WO2012111850A1

WO2012111850A1 - Polycrystalline wafer, method for producing same and method for casting polycrystalline material

Info

Publication number: WO2012111850A1
Application number: PCT/JP2012/054338
Authority: WO
Inventors: 義男村上
Original assignee: 株式会社Sumco
Priority date: 2011-02-18
Filing date: 2012-02-16
Publication date: 2012-08-23
Also published as: JP2012171821A; TW201247949A

Abstract

This method for casting a polycrystalline material is characterized by controlling the cooling speed of a melt in an electromagnetic casting. This method for producing a polycrystalline wafer is characterized by cutting polycrystalline ingots obtained by this casting method, dividing the ingots into polycrystalline blocks, and slicing the polycrystalline blocks. Furthermore, this polycrystalline wafer is characterized in that the strength (Vickers' hardness) of a rim portion is high compared to a center portion.

Description

Polycrystalline wafer, method for producing the same, and method for casting polycrystalline material

The present invention relates to a polycrystalline wafer, a method for producing the same, and a method for casting a polycrystalline material, and in particular, a method for casting a polycrystalline material capable of obtaining a polycrystalline ingot having high strength, and cutting the polycrystalline ingot. The present invention relates to a polycrystalline wafer which is manufactured by slicing a polycrystalline block and has few cracks in a cell process or a module process.

At present, silicon crystals are mainly used as substrates for manufacturing solar cells.
Silicon crystals include single crystals and polycrystals, and solar cells using single crystal silicon as a substrate have a conversion efficiency that converts incident light energy into electrical energy compared to those using polycrystalline silicon as a substrate. It is characterized by being expensive.
This single crystal silicon is generally produced by the Czochralski method in order to produce dislocation-free high-quality crystals. However, the production by this Czochralski method has a problem of high cost.

On the other hand, a casting method is known as a method for producing polycrystalline silicon (for example, Patent Document 1).
In the casting of polycrystalline silicon by the casting method, high-purity silicon as a raw material is heated and dissolved in a crucible, and a dopant such as boron is added uniformly and solidified in the crucible. Since crucibles are required to have heat resistance and shape stability, quartz is generally used.
By applying a unidirectional solidification method to this casting method, it is possible to obtain polycrystalline silicon having large crystal grains.

However, the polycrystalline silicon cast by the above casting method has low strength, and there are the following problems in manufacturing a polycrystalline silicon wafer by slicing a silicon block formed by cutting a silicon ingot. That is, in the wafer slicing step, a fixed abrasive method in which slicing is mainly performed using a wire in which abrasive grains made of diamond or the like are fixed to the outer peripheral surface of a core wire (elementary wire) made of steel wire or the like, and abrasive grains There is a free abrasive method in which slicing is performed using a slurry containing steel and a steel wire. However, since the polycrystalline silicon cast by the above casting method has low strength, high-speed slicing by the fixed abrasive method causes a crack in the wafer with a high probability. There is a problem that the throughput of the slicing process is low.

In addition, in the above-described slicing process, the polycrystalline wafer manufactured without cracking is also low in strength of the wafer, so that cracking occurs later in the cell process and module process when used as a substrate for solar cells. There was a problem that it was easy.

In particular, in recent years, the thickness of polycrystalline wafers has been reduced, and particularly when the wafer thickness is 200 μm or less, the rate of occurrence of cracks in the wafer slicing process, the above-described cell process, and module process increases. There is an increasing need for countermeasures against such wafer cracks.

Japanese Unexamined Patent Publication No. 6-64913

The objective of this invention is providing the casting method of the polycrystalline material which can obtain a polycrystalline ingot with high intensity | strength.
Another object of the present invention is to provide a polycrystalline wafer that can be produced at a low crack generation rate in the slicing process and that has a low crack generation rate in the cell process and module process.

