JP2013089629A - Etchant and method for processing surface of silicon substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an etchant capable of forming a silicon substrate having fine pyramidal protrusions and recesses (texture structure) stably, without using a conventional etching retarder such as isopropyl alcohol.SOLUTION: There is provided an etchant in which a silicon substrate is immersed in order to form pyramidal protrusions and recesses on the surface of the substrate. The etchant contains a compound (A) represented by following general formula (1) and alkali hydroxide (B). (In the formula (1), X represents a sulfonic acid group or an alkali salt thereof, Rand Rmay be the same or different and each represent one kind selected from a group of hydrogen atom, alkyl group, alkenyl group, and alkynyl group (each having 1 or 2 carbon atoms) provided that the total number of carbon atoms of Rand Ris 0 to 2).

Description

  The present invention relates to an etching solution for forming a pyramidal concavo-convex structure on the surface of a silicon substrate and a surface processing method for a silicon substrate using the etching solution.

  On the surface of a silicon substrate used for a crystalline silicon solar cell, fine pyramid-shaped irregularities called a texture structure are formed. The irradiated light is multiple-reflected on the surface by this texture structure, increasing the chance of incidence on the silicon substrate and being efficiently absorbed inside the solar cell.

A silicon substrate having a texture structure is manufactured by etching a silicon substrate obtained by slicing a silicon ingot with a wire saw or the like.
Etching of the silicon substrate can be performed by wet etching using an alkaline etchant. This etching proceeds by the reaction of the following reaction formula (1).

[Chemical 1]

Si + 4OH ^{over _{+ 4H + → Si (OH)}} 4 + 2H 2 Reaction formula (1)

  In order to form a texture structure on the surface of the silicon substrate, anisotropic etching is usually performed by using an etching solution with a controlled etching rate. The purpose of the etching is to remove a damaged layer in which distortion or defect caused by slicing processing existing on the surface of the silicon substrate after slicing, which is a raw material, and to form a texture structure. The removal of the damage layer and the formation of the texture structure may be carried out with the same etching solution, or from the viewpoint of productivity, a two-step etching process using different etching solutions for the removal of the damage layer and the formation of the texture is performed. Also good.

  In the two-stage etching process, first, damage layer removal etching is performed with an alkaline etching solution having a relatively high etching rate, and then anisotropic etching is performed by using an etching solution with a controlled etching rate as texture etching. It is a processing method.

  In any method, the formation of the texture structure on the surface of the silicon substrate is based on the following mechanism.

  The etching rate of the silicon substrate with the alkaline aqueous solution is the fastest on the (100) plane of silicon and the slowest on the (111) plane. Therefore, when the rate of texture etching is suppressed by adding a specific additive (hereinafter, also referred to as “etching inhibitor”) that can reduce the etching rate to the alkaline aqueous solution, the (100) surface of silicon. A crystal plane that is easily etched is preferentially etched, and a (111) plane having a slow etching rate remains on the surface. Since the (111) plane has an inclination of about 54 degrees with respect to the (100) plane, pyramidal irregularities including the (111) plane and its equivalent plane are formed at the final stage of the process.

  For damage layer removal etching, an etching solution composed of a general strong alkali chemical solution can be used, whereas in texture etching, the above-mentioned etching inhibitor is added to control various conditions such as solution temperature. Therefore, it is necessary to control the etching rate.

  Usually, an etching solution in which isopropyl alcohol (hereinafter sometimes referred to as “IPA”) is added as an etching inhibitor to a sodium hydroxide (NaOH) aqueous solution is used as an etching solution for texture etching. A method has been used in which this etching solution is heated to about 60 to 80 ° C. and a (100) plane silicon substrate is immersed for 10 to 30 minutes (see, for example, Patent Document 1 and Non-Patent Document 1).

  Further, a solution obtained by diluting 45% by weight potassium hydroxide (KOH) 1: 1 with water was heated to 85 ° C., and the silicon substrate was immersed for 30 minutes to remove the damaged layer. Then, potassium hydroxide (KOH) ) A method of performing texture etching using an etching solution obtained by adding IPA as an etching inhibitor to an aqueous solution is also disclosed (for example, see Non-Patent Document 2).

  On the other hand, since IPA has high volatility, it is necessary to add IPA corresponding to the volatilization amount to the texture etching solution as needed, and an increase in etching cost due to an increase in consumption of IPA is a problem. Furthermore, it is not preferable to use a large amount of highly volatile IPA in terms of safety and environment. Even if an apparatus for recovering volatile IPA is added, the manufacturing cost of the etching processing equipment increases and the operating cost of the equipment also increases. There was a problem of increasing.

