JP2010095421A - Method for producing polycrystalline silicon and polycrystalline silicon wafer - Google Patents

Abstract

A method for producing polycrystalline silicon capable of improving the power generation efficiency of a solar power generation device is provided.
Argon gas is introduced from a gas inlet 10a and hydrogen gas is introduced into a chamber 10 from a gas inlet 10b, and a polycrystalline silicon ingot 33 is grown in this state. Thereby, since hydrogen is directly introduced into the silicon melt 32, a large amount of hydrogen is taken into the silicon melt 32. For this reason, the polycrystalline silicon wafer finally obtained contains more hydrogen than before, and its concentration distribution is substantially constant in the in-plane direction and the thickness direction.
[Selection] Figure 1

Description

  The present invention relates to a method for producing polycrystalline silicon and a polycrystalline silicon wafer, and more particularly to a method for producing polycrystalline silicon and a polycrystalline silicon wafer suitable for use in a power generation unit of a solar power generation device.

  In recent years, power generation using natural energy has attracted attention against the background of depletion of fossil fuels and deterioration of the global environment. Above all, solar power generation devices (also called solar cells) are not accompanied by mechanical operation, and can be expanded to a wide range of products from small to large types. It is expected.

  As widely known, a silicon wafer is used for a power generation unit of a solar power generation device. However, since a cost reduction is strongly demanded for a solar power generation apparatus, it is common to use a cheaper polycrystalline silicon wafer instead of a single crystal silicon wafer used for an IC device. Since the quality of polycrystalline silicon wafers greatly affects the power generation efficiency of photovoltaic power generation devices, the silicon dangling bonds that make up the polycrystalline silicon wafer are sufficient to obtain good high power generation efficiency. Must be terminated.

For this reason, conventionally, a process called a “hydrogen passivation method” is performed on a polycrystalline silicon wafer, thereby terminating the dangling bonds of silicon (see Patent Document 1). The hydrogen passivation method is generally a method in which a polycrystalline silicon wafer cut out from a polycrystalline silicon ingot is placed in a plasma processing apparatus and plasma discharge is performed in a low-pressure hydrogen atmosphere. Thereby, ionized active hydrogen atoms are introduced into the inside from the surface layer of the wafer.
JP-A-7-183552

  However, since the conventional hydrogen passivation method introduces hydrogen atoms into polycrystalline silicon in the wafer state, the introduction depth remains at a few μm from the surface layer of the wafer, and hydrogen reaches a depth of more than that. There are very few atoms. For this reason, the end of the unbonded hand inside the wafer is incomplete, which causes the power generation efficiency of the photovoltaic power generation apparatus to be reduced.

  Therefore, an object of the present invention is to provide a polycrystalline silicon manufacturing method and a polycrystalline silicon wafer capable of improving the power generation efficiency of a photovoltaic power generation apparatus.

  A polycrystalline silicon manufacturing method according to the present invention is a polycrystalline silicon manufacturing method for growing polycrystalline silicon from a silicon melt in a chamber, characterized in that hydrogen gas is introduced into the chamber during crystal growth. To do.

According to the present invention, hydrogen is not introduced into the polycrystalline silicon wafer but hydrogen is introduced into the molten silicon, so that more hydrogen is taken into the silicon melt. For this reason, the polycrystalline silicon wafer finally obtained has a hydrogen concentration of 1 × 10 ¹⁵ atoms / cm ³ or more and 1 × 10 ²¹ atoms / cm ³ or less, and more hydrogen than the conventional one. The concentration distribution is almost constant in the in-plane direction and the thickness direction. Therefore, if the polycrystalline silicon manufactured by the method of the present invention is used, it is possible to manufacture a solar power generation device with high power generation efficiency. In addition, since the hydrogen introduction step is not necessary after the polycrystalline silicon wafer is cut out, the manufacturing cost is reduced and the manufacturing efficiency is improved.

  In the present invention, it is preferable to introduce hydrogen gas at a volume concentration of 3 ppm or more and 20% or less with respect to the total amount of gas introduced into the chamber. This is because if the hydrogen gas concentration is less than 3 ppm, the amount of hydrogen taken into the silicon melt may be insufficient. If the hydrogen gas concentration exceeds 20%, there is a risk of detonation. This is because a defect due to hydrogen is generated in the crystal and this increases recombination loss.

  The method for producing polycrystalline silicon according to the present invention can be applied to any of a casting method, an electromagnetic casting method, a CZ method, and a continuous CZ method.

  Thus, according to the present invention, it is possible to provide a polycrystalline silicon manufacturing method and a polycrystalline silicon wafer capable of improving the power generation efficiency of the photovoltaic power generation apparatus.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram showing the structure of a polycrystalline silicon manufacturing apparatus according to a preferred embodiment of the present invention.

