WO2011024910A1 - Silicon wafer for solar cells and production method therefor - Google Patents

Abstract

Disclosed is a silicon wafer for solar cells that has a plurality of similarly-oriented, minute, linear depressions that are saw marks formed upon the front and back surfaces of the wafer, whereby the light receiving area of the wafer is expanded and high photoelectric conversion efficiency can be obtained. Furthermore, projection/depression formation processes other than slicing are simplified, and the production cost for silicon solar cells is reduced.

Description

Silicon wafer for solar cell and manufacturing method thereof

The present invention relates to a silicon wafer for a solar cell and a method for manufacturing the same, and more particularly to a silicon wafer for a solar cell that is obtained by slicing a silicon ingot for a solar cell as a base material for a silicon-based solar cell.

A silicon-based solar cell is manufactured by forming a PN junction on a solar cell silicon wafer obtained by slicing a single crystal silicon ingot or a polycrystalline silicon ingot, and forming electrodes on the front and back surfaces of the wafer. In recent years, wire saws have been used for general slices.
In the slicing method using a wire saw, first, one wire is bridged between two to four groove rollers at a predetermined pitch to form a wire row. Thereafter, each groove roller is rotated to run the wire, and the solar cell silicon ingot is pressed against the wire row while supplying slurry containing loose abrasive grains onto the wire row on both sides of the ingot. Thereby, many silicon wafers for solar cells are obtained simultaneously.

Incidentally, the thickness of a general silicon wafer for solar cells is as thin as about 200 μm. Therefore, in the bi-directional feed of the wire, the silicon wafer for solar cells is cracked or chipped at the time of slicing, and the wire is easily broken. Therefore, in order to solve this problem, unidirectional feed that provides high smoothness of the slice surface has been adopted as one of the wire operating conditions (wire feed direction). As a result, the front and back surfaces of the solar cell silicon wafer were so smooth (Rmax 5 to 10 μm) that the saw marks on the grinding marks could not be confirmed.

In addition, in order to increase photoelectric conversion efficiency, silicon-based solar cells have innumerable fine irregularities formed on the surface (light-receiving surface) of a solar cell silicon wafer, and the light reflectance on this surface is reduced. As a method of forming irregularities on the surface of a silicon wafer for solar cells, conventionally, utilizing the difference in etching rate depending on the silicon surface orientation, an alkaline aqueous solution is brought into contact with the silicon wafer for solar cells, and irregularities are formed on the wafer surface by etching. A method has been developed (for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2006-202831

However, according to such an etching method, the surface roughness of Rmax of 20 μm or more could not be realized with respect to the surface of the solar cell silicon wafer due to the characteristics of crystal anisotropy. As a result, it was not possible to sufficiently expand the light receiving area (surface area) of the silicon wafer for solar cells necessary for obtaining high photoelectric conversion efficiency.
In addition, when an etching process is employed for forming the irregularities, an etching process different from slicing is required when manufacturing a silicon-based solar cell from a silicon wafer for solar cells. Thereby, the silicon wafer for solar cells was expensive. This also applies to the case where irregularities are formed on the surface of the solar cell silicon wafer by a method different from etching (for example, grinding, laser irradiation, etc.).

Therefore, as a result of diligent research, the inventor focused attention on a slice of a silicon ingot by two-way traveling of a wire, which has been feared to induce wafer cracking. That is, if there are a large number of linear concave grooves in the same direction called saw marks formed on the front and back surfaces of the silicon wafer for solar cells when the wire row is run in both directions, It was found that all problems could be solved, and the present invention was completed.

The invention provides a silicon wafer for a solar cell that can increase the light receiving area, obtain high photoelectric conversion efficiency, simplify the unevenness forming process different from slicing, and reduce the manufacturing cost of silicon-based solar cells. And it aims at providing the manufacturing method.

The present invention relates to a silicon wafer for a solar cell in which a PN junction and an electrode are formed to be processed into a silicon-based solar cell, and a large number of linear concave grooves directed in the same direction, which are saw marks that appear at the time of slicing. It is the silicon wafer for solar cells formed in the front and back.