  Therefore, development of a texture etching solution containing an etching inhibitor as an alternative to IPA has been performed. For example, aliphatic carboxylic acids or salts thereof (Patent Documents 2 and 3), inorganic salts (Patent Document 4), and compounds containing a benzene ring (Patent Document 5) have been proposed as etching inhibitors.

JP 61-96772 A JP 2002-57139 A International Publication No. 06/046601 Pamphlet JP 2000-183378 A JP 2007-258656 A

"Uniform Pyramid Formation on Alkaline-etched Polished Monocrystalline (100) Silicon Wafers" Progress in Photovoltaics, Vol.4, 435-438 (1996) "EXPERIMENTAL OPTIMIZATION OF AN ANISOTROPIC ETCHING PROCESS FOR RANDOM TEXTURIZATION OF SILICON SOLAR CELLS" CONFERENCE RECORD OF THE IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE, P303-308 (1991)

  In the method using an aliphatic carboxylic acid, there is a problem that the raw material cost is high, and when neutralizing in the waste liquid treatment, the aliphatic carboxylic acid is liberated, a separate oil-water separation step is required, and a special bad odor is generated. In addition, there is a problem that waste liquid treatment is costly and leads to an increase in manufacturing cost.

In the method using an inorganic salt, it is necessary to use expensive Na ₂ CO ₃ in order to suppress the impurity concentration of heavy metals and salts to a necessary level. In addition, since the salt concentration in the system increases and the amount of silicate dissolved as a by-product during the etching of silicon decreases, the texture etching solution must be frequently replaced.

  In addition, a compound having a chain structure including a benzene ring has a stronger effect of suppressing etching than a compound having a simple chain structure, and thus the additive concentration needs to be reduced. However, if the additive concentration is too low, it is difficult to control the concentration in the etching solution, and it becomes difficult to manufacture a silicon substrate having a fine texture structure with good reproducibility.

  As described above, a conventional etching solution can form a fine texture structure on a silicon substrate with good reproducibility, and has an industrially satisfactory performance including waste liquid treatment and work environment. The fact is that the liquid has not yet been found.

  Under such circumstances, an object of the present invention is to provide an etching solution capable of stably forming a silicon substrate having a fine texture structure without using a conventional etching inhibitor such as IPA. .

  As a result of intensive studies to solve the above-mentioned problems, the present inventor obtained the knowledge that the above-mentioned object can be easily achieved by using a specific compound as an etching inhibitor, leading to the present invention. .

  That is, the first gist of the present invention is an etching solution for immersing a silicon substrate to form pyramidal irregularities on the surface of the substrate, the compound (A) represented by the following general formula (1) and water It exists in the etching liquid characterized by containing an alkali oxide (B).

(In the formula (1), X is a sulfonic acid group or an alkali salt thereof, R ¹ and R ² are the same or different, and each represents a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group (all having 1 or 2), provided that R ¹ and R ² have 0 to 2 carbon atoms in total.

  And the 2nd summary of this invention exists in the silicon substrate surface processing method characterized by including the process of immersing a silicon substrate in said etching liquid, and forming the pyramidal unevenness | corrugation in the said substrate surface. .

  When the etching solution of the present invention is used, a fine texture structure with an average size of 2.0 to 4.0 μm suitable for light confinement for solar cells and efficient cell formation is formed on the surface of a silicon substrate with good reproducibility. can do.

It is an electron micrograph of the silicon substrate surface after etching using the etching solution of Example 1. It is an electron micrograph of the silicon substrate surface after etching using the etching solution of Example 2. It is an electron micrograph of the silicon substrate surface after etching using the etching liquid of Example 3. It is an electron micrograph of the silicon substrate surface after etching using the etching liquid of Example 4. It is an electron micrograph of the silicon substrate surface after etching using the etching liquid of Example 5. It is an electron micrograph of the silicon substrate surface after etching using the etching liquid of Example 6. It is an electron micrograph of the silicon substrate surface after etching using the etching liquid of Example 7. It is an electron micrograph of the silicon substrate surface after etching, using the etching solution of Example 8. It is an electron micrograph of the silicon substrate surface after etching, using the etching solution of Example 9. It is an electron micrograph of the silicon substrate surface after etching using the etching liquid of Example 10. FIG. It is an electron micrograph of the silicon substrate surface after etching, using the etching solution of Example 11. It is an electron micrograph of the silicon substrate surface after etching using the etching liquid of Example 12. It is an electron micrograph of the silicon substrate surface after etching, using the etching solution of Example 13. It is an electron micrograph of the silicon substrate surface after etching, using the etching solution of Comparative Example 1. It is an electron micrograph of the silicon substrate surface after etching, using the etching solution of Reference Example 1.

