JP2004203644A - Seed crystal for manufacturing silicon single crystal, and method for manufacturing silicon single crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a seed crystal for manufacturing a silicon single crystal which can prevent the generation of dislocation when the seed crystal is dipped in a silicon melt and endure the load of a silicon single crystal in large weight during manufacturing the silicon single crystal by Czochralski method, to provide a method for manufacturing the seed crystal, and to provide a method for manufacturing the silicon single crystal by which the non-dislocation ratio can be enhanced. <P>SOLUTION: In the silicon seed crystal to be used for manufacturing the silicon single crystal by the Czochralski method, the preform silicon single crystal from which the silicon seed crystal is sliced off has the boron concentration of ≥4×10<SP>18</SP>atoms/cm<SP>3</SP>and ≤4×10<SP>19</SP>atoms/cm<SP>3</SP>, and the seed crystal for manufacturing the silicon single crystal is prepared by slicing the preform silicon single crystal, griding, polishing and etching the surface. The method for manufacturing the silicon single crystal uses the seed crystal. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００９】
ここで、Ｙはエッチング量（μｍ）、Ａは砥石のＪＩＳ Ｒ ６００１に規定される粒度（＃）を表す。）
（２）チョクラルスキー法によるシリコン単結晶の製造方法において、請求項１に記載の種結晶を使用して、ダッシュネッキング（種絞り）を行わずにシリコン単結晶を引上成長させることを特徴とするシリコン単結晶の製造方法。
【００１０】
（３）チョクラルスキー法によるシリコン単結晶の製造方法において、請求項１に記載の種結晶を使用して、ダッシュネッキング（種絞り）を行わずにシリコン単結晶を引上成長させるシリコン単結晶の製造方法であって、シリコン融液の周期１０秒以上の温度変動の標準偏差が４℃以下を満足することを特徴とするシリコン単結晶の製造方法。
【００１１】
（４）チョクラルスキー法によるシリコン単結晶の製造方法において、請求項１に記載の種結晶を使用して、該種結晶をシリコン融液に一部溶解させた後にダッシュネッキング（種絞り）を行わずにシリコン単結晶を引上成長させるシリコン単結晶の製造方法であって、該種結晶の溶解量が該種結晶の直径以上かつシリコン融液の周期１０秒以上の温度変動の標準偏差が４℃以下の条件を満足することを特徴とするシリコン単結晶の製造方法、
を要旨とするものである。
【００１２】
【発明の実施の形態】
本発明を以下に説明する。
【００１３】
本発明者らは、シリコン単結晶製造時の有転位化について鋭意検討した結果、種結晶の表面状態が重要であり、種結晶の研削及び研磨時に発生する表面の残留加工歪みが、転位の発生原因となっていることを見いだした。このような歪みが表面に残存していると、たとえ種結晶中のホウ素濃度が高くて、種結晶の硬度が大きくなっていようとも、種結晶と融液との接触時の熱応力により、種結晶先端部に転位が発生する。
【００１４】
種結晶中のホウ素濃度は、４×１０¹⁸ａｔｏｍｓ／ｃｍ³より小さいと、種結晶の硬度が足りず、融液との接触時に転位が発生する確率が高くなる。一方、４×１０¹⁹ａｔｏｍｓ／ｃｍ³より大きいと、理由は明確ではないが、同じように融液との接触時に転位が発生する確率が高くなる。
【００１５】
残留加工歪みは、研削あるいは研磨に用いる砥石の番手（粗さ）に依存していることが判った。ここで用いる残留加工歪みとは、以下に説明するＸ線トポグラフで観察される像で定義する。
【００１６】
通常、歪みのないシリコン単結晶では、コントラストの無い、均一なトポグラフのパターンとなる。しかし、加工歪みが存在する場合には、歪みに反映する像が現れる。これは、加工歪みが存在する場合、シリコン原子で構成される格子が歪むことによって、測定上、無歪みの状態よりも高い回折強度が検出されるからである。鋭意研究した結果、研削あるいは研磨に用いる砥石の番手と加工歪み層の厚さは、一定の相関にあることを明らにした。すなわち、砥石の番手をＡ（＃）、加工歪み厚さをＹ（μｍ）とした時、下記（ＩＩ）式の関係があることを見いだした。ここで用いるＡは、砥石のＪＩＳ Ｒ ６００１で規定される粒度（＃）を表す。
