JPH0353274B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an apparatus for growing flat silicon crystals using crystal end supports (strings).

[Technical background of the invention and its problems] In recent years, a pulling method shown in FIG. 3 has been developed in order to manufacture solar cells at low cost. Furthermore, Figure 3a
is a plan view, and figure b is a sectional view taken along line A-A' in the same figure. That is, the heaters 34 and 35 are energized to melt the polycrystalline silicon contained in the carbon crucible 32 to form a silicon melt 33. after that,
Small holes 37a, 3 at the bottom of the carbon crucible 32
The silicon melt 3 is passed through the hot chamber 36 through the strings 31a and 31b with a diameter of 0.5 mm made of carbon fiber as a crystal end support material.
Put the tip out on the surface of 3. then string 3
A seed crystal (not shown) is lowered between 1a and 31b,
When the silicon melt 33 is blended with the ends of the strings 31a, 31b, the silicon melt 33 is supported by the strings 31a, 31b and the seed crystal. By pulling up the two strings 31a and 31b together with the seed crystal, a flat silicon crystal is pulled up. By the way, when the flat silicon crystal pulled by such a device has an uneven thickness and is used as a solar cell, for example, it will crack during the device process, especially the printing process for electrodes, etc., resulting in a low device production yield of 30 to 40%. It was. Further, after the completion of pulling, the silicon melt remaining in the carbon crucible was damaged due to volume expansion when it solidified, making it difficult to reuse it.

After investigating the reason for the non-uniform thickness of the flat silicon crystal, it was found that the temperature distribution of the silicon melt was non-uniform. The temperature of the silicon melt between two strings with a spacing of 100 mm was measured at 10 mm intervals. Measurements were carried out by inserting a thermocouple into a protective tube made of quartz glass or the like. Figure 4a shows the temperature distribution of the silicon melt. In other words, the hottest part (measurement point 4 in the figure) is 1490±2℃, and the coldest part is 1480±3℃.
There was a difference of about 10°C between the two temperatures, and unevenness in the temperature distribution was observed.

Attempts were made to grow the silicon melt at a pulling rate of 16 mm/min to 18 mm/min with this temperature distribution. Although the growth of a flat silicon crystal with a width of 100 mm continued, non-uniformity in thickness occurred in the width direction. The results of measuring the thickness are shown in Figure 4b. 510±20μm at the thickest part (the part with the lowest temperature, measurement point 10 in the figure),
At the thinnest part (the part with the highest temperature, measurement point 4 in the figure), the thickness was 290±20 μm, and unevenness of about 220 μm was observed, and at measurement point 8 in the figure, unevenness of 370±20 μm was observed. Next, the flat silin crystal with uneven thickness was cut into a size of, for example, 100 mm x 100 mm to create a solar cell element. However, the handling of the element,
Furthermore, the yield of creating cracked elements was low at about 30% to 40% due to the printing pressure in the printing process of electrodes, etc. In addition, when the heater is turned off and the temperature is lowered after growth, the silicon melt remaining in the carbon crucible or in the string hole solidifies and expands in volume, causing damage to the carbon crucible and reuse. However, it has been difficult to manufacture solar cell elements at low cost.

[Objective of the Invention] The object of the present invention is to prevent cracking in the solar cell element process by making the thickness of a flat silicon crystal uniform, thereby significantly improving the element production yield, and at the same time, preventing damage to the crucible. , provides a flat silicon crystal growth device that allows reuse.

[Summary of the Invention] The present invention provides at least two crystal edge supports disposed above the crucible through each of the small holes from the lower part of the crucible having at least two small holes in the bottom, and A silicon melt is stored in a crucible, a seed crystal is brought into contact with the silicon melt between the crystal edge supports and the crystal edge support, and the seed crystal is pulled up. In an apparatus for growing a flat silicon crystal by continuously crystallizing a silicon melt stretched by surface tension between the crucible and the inner wall of the crucible or the small hole, carbon is applied via powdered silicon nitride to the inner wall of the crucible or the inner wall of the small hole. It is covered with foil.

[Embodiment of the Invention] FIG. 1 shows an embodiment of the present invention. In the figure, the third
The same parts as those in the figures are given the same reference numerals and the explanation thereof will be omitted. As shown in FIG. 1, a carbon crucible 32
The inner wall was covered with a layer of about 0.5 mm thick with powdered silicon nitride 12, and carbon foil 11 about 2 mm thick was laid inside the layer.

Polycrystalline silicon as a raw material was placed in the carbon crucible 32, and the heaters 34 and 35 were energized to melt the polycrystalline silicon and create a silicon melt 33.

