JP2017118143A - Silicon substrate surface processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon substrate surface processing method applicable to both of a silicon substrate manufactured by a free abrasive grain method and a silicon substrate manufactured by a fixed abrasive grain method, which enables the stable uniform formation of a satisfying texture on a substrate surface, and which never causes the volatilization of an additive agent component in a temperature region of 60-95°C which are normal service temperatures.SOLUTION: A silicon substrate surface processing method comprises the step of immersing a silicon substrate in an etchant which contains (A) an alkaline component, (B) a phosphonic acid derivative or a salt thereof, and (C) a compound having at least one group selected from a group consisting of a carboxyl group, a sulfo group, and the groups each forming a salt, and a carboxymethyl group, thereby forming an uneven structure in a substrate surface.SELECTED DRAWING: None

Description

  The present invention relates to an etching solution and an etching method for forming an uneven structure called texture on the surface of a silicon substrate.

  A crystalline silicon substrate used in a solar cell is subjected to a surface structure treatment called texture in order to reduce light reflectance on the substrate surface and absorb light efficiently.

  Conventionally, for example, a method of forming a pyramidal texture structure by immersing and etching a single crystal silicon substrate in an alkaline solution such as sodium hydroxide or potassium hydroxide is used. It is known that a texture structure having a uniform size or shape can be formed by using an additive other than the above.

  As such an etching solution for forming a texture, for example, a mixture of an aqueous solution of sodium hydroxide or potassium hydroxide and isopropyl alcohol (IPA) is used. A texture is formed by immersing the silicon substrate for 10 to 30 minutes under a warm condition.

  IPA is used to moderately etch silicon with alkali and form a pyramidal texture on the silicon substrate. This effect of IPA is the most widely known substance that serves this purpose. However, since the boiling point of IPA is about 82 ° C., which is about the same as the etching processing temperature, there is a problem that the composition of the etching solution is likely to change due to volatilization of IPA during the processing. In addition, it has been pointed out that the IPA method has a low substrate processing yield. In addition, since IPA has a low flash point, it is not desirable to be careful in handling. Thus, in order to improve the performance of silicon substrates for solar cells, which have been attracting increasing attention in recent years, it has become difficult to satisfy the required performance with an etchant containing IPA as a main component. For example, texture size and texture shape Higher performance is required in various respects, such as being able to control the process, high yield in mass production, easy handling, and high safety.

  In order to satisfy these various demands, various components other than IPA have been proposed as components of the texture-forming etching solution, but it is excellent in texture formation, and quality control such as concentration control during use is easy. The present condition is that the etching solution and additive which can be generally satisfied are not yet obtained (patent documents 1-6).

  Also, in recent years, the silicon substrate slicing method is shifting from the conventional free abrasive method to the fixed abrasive method, and in these methods, the surface state of the substrate after slicing is different from each other. There is also a problem that the etching solution that can be used on the silicon substrate manufactured by the method cannot be used as it is on the silicon substrate manufactured by the fixed abrasive method.

JP 2000-183378 A International Publication No. WO2007 / 129555 JP 2009-123811 A JP 2002-57139 A JP 2007-258656 A JP 2010-141139 A

  The present invention has been made in view of the above problems, and can form a good texture with low reflectivity uniformly and stably on the substrate surface, which is a normal use temperature of 60 ° C. to 95 ° C. In the region, the composition of the etching solution does not volatilize, and it can be applied to both a silicon substrate manufactured by a free abrasive method and a silicon substrate manufactured by a fixed abrasive method. It is an object to provide an etching solution for texture formation.

The silicon substrate surface processing method of the present invention includes a step of immersing the silicon substrate in an etching solution for forming irregularities on the silicon substrate surface containing the following (A) to (C) to form an irregular structure on the substrate surface. Is included.
(A) Alkali component (B) Phosphonic acid derivative or salt thereof (C) In the molecule, at least one selected from the group consisting of a carboxyl group, a sulfo group, these groups forming a salt, and a carboxymethyl group Compound having a group

  According to the etching solution of the present invention, a uniform pyramidal texture structure can be formed on the surface of the silicon substrate. As a result, it is possible to stably mass-produce a low-reflectance silicon substrate, and to provide a high-quality silicon substrate that can improve the performance of the solar cell. In addition, since the blended components do not volatilize in the range of 60 ° C. to 95 ° C., which is the use temperature, stable texture formation is possible and safety is high. Furthermore, the etching solution of the present invention has an excellent effect on a silicon substrate manufactured by either the free abrasive grain method or the fixed abrasive grain method.

