JP2000128692A - Cleaning method for polysilicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove organic substances, fine grains and metallic impurities adhered to the surface of polysilicon at a low cost to increase the yield of single crystal from polysilicon (i.e., polycrystalline silicon. SOLUTION: The method of this invention is to conduct processes 12 and 13 where massive or granular polysilicon is cleaned with an ozone-dissolved aqueous solution and processes 14 and 17 where polysilicon cleaned with an ozone-dissolved aqueous solution is cleaned with hydrofluoric acid in the above order at least once. After the last cleaning process 17 using hydrofluoric acid, processes 18 and 19 where polysilicon is cleaned with pure water and then is dried are preferably added.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for cleaning polysilicon (polycrystalline silicon) which is a raw material for producing a silicon single crystal.

[0002]

2. Description of the Related Art A silicon single crystal for a semiconductor device is manufactured mainly by a Czochralski method (hereinafter, referred to as a CZ method). The CZ method is a method in which massive or granular polysilicon is melted in a quartz crucible in a furnace, a seed crystal is immersed in the obtained melt, and the seed crystal is pulled up to grow a silicon single crystal. Since polysilicon, which is a raw material of the silicon single crystal, has an active property, it is sealed in a plastic bag until it is put into a quartz crucible. However, before and after sealing, an oxide film is easily formed on the surface of the polysilicon by reacting with oxygen in the air. Organic substances, fine particles, or metal impurities may adhere to the surface of the oxide film. If polysilicon is melted while these organic substances and the like are attached, there is a problem that the growth of silicon single crystal is hindered and the free ratio (the ratio of obtaining single crystal silicon from polysilicon) decreases.

[0003]

In order to solve the above-mentioned problem, a method is known in which polysilicon is washed with a mixed acid of hydrofluoric acid and nitric acid before being charged into a quartz crucible. However, in this method, organic substances dissolved in the mixed acid easily adhere to the polysilicon surface. For this reason, it was difficult to remove organic substances. Further, since the amount of the mixed acid used is large, there is a problem that the cleaning cost is increased. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for cleaning polysilicon, which removes organic substances, fine particles and metal impurities attached to the surface of polysilicon at low cost and increases the free ratio.

[0004]

The invention according to claim 1 is
As shown in FIG. 1, steps 12 and 13 for cleaning bulk or granular polysilicon with a dissolved ozone aqueous solution and steps 14 and 17 for cleaning the polysilicon washed with the dissolved ozone aqueous solution with hydrofluoric acid are performed in this order. The method is a method of cleaning polysilicon, which is performed once or repeatedly one or more times.
The surface of the polysilicon is oxidized by washing the dissolved ozone aqueous solution to form an oxide film. The oxide film is removed by washing with hydrofluoric acid, and organic substances, fine particles and metal impurities attached to the oxide film are removed together with the oxide film. Step 1
By repeating steps 2 and 13 and steps 14 and 17, the removal effect is enhanced. The invention according to claim 2 is the invention according to claim 1, wherein after the final hydrofluoric acid cleaning step 17 as shown in FIG. 1, the polysilicon is cleaned with pure water and then dried. This is a method for cleaning polysilicon including 18, 19. Step 18 of cleaning polysilicon with pure water
Is further added, the effect of removing organic substances, fine particles and metal impurities can be further enhanced.

