Abstract:
The present invention addresses the problem of providing a novel silicon substrate having a textured surface by dry-etching the surface of a silicon substrate having (111) orientation and thereby forming a texture thereon. The present invention provides a silicon substrate having (111) orientation, said silicon substrate having a textured surface that includes multiple protrusions which each comprise three slant faces and have heights of 100 to 8000 nm. This process for producing a silicon substrate includes: a step of preparing a silicon substrate having (111) orientation; and a step of blowing an etching gas onto the surface of the silicon substrate, said etching gas containing one or more gases selected from the group consisting of ClF3, XeF2, BrF3, BrF5 and NF3.

Description:
TECHNICAL FIELD 
     The present invention relates to a silicon substrate having a textured surface and a manufacturing method thereof. 
     BACKGROUND ART 
     In a silicon solar cell (photovoltaic device), unevenness referred to as a texture is provided on a light-receiving surface of a silicon substrate so as to prevent reflection of incident light and leakage of light trapped on the silicon substrate. A surface of a silicon substrate is usually textured by a wet process using an alkaline (KOH) solution as an etchant (see PTL 1, PTL 2, PTL 3, and PTL 4). The texturing by a wet process requires post-processes such as a cleaning process using hydrogen fluoride and a thermal process. Not only may the post-processes contaminate a surface of a silicon substrate, but they also have a disadvantage with regard to increased cost. 
     In addition, the wet process only allows texturing a silicon substrate having orientation (100) (see PTL 5, for example), and the wet process cannot form a texture on a surface of a silicon substrate having another orientation. 
     Techniques for forming a texture on a surface of a silicon substrate by a dry process have also been proposed. For example, 1) a technique using reactive ion etching by plasma (see PTL 6 and PTL 7), 2) a technique for etching by a photo-electrolysis reaction (PTL 8), and 3) a technique for etching a surface of a silicon substrate by introducing ClF 3  gas and others into a reaction chamber in which the silicon substrate is placed (see PTL 9, PTL 10, and PTL 11) have been proposed. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2011-515872 
         PTL 2: Japanese Patent Application Laid-Open No. 2008-172279 
         PTL 3: United States Patent Application Publication No. 2009/0280597 
         PTL 4: United States Patent Application Publication No. 2010/0193027 
         PTL 5: Japanese Patent Application Laid-Open No. 2000-150937 
         PTL 6: Japanese Patent Application Laid-Open No. 2002-164555 
         PTL 7: Japanese Patent Application Laid-Open No. 2000-12517 
         PTL 8: United States Patent Application Publication No. 2002/0104562 
         PTL 9: Japanese Patent Application Laid-Open No. 10-313128 
         PTL 10: Japanese Patent Application Laid-Open No. 2005-150614 
         PTL 11: United States Patent Application Publication No. 2005/0126627 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     As described above, by a wet process, a texture can only be formed on a silicon substrate having orientation (100). Accordingly, forming a texture on a silicon substrate having another orientation would result in an unconventional unique texture. 
     For example, the texture formed by a wet process has projections with the height of 10 to 20 μm. The thickness of the entire silicon substrate is naturally greater than the thickness of the projections. As a result, the thickness of the silicon substrate ends up being greater than the thickness required for a silicon substrate used for a solar cell. In contrast, forming a finer texture can reduce the thickness of the entire silicon substrate, increasing material efficiency of silicon. 
     As described above, a method for forming a texture on a surface of a silicon substrate by a dry process has been proposed. However, with this method, a desired structure of texture may not be achieved. One reason is that a gas such as ClF 3 , XeF 2 , BrF 3  and BrF 5  causes a reaction generating heat with a silicon substrate, increasing the temperature of the silicon substrate. Consequently, anisotropic etching may not be performed. Furthermore, since the composition of the etching gas is not optimized either, it was difficult to achieve a proper texture. 
     In view of the problems, it is an object of the present invention to provide a silicon substrate having a new textured surface by forming a texture on a surface of a silicon substrate having orientation (111) preferably by dry etching. It is another object of the present invention to provide a solar cell including the silicon substrate. 
     Solution to Problem 
     The present invention relates to a silicon substrate having a textured surface and a solar cell including the silicon substrate described as follows. 
     [1] A silicon substrate having orientation (111) and a textured surface on which a textured structure is formed, in which the textured surface has a plurality of projections each having three slant faces, and a height of the projections is in a range from 100 nm to 8 μm. 
