# Patent

## Title
Method of making solar cell

## Abstract

A method of manufacturing a solar cell including a silicon wafer is provided. In certain example instances, the method may include flattening fine roughness existing on a side face of a silicon block or a silicon stack used for manufacturing the silicon wafer for use in the solar cell.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part (CIP) of U.S. Ser. No. 10/716,661, filed Nov. 20, 2003, now abandoned, which is a division of 09/956,113, filed Sep. 20, 2001, now U.S. Pat. No. 6,679,759,the entire contents of which are hereby incorporated herein by reference. This application is also related to Japanese application Nos. 2000-296628 and 2001-272356, filed on Sep. 28, 2000 and Sep. 7, 2001, whose priority is claimed under 35 USC § 119, the disclosures of which are hereby incorporated herein by reference.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a solar cell comprising a silicon wafer. In particular, it relates to a polishing technique for flattening fine roughness existing on a side face of a silicon block or a silicon stack.
2. Description of Related Art
Demand for silicon wafers is increasing year by year in accordance with the spread of solar cells and the like. For example, one solar cell requires about 54 silicon wafers of 5×5 inch square, which are much greater than the number of silicon wafers required in IC and LSI.
The silicon wafer includes polycrystalline and single crystalline silicon wafers, which are manufactured by the following method.
The polycrystalline silicon (polysilicon) wafer is obtained by manufacturing a square polysilicon ingot, cutting the ingot into plural polysilicon blocks 1 with a band saw 20 (FIG. 4) and slicing the polysilicon block 1 (FIG. 5). FIGS. 4 and 5 show a side face 19 of a silicon block, an edge 21 of a silicon block and silicon wafers 46.
The single crystalline silicon wafer is obtained by cutting a cylindrical silicon ingot manufactured by a crystal pulling method (generally 1 m in length) into cylindrical single crystalline silicon blocks in a suitable size (generally 40 to 50 cm in length), grinding the single crystalline silicon block to form a flat portion called an orientation flat and slicing the silicon block.
Where the silicon wafer of high dimensional accuracy is required, grinding is carried out in both cases of processing the polycrystalline and single crystalline silicon blocks. Specifically, the grinding is performed by rotating a polishing wheel 45 such as a circular grindstone containing abrasive grains or a diamond wheel at high speed, pressing the silicon block 1 onto the polishing wheel and moving them relatively to each other. FIG. 6 shows a one-axis stage 7, a direction 11 along which the stage 7 moves, a motor 5 for rotating the polishing wheel, a two-axes stage 6 and a direction 10 along which the stage 6 moves laterally.
In a conventional process of manufacturing the silicon wafer, a process of improving dimensional accuracy of the silicon block or the silicon stack, or a process of erasing unevenness on the surface of the silicon block or the silicon stack has been carried out. However, flattening of the fine surface roughness existing on its side faces has not been conducted.
The thus obtained silicon wafer is subjected to processing of a side face (may be referred to as a periphery face or a circumferential face).
The periphery processing is carried out by grinding the periphery surfaces of the silicon wafers one by one into a desired configuration in the same manner as a method of processing a glass substrate described in Japanese Unexamined Patent Publication No. Hei 10 (1998)-154321, or by chemical polish (etching).
Since the solar cell requires a large number of silicon wafers as compared with IC and LSI, the above-described periphery processing with respect to each of the silicon wafers consumes a lot of time, investment in equipment and labor. This may delay the supply of the silicon wafers behind the demand. Further, the etching requires equipment for liquid waste treatment, which also involves a problem of equipment costs.
However, without the periphery processing, the silicon wafer may be cracked in a later step for manufacturing the solar cell, which reduces a product yield. Accordingly, there has been demanded development of an efficient method for the periphery processing.
SUMMARY OF THE INVENTION
In certain example embodiments of this invention, there is provided a method of making a solar cell comprising a silicon wafer, the method comprising the following steps in the order recited: polishing side faces of a silicon block used for manufacturing the silicon wafer; slicing the silicon block, so that said slicing is performed after said polishing, and making solar cell(s) using sliced pieces from the silicon block. Accordingly, the present invention provides a method of manufacturing a silicon wafer comprising the step of flattening fine roughness existing on a side face of a silicon block or a silicon stack used for manufacturing the silicon wafer.
According to certain example embodiments of the present invention, the side face of the silicon block or the silicon stack is flattened to such an extent that dimensional accuracy is improved and surface unevenness is eliminated, i.e., the side face is flattened so that it has surface roughness Ry of 8 μm or less, preferably 6 μm or less.
However, it should be understood that the detailed description and specific examples, while indicating example preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view illustrating a method of manufacturing a silicon wafer according to Method 1 of the present invention;
FIG. 2 is a schematic view illustrating a method of manufacturing a silicon wafer according to Method 2 of the present invention;
FIG. 3 is a graph illustrating a relationship between surface roughness of a circumferential surface of a silicon wafer and cracking reduction ratio of a solar cell from the silicon wafer;
FIG. 4 is a schematic view illustrating a method of cutting a silicon ingot into silicon blocks;
FIG. 5 is a schematic view illustrating a method of slicing a silicon block into silicon wafers; and
FIG. 6 is a schematic view illustrating a conventional process of grinding a silicon block.