【００１７】
【数３】

【００１８】
これをグラフに表すと、図１のようになる。加工歪み厚さは、研削、研磨処理後の種結晶を段階的にエッチング処理して、Ｘ線トポグラフで歪みが観察されなくなるまでの除去厚さで定義した。従って、研削、研磨後のエッチング厚さを式（ＩＩ）のＹμｍ以上にすれば、Ｘ線トポグラフで計測可能な加工歪み層は除去できる。したがって、シリコン単結晶引き上げの無転位率は、エッチング厚さをＹμｍ以上の値にすることにより、著しく向上する。Ｘ線トポグラフの歪み量分解能は０．２μｍであるので、種結晶の残留表面加工歪みの厚みを０．２μｍ以下にすれば、シリコン単結晶引き上げの無転位率が向上することになる。
【００１９】
なお、エッチング液としては、通常、フッ酸、硝酸の混酸水溶液が用いられるが、エッチングにより表面の平滑度が確保できれば、他の組成を用いても式（ＩＩ）で規定される加工歪み層Ｙが除去できればよい。フッ酸、硝酸の混酸水溶液を用いる場合には、フッ酸によるシリコンのエッチング速度が高いため、配合比は、硝酸過多の方が好ましく、容積比としてフッ酸：硝酸＝１：３〜８がより望ましい。また、エッチングによる表面荒れが大きい場合や、シリコン表面に皮膜が形成する問題が生じた時には、酢酸等の緩和剤を混ぜることにより回避することができる。酢酸の添加量としては、上記のフッ酸と硝酸の配合比に対して２〜８が好ましい。すなわち、フッ酸：硝酸：酢酸＝１：（３〜８）：（２〜８）の配合比が表面荒れや表面皮膜を回避するのに望ましい。この理由は、酢酸の添加量が２未満であると添加の効果が低く、８より大きくなるとシリコンのエッチング速度が著しく低下するためである。
【００２０】
また、研削、研磨に用いる砥石は、Ｓｉと化学的な反応性の低いＳｉＣやＡｌ₂Ｏ₃等のセラミックスやダイヤモンドが好ましい。これらの砥石を使用した場合、加工歪み層の厚さは、ほぼ材質によらず砥石の番手で決まるので、番手に応じた加工歪み層以上の厚さをエッチングで除去すればよい。一方、種結晶の融液に浸かる先端形状は、必ずしも下に凸状でなくてもよく、平坦でもよい。この時、先端形状が平坦の場合の先端面、及び下に凸の場合の径が小さくなる部位の外周部は、面取りされていることが望ましい。これは、外周部では研削加工時にチッピングとよばれる欠けが生じやすく、砥石番手で形成される加工歪み層よりも厚い領域まで歪みが形成される場合があるからである。面取り加工をする場合、面取りの曲率半径を１００μｍ以上とすれば、チッピングによる歪みを除去することができる。面取り部の加工は、チッピングが生じた場合に、粗く低い番手の砥石で研削してから、さらに細かく高い番手の砥石で研磨することにより、式（ＩＩ）で規定される加工歪み層にすることができる。また、実用上の観点からは、面取りの曲率半径を５ｍｍにすることが望ましい。面取りの曲率半径を必要以上に大きくすると、シリコン単結晶は脆いため、加工が困難であるので、必要以上に加工時間を要するからである。ところで、加工時にチッピングが発生した場合には、チッピング部と種結晶の他の表面を、同じ加工状態にすることが望ましい。これは、加工後にエッチングする厚さを加工歪み層の一番厚い領域に合わせなければならないからである。
【００２１】
本発明では、シリコン単結晶の製造において、上述した本発明の種結晶を用いることにより、表面加工歪み層が無いために、シリコン溶液に種結晶を浸漬した際に、種結晶及び育成するシリコン単結晶に熱応力による転位が格段に発生し難い。したがって、ダッシュネッキングによる絞り部を形成することなく、大重量の負荷に耐え得る無転位の大口径シリコン単結晶製造が可能となる。
【００２２】
さらに、本発明者らは、シリコン単結晶製造時の有転位化について鋭意検討した結果、上記のような残留加工歪の無い種結晶を使用しても、シリコン融液の温度変動を抑えないと転位が発生する場合があることを見出した。以下にこの理由を説明する。
【００２３】
シリコン融液、特に大型結晶を引き上げるための大型坩堝内の融液は乱流状態になっており、様々な周期の温度変動を含んでいる。このうち周期１０秒以上の温度変動が転位の発生に大きな影響を及ぼす。この温度変動により、種結晶の浸漬中に急速な成長と溶解が生じる。すなわち低温の融液が種結晶の近傍に来たときには結晶は急速に成長し、高温の融液が種結晶の近傍に来たときには結晶は急速に溶解する。このような急成長、急溶解により転位が発生する場合がある。周期が１０秒よりも小さい温度変動の影響は小さい。なぜならば、融液と種結晶との間に存在する温度境界層がローパスフィルターとして働くために、速い温度変動は種結晶には伝わりにくく、結晶の急成長や急溶解は起こりにくいためである。
【００２４】
引き上げ条件によっては融液との接触時に熱応力が非常に大きく、このような加工歪の無い種結晶を使っても転位が入る場合がある。その場合は例えば特開平９−２４９４９２号公報で開示されているように、種結晶を融液にある一定量溶解させることにより、この融液との接触時に導入された転位を溶解消滅させることができる。なぜならば、種結晶中のホウ素濃度が高い場合は、種結晶が硬いために、融液との接触時に発生する転位の長さは短く、そして種結晶の浸漬中もこの転位は長くならないためである。しかしながら、種結晶側面に加工歪が残っている場合は、種結晶を浸漬していく段階で、種結晶側面の結晶−融液−ガス三重点（種結晶の外周線）から新たな熱応力により転位が導入される確率が高い。この転位は種結晶を浸漬して、溶解消滅させていっても、新しい三重点から次々に転位が発生するため、最終的に消滅させることができない。従って、初めから種結晶の側面の残留加工歪を除去して、種結晶の浸漬中に転位の発生が起こらないようにしておく必要がある。すなわち表面加工歪の無い層は、少なくとも種結晶が融液に浸漬される領域全部にわたって形成されていなければならない。