     [2] The silicon substrate according to [1], in which the height of the projections is in a range from 100 nm to 5 μm. 
     [3] The silicon substrate according to [1] or [2], in which a side of a bottom surface of each of the projections has a length in a range from 100 nm to 8 μm. 
     [4] The silicon substrate according to any one of [1] to [3], in which a density of the projections on the textured surface is 10 to 1,000/100 μm 2 . 
     [5] The silicon substrate according to any one of [1] to [4], in which an absorbance of incident light having a wavelength of 0.5 μm to 10 μm on the textured surface of the silicon substrate is 80% or higher. 
     [6] A solar cell including the silicon substrate according to any one of [1] to [5], in which the textured surface is used as a light-receiving surface of the solar cell. 
     The present invention also relates to a method for manufacturing a silicon substrate having a textured surface described as follows. 
     [7] A method for manufacturing the silicon substrate according to [1], the method including: providing a silicon substrate having orientation (111); and blowing an etching gas on a surface of the silicon substrate, in which the etching gas contains (i) at least one gas selected from the group consisting of ClF 3 , XeF 2 , BrF 3 , BrF 5 , and NF 3 , and (ii) a gas containing an oxygen atom in a molecule thereof. 
     [8] The method according to [7], in which the etching gas further contains an inert gas. 
     [9] The method according to [7] or [8], in which a temperature of the silicon substrate is maintained at a temperature of 200° C. or lower. 
     [10] The method according to any one of [7] to [9], in which etching on the silicon substrate is performed under a reduced-pressure environment. 
     Advantageous Effects of Invention 
     The silicon substrate according to the present invention has a textured surface with a low reflectance. Accordingly, by setting the textured surface as a light-receiving surface, the silicon substrate can be suitably used as a silicon substrate for a solar cell. Furthermore, since the texture according to the present invention is a fine-structure, it is possible to reduce the thickness of the silicon substrate, increasing the material efficiency of the silicon substrate, which will increase flexibility in designing a device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A to 1F  are electron micrographs of a textured surface on a silicon substrate according to Examples; 
         FIGS. 2A and 2B  show an example in which a desired textured surface is not formed by dry-etching on a silicon substrate; 
         FIGS. 3A and 3B  show an example in which a desired textured surface is not formed by dry-etching on a silicon substrate; 
         FIG. 4  schematically illustrates a projection having three slant faces; 
         FIGS. 5A and 5B  illustrate an overview of a texturing apparatus used in Examples; 
         FIG. 6A  is a graph representing reflectance of a non-textured surface on a silicon substrate having orientation (111) (Reference Example) and a textured surface on a silicon substrate obtained in Example; 
         FIG. 6B  is a graph representing absorbance of a non-textured surface on a silicon substrate having orientation (111) (Reference Example) and a textured surface on a silicon substrate obtained in Example; 
         FIGS. 7A to 7C  show an assumption on reaction mechanism by which a textured structure is formed; 
         FIGS. 8A to 8D  show textured surfaces of a silicon substrate obtained in Example 2; and 
         FIGS. 9A to 9D  show textured surfaces of a silicon substrate obtained in Example 3. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     1. Silicon Substrate Having Textured Surface 
     A silicon substrate according to the present invention has a feature in a textured surface on the substrate. The textured surface refers to a surface of the substrate having the textured structure. 
     It is preferable that the silicon substrate is made of single-crystal silicon; the silicon substrate may be doped with a p-type impurity or an n-type impurity, or may be intrinsic silicon. The silicon substrate has orientation (111). A silicon substrate having orientation other than (111) is not likely to have the textured surface according to the present invention. 
     One feature of the silicon substrate having the textured surface according to the present invention is that the orientation of the silicon substrate is (111). Conventional wet etching allows a silicon substrate with orientation (100) to have a textured surface. However, the conventional wet etching cannot form the textured surface on the silicon substrate having the orientation (111), and such a substrate is isotropically etched instead. 