The present inventor has intensively studied to solve the above problems.
As a result, the present inventor increases the strength of the border by reducing the crystal grain size only at the border of the polycrystalline ingot, thereby reducing the incidence of cracking during slicing of the polycrystalline block, Moreover, the knowledge that the incidence rate of the crack of the polycrystalline wafer in a cell process and a module process can be reduced was acquired.
Specifically, the present inventor has found that, in the electromagnetic casting method described later, by controlling the cooling rate of the melt of the polycrystalline material, it is possible to reduce the crystal grain size of only the edge portion of the polycrystalline ingot. I found it.

The present invention is based on the above findings, and the gist of the present invention is as follows.
(1) A rectangular flat plate-shaped polycrystalline wafer having an edge on at least one side of the rectangle, the crystal grain size of the edge being 0.05 to 0 of the crystal grain size in the center of the rectangle A polycrystalline wafer characterized by being 5 times larger.

(2) The polycrystalline wafer according to (1) above, wherein the Vickers hardness (Hv) at the border portion is 1.2 to 5 times the Vickers hardness (Hv) at the central portion of the rectangle. .

(3) The polycrystalline wafer according to (1) or (2) above, wherein the crystal grain size at the border is 5 mm or less.

(4) The polycrystalline wafer according to any one of (1) to (3) above, wherein a Vickers hardness (Hv) of the edge portion is 1100 Hv or more.

(5) A bottomless cooling mold in which at least a part in the axial direction is divided into a plurality of parts in the circumferential direction is arranged in the induction coil of the chamber, and polycrystalline is formed in the cooling mold by electromagnetic induction heating by the induction coil. A method of casting a polycrystalline material, in which a melt of a material is melted, and the melt is cooled and solidified to be drawn downward,
The method for casting a polycrystalline material, wherein a rate of cooling the melt is 100 ° C./sec or more.

(6) The method for casting a polycrystalline material according to (5) above, wherein the crystal grain size of the border is controlled to 5 mm or less by setting the cooling rate to 100 ° C./sec or more.

(7) The polycrystalline ingot obtained by the polycrystalline material casting method according to the above (5) or (6) is cut, divided into polycrystalline blocks, and the polycrystalline blocks are sliced. A method for producing a polycrystalline wafer.

According to the casting method of the present invention, it is possible to obtain a polycrystalline ingot having a strong edge portion.
Since the polycrystalline block obtained by cutting the polycrystalline ingot has a high edge portion strength, the occurrence rate of cracks in the wafer slicing step can be reduced.
Therefore, high-speed slicing by the fixed abrasive method is possible, and the throughput of the slicing process is improved.
In addition, since the strength of the edge of the polycrystalline wafer obtained by slicing the polycrystalline block is high, the rate of occurrence of wafer cracking in the cell process and module process, which are the manufacturing processes of solar cells, is reduced. Yield can also be improved.
Furthermore, even in a process where the thickness of the wafer has been reduced in recent years, the cracking rate during the slicing step can be kept low.
In addition, since the polycrystalline wafer of the present invention has a small crystal grain size only at the edge portion and a large crystal grain size at other portions, the conversion efficiency of the solar cell can be sufficiently increased.

It is sectional drawing which shows an example of the apparatus used for an electromagnetic casting method. (A) It is a schematic perspective view of the silicon ingot obtained by the casting method of this invention. (B) It is a schematic top view of the silicon ingot obtained by the casting method of this invention. It is the figure which showed typically a mode that the silicon ingot obtained by the casting method of this invention was cut | disconnected and divided | segmented into several silicon blocks. (A) (b) It is the figure which showed typically a mode that the silicon block was sliced and a silicon wafer was manufactured. (A) (b) It is a figure which shows the photograph of the polycrystalline silicon wafer of this invention. (C) It is a figure which shows the photograph of the polycrystalline silicon wafer obtained by the casting method.