【００２５】
以下に、本発明に係る種結晶中のホウ素濃度、表面加工処理、融液温度変動と育成した単結晶の無転位化率（ＤＦ［Ｄｉｓｌｏｃａｔｉｏｎ Ｆｒｅｅ］率）の関係について、実施例を用いて説明する。
【００２６】
【実施例】
ここで用いる種結晶は、シリコン単結晶インゴットから切断加工を行った後、円筒研削及び後述する表面処理を施した直径１３ｍｍの円柱形状である。
【００２７】
チョクラルスキー法によるシリコン単結晶育成は、この種結晶をホルダーに装着した後にアルゴンガス雰囲気中で行われる。加熱した結晶育成炉内で、多結晶シリコンを溶解した後、種結晶をシリコン融液表面に向かってゆっくりと下降させる。特に１３００℃温度域から融液接触位置までは、種結晶の急激な温度変化による熱応力の発生を避けるために、降下速度を１ｍｍ／ｍｉｎとした。種結晶が融液に接触した後は、場合によっては種結晶の一定長さを融液に浸漬溶解し、種結晶を融液に馴染ませた後、結晶育成を開始した。結晶育成では、ダッシュネックを行わずに、直径を徐々に拡げながらコーン部を作製し、直径３００ｍｍで肩入れをして、この直径を維持したまま５００ｍｍの直胴部を形成した。
【００２８】
実施例、比較例では、上記の育成条件でそれぞれ１０回の育成を行い、ＤＦ率を評価した。ＤＦ率は、育成した単結晶を成長方向に沿って縦切りのスライス加工し、Ｘ線トポグラフによって、種結晶部を含んだインゴット全体の転位の有無を評価した。Ｘ線トポグラフで単結晶の縦断面に少しでも転位が観察された場合には、ＤＦを０とし、ＤＦ率は、１０回の育成における転位が存在していないインゴットの個数の割合で算出した。
【００２９】
種結晶の表面加工は、円筒研削を用いて円柱状に仕上げるとともに先端形状を平坦あるいは下に凸の形状とした。この時、表面加工での砥石としてＳｉＣあるいはダイヤモンドを用いて所定の砥石番手まで研削、研磨加工を行った。一方、面取り加工として先端形状が平坦の場合の先端面、及び下に凸の場合の径が小さくなる部位の外周部において面取りの曲率半径が１ｍｍになるように研削、研磨加工を行った。研削、研磨後の表面層の除去処理は、フッ酸と硝酸の容積比が１：８である混合水溶液でエッチングを行い、初期種結晶径から処理後の種結晶径の変分をエッチングによる表面層除去厚みとした。
【００３０】
表１に実施例１〜８および比較例１〜４それぞれにおける、種結晶中のホウ素濃度、仕上げ砥石番手、エッチング除去厚み、周期１０秒以上の融液温度変動標準偏差、種結晶のその直径分以上の溶解有無の各条件の組み合わせでＤＦ率がどう変化したか示す。
【００３１】
実施例１〜２では、ホウ素濃度が適当な範囲内で、エッチングにより表面の加工歪み層も除去されているため、７０％の高いＤＦ率となっている。また実施例３〜４では上記条件に加え、融液の周期１０秒以上の温度変動の標準偏差を４℃以下に抑えたため、ＤＦ率が９０％とより大きくなっている。実施例５〜８では、実施例３〜４の条件に加えて、種結晶先端をその種結晶の直径と同じ長さ溶解したところ、ＤＦ率は１００％と更に大きくなった。
【００３２】
一方、比較例１〜２ではエッチング量は十分で、融液温度変動が４℃と小さく、種結晶の先端もその直径分溶解しているにもかかわらず、種結晶中のホウ素濃度が３×１０¹⁸ａｔｏｍｓ／ｃｍ³と小さいまたは５×１０¹⁹ａｔｏｍｓ／ｃｍ³と大きいために、種結晶表面から転位が発生し、成長したインゴット直胴部にも転位が成長し、ＤＦ率が０％となった。比較例３〜４では、種結晶中のホウ素濃度も適当な範囲に入っており、融液温度変動が４℃と小さく、種結晶の先端もその直径分溶解しているにもかかわらず、エッチング量が少ないため、ＤＦ率が１０％と小さくなっている。
【００３３】
【表１】

【００３４】
【発明の効果】
本発明の種結晶は、ホウ素濃度が高いために硬く、表面処理を行って加工歪み層を除去した種結晶であるため、熱応力の影響を受け難く、したがって、ダッシュネッキングを行わなくても、転位の発生し難い種結晶を提供できる。
【００３５】
また、この種結晶は、従来の種結晶製造工程を変えることなく製造することができ、安価な種結晶の製造方法を提供できる。
【００３６】
さらに、この種結晶をシリコン単結晶製造に用いれば、大口径、大重量のシリコン単結晶を無転位で容易に育成することができる。その結果、大口径、大重量のシリコン単結晶の製造コストを大幅に低減することに効果が顕れるのみならず、種結晶径が大きくなることにより、耐荷重が大きくなり、シリコン単結晶育成中にインゴットの破断・落下するという事故も防止でき、大口径・大重量のシリコン単結晶製造時の操業安全性が格段に向上するものである。
【図面の簡単な説明】
【図１】種結晶加工時の砥石番手と導入される加工歪み層厚さとの関係図[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a silicon seed crystal used when manufacturing a silicon single crystal by the Czochralski method and a method for manufacturing a silicon single crystal using the seed crystal.
[0002]
[Prior art]