The invention according to claim 3 is the invention according to claim 1 or 2, wherein the dissolved ozone aqueous solution has an ozone concentration of 3%.
２０20 ppm, and the concentration of hydrofluoric acid is 0.1-5 wt%
This is a method for cleaning polysilicon. By setting the ozone concentration of the dissolved ozone aqueous solution and the concentration of hydrofluoric acid in the above ranges, the effect of removing organic substances, fine particles and metal impurities can be further enhanced. The invention according to claim 4 is the invention according to claims 1 to 3, wherein FIG.
The washing step with the dissolved ozone aqueous solution, as shown in FIG. 3, injects the dissolved ozone aqueous solution into the bulk or granular polysilicon put in the chemical-resistant basket, and then puts the basket containing the polysilicon into the dissolved ozone aqueous solution. Immersion process 1
This is a method for cleaning polysilicon including No. 2 and No. 13. By combining the injection step 12 of the dissolved ozone aqueous solution and the immersion step 13 of the dissolved ozone aqueous solution, the effect of removing organic substances, fine particles and metal impurities can be further enhanced. The invention according to claim 5 is the invention according to claim 4, wherein the cleaning step using hydrofluoric acid is performed in a chemical-resistant basket, and the polysilicon that has been cleaned with a dissolved ozone aqueous solution is subjected to fluorine cleaning. This is a method for cleaning polysilicon including steps 14 and 17 of immersing in polysilicon. The effect of removing oxide films, organic substances, fine particles and metal impurities by hydrofluoric acid is enhanced.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The dissolved ozone aqueous solution used in the present invention has high purity, low concentration, high oxidizing power, and easy availability. The dissolved ozone aqueous solution preferably has an ozone concentration of 3 ppm or more. If it is less than 3 ppm, the ability to oxidize the surface of polysilicon is insufficient,
If it exceeds 20 ppm, the container in contact with the dissolved ozone aqueous solution is damaged, which is not preferable. The ozone concentration of the dissolved ozone aqueous solution is more preferably 5 to 15 ppm. The concentration of hydrofluoric acid used in the present invention is 0.1 to
5% by weight. In particular, 0.1 to 1% by weight is preferable,
0.5% by weight is more preferred. If it is less than 0.1% by weight,
Insufficient for removing organic matter, fine particles and metal impurities,
On the other hand, if it exceeds 5% by weight, the washing cost increases, which is not preferable.

An embodiment of the polysilicon cleaning method of the present invention will be described with reference to FIG. As shown in step 11 of FIG. 1, massive or granular polysilicon is put into a chemically resistant cage. Examples of the substance that forms the chemical-resistant cage include fluorine resins such as polyethylene, polypropylene, and Teflon. Next, as shown in step 12, a dissolved ozone aqueous solution is injected into the polysilicon housed in the car. This injection is performed using an injection nozzle or a shower. In addition, when performing this injection, by installing a basket in the container and injecting the dissolved ozone aqueous solution,
After the injection, the dissolved ozone aqueous solution flowing out of the basket can be used as a part of the dissolved ozone aqueous solution used in the next step 13. Next, as shown in step 13, the cage containing the polysilicon is immersed in a dissolved ozone aqueous solution.
This immersion is performed by immersing the car in a first tank (not shown) containing a dissolved ozone aqueous solution.

Next, as shown in step 14, the cage containing the polysilicon is immersed in hydrofluoric acid. This immersion is performed by immersing the basket in a second tank (not shown) containing hydrofluoric acid. Hydrofluoric acid is put in such a way that when the basket is immersed in the second tank in which the hydrofluoric acid has been put, it overflows. It is preferable that the hydrofluoric acid overflowed by the immersion of the basket is returned to the second tank by the pump and circulated. Next, as shown in step 16, a dissolved ozone aqueous solution is injected into the polysilicon housed in the basket pulled up from the second tank. Next, as shown in step 17, the cage containing the polysilicon is immersed in hydrofluoric acid. This immersion is performed by immersing the basket in a third tank (not shown) containing hydrofluoric acid. In this case, it is preferable that the hydrofluoric acid overflowing from the third tank when the car is immersed is returned to the third tank by a pump and circulated as in step 14. Further, a part of the overflowed hydrofluoric acid can be returned to the second tank used in the step 14 by a pump and circulated. Next, as shown in step 18, the cage containing the polysilicon is immersed in pure water. This immersion is performed by immersing the car in a fourth tank (not shown) in which pure water is put.
In this case, the pure water is heated to a predetermined temperature (for example, 25 to 60).
C). Next, as shown in step 19, the polysilicon contained in the car is dried. This drying is performed by hot air drying by blowing hot air or vacuum drying in an inert gas such as argon. Finally, as shown in step 21, the dried polysilicon is removed from the basket.

In the above-described embodiment, the method of cleaning by placing polysilicon in a chemical-resistant basket has been described. However, it is also possible to perform cleaning by placing polysilicon on a chemical-resistant conveyor instead of the basket. it can.