DESCRIPTION OF EXAMPLE PREFERRED EMBODIMENTS
An object of the present invention is to provide a polishing technique for flattening fine roughness existing on a side face of a silicon block or a silicon stack in a short period so that the silicon wafer is prevented from cracking and improved in yield. This may be used in making a solar cell according to certain example embodiments of this invention.
As a result of eager researches for solving the above problems, it has been found that the fine roughness on the side face of the silicon block or the silicon stack causes the cracking of the silicon wafer and decreases the yield. Then the inventors have found that the cracking is prevented and the yield is improved efficiently by flattening the fine roughness before slicing the silicon block or the silicon stack into the silicon wafer. Thus, the present invention has been achieved.
The “silicon stack” mentioned in the present application signifies a silicon block in the shape of cylinder or quadratic prism in which two or more silicon wafers are stacked. The “side face of the silicon block or the silicon stack” mentioned in the present application signifies a face which will constitute a circumferential surface of the silicon wafer.
Method 1
According to Method 1 of the present invention, a mixture of abrasive grains and a medium is sprayed on a side face of the silicon block or the silicon stack, a polishing member is shifted closer to or contacted with the side face to be polished, and the silicon block or the silicon stack is moved relatively to the polishing member in the presence of the abrasive grains so that the side face of the silicon block or the silicon stack is mechanically and physically polished. Thereby the fine roughness existing on the side face of the silicon block or the silicon stack is flattened.
The abrasive grains may be known abrasive grains, e.g., diamond, GC (Green Carborundum), C (Carborundum), CBN (cubic boron nitride) and the like.
The medium to spray the abrasive grains may be a liquid such as water, alkaline solution, mineral oil, glycols (polyethylene glycol, propylene glycol (PG)) or the like, or a gas such as air or inert gas, e.g., nitrogen, helium, neon, argon or the like. The abrasive grains may be mixed in a ratio of about 0.5-1.5 kg with respect to 1 kg of the liquid medium or about 0.01-2:1 kg with respect to 1 liter of the gaseous medium.
The polishing member may be made of steel, resin, cloth, sponge or the like. More specifically, it may be a steel brush, a resin brush, a sponge wheel or the like. The polishing member may or may not have the abrasive grains on its surface and/or in the inside thereof.
Method 1 will be detailed with reference to FIG. 1.
A polishing member 13 is arranged on a polishing wheel 4 so that it contacts with a side face 9 of a silicon block 1 to be polished, and then rotated at high speed by a motor 5 for rotating the polishing wheel along a direction 12 shown in FIG. 1. At this time, a mixture 8 of abrasive grains 14 and a medium 15 (may be referred to as “slurry” or “dispersed abrasive grains”) is sprayed from a nozzle 3. Further, the silicon block 1 is reciprocated by a one-axis stage 7 along a direction 11 shown in FIG. 1. According to the rotational movement of the polishing wheel 4 and the reciprocal movement of the one-axis stage 7, the side face 9 is entirely polished and the fine roughness is removed. The slurry 8 is used to let the abrasive grains 14 into the polishing member 13 of the polishing wheel 4 so that the side face 9 is polished with the abrasive grains 14. Further, the medium 15 in the slurry 8 serves to discharge silicon shavings and unnecessary abrasive grains 14, and cool the side face 9.
FIG. 1 shows a two-axis stage 6 capable of moving in a lateral direction 10 and a vertical direction 31, which is used to shift the polishing wheel 4.
Method 2
According to Method 2 of the present invention, a medium is sprayed on a side face of the silicon block or the silicon stack, a polishing member having abrasive grains on its surface and/or in the inside thereof is shifted closer to or contacted with the side face to be polished, and the silicon block or the silicon stack are moved relatively to the polishing member so that the side face of the silicon block or the silicon stack is mechanically and physically polished. Thereby the fine roughness existing on the side face of the silicon block or the silicon stack is flattened.
The medium to spray the abrasive grains may be the above-described liquid or gas. The liquid or the gas may not contain the abrasive grains.
The polishing member having the abrasive grains on its surface and/or in the inside thereof may be made of steel, resin, cloth, sponge or the like having, on its surface and/or in the inside thereof, abrasive grains such as diamond, GC (Green Carborundum), C (Carborundum), (CBN (cubic boron nitride) or the like. More particularly, the polishing member may be a steel brush, a resin brush, a sponge wheel or the like.
The liquid or the gas to be sprayed serves to remove, from the surface of the silicon block, silicon shavings and the abrasive grains fallen from the surface and/or the inside of the polishing member. Where the liquid or the gas containing no abrasive grains is used, the liquid or the gas is easily recycled and the abrasive grains and the silicon shavings are easily separated.
Method 2 will be detailed with reference to FIG. 2.