FIG. 1 is a cross-sectional view schematically showing an example of an electromagnetic casting apparatus used in the electromagnetic casting method.
As shown in FIG. 1, the chamber 1 is a double-walled cooling container that is protected from heat generation inside, and a cooling mold 2, an induction coil 3, and a heater 4 are arranged in the center. .
In the illustrated example, the cooling mold 2 is a copper water-cooled cylinder, and is divided into a plurality of parts in the circumferential direction except the upper part, and has no bottom.
In the illustrated example, the induction coil 3 is concentrically provided on the outer peripheral side of the cooling mold 2 and is connected to a power source by a coaxial cable (not shown).
In the illustrated example, the heater 4 is provided concentrically below the cooling mold 2, heats the ingot 5 pulled down from the cooling mold 2, and gives a predetermined temperature gradient in the downward axis direction of the ingot 5.
Hereinafter, the casting method of the present invention will be described by taking polycrystalline silicon as an example of the polycrystalline material.

To cast polycrystalline silicon using the apparatus shown in FIG. 1, first, the silicon material 6 is charged into the cooling mold 2, and then an alternating current is passed through the induction coil 3.
The cooling mold 2 is divided in the circumferential direction, and the respective pieces are electrically separated from each other. Therefore, a current loop is formed in each piece, and the current generates a magnetic field in the cooling mold 2.
Thereby, the silicon material is melted by electromagnetic induction heating, and the silicon melt 7 is melted.

Here, the silicon material in the cooling mold 2 receives a force inward in the radial direction of the cooling mold 2 due to electromagnetic interaction between the magnetic field generated by the inner wall of the cooling mold 2 and the current on the surface of the molten silicon. 2 is melted in a non-contact state, impurity contamination from the cooling mold is prevented, and the ingot 5 can be easily pulled down.

Here, in solidifying the molten silicon, the lowering device 8 that holds the molten silicon and the ingot at the lower part is moved downward. As the induction coil 3 is separated from the lower end of the induction coil 3, the induction magnetic field is reduced, the amount of generated heat and the above-mentioned radial inward force are reduced, and the molten silicon 7 is solidified from the outer peripheral side by the cooling effect of the cooling mold 2. , Pull this down.
Along with the downward movement of the pulling device, the silicon material 6 is continuously added to the cooling mold 2 and the melting and solidification of the silicon material 6 is continued to allow continuous casting of polycrystalline silicon. It becomes.
The conductivity of the polycrystalline silicon wafer can be controlled by inserting a silicon material 6 to which a dopant is added.
For casting a p-type polycrystalline silicon wafer, a molten raw material such as boron, gallium, or aluminum is used as a dopant. For casting an n-type polycrystalline silicon wafer, a molten raw material such as phosphorus, arsenic, or antimony is used as a dopant. it can.

Here, in the casting method of the present invention, in order to solidify the polycrystalline material, it is important to set the cooling rate of the polycrystalline material melt to 100 ° C./sec or more. The polycrystalline material can be, for example, polycrystalline silicon.
The cooling rate means an average value (° C./sec) of temperature change per unit time until the silicon melt is solidified.

Specifically, the above cooling rate can be achieved by using a water-cooled mold using copper having good thermal conductivity and setting the temperature of the cooling water to 30 ° C. or lower.
In addition, it is preferable that a cooling rate shall be 500 degrees C / sec or less.

As a result, due to the rapid cooling effect of the water-cooled copper mold on the outer peripheral portion, as shown in FIGS. 2 (a) and 2 (b), it is possible to manufacture the silicon ingot 9 in which the crystal grain size of the rim portion 10 is 5 mm or less. it can.
At this time, the crystal grain size of the center portion 11 of the silicon ingot 9 is about 10 to 20 mm, and the crystal grain size in the rim portion 10 is 0.05 to 0.5 times the crystal grain size in the center portion 11. .
Here, the particle diameter means the average of the diameters of the particles in the entire area of each of the border part 10 or the center part 11, and the diameter of a single particle is defined by the length in the longest direction of the grains. And
The crystal grain size can be measured by observing with an optical microscope.
The crystal grains in the central part 11 have a radial regular shape.

