In-furnace capsule for housing silicon single crystal, and method and apparatus for in-furnace annealing of NTD silicon using the same

Abstract

To eliminate the need for out-of-reactor annealing of NTD silicon.SOLUTION: In the in-furnace capsule 30 for housing the silicon single crystal for irradiating the silicon single crystal 20 with neutrons 12 in a nuclear reactor 10, the irradiation of the neutrons 12 and annealing are simultaneously performed in the nuclear reactor 10 by using the in-furnace capsule 30 for housing the silicon single crystal having a structure, a material and a sealed gas for raising the temperature of the silicon single crystal 20 to a temperature suitable for eliminating defects generated by the irradiation of the neutrons 12.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an in-furnace capsule for storing silicon single crystals, and an in-furnace annealing method and apparatus for NTD silicon using the same, and in particular to an in-furnace capsule for storing silicon single crystals that can turn phosphorus-doped NTD silicon semi-finished products into NTD silicon products without annealing them outside a nuclear reactor, and an in-furnace annealing method and apparatus for NTD silicon using the same. [Background technology]

[0002] Conventionally, as shown in FIG. 1, a nuclear reactor 10, which is a research reactor used around the world (hereinafter referred to as a research reactor), has been used to transmute silicon Si by neutron irradiation to introduce a different element, phosphorus, into a high-purity silicon single crystal 20 to produce NTD silicon semi-finished products 22, which are then annealed outside the reactor to produce NTD silicon products 24. In this neutron transmutation method, as described in Patent Document 1, as shown in FIG. 2, a high-purity silicon single crystal 20 is irradiated with neutrons 12 to generate 31Si21 from 30Si20 in the silicon. 31Si21 undergoes beta decay to produce 31P22. NTD silicon produced by this neutron transmutation has better resistivity uniformity than silicon produced by other methods, such as gas doping, and is therefore used in products such as power thyristors, power transistors, and diodes. This manufacturing method is usually called the NTD (Neutron Transmutation Doping) method, and silicon semiconductors manufactured using this method are called NTD silicon. Because the NTD method involves adding phosphorus, it is used to manufacture n-type semiconductors.

[0003] When silicon is irradiated with neutrons 12, the 30Si20 in the silicon is irradiated with the neutrons 12, producing 31Si21, as shown in Figure 2. This 31Si21 emits beta rays, undergoes nuclear transmutation, and is converted into the stable isotope of phosphorus 31P22.

[0004] The NTD method utilizes this phenomenon by irradiating a silicon single crystal 20 with neutrons 12 to uniformly dope phosphorus (P) into the single crystal. The distribution of phosphorus in the silicon single crystal exhibits a uniformity in resistance distribution that cannot be obtained by conventional methods of adding impurity elements, making this method a new field in semiconductor manufacturing. [n+30Si] →31Si→[31P+beta ray]-(1) [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-303146 [Non-Patent Document 1] M. Wakatsuki, Y. Matsushita and M. Tajima, “Thermal behavior of MCZ-NTD Silicon”, Extended Abstracts of the 18th (1986 International Conference on Solid State Devices and Materials, Tokyo, 1986, pp.525-528(Fig.2) Summary of the Invention [Problem to be solved by the invention]

[0006] However, when 31P is produced by thermal neutrons, irradiation defects are generated by fast neutrons, resulting in defective NTD silicon semi-finished products 22. These irradiation defects must be eliminated to produce NTD silicon products 24, so the material must be irradiated with neutrons 12 in cooling water at 40-50°C in the reactor 10, and then annealed at a temperature of over 700°C outside the reactor to eliminate the irradiation defects and produce NTD silicon products 24.

[0007] The present invention has been made to solve the above-mentioned conventional problems, and has an object to produce NTD silicon products from phosphorus-doped NTD silicon without annealing it outside a nuclear reactor. [Means for solving the problem]

[0008] The present invention solves the above-mentioned problems by providing an in-reactor capsule for storing silicon single crystals for irradiating the silicon single crystals with neutrons in a nuclear reactor, which has a structure, material, and enclosed gas that raises the temperature of the silicon single crystals to a temperature suitable for eliminating defects that occur due to neutron irradiation.

[0009] Here, the structure can be any one of a single structure containing no heat medium, a double structure containing a single heat medium, and a triple structure containing a double heat medium.

