JP2000302594A - Cleaning method of polycrystalline silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polycrystalline silicon cleaning method excellent in cleaning effect. SOLUTION: The system used for this cleaning method is a multistage cleaning system provided with at least two cleaning tanks each having a circulation passage for withdrawing a cleaning liquid from the bottom of the cleaning tank, filtering the withdrawn cleaning liquid and thereafter returning the filtered cleaning liquid to the cleaning tank. More specifically, in this multistage cleaning system, a first cleaning tank 10 provided with a filter having >=0.1 μm pore size in a filtration section 13 installed in a circulation passage 11, and a second cleaning tank 30 provided with a filter having <0.1 μm pore size in a filtration section 33 installed in a circulation passage 31, are placed. The cleaning method comprises firstly performing cleaning in the first cleaning vessel 10 and thereafter performing cleaning in the second cleaning tank 30, to subject fine particles sticking to polycrystalline silicon to acid cleaning and stepwise removal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cleaning method for obtaining polycrystalline silicon having a high surface cleanliness used for producing a silicon single crystal. More specifically, the concentration of the particles adhering to the surface and the impurity metal contained in the surface oxide layer is less than 0.8 wt ppb and the carbon concentration is 0.3 × 10 ¹⁶ ato.
The present invention relates to a cleaning method for obtaining highly purified polycrystalline silicon of less than ms / cc.

[0002]

2. Description of the Related Art At present, single crystal silicon is mainly manufactured by the Czochralski method (CZ method) or the floating zone method (FZ method). Each of these methods uses polycrystalline silicon as a raw material, and this polycrystalline silicon is generally manufactured in a rod shape by a method called a Siemens method. In the CZ method, the rod-shaped polycrystalline silicon is crushed or cut into an arbitrary size to form a lump, which is dissolved in a crucible, and single-crystal silicon is pulled up from the silicon melt. On the other hand, in the FZ method, after the surface of the polycrystalline silicon rod is ground to adjust its shape, the rod is heated from one end to the other end to form a single crystal.

[0003] Since the silicon single crystal used as a semiconductor material is manufactured using polycrystalline silicon as a raw material, the cleanliness of the polycrystalline silicon is reduced by the yield of the single crystal silicon.
(Single crystallization ratio). Therefore, conventionally, in order to increase the cleanliness of the polycrystalline silicon as much as possible, the polycrystalline silicon is washed with a chemical solution to reduce fine powder and impurity metals on the surface.

[0004]

The surface of polycrystalline silicon has many fine irregularities even though it looks smooth. Fine particles of impurity metal and silicon are contained in these irregularities. Must be sufficiently removed. When the fine particles remain, when polycrystalline silicon is melted and a single crystal is pulled up, this becomes a starting point to cause crystal dislocation, which causes a reduction in the single crystallization ratio. However, there is a problem that the fine particles are not sufficiently removed by the conventional cleaning of polycrystalline silicon.

For example, US Pat. No. 5,820,688 describes a method in which a cleaning liquid is withdrawn from the bottom of a tank, and the cleaning liquid is again jetted from a bottom of the tank to a semiconductor material immersed in the cleaning tank for cleaning. When cleaning crystalline silicon, such a method of blowing up the cleaning liquid from the bottom of the tank,
Since the fine particles washed out of the polycrystalline silicon stay in the cleaning tank and re-adsorb to the silicon surface, a sufficient cleaning effect cannot be obtained.

On the other hand, in the case of cleaning a silicon wafer, a cleaning method is also known in which a cleaning liquid extracted from a cleaning tank is filtered and returned to the upper part of the tank so that the liquid flow in the tank is directed downward. Since the state of the fine particles attached to the wafer is different from that of the polycrystalline silicon, even if the extracted cleaning liquid is filtered in the same manner as in the case of the wafer, the cleaning effect of the polycrystalline silicon does not sufficiently remove the fine particles. I can't get it. It is necessary to use a filter having a pore size corresponding to the size of the particles attached to the polycrystalline silicon, and it is necessary to devise a cleaning method such as multi-stage cleaning.