In addition, as shown in FIGS. 2 (a) and 2 (b), the edging portion 10 is a rectangle (square or rectangular) surface or cross section perpendicular to the casting direction of a rectangular silicon ingot 9 that is rectangular from four sides. This is a portion having a width of 5 mm to 5 cm on the inner side in the radial direction of the cross section.
As shown in FIG. 2B, the radial direction herein refers to the normal direction of each side of the rectangular surface, and the center position O of the rectangular surface (two diagonal lines of the rectangle) with respect to each side. The position of the intersection point) is the inside.
Further, as shown in FIGS. 2A and 2B, the center portion 11 includes the center position O in a rectangular (square or rectangular) surface or cross section of a rectangular silicon ingot perpendicular to the casting direction. It is a part, and is a part on the center O side from the edge part 10 described above.
In addition, a rectangular parallelepiped and a rectangle (square, rectangle) are not mathematically exact and include shapes having manufacturing errors.

As described above, in the silicon ingot 9 obtained by the casting method of the present invention, the crystal grain size of the edge portion 10 is smaller than that of the central portion 11. For this reason, as for this silicon ingot 9, the intensity | strength of the edge part 10 becomes high compared with the center part 10. FIG.
Here, Vickers hardness (Hv) is used as an index indicating strength. The Vickers hardness is obtained based on JIS Z 2244 (2009) using a Vickers hardness meter (manufactured by Mitutoyo Corporation: micro hardness meter MVK-G3) with a load of 9.8 N.
In the silicon ingot 9 obtained by the casting method of the present invention, the Vickers hardness (Hv) of the edge portion 10 is 1.2 to 5 times the Vickers hardness (Hv) of the center portion 11.

Next, as shown in FIG. 3, by cutting the silicon ingot 9, a plurality of rectangular parallelepiped silicon blocks 12 can be manufactured.
As shown in FIG. 3, the silicon block 12 includes an edge portion 10 and a center portion 11 of the silicon ingot 9. That is, the silicon block 12 has a surface or a cross section perpendicular to the casting direction (the direction in which the ingot is cast) of 5 mm from the one side of the outer periphery or two adjacent sides to the inside of the rectangular surface in the radial direction. It comprises an edge portion 10 having a width of ˜5 cm and a center portion 11 which is a portion other than the edge portion and is closer to the center of the rectangular surface than the edge portion.
Since the silicon block 12 is obtained by dividing the silicon ingot 9, similarly to the silicon ingot 9, the Vickers hardness of the edge portion 10 is 1.2 to 5 times larger than the Vickers hardness of the center portion 11.

Next, FIGS. 4A and 4B are views schematically showing a state in which the silicon wafer 12 is manufactured by slicing the silicon block 12 so that the cross section is perpendicular to the casting direction.
As described above, the edge portion 10 of the silicon block 12 obtained according to the present invention is positioned at the start of slicing, so that the strength of the edge portion 10 is high. However, the occurrence of wafer cracking during slicing can be reduced, or the occurrence of wafer cracking during slicing can be reduced even if the wafer is thinned and sliced.

