, wherein the auxiliary material is MgO. The crystal preparation apparatus according to claim 2, wherein the auxiliary material is Al ₂ O ₃ .  The crystal manufacturing apparatus according to claim 2, wherein the crucible has a plurality of regions separated by a plate having a small hole in the center of the crucible. 6. The crystal according to claim 5 , wherein a small hole for containing a seed crystal is provided at a bottom of the crucible forming a lowermost region containing the raw material among a plurality of regions constituting the crucible. Production device.

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