The present invention solves the above-mentioned problems in the conventional cleaning method for polycrystalline silicon, and effectively removes fine particles adhering to the polycrystalline silicon surface.
It is an object of the present invention to provide a cleaning method capable of achieving a high single crystallization rate in pulling a silicon single crystal or the like.

[0008]

Means for Solving the Problems The present inventors have repeatedly studied a method for cleaning polycrystalline silicon, and the amount of fine particles on the surface of polycrystalline silicon is greatly affected by the re-adsorption of fine particles in the cleaning tank, The diameter has also been found to have a significant effect. The cleaning method of the present invention is based on the above findings, and in the cleaning tank, while preventing the impurity metal fine particles and silicon fine particles contained in the liquid from re-adsorbing to the polycrystalline silicon surface, a specific particle size range. Are effectively removed to enhance the cleaning effect.

That is, the present invention relates to (1) a method for acid-cleaning a polycrystalline silicon lump, wherein the cleaning liquid is extracted from the bottom of the cleaning tank, filtered, and then circulated again to the cleaning tank. A multi-stage washing system with at least two stages,
The washing system is provided with a first washing tank having a filter having a pore size of 0.1 μm or more in a filtration section of a circulation path, and a second washing tank having a filter having a pore diameter of less than 0.1 μm in a filtration section of the circulation path. The present invention relates to a method for cleaning polycrystalline silicon, characterized in that by cleaning a second cleaning tank after cleaning one cleaning tank, fine particles adhering to the polycrystalline silicon are removed stepwise together with acid cleaning.

[0010] The cleaning method of the present invention includes the following aspects.
(2) A filter having a pore size of 0.2 to 0.5 μm is provided in the filtration section of the first cleaning tank, and a filter having a pore size of 0.05 to 0.1 μm is provided in the filtration section of the second cleaning tank. the above
(1) The washing method of (1), a pre-filter having a pore diameter of 0.5 mm or more is provided in the filtration section of the first washing tank and the second washing tank, and a pre-filter of 0.2 mm in the filtration section of the first washing tank. ~ 0.3
(1) or one or more filters provided with one or two or more filters of μm, 0.3 to 0.4 μm, 0.4 to 0.5 μm.
The washing method of (2). (4) As a cleaning liquid, a concentration of 50 wt% hydrofluoric acid is 0.01 to 0.5 with respect to a nitric acid concentration of 70 wt%.
And a hydrofluoric acid solution prepared with a volume ratio of water less than 10 or a hydrofluoric acid solution prepared with a volume ratio of water of 5 to 200 with respect to 1 of hydrofluoric acid having a concentration of 50 wt%. )
Or the method for cleaning polycrystalline silicon according to (3).
(5) The ratio of the circulation amount B of the cleaning liquid to the cleaning tank capacity A (B /
The method for cleaning polycrystalline silicon according to any one of the above (1) to (4), wherein the cleaning is performed with A) being 0.05 to 1.0. (6)
The above (1) to (1), wherein the metal concentration of impurities contained in the oxide layer on the surface of the polycrystalline silicon after the cleaning is less than 0.8 wt ppb and the carbon concentration is less than 0.3 × 10 ¹⁶ atoms / cc. 5) The method for cleaning polycrystalline silicon according to any one of the above items.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments. FIG. 1 shows an example of an apparatus configuration for implementing the cleaning method of the present invention. As shown in the figure, the cleaning method of the present invention forms a multi-stage cleaning system in which a plurality of cleaning tanks 10, 20, 30, and 40 for cleaning a polycrystalline silicon lump 50 are arranged in series along a traveling direction. ing. FIG. 1 shows four washing tanks, but the number of washing tanks may be increased or decreased as necessary. Of these cleaning tanks, the cleaning tanks 10, 20, and 30 are acid cleaning tanks, and are filled with the acid of the cleaning liquid. The last washing tank 40 is a finished water washing tank.

