TW201444955A - Silicon wafer for solar cells and method for producing same - Google Patents

Abstract

Provided are: a silicon wafer for solar cells, which is obtained using a slice that is obtained by slicing a polycrystalline silicon ingot by bonded abrasive machining, and which has low reflectance and less gloss unevenness; and a method for producing the silicon wafer for solar cells. A method for producing a silicon wafer for solar cells, wherein a polycrystalline silicon slice, which has been sliced out with use of a fixed abrasive wire saw, is etched by means of an etching liquid that contains a mixed acid composed of hydrofluoric acid, nitric acid and sulfuric acid. The composition range of the mixed acid is within a region that is surrounded by four line segments connecting point A at which hydrofluoric acid is 2.82% by weight, nitric acid is 0.18% by weight and sulfuric acid is 97% by weight, point B at which hydrofluoric acid is 0.18% by weight, nitric acid is 2.82% by weight and sulfuric acid is 97% by weight, point C at which hydrofluoric acid is 8.47% by weight, nitric acid is 0.53% by weight and sulfuric acid is 91% by weight, and point D at which hydrofluoric acid is 0.53% by weight, nitric acid is 8.47% by weight and sulfuric acid is 91% by weight in this order in a ternary diagram that expresses the composition in terms of weight percentage. The density of water in the etching liquid is 0-10.5% by weight.

Description

Silicon wafer for solar cell and method of manufacturing the same

The present invention relates to a tantalum wafer for a solar cell obtained by slicing a silicon ingot in a fixed abrasive grain manner and a method of manufacturing the same.

From the perspective of energy exhaustion and environmental issues, solar cells are being put into practical use as new energy sources. As a solar cell for the solar cell, it is common to form an pn junction by diffusing impurities to the light-receiving surface of the germanium wafer, and forming electrodes on the back surface opposite to the light-receiving surface and the light-receiving surface.

A tantalum wafer used in a solar cell using such a solar cell or the like is obtained by slicing a crystal ingot and then treating the surface thereof.

The wire saw is generally used for the slicing of the ingot. As a method of the wire saw, a suspension (a slurry) in which abrasive grains are supplied to the wire and a free abrasive grain which moves while being pressed against the twin ingot (for example, refer to Patent Document 1); The surface is then fixed with a saw wire of abrasive grains, and is fixed to the abrasive grains while being pressed against the twin ingot (for example, refer to Patent Documents 2 and 3).

The saw wire used for the wire saw of the fixed abrasive grain type is a resin bonded wire in which the abrasive grain is fixed to the surface of the core wire by an adhesive resin (for example, see Patent Document 4), and the abrasive grain is used. An electroplating line that is plated on the surface of the core wire and fixed via a plating layer (for example, see Patent Document 5).

The slice obtained by slicing the twin ingot can be surface-treated by etching to adjust the surface. The processed metamorphic layer of the sliced surface caused by the slicing is removed by etching, and And fine irregularities are formed on the surface. By this unevenness, multiple reflection occurs on the surface of the wafer, the reflectance is lowered, and the absorption of light is improved, and as a result, the incident light can be utilized efficiently.

However, by the dicing process accompanied by mechanical and thermal action, the surface layer in which the material changes is generated in the wafer, that is, the destruction or disorder of the crystal structure, polycrystallization, amorphization, and microscopic buildup defects. A process metamorphic layer is produced, and strain or stress remains in the portion.

In the free abrasive grain method, since the thickness of the processed metamorphic layer caused by slicing on the surface of the slice is approximately 10 to 20 μm and relatively thick, it is easy to obtain if the etching is easy to be self-strain or the residual stress is large. The effect of etching. That is, the unevenness is formed by removing the processed altered layer by etching.

In the fixed abrasive grain method, the cutting loss of the ingot due to slicing is relatively smaller than that of the free abrasive grain method, so that the raw material yield is improved. On the other hand, the fixed abrasive grain method has the following problems: Since the thickness of the processed metamorphic layer of the sliced surface obtained by the slicing is less than 10 μm and relatively thin, it is difficult to form irregularities sufficient to reduce the reflectance by etching.

Further, in the surface unevenness formed by etching the slice containing the polycrystalline silicon, since the crystal orientation of the exposed crystal grains is not fixed, there is a difference in the dissolution speed due to the crystal surface, and the gloss is different on the wafer surface. Both (the difference in crystal grain contrast). This problem can be a cause of poor appearance when the solar cell is made.

However, as a twin ingot, a single crystal crucible has been conventionally used. In contrast, in recent years, the use of polycrystalline germanium has been increasing in view of improvement in performance of polycrystalline germanium and production cost.