In the production of a silicon single crystal by the Czochralski method, a silicon seed crystal is immersed in a silicon melt and then grown to a target diameter. Usually, when a silicon seed crystal is immersed in a silicon melt, dislocations, which are crystal defects, occur around the immersed portion. It is considered that the main cause of dislocation generation is thermal stress caused by the temperature difference of the seed crystal before and after immersion in the melt. In the production of silicon single crystals, it is indispensable to prevent the generation of dislocations or to eliminate dislocations. Conventionally, as a means for removing dislocations generated during immersion, a dash that once reduces the seed crystal diameter after immersion is used. Necking has been done. Due to this dash necking, the maximum drawing diameter that can eliminate dislocations is about 4 mm. If the diameter becomes larger than that, dislocations cannot be completely removed.
[0003]
With a recent increase in the diameter of a silicon single crystal, there is a risk that the silicon having a reduced diameter of 4 mm may break as the strength of silicon supporting the increased single crystal. Therefore, a method of manufacturing a single crystal without forming a narrowed portion due to necking, which is a problem in the strength of silicon, is disclosed (for example, see Patent Document 1). The present invention increases the strength of the seed crystal by increasing the boron concentration in the seed crystal, and does not generate dislocations due to thermal stress during immersion with the melt, or the length of the dislocations is small even if it occurs, This is a technique in which dislocations can be removed by dissolving a seed crystal at a diameter equal to or greater than that of the seed crystal, and a silicon single crystal can be grown without forming a narrowed portion due to necking.
[0004]
[Patent Document 1]
JP-A-9-249492 [0005]
[Problems to be solved by the invention]
Even when the boron concentration in the seed crystal is high as in the above-described conventional technique, when the present inventors performed additional tests, dislocations were sometimes generated in the grown silicon crystal. In other words, although dislocation formation of the single crystal was suppressed by these conventional techniques, dislocation generation was not completely prevented, and a quantitative causal relationship between factors other than boron concentration and dislocation generation was clarified. Did not.