  Hereinafter, the present invention will be described in detail.

First, the etching solution of the present invention will be described.
An etching solution for immersing a silicon substrate in the etching solution of the present invention to form pyramidal irregularities on the surface of the substrate, comprising a compound (A) represented by the following general formula (1) and an alkali hydroxide (B) Containing.

(In the formula (1), X is a sulfonic acid group or an alkali salt thereof, R ¹ and R ² are the same or different, and each represents a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group (all having 1 or 2), provided that R ¹ and R ² have 0 to 2 carbon atoms in total.

  In the present invention, the “silicon substrate” includes a single crystal silicon substrate and a polycrystalline silicon substrate. The etching solution of the present invention is a single crystal silicon substrate, particularly a single crystal silicon substrate having a (100) plane on the surface. Suitable for etching.

  The compound (A) represented by the general formula (1) has an advantage of exhibiting an etching suppression effect equivalent to or higher than that of IPA which is a conventional etching inhibitor and having a wide applicable concentration range as described later. . Therefore, by using the etching solution of the present invention, the size and shape of pyramidal irregularities on the surface of the silicon substrate can be controlled within a suitable range. In addition, as a means for improving the efficiency of solar cells, items related to the texturing process are “improvement of light confinement amount (low reflectance)” and “reduction of resistance between Si substrate and electrode in cell formation”. By using the etching solution of the present invention, it becomes possible to form fine pyramidal irregularities having an average size of 2.0 to 4.0 μm, and these items can be improved and contribute to higher efficiency of the solar cell. It is a feature.

As R ¹ and R ² in the general formula (1), a hydrogen atom or an alkyl group is preferable, and a compound having substitution at least at the 4-position is preferable. Specific examples of the compound (A) include sodium benzenesulfonate, sodium p-toluenesulfonate, sodium 4-ethylbenzenesulfonate, sodium 2,4-dimethylbenzenesulfonate, sodium 4-styrenesulfonate, and the like. .

  In addition, as an alkali component in the alkali salt of the compound (A), a Group 1 element and a Group 2 element can be used. Among these, lithium hydroxide, sodium hydroxide, and potassium hydroxide are particularly easily available and costly. It is suitable because it is excellent in terms of surface.

  The concentration of the compound (A) is capable of forming a fine texture structure on the surface of the silicon substrate, and is selected within the range of industrially effective etching rates, and is usually 0.01 to 30% by weight, preferably It is 0.1 to 20 weight%, More preferably, it is 1 to 15 weight%.

  Within the above range, the surface of the substrate can be anisotropically etched to form a fine texture structure on the surface of the substrate. When the concentration of the compound (A) is less than 0.01% by weight, the effect of suppressing etching may be insufficient, and the concentration is too low, making it difficult to control the concentration in the etching solution with good reproducibility. It becomes difficult to manufacture a silicon substrate having a fine texture structure. On the other hand, when it is larger than 30% by weight, the chemical cost increases, and the number of water washings and waste liquid treatment costs increase, which is not preferable.

  As the alkali hydroxide (B), a hydroxide of a Group 1 element or a hydroxide of a Group 2 element can be used. Examples include lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide, beryllium hydroxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide, ammonium hydroxide, and the like. These can be used alone or in admixture of two or more. Among these, lithium hydroxide, sodium hydroxide, and potassium hydroxide are particularly preferable because they are readily available and excellent in cost. Further, these alkali hydroxides may be mixed and used at an arbitrary ratio.

  In the etching solution of the present invention, the concentration of alkali hydroxide (B) in the etching solution is usually 0.1 to 50% by weight, preferably 0.5 to 10% by weight. If it is this range, an etching will advance suitably and a fine uneven structure can be formed in the surface of a silicon substrate. When the alkali concentration is less than 0.1% by weight, the etching rate may not be sufficient, and when it is more than 50% by weight, alkali hydroxide may be precipitated.

  The etching solution of the present invention may contain a silicate compound (C) in addition to the compound (A) and the alkali hydroxide (B).