     The textured surface refers to a surface having a low reflectance. A low reflectance surface is a surface preferably having a reflectance of approximately 20% or less, more preferably 10% or less, and refers to a surface having a reflectance of substantially 0%, when a reflectance of a mirror surface for light having a wavelength from 0.5 to 1.0 μm is set as 100%. The silicon substrate having the textured surface according to the present invention preferably has an absorbance (a wavelength range of 0.5 to 1.0 μm) of 80% or higher, and more preferably 85% or higher. The absorbance can be measured by a sphere spectrophotometer, and can be calculated by the following equation; absorbance(%)=100×{intensity of incident light−(intensity of reflected light+intensity of transmitted light)}/intensity of incident light. 
     More specifically, the textured surface according to the present invention has projections each having three slant faces (see the schematic diagram in  FIG. 4  for details). The projection having the three slant faces may be a projection having the shape of a triangular pyramid. It is preferable that the textured surface has the projections with three slant faces closely formed with each other. Each of the three slant faces has orientation (110), (101), or (011), which is different from orientation (111). It is preferable that the projection has an apex. 
     The height H of the projection having three slant faces (see  FIG. 4 ) is usually 100 nm to 8 μm, preferably 100 nm to 3 μm, more preferably 100 nm to 1.5 μm, and even more preferably 100 nm to 1 μm. In addition, the length L of a side of the bottom surface of the projection having three slant faces (see  FIG. 4 ) is usually 100 nm to 8 μm, preferably 100 nm to 3 μm, more preferably 100 nm to 1.5 μm, and even more preferably 100 nm to 1 μm. 
     The angle θ of the apex of the projection having three slant faces (see FIG.  4 ) is preferably 54.7°±10°. 
     A feature of the silicon substrate having the textured surface according to the present invention is that the projections composing the texture are small. Projections in the textured structure formed by the conventional wet etching or ion plasma etching are much higher (for example, the height of a textured structure formed by wet etching is from 10 to 20 μm), and it was not possible to form miniaturized projections as in the present invention. The more miniaturized the structure of the texture, the more suppressed reflection of light on the textured surface. For example, if the processing accuracy of the textured surface is 1 μm or less, the reflection of light having the wavelength of 1 μm is set to be nearly zero. 
     Another feature of the silicon substrate having the textured surface according to the present invention is that the thickness of the silicon substrate can be reduced. More specifically, since the projections composing the textured structure are miniaturized, the thickness of the silicon substrate can be reduced as much as the reduced thickness of the projections. The thickness of the silicon substrate according to the present invention is 20 μm or less including the height of the projections, preferably 15 μm or less, and even more preferably 10 μm or less. The lower limit on the thickness of the silicon substrate is not particularly set as long as a sufficient strength as a substrate is maintained, and is usually 10 μm or more. 
     The textured surface has a plurality of projections having three slant faces, and the projections having three slant faces may each have a different shape. A preferable density of the projections on the textured surface is 10 to 1,000 per unit area (100 μm 2 ). 
     The surface of the silicon substrate may be entirely textured; alternatively, the silicon substrate may be partially textured. For example, when the silicon substrate according to the present invention is used as a silicon substrate for a solar cell, it is preferable that an area for a front surface electrode (including a connector electrode, a bar electrode, a grid electrode and others) provided on the light-receiving surface side preferably remains flat, without the textured structure. 
     2. Method for Manufacturing Silicon Substrate Having Textured Surface 
     The method for manufacturing the silicon substrate according to the present invention includes: providing a silicon substrate having orientation (111), and blowing an etching gas on the silicon substrate. Preferably, the manufacturing method further includes blowing a cooling gas on the silicon substrate, and blowing the etching gas and the cooling gas may be alternately repeated. 
     The silicon substrate having the orientation (111) is a single-crystal silicon substrate having a main surface with orientation (111). The silicon substrate may be a semiconductor wafer, or a semiconductor layer stacked on another substrate. In any case, the textured structure is formed on the surface having orientation (111), which is the orientation of the main surface. 
     The silicon substrate provided may be made of intrinsic silicon, or silicon doped with a p-type or n-type impurity. A silicon substrate doped with a p-type impurity is usually provided for a silicon substrate for a solar cell. 
     The etching gas is blown on the silicon substrate under a condition in which the pressure is in a range from the atmospheric pressure to 80 KPa. The pressure is preferably 30 KPa or lower, more preferably 20 KPa or lower, and even more preferably 10 KPa or lower, and may be 50 Pa or lower. The lower the pressure at the time of etching is, the finer the structure becomes; however, a higher pressure is likely to provide a smaller structure of texture. 