Regarding etching, etching of a slice containing single crystal germanium by an etching liquid containing hydrofluoric acid, nitric acid, or sulfuric acid is disclosed (for example, refer to Patent Documents 6, 7, 8, and 9). also, It is disclosed that etching of a slice containing polycrystalline germanium by a mixture of hydrofluoric acid and nitric acid (for example, refer to Patent Document 10). However, regarding the etching of a slice including polycrystalline germanium, in particular, a slice obtained by slicing by a fixed abrasive grain method, it has not been found that the reflectance is sufficiently lowered and the surface of the wafer is not caused by the size or shape of the crystal grain. An etching method that causes uneven gloss.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2012-24866

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2013-12688

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2013-43268

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2000-052226

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2011-255475

[Patent Document 6] Japanese Patent Laid-Open Publication No. WO2005/036629

[Patent Document 7] Japanese Patent Laid-Open Publication No. 2004-503081

[Patent Document 8] Japanese Patent Laid-Open No. 09-270400

[Patent Document 9] Japanese Patent Laid-Open Publication No. 2004-63954

[Patent Document 10] Japanese Patent Laid-Open Publication No. 2006-73832

The present invention has been made in view of the above-described actual circumstances, and is intended to provide a silicon wafer for solar cells having low reflectance and low gloss unevenness, and a method for manufacturing the same, which is used in a fixed abrasive grain method for a solar cell. The polycrystalline germanium ingot is obtained by sectioning the obtained slice.

The inventors of the present invention have found that a low reflectance can be obtained by etching a polycrystalline germanium slice obtained by slicing a wire saw with a fixed abrasive grain by using a specific etching liquid. The solar cell with less uneven gloss is used for the wafer, thereby completing the invention of the present invention. In other words, the present invention is a method for producing a silicon wafer for a solar cell in which a polycrystalline silicon wafer sliced by a wire saw with a fixed abrasive particle is etched by an etching solution containing a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid at a specific ratio. .

In other words, a method for producing a tantalum wafer for a solar cell, which is a method for producing a tantalum wafer for a solar cell, in which a slice of a polycrystalline silicon is etched by an etching liquid containing a mixed acid as a main component, is a part of the present invention. The slice is a slice obtained by slicing with a wire saw having a fixed abrasive particle type, and the mixed acid includes hydrofluoric acid represented by Chemical Formula HF, nitric acid represented by Chemical Formula HNO ₃ , and sulfuric acid represented by Chemical Formula H ₂ SO ₄ , and the above mixture The composition of the acid is in a triangular diagram in which the composition is expressed by weight %, in a region surrounded by four line segments sequentially connected to the following four points, that is, point A: the hydrofluoric acid is 2.82% by weight, the above Nitric acid was 0.18% by weight, and the sulfuric acid was 97% by weight. Point B: the hydrofluoric acid was 0.18% by weight, the nitric acid was 2.82% by weight, the sulfuric acid was 97% by weight, and Point C: the hydrofluoric acid was 8.47% by weight. The nitric acid is 0.53% by weight, the sulfuric acid is 91% by weight, and the D point is 0.53 wt% of the hydrofluoric acid, the nitric acid is 8.47% by weight, and the sulfuric acid is 91% by weight. The water concentration of the etching solution is 0~10 .5 wt%.

In the above method for manufacturing a silicon wafer for a solar cell, when the slice is a polycrystalline silicon wafer obtained by slicing a resin bonded wire saw, a low reflectance and uneven gloss are particularly obtained. Less solar cells are used for wafers.

Further, according to the method for producing a silicon wafer for a solar cell, the following silicon wafer for solar cells can be obtained, and the silicon wafer for solar cells has a surface over the entire surface. a plurality of concave and convex holes formed in a substantially bowl bottom shape, wherein the opening diameter of the concave hole is 2 to 15 μm, and one or a plurality of opening diameters are 0.1 to 1.5 μm on the inner wall of each of the concave holes. Micropores.

Further, a part of the gist of the present invention is an etching liquid for etching a slice of polycrystalline silicon obtained by slicing a wire saw with a fixed abrasive grain method, and a mixed acid containing a main component, wherein the mixed acid is represented by a chemical formula HF Hydrofluoric acid, nitric acid represented by the chemical formula HNO ₃ and sulfuric acid represented by the chemical formula H ₂ SO ₄ , wherein the composition of the mixed acid is represented by a triangle in which the composition is represented by weight %, and the following four are sequentially connected In the region surrounded by the four line segments, that is, point A: the hydrofluoric acid is 2.82% by weight, the nitric acid is 0.18% by weight, the sulfuric acid is 97% by weight, and the point B: the hydrofluoric acid is 0.18% by weight. The nitric acid was 2.82% by weight, the sulfuric acid was 97% by weight, the C point was 8.47% by weight of the hydrofluoric acid, the nitric acid was 0.53% by weight, the sulfuric acid was 91% by weight, and the D point: the hydrofluoric acid was 0.53. The weight %, the nitric acid was 8.47% by weight, the sulfuric acid was 91% by weight, and the concentration of the etching liquid was 0 to 10.5% by weight.

