JP7200921B2 - Method for irradiating silicon ingot with neutrons, method for manufacturing silicon ingot, and method for manufacturing silicon wafer - Google Patents

Description

The present invention relates to a neutron irradiation method for a silicon ingot, a method for manufacturing a silicon ingot, and a method for manufacturing a silicon wafer.

In recent years, thyristors, bipolar transistors, MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), insulated gate bipolar transistors (IGBTs), and the like have been actively developed as switching devices for power applications. Among them, the IGBT is a device that combines the high speed of a MOSFET with the low saturation voltage characteristics of a bipolar transistor, and is required to have a large capacity, high withstand voltage, and high-speed switching, such as power sources for power motors in hybrid and electric vehicles. It is attracting attention for its uses.

An IGBT is a gate voltage-driven switching device (also referred to as a "vertical IGBT") having a gate electrode and an emitter electrode formed on the front side of a silicon wafer and a collector electrode formed on the back side of the silicon wafer, The switching current flows across the silicon wafer between the frontside emitter and the backside collector. Therefore, high quality is required for the entire silicon wafer.

For example, when the resistivity of a silicon wafer varies greatly in the in-plane direction of the wafer, this leads to variations in the characteristics of devices formed on the silicon wafer. Therefore, it is necessary that the variation in resistivity is small.

RRG (Radial Resistivity Gradient) is one of indices for evaluating variations in resistivity in the wafer in-plane direction. RRG can be obtained by measuring the resistivity in the wafer surface at predetermined intervals, for example, using a four-probe resistance measuring instrument or the like, and using the following equation (1) from the measured resistivity.
RRG (%) = (( _Rmax - _Rmin )/ _Rmin ) x 100 (1)
Here, R _max represents the maximum measured resistivity value, and R _min represents the minimum measured resistivity value.

Neutron Transmutation Doping (NTD) technology is available as a technology for obtaining n-type silicon wafers with reduced variations in resistivity. The NTD technology is a technology that utilizes the fact that ³⁰ Si, which is one of the isotopes contained in silicon, is finally converted to ³¹ P when thermal neutrons collide with ³⁰ Si.

Silicon has three isotopes ²⁸ Si, ²⁹ Si, and ³⁰ Si with different mass numbers at abundance ratios of 92.23%, 4.67%, and 3.10%, respectively. When such silicon is irradiated with neutrons and thermal neutrons collide with ³⁰ Si, γ-rays are emitted and the thermal neutrons are replaced with protons to form ³¹ Si.

Since the generated ³¹ Si is extremely unstable with a half-life of 2.62 hours, it undergoes β-decay to emit β-rays, and ³¹ Si becomes phosphorus ( ³¹ P). Since ³⁰ Si exists uniformly in silicon, in principle, phosphorus can be uniformly distributed in silicon by neutron irradiation, and an n-type silicon wafer with small resistivity variation can be obtained.

FIG. 1 shows a schematic diagram of an example of a neutron irradiation apparatus. A neutron irradiation apparatus 1 shown in FIG. 1 includes a core 11 that emits neutrons, and a storage container 12 that stores and holds a silicon ingot I. The storage container 12 is rotatable so that the ingots I stored therein are uniformly irradiated with neutrons.

FIG. 2 shows an example of the storage container 12. As shown in FIG. The storage container 12 shown in FIG. 2 has a cylindrical portion 13a and a bottom portion 13b. have The containment vessel 12 is constructed of a suitable metal, for example aluminum.

When the ingot I is stored in the storage container 12, the ingot I may be stored in the storage container 12 in a single state. It may be stored in a divided state into portions I ₁ , I ₂ , and I ₃ ). During transportation and neutron irradiation, the storage container 12 is filled with water, and during neutron irradiation, the surroundings of the storage container 12 inside the neutron irradiation apparatus 1 are filled with water.

In this state, the ingot I is irradiated with neutrons from the side surface of the ingot I (in a direction perpendicular to the extending (longitudinal) direction) from the core 11 while rotating the container 12 . As a result, ³⁰ Si in the ingot I is changed to ³¹ P to obtain an n-type silicon ingot I with small resistivity variation in the radial direction and an n-type silicon wafer with small resistivity variation in the wafer in-plane direction. be done.

Various techniques have been proposed for obtaining silicon wafers with less variation in resistivity in the in-plane direction of the wafer using the NTD technique. For example, in Patent Document 1, a silicon single crystal containing a COP (Crystal Originated Particle) generation region whose interstitial oxygen concentration, size and density are adjusted to predetermined ranges by the Czochralski (CZ) method is pulled up. , describes a technique for obtaining a silicon wafer having an RRG of less than 5% by irradiating the obtained silicon single crystal with neutrons.