The difference from Method 1 is that the polishing member 17 having the abrasive grains on its surface or in the inside thereof is arranged on the polishing wheel 4 so that it contacts with the side face 9 of the silicon block 1 to be polished, and then a polishing liquid or polishing gas 16 comprising a medium 18 is sprayed. That is, the side face 9 of the silicon block 1 is polished by the abrasive grains 14 (not shown) of the polishing member 17. The polishing liquid or polishing gas 16 is sprayed onto the side face 9 of the silicon block 1 to be polished in order to remove the silicon shavings, unnecessary abrasive grains (grain scraps) and waste generated during the polishing, and to cool the side face 9. Other components than the above-mentioned ones are indicated by the same reference numbers shown in FIG. 1.
This method prevents contamination of the side face by the silicon shavings, grain scraps and waste, and sticking of such unnecessary wastes to the side face after polishing. Accordingly, reduction of processing quality is prevented. Where the polishing liquid is used, the removal of the shavings and waste can be easily carried out by using a filter or the like, which eliminates the need to exchange the liquid in every polishing process.
The side face of the silicon block or the silicon stack flattened by the above method preferably shows surface roughness Ry of 8 μm or less, more preferably 6 μm or less. Where the thus obtained silicon block or the silicon stack having the surface roughness of 8 μm is sliced into silicon wafers for manufacturing a solar cell, the yield of the solar cells increases because damage of the silicon wafers is small.
In the method of manufacturing the silicon wafer according to the present invention, the section of the silicon block or the silicon stack, i.e., the shape of the silicon wafer in a front view, is not particularly limited. However, it is preferred that the section comprises four main lines and the lines form angle of about 90° with adjacent lines, respectively. That is, the section is preferably a rectangle or almost rectangle constituted of sides parallel to opposite sides, respectively. The silicon block or the silicon stack having such a section is preferred because two opposite side faces can be polished and flattened simultaneously. This allows high-speed processing. Further, where the silicon block or the silicon stack has a rectangular or almost rectangular section, accuracy in positioning the polishing wheel and the silicon block or the silicon stack is not required, which eliminates the need of expensive equipment.
Alternatively, the rectangular or almost rectangular section of the silicon block or the silicon stack may be formed of four lines connected to adjacent lines via another line or curve, respectively. That is, the section may have rounded corners each having a curve or an arc.
EXAMPLES
Hereinafter, the present invention will be further detailed by way of examples, but the invention is not limited thereto.
Example 1
Cutting a Silicon Block
As shown in FIG. 4, a silicon block 1 was cut from a silicon ingot using a band saw 20. FIG. 4 shows a side face 19 of the silicon block and an edge 21 of the silicon block.
Four side faces 19 of the silicon block 1 were flattened by the method of the present invention to reduce defective wafers cracked in a later step and thus improve yield of the silicon wafer.
Example 2
Method 1
The silicon block 1 of 125×125×250 mm obtained in Example 1 was polished by Method 1 to confirm the effect of the invention. A sponge wheel and a mixture of GC abrasive grains (#800) with polish oil were used as the polishing member 13 and the slurry 8, respectively.
As a result, four side faces 9 were polished in 16 minutes. Surface roughness Ry of the side faces was reduced from 20 μm to 5.8 μm by the polishing.
Example 3
Method 1, Using Resin Brush
The silicon block 1 of 125×125×250 mm obtained in Example 1 was polished by Method 1 to confirm the effect of the invention. As the polishing member 13, a wheel (240 mm in diameter) provided with nylon resin hairs (0.5 mm in diameter, 20 mm in length) densely fixed with an epoxy adhesive on a bottom region of 160-240 mm diameter was used. As the slurry 8, a mixture of GC abrasive grains (#800) and polish oil (weight ratio 1:1.28) was used.
The polishing member 13 was pressed on the surface of the silicon block 1 to such a degree that the distal ends of the nylon resin hairs reach 1.5 mm below a position where the distal ends contact the surface of the silicon block 1. Then, the polishing member was rotated at 1800 rpm.
After the polishing member 13 contacted the surface of the silicon block 1, the silicon block 1 was moved along a lengthwise direction of the silicon block, which is orthogonal to a rotation axis of the polishing member 13. The silicon block 1 was moved at 0.6 mm/sec.
From the circumference of the polishing member 13, the slurry 8 of 150 l/min was sprayed onto the side face 9 of the silicon block 1 to be polished.
As a result, four side faces 9 were polished in 12 minutes. The surface roughness Ry was reduced from 12 μm to 2.8 μm by the polishing. Cracking reduction ratio was 2.5 fold (ratio of cracked defective wafers was reduced by 60%, i.e., reduction of yield due to wafer cracking was decreased by 60%).
The cracking reduction ratio signifies a value obtained by dividing a ratio (XA) of cracked silicon wafers to silicon wafers having reference surface roughness Ry=A μm used for the manufacture of a solar cell panel by a ratio (XB) of cracked silicon wafers to silicon wafers having surface roughness of Ry=B μm used for the manufacture of a solar cell panel (provided that A>B).

