JP7522065B2 - Method for cleaning polycrystalline silicon chunks - Google Patents

Description

The present invention relates to a method for cleaning polycrystalline silicon chunks used as a semiconductor raw material. More specifically, the present invention relates to a method for cleaning the surface of polycrystalline silicon chunks using laser light.

In recent years, as the performance of semiconductor devices has improved, there has been a demand to reduce various impurities in the silicon single crystal substrates that are widely used in these devices, and there is also an increasing demand to reduce impurities in the polycrystalline silicon blocks that are the raw material for these devices in order to improve quality. Among these, there is an increasing demand to reduce surface impurities in the polycrystalline silicon blocks, especially the surface carbon concentration.

Generally, as a method for reducing surface impurities in polycrystalline silicon chunks, a method and apparatus for producing high-purity classified polycrystalline silicon fragments (polycrystalline silicon chunks) have been disclosed (see, for example, Patent Document 1 (paragraphs [0045] and [0054])). In this method, polycrystalline silicon rods produced by the Siemens process are crushed by a shredding tool to produce polycrystalline silicon chunks, which are then classified by a screening device. In the process, if foreign particles from the tools or equipment that come into contact with the polycrystalline silicon chunks adhere to the polycrystalline silicon chunks, the foreign particles are immersed in a chemical solution such as an acid solution, dissolved in a cleaning bath, and removed from the polycrystalline silicon chunks, or removed together with the dissolved silicon as the surface of the polycrystalline silicon chunks dissolves.

As another method for reducing surface impurities in polycrystalline silicon chunks, a method for cleaning polycrystalline silicon chunks has been disclosed (see, for example, Patent Document 2 (paragraphs [0016], [0017], and [0062] to [0074])). Patent Document 2 shows that metal contamination and contamination by organic molecules occur during mechanical processing of polycrystalline silicon, and indicates that contact with parts made of organic polymers or plastics is one cause of organic contamination. In this method, in order to reduce surface contamination (organic contamination) of the polycrystalline silicon chunks, the polycrystalline silicon chunks are heat-treated at high temperatures in an inert gas atmosphere, and then cooled while purging the inert gas.

Meanwhile, a laser cleaning method has been disclosed in which a pulsed laser beam is irradiated onto the surface of a workpiece to be cleaned via a scan mirror while scanning, thereby removing objects adhering to the surface to be cleaned (see, for example, Patent Document 3 (Claim 1, paragraph [0007])). In this method, objects adhering to the surface to be cleaned include moisture and oil adhering to the cutting surface of metal after machining.

Special Publication No. 2009-544564 JP 2013-170122 A JP 2015-217427 A

The method of Patent Document 1 assumes that the shredding tool and screen device will come into contact with and adhere to the polysilicon fragments (polycrystalline silicon chunks), and must either dissolve the foreign particles in the cleaning bath or remove them together with the dissolved silicon as the surface of the polycrystalline silicon chunks dissolves, which poses the problem of certain restrictions being placed on the selection of the type of chemical solution used in the cleaning bath and its use conditions.

In addition, in the method of Patent Document 2, when the polycrystalline silicon chunks are heated at high temperatures for a certain period of time for cleaning, the process is carried out under a constant reduced pressure and inert gas flow rate, so if the processing volume is large, the equipment needs to be large in scale. After the heating process, the polycrystalline silicon chunks are cooled to room temperature outside the oven by purging with inert gas, but since the heating process of the entire silicon requires heating and cooling processes, there is an issue that it takes a considerable amount of processing time. Also, to prevent contamination from the processing space, it is necessary to make the environment in which the cleaning process is carried out free of impurities.

In addition, polycrystalline silicon is crushed and processed to an appropriate size for easy use, but since silicon is a brittle material and polycrystalline, its surface is prone to unevenness and sharp blade marks. For this reason, when parts or jigs come into contact with the polycrystalline silicon surface during the processing process, wear powder from the parts or jigs is likely to adhere as foreign matter. In addition, when silicon is crushed, fine cracks may form on the surface, and if foreign matter adheres to such cracks, it is difficult to remove the foreign matter. For this reason, in the above-mentioned processing with the chemical solution, it is necessary to increase the amount of silicon dissolved by etching the silicon surface, and the processing time also increases. As a result, the cost increases and productivity decreases due to the increase in the amount of chemical solution used. In addition, silicon loss due to etching also increases, which leads to a decrease in the yield of polycrystalline silicon and an increase in processing costs. In addition, even with the method using heat treatment, if the amount of adhesion is large or the adhesion is strong, it takes time to heat and evaporate, and as a result, the processing time is likely to be long.

The laser cleaning method in Patent Document 3 is intended to clean the cutting surface of metal after machining, and the objects to be removed are moisture, oil, etc. adhering to the cutting surface, and is not used to remove foreign matter from the surface of polycrystalline silicon chunks, which require a high-quality surface condition.

