JP2014232829A - Method for manufacturing silicon wafer for solar batteries - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a silicon wafer for solar batteries which uses, as a silicon wafer, a polycrystalline silicon wafer obtained by slicing a polycrystalline silicon ingot according to a fixed abrasive-grain-wire-saw method, and adopts a wet etching as an etching method, and is arranged so that a surface of the wafer can be sufficiently roughened without the need for a pretreatment such as a sand blast treatment.SOLUTION: A method for manufacturing a silicon wafer for solar batteries comprises the step of isotropically etching a surface of a polycrystalline silicon wafer with an etchant, provided that has a three dimensional surface roughness of 1.0-1.3, which is measured by a laser-microscope. The etchant includes 55-85 wt.% of a sulfuric acid, 4-21 wt.% of a nitric acid, 2-10 wt.% of a hydrofluoric acid, and 2-14 wt.% of water (to a total of 100 wt.% of the etchant); and the weight ratio of the water to the sulfuric acid is 0.20 or less.

Description

  The present invention relates to a method for producing a silicon wafer for solar cells.

  Polycrystalline silicon wafers having various crystal plane orientations within the wafer are less expensive than single crystal silicon wafers, and are expected to be used as large-area solar cell substrates.

  By the way, the roughening of a single crystal silicon wafer (formation of a surface texture structure) can form a pyramid shape by anisotropic etching. An isotropic etching method has been proposed (Patent Document 1). However, since the polycrystalline silicon wafer cannot be anisotropically etched unlike the single crystal silicon wafer, the surface is roughened by forming a fine hole shape by isotropic etching. Conventionally, many proposals for isotropic etching by plasma via a pattern film have been made (Patent Documents 2 to 5). However, such a method is inferior in operability as compared with the previous wet etching.

  Furthermore, as a cost reduction of the silicon solar cell, an attempt has been made to change the slicing method of the polycrystalline silicon ingot from the free abrasive wire saw method to the fixed abrasive wire saw method with less processing time. However, the surface of the polycrystalline silicon wafer adopting the fixed abrasive wire saw method is relatively smooth with few cut marks. Therefore, it is difficult to sufficiently roughen the surface by wet etching. Therefore, a method of performing wet etching after the surface of the polycrystalline silicon wafer is sandblasted is also being studied.

  Incidentally, the surface roughness of the polycrystalline silicon wafer adopting the fixed abrasive wire saw method is 1.0 to 1.3 in terms of the three-dimensional surface roughness (three-dimensional surface roughness) described later. On the other hand, the surface roughness of the polycrystalline silicon wafer adopting the free abrasive wire saw method is 1.5 to 1.9 in terms of the above-mentioned three-dimensional surface roughness (three-dimensional surface roughness).

JP-A-8-124894 JP 2005-252210 A JP 2008-198269 A JP 2011-077359 A JP 2011-077370 A

  The present invention has been made in view of the above circumstances, and its purpose is to use a polycrystalline silicon wafer obtained by slicing a polycrystalline silicon ingot by a fixed abrasive wire saw method as a silicon wafer, and wet as an etching method. An object of the present invention is to provide a method for producing a silicon wafer for solar cells, which employs etching and does not require a pretreatment such as a sandblasting process and can sufficiently roughen the surface of the wafer.

  As a result of intensive studies, the present inventors have found that the above object can be easily achieved by using a mixed acid adjusted to a predetermined concentration using sulfuric acid, nitric acid and hydrofluoric acid as an etchant. Obtained knowledge.

  The present invention has been completed based on the above findings, and the gist thereof is the surface of a polycrystalline silicon wafer having a three-dimensional surface roughness of 1.0 to 1.3 measured using a laser microscope. The sulfuric acid concentration is 55 to 85% by weight, the nitric acid concentration is 4 to 21% by weight, the hydrofluoric acid concentration is 2 to 10% by weight, and the water concentration is 2 to 14% by weight (the total amount is 100% by weight). In the method of manufacturing a solar cell substrate, isotropic etching is performed with an etchant having a water / sulfuric acid weight ratio of 0.20 or less.