JP 2010-265143 A

A silicon wafer having a small variation in resistivity in the in-plane direction of the wafer can be obtained by the technique disclosed in Patent Document 1 or the like. However, as a result of studies by the present inventors, it was found that the variation (specifically, RRG) in the wafer in-plane direction of the resistivity of the silicon wafer sampled from the upper and lower ends of the ingot I after neutron irradiation It was found to be larger than those taken from the central part. In other words, it was found that, in the longitudinal direction of the ingot I after neutron irradiation, the variation in resistivity in the radial direction was greater at the upper and lower ends than at the central portion.

The present invention has been made in view of the above-mentioned problems, and its object is to reduce the radial variation in resistivity in the longitudinal direction of a silicon ingot after neutron irradiation to the upper and lower ends than in the central portion. To propose a method of irradiating a silicon ingot with neutrons capable of suppressing an increase in neutrons.

The present invention for solving the above problems is as follows.
[1] A single-crystal silicon ingot is placed in an aluminum storage container having a cylindrical container body consisting of a cylinder and a bottom and a lid, and the ingot is irradiated with neutrons from the side surface of the ingot. in the method of
A silicon ingot characterized in that the neutron irradiation is performed in a state in which a cylindrical dummy block made of silicon is arranged at the bottom of the container body, and the ingot is arranged on the dummy block. neutron irradiation method.

[2] The method for irradiating a silicon ingot with neutrons according to [1], wherein the neutron irradiation is performed in a state where another cylindrical dummy block material made of silicon is further arranged on the ingot.

[3] The neutron irradiation is performed by placing a silicon ingot for evaluation longer than a neutron irradiation range in the vertical direction of the neutron irradiation device in advance in the container main body and placing the dummy block in the neutron irradiation device used for the neutron irradiation. A longitudinal region of the evaluation ingot deviating from the desired resistivity range by irradiating the evaluation ingot with neutrons from the neutron irradiation device. is specified, and the method for irradiating a silicon ingot with neutrons according to [2] is performed with the dummy block material placed in the region in the container body where the specified region was placed.

[4] A silicon ingot obtained by a predetermined method is irradiated with neutrons by the method for neutron irradiation of a silicon ingot according to any one of [1] to [3]. A method for manufacturing a silicon ingot, characterized by:

[5] A method for producing a silicon wafer, comprising subjecting the silicon ingot produced by the method for producing a silicon ingot according to [4] to wafer processing to obtain a silicon wafer.

According to the present invention, in the longitudinal direction of the silicon ingot after neutron irradiation, it is possible to prevent the radial variation in resistivity from becoming larger at the upper and lower ends than at the central portion.

It is a schematic diagram of an example of a neutron irradiation apparatus. FIG. 2 is a diagram showing an example of a storage container for storing silicon ingots to be irradiated with neutrons. FIG. 4 is a diagram illustrating an example of storing a silicon ingot to be irradiated with neutrons in a storage container in the present invention. FIG. 10 is a diagram illustrating another example of storing a silicon ingot to be irradiated with neutrons in a storage container in the present invention. FIG. 4 is a diagram showing the relationship between the sampling position of the silicon wafer from the silicon ingot and the RRG value of the silicon wafer.

(Neutron irradiation method for silicon ingot)
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the neutron irradiation method for a silicon ingot according to the present invention, a single crystal silicon ingot is stored in an aluminum storage container having a cylindrical container main body consisting of a cylindrical portion and a bottom portion, and a lid portion. In this method, the ingot is irradiated with neutrons from the side surface. Here, the neutron irradiation is performed in a state in which a cylindrical dummy block made of silicon is arranged at the bottom of the container body, and the ingot is arranged on the dummy block.

As described above, the silicon ingot I was stored in the aluminum storage container 12 shown in FIG. 2, and the ingot I was irradiated with a neutron beam using the neutron beam irradiation apparatus 1 shown in FIG. The variation in the wafer in-plane direction of the resistivity of the silicon wafer sampled from the upper and lower ends of the ingot is larger than that sampled from the central portion of the ingot I, that is, the longitudinal direction of the ingot after neutron irradiation , the radial variation in resistivity is greater at the upper and lower ends than at the center.

The inventors of the present invention have found that the cause of the above-mentioned radial variation in resistivity is the aluminum constituting the storage container 12 and the silicon constituting the ingot I to be irradiated with neutrons. It was inferred that this was due to the difference in

That is, aluminum has a higher neutron absorption rate than silicon. In the storage container 12 shown in FIG. 2, an aluminum lid portion 14 is arranged near the upper end portion of the ingot I, and the lower end portion is arranged on the bottom portion 13b of the aluminum container main body 13. there is

The inventors found that when neutrons are irradiated from the core 11, the neutrons near the upper end of the ingot I are attracted to the lid 14 and absorbed, while near the lower end of the ingot I, the neutrons are emitted from the container body 13. It was thought that it might be attracted to the bottom 13b of the . The present inventors found that neutrons sufficiently reach the outer periphery of the ingot I at the upper and lower ends, but do not sufficiently reach the center. As a result, the resistivity is relatively high at the center. Therefore, it was thought that the variation in resistivity increased.