According to the present invention, the silicon wafer for solar cells has a large number of linear concave grooves in the same direction that appear on the front and back surfaces of the silicon wafer by traveling in both directions. That is, the silicon wafer for solar cells as a product is obtained without performing flattening processing such as etching, grinding, polishing, etc., which is generally performed on the silicon wafer after slicing. Thereby, the light receiving area of the silicon wafer for solar cells is expanded (wafer surface roughness of 20 μm or more is possible at Rmax), and high photoelectric conversion efficiency is obtained. In addition, when a number of linear grooves are formed on the back surface of the silicon wafer in the same direction as the grooves on the wafer surface, when a thin film such as aluminum is formed on the wafer back surface in the solar cell module manufacturing process. In addition, the contact area with the thin film can be increased, and the adhesive strength between them can be increased. In addition, it is possible to simplify the unevenness forming process different from the slicing, and to reduce the manufacturing cost of the silicon-based solar cell. In the present invention, “an unevenness forming process different from a slice can be simplified” means that (1) an unevenness forming process different from a slice is not required, or (2) an unevenness is formed separately from a slice. However, it means that the time for the unevenness forming process can be shortened.

Examples of silicon solar cells include single crystal silicon solar cells and polycrystalline silicon solar cells.
As a material of the silicon wafer for solar cells, single crystal silicon, polycrystalline silicon, or the like can be employed.
As the shape of the silicon wafer for solar cell, a circle, a rectangle with chamfered corners (made of single crystal silicon), a rectangle (made of polycrystalline silicon), or the like can be adopted.

“Straight groove” means a saw mark at the time of slicing consisting of a large number of grooves that are linearly and parallelly formed in the same direction at almost equal intervals over the entire surface of the silicon wafer for solar cells. Grinding marks). This saw mark is formed by slicing a silicon ingot for solar cells while a fixed abrasive wire is traveling in both directions. For this reason, not only the surface of the silicon wafer for solar cells but also the back surface has the same linear groove on the back surface in the same direction as the groove on the wafer surface (for example, the groove on the front and back surfaces of the wafer in the X direction). Further, there may be a curved portion (sag) in one direction at both end portions of the linear groove. Since the groove is a saw mark, there is a finer surface roughness of about 2 to 3 μm in Rmax between the bottom surface of the groove and the top surface of the wafer. Further, since the concave grooves are saw marks, the pitch and depth of the concave grooves on the wafer surface and the pitch and depth of the concave grooves on the back surface of the wafer are substantially the same. Furthermore, in the pitch direction of the silicon wafer, the groove formation position on the wafer surface and the groove formation position on the wafer back surface are substantially the same.

The thickness of the silicon wafer for solar cells is 160 to 220 μm. If it is less than 160 micrometers, the silicon wafer for solar cells is too thin, and the crack of a wafer increases remarkably on wafer handling. Therefore, it is effective to increase the thickness of the silicon wafer for solar cells as a countermeasure against this crack. However, if it exceeds 220 μm, the amount of silicon used is increased, and the production cost of the silicon wafer for solar cells is increased.
On the front and back surfaces of the silicon wafer for solar cells, there is a processing damage of 1 to 5 μm (on one side of the wafer) that appears by slicing the fixed abrasive wire in both directions. Incidentally, when a wire is cut using loose abrasive grains (wires are moved in both directions), a processing damage of 5 to 15 μm generally appears on one side of the wafer.

In the present invention, it is desirable that the concave grooves have a pitch of 0.1 to 5 mm and a depth of 1 to 50 μm.

As a result, the pitch of the concave grooves formed on the front and back surfaces of the wafer is set to 0.1 to 5 mm, and the depth of the concave grooves is set to 1 to 50 μm, thereby preventing a reduction in strength of the silicon wafer for solar cells due to the formation of the concave grooves. However, expansion of the light receiving area of the silicon wafer for solar cells and high photoelectric conversion efficiency can be satisfied at the same time.

If the pitch of the concave grooves is less than 0.1 mm, the strength of the silicon wafer for solar cells due to the formation of the concave grooves tends to decrease. Moreover, if the pitch of a ditch | groove exceeds 5 mm, expansion of the light-receiving area of the silicon wafer for solar cells is inadequate, and high photoelectric conversion efficiency cannot be obtained.
When the depth of the groove is less than 1 μm, the light receiving area of the solar cell silicon wafer is not sufficiently enlarged, and high photoelectric conversion efficiency cannot be obtained. Moreover, if the depth of the ditch | groove exceeds 50 micrometers, there exists a possibility that the intensity | strength of the silicon wafer for solar cells by formation of a ditch | groove may fall. In particular, the thickness where the concave grooves overlap on the front and back surfaces of the wafer is reduced to more than 100 μm, and the wafer is likely to be cracked.

