JP5868437B2 - Method for producing silicon wafer for solar cell - Google Patents

Description

The present invention relates to a manufacturing method of the silicon wafer hard for a solar cell which is obtained by slicing a silicon ingot with a fixed abrasive grain system.

  In view of the problem of depletion of energy resources and environmental problems, solar cells have been put into practical use as a new energy source. As a solar cell used for this, a pn junction is formed by diffusing impurities on the light receiving surface of a silicon wafer, and electrodes are formed on the light receiving surface and the back surface opposite to the light receiving surface, respectively. Yes.

  A silicon wafer used for a solar battery using such a solar battery cell is obtained by slicing a silicon ingot and then processing the surface thereof.

  A wire saw is generally used for slicing a silicon ingot. As a wire saw system, a free abrasive grain system (for example, refer to Patent Document 1) that travels while pressing against a silicon ingot while supplying a suspension (slurry) of abrasive grains to the wire, and abrasive grains on the surface of the core wire The fixed abrasive method (for example, refer patent documents 2 and 3) made to run, pressing against a silicon ingot using the saw wire which adhered and fixed.

  As a saw wire used for a fixed abrasive grain type wire saw, a resin bond wire (for example, refer to Patent Document 4) in which abrasive grains are fixed to the core wire surface with an adhesive resin, or abrasive particles are electrodeposited on the core wire surface. Examples thereof include an electrodeposited wire fixed through a plating layer (see, for example, Patent Document 5).

  The slice piece obtained by slicing the silicon ingot is subjected to a surface treatment by etching to prepare the surface. By etching, a work-affected layer on the surface of the slice piece due to slicing is removed and fine irregularities are formed on the surface. This unevenness causes multiple reflection of light on the wafer surface, resulting in a decrease in reflectivity and an increase in light absorption. As a result, incident light can be used efficiently.

  However, due to the slicing process with mechanical and thermal action, the silicon wafer has a surface layer that has changed in terms of material, that is, destruction and disorder of the crystal structure, polycrystallization, amorphization, and more microscopically. Will cause a work-affected layer accompanied by the occurrence of stacking faults, and strain and stress will remain in that layer.

  In the free abrasive method, the thickness of the work-affected layer due to slicing on the surface of the slice piece is relatively thick, approximately 10 to 20 μm, so that the effect of etching is effective considering that etching is likely to occur from a portion with large strain or residual stress. Easy to obtain. That is, unevenness is formed by removing the work-affected layer by etching.

  In the fixed abrasive method, the ingot cutting loss due to slicing is relatively small compared to the free abrasive method, so there is a great merit that the raw material yield is improved. Since the thickness of the work-affected layer is relatively thin, less than 10 μm, a problem has been pointed out that it is difficult to form unevenness enough to reduce the reflectance by etching.

  Furthermore, the surface irregularities formed by etching slices made of polycrystalline silicon are not uniform in the crystal orientation of the exposed crystal grains. However, there is a problem that uneven brightness (difference in crystal grain contrast) with different brightness occurs. This causes the appearance defect when the solar battery cell is used.

  However, conventional silicon ingots made of single-crystal silicon have been generally used, but in recent years, the use of polycrystalline silicon has increased in terms of manufacturing cost due to the improvement in performance of polycrystalline silicon. .

  As for etching, etching using an etching solution containing hydrofluoric acid, nitric acid, and sulfuric acid is disclosed for slice pieces made of single crystal silicon (see, for example, Patent Documents 6, 7, 8, and 9). In addition, etching using a mixed solution of hydrofluoric acid and nitric acid is disclosed for slice pieces made of polycrystalline silicon. (For example, refer to Patent Document 10). However, etching of slices made of polycrystalline silicon, especially slices sliced by the fixed abrasive method, reduces the reflectivity sufficiently and does not cause uneven gloss on the wafer surface due to the difference in crystal grain size and shape. At present, no method has been found.

JP 2012-24866 A JP 2013-12688 A JP 2013-43268 A JP 2000-052226 A JP 2011-255475 A Reissue WO2005 / 036629 Japanese translation of PCT publication No. 2004-503081 JP 09-270400 A Japanese Patent Application Laid-Open No. 2004-63954 JP 2006-73832 A

The present invention has been made in view of the above situation, and is a solar cell with low reflectance and less gloss unevenness obtained by using a slice piece obtained by slicing a polycrystalline silicon ingot by a fixed abrasive method. It seeks to provide a manufacturing method of use silicon wafer hard.

  The inventors of the present application can obtain a silicon wafer for solar cells with low reflectance and less gloss unevenness by etching a polycrystalline silicon slice piece sliced with a fixed abrasive grain type wire saw with a specific etching solution. As a result, the present invention was reached. That is, the present invention provides a method for manufacturing a silicon wafer for a solar cell in which a polycrystalline silicon slice piece sliced with a fixed abrasive grain type wire saw is etched with an etching solution containing a mixed acid composed of hydrofluoric acid, nitric acid and sulfuric acid in a specific ratio. Is the method.

