JPH09301709A - Method for casting silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the failure of a casting and a casting mold even if the casting is flat by managing the cooling rate in the bottom of the casting mold to specific conditions at the time of casting polycrystalline silicon by unidirectionally solidifying the silicon in the casting mold from the lower part to the upper part by a prescribed method. SOLUTION: The cooling rate in the bottom of the casting mold 40 is managed to <=5 deg.C/min at the time of casting the polycrystalline silicon by lowering the casting mold 40 from the upper part to the lower part, thereby unidirectionally solidifying the silicon 30 in the casting mold 40 from the lower part to the upper part within a solidifying furnace 20 provided with a heating section 21 in the upper part and a cooling section 22 in the lower part. At the time of casting, the front surface of the silicon 30 in the casting mold 40 is preferably maintained at a molten state until the time of the final solidification by increasing the areas of heaters 23a to (c), 24 contributing to the front surface heating of the silicon in the casting mold 40 accompanying with the descending of the casting mold 40. Namely, the failure of the casting and the casting mold by the volumetric expansion occurring in the earlier solidification of the upper part, the confinement of the molten part in the casting and the solidification of the molten part is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for casting polycrystalline silicon, and more particularly to the production of flat polycrystalline silicon products having a lower height than the lateral width such as a silicon electrode for plasma etching and a wafer boat for diffusion treatment. The present invention relates to a silicon casting method suitable for.

[0002]

2. Description of the Related Art In recent years, as the degree of integration of semiconductor devices has increased, semiconductor jigs such as electrodes for plasma etching and wafer boats for diffusion treatment have been made of polycrystalline silicon. These polycrystalline silicon products have traditionally been cut from large ingots,
Alternatively, it is manufactured by cutting out from a rod manufactured by vapor deposition. However, there is an inherent problem that the yield is poor and the silicon raw material is lost a lot because it is entirely cut, and when a vapor deposition rod is used, there is also a problem that it is easily cracked during processing due to residual stress inside the material.

In order to solve these problems, recently, a product having a shape close to a product has been directly cast, but in the casting of polycrystalline silicon, the volume of silicon increases at the time of solidification. I need to escape the minute. For this reason, in general, a casting device in which the heater in the solidification furnace is divided in the vertical direction is used, and there is a method of unidirectionally solidifying the silicon in the mold placed in the solidification furnace from the lower part to the upper part by independent control of the multistage heater. Has been adopted. In some cases, in a solidification furnace with a heating part in the upper part and a cooling part in the lower part,
A method of unidirectionally solidifying the silicon in the mold from the lower part to the upper part by lowering the mold from the upper part to the lower part is also used.

[0004]

However, when the cast product is a flat one having a height lower than the width such as a silicon electrode for plasma etching, a wafer boat for diffusion treatment, etc., it is not unidirectionally solidified by the independent control of the multi-stage heater. , In addition, the amount of casting is essentially small, the vertical length is short, and the temperature lowering heater and the temperature raising heater come close to each other and interfere with each other, so that the cooling of the lower part of the silicon in the mold and the heating of the upper part are insufficient. Becomes As a result, there has been a problem that the upper part is solidified first and the molten part is confined in the casting, and the casting and the mold are damaged due to volume expansion when the molten part is solidified later.

Here, when the temperature in the solidification furnace during casting is lowered, silicon is poured into the mold and at the same time, the silicon solidifies from the lower part to the upper part, but internal stress remains, so that during casting and subsequent processing. Problems such as cracks occur.

On the other hand, in the method of unidirectionally solidifying the silicon in the mold from the lower part to the upper part by pulling the mold from the heating part to the cooling part, since the upper surface is sufficiently heated and a temperature gradient is easily formed, the upper part is Damage due to first solidification is unlikely to occur. However, if the casting is flat with a height lower than the width, the temperature gradient becomes too large and internal stress tends to remain, so there was a problem such as cracking during casting and subsequent processing. .

In order to approximate the casting shape to the product shape,
The core may be installed in the mold, but since the core is attached to the bottom of the mold, when the mold is lowered to cool the silicon in the mold from the bottom, the temperature drop due to heat dissipation of the core occurs. It becomes remarkable. Therefore, if the core penetrates the silicon in the mold and the upper part of the core projects from the upper surface of the silicon, the upper part of the silicon will solidify first and the molten part will also melt even if the mold is pulled out from the heating part to the cooling part However, there is a problem that the casting and the mold are damaged due to being trapped in the casting.

The object of the present invention is to solve the problem that the cast product and the mold are damaged during solidification cooling even when the cast product is a flat product whose height is smaller than the width, and also the problem that the cast product is damaged during processing. It is to provide a silicon casting method which can solve

[0009]

In order to achieve the above object, the silicon casting method of the present invention lowers the mold from the upper part to the lower part in a solidification furnace having a heating part in the upper part and a cooling part in the lower part. Thus, in the method for casting polycrystalline silicon in which the silicon in the mold is unidirectionally solidified from the lower part to the upper part, the cooling rate at the bottom of the mold is controlled to 5 ° C./minute or less.