The present invention relates to a silicon wafer for solar cells obtained by slicing a plurality of silicon wafers for solar cells by pressing a silicon ingot for solar cells while supplying slurry to a wire row traveling between a plurality of groove rollers of a wire saw. In this manufacturing method, a fixed abrasive wire having abrasive grains fixed on an outer peripheral surface is used as a wire constituting the wire row, and the silicon ingot for solar cells is sliced while the wire row is traveling in both directions. This is a method for manufacturing a silicon wafer for solar cells, in which a large number of linear concave grooves directed in the same direction as saw marks are formed on the front and back surfaces of each silicon wafer for solar cells.

According to the present invention, when slicing, a solar cell silicon ingot is pressed against a wire row composed of fixed abrasive wires traveling in both directions (forward and backward directions). As a result, a large number of rough grooves (saw marks) extending in the same direction appearing when the wire row is run in both directions (for example, Rmax of 20 μm or more) are formed on the front and back surfaces of the solar cell silicon wafer. It is formed. At the same time, linear grooves (saw marks) extending in the same direction with a finer Rmax of about 2 to 3 μm are formed by the fixed abrasive grains on the wire surface. As a result, the light receiving area of the solar cell silicon wafer is expanded, and high photoelectric conversion efficiency is obtained. In addition, it is possible to simplify the unevenness forming process different from the slicing, and to reduce the manufacturing cost of the silicon-based solar cell. Further, the use of the fixed abrasive wire increases the cutting efficiency compared with the case of slicing with a wire saw using loose abrasive grains, and the slicing speed of the silicon ingot for solar cells is increased. Even when etching for forming the groove is performed after that, the etching time can be shortened.

The number of wire rollers used in the wire saw is, for example, 2, 3, or 4.
The fixed abrasive wire is a diamond abrasive grain having a particle size of 7 to 25 μm, heat-cured or UV-irradiated with a binder on a piano wire with a concentration of 20 to 55, preferably 50 and a diameter of 100 to 300 μm, etc. Can be adopted. As the binder, an epoxy resin, a phenol resin, an acrylic urethane resin, or the like can be used. Further, a diamond abrasive grain may be electrodeposited together with nickel on the outer peripheral surface of the wire.
The feed rate of the fixed abrasive wire is 500 to 1500 m / min. If it is less than 500 m / min, the slurry (wrapping oil) that does not contain loose abrasive grains is not brought into the cutting portion of the ingot by the wire, and the cutting efficiency decreases. If it exceeds 1500 m / min, the slurry adhering to the wire is blown off, and the cutting efficiency is lowered.

The advance amount of the fixed abrasive wire is 250 to 450 m, and the retract amount of the fixed abrasive wire is 248 to 499 m. The cycle time for forward and reverse is 47.7-88.9 seconds.
The ingot slice average speed is 400 to 1200 μm / min. In order to further improve the wafer quality, a lower speed condition is preferable, but if it is less than 400 μm / min, the productivity is lowered, and the amount of wire used may increase, resulting in an increase in cost. On the other hand, if it exceeds 1200 μm / min, an excessive load acts on the wire, the fixed abrasive grains are worn out or fall off, making ingot cutting impossible, and wire breakage is likely to occur. The preferred slicing speed of the ingot is 500 to 1000 μm / min. Within this range, the flatness of the wafer can be further increased.

As the silicon ingot for solar cells, a single crystal silicon ingot or a polycrystalline silicon ingot can be adopted. The single crystal silicon ingot is grown by, for example, the Czochralski (CZ) method or the floating zone melting (FZ) method. The polycrystalline silicon ingot is manufactured by, for example, the Siemens method.
In particular, when the silicon ingot for solar cells is made of polycrystalline silicon, there is a crystal grain size distribution, so unevenness with uniform size cannot be formed by conventional etching, resulting in variations in power generation efficiency within the wafer surface. There is a risk. However, in the present invention, since the linear concave grooves are mechanically formed by the fixed abrasive wire, a uniform power generation efficiency can be obtained in the wafer plane without depending on the crystal grain size distribution.
A large number of linear concave grooves are formed on the front and back surfaces of the sliced solar cell silicon wafer. Since a linear concave groove is also formed on the back surface of the silicon wafer for solar cells, when a thin film such as aluminum is formed on the back surface of the wafer in the solar cell module manufacturing process, the contact area with the thin film increases, Adhesive strength can be increased. Depending on the process flow in the solar cell module manufacturing process, the concave groove on the back surface side may be removed by a flattening process such as polishing.