The temperature of the etching liquid at the time of the above etching is 0 to 45 °C.

In the above etching liquid, the saw wire used in the wire saw of the fixed abrasive grain type may be a resin adhesive saw wire.

According to the present invention, it is possible to provide a tantalum wafer for a solar cell having a low reflectance and a low gloss unevenness, and a method for producing the same, which is obtained by slicing a polycrystalline tantalum ingot by a fixed abrasive grain method. And get.

2‧‧‧ large recess

3‧‧‧ small recess

D1‧‧‧The upper edge of the large recess

d2‧‧‧The path of the upper edge of the small recess

h1‧‧‧Deep depth

h2‧‧‧Deep depth

P‧‧‧ spaced apart from each other

Fig. 1 is a triangular view showing the composition of an etching solution used in the present invention.

2(a) to (c) are photographs showing the surface of a silicon wafer for a solar cell.

3(a) to 3(c) are explanatory views of the uneven state of the surface of the tantalum wafer for solar cells of the present invention.

4(a) to 4(d) are photographs showing the gloss state of the surface of the silicon wafer for solar cells.

The tantalum wafer for a solar cell of the present invention is obtained by etching a slice of a polycrystalline tantalum ingot by etching with an etching solution to fix the abrasive grains.

The fixed abrasive grain method is a method in which the ingot is sliced on the surface of the core wire and then the saw wire to which the abrasive grain is fixed, and as the surface of the core wire followed by the fixed abrasive grain, a molten metal method, a plating method, and a resin adhesive method are exemplified. .

The molten metal method is a method in which the surface of the core wire is fixed to the surface of the core wire via a low-melting-point metal such as a solder alloy, and the method described in JP-A-2010-201602 can be exemplified.

In the plating method, a plating layer is formed on the surface of the core wire by using a plating solution in which the abrasive grains are mixed, and the abrasive grains are fixed to the surface of the core wire via the plating layer, and the method described in Japanese Patent Laid-Open Publication No. 2003-340729 can be exemplified. .

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

As the core wire for fixing the abrasive grain method, a steel wire is preferably used. The wire diameter is not particularly limited, but is preferably 0.3 to 0.05 mm. The steel wire can be exemplified by a wire obtained by heat-treating spring steel such as high carbon steel or medium carbon low alloy steel; a hard steel wire, a piano wire or a stainless steel wire, a cold rolled steel wire or an oil quenched tempered wire (oil quenched) And tempered wire) and other wires obtained by processing spring steel; low alloy steel, medium alloy steel Or high-alloy steel, weathering steel (maraging steel) and other high toughness, high fatigue strength steel wire.

The abrasive grains used in the fixed abrasive grain method are not particularly limited, and examples thereof include diamond abrasive grains, cubic BN abrasive grains, alumina abrasive grains, and barium carbide abrasive grains. The diamond abrasive particles can be coated with nickel. Among them, it is preferred to use diamond abrasive grains having a particle diameter of 5 to 15 μm.

In the present invention, a tantalum wafer for a solar cell can be produced by etching a polycrystalline tantalum ingot by etching with an etching solution to fix the abrasive grains.

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

Further, the etching liquid may contain an organic acid such as an aliphatic carboxylic acid, an aliphatic sulfonic acid or an aliphatic phosphoric acid, or an oxidizing agent such as perchloric acid, perchloric acid, perchromic acid or perchromate as an auxiliary agent. Further, it may include 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.

In the etching liquid used in the present invention, the mixing ratio of various acids including the mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid with respect to the total weight of the acids is as follows in the triangular diagram shown in FIG. Within the range enclosed by the line segment: A (HF: 2.82% by weight, HNO ₃ : 0.18% by weight, H ₂ SO ₄ : 97% by weight), B (HF: 0.18% by weight, HNO ₃ : 2.82% by weight, H ₂ SO ₄ : 97% by weight), 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).

The water concentration of the etching liquid (the weight concentration in the etching liquid containing all the components including water, that is, the water content ratio) is 0 to 10.5% by weight. When the water concentration is higher than this range, it is difficult to form small pits (small recesses), the reflectance is not sufficiently lowered, and the difference in crystal grain contrast of the wafer becomes remarkable. In terms of the stability of the etching step, it is more preferred that the etching solution has a water concentration of 10% by weight or less.

The etching is performed by immersing a slice obtained by slicing a polycrystalline bismuth in a fixed abrasive grain in an etching solution, and thereafter, the slice is washed with water. Preferably, the temperature of the liquid in the immersion is 0 to 45 ° C, and the time is 1 to 30 minutes. When the liquid temperature is lower than the above range, the progress of etching is insufficient, and the time taken to form the unevenness is excessive. When the liquid temperature is outside the range, it is difficult to form irregularities, particularly small pits, and the reflectance is not sufficiently lowered, and the crystal grain contrast difference is remarkable, and the contrast is poor in gloss, thereby reducing the commercial value. Therefore, it is further preferred that the liquid temperature is 5 to 40 °C.