  Hereinafter, embodiments of the present invention will be described in detail.

  The alkali component (A) used in the present invention is not particularly limited, and those conventionally used for etching solutions for silicon substrates can be used as appropriate. Examples thereof include alkali metal or alkaline earth metal hydroxides such as sodium hydroxide and potassium hydroxide, or salts thereof, and sodium hydroxide is preferred in terms of general availability. These alkali components may be used in combination of two or more as required.

  Next, the phosphonic acid derivative used in the present invention or a salt thereof (hereinafter sometimes abbreviated as “phosphonic acid derivative etc.”) (B) is generally used as a sequestering agent, It can use without being specifically limited.

Preferred examples of the phosphonic acid derivative include 1-hydroxyethylidene-1,1-diphosphonic acid represented by the following formula (1), nitrilotris (methylenephosphonic acid) represented by the following formula (2), and the following formula ( 3) a phosphonobutanetricarboxylic acid represented by the following general formula (4), an alkylenediaminetetra (methylenephosphonic acid) represented by the following general formula (4) (for example, ethylenediaminetetra (methylene Phosphonic acid), hexamethylenediaminetetra (methylenephosphonic acid) in the case of n = 6), dialkylenetriaminepenta (methylenephosphonic acid) represented by the following general formula (5) (for example, in formula (5) , Diethylenetriaminepenta (methylene phosphonic acid), etc., where n = 2), and / or their salts. Examples of these salts include alkali metal salts, alkaline earth metal salts, amine salts, and ammonium salts. Since they are generally easily available, alkali metal salts such as sodium salts and potassium salts are preferred. Used. Two or more of these phosphonic acid derivatives can be used as needed.

  Among the above phosphonic acid derivatives, 1-hydroxyethylidene-1,1-diphosphonic acid is obtained because of high uniformity of the obtained texture structure and easy to obtain a high purity product with few impurities to be avoided in this application. Nitrilotris (methylenephosphonic acid) or phosphonobutanetricarboxylic acid is particularly preferred.

  Next, the compound (C) having at least one group selected from the group consisting of a carboxyl group, a sulfo group, a salt-forming group, and a carboxymethyl group used in the present invention will be described.

  As the compound (C), one or more selected from sulfonic acid, sulfonic acid salt, carboxylic acid, and carboxylic acid salt can be used. The sulfonic acid is a compound having at least one sulfo group in the molecule, and the salt of the sulfonic acid is a compound having at least one sulfonate group in the molecule to form a salt. Is a compound having at least one carboxyl group in the molecule, and a carboxylic acid salt is a compound having at least one carboxylate group in the molecule to form a salt.

  More specifically, for example, a compound represented by the following general formula (I) or a salt thereof can be suitably used as the compound (C).

R ¹ -X (I)
However, R ¹ is an alkyl group having 4 to 12 carbon atoms, an alkenyl group, an alkoxy group, or a aryl group, these groups may have a branched structure in the carbon skeleton, X represents a carboxyl group, One of the sulfo groups.

  The compound represented by the general formula (I) is not particularly limited. Specific examples of the carboxyl group-containing substance include butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, and nonanoic acid having a linear carbon skeleton. , Decanoic acid, undecanoic acid, dodecanoic acid, or carbon skeleton such as 2-ethylhexanoic acid, 2,3-dimethylnonanoic acid, 2-ethyl-3-methylnonanoic acid, 3,5,5-trimethylhexanoic acid, etc. Examples thereof include carboxylic acids having a branched structure. In addition, carboxylic acids having an aromatic ring structure such as a benzene ring and a naphthalene ring can also be used.