     The etching gas includes at least one of ClF 3 , XeF 2 , BrF 3 , BrF 5 , and NF 3  (may be referred to as a “fluorine-containing gas”). The fluorine-containing gas included in the etching gas may be a mixed gas of two or more of the fluorine-containing gases described above. 
     Molecules of the fluorine-containing gas are physisorbed on the surface of the silicon substrate, and migrate to an etching site. The molecules of the gas reached the etching site are decomposed, and a volatile fluoride is generated by a reaction with silicon. With this process, the surface of the silicon substrate is etched, forming a textured surface. 
     It is preferable that the etching gas includes an inert gas, in addition to the fluorine-containing gas. The inert gas is nitrogen, argon, or helium gas, for example, and may be any gas that does not react with silicon. The inert gas included in the etching gas may be a mixed gas of two or more types of gases. 
     The total concentration (volume concentration) of the inert gas in the etching gas is preferably three time or more than the total concentration of the fluorine-containing gas, and may be 10 times or more, or 20 times or more. The higher the total concentration of the fluorine-containing gas in the etching gas, the more likely the size of projections each having three slant faces increases (the height of the projections increases). Accordingly, in order to reduce the size of the projections, it is preferable to increase the concentration of the inert gas and to reduce the concentration of the fluorine-containing gas relative to the inert gas (see the following Examples 1 and 2). 
     If the concentration of the inert gas in the etching gas is low and the concentration of the fluorine-containing gas is relatively high, the surface of the silicon substrate may be etched isotropically, which makes it difficult to form a desired textured structure on the surface of the silicon substrate. 
     Furthermore, it is preferable that the etching gas includes a gas containing an oxygen atom in its molecule, in addition to the fluorine-containing gas. The gas containing oxygen atoms is typically oxygen gas (O 2 ); however, may be carbon dioxide (CO 2 ) or nitrogen dioxide (NO 2 ). 
     The concentration of the gas containing oxygen atoms (volume concentration, at ordinary temperature) is preferably more than twice the total concentration of the fluorine-containing gas, and more preferably four times or more. The concentration of the gas containing oxygen atoms in the etching gas (volume concentration, at ordinary temperature) is preferably 30 to 80% of the total concentration of the fluorine-containing gas and the inert gas. When the concentration of the gas containing oxygen atoms in the etching gas is too low, the desired texture may not be achieved due to over-etching. 
     By including the gas containing oxygen atoms in the etching gas, unevenness suitable for the textured structure in a solar cell may be formed on a surface of a semiconductor substrate. Although the mechanism is not particularly limited, when ClF 3  gas is physisorbed on the surface of silicon, ClF 3  reacts with the silicon, and SiF 4  is generated, which is gas. Here, by oxygen atoms terminating dangling bonds in a silicon network structure, Si—O bond is partially formed. As a result, a portion likely to be etched (Si—Si) and a portion unlikely to be etched (Si—O) are produced. It is assumed that the difference in the etching rates promotes chemical reactions, allowing controlling the shape of the structure. 
       FIGS. 7A to 7C  show an assumption on the mechanism of reactions when the textured structure on the surface of the semiconductor substrate is formed; however, the mechanism for forming the textured structure is not limited to this example. In an early stage of the textured structure formation ( FIG. 7A ), the etching reaction on silicon substrate  100  by the fluorine-containing gas (ClF 3 ) preferentially proceeds, and pores  110  are formed on the surface of silicon substrate  100 , making the surface into a porous structure. In an intermediate stage ( FIG. 7B ), the surface of porous silicon substrate  100  is oxidized by the oxygen-containing gas along the orientation of the surface, and oxidized layer  120  is formed. In a later stage ( FIG. 7C ), oxidized layer  120  serves as an etching mask, forming desired projections  130  (projections each having three slant faces). 
     In the method for manufacturing the silicon substrate according to the present invention, it is important to maintain the temperature of the silicon substrate at a low temperature during etching. It is preferable to maintain the temperature of the silicon substrate at 200° C. or lower, more preferably 180° C. or lower, and even more preferably 160° C. or lower. In order to maintain the temperature of the silicon substrate at a low temperature, it is preferable to maintain the temperature of a stage for placing the silicon substrate approximately at room temperature (25° C.) or lower. The temperature of the silicon substrate during etching may be measured by an infrared temperature sensor or by providing a thermocouple. 