In the etching solution, if the composition ratio of hydrofluoric acid, nitric acid, and sulfuric acid is outside the range enclosed by the line segment connecting the point ABCD in the triangular diagram shown in FIG. 1, the etching rate becomes too slow, and etching is performed. The progress of the reaction is not smooth, so it takes too much time to form the unevenness. Alternatively, it is difficult to form small pits, the reflectance is not sufficiently lowered, and the crystal grain contrast difference becomes remarkable.

Further, in the etching solution, the concentration of hydrofluoric acid / (concentration of hydrofluoric acid + concentration of nitric acid) is preferably 0.059 to 0.94. If it is outside this range, there is a case where the etching rate is lowered, and it is difficult to form small pits, the reflectance is increased, and the crystal grain contrast difference is remarkable.

Further, in the etching solution, the sulfuric acid concentration is 91 to 97% by weight. When the sulfuric acid concentration is less than this range, the etching rate becomes too slow, and the etching reaction progresses unsuccessfully, so that it takes too much time to form the unevenness. If it is out of the range, it is difficult to form small pits, the reflectance is not sufficiently lowered, and the difference in crystal grain contrast becomes remarkable.

Further, in order to obtain a low-reflectance and a low-gloss contrast of crystal grains, the gloss of the solar cell is small, and it is preferable that the etching liquid used in the present invention contains hydrofluoric acid. The blending ratio of the various acids of the mixed acid of nitric acid and sulfuric acid with respect to the total weight of the acids is in the range enclosed by the line connecting the following points in the triangular diagram shown in Fig. 1: A' (HF: 2.62% by weight, HNO ₃ : 0.88% by weight, H ₂ SO ₄ : 96.5 wt%), B' (HF: 0.88% by weight, HNO ₃ : 2.62% by weight, H ₂ SO ₄ : 96.5 % by weight), C' ( HF: 6.75 wt%, HNO ₃ : 2.25 wt%, H ₂ SO ₄ : 91 wt%), D' (HF: 2.25% by weight, HNO ₃ : 6.75 wt%, H ₂ SO ₄ : 91 wt%).

Further, in terms of obtaining a silicon wafer for a solar cell having low reflectance and low gloss unevenness, it is preferable that the etching liquid used in the present invention contains a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid. The blending ratio of the various acids with respect to the total weight of the acids is in the range enclosed by the line connecting the following points in the triangular diagram shown in Fig. 1: A" (HF: 1.98 wt%, HNO ₃ : 1.52) % by weight, H ₂ SO ₄ : 96.5 wt%), B" (HF: 1.44% by weight, HNO ₃ : 2.06 wt%, H ₂ SO ₄ : 96.5 wt%), C" (HF: 5.09 wt%, HNO ₃ : 3.91% by weight, H ₂ SO ₄ : 91% by weight), D" (HF: 3.71% by weight, HNO ₃ : 5.29% by weight, H ₂ SO ₄ : 91% by weight).

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

If the polycrystalline bismuth ingot is sliced by the etchant etching of such a compounding ratio in a fixed abrasive grain, as shown in FIG. 2(a), the processed metamorphic layer is removed and exposed on the surface of the slice. Crystallized grains, thereby forming irregularities on the surface of the slice, and further etching the surface to form a concave shape of the bowl bottom shape, and forming a concave surface on the inner surface of the concave portion of the bowl bottom shape due to the concave and convex portions of the concave portion Bump. Thereby, a silicon wafer for solar cells having a small reflectance of the surface can be obtained. Further, a silicon wafer for solar cells having less uneven gloss can be obtained.

2(a) is a scanning electron micrograph of the uneven state of the surface of the tantalum wafer for solar cells obtained according to the present invention, and FIG. 2(b) is a mixed etching solution containing only hydrofluoric acid and nitric acid. Scanning electron display for obscuring the surface of a silicon wafer for solar cell obtained by slicing a polycrystalline germanium ingot by a free abrasive grain method Microscopic photo.

The tantalum wafer for a solar cell obtained by the present invention shown in Fig. 2(a) has irregularities formed by forming a plurality of concave holes having a curved bottom portion over the entire surface. The opening diameter of the recessed hole is 2 to 15 μm, and one or a plurality of micropores having an opening diameter of 0.1 to 1.5 μm are formed on the inner wall of the recessed hole. That is, the tantalum wafer for a solar cell of the present invention has substantially the following shape: a surface having a plurality of concave holes formed over the entire surface, the opening diameter of the recessed hole being 2 to 15 μm, and the recessed hole The inner wall is formed with one or a plurality of micropores having an opening diameter of 0.1 to 1.5 μm. The properties of such irregularities can be confirmed by a scanning confocal laser microscope or a shape measuring device described in Japanese Patent No. 3810749.

