Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2000-03-29
2001-11-13
Hiteshew, Felisa (Department: 1765)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C117S013000, C117S911000
Reexamination Certificate
active
06315970
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an improvement of a seed crystal used in production of a silicon single crystal in accordance with Czochralski method (CZ method).
BACKGROUND ART
Conventionally, there is known for production of a single crystal, a pulling method called Czochralski method (CZ method) wherein a seed crystal is immersed in a melt of polycrystal raw material and is then pulled to grow a single crystal. In the method, a seed crystal held by a seed crystal holding jig needs to be strong enough to bear the weight of a grown crystal.
Recently, a single crystal, for example, CZ silicon single crystal has been getting heavier with increase in a diameter thereof. For such a tendency of increase in weight of the crystal, there have been proposed an improvement in resistance under load of the apparatus, a necking with a large diameter, or a new method substituted for a necking method, a method of holding a crystal mechanically or the like. However, strength of the crystal itself is one of important factors in common.
There have been various propositions as for a seed crystal and a seed crystal holding jig. For example, there is proposed in Japanese Patent Publication (kokoku) No. 5-36395 a seed crystal holding jig consisting of a body of a seed crystal holding jig with a taper, a piece for pressing a seed crystal and a ring for preventing the piece from dropping off. The seed crystal holding jig holds a seed crystal having a tapered cut to pull a single crystal. With such a structure, it is possible to hold a seed crystal without lowering strength of the seed crystal.
A seed crystal to be held by such a seed crystal holding jig is generally a prismatic seed crystal having a tapered cut
1
as shown in
FIG. 2A
, or a columnar seed crystal having a tapered cut
1
as shown in FIG.
2
B. In order to pull the crystal without conducting necking, a seed crystal having a sharp tip end as shown in
FIG. 2C
can be used in some cases (See Japanese Patent Application No. 9-17687).
However, even when a seed crystal is held by the above-mentioned seed crystal holding jig, it is sometimes broken depending on a shape of a seed crystal or weight of a single crystal to be grown and pulled. Breakage of the seed crystal may lead to fall of the single crystal having a heavy weight in a melt in a crucible, which will be a serious accident, and is problematic. On the other hand, it is not desirable for production of a seed crystal and a necking process that a seed crystal is unnecessarily thick.
There has not been an appropriate standard for defining a shape such as a thickness of a seed crystal so far, and a seed crystal having a shape defined experientially has been used for producing a single crystal.
DISCLOSURE OF INVENTION
The present invention has been accomplished in order to solve the above-mentioned problems, and the main object of the present invention is to provide a seed crystal having a sufficient strength so that it may be surely prevented from being broken, even when a heavy single crystal is produced.
To achieve the above-mentioned object, the invention set forth in claims of the present application relates to a seed crystal used for producing a single crystal according to Czochralski method wherein a relation between a sectional area of the seed crystal S (mm
2
) and a weight W (kg) of the single crystal to be pulled is represented by the formula: S>W.
As described above, when the seed crystal wherein a relation between a sectional area of the seed crystal S (mm
2
) and a weight W (kg) of the single crystal to be pulled is represented by the formula: S>W is used, the seed crystal is never broken even in the case that a heavy single crystal is pulled, so that a silicon single crystal can be safely produced using a seed crystal having a minimum thickness.
In that case, a sectional area means a sectional area of a seed crystal at a section vertical to a pulling direction.
It is preferable that the above-mentioned seed crystal has a tapered cut on the side. In that case, the seed crystal can be held firmly, so that it can be held even if the single crystal to be pulled is heavy. Furthermore, strength of the seed crystal is not lowered so much, and therefore, there is no fear of breakage of the seed crystal.
In this case, it is preferable that an angle of the above-mentioned tapered cut is 3 to 15 degree. It is further preferable that a sectional area of the above-mentioned tapered cut is 25% or less of the sectional area S of a seed crystal.
If the angle is less than 3 degree, depth of the cut at which a seed crystal is held is too small, so that the seed crystal is held weakly. If the angle is more than 15 degree, sectional area of the seed crystal at a position where the cut is formed is too small, so that strength of the seed crystal is lowered, and there is caused a problem of strength of the seed crystal holding rig. In order to prevent strength of the seed crystal from being lowered as a result of too small sectional area of the seed crystal at a position where the cut is formed, a sectional area of the tapered cut is preferably 25% or less of the sectional area S of the seed crystal.
A sectional area of a cut means a maximum sectional area at a section vertical to the pulling direction of a portion removed from a seed crystal by cutting or the like in order to form a tapered cut on the seed crystal.
The present invention also relates to a seed crystal used for producing a single crystal according to Czochralski method wherein a relation between a minimum sectional area A (mm
2
) of the seed crystal at a part where the seed crystal is held and a weight W (kg) of the single crystal to be pulled is represented by the formula: A>W.
As described above, if a relation between a minimum sectional area A (mm
2
) of the seed crystal at a part where the seed crystal is held and a weight W (kg) of a crystal to be pulled is represented by the formula: A>W, the seed crystal is never broken at a part where it is held, so that a silicon single crystal can be safely pulled.
The minimum sectional area of the seed crystal at a part where the seed crystal is held means a minimum sectional area of a section of the part where the seed crystal is held by a holding jig or the like, which is vertical to the pulling direction of the seed crystal.
The above-mentioned seed crystal of the present invention can be silicon. The above-mentioned seed crystal can sufficiently cope with a recent tendency of a CZ silicon single crystal to be larger in diameter and be heavier.
In the method for producing a single crystal wherein the above-mentioned seed crystal is used, there is no fear of breakage of the seed crystal, so that a heavy single crystal can be safely produced. Especially, in the method wherein a single crystal is pulled according to Czochralski method with conforming a sectional area S (mm
2
) of a seed crystal and a weight W (kg) of a single crystal to be pulled to the formula: S>W, the single crystal can be safely pulled without breakage of the seed crystal, even when the heavy single crystal is pulled.
As described above, the seed crystal of the present invention enables the single crystal to be safely pulled without breakage of the seed crystal, in the case that it is pulled according to Czochralski method, even if the single crystal has a large diameter and is heavy.
REFERENCES:
patent: 5714267 (1998-02-01), Machida et al.
patent: 0 854 211 A1 (1998-07-01), None
patent: A-8-290994 (1996-11-01), None
patent: A-9-249492 (1997-09-01), None
patent: A-10-203898 (1998-08-01), None
Hiteshew Felisa
Oliff & Berridg,e PLC
Shin-Etsu Handotai & Co., Ltd.
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