According to the present invention, as the operating condition of the wire at the time of slicing, traveling in both directions of the wire row is adopted. For this reason, a large number of linear fine grooves in the same direction as saw marks are formed on the front and back surfaces of the silicon wafer for solar cell. Thereby, the light receiving area of the silicon wafer for solar cells is expanded, and high photoelectric conversion efficiency is obtained. In addition, it is possible to simplify the unevenness forming process different from slicing, and to reduce the manufacturing cost of the silicon-based solar cell.
Moreover, by adopting a fixed abrasive wire as a wire, at the time of slicing, linear striation is performed on the front and back surfaces of the solar cell silicon wafer by the fixed abrasive. Thereby, a large number of rough linear grooves are formed on the front and back surfaces of the wafer. At the same time, linear grooves having a finer Rmax of about 2 to 3 μm are formed by the fixed abrasive grains on the wire surface. In addition, the use of the fixed abrasive wire increases the cutting efficiency as compared with slicing using loose abrasive grains, and increases the slicing speed of the silicon ingot for solar cells.

In the present invention, it is desirable that the pitch of the groove is 0.1 to 5 mm and the depth of the groove is 1 to 50 μm. Thereby, expansion of the light receiving area of the silicon wafer for solar cells and high photoelectric conversion efficiency can be satisfied at the same time while preventing the strength reduction of the silicon wafer for solar cells due to the formation of the concave grooves.

It is a top view of the silicon wafer for solar cells which concerns on Example 1 of this invention. It is a principal part expanded sectional view of the silicon wafer for solar cells which concerns on Example 1 of this invention. (A) is a perspective view of the use condition of the wire saw used with the manufacturing method of the silicon wafer for solar cells which concerns on Example 1 of this invention. (B) is a principal part enlarged view of Fig.3 (a). It is a principal part enlarged plan view which shows the slice process of the silicon ingot for solar cells which uses the fixed abrasive wire in the manufacturing method of the silicon wafer for solar cells which concerns on Example 1 of this invention. It is a principal part enlarged plan view which shows the slice process of the silicon ingot for solar cells using the slurry containing the loose abrasive grain in the manufacturing method of the silicon wafer for solar cells which concerns on the conventional means. It is a graph which shows the surface roughness profile of the silicon wafer for solar cells sliced by bidirectional | two-way driving | running | working with the wire saw using the fixed abrasive wire which concerns on Example 1 of this invention. It is a distribution map (magnification: 200 times) of the site-like surface roughness of the surface of the silicon wafer for solar cells sliced by using a fixed abrasive wire and feeding the wire in both directions. It is a distribution map (magnification: 1000 times) of the site-like surface roughness of the surface of the silicon wafer for solar cells sliced by using a fixed abrasive wire and feeding the wire in both directions. It is a distribution map (magnification: 200 times) of the site-like surface roughness of the surface of the silicon wafer for solar cells which used the slurry containing loose abrasive grains and sliced it by making wire feeding into one direction. It is a distribution map (magnification: 1000 times) of the site-like surface roughness of the surface of the silicon wafer for solar cells sliced using a slurry containing loose abrasive grains with the wire feeding as one direction.

10 Wire saw,
11 Wire row,
11a Fixed abrasive wire,
11b abrasive grains,
12A, 12B Groove roller,
I polycrystalline silicon ingot (silicon ingot for solar cell),
S slurry,
W Silicon wafer for solar cells,
Wa groove,
a pitch,
b Depth.

Hereinafter, embodiments of the present invention will be described in detail.

1 and 2, W is a silicon wafer for solar cells according to Example 1 of the present invention. This silicon wafer W for solar cells is formed by slicing a silicon ingot for solar cells with a wire saw to form a PN junction and an electrode. And processed into a silicon-based solar cell.
A large number of linear concave grooves Wa appearing in the slicing process and extending in the same direction are formed on the entire front and back surfaces of the silicon wafer W for solar cells. The pitch a of the concave grooves Wa is about 1000 μm, and the depth b of the concave grooves Wa is about 5 μm. The degree of surface roughness (fine groove group) c of the silicon wafer W for solar cells is about 1 μm. In addition, the direction (for example, X direction) of each ditch | groove Wa on the wafer surface and the direction (X direction) of each ditch | groove Wa on the wafer back surface are the same. The same applies to the direction of surface roughness c, which is a fine groove group. Further, since it is a saw mark, the pitch and depth of the concave grooves Wa on the front surface of the silicon wafer W and the pitch and depth of the concave grooves Wa on the rear surface of the silicon wafer W are substantially the same. Furthermore, in the pitch direction of the concave groove Wa of the silicon wafer W, the formation position of the concave groove Wa on the wafer surface and the formation position of the concave groove Wa on the back surface of the wafer are substantially the same.