  The polycrystalline silicon manufacturing apparatus according to the present embodiment is a polycrystalline silicon manufacturing apparatus based on an electromagnetic casting method. As shown in FIG. 1, a cylindrical bottomless copper crucible 11 through which cooling water circulates, and silicon in the copper crucible 11. Induction coil 12 for inductively heating the raw material, cylindrical graphite susceptor 13 for holding the polycrystalline silicon ingot, cylindrical heat insulating material 14 disposed around the graphite susceptor 13, and a heat retaining heater for adjusting the temperature gradient 15, a raw material charging device 16 for charging silicon raw material into the copper crucible 11, and a chamber 10 for storing these.

The chamber 10 is provided with gas inlets 10a and 10b and a gas outlet 10c. Argon gas (Ar) is introduced from the gas introduction port 10a through the valve 21, and hydrogen gas (H ₂ ) is introduced from the gas introduction port 10b through the valve 22. Further, the gas introduced into the chamber 10 is discharged from the gas discharge port 10 c by the pump 23. Thus, by controlling the valves 21 and 22 and the pump 23, the inside of the chamber 10 can be set to a predetermined gas atmosphere.

  Production of polycrystalline silicon using the apparatus shown in FIG. 1 is performed as follows.

  First, the bottom of the cylindrical bottomless copper crucible 11 is closed with a seed ingot (not shown), and the silicon raw material 31 is charged into the copper crucible 11 from the raw material charging device 16 in this state. Next, the silicon raw material 31 in the copper crucible 11 is induction-heated by passing a high-frequency current through the induction coil 12. Thereby, the silicon raw material 31 in the copper crucible 11 is melted, and a silicon melt 32 is obtained. At this time, since the copper crucible 11 is forcibly cooled by the cooling water, the silicon raw material 31 in contact with the copper crucible 11 does not melt, and therefore the silicon melt 32 and the copper crucible 11 are kept in non-contact. . When the silicon melt 32 in the copper crucible 11 is gradually drawn downward, the silicon melt 32 is cooled to obtain a polycrystalline silicon ingot 33. A temperature gradient from the silicon melt 32 to the polycrystalline silicon ingot 33 is adjusted by the heat insulating material 14 and the heat retaining heater 15.

  In such a series of manufacturing steps, the gas atmosphere in the chamber 10 is a mixed atmosphere of argon gas and hydrogen gas. As a result, hydrogen is taken into the silicon melt 32, and as a result, a polycrystalline silicon ingot 33 containing hydrogen can be obtained. Hydrogen gas does not have to be introduced in all steps, and it is sufficient to introduce hydrogen gas into the chamber 10 at least during crystal growth. Therefore, for example, in the state before crystal growth, only the argon gas is introduced into the chamber 10 by opening the valve 21, and then the hydrogen gas is introduced into the chamber 10 by opening the valve 22 at the timing of starting crystal growth. It doesn't matter. According to this, it becomes possible to reduce the usage-amount of hydrogen gas.

  Although the ratio of argon gas and hydrogen gas during crystal growth is not particularly limited, it is preferable that hydrogen gas is 3 ppm or more and 20% or less in volume concentration with respect to the total amount of gas. This is because if the hydrogen gas concentration is less than 3 ppm, the amount of hydrogen taken into the silicon melt 32 may be insufficient. If the hydrogen gas concentration exceeds 20%, there is a risk of detonation. Because it occurs. Further, when the concentration of hydrogen gas exceeds 20%, hydrogen-induced defects generated in the crystal become remarkable. Considering these points, it is particularly preferable that the volume concentration of hydrogen gas is 3% or more and 10% or less with respect to the total amount of gas.

  The gas atmosphere during crystal growth is not limited to a mixed atmosphere of argon gas and hydrogen gas, and other gases may be included, and other inert gas is used instead of argon gas. It doesn't matter. Further, the pressure in the chamber 10 during crystal growth is not particularly limited.

The polycrystalline silicon ingot 33 manufactured by the apparatus of this embodiment is sliced to a predetermined thickness, whereby a polycrystalline silicon wafer is cut out. Unlike the conventional polycrystalline silicon wafer, the dangling bonds are already terminated with hydrogen in the thus produced polycrystalline silicon wafer. In addition, since hydrogen is taken in at the stage of the silicon melt 32, the hydrogen concentration in the in-plane direction and the thickness direction of the polycrystalline silicon wafer becomes substantially constant. Specifically, a hydrogen concentration of 1 × 10 ¹⁵ atoms / cm ³ or more and 1 × 10 ²¹ atoms / cm ³ or less is obtained.