The object of the present invention is to provide a method for cleaning polycrystalline silicon chunks that can quickly and easily remove foreign matter attached to or adsorbed on the surface of the polycrystalline silicon chunks, particularly impurities that are invisible to the naked eye.

The first aspect of the present invention is a method for cleaning polycrystalline silicon chunks 2, which, as shown in Figures 1 to 4, involves irradiating the surface of one or more polycrystalline silicon chunks 2 placed in a container 1 or on a stand 5 with laser light 3, and removing foreign matter 4 attached to or adsorbed on the surface of the polycrystalline silicon chunks 2 by moving the container 1 or stand 5 on which the polycrystalline silicon chunks 2 are placed, or by moving the laser light 3.

The second aspect of the present invention is an invention based on the first aspect, and is a cleaning method in which the laser light is scanned linearly.

The third aspect of the present invention is an invention based on the first or second aspect, and is a cleaning method in which, as shown in FIG. 5, a polycrystalline silicon lump 2 is placed in a container 8 in which a liquid 9 is moving, and the container 8 or the laser light 3 is moved while irradiating the surface of the polycrystalline silicon lump 2 with a laser light 3.

The fourth aspect of the present invention is an invention based on any one of the first to third aspects, and is a cleaning method in which the irradiation of laser light is performed after the polycrystalline silicon block is treated with a chemical solution.

In the method of the first aspect of the present invention, the surface of one or more polycrystalline silicon chunks placed in a container or on a table is irradiated with laser light, and the container or table on which the polycrystalline silicon chunks are placed is moved, or the laser light is moved, so that foreign matter attached to or adsorbed on the surface of the polycrystalline silicon chunks, particularly impurities invisible to the naked eye, can be easily removed in a short time.

In the method according to the second aspect of the present invention, the laser light is scanned linearly, so that the surface of the polycrystalline silicon block can be irradiated uniformly and without unevenness.

In the method of the third aspect of the present invention, the polycrystalline silicon chunk is placed in a container through which liquid moves, and the container or the laser light is moved while irradiating the surface of the polycrystalline silicon chunk with laser light. Therefore, foreign matter removed from the surface of the polycrystalline silicon chunk is transported outside the container by the liquid and does not reattach to the polycrystalline silicon chunk.

In the method according to the fourth aspect of the present invention, the laser light is irradiated onto a polycrystalline silicon block in which the amount of attached impurities has been reduced by treatment with a chemical solution, so that a polycrystalline silicon block of even higher purity can be obtained.

1 is a perspective view showing a state in which a plurality of polycrystalline silicon chunks placed in a container according to a first embodiment of the present invention are being cleaned by a laser cleaning device. FIG. 1 is an enlarged cross-sectional view showing a state in which a polycrystalline silicon chunk placed in a container according to a first embodiment of the present invention is being laser cleaned. FIG. FIG. 2 is a plan view showing a state in which a laser beam of the laser cleaning device according to the first embodiment of the present invention is moved in a direction perpendicular to a scanning straight line. FIG. 2 is a plan view showing a state in which a laser beam of the laser cleaning device according to the first embodiment of the present invention is moved in a direction oblique to a scanning straight line. 11A to 11C are diagrams showing a situation in which laser light is irradiated onto a polycrystalline silicon block placed in a container in which a liquid moves in a second embodiment of the present invention.

Next, a description will be given of an embodiment of the present invention with reference to the drawings. The number of polycrystalline silicon chunks shown in the following Figures 1 to 5 is an example, and is not intended to be limiting, and can be increased as necessary.

First Embodiment
1, a laser cleaning device 10 of this embodiment includes a processing head 11 and a main body control unit 12. The main body control unit 12 includes a laser oscillator 13 and a control unit 14 that controls the processing head 11 and the laser oscillator 13. The processing head 11 includes a built-in galvano scanner (not shown) that rotates and controls a shaft to which a reflection mirror that reflects pulsed laser light from the laser oscillator 13 is attached.

A container 1 with an open top is placed below the laser cleaning device 10, and multiple polycrystalline silicon chunks 2 (six in FIG. 1) are placed in the container 1 without overlapping as objects to be cleaned. As shown in FIG. 2, foreign matter 4 is attached to the surface of these polycrystalline silicon chunks 2. Examples of foreign matter include impurities that cannot be seen with the naked eye, such as organic matter. In FIG. 2, the same elements as in FIG. 1 are given the same reference numerals. Note that instead of the container 1 on which the polycrystalline silicon chunks are placed, a table 5 with a flat surface may be used, as shown in FIGS. 3 and 4. The material of the container 1 or table 5 may be quartz or silicon, or resin, paper, etc.