That is, the gist of the present invention is a method for producing a silicon wafer for solar cells, in which a slice of polycrystalline silicon is etched with an etchant containing a mixed acid as a main component,
The slice piece is a slice piece sliced with a fixed abrasive grain type wire saw,
The mixed acid comprises hydrofluoric acid represented by the chemical formula HF, nitric acid represented by the chemical formula HNO ₃ and sulfuric acid represented by the chemical formula H ₂ SO ₄ ;
In the triangular diagram in which the composition range of the mixed acid represents these compositions in weight%,
Point A, in which the hydrofluoric acid is 2.82% by weight, the nitric acid is 0.18% by weight, and the sulfuric acid is 97% by weight;
Point B where the hydrofluoric acid is 0.18% by weight, the nitric acid is 2.82% by weight, and the sulfuric acid is 97% by weight;
Point C, where the hydrofluoric acid is 8.47 wt%, the nitric acid is 0.53% wt, and the sulfuric acid is 91 wt%;
The hydrofluoric acid is in a region surrounded by four line segments connecting the point D in which the hydrofluoric acid is 0.53% by weight, the nitric acid is 8.47% by weight, and the sulfuric acid is 91% by weight in this order;
It is in the manufacturing method of the silicon wafer for solar cells whose density | concentration of the water of the said etching liquid is 0 to 10.5 weight%.

In the method for producing a silicon wafer for a solar cell, a silicon wafer for a solar cell with a low reflectance and less gloss unevenness can be obtained particularly when the slice piece is a polycrystalline silicon wafer sliced with a resin bond wire saw.
Further, according to the method for manufacturing a silicon wafer for a solar cell, the surface has a plurality of substantially bowl-shaped concave holes formed over the entire surface, the opening diameter of the concave holes is 2 to 15 μm, and A silicon wafer for solar cells is obtained in which one or a plurality of micropores having an opening diameter of 0.1 to 1.5 μm are formed on the inner wall of each concave hole.
Further, the gist of the present invention is an etching solution used for etching a slice piece of polycrystalline silicon sliced with a fixed abrasive grain type wire saw, having a mixed acid as a main component,
The mixed acid comprises hydrofluoric acid represented by the chemical formula HF, nitric acid represented by the chemical formula HNO ₃ and sulfuric acid represented by the chemical formula H ₂ SO ₄ ;
In the triangular diagram in which the composition range of the mixed acid represents these compositions in weight%,
Point A, in which the hydrofluoric acid is 2.82% by weight, the nitric acid is 0.18% by weight, and the sulfuric acid is 97% by weight;
Point B where the hydrofluoric acid is 0.18% by weight, the nitric acid is 2.82% by weight, and the sulfuric acid is 97% by weight;
Point C, where the hydrofluoric acid is 8.47 wt%, the nitric acid is 0.53% wt, and the sulfuric acid is 91 wt%;
The hydrofluoric acid is in a region surrounded by four line segments connecting the point D in which the hydrofluoric acid is 0.53% by weight, the nitric acid is 8.47% by weight, and the sulfuric acid is 91% by weight in this order;
It exists in the etching liquid whose density | concentration of the water of the said etching liquid is 0 to 10.5 weight%. The temperature of the etching solution during the etching is 0 to 45 ° C.

  In the etching solution, a saw wire used in the fixed abrasive grain type wire saw may be a resin bond saw wire.

Manufacturing method of the silicon wafer hard for small solar gloss unevenness is provided with a low reflectance obtained using the slices obtained by slicing a polycrystalline silicon ingot according to the present invention with the fixed abrasive grain system .

The triangular figure which shows the composition of the etching liquid used for this invention. Imaging of the surface of a silicon wafer for solar cells. Explanatory drawing of the uneven | corrugated state of the surface of the silicon wafer for solar cells of this invention. Imaging showing the glossy state of the surface of a silicon wafer for solar cells.

A silicon wafer for solar cells is obtained by etching a slice piece obtained by slicing a polycrystalline silicon ingot by a fixed abrasive method with an etching solution.

  The fixed abrasive method is a method of slicing an ingot using a saw wire in which abrasive grains are bonded and fixed to the surface of the core wire. As an aspect of bonding and fixing the abrasive particles to the surface of the core wire, a molten metal method, an electrodeposition method, a resin Bond method is mentioned.

  The molten metal method is a method in which abrasive grains are fixed to the surface of the core wire via a low melting point metal (brazing material) such as a solder alloy, and a method described in JP 2010-201602 A is exemplified.