[0006]
Thus, the present invention can prevent dislocations that occur when a seed crystal is immersed in a silicon melt when manufacturing a silicon single crystal by the Czochralski method, and in addition, load a heavy silicon single crystal. An object is to provide a seed crystal for producing a silicon single crystal that can withstand. Another object of the present invention is to provide a method for manufacturing a silicon single crystal that can improve the dislocation-free ratio in a silicon single crystal manufacturing process using the Czochralski method.
[0007]
[Means for Solving the Problems]
The present invention focuses on the viewpoint of the surface treatment state of the seed crystal, in particular, on the amount of surface distortion, and as a result of examining the effect of the surface treatment state of the seed crystal on dislocation during the growth of a silicon single crystal, has obtained new findings. Found and completed. Furthermore, the present invention has focused on the temperature fluctuation of the silicon melt, and has found new findings as a result of examining the effect of temperature fluctuation on dislocation of the seed crystal when the seed crystal is in contact with the melt. Thus, the present invention is completed.
(1) A silicon seed crystal used when manufacturing a silicon single crystal by the Czochralski method, wherein the boron concentration in the silicon single crystal of the base material from which the silicon seed crystal is cut is 4 × 10 ¹⁸ atoms / cm ³ or more. A seed crystal for producing a silicon single crystal, wherein the seed crystal is 4 × 10 ¹⁹ atoms / cm ³ or less, and the silicon seed crystal is cut out from a base material silicon single crystal, ground, polished, and then subjected to surface etching. (However, in the step of etching the silicon seed crystal, at least a portion where the silicon seed crystal comes into contact with the silicon melt at the time of manufacturing a silicon single crystal satisfies the following formula (I).
[0008]
(Equation 2)

[0009]
Here, Y represents the etching amount (μm), and A represents the grain size (#) defined in JIS R 6001 of the grindstone. )
(2) A method for producing a silicon single crystal by the Czochralski method, wherein the seed crystal according to claim 1 is used to pull up a silicon single crystal without performing dash necking (seed drawing). Production method of a silicon single crystal.
[0010]
(3) In a method for producing a silicon single crystal by the Czochralski method, a silicon single crystal in which a silicon single crystal is pulled up using the seed crystal according to claim 1 without performing dash necking (seed drawing). The method for producing a silicon single crystal according to claim 1, wherein a standard deviation of a temperature variation of the silicon melt at a cycle of 10 seconds or more satisfies 4 ° C. or less.
[0011]
(4) In the method for producing a silicon single crystal by the Czochralski method, dash necking (seed drawing) is performed after partially dissolving the seed crystal in a silicon melt using the seed crystal according to claim 1. A method for producing a silicon single crystal, in which a silicon single crystal is pulled up without performing the method, wherein a dissolution amount of the seed crystal is equal to or more than a diameter of the seed crystal and a standard deviation of a temperature variation of 10 seconds or more of a silicon melt is reduced. A method for producing a silicon single crystal, which satisfies a condition of 4 ° C. or less;
It is the gist.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention will be described below.
[0013]
The present inventors have conducted intensive studies on the formation of dislocations during the production of silicon single crystals.As a result, the surface state of the seed crystal is important, and the residual processing strain on the surface generated during grinding and polishing of the seed crystal causes dislocation generation. I found out what was causing it. If such strains remain on the surface, even if the concentration of boron in the seed crystal is high and the hardness of the seed crystal is high, the seed will be affected by thermal stress at the time of contact between the seed crystal and the melt. Dislocations occur at the tip of the crystal.
[0014]
If the boron concentration in the seed crystal is less than 4 × 10 ¹⁸ atoms / cm ³ , the hardness of the seed crystal is insufficient, and the probability of dislocation occurring upon contact with the melt increases. On the other hand, if it is larger than 4 × 10 ¹⁹ atoms / cm ³ , although the reason is not clear, similarly, the probability of dislocation occurring upon contact with the melt increases.
[0015]
It has been found that the residual processing strain depends on the number (roughness) of the grindstone used for grinding or polishing. The residual processing strain used here is defined by an image observed by an X-ray topograph described below.
[0016]
Normally, a silicon single crystal without distortion has a uniform topographic pattern without contrast. However, when processing distortion exists, an image reflected on the distortion appears. This is because, when processing strain is present, the diffraction intensity higher than that in the unstrained state is detected due to distortion of the lattice composed of silicon atoms. As a result of intensive research, it has been clarified that the number of grinding wheels used for grinding or polishing and the thickness of the work-strained layer have a certain correlation. That is, when the count of the grindstone is A (#) and the thickness of the processing strain is Y (μm), it has been found that there is a relationship of the following formula (II). A used here represents a grain size (#) defined in JIS R 6001 of the grindstone.