The cleaning method of the present invention uses at least two of the acid cleaning tanks, in the illustrated example, the first cleaning tank 10.
The (first washing tank) and the third washing tank 30 (second washing tank) are provided with circulation paths 11, 31 for extracting the washing liquid from the tank bottom, filtering the washing liquid, and then circulating the washing liquid again to the washing tank. Filters 12, 13, 32, and 33 for removing fine particles contained in the cleaning liquid are provided in the circulation paths 11, 31, respectively. The filter is provided with a filter having a predetermined pore size. The circulation paths 11 and 31 are provided with liquid feed pumps 14 and 34 for extracting and circulating the cleaning liquid from the bottom of the tank. An intermediate cleaning tank 20 is provided between the first cleaning tank 10 and the second cleaning tank 30.

The polycrystalline silicon lump 50 is first immersed in the first cleaning tank 10 to be pickled and then picked up and transferred to the intermediate cleaning tank 20, where it is pickled and then washed in the second cleaning tank 30. After being further washed with an acid, it is lifted and then moved to the last water washing tank 40, where it is lifted and sent out of the system. The acid cleaning liquid in the first cleaning tank 10 and the acid cleaning liquid in the second cleaning tank 30 are drawn out of the cleaning tanks 10 and 30 by the liquid feed pumps 14 and 34, respectively, and are filtered.
The cleaning tank 10 is returned to the cleaning tanks 10 and 30 again via the line 3. At this time, the cleaning liquid is withdrawn from the bottom of the cleaning tank so that the liquid flow in the cleaning tank is directed downward, and returned to the upper part of the cleaning tank via the filtration unit.

In the conventional method in which the cleaning liquid in the cleaning tank is replaced by overflow, the cleaning liquid on the liquid surface is frequently changed, but the cleaning liquid on the tank bottom stays, and fine particles washed out from the silicon surface gradually accumulate on the tank bottom. Then, the fine particles float on the liquid surface and re-adsorb to the polycrystalline silicon surface. The fine particles are adsorbed on the silicon surface by an electric interaction and are relatively strong. For this reason, it is difficult to sufficiently wash the fine particles re-adsorbed on the silicon surface by water washing, and it is necessary to prevent the fine particles from being re-adsorbed as much as possible.

In the cleaning method of the present invention, the cleaning liquid is extracted from the bottom of the tank, filtered, and then circulated again to the upper part of the cleaning tank. Since the fine particles washed from the silicon surface are guided to the circulation path without floating on the liquid surface and efficiently discharged out of the tank, the fine particles washed from the silicon surface do not stay in the tank, and therefore, Re-adsorption of the fine particles is prevented.

Further, in the cleaning method of the present invention, the effect of removing fine particles is enhanced by specifying the pore size of the filter provided in the filtration section of the circulation path. In the cleaning of polycrystalline silicon, impurity metal particles and silicon fine particles having various particle sizes washed out from the silicon surface are contained in the cleaning waste liquid.
The conventional cleaning method for such fine particles is 0.1 μm.
The cleaning effect of the following fine particles is insufficient. If the fine particles are to be filtered at once using a filter having a small pore size, the load on the filter increases, and clogging occurs in a short time.

Therefore, in the cleaning method of the present invention, a multi-stage cleaning system having at least two stages of cleaning tanks having circulation paths is provided.
The effect of removing fine particles is enhanced by installing a filter having a predetermined pore size stepwise so that the pore size of the filter gradually decreases in accordance with the progress of the washing. Specifically, in the present invention, the filtration unit 13 of the first cleaning tank 10 has a pore size of 0.
A filter having a diameter of 1 μm or more is provided, and a filter having a pore diameter of less than 0.1 μm is provided in the filtration section 33 of the second washing tank 30.
In the present invention, the filtration part of the circulation path has a pore size of 0.
The one provided with a filter of 1 μm or more is referred to as a first cleaning tank, and the one provided with a filter having a pore size of less than 0.1 μm in a filtration section of a circulation path is referred to as a second cleaning tank. The filters of the first washing tank or the second washing tank may be used in combination of those having a plurality of pore sizes.Also, a plurality of washing tanks having different filter pore sizes may be used as the first washing tank. A plurality of cleaning tanks having different hole diameters may be used as the second cleaning tank.