3 is a schematic view showing a state of unevenness on the surface of a silicon wafer for solar cells in a cut surface in the thickness direction of the wafer, and FIG. 3(a) is a wafer for solar cell shown in FIG. 2(b). Fig. 3(b) shows the uneven state of the surface of the silicon wafer for solar cell shown in Fig. 2(a), and Fig. 3(c) is a partially enlarged schematic view of Fig. 3(b). As shown in FIG. 3, when the ruthenium wafer for a solar cell of the present invention is enlarged, it has irregularities in the following state: a recess having a large shape due to a substantially bowl-shaped recess (a recessed hole having a substantially bowl bottom shape) 2 The unevenness of the large cycle (a shape in which the shape of the bottom of the bowl or the shape of the substantially bottom of the bowl overlaps) is superposed on the small period of the small recess (microporous) 3 caused by the etching of the etching liquid. The diameter d1 of the upper edge of the large recess 2 is 2-15 μm, the depth h1 is 2-15 μm, the diameter d2 of the upper edge of the small recess 3 is 0.1-1.5 μm, and the depth h2 is 0.1-1.5 μm. Further, the interval p between the mutually adjacent large recesses 2 is 0 to 10 μm. Further, a plurality of small recesses are present on the inner surface of one large recess 2. On the other hand, in the etching shown in the micrograph of Fig. 2(b), the surface of the wafer for solar cell obtained by slicing the polycrystalline germanium in the form of free abrasive grains is obtained, and the inner surface of the large recess 2 is almost There is no small recess corresponding to the small recess 3 in Fig. 3(b). Further, the reflectance was 30%, and a surface state which was sufficiently lower than that of Fig. 2(a) using the etching liquid of the present invention was not formed.

Furthermore, the term "over the entire surface" refers to the upper edge of the concave hole adjacent to the concave hole. The interval between the portions, that is, the p is 0 to 10 μm, exists on the surface of the wafer. Further, the adjacent recessed holes can enter each other and overlap.

The large recess 2 is similar in shape to the recess of the bowl, and has a straight line that is vertically erected from the deepest portion of the recess toward the surface of the wafer as a center line of symmetry, or from the deepest portion of the recess to the wafer. The plane in which the plane direction is vertically erected serves as a substantially symmetrical shape of the plane of symmetry. It is shown that a relatively thin process-affected layer of about 5 μm is removed by etching and further etched in the thickness direction of the wafer to form a recess of such a symmetrical shape. With such a symmetrical shape recess, it is difficult for the wafer surface to generate irregular reflected light, and a wafer with less uneven gloss can be obtained. That is, the etching system using the etching liquid of the present invention can be etched regardless of the presence or absence of the work-affected layer. Therefore, a recess of the shape of the bowl bottom having a shape symmetrical in the thickness direction is formed on the surface of the wafer.

The etching liquid of the present invention can newly form the etching liquid of the large recess 2 and the small recess 3 together on the crucible after the processing of the deteriorated layer. In other words, the etching liquid of the present invention can form the etching liquid of the large recess 2 and the small recess 3 together on the wafer surface regardless of the presence or absence of the processing and modifying layer.

On the other hand, since the slice obtained by slicing from the polycrystalline bismuth in the form of free abrasive grains is thick, the unevenness of the crystal grains is generated on the surface by removing the work-affected layer by etching. The removal of the processing metamorphic layer can be carried out relatively easily using an etching solution previously using hydrofluoric acid or nitric acid. However, the recess obtained in this manner is caused by the crystal orientation of the crystal grains, and thus has an irregular shape and a shape asymmetrical in the thickness direction of the wafer. Therefore, irregularities are easily generated on the surface of the wafer, and a wafer having uneven gloss is obtained. In this way, the previous etching liquid is formed in the process of removing the processed metamorphic layer, and the unevenness is formed on the crucible after removing the modified layer. Further, in the case of the conventional etching liquid, it is difficult to perform etching to the extent that the small recesses 3 are formed on the crucible after the processing of the altered layer.

Further, there has been disclosed a method in which a chemical etching is performed in an acidic etching solution containing hydrofluoric acid, nitric acid, phosphoric acid or the like as a main component, even if polycrystalline germanium is inconsistent with respect to crystal faces In the wafer, a work-affected layer having a thickness of 10 μm or more can be removed by etching to obtain irregularities (for example, Japanese Patent Laid-Open No. Hei 10-303443, Japanese Patent No. 4766880, and the like).

It is considered to be due to the following reaction.

HNO ₃ +H ₂ O+HNO ₂ →2HNO ₂ +2OH ^- +2h ⁺ (hole)

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 the nitric acid oxidized nitric acid to the hydrofluoric acid which dissolves the cerium oxide oxide, the reaction rate may be changed, or the shape or size of the concavities and convexities may also vary, but in order to achieve more stable control, it is preferred to Sulfuric acid is added internally. The reason is as follows.