  Specific examples of the sulfo group-containing substance include butanesulfonic acid, pentanesulfonic acid, hexanesulfonic acid, heptanesulfonic acid, octanesulfonic acid, nonanesulfonic acid, decanesulfonic acid, undecanesulfonic acid, and dodecanesulfonic acid. Can be mentioned. In addition, sulfonic acid having a branched structure in the carbon skeleton and sulfonic acid having an aromatic ring structure can also be used in the sulfo group-containing substance, similarly to the carboxyl group-containing substance. These may be used alone or in combination of two or more, including a mixture of carboxylic acid and sulfonic acid. These carboxylic acids and sulfonic acids may be used in the form of alkali metal salts, alkaline earth metal salts, amine salts, and ammonium salts.

  Among the compounds represented by the general formula (I), the carbon number of the longest carbon chain length portion such as 3,5,5-trimethylhexanoic acid is such that a synergistic effect with the phosphonic acid is easily obtained. Carboxylic acid having 6 to 8 and 1 to 3 branched side chains of methyl group and ethyl group is preferable.

As the compound (C), a compound having a structure represented by the following general formula (II) or a salt thereof is also preferably used.

However, in the formula (II), n is an integer of 2 or more, m is an integer of 1 to 5, and when m is 2 or more, each m value is the same, Y ⁰ represents a hydrogen atom, Y ^{1 to} Y ⁿ each represents a hydrocarbon group having 1 to 30 carbon atoms, and the structure of these hydrocarbon groups is a single bond, a double bond, Any of triple bonds may be included, and any of an aliphatic hydrocarbon group and an aromatic hydrocarbon group may be sufficient, and all may be the same or may mutually differ. X ^{1 to} X ⁿ each represent any one of a sulfo group, a carboxyl group, and a hydrogen atom, and may be all the same or different from each other, but at least one of them is a sulfo group or a carboxyl group It is a group.

In the general formula (II), at least two or more of ⁿ of X ^{1 to} X ⁿ in the general formula (II) have a high effect of forming an excellent etching solution when used in combination with a phosphonic acid derivative as described above. A sulfonic acid group and / or a salt thereof, a polysulfonic acid and a salt thereof, and a polycarboxylic acid such that at least two of n X's of X ^{1 to} X ⁿ are a carboxyl group and / or a salt thereof Acids and their salts are preferred. Here, the salt is not particularly limited, and examples thereof include sodium salt and potassium salt.

As a preferable specific example of the compound represented by the general formula (II) as described above, it is produced by polycondensation of polyacrylic acid or naphthalenesulfonic acid and formalin produced by polymerization of acrylic acid represented by the following formula. Naphthalene sulfonic acid formalin condensate, polystyrene sulfonic acid and the like. Of these, naphthalenesulfonic acid formalin condensates and salts thereof are particularly preferable in that a synergistic effect with the above-described phosphonic acid is remarkably obtained.

  However, the compound that can be used is not limited to the polymer in which the monomer is polymerized, and the type and the number of polymerization of the monomer to be polymerized are not limited. Further, the polymerization may be carried out either in a state where one kind of monomer is continuously polymerized (block) or in a state where a plurality of monomers are complicatedly polymerized (random). The number of polymerizations n is not particularly limited as long as it is 2 or more. In the case of polyacrylic acid, about 130 molecules of acrylic acid produced by polymerizing about 130 molecules of acrylic acid is about 40 molecules of acrylic acid. Even a polyacrylic acid having a molecular weight of about 3000 produced by polymerization is applicable.

  As said compound (C), the compound which has a carboxymethyl group in a molecule | numerator can also be used suitably. Examples of compounds having a carboxymethyl group in the molecule include carboxymethyl cellulose salt, carboxymethyl starch, carboxymethyl sucrose, carboxymethyl inulin, carboxymethyl chitin, carboxymethyl chitosan, carboxymethyl dextran, carboxymethyl lignin, carboxymethyl guar gum, etc. And so on. Not only these existing compounds having a carboxymethyl group in the molecule but also a compound in which a carboxymethyl group is newly introduced into the molecule can be used for the purpose of the present invention. These may be used alone or in combination of two or more.

  Among the compounds having a carboxymethyl group in the molecule, as a composition component of the etching solution for texture formation, monosaccharides such as glucose and fructose are used as a basic structural unit in that a synergistic effect with the phosphonic acid can be obtained. Compounds are preferred, among which carboxymethylcellulose salt and carboxymethyl starch are preferred, and carboxymethylcellulose salt is particularly preferred. As the salt, an alkali metal salt is preferable, and among them, a lithium salt, a sodium salt, and a potassium salt are preferable.