In the illustrated example, the filtration units 13 and 33
The pre-filtration units 12 and 32 are provided in front of. The filter pore diameter of the pre-filtration sections 12 and 32 is suitably 0.5 mm or more, and usually about 1 mm. By removing relatively coarse silicon particles and the like here, the filtration units 13 and 33 are removed.
To reduce the burden of When transferring polycrystalline silicon to a plurality of cleaning tanks, silicon chunks are pressed against each other inside a basket or the like, causing silicon particles on the surface to be rubbed off and relatively coarse particles may be generated. Should be removed in advance. The cleaning liquid extracted from the tank bottoms of the cleaning tanks 10 and 30 passes through the pre-filtration units 12 and 32 and the filtration unit 1.
It is led to 3,33.

As described above, by performing multi-stage cleaning by the first cleaning tank 10 provided with a filter having a pore diameter of 0.1 μm or more and the second cleaning tank 30 provided with a filter having a pore diameter of less than 0.1 μm, a relatively rough surface is obtained. Since the particles are removed in the first washing tank 10 and the fine particles having a particle diameter of 0.1 μm or less are removed in the second washing tank 30, the load on each filter is small, and the fine particles having a particle diameter of 0.1 μm or less are removed. The cleaning effect can be enhanced.

The filter provided in the filtration section 13 of the circulation path of the first washing tank 10 has a pore size of 0.2 to 0.3 μm, and 0.3 to 0.3 μm.
An appropriate range is 0.4 μm, or a range of 0.4 to 0.5 μm. It is preferable to select the pore size of the filter in accordance with the particle size of the fine particles washed out from the surface of the polycrystalline silicon, and to increase the effect of removing the fine particles by providing filters having two or more different types of pore sizes in multiple stages.

The pore size of the filter provided in the filtration section 33 in the circulation path of the second washing tank 30 is suitably 0.05 to 0.1 μm. With a filter having a pore size of less than 0.05 μm, the burden of filtration increases, and the frequency of filter replacement increases.

In each of the first cleaning tank 10 and the second cleaning tank 30, the circulation amount B (liter / min) of the cleaning liquid is such that the ratio (B / A) to the cleaning tank capacity A (L) is 0.05 to 5%. An appropriate range is 1.0. If this ratio is less than 0.05, the removal efficiency of the filter is greatly reduced, and if it is more than 1.0, the load on the filter and the pump is increased, which is not preferable.

The composition of the acid washing solution is not particularly limited.
Hydrofluoric nitric acid solution prepared with a concentration ratio of 0.01 to 0.5 and a water concentration of less than 10 with a concentration of 50 wt% hydrofluoric acid to a concentration of 70 wt% nitric acid 1 or a water solution with a concentration of 50 wt% hydrofluoric acid to 1 Hydrofluoric acid solutions prepared at a volume ratio of 5-200 are preferred. Further, the temperature of the cleaning liquid is suitably 30 ° C. or higher.

The illustrated cleaning system is an example in which a first cleaning tank 10, an intermediate cleaning tank 20, a second cleaning tank 30, and a water cleaning tank 40 are arranged in series. Not limited to For example, in a water cleaning system in which two cleaning tanks are connected to an acid cleaning system in which five cleaning tanks are arranged in a row, at least two of the acid cleaning systems are provided with the circulation path of the present invention, or are further provided with water. The present invention can be applied to various cleaning systems, such as using the cleaning tank having the circulation path of the present invention as the cleaning tank of the cleaning system.

[0025]

EXAMPLES The present invention will be specifically described below with reference to examples. Note that these examples do not limit the scope of the present invention.