That is, the oxidation reaction of hydrazine is slightly reacted as described above even with concentrated nitric acid. However, when concentrated sulfuric acid is added, an acid-base reaction in which sulfuric acid is an acid and nitric acid is an alkali is generated. As a result, "-O-SO ₂ -OH" anion and "H ₂ O(+)-NO ₂ " cation are formed, and water is desorbed from "H ₂ O(+)-NO ₂ "cation, "(+)NO ₂ cations (nitrates and cations) increase in the system. That is, in the following reaction formula, the equilibrium shifts to the right when the concentrated sulfuric acid is not added, the reaction becomes faster, and the uneven shape of the surface of the ruthenium sheet is controlled.

In view of this phenomenon, in the present invention, the concentration of an acid, particularly sulfuric acid, which is used to form a good uneven shape on the surface of the crepe sheet has been found.

[Examples]

The embodiments of the present invention are described below, but the present invention is not limited to the embodiments.

The reflectance of the tantalum wafer in the examples and the comparative examples 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 the nine positional measurements of the value of the wavelength of 600 nm was determined as the reflectance.

Example 1 <矽晶锭>

Polycrystalline germanium ingots manufactured by GET Corporation were used.

<saw line>

A resin adhesive saw wire manufactured by TKX Co., Ltd. (product number: MW-100-8-16) was used.

(Binder composition for resin adhesive saw wire... phenol resin composition

Abrasive grain...Diamond abrasive grain: Grinding stone diameter 8-16μm (10.5μm±1μm)

line... 100μm steel wire)

<Slice step>

The twin ingot was attached to a pulley which was wound around a groove on the circumferential surface to form a loop, and a cutting device was formed to form a loop, and was moved at a speed of 600 m/min to obtain a slice.

<etching solution>

Hydrofluoric acid aqueous solution (concentration: 47% by weight)...6.2% by weight

Aqueous nitric acid solution (concentration 67 wt%)...5.0% by weight

95% by weight of sulfuric acid...88.8 wt%

This composition is indicated by the point of Example 1 in Fig. 1.

<etching>

The chips were immersed in an etching solution at 10 ° C for 20 minutes and then washed with water to obtain a tantalum wafer.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 18.6%. D1 is 3 to 12 μm (average 5.4 μm), and d2 is 0.1 to 1 μm. As shown in the photograph of the surface state shown in Fig. 4 (a), uneven gloss is hardly observed on the surface of the wafer. Further, Fig. 2(a) is a 顕 micromirror photograph of the uneven state of the surface of the wafer of Example 1.

Example 2

The composition of the etching solution was set to: aqueous hydrofluoric acid solution (concentration: 47 wt%), 6.3 wt%, aqueous solution of nitric acid (concentration: 67 wt%), 4.2 wt%, sulfuric acid having a concentration of 95 wt%, 89.5% by weight, Except for this, a tantalum wafer was obtained in the same manner as in Example 1.

This composition is represented by the point of Example 2 in Fig. 1.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 18.0%. D1 is 3 to 10 μm (average 4.8 μm), and d2 is 0.1 to 1 μm. It can be said that the surface of the wafer is completely invisible.

Example 3

The composition of the etching solution was set to: aqueous hydrofluoric acid solution (concentration: 47 wt%), 6.6 wt%, aqueous solution of nitric acid (concentration: 67 wt%), 3.4 wt%, sulfuric acid having a concentration of 95 wt%, 90 wt%, Except for this, a tantalum wafer was obtained in the same manner as in Example 1.

This composition is represented by the point of Example 3 in Fig. 1.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 20.8%. D1 is 2 to 8 μm (average 3.9 μm), and d2 is 0.1 to 1 μm. It can be said that the surface of the wafer is completely invisible.

Example 4

The composition of the etching solution is set to: hydrofluoric acid aqueous solution (concentration: 47% by weight), 6% by weight, aqueous solution of nitric acid (concentration: 67% by weight), 6% by weight, sulfuric acid having a concentration of 95% by weight, 88% by weight, Except for this, a tantalum wafer was obtained in the same manner as in Example 1.

This composition is represented by the point of Example 4 in Fig. 1.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 21.0%. D1 is 3 to 10 μm (average 3.9 μm), and d2 is 0.1 to 0.5 μm. A few gloss unevenness can be seen on the surface of the wafer, but it does not reach the extent of the loss of the value of the commodity.

Example 5

A tantalum wafer was obtained in the same manner as in Example 1 except that a saw wire of a molten metal type obtained by the method described in Example 1 of Japanese Patent Laid-Open No. 2010-201602 was used as the saw wire.

<Method of manufacturing saw wire>

The metal core wire of the wire is set to be covered by brass. 100μm piano line.

Sn-3.0% Ag-0.5% Cu (solid phase line: 218 ° C, liquidus: 220 ° C) was used as a feed. 0.2% of aluminum (Al) powder was added thereto and melted.