  As a carboxymethyl group-containing compound that can be suitably used in the present invention, the details thereof will be described using a carboxymethyl cellulose salt as an example. Cellulose formed by polymerization of glucose has three hydroxyl groups per unit glucose, and a substance in which these hydroxyl groups are substituted with carboxymethyl groups is called carboxymethyl cellulose. The number of substitutions of these hydroxyl groups with carboxymethyl groups is called the degree of etherification, and the degree of etherification can theoretically be up to 3.

  As the carboxymethyl cellulose salt, those having any degree of etherification can be used, but a carboxymethyl cellulose salt having a degree of etherification of 0.4 to 2.0 is preferred, and a degree of etherification of 0.7 to 1.5 is preferred. Those are particularly preferred. Further, the molecular weight of the carboxymethyl cellulose salt can be applied without any particular limitation, but those having an average degree of polymerization of 1000 or less and an average molecular weight of 220,000 or less are preferred, and those having an average degree of polymerization of 250 or less and an average molecular weight of 54000 or less are more preferred. .

  The compound (C) usable in the present invention can be used without particular limitation as long as it has a structure that can be dissolved in a high-temperature alkaline aqueous solution to which the present invention is applied as well as an existing compound. If it cannot be dissolved, the etching solution becomes non-uniform, and there is a concern that it may adversely affect the uniform processing of the substrate. These may be used alone or in combination of two or more.

  According to the etching solution of the present invention, the phosphonic acid derivative (B) is regularly arranged and adsorbed on the surface of the silicon substrate, and etching by the alkali component (A) is moderately suppressed, so that a texture is formed on the substrate surface. It is thought that it is done. However, in the case of an etching solution consisting only of an alkali and a phosphonic acid derivative, etching unevenness may occur on the surface of the silicon substrate, and this unevenness is caused by uneven hydrogen bubbles generated on the surface of the silicon substrate. It is thought that this occurs because the reaction proceeds while adhering to the surface. By using the various compounds (C) as described above in combination with the phosphonic acid derivative (B), the generation of bubbles is suppressed, and more uniform. It is considered that etching can be performed.

  The etching solution of the present invention can be used as an aqueous solution in which the alkali component (A), the phosphonic acid derivative or the like (B), and the compound (C) are dissolved in water. The water used is preferably water from which impurities have been removed, such as ion exchange water or distilled water. Moreover, when an insoluble component is contained in the etching solution, it is preferable to use after removing by performing a known fractionation operation such as filtration.

  In the aqueous solution, the concentration of the alkali component (A) is preferably in the range of 0.3 to 25% by mass, and more preferably 1 to 15% by mass. When the concentration of the alkali component is 0.3% by mass or more, the texture can be formed efficiently in a short time, and even if it exceeds 25% by mass, the etching effect does not decrease, but an etching inhibitor used as an additive is necessary. Since the amount increases, it is disadvantageous in terms of cost.

  Moreover, 0.1-25 mass% is preferable, as for content of the phosphonic acid derivative (B) in this etching liquid, 0.3-15 mass% is more preferable, and 0.5-15 mass% is especially preferable. When the content of the phosphonic acid derivative is within the above range, a moderate inhibitory effect on alkali etching can be obtained, and texture formation without unevenness can be performed in a short time.

  Further, the content of the compound (C) is preferably in the range of 0.0001 to 25% by mass in order to efficiently perform etching without unevenness. However, the preferable content varies depending on the type of the compound (C), and when the compound (C) is a compound represented by the formula (I), 0.1 to 25% by mass is more preferable, and 0.3 to 15% by mass. Is particularly preferred. When the compound (C) is a compound represented by the formula (II), 0.0001 to 10% by mass is more preferable, and 0.01 to 5% by mass is particularly preferable. When the compound (C) is a compound having a carboxymethyl group, 0.001 to 10% by mass is more preferable, and 0.01 to 3% by mass is particularly preferable.