  As described above, according to this embodiment, since hydrogen is taken in at the stage of the silicon melt 32, it is possible to obtain a polycrystalline silicon wafer in which the hydrogen concentration in the in-plane direction and the thickness direction is substantially constant. It becomes. Therefore, it is not necessary to introduce hydrogen by the hydrogen passivation method after cutting the polycrystalline silicon wafer, and it is possible to realize a reduction in manufacturing cost and an improvement in manufacturing efficiency. Moreover, if the polycrystalline silicon wafer according to the present embodiment is used, a solar power generation device with high power generation efficiency can be manufactured.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. Needless to say, it is included in the range.

  For example, in the above embodiment, the method for producing polycrystalline silicon by the electromagnetic casting method has been described as an example. However, the present invention is not limited to this, and the casting method, CZ method (Czochralski method), continuous The present invention can also be applied to other methods such as the CZ method.

  In the above embodiment, hydrogen gas and argon gas are introduced into the chamber from separate gas inlets. However, hydrogen gas and argon gas are mixed in advance at a predetermined ratio, and the mixed gas is added to the chamber. It may be introduced inside. According to this, it becomes possible to reduce the risk of detonation by hydrogen gas.

  Further, the usage destination of the polycrystalline silicon wafer manufactured by the method of the present invention is not limited to the solar power generation apparatus, and may be used for other devices.

  Hereinafter, although the Example of this invention is described, this invention is not limited to this Example at all.

  First, a polycrystalline silicon ingot was manufactured using a polycrystalline silicon manufacturing apparatus having the structure shown in FIG. As the copper crucible, a square copper water-cooled crucible having an inner diameter of 220 mm on one side was used. While supplying a granular silicon raw material having a diameter of 0.3 to 2.0 mm into such a copper crucible at an input speed of 200 g / min, the silicon raw material in the copper crucible was dissolved by flowing an alternating current through the induction coil. .

  Then, the silicon melt was solidified by moving the closing lid at the bottom of the crucible downward at a constant speed. Thereafter, while continuously supplying silicon raw material, the solidified crystal was drawn downward to continuously grow polycrystalline silicon. The drawing speed of the polycrystalline silicon ingot was set to 3 mm / min.

  Argon gas was supplied as an inert gas into the crystal growing chamber, and the mixing ratio of hydrogen gas was set to a different ratio for each of the comparative examples and the examples.

  A polycrystalline silicon wafer was cut out from the substantially central portion in the axial direction of the p-type polycrystalline silicon ingot thus produced, and the photoelectric conversion efficiency was measured to investigate the dependence of the conversion efficiency on the hydrogen concentration. The photoelectric conversion efficiency was measured by the following method.

First, as shown in FIG. 2A, a 20 mm square chip 40 was taken out by dicing the polycrystalline silicon wafer. Then, as shown in FIG. 2B, the damaged layer 41 on the surface of the chip 40 was removed using a mixed acid composed of boiling acid, nitric acid and acetic acid. Next, the chip 40 was introduced into a phosphorus (P) deposition furnace, and phosphorus diffusion using POCl ₃ as a diffusion source was performed. As a result, as shown in FIG. 2C, an n ⁺ layer 42 was formed on the surface layer of the chip 40, and a PSG film 43 was formed on the surface.

Next, as shown in FIG. 2D, a protective film 44 was applied to the front surface of the chip 40, and in this state, the n ⁺ layer 42 was removed by etching. As a result, as shown in FIG. 2E, the back surface of the chip 40 is in a state where silicon is exposed. Next, as shown in FIG. 2 (f), an aluminum (Al) electrode 45 is deposited on the exposed back surface of the chip 40, and as shown in FIG. 2 (g), a paste-like material is formed on the front surface of the chip 40. A silver (Ag) electrode 46 was applied. Finally, as shown in FIG. 2H, the paste-like silver electrode 46 was baked by heat treatment. Thereby, since the filler contained in the silver electrode 46 penetrates the PSG film 43, the silver electrode 46 and the n ⁺ layer 42 are connected. The reason why the aluminum electrode 45 on the back surface is formed first is to prevent junction breakdown due to silver diffusion.

  The photoelectric conversion efficiency was measured with the solar simulator and the curve tracer measuring machine with respect to the sample created by such a method. The results are shown in Table 1.