  The electrodeposition method is a method in which a plating layer is formed on the surface of the core wire by using a plating solution mixed with abrasive particles, thereby fixing the abrasive particles on the surface of the core wire via the plating layer, as described in JP-A-2003-340729. This method is exemplified.

  The resin bond method is a method in which abrasive grains are fixed to the surface of the core wire through a resin adhesive.

  A steel wire is preferably used as the core wire used in the fixed abrasive system. The wire diameter is not particularly limited, but is preferably 0.3 to 0.05 mm. Steel wire includes heat-treated spring steel wire such as high carbon steel and medium carbon low alloy steel, hard steel wire, piano wire and stainless steel wire, cold rolled steel wire and oil tempered wire wire made of spring steel, low Examples include steel wires with high toughness and high fatigue strength such as alloy steel, medium alloy steel, high alloy steel, and maraging steel.

  Although it does not specifically limit as an abrasive grain used in a fixed abrasive system, A diamond abrasive grain, cubic-type BN abrasive grain, an alumina abrasive grain, a silicon carbide abrasive grain etc. are illustrated. The diamond abrasive grains may be nickel-coated. Among them, it is preferable to use diamond abrasive grains having a particle diameter of 5 to 15 μm.

  In the present invention, a silicon wafer for a solar cell is manufactured by etching a slice piece obtained by slicing a polycrystalline silicon ingot by a fixed abrasive method using an etching solution.

The etching solution used in the present invention contains a mixed acid composed of hydrofluoric acid (HF), nitric acid (HNO ₃ ), and sulfuric acid (H ₂ SO ₄ ) as a main component. This etchant may further contain water.

  This etchant also aids organic acids such as aliphatic carboxylic acids, aliphatic sulfonic acids and aliphatic phosphoric acids, and oxidizing agents such as perchloric acid, perchlorate, perchromic acid and perchromate. May be included. Further, nitrates such as sodium nitrate, potassium nitrate and ammonium nitrate, nitrites such as sodium nitrite, potassium nitrite and ammonium nitrite, and fluoride salts such as sodium fluoride, potassium fluoride and ammonium fluoride may be contained.

In the etching solution used in the present invention, the mixing ratio of each acid of the mixed acid composed of hydrofluoric acid, nitric acid, and sulfuric acid with respect to the total weight of these acids is point A (HF: 2.82 weight in the triangular diagram shown in FIG. %, HNO ₃ : 0.18 wt%, H ₂ SO ₄ : 97 wt%),
B (HF: 0.18 _wt%, HNO 3: 2.82 _wt%, H 2 _SO 4: 97 wt%),
C (HF: 8.47% by weight, HNO ₃ : 0.53% by weight, H ₂ SO ₄ : 91% by weight),
D (HF: 0.53% by weight, HNO ₃ : 8.47% by weight, H ₂ SO ₄ : 91% by weight)
It is in the range surrounded by the line segment connecting.

  The moisture concentration of the etching solution (weight concentration in the etching solution including all components including water, that is, the content ratio of water) is 0 to 10.5% by weight. If the moisture concentration is higher than this, it becomes difficult to form small concaves (small dents), the reflectance is not sufficiently lowered, and the crystal grain contrast difference of the wafer becomes remarkable. In view of stability of the etching process, the moisture concentration of the etching solution is more preferably 10% by weight or less.

  Etching is performed by immersing a slice piece obtained by slicing a polycrystalline silicon ingot by a fixed abrasive method in an etching solution, and then washing the slice piece with water. It is preferable that the liquid temperature in immersion is 0 to 45 ° C. and the time is 1 to 30 minutes. When the liquid temperature falls below this range, the etching proceeds insufficiently and it takes too much time until the irregularities are formed. If the liquid temperature exceeds this range, it will be difficult to form irregularities, especially small depressions, the reflectivity will not be lowered sufficiently, and the difference in crystal grain contrast will be significant, resulting in a contrast difference in gloss and commercial value. drop down. Therefore, the liquid temperature is more preferably 5 to 40 ° C.

  In this etching solution, if the composition ratio of hydrofluoric acid, nitric acid, and sulfuric acid is outside the range surrounded by the line segment connecting the points ABCD in the triangular diagram shown in FIG. 1, the etching rate becomes too slow and the etching reaction proceeds. Therefore, it takes too much time until the irregularities are formed. Alternatively, it becomes difficult to form small concaves, the reflectance is not sufficiently lowered, and the crystal grain contrast becomes remarkable.

  In this etching solution, the concentration of hydrofluoric acid / (concentration of hydrofluoric acid + concentration of nitric acid) is preferably 0.059 to 0.94. Outside this range, the etching rate decreases, small cavities are hardly formed, the reflectivity increases, and the crystal grain contrast difference may become significant.