When the cooling rate at the bottom of the mold is controlled to 5 ° C./min or less, the temperature gradient in the vertical direction of silicon in the mold is relaxed. Therefore, even if the cast product is a flat product having a height lower than the lateral width, damage to the cast product due to internal residual stress is prevented. If this cooling rate exceeds 5 ° C./minute, damage to the cast product and mold due to the solidification of the upper part of silicon first is prevented, but damage to the cast product due to internal residual stress is not prevented.

It is desirable that the heating section has heaters arranged vertically or stepwise or continuously. When the mold is pulled out from such a heating unit to the cooling unit, the heater area that contributes to the upper surface heating of the silicon in the mold increases as the mold is pulled out. Therefore, if the heater output of each stage is not particularly reduced during the lowering of the mold, with the lowering of the mold,
The amount of heat received on the upper surface of silicon increases. Therefore, even if the solidification of silicon in the mold proceeds, the upper surface of the silicon can be maintained in a molten state until the final solidification. Thus, even if the cast product is a flat product having a height smaller than the width, the silicon in the mold can be surely unidirectionally solidified from the lower part to the upper part.

When a core having a specific heat of silicon or more is used as the core when the core is installed in the mold, the temperature drop due to heat dissipation of the core is suppressed. Even when it projects from the upper surface of the mold, damage to the cast product and the mold due to the upper part of the silicon solidifying first is prevented. By the way, the specific heat of silicon is
It is 1.0 J / gK.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of a casting apparatus, and FIG. 2 is a sectional view of a mold.

As shown in FIG. 1, the casting apparatus comprises a melting furnace 1
0 and a solidification furnace 20. These are installed in a vacuum chamber (not shown). The melting furnace 10 melts silicon charged in a crucible 11 made of high-purity quartz. The molten silicon 30 is used in the melting furnace 10.
Is poured into the mold 40 in the solidification furnace 20.

The solidification furnace 20 has a rectangular tube-shaped furnace body having an open upper surface and a lower surface, and a vertical cylinder 26 for raising and lowering the mold 40 in the furnace body. The upper part of the furnace body is a heating part 21, and the lower part is a cooling part 22. Inside the heating section 21, side heaters 23a, 23b, and 23c having a rectangular frame shape are arranged in three stages. Further, a small top heater 24 in the shape of a square frame is arranged above these heaters and positioned just below the opening 25.

The cylinder 26 is inserted into the furnace body from below, and supports the mold 40 in the furnace body through the four columns 27 above. And by being driven in the axial direction,
The mold 40 is moved up and down in the furnace. 28 is a lid that closes the opening 25.

As shown in FIG. 2, the mold 40 is used to manufacture a polycrystalline silicon electrode for plasma etching, and a separable dish-shaped silicon holding portion 42 is placed on a base 41, and the inside thereof It has a structure in which a thin core 43 is laid. When the molten silicon 30 is injected into the silicon holder 42, the core 43 is completely immersed in the molten silicon 30. The material of the silicon holder 42 is carbon, and the material of the core 43 is also carbon.

Next, an operation example using the above casting apparatus will be described. In the casting operation, the inside of the chamber is about 13
Evacuate to 3 Pa. In this state, the silicon raw material charged in the crucible 11 is melted. On the other hand, in the solidification furnace 20, the mold 40 is supported in the middle position of the heating unit 21, and the heaters 23a, 23b, 23c installed in the heating unit 21 are installed.
The temperature of 24 and 24 is raised to 1455 ° C. to preheat the mold 40.

With respect to the preheating temperature of the mold 40, if the temperature is low, for example, 1400 ° C. or less, internal residual stress is generated during solidification of silicon, and in the worst case, cracks are generated in the silicon casting, so that it is generally higher than the melting point of silicon. Temperature is required and the cast product is thin like a polycrystalline silicon electrode for plasma etching, the heat capacity is small.
It is desired that the temperature be 50 ° C. or higher.

When the mold 40 is preheated to a predetermined temperature, the molten silicon 30 in the crucible 11 is cast into the mold 40. When the casting is completed, the mold 40 is gradually lowered until the upper surface of the silicon in the mold 40 comes out of the heating section 21.

As a result, the mold 40 moves from the middle position to the lower position in the heating section 21 and is further drawn out of the heating section 21 into the cooling section 22. As a result, the mold 40 is gradually cooled from the bottom. On the other hand, the upper surface of the silicon in the mold 40 is initially heated by the top heater 24 for heating the upper surface and the upper side heater 23a arranged in the uppermost stage, but as the mold 40 goes down, the middle side heater is heated. 23b and the lower side heater 23c also receive heat, and the amount of heat received increases.