Therefore, based on the above conjecture, the inventors of the present invention, in a state in which the ingot I is housed in an aluminum container 12 as shown in FIG. In the longitudinal direction of the ingot I after neutron irradiation, a method for suppressing the radial variation in resistivity from becoming larger at the upper and lower ends than at the central portion was studied earnestly.

As a result, as shown in FIG. 3, a cylindrical dummy block B1 made of silicon is placed at the bottom of the storage container 12, and an ingot _I is placed on the dummy block _B1 . The inventors have found that it is extremely effective to carry out the operation in a state in which they are held in place, and have completed the present invention. Each requirement of the present invention will be described below.

The single crystal silicon ingot I to be irradiated with neutrons can be obtained by the CZ method or the Floating Zone (FZ) method.

The diameter of the single crystal silicon ingot I is not particularly limited, and may be 150 mm, 200 mm, 300 mm, 450 mm, or the like. From the viewpoint of further reducing resistivity variations, it is desirable to use a single crystal silicon ingot grown without adding dopants for adjusting electrical resistivity as the single crystal silicon ingot I. A single crystal silicon ingot doped with a small amount of p-type dopant or n-type dopant may be used as long as it does not affect the variation in resistivity.

The dummy block material _B1 arranged at the bottom of the storage container 12 is not particularly limited as long as it is made of silicon and has a cylindrical shape. can be done. Also, a silicon block material or the like cut out from a silicon ingot having non _- standard resistivity, oxygen concentration, etc. can be used as the dummy block material B1.

By arranging the dummy block material _B1 on the bottom portion 13b in the container body 13 itself, the influence of neutron absorption by the aluminum constituting the bottom portion 13b of the container body 13 can be suppressed, and the silicon ingot I after neutron irradiation can be suppressed. The radial variation in resistivity can be reduced at the lower end of the .

In terms of suppressing variations in resistivity in the in-plane direction of the wafer, as shown in FIG. It is not always necessary to arrange the dummy block material on the ingot I if a sufficient space is provided so as not to be affected by the ingot. However, as described above, the interior of the container 12 is filled with water during transportation and neutron irradiation. Therefore, buoyancy acts on the ingot I, and the lid portion 14 and the ingot I come into contact with each other during transportation. Ingot I may be damaged.

Therefore, it is preferable to arrange another dummy block material B2 on the ingot I as shown in _FIG . As a result, contact between the lid portion 14 and the ingot I during transportation is suppressed, and when the ingot I is divided into a plurality of pieces and stored, contact between the block portions is suppressed. It can prevent breakage.

In addition, for the neutron irradiation, a silicon ingot for evaluation longer than the neutron irradiation range in the vertical direction of the neutron irradiation device 1 is placed in the container body 13 in advance with respect to the neutron irradiation device ₁ used for neutron irradiation. , B ₂ are not placed in the storage container 12, and the evaluation ingot is irradiated with neutrons from the neutron irradiation device 1, and the longitudinal region of the evaluation ingot deviating from the desired resistivity range. is specified in advance, and the dummy blocks B ₁ and B ₂ are placed in the region in the container body 13 where the specified region was placed. This makes it possible to obtain an ingot I that satisfies the desired resistivity range in all regions.

The resistivity of the ingot I can be adjusted by the neutron irradiation time. As described above, silicon contains ³⁰ Si at 3.10%, and theoretically, phosphorus atoms can be contained up to about 10 ²¹ atoms/cm ³ . can be adjusted to the desired resistivity range.

(Method for manufacturing silicon ingot and method for manufacturing silicon wafer)
In the method for manufacturing a silicon ingot according to the present invention, a silicon ingot to be irradiated with neutrons obtained by a predetermined method is subjected to the above-described method for irradiating a silicon ingot with neutrons according to the present invention. It is characterized by irradiating with neutrons.

Manufacture of an ingot I in which, after irradiation with neutrons, the variation in resistivity in the radial direction is suppressed from becoming greater at the upper and lower ends than at the center, by the method for irradiating a silicon ingot with neutrons according to the present invention. can be done.

Then, the silicon wafers obtained by subjecting the ingot I manufactured as described above to the wafer processing treatment are taken from the upper and lower ends of the ingot I and from the central part. This suppresses variations in resistivity in the in-plane direction of the wafer.

Examples of the present invention will be described below, but the present invention is not limited to these examples.