In this etching solution, the sulfuric acid concentration is 91 to 97% by weight. If the sulfuric acid concentration exceeds this range, the etching rate becomes too slow and the etching reaction does not proceed, so it takes too much time for the unevenness to be formed. Below the range as small concave is hardly formed, without fall reflectance enough, and the crystal grains contrast difference becomes remarkable.

Further, the blending ratio of each of the hydrofluoric acid, nitric acid, and sulfuric acid in the etching solution used in the present invention with respect to the total weight of these acids is point A ′ (HF: 2.62 wt. %, HNO ₃ : 0.88 wt%, H ₂ SO ₄ : 96.5 wt%),
B ′ (HF: 0.88 wt%, HNO ₃ : 2.62 wt%, H ₂ SO ₄ : 96.5 wt%),
C ′ (HF: 6.75 wt%, HNO ₃ : 2.25 wt%, H ₂ SO ₄ : 91 wt%),
D ′ (HF: 2.25 wt%, HNO ₃ : 6.75 wt%, H ₂ SO ₄ : 91 wt%)
It is more preferable to obtain a silicon wafer for a solar cell having a low reflectivity and less gloss unevenness due to a difference in crystal grain contrast.

Further, the blending ratio of each of the hydrofluoric acid, nitric acid, and sulfuric acid in the etching solution used in the present invention with respect to the total weight of these acids is indicated by a point A ″ (HF: 1.98) in the triangular diagram shown in FIG. Wt%, HNO ₃ : 1.52 wt%, H ₂ SO ₄ : 96.5 wt%),
B ″ (HF: 1.44 wt%, HNO ₃ : 2.06 wt%, H ₂ SO ₄ : 96.5 wt%),
C ″ (HF: 5.09 wt%, HNO ₃ : 3.91 wt%, H ₂ SO ₄ : 91 wt%),
D ″ (HF: 3.71 wt%, HNO ₃ : 5.29 wt%, H ₂ SO ₄ : 91 wt%)
It is most preferable to obtain a silicon wafer for solar cells with low reflectance and less gloss unevenness within the range surrounded by the line segments connecting the two.

The etching solution used in the present invention is, for example, an aqueous solution of hydrofluoric acid having a concentration of 40 to 55 wt% or higher, an aqueous solution of nitric acid having a concentration of 59 to 75 wt% or higher, preferably a concentration of 95. 98 wt% of sulfuric acid (acid concentration x% refers to a mixture of acid x parts by weight of water (100-x) parts by weight) and can be a obtained by mixing.

  When a slice piece obtained by slicing a polycrystalline silicon ingot by the fixed abrasive method is etched with an etching solution having such a blending ratio, the work-affected layer is removed as shown in FIG. As the grains are exposed on the surface of the slice piece, irregularities are formed on the surface of the slice piece, and the surface is further etched to form a basal-shaped depression, and moreover than the irregularities caused by the depression. Fine irregularities are formed on the inner surface of the bowl-shaped depression. Thereby, the silicon wafer for solar cells with a small surface reflectance is obtained. Moreover, the silicon wafer for solar cells with little gloss unevenness is obtained.

  FIG. 2 (a) is a scanning electron microscope image of the surface of the silicon wafer for solar cells obtained by the present invention, and FIG. 2 (b) is a slice of a polycrystalline silicon ingot by the free abrasive grain method. It is a scanning electron microscope image of the uneven | corrugated state of the surface of the silicon wafer for solar cells obtained by etching the slice piece obtained by making only the conventional hydrofluoric acid and nitric acid mixed etching liquid.

  The silicon wafer for solar cells obtained by the present invention shown in FIG. 2 (a) has irregularities formed by forming a plurality of concave holes having a curved bottom surface on the entire surface. The opening diameter of the concave hole is 2 to 15 μm, and one or a plurality of micro holes having an opening diameter of 0.1 to 1.5 μm are formed on the inner wall of the concave hole. That is, the silicon wafer for solar cells of the present invention has irregularities formed by forming a plurality of concave holes over the entire surface, the opening diameter of the concave holes is 2 to 15 μm, and the inner wall of the concave holes Substantially has a shape in which one or a plurality of micropores having an opening diameter of 0.1 to 1.5 μm are formed. Such unevenness can be confirmed by a scanning confocal laser microscope or a shape measuring apparatus described in Japanese Patent No. 3810748.