The temperature distribution was measured using the same method as described above, and the results are shown in FIG. 2a. The highest temperature part (measurement point 4) was 1488 ± 2°C, and the lowest temperature part (measurement point 7) was 1485 ± 2°C, the temperature difference was significantly reduced to 3°C, and the uniformity was improved. Next, add the seed crystal 39 to 16~
A flat silicon crystal was created by pulling at a rate of 18 mm/min, and its thickness was measured. The results are shown in Figure 2b. The thickest part is 420 ± 20 μm, the thinnest part is 390 ± 20 μm, and the difference in thickness has decreased to 30 μm.
The level of unevenness was small and the thickness uniformity was improved by about three times or more.

The above flat silicon crystal was cut into a size of 100 mm x 100 mm to create a solar cell element. By improving the uniformity and flatness of the flat silicon crystal, cracks due to handling during device fabrication or printing pressure during the printing process of electrodes, etc. are significantly reduced, and the device production yield is improved to 80-85%. did.

Further, after the pulling was completed, the heaters 34 and 35 were de-energized to solidify the silicon melt 33. After cooling the carbon crucible 32 to room temperature, the carbon foil 11 was taken out, and the solid silicon remaining in the crucible 32 or in the string holes 37a and 37b was easily removed from the crucible 32 along with the carbon foil 11. I was able to remove it. As a result of reuse, it was possible to reuse it several times without any problems, and it also met the purpose of producing solar cell elements at low cost.

As described in the example above, by covering the inner wall of the carbon crucible with carbon foil via powdered silicon nitride, the temperature distribution of the silicon melt is made uniform, and the thickness of the flat silicon crystal is thereby made uniform. It has become possible to improve the uniformity and flatness of the solar cell, and to significantly improve the yield of solar cell devices. Furthermore, the carbon crucible can now be reused several times, achieving the goal of producing solar cell elements at low cost.

After studying various ways to make the temperature distribution of silicon melt uniform, we found a simple method.
This method involves covering the inner wall of a carbon crucible with carbon foil and interposing powdered silicon nitride between the carbon foil and the carbon crucible. Carbon foil has a layered structure, and its in-plane thermal conductivity is 36 k・cal/m・hr・at approximately 1100°C.
℃, uniform, and the thermal conductivity in the thickness direction is approximately
It is as small as 2.99k・cal/m・hr・℃ at 1100℃. Carbon foil is also flexible and can withstand temperatures above the melting point of silicone (1420°C).

Therefore, by placing carbon foil in the inner wall or string hole of a carbon crucible, it is possible to make the temperature of the silicon melt in the crucible uniform by using high and uniform thermal conductivity in the planar direction. . Furthermore, due to the flexibility of carbon foil, the force applied to the carbon crucible due to volume expansion can be alleviated, and even if the silicon melt penetrates into the carbon foil, there is a gap between the inner wall of the carbon crucible and the carbon foil. Since silicon nitride does not wet the intervening silicon melt, it is possible to prevent the silicon melt from adhering to the inner wall of the carbon crucible. Therefore, the carbon crucible can be reused by removing the carbon foil after finishing pulling.

[Effects of the Invention] As described above, according to the present invention, the inner wall of the crucible or the inside of the string small hole is covered with carbon foil, and powdered silicon nitride is interposed between the inner wall of the crucible and the carbon foil. ,
It is possible to make the thickness of the flat silicon crystal uniform, reduce unevenness, prevent cracks in the solar cell element process, and improve element production yield. In addition, after the pulling is completed, the crucible is prevented from being damaged and can be reused, making it possible to produce solar cell elements at low cost.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing one embodiment of the present invention, and FIG.
Figure a is a characteristic diagram showing an example of the temperature distribution of the silicon melt according to the present invention, Figure 2 b is a characteristic diagram showing an example of the thickness of the flat silicon crystal according to the present invention, and Figure 3 a is a characteristic diagram showing an example of the temperature distribution of the silicon melt according to the present invention. FIG. 3b is a sectional view of the flat silicon crystal growth apparatus, FIG. 4a is a characteristic diagram showing the temperature distribution of silicon melt in the conventional apparatus, and FIG. 4b is the conventional flat silicon crystal growth apparatus. FIG. 3 is a characteristic diagram showing the thickness of silicon crystal. 11...Carbon foil, 12...Silicon nitride, 31a, 31b...Carbon string,
32... Carbon crucible, 33... Silicon melt, 34, 35... Heater, 36... Hot chamber, 39... Seed crystal.

Claims (1)

[Claims] 1 At least two crystal edge supports are placed from the bottom of a crucible with at least two small holes in the bottom.
A silicon melt is placed in the crucible through each of the small holes, and a seed crystal is brought into contact with the silicon melt between the crystal edge supports and the crystal edge supports. , in an apparatus for growing a flat silicon crystal by continuously crystallizing a silicon melt stretched between the crystal end supports by surface tension by pulling up the seed crystal, the inner wall in the crucible Alternatively, an apparatus for growing a flat silicon crystal, characterized in that the inner wall of the small hole is covered with carbon foil via powdered silicon nitride.