[0010] The material may be a metal containing aluminum, stainless steel, or tungsten.

[0011] The sealed gas may be any of inert gases including helium gas, neon gas, and nitrogen gas, and may be at normal pressure, elevated pressure, or reduced pressure.

[0012] The heat transfer medium may be a triple structure having two layers, the material may be stainless steel, and the sealed gas may be nitrogen gas.

[0013] The present invention also provides an in-furnace annealing method for NTD silicon, characterized in that neutron irradiation and annealing of silicon single crystals are carried out simultaneously in a nuclear reactor using the in-furnace capsule for storing the silicon single crystals.

[0014] The present invention also provides an NTD silicon furnace annealing apparatus comprising an inner furnace capsule for storing the silicon single crystal. [Effects of the Invention]

[0015] According to the present invention, annealing of phosphorus-doped NTD silicon semi-finished products outside a nuclear reactor is not required, and the equipment and time required for heating NTD silicon semi-finished products outside a nuclear reactor are not required, making it possible to produce NTD silicon products cheaply and quickly. [Brief explanation of the drawings]

[0016] [Figure 1] Schematic diagram showing the conventional NTD silicon manufacturing process [Figure 2] Diagram showing the changes in silicon caused by neutron irradiation [Figure 3] Schematic diagram showing the manufacturing process of NTD silicon according to the present invention. [Figure 4] FIG. 1 is an exploded perspective view showing an embodiment of an in-core capsule according to the present invention disposed in an irradiation vessel of a nuclear reactor. [Figure 5] FIG. 1 is a cross-sectional view showing another embodiment of the in-core capsule according to the present invention. [Figure 6] FIG. 1 is a cross-sectional view showing an example of the structure of an inner capsule according to an embodiment of the present invention. [Figure 7] A diagram showing the results of thermal calculations for each of the example structures shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the contents described in the following embodiments. Furthermore, the components in the embodiments described below include those that can be easily imagined by a person skilled in the art, those that are substantially the same, and those that are within the so-called equivalent range. Furthermore, the components disclosed in the embodiments described below may be combined as appropriate, or may be selected and used as appropriate.

[0018] In an embodiment of the present invention, as shown in FIG. 3 , when a high-purity silicon single crystal 20 is inserted into a nuclear reactor 10 and irradiated with neutrons in a cooling water flow path, the silicon single crystal 20 is stored in an in-reactor capsule 30 and irradiated with neutrons in the reactor to generate 31P while raising the temperature with gamma rays in the reactor, thereby eliminating irradiation defects, and converting an NTD silicon semi-finished product 22 in a state in which irradiation defects exist in the silicon into a defect-free state, thereby producing an NTD silicon product 24 in the reactor.

[0019] In the past, silicon single crystals were immersed directly in cooling water and irradiated with neutrons without using an in-furnace capsule. In contrast, in the present invention, silicon single crystals 20 are housed in an in-furnace capsule 30, such as that shown in Figure 4, which is composed of an outer tube 32 and an inner tube 34 made of a heat-generating heat medium and open at the top and bottom, and placed in the irradiation vessel 14. Using gamma rays from the furnace, the silicon single crystal 20 is heated to a temperature suitable for eliminating defects caused by neutron irradiation, depending on the structure (including the presence or absence, number, and thickness of a heat-generating heat medium) and material of the in-furnace capsule 30, and the type and pressure of the gas (referred to as gap-filling gas) filled in the gap 33 between the outer tube 32 and inner tube 34 or the silicon single crystal 20.

[0020] The in-core capsule 30 is placed in an irradiation vessel 14 that is suspended from above in reactor cooling water and rotates so as to ensure uniform neutron irradiation, and that has holes through which cooling water can flow.

[0021] The temperature of the silicon single crystal 20 can be changed by changing the structure (including the presence or absence, number, and thickness of a heat transfer medium for heat generation) and material of the furnace capsule 30, and the type and pressure of the gap filling gas.

[0022] Another embodiment in which the inner cylinders 34 are housed in the outer cylinder 32 in five layers and placed in the cooling water flow path is shown in Fig. 5. In this embodiment, the inner cylinders 34 are stacked in five layers, but the number of layers and arrangement can be changed to one layer, two layers, or any other number of layers as shown in Fig. 4, depending on the size of the neutron beam irradiation area in the reactor.