Here, as shown in FIGS. 4A and 4B, the polycrystalline wafer 13 has a rectangular flat plate shape.
As shown in FIGS. 4A and 4B, the polycrystalline wafer 13 of the present invention includes an edge portion 10 and a center portion 11 of the silicon ingot 9. That is, the polycrystalline wafer 13 of the present invention includes an edge portion 10 having a width of 5 mm to 5 cm and an edge portion from one side of the outer periphery or two adjacent sides radially inward of the rectangular wafer. The center portion 11 is a portion other than 10 and located on the center side of the rectangle from the edge portion 10.
Here, the radial direction of the wafer means a normal direction of each side of the rectangular surface or cross section in a rectangular surface or cross section perpendicular to the casting direction (the direction in which the ingot 9 is cast), and for each side, The center position (position of the intersection of two diagonal lines of the rectangle) of the rectangular surface is the inner side.
Since the silicon wafer 13 is manufactured by slicing the silicon block 12 perpendicularly to the casting direction, the crystal grain size of the rim portion 10 is the crystal of the center portion 11 as in the silicon ingot 9 and the silicon block 12. 0.05 to 0.5 times the particle size. Specifically, the crystal grain size of the edge portion 10 of the silicon wafer 13 is 5 mm or less.
Further, the Vickers hardness (Hv) of the edge portion 10 of the silicon wafer 13 is 1.2 to 5 times the Vickers hardness (Hv) of the center portion 11. Specifically, the Vickers hardness (Hv) of the edge portion 10 is 1100 (Hv) or more.
As shown in the photographs of the upper surface of the polycrystalline silicon wafer in FIGS. 5A and 5B, in the example shown in FIG. 5A, the central portion 11 has radial regular crystal grains. In the example shown in FIG. 5 (b), the central portion 11 has straight regular crystal grains.

As described above, since the polycrystalline wafer of the present invention has a high strength at the edge portion, the occurrence rate of cracks in the manufacturing process of the solar cell is also low.
In addition, since the central portion 11 has a large crystal grain size, the polycrystalline wafer of the present invention has a high conversion efficiency of the solar cell equivalent to the silicon wafer manufactured by the casting method shown in FIG.

In order to confirm the effects of the present invention, a polycrystalline silicon ingot having a rectangular shape with a cross section of 350 mm × 500 mm is manufactured by the above-described casting method using the casting apparatus shown in FIG. 1, and a large number of polycrystalline silicon blocks are produced from the ingot. Manufactured.
Here, the polycrystalline silicon is, as Invention Examples 1 and 2, using a water-cooling mold using copper having good thermal conductivity, and setting the cooling water temperature to 29 ° C., so that the cooling rate is 150 ° C. / Casting was performed as sec.
When the cast silicon ingot was divided into silicon blocks, the silicon block having the cross section shown in FIG. 5A was designated as invention example 1, and the silicon block having the cross section shown in FIG.
Further, as Comparative Example 1, a water-cooled mold using copper having good heat conductivity was used, and the temperature of the cooling water was set to 80 ° C., so that the above cooling rate was 20 ° C./sec. A polycrystalline silicon wafer was produced in which the crystal grain size of the borders on two adjacent sides was slightly smaller than the central part.
Further, as Comparative Example 2, a polycrystalline silicon block obtained by cutting a polycrystalline silicon ingot manufactured by a conventional method using a casting method was prepared.

First, in order to evaluate the rate of occurrence of cracks in the wafer slicing process, 10 polycrystalline silicon blocks according to Invention Examples 1 and 2 and Comparative Examples 1 and 2 were produced, respectively, 500 crystal silicon wafers were manufactured, and the probability of occurrence of cracks was evaluated.
The slicing was performed by a fixed abrasive method, and a wire in which diamond having an average particle size of 20 μm was fixed as an abrasive was used. Note that the edge portion of the wafer was set as the starting point of slicing.
First, in the case of cutting into a polycrystalline silicon wafer having a thickness of 200 μm, the slicing speed was changed in the range of 0.2 to 1.0 mm / s (Table 1).
Next, the rate of cracking was evaluated by changing the thickness of the wafer to be manufactured in the range of 100 to 200 μm at a slice speed of 0.6 mm / s (Table 2).
The evaluation results are summarized in Tables 1 and 2, respectively.

Next, the test which measures the intensity | strength of each 10 wafer concerning invention example 1 and 2 and comparative examples 1 and 2 was done. The strength was evaluated based on the above-mentioned Vickers hardness (JIS Z 2244: 2009) for the edge portion and the center portion of the wafer.
Further, the crystal grain sizes of the edge portion and the center portion of each of 10 wafers according to Invention Examples 1 and 2 and Comparative Examples 1 and 2 were evaluated by the measurement method described above.
Table 3 shows the average value of Vickers hardness (Hv) of 10 wafers and the average value of crystal grain size.

