Quartz crucible reproducing method

Details Quartz crucible reproducing method Quartz crucible reproducing method

2000-09-28

2001-10-16

Hiteshew, Felisa (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Processes of growth from liquid or supercritical state

Having pulling during growth

C065S091000, C065SDIG008, C117S200000, C117S900000

Reexamination Certificate

active

06302957

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a quartz crucible reproducing method for reusing a quartz crucible after being used for producing single crystal silicon by the Czochralski (CZ) method.
2. Description of the Prior Art
In producing a high purity single crystal silicon for a semiconductor by the CZ method, a quartz glass based crucible is essential to maintain silicon melt during a single crystal producing. The reason is as follows. In order to maintain silicon with a high activity at a high temperature in a molten state, it is required to maintain a container shape even at around 1450° C. The quartz glass based crucible has a variety of advantageous characteristics, i.e., high heat resistance characteristics, high purity characteristics in which impurities are controlled at a level of ppm, and further, possession as a constituent element of oxygen that can be efficiently utilized even if it is dissolved in a silicon melt.
However, a high purity quartz crucible used for producing single crystal silicon by the CZ method is very expensive. Thus, the rate of the crucible cost in the manufacturing cost of single crystal silicon is large, which becomes a large obstacle in reducing the manufacturing cost.
A repeated use of the quartz crucibles is the most efficient to reduce the crucible cost. However, as it will be described later, crucible reusing technique has not been established yet. Therefore, there is proposed a continuous CZ method in which, while a crystal producing is carried out, a silicon raw material is additionally supplied, thereby increasing a crystal producing quantity per batch or a multiplying technique in which, after crystal producing has been completed, a silicon raw material is supplied to a residual silicon melt contained in the crucible, thereby repeating crystal producing.
However, in these techniques, there is a problem that, since crystal producing per batch takes more time, probability of causing dislocation is increased during this period, thus making it difficult to cause dislocation free producing. This problem has not been essentially solved.
BRIEF SUMMARY OF THE INVENTION
OBJECT OF THE INVENTION
In the meantime, an attempt to reuse a quartz crucible is described in Japanese Utility Model Publication No. 48-41319. In this publication, there is proposed a specific method for, after using a quartz crucible, removing the inner quartz crucible from an outer support crucible.
In producing a single crystal silicon using the CZ method, however, a part of silicon melt remains in the quartz crucible after the producing has been completed, the residual melt coagulates, and the temperature is lowered. In this process, there is an essential problem that a quartz crucible cracks due to a difference in a thermal expansion coefficient between silicon and quartz, and the crucible cannot be reused after a crystal producing for a batch.
In addition, even if the residual melt in the crucible is removed and the quartz crucible can be removed from the outer support crucible without causing cracks, the crucible in producing is softened and deformed or dislocation free ratio is very low in the second use or later. Thus, actual reuse is impossible at a present stage.
It is an object of the present invention to provide a quartz crucible reproducing method capable of repairing a quartz crucible after being used up to a level at which crystal producing substantially equal to a new crucible is possible.
SUMMARY OF THE INVENTION
In the case where a quartz crucible after being used is reused, a low dislocation free ratio is problematic in particular. According to the present inventor's investigation, a single crystallization ratio is merely 30% when a quartz crucible after being used is reused without any treatment. Alternatively, a single crystallization ratio exceeds 90% when a new quartz crucible is used. A single crystallization ratio is obtained by the equation below.
(Dislocation free crystal length)/(cylindrical crystal length)×100
From such circumstances, the present inventor made a variety of discussions as to the causes of, and countermeasure against, the low dislocation free ratio which inhibits reuse of the quartz crucible. As a result, the following fact is clarified.
In the quartz crucible after being used, even if the residual melt is removed, the extraneous material such as silicon adheres to the inner surface of the crucible. In addition, grain shaped extraneous material such as silicon mono-oxide or silicon dioxide adheres thereto. These extraneous materials must be removed in order to reuse the quartz crucible, but dislocation free ratio is not remarkably improved by removing the extraneous material. This is because the inner surface of the crucible is eroded by crystallization called cristobalite formation and its inside is stripped out during reuse. This erosion is called devitrification. As a result of cristobalite formation, the inside is brittle as if the powder were blowing, silicon adheres to a deep erosion portion, and dislocation frequently occurs during reuse.
Therefore, for reuse of the quartz crucible, it is indispensable to carry out the repair of the inner surface where cristobalite formation occurs as well as removal of the extraneous material adhering to the inner surface of the crucible. Alternatively, a new quartz crucible consists of amorphous silicon dioxide.
As a result of further investigation, it was found that chemical etching treatment such as acid wash treatment or gas etching treatment can be effectively employed as treatment for removing extraneous materials adhering to the inner surface of the crucible; non-crystallization of the inner surface using high temperature heat treatment can be effectively employed as treatment for repairing the inside where cristobalite formation occurs; and the quartz crucible after being used can be repaired to performance substantially equal to that of a new crucible by using composite treatment in which these treatments are combined with each other.
In addition, according to chemical etching treatment or mechanical grinding treatment, the extraneous material adhering to the inner surface of the crucible is removed; an inside layer portion where cristobalite formation occurs is removed; a non-crystal quartz appears in the inner surface of the crucible, and its inner surface is non-crystallized. Therefore, chemical etching treatment or mechanical grinding treatment can be effectively employed as treatment for repairing the inner surface where cristobalite formation occurs.
The quartz crucible reproducing method according to the present invention has been developed based on these findings. In this method, a quartz crucible is reproduced by carrying out the repair treatment for the inner surface employing at least one of the chemical etching treatment, mechanical grinding treatment, and high temperature heat treatment for the inner surface of the quartz crucible after being used for producing the single crystal silicon by the CZ method.
By using at least one of the chemical etching treatment, mechanical grinding treatment, and high temperature heat treatment, the inner surface of the quartz crucible after being used is non-crystallized, thereby making it possible to repair the crucible up to a level at which crystal producing substantially equal to a new crucible is possible.
The non-crystallization ratio on the inner surface of the crucible is preferably 70% or more. If the non-crystallization ratio is less than 70%, the single crystallization ratio in reuse is not remarkably improved.
There is provided an advantageous effect that high temperature heat treatment not only makes the inner surface of the crucible inside non-crystalline, but also lowers the infrared ray transmittance on the bottom portion of the crucible and inhibits dislocation at the time of reuse, in particular, at the latter period of pull-up.
That is, the quartz crucible is made of a transparent layer whose inside layer is free of air bubbles in order to prevent dislocation

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