Example 1 A first washing tank having a capacity of 100 liters, in which a PTFE filter having a pore size of 0.3 μm was installed in the filtration section, and a pore size of 0.1 in the filtration section.
A cleaning apparatus having the structure shown in FIG. 1 was formed using a 100-liter second cleaning tank provided with a μm PTFE filter. Using this washing apparatus, 10 kg of polycrystalline silicon was washed. Hydrofluoric acid solution (concentration 70 wt% nitric acid: concentration 50 wt% hydrofluoric acid =
(10: 1 vol ratio), the circulation rate of the washing liquid extracted from the bottom of the tank was set to 20 l / min, and the liquid temperature was maintained at 30 ° C. The polycrystalline silicon lump is immersed in the hydrofluoric nitric acid solution of the first cleaning tank for 5 minutes, then pulled up and immersed in the cleaning water of the intermediate cleaning tank, and then immersed in the hydrofluoric nitric acid solution of the second cleaning tank 30 for 5 minutes to perform acid cleaning. Was. Subsequently, the polycrystalline silicon mass is lifted from the second cleaning tank,
The plate was washed with 5 ° C. water for 5 minutes and dried in a 70 ° C. hot air drier. After the dried polycrystalline silicon was immersed in diluted hydrofluoric acid to dissolve the surface oxide film, the heavy metal concentration in the diluted fluorine was measured by an ICP mass spectrometer. Further, the carbon concentration was measured by an FT-IR meter. Further, a pulling test of a silicon single crystal was performed, and an average value (average of 5 times) of a single crystal yield (single crystallization ratio) was obtained. The results are shown in Table 1.

Example 2 A PTFE filter having a pore size of 0.05 μm and a PTFE filter having a pore size of 0.5 μm were provided in the filtration section, and a hydrofluoric acid solution (concentration of 50 wt% hydrofluoric acid: water = 1: 20 vol.) Was used as a cleaning solution.
Ratio), the polycrystalline silicon was cleaned in the same manner as in Example 1 except that the circulation amount of the cleaning liquid was changed to 10 liter / min, and the amount of impurities, the rate of single crystallization, and the like were examined. Table 1 shows the results.
It was shown to.

COMPARATIVE EXAMPLE 1 10 kg of polycrystalline silicon was immersed in a cleaning tank having a capacity of 100 liters, and the cleaning liquid was circulated at a rate of 10 liters / min using a circulating pump. Filtered. A hydrofluoric nitric acid solution (concentration 70 wt% nitric acid: concentration 50 wt% hydrofluoric acid = 10: 1 vol ratio) was used as a cleaning solution, and 3
Washing was performed at 0 ° C. for 3 minutes (one-step washing). After acid cleaning, 2
The plate was washed in a 5 ° C. water bath for 5 minutes, and dried in a 70 ° C. hot air drier. The amount of impurities and the rate of single crystallization were examined in the same manner as in Example 1. The results are shown in Table 1.

Comparative Example 2 Polycrystalline silicon was prepared in the same manner as in Example 1 except that a PTFE filter having a pore size of 0.3 μm and a PTFE filter having a pore size of 0.6 μm were used, and the circulation rate of the cleaning solution was 10 liter / min. Was washed, and its impurity amount, single crystallization ratio, and the like were examined. The results are shown in Table 1.

Comparative Example 3 Polycrystalline silicon was cleaned in the same manner as in Example 1 except that the circulation amount of the cleaning liquid was changed to 1 liter / min, and the amount of impurities and the rate of single crystallization were examined. The results are shown in Table 1.

As shown in Table 1, the amounts of residual metals in Comparative Examples 1 to 3 are much larger than those in Examples 1 and 2,
Is about 3 to 4 times that of Examples 1 and 2 and the residual carbon concentration is about 2 times, and the cleaning effect is greatly different. For this reason, the single crystallization ratios of Comparative Examples 1 to 3 remain at the 60% level,
The single crystallization ratio of Examples 1 and 2 is significantly lower than 70%.