Next, using 500 wafers according to Invention Examples 1 and 2 and Comparative Examples 1 and 2, the rate of occurrence of wafer cracking in the cell process or module process when a solar cell was manufactured was evaluated. did.
The evaluation results are shown in Table 4.

Next, the conversion efficiency of solar cells manufactured using 500 wafers according to Invention Examples 1 and 2 and Comparative Examples 1 and 2 was evaluated.
Here, the conversion efficiency of the solar cell is a ratio (E2 / E1) × 100 (%) of the light energy E1 irradiated per unit cell area of the solar cell and the converted electric energy E2 taken out per unit cell area. Defined by
Table 5 shows the evaluation results.

As shown in Tables 1 and 2, when the silicon blocks according to Invention Examples 1 and 2 are sliced at high speed, the crack generation rate can be kept low, and when slicing into a wafer having a thickness of 200 μm or less However, the occurrence rate of cracks can be kept low.
Further, as shown in Table 3, it can be seen that the polycrystalline silicon wafers according to Invention Examples 1 and 2 have high Vickers hardness only at the edge portion and 1.2 to 5 times the center portion. Therefore, it can be seen that the polycrystalline silicon wafers according to Invention Examples 1 and 2 have high strength.
Furthermore, since the polycrystalline wafers according to Invention Examples 1 and 2 have high strength, as shown in Table 4, it can be seen that the rate of occurrence of wafer cracking in the cell process and module process during the production of solar cells is low.
Furthermore, the wafers according to Invention Examples 1 and 2 have high strength only at the edge portion and low strength at the center portion. That is, since the crystal grain size is small only at the border and the crystal grain size at the center is large, as shown in Table 5, the conversion efficiency of the solar cells manufactured using the wafers according to Invention Examples 1 and 2 is a comparative example. It is equivalent to the conversion efficiency of the solar cell manufactured using the wafer concerning.

DESCRIPTION OF SYMBOLS 1 Chamber 2 Cooling mold 3 Induction coil 4 Heater 5 Ingot 6 Silicon material 7 Molten silicon 8 Pulling-down apparatus 9 Ingot 10 Edge part 11 Center part 12 Polycrystalline block 13 Polycrystalline wafer

Claims

A rectangular flat plate-shaped polycrystalline wafer having a border on at least one side of the rectangle, the crystal grain size of the border being 0.05 to 0.5 times the crystal grain size in the center of the rectangle A polycrystalline wafer characterized by the above.
2. The polycrystalline wafer according to claim 1, wherein a Vickers hardness (Hv) in the border portion is 1.2 to 5 times a Vickers hardness (Hv) in the central portion of the rectangle.
3. The polycrystalline wafer according to claim 1, wherein a crystal grain size in the border portion is 5 mm or less.
The polycrystalline wafer according to any one of claims 1 to 3, wherein a Vickers hardness (Hv) of the border portion is 1100 Hv or more.
A bottomless cooling mold in which at least a part in the axial direction is divided into a plurality of portions in the circumferential direction is arranged in the induction coil of the chamber, and the polycrystalline material is melted in the cooling mold by electromagnetic induction heating by the induction coil. A casting method of a polycrystalline material, in which a liquid is melted and the melt is cooled and solidified to be drawn downward,
The method for casting a polycrystalline material, wherein a rate of cooling the melt is 100 ° C./sec or more.
6. The method for casting a polycrystalline material according to claim 5, wherein the crystal grain size of the border is controlled to 5 mm or less by setting the cooling rate to 100 ° C./sec or more.
A polycrystalline ingot obtained by the method for casting a polycrystalline material according to claim 5 or 6 is cut, divided into polycrystalline blocks, and the polycrystalline blocks are sliced. Production method.