(Cracking reduction ratio)Ry=B=(XA/XB)

For example, provided that X20=1 and X8=0.66, the cracking reduction ratio is calculated as follows:

(Cracking reduction ratio)Ry=B=(X20/X8)=1/0.66=1.52

Example 4
Method 2
The silicon block 1 of 125×125×250 mm obtained in Example 1 was polished by Method 2 to confirm the effect of the invention. A sponge wheel provided with diamond grains (#800) was used as a polishing member 17 and polish oil was used as a polishing liquid 16 containing no abrasive grains.
As a result, four side faces 9 of the silicon block were polished in 14 minutes. The surface roughness Ry was reduced from 12 μm to 5.8 μm by the polishing.
Example 5
Method 2, Using Resin Brush
The silicon block 1 of 125×125×250 mm obtained in Example 1 was polished by Method 2 to confirm the effect of the invention. As the polishing member 17, a wheel (220 mm in diameter) provided with nylon resin hairs (0.4 mm in diameter, 15 mm in length) containing diamond grains (#320) densely fixed with an epoxy adhesive on a bottom region of 160-240 mm diameter was used. The slurry 8 used in Example 3 was used as the polishing liquid 16.
The polishing member 17 was pressed on the surface of the silicon block 1 to such a degree that the distal ends of the nylon resin hairs reach 1.5 mm below a position where the distal contact the surface of the silicon block. Then, the polishing member was rotated at 600 rpm.
After the polishing member 17 contacted the surface of the silicon block 1, the silicon block 1 was moved along a lengthwise direction of the silicon block 1 which is orthogonal to a rotation axis of the polishing member 17. The silicon block 1 was moved at 5 mm/sec.
From the circumference of the polishing member 17, the slurry 8 of 150 l/min was sprayed onto the side face 9 of the silicon block 1 to be polished.
As a result, four side faces of the silicon block were polished in 4 minutes. The surface roughness Ry was reduced from 12 μm to 5 μm by the polishing. The cracking reduction ratio was 2 fold (ratio of cracked defective wafers was reduced by 50%, i.e., reduction of yield due to wafer cracking was decreased by 50%).
Example 6
Method 2, Using Resin Brush
The silicon block 1 polished in Example 5 was further polished for 4 minutes to confirm the effect of the invention in the same manner as in Example 5, except that a wheel (220 mm in diameter) provided with nylon resin hairs (0.4 mm in diameter, 15 mm in length) containing diamond grains (#800) and fixed densely with an epoxy adhesive on a bottom region of 160-220 mm diameter was used as the polishing member 17.
As a result, the surface roughness Ry was reduced from 12 μm to 1 μm by the polishing. The cracking reduction ratio was 2.5 fold (ratio of cracked defective wafers was reduced by 60%, i.e., reduction of yield due to wafer cracking was decreased by 60%).
Example 7
Surface Roughness and Cracking Reduction Ratio
A silicon block polished by the method of the present invention was sliced into silicon wafers by a known method. With the thus obtained silicon wafers, a solar cell panel was manufactured and the cracking reduction ratio in the solar cell panel was obtained with respect to that of a solar cell panel manufactured by a conventional method. Surface roughness Ry of 20 μm was determined as a reference for the cracking reduction ratio.
Sets of 10,000 silicon wafers having the surface roughness Ry of 0.1, 1, 2, 4, 6, 8, 10, and 20 μm, respectively, were manufactured and solar cell modules were manufactured through a solar cell module manufacture line. FIG. 4 shows the results. In FIG. 4, the surface roughness Ry (μm) is plotted in a vertical axis and the cracking reduction ratio (fold) of the solar cell panel is plotted in a horizontal axis.