As described above, since a large number of linear concave grooves Wa having a pitch of about 1000 μm and a depth of about 5 μm are formed on the entire front and back surfaces of the silicon wafer W for solar cells, particularly on the entire surface serving as the light receiving surface. The light receiving area of the silicon wafer W is expanded, and high photoelectric conversion efficiency is obtained. In addition, it is possible to simplify the unevenness forming process different from the slicing as in the prior art, and to reduce the manufacturing cost of the silicon solar cell.

Next, the manufacturing method of the silicon wafer W for solar cells of Example 1 of this invention is demonstrated.
First, by reducing trichlorosilane which is an intermediate compound (SiHCl ₃₎ with hydrogen, Siemens method to obtain a polycrystalline silicon manufacturing method of a silicon ingot for a solar cell using (Siemens Method) will be described.
This is done by installing a silicon seed rod in a water-cooled bell jar type reactor, applying a predetermined voltage to the seed rod and heating the seed rod to 1100 ° C., and trichlorosilane (SiHCl ₃ ) in the reactor. Then, hydrogen as a reducing agent and boron gas as a dopant are introduced from below. Thereby, silicon chloride is reduced, and the generated silicon selectively adheres to the surface of the seed rod, so that rod-shaped polycrystalline silicon is vapor-phase grown.
Next, the polycrystalline silicon ingot is crushed into a lump of a predetermined size to obtain a molten raw material for casting a polycrystalline silicon ingot for a solar cell.

The obtained polycrystal silicon lump is put into a crucible, and a 160 mm square polycrystal silicon ingot is manufactured by an electromagnetic melting continuous casting method. In this method, a conductive bottomless crucible having an induction coil arranged on the outer periphery is used. The raw material silicon inserted into the bottomless crucible is heated and melted above the melting point of silicon in a non-contact state on the inner wall of the crucible by electromagnetic induction (15 kHz, 300 kW) of the induction coil. After that, while supplying raw material silicon to the bottomless crucible, the melt in the bottomless crucible is gradually lowered downward by a drawing device and solidified by a slow cooling device disposed immediately below the bottomless crucible. Thereby, a polycrystalline silicon ingot is continuously manufactured. The continuously cast polycrystalline silicon ingot is cut every 400 mm in length and finished to a prism having a side of 156 mm by grinding.

Next, the wire saw will be specifically described with reference to FIG.
In FIG. 3A, 10 is a wire saw, and this wire saw 10 is an apparatus for slicing a polycrystalline silicon ingot I cast by an electromagnetic melting continuous casting method into a silicon wafer W for solar cells made of a large number of polycrystalline silicon. It is.

The wire saw 10 has two groove rollers 12A and 12B arranged in a rectangular shape when viewed from the front. Among these, the groove roller 12A is a driving roller connected so that the rotational force of the driving motor can be transmitted, and the groove roller 12B is a driven roller. A single fixed abrasive wire 11a is wound around the groove rollers 12A and 12B in parallel with each other at a pitch of 370 μm. Thereby, the wire row 11 appears between the groove rollers 12A and 12B.
The wire row 11 is reciprocated between the two groove rollers 12A and 12B by a drive motor. The middle of the groove rollers 12A and 12B is the ingot cutting position a1 of the wire row 11 that cuts the polycrystalline silicon ingot I.

The polycrystalline silicon ingot I is fixed to the lower surface of the lifting platform 19 for raising and lowering the polycrystalline silicon ingot I through the carbon bed 19a. A pair of slurry nozzles 30 for continuously supplying the slurry S onto the wire row 11 are disposed above both sides of the ingot cutting position a1. As the slurry S, wrapping oil (100 liters / min) that does not contain loose abrasive grains is employed.
The groove rollers 12A and 12B have a cylindrical shape, and their outer peripheral surfaces are covered with a lining material having a predetermined thickness made of urethane rubber. A wire groove 12d is formed on the outer peripheral surface of each lining material (FIG. 3B).