FIG. 3 is a schematic diagram showing the uneven state of the surface of the silicon wafer for solar cells by a cut surface in the thickness direction of the wafer. FIG. 3A is a silicon wafer for solar cells shown in FIG. 3B is a partially enlarged schematic view of FIG. 3B. FIG. 3B is a partially enlarged schematic view of the surface of the silicon wafer for solar cells shown in FIG. 2A. . As shown in FIGS. 3 (b) and 3 (c) , the silicon wafer for solar cells of the present invention, when enlarged, results from a large depression (substantially bowl-shaped recess) 2 having a substantially bowl-shaped depression. It has an irregularity in a state in which the irregularities of a small period caused by small depressions (micropores) 3 caused by erosion by the etching solution are superimposed on the irregularities of a large cycle (substantially basal shape or a shape in which the substantially basal shape overlaps). . The diameter d1 of the upper edge of the large depression 2 is 2 to 15 μm and the depth h1 is 2 to 15 μm. The diameter d2 of the upper edge of the small depression 3 is 0.1 to 1.5 μm and the depth h2 is 0.1. ˜1.5 μm. Moreover, the space | interval p of the upper edge of the big dents 2 adjacent to each other is 0-10 micrometers. In addition, a plurality of small depressions exist on the inner surface of one large depression 2. On the other hand, the surface of the silicon wafer for solar cells obtained by etching the slice piece obtained by slicing the polycrystalline silicon ingot by the free abrasive grain method shown in the microscopic image of FIG. In FIG. 3, there is almost no small depression corresponding to the small depression 3 in FIG. The reflectance was 30%, and a sufficiently low surface state could not be formed as compared with FIG. 2 (a) using the etching solution of the present application.

  “Over the entire surface” means that the concave holes are present on the surface of the wafer so that the distance between the upper edge portions of adjacent concave holes, that is, p is 0 to 10 μm. Moreover, adjacent concave holes may enter and overlap each other.

  The large indentation 2 is similar to the shape of the indentation of the ridge, and a straight line standing perpendicularly to the wafer surface direction from the deepest part of the depression is used as a symmetric center line or perpendicular to the wafer surface direction from the deepest part of the depression. It has a substantially symmetric shape with a flat plane as a symmetry plane. This indicates that a thin work-affected layer of about 5 μm is removed by etching, and further etching proceeds in the thickness direction of the wafer to form such a symmetric recess. Due to such symmetrical recesses, irregular reflected light is hardly generated on the wafer surface, and a wafer with less gloss unevenness can be obtained. That is, the etching using the etching solution of the present invention can be performed regardless of the presence or absence of a work-affected layer. Accordingly, a basal depression having a symmetrical shape with respect to the thickness direction is formed on the wafer surface.

  The etching solution of the present invention is an etching solution that can form both such a large depression 2 and a small depression 3 on the silicon after the work-affected layer has been removed. In other words, the etching solution of the present invention is an etching solution that can form both the large dent 2 and the small dent 3 on the wafer surface regardless of the presence or absence of the work-affected layer.

  On the other hand, since the sliced piece sliced from the polycrystalline silicon ingot by the free abrasive grain method has a thick work-affected layer, unevenness due to crystal grains is generated on the surface only by removing the work-affected layer by etching. It will be. The process-affected layer can be removed relatively easily using a conventional etching solution using hydrofluoric acid or nitric acid. However, the depressions obtained in this way are due to the crystal orientation of the crystal grains, and therefore have irregular shapes and asymmetric shapes with respect to the thickness direction of the wafer. For this reason, irregular reflected light tends to be generated on the wafer surface, and a wafer having gloss unevenness can be obtained. As described above, the conventional etching solution forms unevenness in the process of removing the work-affected layer, and does not form unevenness on the silicon after the work-affected layer is removed. In addition, it is difficult to perform etching to the extent that a smaller recess 3 is formed in silicon after the work-affected layer is removed with a conventional etching solution.

By performing chemical etching in an acidic etching solution mainly composed of hydrofluoric acid, nitric acid, phosphoric acid, etc., a work-affected layer having a thickness of 10 μm or more can be formed even for a polycrystalline silicon wafer having irregular crystal faces. A method for obtaining unevenness by etching removal is disclosed (for example, Japanese Patent Laid-Open No. 10-303443, Japanese Patent No. 4766880, etc.).
This is because the following reaction proceeds.
_{HNO 3 + H 2 O + HNO} 2 → 2HNO 2 + 2OH - + 2h + ( holes)
Si + 4h ⁺ → Si ^{4+
_{Si 4+ + 2OH - → SiO 2}} + H 2
SiO ₂ + 6HF → H ₂ SiF ₆ + H ₂ O

  Therefore, depending on the ratio of nitric acid that oxidizes silicon and hydrofluoric acid that dissolves silica oxide, the reaction rate can be changed, and the shape and size of the unevenness can also be changed. It is preferable to add to. The reason for this is as follows.