[0010]

Next, examples of the present invention will be described together with comparative examples. <Example 1> A lump of polysilicon was subjected to a cleaning process based on the cleaning process shown in FIG. First, the massive polysilicon was put into a polyethylene basket (step 11). This basket was placed in tank A, and the ozone concentration was adjusted to 20 from the injection nozzle.
A ppm dissolved ozone aqueous solution was sprayed onto the polysilicon at a rate of 5000 cc / min for 3 minutes (step 12). Next, a dissolved ozone aqueous solution having the same ozone concentration as described above was stored in another tank B, and a basket in which the jet-treated polysilicon was placed was immersed in this aqueous solution for 5 minutes (step 13). Next, the basket was pulled out of the tank B and immersed in hydrofluoric acid having a concentration of 0.5% by weight stored in another tank C for 5 minutes (step 14). Then, the car is pulled out of the tank C and another tank D
Then, a dissolved ozone aqueous solution having an ozone concentration of 20 ppm was sprayed onto the polysilicon at a rate of 5000 cc / min for 3 minutes from the spray nozzle (step 16). Then, the car was pulled out of the tank D, and the concentration stored in another tank E was 0.5.
It was immersed in hydrofluoric acid of 5% by weight for 5 minutes (step 17). Next, the basket was pulled out of the tank E and immersed in pure water contained in another tank F for 15 minutes (step 18). Next, the basket was pulled out of the tank F and introduced into a hot air drier to dry the polysilicon (Step 19). Finally, the dried polysilicon was removed from the basket (step 21).

<Comparative Example 1> A bulk polysilicon was cleaned by substantially repeating the cleaning method of Example 1 except that the step of cleaning with a dissolved ozone aqueous solution was omitted. That is,
Step 11, Step 17, Step 18, Step 19 and Step 21
Was performed in this order.

<Comparative Test and Evaluation> Comparative Example 1 removes a natural oxide film having a non-uniform thickness on the polysilicon surface only with hydrofluoric acid, whereas Example 1 dissolves dissolved ozone water into the polysilicon surface. An oxide film with a uniform thickness was formed, and this oxide film was removed with hydrofluoric acid.
In Example 1, the etching amount could be increased, and the formation of the oxide film on the polysilicon surface and the removal of the oxide film could be completely performed. Pure water was applied to the bulk polysilicon washed by the method of Comparative Example 1 and the bulk polysilicon washed by the method of Example 1 and the water repellency was examined. As a result, Example 1 was superior in water repellency, confirming the above phenomenon.

[0013]

As described above, in the method for cleaning polysilicon according to the present invention, the step of cleaning lump or granular polysilicon with a dissolved ozone aqueous solution and then cleaning with hydrofluoric acid is one.
By performing the process once or repeating at least once, it is possible to remove the organic substances, fine particles, and metal impurities attached to the surface of the polysilicon at low cost, and as a result, it is possible to increase the free ratio.

[Brief description of the drawings]

FIG. 1 is a view showing a polysilicon cleaning step of the present invention.

[Description of Signs] 12 Injection of aqueous solution of dissolved ozone to polysilicon in cage 13 Immersion of polysilicon in cage in aqueous solution of dissolved ozone 14 Immersion of polysilicon in cage in hydrofluoric acid 16 To polysilicon in cage Of dissolved ozone aqueous solution of water 17 Soaking of polysilicon in hydrofluoric acid 18 Soaking of polysilicon in basket in pure water 19 Drying of polysilicon in basket

Claims (5)

[Claims] A step of washing the lump or granular polysilicon with a dissolved ozone aqueous solution; and a step of washing the polysilicon washed with the dissolved ozone aqueous solution with hydrofluoric acid.
4, 17) is performed once in this order or is repeated one or more times. 2. The polysilicon cleaning method according to claim 1, further comprising a step of cleaning the polysilicon with pure water and drying after the last hydrofluoric acid cleaning step. . 3. The dissolved ozone aqueous solution has an ozone concentration of 3 to 2.
3. The method for cleaning polysilicon according to claim 1, wherein the concentration is 0 ppm and the concentration of hydrofluoric acid is 0.1 to 5% by weight. 4. A cleaning step using a dissolved ozone aqueous solution, wherein the dissolved ozone aqueous solution is sprayed on a lump or granular polysilicon placed in a chemically resistant cage, and the cage containing the polysilicon is placed in the dissolved ozone aqueous solution. Immersion process
4. The method for cleaning polysilicon according to claim 1, further comprising (12, 13). 5. The method according to claim 4, wherein the step of washing with hydrofluoric acid includes a step of immersing the polysilicon, which has been placed in a chemical-resistant basket and washed with a dissolved ozone aqueous solution, in hydrofluoric acid. How to clean silicon.