[0023] FIG. 7 shows the results of thermal calculations for the in-furnace capsules with cross sections of (A) a single structure without a heat transfer medium, (B) a double structure with a single heat transfer medium, and (C) a triple structure with a double heat transfer medium, as shown in FIG. 6. FIG. 7(A) shows the results of calculations for an aluminum single cylinder (no heat transfer medium). FIG. 7(B) shows the results of calculations for a stainless steel double cylinder (inner cylinder 34 as a single heat transfer medium). FIG. 7(C) shows the results of calculations for a stainless steel triple cylinder (inner cylinders 34, 36 as double heat transfer mediums). Here, the diameter (outer diameter) of the silicon single crystal 20 is R10×2=20.0 mm, and the diameter (outer diameter) of the in-furnace capsule 30 is R16.0×2=32.0 mm.

[0024] The temperature at which silicon atoms begin to diffuse within a single crystal is roughly half the melting point (melting point 1687K x 0.5 = 844K (570°C)), so it is thought that annealing can be performed at temperatures above that temperature. Additionally, Figure 2 in Non-Patent Document 1 also indicates that annealing can be performed at temperatures above 700°C.

[0025] If the resistivity of NTD silicon is about 500 Ωcm, the neutron irradiation time is 5 to 10 hours, which is sufficient for annealing.

[0026] In the example shown in Figure 6, when nitrogen gas is used as the gap filling gas and a stainless steel triple cylinder is used, as shown in Figure 7(C), the temperature of the silicon can be raised to approximately 766°C to 768°C, and annealing can be performed.

[0027] The structure of the furnace capsule is not limited to that of the embodiment, and it is possible to use a number of layers other than one or five layers, and to adopt a structure or thickness other than one to three layers.

[0028] The materials for the reactor capsule and heat transfer medium are not limited to aluminum or stainless steel, and can be, for example, tungsten, as long as they are heat-generating due to gamma rays in the reactor, have a high atomic number, are heat-resistant enough to withstand the annealing of silicon, and do not adversely affect the silicon. Iron is not preferred because it oxidizes.

[0029] The filled gas is not limited to inert gases including helium gas, neon gas, and nitrogen gas, and the pressure can be atmospheric pressure, reduced pressure or vacuum to reduce thermal conductivity, or pressurized to increase thermal conductivity. [Explanation of symbols]

[0030] 10…Nuclear reactor 12...Neutron 14...Irradiation container 20...Silicon single crystal 22...NTD silicon semi-finished product (with irradiation defects) 24...NTD silicon products (no irradiation defects) 30...Inner furnace capsule 32...Outer cylinder 33...Gap 34, 36...Inner cylinder (heat medium)

Claims

1. An in-furnace capsule for storing silicon single crystals for irradiating the silicon single crystals with neutrons in a nuclear reactor, characterized in that the in-furnace capsule for storing silicon single crystals has a structure, material, and enclosed gas that raise the temperature of the silicon single crystals to a temperature suitable for eliminating defects that occur due to neutron irradiation.

2. 2. The furnace capsule for storing silicon single crystals as described in claim 1, characterized in that the structure is either a single structure without a heat transfer medium, a double structure with a single heat transfer medium, or a triple structure with a double heat transfer medium.

3. 2. The in-furnace capsule for storing silicon single crystals according to claim 1, wherein the material is a metal containing aluminum, stainless steel, or tungsten.

4. 2. The furnace capsule for storing silicon single crystals according to claim 1, wherein the sealed gas is any of an inert gas including helium gas, neon gas, or nitrogen gas, and is kept at normal pressure, pressurized, or reduced pressure.

5. 2. The furnace capsule for storing silicon single crystals according to claim 1, wherein the structure is a triple structure with a double heat transfer medium, the material is stainless steel, and the sealed gas is nitrogen gas.

6. 6. A method for in-furnace annealing of NTD silicon, comprising simultaneously irradiating a silicon single crystal with neutrons and annealing the silicon single crystal in a nuclear reactor using an in-furnace capsule for storing the silicon single crystal according to claim 1.

7. An NTD silicon in-furnace annealing apparatus comprising an in-furnace capsule for storing silicon single crystals according to any one of claims 1 to 5.

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