That is, the oxidation reaction of silicon reacts a little with concentrated nitric acid as shown above, but when concentrated sulfuric acid is added, an acid / base reaction in which sulfuric acid is an acid and nitric acid is a base occurs. As a result, the “—O—SO ₂ —OH” anion and the “H ₂ O (+)-NO ₂ ” cation are formed, and the water is released from the “H ₂ O (+)-NO ₂ ” cation to “(+ ) NO ₂ ”cation (nitronium cation) increases in the system. In other words, in the following reaction formula, the equilibrium is biased to the right as compared with when concentrated sulfuric acid is not added, the reaction becomes faster, and the surface roughness of the silicon piece is governed.
_{2H 2 SO 4 + HNO 3 ⇔} NO 2 + + H 3 O + + 2HSO 4 ^over
In view of such a phenomenon, in the present invention, the concentration of acid for forming a good uneven shape on the surface of the silicon piece, in particular, the concentration of sulfuric acid was found.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

  In Examples and Comparative Examples, the reflectance of silicon wafers was measured using an ultraviolet-visible near-infrared spectrophotometer Solidspec-3700 and an integrating sphere BIS-3700 manufactured by Shimadzu Corporation. The average value of nine measurements at a wavelength of 600 nm was determined and used as the reflectance.

Example 1
<Silicon ingot>
A polycrystalline silicon ingot manufactured by GET was used.
<Saw wire>
A resin bond saw wire (product number: MW-100-8-16) manufactured by TKX Corporation was used.
(Adhesive composition for resin bond saw wire: phenolic resin composition abrasive grains: diamond abrasive grains: grinding wheel diameter 8-16 μm (10.5 μm ± 1 μm)
Wire ... φ100μm steel wire)
<Slicing process>
A saw wire was wound around a pulley having a groove cut on its peripheral surface and attached to a cutting device for forming a loop. The slice was obtained by running at a speed of 600 m / min.
<Etching solution>
Hydrofluoric acid aqueous solution (concentration 47 wt%) ... 6.2 wt%
Nitric acid aqueous solution (concentration 67 wt%) ... 5.0 wt%
Sulfuric acid with a concentration of 95 wt% ... 88.8 wt%
This composition is displayed as the point of Example 1 in FIG.
<Etching>
The slice piece was immersed in an etching solution at 10 ° C. for 20 minutes and then washed with water to obtain a silicon wafer.
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 18.6%. d1 was 3 to 12 μm (average 5.4 μm), and d2 was 0.1 to 1 μm. As in the imaging of the surface state shown in FIG. 4A, almost no gloss unevenness was observed on the surface of the silicon wafer. FIG. 2A is a microscopic image of the unevenness of the surface of the silicon wafer of Example 1. FIG.

Example 2
The composition of the etching solution is hydrofluoric acid aqueous solution (concentration 47 wt%) ... 6.3 wt%
Nitric acid aqueous solution (concentration 67 wt%) ... 4.2 wt%
Sulfuric acid with a concentration of 95 wt% ... 89.5 wt%
A silicon wafer was obtained in the same manner as in Example 1 except that
This composition is displayed as the point of Example 2 in FIG.
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 18.0%. d1 was 3 to 10 μm (average 4.8 μm), and d2 was 0.1 to 1 μm. There was almost no gloss unevenness on the surface of the silicon wafer.

Example 3
The composition of the etching solution is a hydrofluoric acid aqueous solution (concentration 47 wt%)... 6.6 wt%
Nitric acid aqueous solution (concentration 67wt%) ... 3.4wt%
Sulfuric acid with a concentration of 95 wt.
A silicon wafer was obtained in the same manner as in Example 1 except that
This composition is displayed as the point of Example 3 in FIG.
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 20.8%. d1 was 2 to 8 μm (average 3.9 μm), and d2 was 0.1 to 1 μm. There was almost no gloss unevenness on the surface of the silicon wafer.

Example 4
The composition of the etching solution is a hydrofluoric acid aqueous solution (concentration 47 wt%) ... 6 wt%
Nitric acid aqueous solution (concentration 67 wt%) ... 6 wt%
Sulfuric acid with a concentration of 95 wt.
A silicon wafer was obtained in the same manner as in Example 1 except that
This composition is displayed as the point of Example 4 in FIG.
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 21.0%. d1 was 3 to 10 μm (average 3.9 μm), and d2 was 0.1 to 0.5 μm. Although some uneven gloss was observed on the surface of the silicon wafer, it did not impair the commercial value.

Example 5
A silicon wafer was obtained in the same manner as in Example 1 except that the saw wire obtained by the method described in Example 1 of JP2010-201602A was used as a saw wire.
<Saw wire manufacturing method>
The metal core wire of the wire was a piano wire with a wire diameter of φ100 μm covered with brass.

  As the brazing material, Sn-3.0% Ag-0.5% Cu (solidus: 218 ° C., liquidus: 220 ° C.) was used. To this, 0.2% aluminum (Al) powder was added and melted.