A powder of nickel-coated diamond was used as the abrasive grain 2. The particle size of the abrasive grains is 20 to 35 μm. It is kneaded with the above-mentioned pigment powder and diamond powder at a ratio of 70 to 30 (% by weight) with an organic amine-based reactive rosin flux, and the viscosity is adjusted to 300 Pa.s by rosinol. This is filled as a paste into the dispenser (syringe).

Then, using a dispenser having a nozzle diameter of 100 μm, the solder paste was uniformly applied to the piano core material at a film thickness of 22 to 20 μm. It is melted by laser light having an irradiation power of 1 W, a beam diameter of 600 to 1300 μm, and a wavelength of 808 nm, and then naturally cooled.

While determining the molten state, the ratio of the diamond to the crucible is set such that the thickness of the molten solidified layer is controlled to be 5 to 40% of the particle diameter of the abrasive grains 2.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 22.0%. D1 is 3 to 14 μm (average 4.0 μm), and d2 is 0.1 to 0.5 μm. A few gloss unevenness can be seen on the surface of the wafer, but it does not reach the extent of the loss of the value of the commodity.

Example 6

The composition of the etching solution was set to: hydrofluoric acid aqueous solution (concentration: 47% by weight), 4.2% by weight, aqueous solution of nitric acid (concentration: 69% by weight), 7.8% by weight, sulfuric acid having a concentration of 95% by weight, 88.0% by weight, The chips were immersed in an etching solution at 25 ° C for 142 seconds, and then washed with water to obtain a tantalum wafer. Other than this, it is the same as that of the first embodiment.

This composition is indicated by the point of Example 6 in Fig. 1.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 23.0%. D1 is 2 to 12 μm (average 4.1 μm), and d2 is 0.4 to 1.0 μm. A few gloss unevenness can be seen on the surface of the wafer, but it does not reach the extent of the loss of the value of the commodity.

Example 7

The composition of the etching solution was set to: aqueous hydrofluoric acid solution (concentration: 47 wt%), 4.6 wt%, aqueous solution of nitric acid (concentration: 69 wt%), 6.9 wt%, sulfuric acid having a concentration of 95 wt%, 88.5 wt%, The chips were immersed in an etching solution at 25 ° C for 142 seconds, and then washed with water to obtain a tantalum wafer. Other than this, it is the same as that of the first embodiment.

This composition is represented by the point of Example 7 in Fig. 1.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 19.4%. D1 is 3 to 8 μm (average 4.8 μm), and d2 is 0.4 to 1.1 μm. Some gloss unevenness can be seen on the surface of the wafer, but not To the extent that it is detrimental to the value of the goods.

Example 8

Set the composition of the etchant to: Hydrofluoric acid aqueous solution (concentration: 47% by weight)...4.95% by weight, aqueous solution of nitric acid (concentration: 69% by weight), 6.05% by weight, sulfuric acid having a concentration of 95% by weight, 89.0% by weight, and an etching solution slicing at 25 ° C After immersing for 142 seconds, it was washed with water to obtain a tantalum wafer. Other than this, it is the same as that of the first embodiment.

This composition is represented by the point of Example 8 in Fig. 1.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 19.4%. D1 is 3 to 10 μm (average 6.5 μm), and d2 is 0.5 to 1.3 μm. A few gloss unevenness can be seen on the surface of the wafer, but it does not reach the extent of the loss of the value of the commodity.

Example 9

The composition of the etching solution was set to: aqueous hydrofluoric acid solution (concentration: 47% by weight), 5.25% by weight, aqueous solution of nitric acid (concentration: 69% by weight), 5.25% by weight, sulfuric acid having a concentration of 95% by weight, 89.5% by weight, The chips were immersed in an etching solution at 25 ° C for 142 seconds, and then washed with water to obtain a tantalum wafer. Other than this, it is the same as that of the first embodiment.

This composition is indicated by the point of Example 9 in Fig. 1.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 19.8%. D1 is 3 to 11 μm (average 5.6 μm), and d2 is 0.5 to 1.1 μm. The surface of the wafer is substantially dull and uneven.

Comparative example 1

Set the composition of the etchant to: Hydrofluoric acid aqueous solution (concentration: 47% by weight), 4.5% by weight, aqueous solution of nitric acid (concentration: 67% by weight), 9% by weight, sulfuric acid having a concentration of 95% by weight, of 8.65% by weight, and other examples. 1 obtain the germanium wafer in the same way.

This composition is represented by the point of Comparative Example 1 in Fig. 1.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 26.1%. D1 is 3 to 15 μm (average 5.1 μm), and d2 is 0.1 to 1 μm. On the surface of the wafer, the unevenness of the gloss of the value of the product can be seen. Figure 4(b) is a photograph showing the gloss state of the obtained germanium wafer.