[0017]
[Equation 3]

[0018]
This is represented in a graph as shown in FIG. The processing strain thickness was defined as the thickness of the seed crystal after the grinding and polishing treatments, which was etched in stages, until the strain was no longer observed in the X-ray topograph. Therefore, if the etching thickness after grinding and polishing is set to be not less than Y μm in the formula (II), the work distortion layer that can be measured by the X-ray topograph can be removed. Therefore, the dislocation-free ratio in pulling a silicon single crystal is significantly improved by setting the etching thickness to a value of Y μm or more. Since the resolution of the strain amount of the X-ray topograph is 0.2 μm, if the thickness of the residual surface processing strain of the seed crystal is set to 0.2 μm or less, the dislocation-free rate in pulling a silicon single crystal is improved.
[0019]
As the etching solution, an aqueous solution of a mixed acid of hydrofluoric acid and nitric acid is usually used. However, as long as the surface smoothness can be ensured by etching, the processing strained layer Y defined by the formula (II) can be used even if another composition is used. As long as it can be removed. When a mixed acid aqueous solution of hydrofluoric acid and nitric acid is used, since the etching rate of silicon by hydrofluoric acid is high, the compounding ratio is preferably an excess of nitric acid, and the volume ratio of hydrofluoric acid: nitric acid = 1: 3 to 8 is more preferable. desirable. Further, when the surface roughness due to etching is large, or when a problem of forming a film on the silicon surface occurs, it can be avoided by mixing a moderating agent such as acetic acid. The amount of acetic acid to be added is preferably 2 to 8 based on the mixing ratio of hydrofluoric acid and nitric acid. That is, the mixing ratio of hydrofluoric acid: nitric acid: acetic acid = 1: (3 to 8) :( 2 to 8) is desirable to avoid surface roughness and surface coating. The reason is that if the amount of acetic acid is less than 2, the effect of the addition is low, and if it is more than 8, the etching rate of silicon is significantly reduced.
[0020]
Further, as a grindstone used for grinding and polishing, ceramics such as SiC or Al ₂ O _{3 having} low chemical reactivity with Si, or diamond is preferable. When these grindstones are used, the thickness of the work-strained layer is determined by the number of the grindstones irrespective of the material, and therefore, the thickness of the work-strained layer or more according to the workmany may be removed by etching. On the other hand, the shape of the tip immersed in the melt of the seed crystal does not necessarily have to be convex downward and may be flat. At this time, it is desirable that the distal end surface when the distal end shape is flat and the outer peripheral portion where the diameter decreases when the distal end shape is convex downward be chamfered. This is because chipping called chipping is likely to occur in the outer peripheral portion during grinding, and strain may be formed up to a region thicker than the work strain layer formed by the grindstone count. In the case of chamfering, if the radius of curvature of the chamfer is 100 μm or more, distortion due to chipping can be removed. In the processing of the chamfered part, when chipping occurs, after grinding with a coarse and low-numbered grindstone, and then polishing with a finer and higher-numbered grindstone, a processing distortion layer defined by the formula (II) is obtained. Can be. Further, from a practical viewpoint, it is desirable to set the radius of curvature of the chamfer to 5 mm. If the radius of curvature of the chamfer is made larger than necessary, the silicon single crystal is brittle and processing is difficult, so that processing time is required more than necessary. By the way, when chipping occurs during processing, it is desirable that the chipping portion and the other surface of the seed crystal be in the same processing state. This is because the thickness to be etched after processing must be adjusted to the thickest region of the processing strain layer.
[0021]
In the present invention, in the production of a silicon single crystal, by using the seed crystal of the present invention described above, since there is no surface processing strain layer, when the seed crystal is immersed in a silicon solution, Dislocation due to thermal stress in the crystal is extremely unlikely to occur. Therefore, it is possible to produce a dislocation-free large-diameter silicon single crystal that can withstand a heavy load without forming a narrowed portion due to dash necking.
[0022]
Furthermore, the present inventors have conducted intensive studies on dislocations during the production of silicon single crystals.As a result, even if a seed crystal having no residual processing strain as described above is used, it is necessary to suppress the temperature fluctuation of the silicon melt. It has been found that dislocations may occur. The reason will be described below.