  As the abrasive grains 2, diamond powder coated with nickel was used. The grain size of the abrasive grains is 20 to 35 μm. This was kneaded with an organic amine-based active rosin flux at a ratio of 70:30 (% by weight) to the brazing filler metal powder and diamond powder, and the viscosity was adjusted to 300 Pa · s with terpineol. Syringe).

  Subsequently, using a dispenser having a nozzle diameter of 100 μm, the paste was uniformly applied to a piano wire core material with a film thickness of 22 to 20 μm. It was melted by irradiating it with laser light having an output of 1 W, a beam diameter of 600 to 1300 μm, and a wavelength of 808 nm, and then naturally cooled.

  While judging the melted state, the ratio of diamond to brazing material was set so that the thickness of the melt-solidified layer was 5 to 40% of the grain size of the abrasive grains 2.

<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 22.0%. d1 was 3 to 14 μm (average 4.0 μm), and d2 was 0.1 to 0.5 μm. Although some uneven gloss was observed on the surface of the silicon wafer, it did not impair the commercial value.

Example 6
The composition of the etching solution is hydrofluoric acid aqueous solution (concentration 47 wt%) ... 4.2 wt%
Nitric acid aqueous solution (concentration 69 wt%) 7.8 wt%
Sulfuric acid with a concentration of 95 wt% ... 88.0 wt%
Then, the slice piece was immersed in an etching solution at 25 ° C. for 142 seconds and then washed with water to obtain a silicon wafer. Others are the same as in the first embodiment.
This composition is displayed as the point of Example 6 in FIG.
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 23.0%. d1 was 2 to 12 μm (average 4.1 μm), and d2 was 0.4 to 1.0 μm. Although some uneven gloss was observed on the surface of the silicon wafer, it did not impair the commercial value.

Example 7
The composition of the etching solution is a hydrofluoric acid aqueous solution (concentration 47 wt%) ... 4.6 wt%
Nitric acid aqueous solution (concentration 69 wt%) ... 6.9 wt%
Sulfuric acid with a concentration of 95wt% ... 88.5wt%
Then, the slice piece was immersed in an etching solution at 25 ° C. for 142 seconds and then washed with water to obtain a silicon wafer. Others are the same as in the first embodiment.
This composition is displayed as the point of Example 7 in FIG.
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 19.4%. d1 was 3 to 8 μm (average 4.8 μm), and d2 was 0.4 to 1.1 μm. Although some uneven gloss was observed on the surface of the silicon wafer, it did not impair the commercial value.

Example 8
The composition of the etching solution is hydrofluoric acid aqueous solution (concentration 47 wt%) ... 4.95 wt%
Nitric acid aqueous solution (concentration 69 wt%) ... 6.05 wt%
Sulfuric acid with a concentration of 95 wt% ... 89.0 wt%
Then, the slice piece was immersed in an etching solution at 25 ° C. for 142 seconds and then washed with water to obtain a silicon wafer. Others are the same as in the first embodiment.
This composition is displayed as the point of Example 8 in FIG.
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 19.4%. d1 was 3 to 10 μm (average 6.5 μm), and d2 was 0.5 to 1.3 μm. Although some uneven gloss was observed on the surface of the silicon wafer, it did not impair the commercial value.

Example 9
The composition of the etching solution is hydrofluoric acid aqueous solution (concentration 47 wt%) ... 5.25 wt%
Nitric acid aqueous solution (concentration 69 wt%) ... 5.25 wt%
Sulfuric acid with a concentration of 95 wt% ... 89.5 wt%
Then, the slice piece was immersed in an etching solution at 25 ° C. for 142 seconds and then washed with water to obtain a silicon wafer. Others are the same as in the first embodiment.
This composition is displayed as the point of Example 9 in FIG.
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 19.8%. d1 was 3 to 11 μm (average 5.6 μm), and d2 was 0.5 to 1.1 μm. There was almost no uneven gloss on the surface of the silicon wafer.

Comparative Example 1
The composition of the etching solution is hydrofluoric acid aqueous solution (concentration 47 wt%) ... 4,5 wt%
Nitric acid aqueous solution (concentration 67 wt%) ... 9 wt%
Sulfuric acid with a concentration of 95 wt% ... 86.5 wt%
A silicon wafer was obtained in the same manner as in Example 1 except that
This composition is displayed as the point of Comparative Example 1 in FIG.
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 26.1%. d1 was 3 to 15 μm (average 5.1 μm), and d2 was 0.1 to 1 μm. Gloss unevenness to the extent that the commercial value is impaired was observed on the surface of the silicon wafer. FIG. 4B is an image showing the gloss state of the obtained silicon wafer.

Comparative Example 2
The composition of the etching solution is a hydrofluoric acid aqueous solution (concentration 47 wt%)... 3.6 wt%
Nitric acid aqueous solution (concentration 67wt%) ... 16wt%
95wt% sulfuric acid concentration 80.4wt%
A silicon wafer was obtained in the same manner as in Example 1 except that
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 27%. Gloss unevenness to the extent that the commercial value is impaired was observed on the surface of the silicon wafer.

