Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
1999-04-06
2003-12-30
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C424S200100, C424S200100, C423S348000, C423S349000, C423S350000, C117S020000, C117S902000, C117S911000, C117S916000, C023S301000
Reexamination Certificate
active
06670036
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention related to a method for producing a silicon single crystal, wherein the silicon single crystal is grown by the Czochralski method (CZ method) with or without performing necking operation, and a silicon seed crystal.
2. Related Art
In the conventional production of silicon single crystals according to the CZ method, a silicon single crystal is used as a seed crystal, which is brought into contact with silicon melt and then slowly pulled while being rotated to grow a single crystal ingot. In such a method, after the seed crystal is brought into contact with the silicon melt, the so-called necking is performed to form a neck portion having a smaller diameter of around 3 mm to eliminate dislocation propagated from slip dislocations generated in the seed crystal in high density due to thermal shock, then the diameter of the crystal is increased to a predetermined diameter, and a dislocation-free silicon single crystal is pulled. The necking operation performed as described above is widely known as the Dash Necking method, and has commonly been used for pulling a silicon single crystal ingot by the CZ method.
That is, conventionally used seed crystals have, for example, a cylindrical or prismatic shape with a diameter or side length of about 8-20 mm, and have a cut-away portion or a notch for attaching the seed crystal to a seed holder, and a flat bottom surface in a tip end thereof, which is initially brought into contact with the silicon melt. In order to safely pull a single crystal ingot while withstanding the weight of heavy single crystal ingot, it is difficult to use a thickness of the seed crystal smaller than the range mentioned above, considering the strength of the material.
Because a seed crystal having such a shape as described above has a large heat capacity of the tip end which is brought into contact with the melt, a large temperature difference is suddenly generated in the crystal upon the contact with the melt, and thus slip dislocations are generated at a high density. Accordingly, the aforementioned necking operation becomes necessary in order to eliminate these dislocations to grow a single crystal.
However, under the circumstance described above, the necking operation must be performed to a minimum diameter of 3-5 mm in order to completely eliminate the dislocations even if the other necking conditions are selected variously. The mechanical strength obtained by such a diameter has become insufficient for supporting a single crystal ingot getting heavier with recent use of a larger diameter of the silicon single crystals, and thus a serious accident threatens to occur, for example, the single crystal ingot falls due to breakage of the neck portion of a small diameter.
To solve these problems, the applicants of this application have previously suggested such invention as disclosed in Japanese Patent Laid-Open Publication No. 5-139880, and Japanese Patent Application No. 8-87187 (Japanese Patent Laid-Open Publication No. 9-255485). These inventions relate to techniques employing a seed crystal whose tip end has wedge shape or a hollow to reduce as far as possible the slip dislocations generated upon the contact of the seed crystal with the silicon melt, and thereby allowing dislocation-free production even when a relatively large diameter of the neck portion is used to improve the mechanical strength.
Though these methods are expected to improve the mechanical strength of the neck portion to some extent because of the use of a large diameter of the neck portion, they still perform the necking operation as ever, and hence form a necking portion containing slip dislocations. For the pulling of recent single crystals whose length and diameter are increasingly getting longer and larger, for example, which have a weight of 150 kg or more, the mechanical strength of the neck portion obtained even in these methods may become insufficient, and therefore they cannot be considered ultimate solutions.
Therefore, the applicants of the present application previously developed a method for converting crystals into single crystals without forming a neck portion, which is the most problematic factor as for ensuring the mechanical strength, and filed a patent application therefor (Japanese Patent Application No. 9-17687). This method uses a seed crystal having a tip end in a sharp-pointed shape or truncated sharp-pointed shape, which tip end is brought into contact with the silicon melt as the seed crystal. First, the tip end of the seed crystal is carefully brought into contact with the silicon melt, the seed crystal was let down at a low rate to melt the tip end of the seed crystal until it gets a desired diameter, and then the seed crystal is slowly pulled upwardly to grow a silicon single crystal ingot of a desired diameter without performing necking operation.
According to this method, because the contact area when the tip end of the seed crystal is initially brought into contact with the silicon melt and heat capacity of the tip end are small, thermal shock or steep temperature gradient does not occur in the seed crystal, and hence the dislocations are not introduced. Then, by letting down the seed crystal at a low rate to melt it down until the tip end of the seed crystal gets a desired diameter, steep temperature gradient is prevented, and the slip dislocations are not introduced into the seed crystal also during the melting down process. Finally, a silicon single crystal ingot can be grown to a desired diameter by slowly pulling the seed crystal as it is with no need to perform the necking, because the seed crystal has the desired diameter, no dislocation, and sufficient strength.
As described above, while temperature holding or heating of seed crystals above the melt, shapes or methods for reducing thermal shock upon seeding and the like have been suggested as means for lowering the initial dislocation density for the conventional necking seeding method, the diameter of the neck portion, which has a certain upper limit, has become to be unable to follow the production of larger and heavier single crystal ingots. In addition, such conditions do not necessarily afford a high rate of success in making crystals dislocation-free.
Therefore, the dislocation-free seeding method without performing the necking operation, which can cope with the use of such a larger diameter and heavier weight as mentioned above, has been established.
However, it is the rate of success in making crystals dislocation-free that may be a difficulty in the dislocation-free seeding method. That is, according to this method, if dislocations are once introduced, the operation cannot be reattempted unless the seed crystal is changed. Therefore, it is particularly important to improve the rate of success in making crystals dislocation-free. In addition, even though the seeding is performed in a dislocation-free state in this method, slip dislocations may be generated when the seed crystal is left at a temperature around the melting point of silicon for a certain period of time after a predetermined length of the tapered tip end of the seed crystal is melted, or depending on the period requiring for starting the crystal growth, and such dislocations may further increase. Investigations of the cause of this phenomenon revealed that the control of the factors which had conventionally been controlled, for example, shape of the seed crystal, temperature holding time above the melt surface, melting down rate, single crystal growing rate and the like, is not sufficient for eliminating the phenomenon, and such control could not afford so high rate of success in making crystals dislocation-free and sufficient reproducibility.
Further, as shown in FIG.
6
(
b
), a conventional seed crystal holder have a structure where a straight body
2
of a seed crystal
1
is inserted into a cylindrical member of the seed crystal holder body, and the seed crystal is fixed with a taper pin
16
inserted from the side face of the cylindrical member into a
Iino Eiichi
Kimura Masanori
Hogan & Hartson L.L.P.
Kiliman Leszek
Shin-Etsu Handotai & Co., Ltd.
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