A fixed abrasive wire 11a having a large number of abrasive grains 11b fixed to the outer peripheral surface is used as the wire. Specifically, a fixed abrasive wire 11a in which abrasive grains 11b made of diamond having a particle diameter of 10 to 25 μm are fixed to a wire having a diameter of 0.12 mm by Ni plating by an electrodeposition method is used. The wire 11a is led out from the bobbin 20 of the feeding device 13, and is bridged over the groove rollers 12A and 12B via the supply-side guide roller. After that, it is wound around the bobbin 21 of the winding device 15 via the guide roller on the outlet side. The rotating shafts of the bobbins 20 and 21 are connected to corresponding output shafts of the drive motors 16 and 17, respectively.
By driving the drive motors 16 and 17 synchronously, the bobbins 20 and 21 pivotally supported by the pair of bearings 18 are rotated clockwise or counterclockwise in FIG. The wire 11a travels in both directions.

As shown in FIG. 3A, when the polycrystalline silicon ingot I is sliced, the bobbin 20 of the feeding device 13 is driven by the drive motor 16 while supplying the slurry S from the slurry nozzle 30 to the wire row 11 at 100 liters / minute. Rotate. Thereby, the wire 11a is supplied to the groove rollers 12A and 12B. At the same time, the bobbin 21 of the winding device 15 is rotated by the drive motor 17 to wind the wire 11a via the groove rollers 12A and 12B.
At that time, the rotation direction of each of the bobbins 20 and 21 is changed at a constant cycle, and the wire 11a is caused to travel in both directions. Specifically, the advance amount of the fixed abrasive wire 11a is 250 m, the retreat amount of the fixed abrasive wire 11a is 248 m, and the cycle time for changing between advance and retreat is 47.7 seconds. The feed rate of the fixed abrasive wire 11a is 900 m / min. The slice speed of the polycrystalline silicon ingot I is 700 μm / min.

During the reciprocating traveling of the wire row 11, the polycrystalline silicon ingot I is pressed against the wire row 11 from above. As a result, the polycrystalline silicon ingot I has a rectangular shape with a length of 156 mm and a width of 120 mm, a boron concentration of 1.4 × 10 ¹⁶ atoms / cm ³ , and a specific resistance of 1.0 mΩ · cm (P-type). It slices into the silicon wafer W for solar cells. That is, during the reciprocating traveling of the wire row 11, a large number of abrasive grains 11b are rubbed against the bottom of the cutting groove by the fixed abrasive wire 11a of the wire row 11, and the bottom is gradually scraped off by a grinding action (FIG. 4).
Thereafter, a PN junction is formed on the silicon wafer W for solar cells, and electrodes are formed on the front and back surfaces of the wafer. Specifically, phosphorus (P) is thermally diffused on the wafer surface to form an N-type diffusion layer, and then a back electrode made of aluminum is formed on the back surface of the silicon wafer W for solar cells, and silicon for solar cells. A surface electrode made of silver is formed on the surface of the wafer W.

As described above, since the polycrystalline silicon ingot I is sliced by reciprocating the wire row 11 in both directions at the time of slicing, the wire row 11 is caused to travel in both directions over the entire front and back surfaces of the silicon wafer W for solar cells. In this case, a large number of linear fine grooves Wa extending in the same direction (for example, the X direction) appear. Each concave groove Wa has a pitch a of about 1000 μm, a depth b of about 5 μm, and a surface roughness c of about 1 μm. As a result, the light receiving area of the solar cell silicon wafer W is expanded, and high photoelectric conversion efficiency is obtained. In addition, an unevenness forming step (for example, an etching step) different from slicing as in the conventional method can be simplified, and the manufacturing cost of the silicon-based solar cell can be reduced.

In addition, since the fixed abrasive wire 11a is moved in both directions to slice the solar cell silicon ingot I, the solar cell silicon wafer is compared with the case of slicing using slurry containing loose abrasive grains. A rough linear groove Wa can be formed on the front and back surfaces of W. That is, the pitch a of the concave grooves Wa is about 1000 μm, and the depth b of the concave grooves Wa is about 5 μm. Further, the degree of surface roughness c of the solar cell silicon wafer W is about 1 μm.