  Furthermore, it is preferable to control the ratio of the content of the phosphonic acid derivative (B) and the sulfonic acid / carboxylic acid (C) to the content of the alkali component (A), and the alkali component (A) and the phosphonic acid derivative ( The blending ratio (mass ratio) of B) and compound (C) is preferably such that the value of A / (B + C) is in the range of 0.1 to 10, more preferably in the range of 0.2 to 5. And By making the ratio of the two in the above range, anisotropic etching of silicon due to the alkali component can be moderately suppressed by the phosphonic acid derivative (B) and the compound (C), contributing to good texture formation. I think that. When the amount of the phosphonic acid derivative (B) and the compound (C) with respect to the alkali component (A) is too small, the effect of suppressing the etching is insufficient, the etching rate is increased, and as a result, the shape and size of the texture structure are increased. Control becomes difficult and the surface tends to be non-uniform. On the other hand, when the amount of the phosphonic acid derivative (B) and the compound (C) is too large relative to the alkali component (A), the effect of suppressing the etching becomes too large, so that the etching rate becomes too small and a texture structure is formed. Difficult to do.

  In the etching solution of the present invention, additives such as isopropyl alcohol, fatty acid, silicate and the like that are usually used in the etching solution for texture formation of a silicon substrate, if necessary, are within the scope of the present invention. Can be added.

  In the case of a silicon substrate manufactured by a conventional loose abrasive method, even if an appropriate texture can be formed with an etching solution prepared to obtain the desired performance, this etching solution is manufactured as it is by a fixed abrasive method. Similarly, it is not always possible to form an appropriate texture when applied to a silicon substrate. Various factors can be considered for this, for example, because the cutting trace on the substrate surface after slicing is different, or because the coolant used at the time of slicing is different. May have an effect. In recent years, there has been a demand for an etching solution that exhibits excellent etching performance with respect to a silicon substrate manufactured by either a free abrasive method or a fixed abrasive method. It is excellent in that it can be applied to a silicon substrate. Further, the etching solution of the present invention can be applied not only to a single crystal silicon substrate but also to a polycrystalline silicon substrate, and any silicon substrate including a slicing method such as a free abrasive method or a fixed abrasive method. Is applicable. Among these, it can be preferably used particularly for a single crystal silicon substrate having a surface orientation of (100) on the substrate surface.

  The etching method using the etching solution of the present invention is not particularly limited, and as in the conventional method, the etching solution is heated to about 60 ° C. to 95 ° C., and the target silicon substrate is heated for 10 to 30 minutes. A dipping method can be used.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to a following example.

[Examples and Comparative Examples]
An aqueous solution in which the alkali component (A), the phosphonic acid derivative and the like (B), and the compound (C) were mixed in the proportions shown in the following tables was prepared and used as an etching solution. Ion exchange water was used as water for preparation.

  These etching solutions are heated to 80 ° C., and single crystal silicon substrate wafers (hereinafter referred to as “wafers”) sliced by either the free abrasive method or the fixed abrasive method shown in the table, respectively. Etching was performed by dipping for 20 minutes, washing with water and drying, and the texture structure was evaluated and the reflectance was measured as follows. The results are shown in each table.

(1) Appearance evaluation of texture structure and measurement of reflectance were performed as follows.

  The texture structure was observed using a scanning electron microscope (JSM-6380LV, manufactured by JEOL Ltd.). When observing at a magnification of 1000 times, a flat area having no texture structure on the substrate surface is 5% or less with respect to the total area, and “◯” is exceeded, and exceeds 5% with respect to the total area. And a flat region is indicated as “x”.

(2) Wafer reflectivity measurement The wafer reflectivity was measured with an ultraviolet / visible spectrophotometer (U-3900H, manufactured by Hitachi High-Technologies Corporation). Light having a wavelength of 600 nm was adopted as an evaluation standard.

The etching solution of the present invention can be used for etching a silicon substrate of a solar cell, for example.

Claims (1)

The surface of a silicon substrate comprising the step of immersing the silicon substrate in an etching solution for forming irregularities on the silicon substrate surface containing the following (A) to (C) to form an irregular structure on the substrate surface: Processing method.
(A) Alkali component (B) Phosphonic acid derivative or salt thereof (C) In the molecule, at least one selected from the group consisting of a carboxyl group, a sulfo group, these groups forming a salt, and a carboxymethyl group Compound having a group