     As described above, the method for manufacturing the silicon substrate according to the present invention may include blowing the cooling gas on the silicon substrate. The cooling gas is similar to the inert gas described above, and refers to nitrogen, argon, helium, or other gases. By blowing the cooling gas on the silicon substrate generating heat by the reaction with the etching gas, the heated substrate is cooled. 
     In the method for manufacturing the silicon substrate according to the present invention, blowing the etching gas on the silicon substrate and blowing the cooling gas on the silicon substrate may be alternately repeated. By controlling the processing time for blowing the etching gas on the silicon substrate, the temperature of the substrate is maintained at a low temperature. Although the processing time is not particularly limited, the processing time may be approximately 1 minute to 10 minutes. After the etching gas is blown on the silicon substrate, the temperature of the substrate is decreased by blowing the cooling gas, and then the etching gas may be blown on the silicon substrate. 
     When the desired textured structure is formed on the surface of the silicon substrate by the etching gas, it is preferable to remove the etching gas or a degradation product of the etching gas remaining on the silicon substrate. For example, the remaining fluorine component on the silicon substrate may be removed in a hydrogen gas atmosphere. 
     In the method for manufacturing the silicon substrate according to the present invention, the blowing of the etching gas on the silicon substrate may be performed in two steps. More specifically, in the first step, processing using a fluorine-containing gas that does not contain gas containing oxygen atoms and an inert gas is performed, and a roughened surface as shown in  FIG. 2A, 2B, 3A or 3B , for example, is formed on the silicon substrate. In the second step, a chemical reaction is promoted by the difference in the etching rates generated through the process using the gas containing oxygen atoms, allowing controlling the shape of the surface of the substrate. 
     3. Use of Silicon Substrate Having Textured Surface 
     As described above, the silicon substrate according to the present invention is preferably used as a silicon substrate for a solar cell. For adapting the silicon substrate to a solar cell, a p-n junction is preferably provided by forming an emitter layer on the textured surface of the silicon substrate. For example, when the textured surface is formed on a p-type silicon substrate, the textured surface is heated under a phosphorus oxychloride atmosphere, and an n-type emitter layer is formed on the textured surface so as to form a p-n junction. Stacking an anti-reflective layer on the emitter layer further reduces reflectance of the solar cell, improving a photoelectric conversion rate. The anti-reflective layer may be a silicon nitride layer, a silicon oxide layer, or a titanium oxide layer, for example. 
     Subsequently, a front electrode is provided on a light-receiving surface which is the textured surface, and a back electrode is provided on the non-light-receiving surface, and thus a solar cell is implemented. Needless to say, an embodiment of the solar cell is not limited to the example described above. 
     EXAMPLES 
     Example 1 
       FIGS. 5A and 5B  illustrate an overview of a texturing apparatus used in Examples.  FIG. 5A  is an external perspective view of texturing apparatus  10 , and  FIG. 5B  is a perspective view seeing through reduced-pressure chamber  20 . Texturizing apparatus  10  illustrated in  FIGS. 5A and 5B  includes, in reduced-pressure chamber  20 , nozzle  30  for blowing an etching gas, nozzle  40  for blowing a cooling gas, and stage  50  for placing silicon substrate  100 . Nozzle  30  for blowing the etching gas is connected to etching gas supply pipe  31 , and nozzle  40  for blowing the cooling gas is connected to cooling gas supply pipe  41 . A silicon substrate having a textured surface is manufactured by blowing the etching gas and the cooling gas on silicon substrate  100  placed on stage  50 . 
     Silicon substrate  100  having orientation (111) is placed on stage  50  of texturing apparatus  10  illustrated in  FIGS. 5A and 5B . The distance between nozzle  30  that blows the etching gas and silicon substrate  100  is set to be 5 mm The area of the surface of silicon substrate  100  is 125 mm×125 mm The temperature of stage  50  is set to 25° C. After the pressure of reduced-pressure chamber  20  is adjusted to 90 KPa, the etching gas through nozzle  30  is blown on the entire surface of silicon substrate  100  for 10 to 60 seconds. The composition of the etching gas blown is “ClF 3 /O 2 /N 2 =500 to 1,000 cc/0 cc/2,000 to 10,000 cc”. 