[0023]
A silicon melt, particularly a melt in a large crucible for pulling up a large crystal, is in a turbulent state and includes temperature fluctuations of various periods. Of these, temperature fluctuations of a period of 10 seconds or more greatly affect the occurrence of dislocation. This temperature fluctuation causes rapid growth and dissolution during seed crystal immersion. That is, when the low temperature melt comes near the seed crystal, the crystal grows rapidly, and when the high temperature melt comes near the seed crystal, the crystal rapidly dissolves. Dislocation may occur due to such rapid growth and rapid dissolution. The effect of temperature fluctuations with a cycle smaller than 10 seconds is small. This is because the temperature boundary layer existing between the melt and the seed crystal functions as a low-pass filter, so that a rapid temperature change is not easily transmitted to the seed crystal, and rapid growth and rapid melting of the crystal are unlikely to occur.
[0024]
Depending on the pulling conditions, thermal stress is very large at the time of contact with the melt, and dislocations may occur even when a seed crystal having no such processing strain is used. In that case, for example, as disclosed in Japanese Patent Application Laid-Open No. 9-249492, by dissolving a certain amount of a seed crystal in a melt, dislocations introduced at the time of contact with the melt can be dissolved and annihilated. it can. This is because when the boron concentration in the seed crystal is high, the length of the dislocation generated upon contact with the melt is short because the seed crystal is hard, and the dislocation does not become long during the immersion of the seed crystal. is there. However, if processing strain remains on the side of the seed crystal, a new thermal stress is applied from the crystal-melt-gas triple point (peripheral line of the seed crystal) at the stage of immersing the seed crystal. The probability of dislocations being introduced is high. Even if these dislocations are dissolved and annihilated by dipping the seed crystal, the dislocations are generated one after another from the new triple point, and thus cannot be finally eliminated. Therefore, it is necessary to remove the residual processing strain on the side surface of the seed crystal from the beginning to prevent the occurrence of dislocation during the immersion of the seed crystal. That is, the layer having no surface processing strain must be formed at least over the entire region where the seed crystal is immersed in the melt.
[0025]
Hereinafter, the relationship between the boron concentration in the seed crystal, the surface processing treatment, the fluctuation in the melt temperature, and the dislocation-free ratio (DF [Dislocation Free] ratio) of the grown single crystal according to the present invention will be described using examples. I do.
[0026]
【Example】
The seed crystal used here has a cylindrical shape with a diameter of 13 mm that has been subjected to a cutting process from a silicon single crystal ingot, and then subjected to cylindrical grinding and surface treatment described later.
[0027]
The silicon single crystal is grown by the Czochralski method in an argon gas atmosphere after the seed crystal is mounted on a holder. After melting the polycrystalline silicon in the heated crystal growing furnace, the seed crystal is slowly lowered toward the surface of the silicon melt. In particular, from the 1300 ° C. temperature range to the melt contact position, the descent speed was set to 1 mm / min in order to avoid generation of thermal stress due to a rapid temperature change of the seed crystal. After the seed crystal was brought into contact with the melt, a certain length of the seed crystal was immersed and dissolved in the melt in some cases, and the seed crystal was adapted to the melt and crystal growth was started. In the crystal growth, a cone portion was formed while gradually expanding the diameter without performing a dash neck, and shouldering was performed at a diameter of 300 mm, and a 500 mm straight body portion was formed while maintaining this diameter.
[0028]
In the example and the comparative example, cultivation was performed 10 times under the above cultivation conditions, and the DF rate was evaluated. The DF rate was determined by cutting the grown single crystal in a vertical slice along the growth direction, and evaluating the presence or absence of dislocation in the entire ingot including the seed crystal part by X-ray topography. If any dislocation was observed in the vertical section of the single crystal by X-ray topography, DF was set to 0, and the DF ratio was calculated by the ratio of the number of ingots having no dislocation in 10 times of growth.
[0029]
The surface treatment of the seed crystal was finished in a cylindrical shape using cylindrical grinding and the tip shape was made flat or convex downward. At this time, grinding and polishing were performed to a predetermined grinding wheel number using SiC or diamond as a grinding wheel in the surface processing. On the other hand, as the chamfering, grinding and polishing were performed such that the radius of curvature of the chamfer was 1 mm at the tip end surface when the tip shape was flat and at the outer peripheral portion of the portion where the diameter was reduced when it was convex downward. The removal of the surface layer after grinding and polishing is performed by etching with a mixed aqueous solution in which the volume ratio of hydrofluoric acid and nitric acid is 1: 8, and the variation of the seed crystal diameter after treatment from the initial seed crystal diameter is determined by etching. The layer removal thickness was used.
[0030]
Table 1 shows the boron concentration in the seed crystal, the finish grinding wheel number, the thickness of the removed etching, the standard deviation of the melt temperature fluctuation of 10 seconds or more, and the diameter of the seed crystal in Examples 1 to 8 and Comparative Examples 1 to 4, respectively. The following shows how the DF ratio changes with the combination of the above conditions of the presence or absence of dissolution.