Thus, the molten silicon 30 injected into the mold 40 gradually radiates heat from the lower portion and is cooled, whereby it is unidirectionally solidified from the lower portion to the upper portion, and the upper surface thereof is maintained in a molten state until the final solidification. It

At this time, the descending speed of the mold 40 is 3 mm /
Minutes. As a result, the cooling rate at the bottom of the mold 40 is 5
The temperature was 4.2 ° C / min, which was below ° C / min. Casting amount is 3.5k
g, the descending amount of the mold 40 is 290 mm. in this case,
The casting and the mold 40 did not break during casting. After casting, the cast product was processed into a polycrystalline silicon electrode for plasma etching, but the cast product was not damaged during processing.

For comparison, the lowering speed of the mold 40 was 5 m.
m / min. As a result, the cooling rate at the bottom of the mold 40 was 5.9 ° C./min, which exceeded 5 ° C./min. in this case,
The cast product taken out from the mold 40 was broken due to internal residual stress due to rapid solidification.

As another comparative example, the mold 4 is used after the casting is completed.
0 is fixed to the middle position in the heating unit 21, the top heater 24 and the upper side heater 32a maintain 1455 ° C., the middle side heater 23b lowers the temperature at a rate of 275 ° C./H, and the lower side heater 23c. The heaters in the respective stages were controlled so as to be turned off to solidify the silicon in the mold 40. However, due to temperature interference between the heaters, the actual heater temperature dropped to 1420 ° C. for the top heater 24 and the upper side heater 32a, and dropped to 400 ° C./H for the middle side heater 23b. When the cast product was taken out from the mold 40 after casting, the cast product was damaged due to the containment of the molten portion due to the solidification of the upper surface first.

As the mold 40, the one for the wafer boat shown in FIG. 3 was used. Unlike the one for the electrode, this mold 40 has a core 43 that penetrates the molten silicon 30 in the silicon holder 42. The silicon holding part 42 includes a split type cylindrical part 42a and a bottom plate 42 fitted in the lower part thereof.
b, which are integrated by upper and lower fixing rings 44 and 45. The core 43 is placed on the bottom plate 42b, and its material is a carbon material. The specific heat of the carbon material is 2.02 J / gK at the melting point of silicon. Reference numeral 46 is an annular hot water leak receiver.

The same method as the first method (the mold descending speed is 3
mm / min, cooling rate at the bottom of the mold is 4.5k / min) 2.0k
g of a wafer boat casting was produced. Even though the upper part of the core 43 projects above the silicon, the core 43
Due to the heat storage of 1, the situation in which the upper surface of the silicon solidified first was avoided, so the cast product was not damaged.

For comparison, the material of the core 43 was changed to silicon nitride (specific heat at the silicon melting point temperature was 0.7 J / gK). The casting was damaged because it solidified.

[0029]

As described above, the method of casting silicon according to the present invention, by lowering the mold from the upper part to the lower part in the solidification furnace having the heating part in the upper part and the cooling part in the lower part,
In the method of casting polycrystalline silicon in which the silicon in the mold is solidified from the bottom to the top, the cooling rate of the mold bottom is set to 5 ° C.
Even if the casting is flat with a height lower than the width, it can be prevented from damaging the casting and the casting due to the solidification of the upper part of the silicon in the casting first by controlling to less than 1 / min. In addition, it is possible to prevent damage to the cast product due to residual stress.

When a core whose specific heat is higher than that of silicon is placed in the mold, even if the upper part of the core protrudes from the upper surface of the silicon in the mold, the upper part of the silicon solidifies first to form a cast product and Damage to the mold is prevented.

Therefore, according to the silicon casting method of the present invention, a polycrystalline silicon cast product having a shape close to that of the product can be economically manufactured with high yield.

[Brief description of drawings]

FIG. 1 is a schematic view showing the structure of an example of a casting apparatus used in the silicon casting method of the present invention.

FIG. 2 is a vertical cross-sectional view illustrating a mold structure for a plasma etching electrode.

FIG. 3 is a vertical cross-sectional view illustrating the structure of a mold for a wafer boat.

[Explanation of symbols]

 10 Melting Furnace 20 Solidification Furnace 21 Heating Part 22 Cooling Part 23, 24 Heater 30 Molten Silicon 40 Mold 42 Silicon Holding Part 43 Core

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenji Fujita 1 Higashihama-cho, Amagasaki-shi, Hyogo Sumitomo Citicus Co., Ltd.

Claims (3)

[Claims] 1. A polycrystalline silicon for unidirectionally solidifying silicon in a mold from the lower part to the upper part by lowering the mold from the upper part to the lower part in a solidification furnace having a heating part in the upper part and a cooling part in the lower part. In the casting method, the cooling rate of the bottom of the mold is controlled to 5 ° C./min or less. 2. The upper surface of the silicon in the mold is kept in a molten state until the final solidification by increasing the heater area contributing to the upper surface heating of the silicon in the mold as the mold descends. The silicon casting method according to claim 1. 3. The silicon casting method according to claim 1, wherein when the core is installed in the mold, a core having a specific heat of silicon or more is used as the core.