From the viewpoint of processing efficiency, a laser light source having a narrow pulse width and high absorption efficiency by silicon or contaminants is suitable as the processing laser light source of the laser cleaning device 10. From this viewpoint, pulsed lasers such as Nd:YAG (1064 nm) and Yb:fiber (1060 nm to 1070 nm), which are widely used in industry, can be cited as practically usable laser light sources.
The wavelengths of the Nd:YAG and Yb:fiber lasers are in a region that exhibits an absorption rate of about 50% in the silicon chunk, and have the effect of essentially heating the surface of the silicon chunk, thereby making it possible to volatilize and remove impurities adsorbed on the surface of the silicon chunk.

The intensity of the laser light of the laser cleaning device 10 is preferably 500 kJ/ ^m2 or less. It is believed that the impurities are volatilized by the surface temperature of the silicon chunk rising instantaneously to 300°C to 600°C due to the irradiation of the laser light, and if a laser light with a short pulse width is used, the heating area is limited, so that even if the intensity of this laser light is about 1 kJ/ ^m2 , the effect of removing impurities can be expected.
Since silicon chunks are irregularly shaped fragments, it is practically difficult to irradiate the entire surface of the silicon chunks with laser light uniformly and completely. In order to remove impurities on the surface of the silicon chunks, the area of the silicon chunks irradiated with the laser light is preferably at least 80% or more of the surface area of the silicon chunks. More preferably, it is 95% or more. For this purpose, for example, a method of arranging the silicon chunks without overlapping on a table that transmits laser light, such as quartz glass, and irradiating the silicon chunks with laser light from both the top and bottom can be used.

In the laser cleaning device 10 configured as described above, it is preferable to scan the laser light 3 reflected by the galvano scanner in the processing head 11 in the Y-axis direction in FIG. 1 by rotating and controlling the axis in the galvano scanner, thereby forming a scanning line 6. In the cleaning method of this embodiment, the laser light 3 is irradiated so that the scanning line 6 covers the entire Y-axis direction of multiple (two in FIG. 1) polycrystalline silicon chunks 2 as shown in FIG. 1. While forming the scanning line 6, the processing head 11 is moved in the X-axis direction in FIG. 1, which is perpendicular to the scanning line 6, to laser clean the surfaces of all the multiple (six in FIG. 1) polycrystalline silicon chunks 2. The processing head 11 may be moved back and forth in the X-axis direction, or the polycrystalline silicon chunks 2 in the container may be turned over, and irradiation with the scanning line 6 of the laser light 3 may be repeated.

As the scanning line 6 of the laser light 3 moves in the X-axis direction, as shown in FIG. 2, the foreign matter 4 adhering to or adsorbed on the surface of the polycrystalline silicon block 2 either absorbs the laser light 3 and evaporates, or is subjected to the impact pressure of the plasma 7 and is removed from the surface of the polycrystalline silicon block 2.

The processing head 11 may be moved not only in a direction perpendicular to the scanning line 6 of the laser light 3 as shown in FIG. 3, but also in a direction diagonal to the scanning line 6 as shown in FIG. 4, depending on the position of the polycrystalline silicon block 2. In FIGS. 3 and 4, the same elements as in FIG. 1 are given the same reference numerals. Although not shown, instead of moving the processing head 11, the processing head 11 may be fixed and the stage 5 may be moved.

Second Embodiment
As shown in Fig. 5, a plurality of polycrystalline silicon chunks 2 (six in Fig. 5) are arranged in a container 8 so as not to overlap each other. In Fig. 5, the same elements as in Fig. 1 are given the same reference numerals. A liquid 9 such as pure water, ultrapure water, ion-exchanged water, or the like is stored in the container 8. A supply pipe 8a for the liquid 9 is connected to the lower right side of the container 8, and a discharge pipe 8b for the liquid 9 is connected to the upper left side of the container 8. The height of the discharge pipe 8b is set so that the liquid level of the container 8 is higher than the uppermost ends of the plurality of polycrystalline silicon chunks 2 arranged in the container, so that the polycrystalline silicon chunks 2 in the container are always present in the liquid.
A processing head (not shown in Figure 5) of the laser cleaning device is provided above this container 8, and the laser light 3 reflected by the processing head is configured to move in the direction indicated by the arrow in Figure 5 while forming a straight scanning line.

In a laser cleaning device configured as described above, first, liquid 9 is supplied to the container 8 from the supply pipe 8a, and the liquid 9 is discharged from the discharge pipe 8b, so that the liquid 9 moves within the container 8. Next, the surface of the multiple polycrystalline silicon chunks 2 arranged in the container 8 is irradiated with laser light 3 from the processing head. The laser light 3 spreads over the upper surface of all the polycrystalline silicon chunks 2 in the container 8 as the processing head moves, forming a scanning line. This removes foreign matter from the upper surface of all the polycrystalline silicon chunks 2. The removed foreign matter floats in the liquid, but is transported by the moving liquid and discharged from the discharge pipe 8b. This prevents the foreign matter from reattaching to the surface of the polycrystalline silicon chunks 2.