Comparative Example 3
The composition of the etching solution is a hydrofluoric acid aqueous solution (concentration 47 wt%) ... 6 wt%
Nitric acid aqueous solution (concentration 67 wt%) ... 29 wt%
95% by weight sulfuric acid ... 65% by weight
A silicon wafer was obtained in the same manner as in Example 1 except that
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was 30%. Gloss unevenness to the extent that the commercial value is greatly impaired was observed on the surface of the silicon wafer.

Comparative Example 4
<Slicing process>
An ingot similar to that used in Example 1 was sliced using a multi-wire saw method of loose abrasive grains.
Wire diameter: 0.1 mm (manufactured by JFE Steel, model SRH)
Abrasive grains: Silicon carbide (manufactured by Fujimi Incorporated, GC # 1500, average particle diameter of about 8 μm)
Cutting speed: 0.35 mm / min (ingot feed speed)
Wire traveling speed: 600m / min <etching>
The composition of the etching solution is hydrofluoric acid aqueous solution (concentration 47 wt%) ... 25 wt%
Nitric acid aqueous solution (concentration 67 wt%) 45% by weight
Water ... 30% by weight
A silicon wafer was obtained by the same etching as in Example 1 except that. However, etching was performed at 10 ° C. × 2 minutes.
<Characteristics of silicon wafer>
d1 was 10 μm on average, and only the large dent 2 was recognized, and the small dent 3 was not recognized. For this reason, the reflectance of the obtained silicon wafer remained as high as 30.8%. FIG. 4C is an image showing the glossy state of this silicon wafer.

Comparative Example 5
The composition of the etching solution is hydrofluoric acid aqueous solution (concentration 47 wt%) ... 25 wt%
Nitric acid aqueous solution (concentration 67 wt%) 45% by weight
Water ... 30% by weight
A silicon wafer was obtained in the same manner as in Example 1 except that However, etching was performed at 10 ° C. × 2 minutes.
<Characteristics of silicon wafer>
The reflectance of the obtained silicon wafer was as high as 32.5%. FIG. 2C is a microscopic image of the surface irregularity of the silicon wafer, and FIG. 4D is an image showing the glossy state of the silicon wafer. d1 was 3 to 15 μm (average 5.5 μm), but no small depression 3 was observed. Further, gloss unevenness to the extent that the commercial value was impaired was observed on the surface of the silicon wafer.
The reflectance of the obtained silicon wafer was 32.5%.

  The present invention is a useful technique that can be widely applied to the manufacture of silicon wafers for solar cells and other photoelectric conversion elements.

2: Large depression 3: Small depression

Claims (3)

A method for producing a silicon wafer for a solar cell, comprising slicing polycrystalline silicon with a fixed-abrasive wire saw to obtain a slice piece, and etching the slice piece with an etchant containing a mixed acid as a main component,
The mixed acid comprises hydrofluoric acid represented by the chemical formula HF, nitric acid represented by the chemical formula HNO ₃ and sulfuric acid represented by the chemical formula H ₂ SO ₄ ;
In the triangular diagram in which the composition range of the mixed acid represents these compositions in weight%,
Point A, in which the hydrofluoric acid is 2.82% by weight, the nitric acid is 0.18% by weight, and the sulfuric acid is 97% by weight;
Point B where the hydrofluoric acid is 0.18% by weight, the nitric acid is 2.82% by weight, and the sulfuric acid is 97% by weight;
Point C, where the hydrofluoric acid is 8.47 wt%, the nitric acid is 0.53% wt, and the sulfuric acid is 91 wt%;
The hydrofluoric acid is in a region surrounded by four line segments connecting the point D in which the hydrofluoric acid is 0.53% by weight, the nitric acid is 8.47% by weight, and the sulfuric acid is 91% by weight;
The water concentration of the etching solution is 0 to 10.5% by weight,
A sliced piece of polycrystalline silicon is obtained by slicing with the fixed abrasive grain type wire saw , and by etching the slice piece with the etching solution, a plurality of substantially bowl-shaped concave holes are formed on the entire surface. 1 or a plurality of micro holes having an opening diameter of 2 to 15 μm and an opening diameter of 0.1 to 1.5 μm are formed on the inner wall of each of the recessed holes. To manufacture a silicon wafer for solar cells. The manufacturing method of the silicon wafer for solar cells of Claim 1 whose saw wire used for the wire saw of the said fixed abrasive system is a resin bond saw wire. The manufacturing method of the silicon wafer for solar cells according to claim 1 or 2, wherein the temperature of the etching solution during the etching is 0 to 45 ° C.

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