  As shown in Table 1, in the sample of Comparative Example 1A in which no hydrogen gas was mixed into the chamber during crystal growth, the conversion efficiency was 10.42%, whereas in the chamber during crystal growth, In the samples of Examples 1B to 1I mixed with hydrogen gas, the conversion efficiency higher than that of Comparative Example 1A was obtained, and in particular, the samples of Examples 1E to 1I having a hydrogen concentration of 3% or more obtained high conversion efficiency. It was. Moreover, in the region where the hydrogen concentration was up to 10%, the conversion efficiency increased as the hydrogen concentration increased. However, in the region where the hydrogen concentration exceeded 10%, the conversion efficiency was saturated, and a higher conversion efficiency could not be obtained. Rather, the conversion efficiency clearly decreased when the hydrogen concentration exceeded 20%. This is presumably that when the hydrogen concentration exceeds 20%, defects due to hydrogen are newly generated in the crystal, which increases the recombination loss. In addition, considering the risk of detonation, Examples 1E to 1G having a hydrogen concentration of 3% or more and 10% or less are considered to be most suitable. Moreover, it was a sample of Examples 1C-1I whose hydrogen concentration is 3 ppm or more that the remarkable effect was confirmed compared with the comparative example 1A. This is considered to be because when the hydrogen concentration is less than 3 ppm, the hydrogen bonding with the silicon dangling bonds at the polycrystalline grain boundary is insufficient.

  In Example 2, a polycrystalline silicon ingot was grown by the CZ method. In order to pull up the polycrystalline silicon ingot by the CZ method, the NECK process was intentionally omitted, and the polycrystalline silicon ingot was pulled up in the same manner as the pulling up of the ordinary single crystal silicon ingot. By omitting the NECK process, dislocations generated by thermal damage to the seed during immersion in the melt remain in the crystal as they are without being removed before the shoulder formation process. As a result, the pulled ingot becomes polycrystalline.

  By such a method, a polycrystalline silicon ingot having a diameter of 155 mm, a charge amount of 45 kg, a growth rate of 1 mm / min, and a BODY length of 800 mm was pulled up. The gas atmosphere at the time of pulling was mainly composed of argon, and the hydrogen concentration was changed stepwise from the TOP part to the TAIL part in increments of 50 mm or 100 mm. And the polycrystalline silicon wafer was cut out from each site | part, and the conversion efficiency was measured in the procedure similar to Example 1. FIG. The results are shown in Table 2.

  As shown in Table 2, in the sample of Comparative Example 2A in which no hydrogen gas was mixed in the chamber during crystal growth, the conversion efficiency was 12.24%, whereas in the chamber during crystal growth, In the samples of Examples 2B to 2I mixed with hydrogen gas, conversion efficiency higher than that of Comparative Example 2A was obtained, and in particular, the samples of Examples 2E to 2H having a hydrogen concentration of 3% to 20% were particularly high. Conversion efficiency was obtained. In addition, in the region where the hydrogen concentration was up to 20%, the conversion efficiency increased as the hydrogen concentration increased. However, in the region where the hydrogen concentration exceeded 20%, the conversion efficiency was saturated, and a higher conversion efficiency could not be obtained. Rather, conversion efficiency decreased when the hydrogen concentration exceeded 20%. The reason is the same as the reason described in the first embodiment. Therefore, for the same reason as in Example 1, it is considered that Examples 2E to 2G having a hydrogen concentration of 3% or more and 10% or less are most suitable. Moreover, it was a sample of Examples 2C-2I whose hydrogen concentration is 3 ppm or more that the remarkable effect was confirmed compared with the comparative example 2A. The reason is the same as the reason described in the first embodiment.

It is a schematic diagram which shows the structure of the polycrystalline-silicon manufacturing apparatus by preferable embodiment of this invention. It is process drawing which shows the preparation procedures of the sample of Example 1,2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Chamber 10a, 10b Gas inlet 10c Gas outlet 12 Induction coil 13 Graphite susceptor 14 Heat insulating material 15 Heat retention heater 16 Raw material input device 21, 22 Valve 23 Pump 31 Silicon raw material 32 Silicon melt 33 Polycrystalline silicon ingot 40 Chip 41 Damage layer 42 n ⁺ layer 43 PSG film 44 Protective film 46 Silver electrode

Claims (4)

  A method for producing polycrystalline silicon, comprising growing polycrystalline silicon from a silicon melt in a chamber, wherein hydrogen gas is introduced into the chamber during crystal growth.   2. The method for producing polycrystalline silicon according to claim 1, wherein the hydrogen gas is introduced at a volume concentration of 3 ppm or more and 20% or less with respect to the total amount of gas introduced into the chamber.   3. The method for producing polycrystalline silicon according to claim 1, wherein the polycrystalline silicon is obtained by any one of a casting method, an electromagnetic casting method, a CZ method, and a continuous CZ method. A polycrystalline silicon wafer in which dangling bonds are terminated with hydrogen, and the hydrogen concentration is 1 × 10 ¹⁵ atoms / cm ³ or more and 1 × 10 ²¹ atoms / cm ³ or less in the in-plane direction and the thickness direction. A polycrystalline silicon wafer having a substantially constant hydrogen concentration.

Images