[0031]
In Examples 1 and 2, the DF ratio was as high as 70% because the processed strain layer on the surface was removed by etching when the boron concentration was within an appropriate range. In addition, in Examples 3 and 4, in addition to the above conditions, the standard deviation of the temperature fluctuation of the melt for a period of 10 seconds or more was suppressed to 4 ° C. or less, so that the DF ratio was further increased to 90%. In Examples 5 to 8, in addition to the conditions of Examples 3 and 4, when the tip of the seed crystal was dissolved by the same length as the diameter of the seed crystal, the DF ratio was further increased to 100%.
[0032]
On the other hand, in Comparative Examples 1 and 2, the etching amount was sufficient, the variation in the melt temperature was as small as 4 ° C., and the boron concentration in the seed crystal was 3 × despite the fact that the tip of the seed crystal was also dissolved by its diameter. Since it is as small as 10 ¹⁸ atoms / cm ³ or as large as 5 × 10 ¹⁹ atoms / cm ³ , dislocations are generated from the surface of the seed crystal, dislocations also grow in the straight body of the grown ingot, and the DF ratio is 0%. became. In Comparative Examples 3 and 4, the boron concentration in the seed crystal was also within an appropriate range, the temperature variation of the melt was as small as 4 ° C., and the tip of the seed crystal was dissolved by the diameter of the seed crystal. Since the amount is small, the DF ratio is as small as 10%.
[0033]
[Table 1]

[0034]
【The invention's effect】
The seed crystal of the present invention is hard due to a high boron concentration, and is a seed crystal that has been subjected to a surface treatment to remove a work-strained layer, so that it is hardly affected by thermal stress, and therefore, even without performing dash necking, A seed crystal in which dislocations hardly occur can be provided.
[0035]
Further, the seed crystal can be manufactured without changing the conventional seed crystal manufacturing process, and an inexpensive seed crystal manufacturing method can be provided.
[0036]
Furthermore, if this seed crystal is used for the production of a silicon single crystal, a large-diameter and heavy silicon single crystal can be easily grown without dislocations. As a result, not only is the effect of significantly reducing the manufacturing cost of large-diameter, large-weight silicon single crystals significant, but also because the seed crystal diameter is increased, the load bearing capacity increases, and during the growth of silicon single crystals. Accidents such as breakage and dropping of the ingot can also be prevented, and the operational safety during the production of large-diameter and heavy-weight silicon single crystals is greatly improved.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the grinding wheel number and the thickness of a work strain layer introduced during seed crystal processing.

Claims (4)

A silicon seed crystal used for producing a silicon single crystal by the Czochralski method, wherein the boron concentration in the silicon single crystal of the base material from which the silicon seed crystal is cut is 4 × 10 ¹⁸ atoms / cm ³ or more and 4 × 10 4 ^A seed crystal for producing a silicon single crystal, wherein the seed crystal is not more than ¹⁹ atoms / cm ³ , and the silicon seed crystal is cut out from a base material silicon single crystal, ground and polished, and then subjected to surface etching. In the step of etching the silicon seed crystal, the amount of etching of at least the portion where the silicon seed crystal comes into contact with the silicon melt during the production of the silicon single crystal satisfies the following formula (I).

Here, Y represents the etching amount (μm), and A represents the grain size (#) defined in JIS R 6001 of the grindstone. ) A method for producing a silicon single crystal by the Czochralski method, wherein a silicon single crystal is pulled up using the seed crystal according to claim 1 without performing dash necking (seed drawing). Single crystal production method. In the method for producing a silicon single crystal by the Czochralski method, a method for producing a silicon single crystal by pulling up a silicon single crystal without performing dash necking using the seed crystal according to claim 1. A method for producing a silicon single crystal, wherein a standard deviation of a temperature fluctuation of a silicon melt at a cycle of 10 seconds or more satisfies 4 ° C. or less. In the method for producing a silicon single crystal by the Czochralski method, the method uses the seed crystal according to claim 1 and partially dissolves the seed crystal in a silicon melt without performing dash necking (seed drawing). A method for producing a silicon single crystal by pulling and growing a silicon single crystal, wherein the amount of dissolution of the seed crystal is equal to or more than the diameter of the seed crystal and the standard deviation of temperature fluctuations of 10 seconds or more of the silicon melt is 4 ° C. or less. A method for producing a silicon single crystal, characterized by satisfying the following conditions:

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