  According to the present invention, the aforementioned problems are achieved.

  Hereinafter, the present invention will be described in detail.

  In the present invention, a polycrystalline silicon wafer obtained by slicing a polycrystalline silicon ingot by a fixed abrasive wire saw method is used as the silicon wafer. Polycrystalline silicon has various crystal plane orientations in a wafer, that is, the direction of adjacent crystals is different and the arrangement of atoms as a whole is not constant like a single crystal. When sliced by a fixed abrasive wire saw method (for example, a method using a wire saw in which diamond abrasive grains are fixed to the core wire), the slice surface is relatively smooth with few cutting marks, as described above. It is characterized by a three-dimensional surface roughness measured using a scope of 1.0 to 1.3.

The above three-dimensional surface roughness was measured using a laser microscope (for example, “VK-9700” manufactured by KEYENCH) under the conditions of magnifying power: 3,000 times and observation field of view: 6,512 μm ^2. It is defined as the value obtained by measuring the surface area of the uneven surface of the wafer and dividing the value by the observation field.

  Laser microscope: “VK-9700” manufactured by KEYENCH employs a two-way light source system using a short-wavelength laser light source and a white light source. It is a device that can obtain the information of color, light quantity, and height necessary for constructing. In this apparatus, since the laser light source is a point light source, the observation field is divided into 1024 × 768 pixels and scanned through an XY scanning optical system, and the reflected light for each pixel is detected by a light receiving element. Then, the objective lens is driven in the Z-axis direction, and scanning is repeated to obtain the reflected light amount for each Z-axis position of each pixel. Accordingly, the unevenness can be measured in three dimensions, and the three-dimensional surface roughness of the uneven surface can be grasped. In addition, height information and the amount of reflected light can be detected with the Z-axis position having the highest amount of reflected light as a focal point. Thereby, an ultra-deep light amount image and a high / low image (information) focused on the whole are obtained.

In the present invention, as a three-dimensional measurement using the above-described apparatus, the surface area of an object is measured in an arbitrarily designated area (observation field of view: 6,512 μm ² ) on an image having a measurement magnification of 3,000 times. The value obtained by dividing the value by the observation field is defined as the three-dimensional surface roughness.

The polycrystalline silicon substrate may be a p-type polycrystalline silicon substrate or an n-type polycrystalline silicon substrate. In this case, the impurity contained in the silicon substrate is, for example, boron or aluminum in the case of p-type, and phosphorus, arsenic, antimony or the like in the case of n-type. When the polycrystalline silicon substrate is a p-type polycrystalline silicon substrate or an n-type polycrystalline silicon substrate, the concentration of impurities is not particularly limited, but is, for example, 10 ¹³ / cm ³ to 10 ²¹ / cm ³ .

  The thickness of the polycrystalline silicon substrate is not particularly limited, but is usually 100 to 300 μm. By setting it as 100 micrometers or more, a silicon substrate can have sufficient intensity | strength, and a solar cell etc. can be manufactured at low cost by setting it as 300 micrometers or less. The size of the polycrystalline silicon substrate is not particularly limited, but is, for example, 126 mm × 126 mm or 156 mm × 156 mm.

  In the present invention, a mixture of sulfuric acid, nitric acid and hydrofluoric acid is used as an etchant. As the raw material acid used for the preparation of the etchant, those having various concentrations can be used. Examples of the sulfuric acid raw material include dilute sulfuric acid, concentrated sulfuric acid, fuming sulfuric acid and the like. Concentrated sulfuric acid is 96-98% by weight sulfuric acid, and fuming sulfuric acid is concentrated sulfuric acid in which excess sulfur trioxide is absorbed. Examples of nitric acid include dilute nitric acid, concentrated nitric acid, and fuming nitric acid. Concentrated nitric acid is 70 to 98% by weight nitric acid, and fuming nitric acid is obtained by blowing gaseous nitrogen dioxide into concentrated nitric acid. As the raw material hydrofluoric acid, hydrogen fluoride gas (anhydrous hydrofluoric acid) can be used in addition to hydrofluoric acid.