In the range of Ry =6-8 μm, reduction of cracked defective wafers of 1.5 or more was observed. That is, the surface roughness Ry of 8 μm or less is effective in reduction of cracked defective wafers.
Example 8
As shown in FIG. 4, a rectangular polysilicon ingot (250 mm in length) was cut into silicon blocks 1 in the form of quadratic prism (125×125 mm) using a band saw 20. If the band saw has enough accuracy, it is not necessary to grind the surface of the silicon block. Edges 21 of the silicon block 1 were cut off and rounded to complete the silicon block.
Surfaces of the thus obtained silicon block that would serve as circumferential surfaces of the silicon wafer were polished mechanically and physically by the method of the invention. Then, as shown in FIG. 5, the silicon block 1 was sliced with a wire saw (not shown) to obtain about 470 silicon wafers 46.
The present invention provides a polishing technique for flattening the fine roughness on the side face of the silicon block or the silicon stack in a short period and allows reduction of cracked defective the silicon wafer and improvement in yield of the silicon wafer.

## Claims

1. A method of making a solar cell comprising a silicon wafer, the method comprising the following steps in the order recited:
polishing side faces of a silicon block used for manufacturing the silicon wafer, wherein said polishing is performed using a polishing member comprising material selected from the group consisting of steel, resin, cloth and sponge, and wherein the polished side faces correspond to side faces of the silicon wafer;
slicing the silicon block, so that said slicing is performed after said polishing, and
making the solar cell using the wafer, wherein
the silicon block is sliced starting at a polished side face with the slicing extending to an opposing side face.

2. The method of claim 1, wherein the polished side faces of the silicon block have a surface roughness Ry of 8 μm or less.

3. The method of claim 1, where after said polishing the polished side faces of the silicon block have a glossy mirror-like finish.

4. The method of claim 1, wherein the silicon block is a rectangular and/or square block obtained by cutting a silicon ingot.

5. The method of claim 1, further comprising chamfering edges of the silicon block prior to said polishing.

6. A method of manufacturing a silicon wafer, the method comprising:
polishing side faces of a silicon block used for manufacturing the silicon wafer, wherein said polishing is performed using a polishing member comprising material selected from the group consisting of steel, resin, cloth and sponge, and wherein the polished side faces correspond to side faces of the silicon wafer; and
slicing the silicon block, so that said slicing is performed after said polishing, wherein
the silicon block is sliced starting at a polished side face with the slicing extending to an opposing side face.

7. The method of claim 6, wherein the polished side faces of the silicon block have a surface roughness Ry of 8 μm or less.

8. The method of claim 6, where after said polishing the polished side faces of the silicon block have a glossy mirror-like finish.

9. The method of claim 6, wherein the silicon block is a rectangular and/or square block obtained by cutting a silicon ingot.

10. The method of claim 6, further comprising chamfering edges of the silicon block prior to said polishing.