[0032]

[Table 1]

[0033]

The cleaning method of the present invention exerts an excellent cleaning effect on a polycrystalline silicon lump, and is particularly excellent in an effect of removing adhering fine particles, so that polycrystalline silicon having high cleanliness can be obtained. . Specifically, for example, the metal concentration of impurities contained in the oxide layer on the surface of the polycrystalline silicon after cleaning is less than 0.8 wt. Ppb, and the carbon concentration is 0.3 × 1.
Polycrystalline silicon of less than 0 ¹⁶ atoms / cc can be obtained. Further, it is possible to obtain polycrystalline silicon having very few silicon microparticles of 0.1 μm or less attached to the surface.
By using the polycrystalline silicon washed by the method of the present invention, the single crystallization ratio is greatly improved in pulling a silicon single crystal.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a cleaning system of the present invention.

[Explanation of symbols]

10-first washing tank, 11-circulation path, 12-pre-filtration unit,
13-filtration unit, 20-intermediate washing tank, 30-second washing tank,
31-circulation path, 32-pre-filtration section, 33-filtration section, 40
-Water wash tank, 50-polycrystalline silicon.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroyuki Takashina 5 Mitacho, Yokkaichi-shi, Mie Mitsubishi Materi AlPolysilicon Co., Ltd. (72) Inventor Kenji Hori 1-5-1, Otemachi, Chiyoda-ku, Tokyo Mitsubishi Materials Silicon Co., Ltd.

Claims (6)

[Claims] An acid cleaning method for a polycrystalline silicon lump, wherein a cleaning liquid is extracted from the bottom of a cleaning tank, filtered, and then provided with at least two cleaning tanks having a circulation path for recirculating the cleaning liquid to the cleaning tank. A washing system, wherein the washing system has a first washing tank having a filter having a pore size of 0.1 μm or more in a filtration section of a circulation path, and a second washing tank having a filter having a pore size of less than 0.1 μm in a filtration section of the circulation path. And cleaning the second cleaning tank after cleaning the first cleaning tank, thereby removing fine particles adhering to the polycrystalline silicon step by step together with the acid cleaning. Method. 2. The filtration part of the first washing tank has a pore size of 0.2 to 0.5.
2. The method for cleaning polycrystalline silicon according to claim 1, wherein a filter having a pore size of 0.05 to 0.1 [mu] m is provided in the filtration section of the second cleaning tank, wherein a filter having a pore size of [mu] m is provided. 3. A pre-filter having a pore diameter of 0.5 mm or more is provided in a filtration section of the first washing tank and the second washing tank,
Further, the filtration part of the first washing tank has a pore size of 0.2 to 0.3 μm and a pore size of 0.2 μm.
3. The method for cleaning polycrystalline silicon according to claim 1, wherein one or more filters of 3 to 0.4 [mu] m and 0.4 to 0.5 [mu] m are provided. 4. A hydrofluoric nitric acid solution prepared at a volume ratio of 0.01 to 0.5 and a water concentration of less than 10 to 50% by weight of hydrofluoric acid and 1 to 70% by weight of nitric acid as a cleaning solution, or a concentration of 5%.
4. A hydrofluoric acid solution prepared in a volume ratio of water of 5 to 200 with respect to 1% of hydrofluoric acid of 0 wt%.
3. The method for cleaning polycrystalline silicon described in 1. above. 5. A circulation amount B of the cleaning liquid with respect to a cleaning tank capacity A.
The cleaning is performed with the ratio (B / A) of 0.05 to 1.0.
5. The method for cleaning polycrystalline silicon according to any one of items 1 to 4. 6. The method according to claim 1, wherein the metal concentration of impurities contained in the oxide layer on the surface of the polycrystalline silicon after the cleaning is less than 0.8 wt. Ppb, and the carbon concentration is less than 0.3 × 10 ¹⁶ atoms / cc. Item 6. The method for cleaning polycrystalline silicon according to any one of Items 1 to 5.