The polycrystalline silicon chunks described in the first embodiment may be subjected to a surface cleaning treatment using a chemical solution such as fluoronitric acid before being laser cleaned. This makes it possible to make the polycrystalline silicon chunks after laser cleaning into silicon of even higher purity.
In addition, in the above-mentioned container 8, the discharge pipe 8b is provided at the top of the container, but as an example, the discharge pipe 8c may also be provided at the bottom end of the container, as shown by the dashed line in Fig. 5. By using this discharge pipe 8c, foreign matter that does not float in the liquid can be transported from the bottom side of the container to the outside of the container 8 by the moving liquid.

Next, we will explain in detail the examples of the present invention along with comparative examples.

Example 1
Seven polycrystalline silicon chunks with long side lengths of 10 mm to 40 mm were etched and cleaned with fluoronitric acid, rinsed with pure water, and then dried. As shown in Figure 1, the seven dried polycrystalline silicon chunks were arranged on a silicon table and cleaned using a laser cleaning device. In this way, seven samples of Example 1 were obtained.

As shown in Figure 1, during cleaning, the surface of the polycrystalline silicon block was irradiated with a pulsed laser beam (wavelength 1060 nm to 1080 nm, pulse frequency 55 kHz) with an average output of 63 W while scanning linearly at a speed of 3900 mm/s, and the container was moved back and forth twice at a speed of 165 mm/min in a direction perpendicular to the linear scanning line of the laser beam. After two round trips were completed, the polycrystalline silicon block was flipped over and irradiated with the laser beam in the same manner.

<Comparative Example 1>
As in Example 1, seven polycrystalline silicon chunks with long side lengths of 10 mm to 40 mm sampled from the same production lot as the seven samples of Example 1 were etched and cleaned with fluoronitric acid, rinsed with pure water, and then dried. The seven dried polycrystalline silicon chunks were not laser cleaned. As a result, seven samples of Comparative Example 1 were obtained.

<Comparative testing and evaluation>
Of the seven samples obtained in Example 1 and Comparative Example 1, the surface carbon concentrations of three samples from each were measured using a thermal desorption gas chromatograph mass spectrometer (TD-GC/MS), the surface boron concentrations of two samples from each were measured using ICP-MS (Inductively Coupled Plasma Mass Spectrometry) after elution with 2 wt % HF, and the surface phosphorus concentrations of the remaining two samples were measured in the same manner as for the surface boron concentrations. The average values of the samples under each condition are shown in Table 1 below.

As is clear from Table 1, the average carbon concentration of the sample surface in the measured samples was 0.57 ppbw in Comparative Example 1, whereas it was 0.20 ppbw in Example 1, which was low. In addition, the average boron concentration of the sample surface in the measured samples was 0.045 ppbw in Comparative Example 1, whereas it was 0.031 ppbw in Example 1, which was low. In addition, the average phosphorus concentration of the sample surface in the measured samples was 0.051 ppbw in Comparative Example 1, whereas it was 0.041 ppbw in Example 1, which was low. From these findings, it was found that the carbon concentration of the sample surface in Example 1, in which laser cleaning was performed, was significantly lower than that in Comparative Example 1, in which laser cleaning was not performed, and the boron concentration and phosphorus concentration of the sample surface were also lower.

The method for cleaning polycrystalline silicon chunks of the present invention can be used to purify polycrystalline silicon chunks produced by crushing polycrystalline silicon rods produced by the Siemens process.

Reference Signs List 1, 8 Container 2 Polycrystalline silicon block 3 Laser light 4 Foreign object 5 Table 6 Scanning line 7 Plasma 9 Liquid 10 Laser cleaning device 11 Processing head 12 Main body control unit 13 Laser oscillator 14 Control unit

Claims (4)

A method for cleaning polycrystalline silicon chunks, which involves irradiating the surface of one or more polycrystalline silicon chunks placed in a container or on a stand with laser light, and removing foreign matter adhering to or adsorbed on the surface of the polycrystalline silicon chunks by moving the container or stand on which the polycrystalline silicon chunks are placed, or by moving the laser light. The cleaning method according to claim 1, in which the laser light is scanned linearly. The cleaning method according to claim 1 or 2, in which the polycrystalline silicon chunk is placed in the container in which the liquid moves, and the container or the laser light is moved while irradiating the surface of the polycrystalline silicon chunk with the laser light. The cleaning method according to any one of claims 1 to 3, wherein the irradiation of the laser light is performed after the polycrystalline silicon block is treated with a chemical solution.

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