Furthermore, since the fixed abrasive wire 11a is used, the cutting efficiency of the polycrystalline silicon ingot I is higher than when slicing using a slurry containing loose abrasive grains. Therefore, the slice speed of the polycrystalline silicon ingot I is increased. In the case of slicing using the fixed abrasive wire 11a, the abrasive grains 11b and the fixed abrasive wire 11a are integrated, and the moving speed of the fixed abrasive wire 11a and the moving speed of the abrasive grains 11b in the slice are the same. Because it becomes. As a result, a uniform slicing speed is obtained over the entire cutting groove of the polycrystalline silicon ingot I (from one end to the other end of the ingot I), and the cutting efficiency of the polycrystalline silicon ingot I is increased. On the other hand, in the case of the slice using the free abrasive grains 11d, the free abrasive grains 11d and the wire 11c are not integrated, and therefore the movement of the free abrasive grains 11d from the moving speed (feed speed) of the wire 11c in the slice. The speed is slow. As a result, in the case of slicing using the free abrasive grains 11d, the cutting efficiency of the polycrystalline silicon ingot I gradually decreased from the cutting start end toward the cutting end end (FIG. 5).

Actually, the solar cell silicon ingot was sliced in accordance with the method for producing the solar cell silicon wafer of Example 1. The data of the surface roughness of the silicon wafer for solar cells at that time are shown in FIG.
For the measurement, a contact roughness meter (Surfcom 130A) manufactured by Tokyo Seimitsu Co., Ltd. was used. As measurement conditions, the measurement length was 5 mm, the measurement speed was 0.3 mm / s, and CutOFF was 0.8 mm.
As apparent from the wafer surface roughness profile in the graph of FIG. 6, compared with the conventional method (surface roughness Rmax of about 5 μm) using a slurry containing loose abrasive grains and running the wire in one direction, it is for solar cells. On the front and back surfaces of the silicon wafer W, rough linear grooves Wa appeared.

Moreover, about the silicon wafer for solar cells obtained by slicing the same silicon ingot for solar cells, the result of having measured the site-like roughness distribution of the wafer surface using Keyence VK8500 is shown in FIG. FIG. 7a shows a magnification of 200 times and FIG. As is clear from these three-dimensional distribution diagrams, a large concave groove (wire running trace) in both directions of the wire could be confirmed in the observation at a low magnification (200 times). On the other hand, at a high magnification (1000 times), it was possible to confirm a linear concave groove (a grinding mark by fixed abrasive grains).
For comparison, using a general wire (diameter 160 μm, made of high-tensile steel wire) instead of a fixed abrasive wire, and similarly for a solar cell silicon wafer obtained by cutting a solar cell silicon ingot, FIG. 8 shows the results of measuring the surface roughness of the silicon wafer for solar cells using Keyence VK8500. Here, instead of the non-abrasive slurry, a slurry in which 110 kg of free abrasive grains (GC abrasive grains) having an average particle size of 7 to 8 μm are mixed with 100 liters of wrapping oil is used. The magnification of the site on the wafer surface is 200 times in FIG. 8a and 1000 times in FIG. 8b. As is apparent from these three-dimensional distribution diagrams, no linear grooves (irregularities) were observed on the surface of the solar cell silicon wafer at both low magnification and high magnification.

This invention is useful, for example, for silicon wafers for solar cells for power generation.

Claims (3)

1. In a silicon wafer for solar cells in which a PN junction and an electrode are formed and processed into a silicon-based solar cell,
   A silicon wafer for solar cells in which a large number of linear concave grooves directed in the same direction, which are saw marks appearing during slicing, are formed on the front and back surfaces.
2. 2. The silicon wafer for a solar cell according to claim 1, wherein the groove has a pitch of 0.1 to 5 mm and a depth of 1 to 50 μm.
3. In a method for producing a silicon wafer for a solar cell, a silicon cell ingot for a solar cell is pressed against a wire row traveling between a plurality of groove rollers of a wire saw while supplying slurry, and a plurality of silicon wafers for a solar cell are obtained by slicing. ,
   As a wire constituting the wire row, a fixed abrasive wire in which abrasive grains are fixed to the outer peripheral surface is used,
   By slicing the solar cell silicon ingot while running the wire row in both directions, a large number of linear concave grooves directed in the same direction as saw marks are formed on the front and back surfaces of each solar cell silicon wafer. Manufacturing method of silicon wafer for solar cell.

Images