     Next, an etching gas through nozzle  30  having a different composition is blown on the entire surface of silicon substrate  100  for 60 to 120 seconds. The composition of the etching gas blown is “ClF 3 /O 2 /N 2 =500 to 1,000 cc/500 to 5,000 cc/2,000 to 10,000 cc”. 
       FIGS. 1A to 1D  show electron micrographs of the textured surface of the silicon substrate obtained. As shown in  FIG. 1A  (1,000×) and  FIG. 1B  (3,000×), or  FIG. 1D  (1,000×) and  FIG. 1E  (3,000×), projections each having three slant faces are closely formed. Note that,  FIG. 1B  is an enlarged view of  FIG. 1A , and  FIG. 1E  is an enlarged view of  FIG. 1D . As shown in  FIG. 1C  and  FIG. 1F  (3000×), the height of the projection having three slant faces is approximately 1.6 μm (represented as 1.5 μm to 1.7 μm in the drawings). Note that,  FIG. 1A  is a perspective view of an electron micrograph of the periphery of the silicon substrate surface having the texture, and  FIG. 1D  is a perspective view of an electron micrograph of a central part of the silicon substrate surface. 
     Comparative Example 1 
     Silicon substrate  100  having orientation (111) is placed on stage  50  of texturing apparatus  10  illustrated in  FIGS. 5A and 5B . The area of the surface of silicon substrate  100  is 125 mm×125 mm. The temperature of stage  50  is set to 80° C. After the pressure of reduced-pressure chamber  20  is adjusted to 90 KPa, the etching gas through nozzle  30  is blown on the entire surface of silicon substrate  100  for 75 to 180 seconds. The composition of the etching gas blown is “ClF 3 /O 2 /N 2 =500 to 1,000 cc/0 cc/2,000 to 5,000 cc”. 
       FIGS. 2A and 2B  show the shape of surface of the silicon substrate obtained. As shown in  FIG. 2A  (top view, 10,000×) and  FIG. 2B  (cross-sectional view, 10,000×), although the surface of the silicon substrate is roughened, the shape is irregular, and no projection having three slant faces is formed. This is because the temperature of the silicon substrate is not maintained at a low temperature. 
     Comparative Example 2 
     Silicon substrate  100  having orientation (111) is placed on stage  50  of texturing apparatus  10  illustrated in  FIGS. 5A and 5B . The area of the surface of silicon substrate  100  is 125 mm×125 mm. The temperature of stage  50  is set to 25° C. After the pressure of reduced-pressure chamber  20  is adjusted to 90 KPa, the etching gas through nozzle  30  is blown on the entire surface of silicon substrate  100  for 75 to 180 seconds. The composition of the etching gas blown is “ClF 3 /O 2 /N 2 =500 to 1,000 cc/0 cc/2,000 to 5,000 cc”. 
       FIGS. 3A and 3B  show the shape of the surface of the silicon substrate obtained. As shown in  FIG. 3A  (top view, 10,000×) and  FIG. 3B  (cross-sectional view, 10,000×), although the surface of the silicon substrate is finely roughened, no projection having three slant faces is formed. 
     Example 2 
     Silicon substrate  100  having orientation (111) is placed on stage  50  of texturing apparatus  10  illustrated in  FIGS. 5A and 5B . The area of the surface of silicon substrate  100  is 125 mm×125 mm. The temperature of stage  50  is set to 25° C. After the pressure of reduced-pressure chamber  20  is adjusted to 90 KPa, the etching gas is blown in two steps. 
     In the first step, the etching gas through nozzle  30  is blown on the entire surface of silicon substrate  100  for 10 to 30 seconds. The composition of the etching gas blown is “ClF 3 /O 2 /N 2 =500 to 1,000 cc/0 cc/2,000 to 10,000 cc”. 
     In the second step, the etching gas through nozzle  30  is blown on the entire surface of the silicon substrate  100  for 60 to 120 seconds. The composition of the etching gas blown is “ClF 3 /O 2 /N 2 =500 to 1,000 cc/500 to 5,000 cc/2,000 to 10,000 cc”. 
       FIGS. 8A to 8D  (electron micrograph, 3000×) show the shape of the surface of the silicon substrate obtained.  FIGS. 8A and 8B  show the shape of the surface of the silicon substrate after the first step is complete. As shown in  FIG. 8A  (top view) and  FIG. 8B  (cross-sectional view), although the surface of the silicon substrate is finely roughened, no projection having three slant faces is formed. 