  In the present invention, the composition of the etchant is adjusted as follows. That is, the sulfuric acid concentration is 55 to 85 wt%, preferably 60 to 80 wt%, the nitric acid concentration is 4 to 21 wt%, preferably 10 to 21 wt%, and the hydrofluoric acid concentration is 2 to 10 wt%, preferably 2 to 2 wt%. 5% by weight and water concentration is 2 to 14% by weight, preferably 7 to 14% by weight (however, the total amount is 100% by weight). It is also important that the water / sulfuric acid weight ratio is 0.20 or less.

When the sulfuric acid concentration is less than 55% by weight, the etching rate tends to be too slow, and when it exceeds 85% by weight, mixing of appropriate amounts of nitric acid, hydrofluoric acid and water is hindered. When the nitric acid concentration is less than 4% by weight, the etching rate is too slow, and when it exceeds 21% by weight, the etching rate tends to be too fast and difficult to control. The moisture concentration is particularly important. When the water concentration is less than 2% by weight, the etching rate is too slow. That is, in wet etching, the surface of the silicon wafer is oxidized by nitric acid, and etching proceeds due to the reaction between the generated SiO ₂ and HF. However, if the water content is too small, HF is not ionized and SiO ₂ is not removed. Does not progress. On the other hand, when the water concentration exceeds 14% by weight or when the weight ratio of water / sulfuric acid exceeds 0.20, the oxidizing power of nitric acid becomes low and etching becomes difficult.

  Incidentally, the wafer that has been sliced has a layer with mechanical damage (defects) remaining on the surface, but according to the wet etching using the above etchant, the damage layer is removed and the surface texture structure is formed. Can be performed in the same process. For this reason, in the present invention, it is not necessary to perform in advance the step of removing the damaged layer by etching with acid or alkali.

  The time required for etching is determined by the etchant composition and temperature, the thickness of the polycrystalline silicon substrate, and the desired post-etching thickness. For example, in the case of a dip method, it is usually about 0.1 to 10 minutes. In addition, the etching amount is optimally an etching amount that is about 1 to 20 μm on the wafer surface average on one side. The etching temperature (etchant temperature) is usually 0 to 30 ° C.

  EXAMPLES Next, although an Example is given and this invention is demonstrated further in detail, this invention is not restrict | limited to these at all. Note that 97 wt% sulfuric acid, 98 wt% nitric acid, and 50 wt% hydrofluoric acid were used as acids for preparing the etchant.

Examples 1-4 and Comparative Examples 1-2:
A P-type polycrystalline silicon wafer having a size of 156 mm × 156 mm ± 0.5 mm and a thickness of 200 μm ± 20 μm was etched with an etchant having the composition shown in Table 1, and the three-dimensional surface roughness of the resulting uneven surface was measured. The silicon wafer is obtained by slicing a polycrystalline silicon ingot by a fixed abrasive wire saw method, and the above-mentioned three-dimensional surface roughness is 1.1. The etching temperature was 25 ° C. and the etching time was 60 seconds. Table 1 shows the three-dimensional surface roughness measurement results.

(1) Measurement of three-dimensional surface roughness of uneven surface:
Laser microscope: Using “VK-9700” manufactured by KEYENCH, measuring the surface area of the concavo-convex surface of the polycrystalline silicon wafer under the conditions of the magnification at the time of measurement: 3,000 times and the viewing field: 6,512 μm ² , The value was divided by the observation field, and the three-dimensional surface roughness value was calculated.

Claims (2)

  The surface of a polycrystalline silicon wafer having a three-dimensional surface roughness of 1.0 to 1.3 measured by using a laser microscope, a sulfuric acid concentration of 55 to 85% by weight, a nitric acid concentration of 4 to 21% by weight, An etchant having a hydrofluoric acid concentration of 2 to 10% by weight, a water concentration of 2 to 14% by weight (the total amount of which is 100% by weight), and a water / sulfuric acid weight ratio of 0.20 or less. The manufacturing method of the board | substrate for solar cells characterized by performing etching.   The manufacturing method according to claim 1, wherein the three-dimensional surface roughness of the surface of the silicon wafer after isotropic etching is 1.8 to 4.0.