(Test example 1)
In order to investigate the neutron irradiation variation of the neutron irradiation equipment to be used, the following experiments were conducted.
By the CZ method, a single crystal silicon ingot is grown without adding a dopant, and the single crystal silicon ingot is cut and peripherally ground to obtain a single crystal silicon block with a diameter of 200 mm and a length of 600 mm for neutron irradiation. made.
Next, the produced single crystal silicon block is placed on the bottom of an aluminum storage container, and the neutron irradiation apparatus shown in FIG. Irradiated. After that, the neutron-irradiated single-crystal silicon block was subjected to wafer processing to obtain a plurality of silicon wafers.
Subsequently, the resistivity of each silicon wafer was measured using a four-probe resistance meter, and RRG was obtained using the above-described formula (1).

FIG. 5 shows the relationship between the RRG value of each silicon wafer and the sampling position of each silicon wafer cut from a single crystal silicon block irradiated with neutrons. Here, the zero position on the horizontal axis corresponds to the height position of the neutron irradiation hole provided at the lowest height position in the core 11 of the neutron irradiation device 1 shown in FIG. It can be seen from FIG. 5 that the RRG values of the silicon wafers sampled from the upper and lower ends of the silicon ingot are larger than the RRG values of the silicon wafers sampled from the central portion.

(Invention Example 1)
A single crystal silicon ingot was grown by the CZ method, and the grown single crystal silicon ingot was cut and peripherally ground to produce a single crystal silicon block having a diameter of 200 mm and a length of 300 mm. The conditions for growing and processing the single crystal silicon ingot were all the same as in Test Example 1.
Next, a cylindrical dummy block made of single crystal silicon with a diameter of 200 mm and a height of 150 mm was placed on the bottom of the aluminum storage container used in Test Example 1, and the single crystal silicon block was placed on the dummy block. A dummy block material made of single crystal silicon having a diameter of 200 mm and a height of 150 mm was further placed on the single crystal silicon block. The dummy blocks placed at the upper and lower ends of the monocrystalline silicon blocks are manufactured by processing waste silicon ingots that do not meet the standards for product ingots.
Thereafter, the single-crystal silicon block was irradiated with neutrons, and then wafer processing was performed on the single-crystal silicon block that had been irradiated with neutrons to obtain a plurality of silicon wafers. Subsequently, in the same manner as in Test Example 1, the resistivity of each silicon wafer was measured using a four-probe resistance meter. Neutron irradiation conditions, wafer processing conditions, and resistivity measurement conditions were all the same as in Test Example 1.
As a result, the RRG of the obtained silicon wafers was all 5% or less.

(Invention Example 2)
As in Invention Example 1, the single crystal silicon block was irradiated with neutrons. However, the length of the single crystal silicon block was set to 200 mm, and the height of the cylindrical dummy block material was set to 200 mm. All other conditions are the same as in Invention Example 1. When the RRG of each silicon wafer after neutron irradiation was measured, all were 4.5% or less.

According to the present invention, in the longitudinal direction of the silicon ingot after neutron irradiation, it is possible to suppress the variation in resistivity in the radial direction from becoming larger at the upper and lower ends than at the center, so that semiconductor wafer production useful in industry.

1 Neutron Irradiator 11 Core 12 Storage Container 13 Container Body 13a Cylindrical Part 13b Bottom 14 Lid _B1 , B2 Dummy Block Material _I Silicon Ingot I1, _{I2 , I3} Block Part N Neutron

Claims (5)

A method of irradiating a single crystal silicon ingot with neutrons from the side surface of the ingot in a state where the single crystal silicon ingot is housed in an aluminum storage container having a cylindrical container body consisting of a cylinder and a bottom and a lid. ,
A silicon ingot characterized in that the neutron irradiation is performed in a state in which a cylindrical dummy block made of silicon is arranged at the bottom of the container body, and the ingot is arranged on the dummy block. neutron irradiation method. 2. The method of irradiating a silicon ingot with neutrons according to claim 1, wherein said neutron irradiation is performed in a state in which another cylindrical dummy block material made of silicon is further arranged on said ingot. For the neutron irradiation, the neutron irradiation device used for the neutron irradiation is prepared by disposing a silicon ingot for evaluation having a length longer than a neutron irradiation range in the vertical direction of the neutron irradiation device and placing the dummy block material in the container body in advance. Then, the evaluation ingot is irradiated with neutrons from the neutron irradiation device, and the longitudinal region of the evaluation ingot outside the desired resistivity range is specified. 3. The method of irradiating a silicon ingot with neutrons according to claim 2, wherein said dummy block material is arranged in said region in said container body in which said specified region was arranged. Silicon characterized by irradiating a silicon ingot obtained by a predetermined method with neutrons by the method for neutron irradiation of a silicon ingot according to any one of claims 1 to 3. Ingot manufacturing method. A method for producing a silicon wafer, comprising subjecting a silicon ingot produced by the method for producing a silicon ingot according to claim 4 to wafer processing to obtain a silicon wafer.

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