       FIGS. 8C and 8D  show the shape of the surface of the silicon substrate after the second step is complete. As shown in  FIG. 8C  (top view) and  FIG. 8D  (cross-sectional view), projections each having three slant faces and a height of 1.0 μm to 3.0 μm are closely formed on the surface of the silicon substrate. 
     Example 3 
     Silicon substrate  100  having orientation (111) is placed on stage  50  of texturing apparatus  10  illustrated in  FIGS. 5A and 5B . The area of the surface of silicon substrate  100  is 125 mm×125 mm. The temperature of stage  50  is set to 25° C. After the pressure of reduced-pressure chamber  20  is adjusted to 90 KPa, the etching gas is blown in two steps. 
     In the first step, the etching gas through nozzle  30  is blown on the entire surface of silicon substrate  100  for 40 to 60 seconds. The composition of the etching gas blown is “ClF 3 /O 2 /N 2 =500 to 1,000 cc/0 cc/2,000 to 10,000 cc”. In the second step, the etching gas from nozzle  30  is blown on the entire surface of silicon substrate  100  for 60 to 120 seconds. The composition of the etching gas blown is “ClF 3 /O 2 /N 2 =500 to 1,000 cc/500 to 5,000 cc/2,000 to 10,000 cc”. 
       FIGS. 9A to 9D  (electron micrograph, 3,000×) show the shape of surface of the silicon substrate obtained.  FIGS. 9A and 9B  show the shape of the surface of the silicon substrate after the first step is complete. As shown in  FIG. 9A  (top view) and  FIG. 9B  (cross-sectional view), although the surface of the silicon substrate is finely roughened, no projection having three slant faces is formed. In addition, the surface of the silicon substrate after the first step in Example 3 is rougher than the surface of the silicon substrate after the first step in Example 2 (see  FIGS. 8A and 8B ). 
       FIGS. 9C and 9D  show the shape of the surface of the silicon substrate after the second step is complete. As shown in  FIG. 9C  (top view) and  FIG. 9D  (cross-sectional view), projections each having three slant faces and a height of 1.0 μm to 6.0 μm are closely formed on the surface of the silicon substrate. The height of the projections formed on the surface of the silicon substrate after the second step in Example 3 is greater than the height of the projections formed on the surface of the silicon substrate after the second step in Example 2 (see  FIGS. 8C and 8D ). 
     The reflectance and the absorbance of the silicon substrate on the textured surface obtained in Example 1 are measured. As Reference Example, the reflectance and absorbance of a non-textured surface of the silicon substrate having orientation (111) are measured. The reflectance and absorbance are measured by a sphere spectrophotometer (U4000, manufactured by Hitachi High-Tech Fielding Corporation). 
       FIG. 6A  is a graph representing reflectance at the non-textured surface of the silicon substrate having orientation (111) (Reference Example) and on the textured surface of the silicon substrate obtained in Example; and  FIG. 6B  is a graph representing absorbance on the non-textured surface of the silicon substrate having orientation (111) (Reference Example) and on the textured surface of the silicon substrate obtained in Example. 
     As illustrated in  FIG. 6A  and  FIG. 6B , the reflectance (wavelength of 500 nm to 1,000 nm) on the textured surface of the silicon substrate according to Example 1 is reduced to 20% or lower, and the absorbance (wavelength of 500 nm to 1,000 nm) is increased to 80% or higher. 
     Industrial Applicability 
     The silicon substrate according to the present invention has a textured surface with low reflectance. In addition, the textured surface is more finely formed than in the conventional technique, and the thickness of the silicon substrate may be reduced. Accordingly, by setting the textured surface as the light-receiving surface, the silicon substrate can be suitably used as a silicon substrate for a solar cell. Accordingly, the silicon substrate contributes to an increased photoelectric conversion rate of a solar cell. 
     REFERENCE SIGNS LIST 
     
         
           10  Texturing apparatus 
           20  Reduced-pressure chamber 
           30  Nozzle that blows etching gas 
           31  Etching gas supply pipe 
           40  Nozzle that blows cooling gas 
           41  Cooling gas supply pipe 
           50  Stage 
           100  Silicon substrate 
           110  Pore 
           120  Oxidized layer 
           130  Projection