Comparative example 2

The composition of the etching solution was set to: hydrofluoric acid aqueous solution (concentration: 47% by weight), 3.6% by weight, aqueous solution of nitric acid (concentration: 67% by weight), 16% by weight, sulfuric acid having a concentration of 95% by weight, 80.4% by weight, Except for this, a tantalum wafer was obtained in the same manner as in Example 1.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 27%. On the surface of the wafer, the unevenness of the gloss of the value of the product can be seen.

Comparative example 3

The composition of the etching solution was set to: aqueous solution of hydrofluoric acid (concentration: 47% by weight), 6% by weight, aqueous solution of nitric acid (concentration: 67% by weight), 29% by weight, sulfuric acid having a concentration of 95% by weight, ...5% by weight, Except for this, a tantalum wafer was obtained in the same manner as in Example 1.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was 30%. On the surface of the wafer, you can see that there is a serious The degree of loss of the value of the product is uneven.

Comparative example 4 <Slice step>

The same ingot as the one used in Example 1 was sliced using a multi-wire saw method of a free abrasive grain method.

Wire diameter: 0.1mm (manufactured by JFE Steel, model SRH)

Abrasive grain: bismuth carbide

(Manufactured by Fujimi Incorporated, GC#1500, average particle size is about 8μm)

Cutting speed: 0.35mm/min (ingot feed rate)

Line moving speed: 600m/min

<etching>

Set the composition of the etchant to: Hydrofluoric acid aqueous solution (concentration: 47% by weight)...25% by weight, An aqueous solution of nitric acid (concentration: 67% by weight)...45% by weight, Water... 30% by weight, Except for this, a tantalum wafer was obtained by the same etching as in Example 1. Among them, Eclipse The engraving was carried out at 10 ° C for 2 minutes.

<Characteristics of wafers>

The average d1 is 10 μm, only the large recess 2 can be seen, and the small recess 3 is not visible. Therefore, the reflectance of the obtained germanium wafer is 30.8%, which is still high. Fig. 4(c) is a photograph showing the gloss state of the germanium wafer.

Comparative Example 5

The composition of the etching solution was set to: hydrofluoric acid aqueous solution (concentration: 47% by weight), 25% by weight, aqueous nitric acid solution (concentration: 67% by weight), 45% by weight, water, 30% by weight, Except for this, a tantalum wafer was obtained in the same manner as in Example 1. Among them, the etching was carried out under the conditions of 10 ° C × 2 minutes.

<Characteristics of wafers>

The reflectance of the obtained germanium wafer was a high value of 32.5%. Fig. 2(c) is a photographic micrograph of the surface of the ruthenium wafer in a concave-convex state, and Fig. 4(d) is a photograph showing the gloss state of the ruthenium wafer. D1 is 3 to 15 μm (average 5.5 μm), but the small recess 3 is not visible. Moreover, on the surface of the wafer, the unevenness of the gloss of the value of the product can be seen.

The reflectance of the obtained germanium wafer was 32.5%.

[Industrial availability]

The present invention is a useful technique widely applicable to the manufacture of tantalum wafers or other photoelectric conversion elements for solar cells.

Claims (4)

A method for producing a tantalum wafer for a solar cell, which is a method for producing a tantalum wafer for a solar cell in which a slice of a polycrystalline silicon is etched by using an etching liquid containing a mixed acid as a main component, and the above-mentioned slicing is performed by a fixed abrasive grain method. a slice obtained by slicing a wire saw, wherein the mixed acid comprises hydrofluoric acid represented by chemical formula HF, nitric acid represented by chemical formula HNO ₃ and sulfuric acid represented by chemical formula H ₂ SO ₄ , and the composition range of said mixed acid is expressed by weight % In the triangular diagram of the composition, in the region surrounded by four line segments which are sequentially connected to the following four points, that is, point A: 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: 0.18% by weight of the hydrofluoric acid, 2.82% by weight of the above nitric acid, 97% by weight of the sulfuric acid, C point: 8.47% by weight of the hydrofluoric acid, and 0.53% by weight of the above nitric acid, The sulfuric acid was 91% by weight, and the D point: the hydrofluoric acid was 0.53% by weight, the nitric acid was 8.47% by weight, the sulfuric acid was 91% by weight, and the water concentration of the etching liquid was 0 to 10.5% by weight. The method of manufacturing a silicon wafer for a solar cell according to claim 1, wherein the saw wire used in the wire saw of the fixed abrasive grain method is a resin adhesive saw wire. The method for producing a silicon wafer for a solar cell according to claim 1 or 2, wherein the temperature of the etching solution during the etching is 0 to 45 °C. A silicon wafer for a solar cell, which is obtained by etching a etchant solution according to any one of claims 1 to 3 by using a wire saw with a fixed abrasive grain method. And the surface has a concave and convex surface formed by forming a plurality of concave holes having a substantially bottom shape over the entire surface, and the opening diameter of the concave hole is 2 to 15 μm, and the inner wall of each of the concave holes is formed with 1 Or a plurality of micropores having an opening diameter of 0.1 to 1.5 μm.