Specific examples of the silicate compound (C) include lithium orthosilicate (Li ₄ SiO ₄ .nH ₂ O), lithium metasilicate (Li ₂ SiO ₃ .nH ₂ O), lithium pyrosilicate (Li ₆ Si ₂ O _7). · NH ₂ O), lithium metadisilicate (Li ₂ Si ₂ O ₅ · nH ₂ O), lithium metatrisilicate (Li ₄ Si ₃ O ₈ · nH ₂ O), sodium orthosilicate (Na ₄ SiO ₄ · nH) ₂ O), sodium metasilicate (Na ₂ SiO ₃ · nH ₂ O), sodium pyrosilicate (Na ₆ Si ₂ O ₇ · nH ₂ O), sodium metadisilicate (Na ₂ Si ₂ O ₅ · nH ₂ O) , Sodium metatrisilicate (Na ₄ Si ₃ O ₈ .nH ₂ O), potassium orthosilicate (K ₄ SiO ₄ .nH ₂ O), potassium metasilicate (K ₂ SiO ₃ .nH ₂ O), potassium pyrosilicate ( _{K 6 Si 2 O 7 · nH} 2 O), meta Ni珪Potassium _{(K 2 Si 2 O 5 ·} nH 2 O), meta trisilicate potassium _{(K 4 Si 3 O 8 ·} nH 2 O) and the like.

  Since the silicate compound (C) has an action of suppressing the dissolution of silicon, by adding the silicate compound (C) to the etching solution of the present invention, the etching suppression action of the compound (A) can be assisted. Thus, the etching rate suitable for forming the texture structure can be controlled more appropriately.

  As the silicate compound (C), in addition to the above compounds, a product obtained by etching a silicon substrate with an alkali hydroxide (B) aqueous solution, a reaction product of alkali hydroxide (B) and silicon or silicon dioxide It may be a thing.

  When the concentration of the silicate compound (C) is too high, the etching rate is remarkably reduced, the liquid viscosity is increased, and further, the precipitation of the silicate compound (C) is likely to occur, and the substrate can be etched normally. It becomes difficult to use as a solar cell substrate. For this reason, the silicate concentration in the etching solution is preferably in the range of 10 wt% or less in terms of Si. Here, the “Si equivalent concentration” means a concentration in terms of silicon (Si) atoms contained in the silicate.

  In addition, since the silicate compound (C) is also generated as a by-product of etching the silicon substrate, the concentration of the silicate compound (C) in the etching solution is 10% by weight in terms of Si by repeated etching operations. When it exceeds, it is preferable to replenish and dilute other components or change the solution.

  In addition, in the etching liquid of this invention, components other than a compound (A), an alkali hydroxide (B), and a silicate compound (C) are included as other components in the range which does not impair the objective and effect of this invention. But you can. Examples of such components include a buffer, a pH adjuster, a viscosity adjuster, and a surface tension adjuster.

  The etching liquid of the present invention can be obtained by dissolving the above compound (A), alkali hydroxide (B), and, if necessary, silicate compound (C) in water as a solvent by a conventional method. In addition, the temperature which manufactures etching liquid is 0 to 100 degreeC normally, Preferably it is 20 to 40 degreeC, and is generally room temperature.

  The water as the solvent of the etching solution of the present invention is preferably water from which impurities are removed, and usually ion-exchanged water or distilled water is preferably used. Specifically, ion-exchanged water or distilled water having an electric conductivity measured at 25 ° C. of 1 mS / cm or less (particularly 100 μS / cm or less) is suitable.

Next, the surface processing method of the silicon substrate according to the present invention will be described.
As the silicon substrate, single crystal and polycrystalline silicon substrates formed by any manufacturing method may be used, but a single crystal silicon substrate is preferable, and a single crystal silicon substrate having a surface orientation of (100) is particularly preferable. As described above, since the etching of the silicon substrate with the alkaline aqueous solution is anisotropic etching, the silicon substrate having a surface orientation of (100) forms a fine texture structure and has a low reflectance. This is because the energy conversion efficiency is increased when the cell is formed.

  The etching method is not particularly limited, and a fine uneven structure is formed on the surface of the silicon substrate by immersing the silicon substrate for a predetermined time using the etching solution of the present invention heated and held at a predetermined temperature. Can be formed.

  Although the use temperature of an etching liquid is not specifically limited, Usually, it is 0-100 degreeC, and it is preferably 80-100 degreeC from a viewpoint of etching efficiency. The etching time is not particularly limited, but is usually 1 to 120 minutes, preferably 10 to 40 minutes.

  With the silicon substrate surface processing method according to the present invention, a silicon substrate having a pyramidal uneven structure on the silicon substrate surface can be obtained.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded.

Examples 1 to 13, Comparative Example 1 and Reference Example 1:
<Preparation of texture etchant>
The etching solution used in each example was added to the alkali hydroxide (B) dissolved in water at room temperature and stirred until the compound (A) shown in Table 1 was completely dissolved. It was produced by heating to the etching temperature. The silicate compound (C) was prepared by repeatedly etching until the silicate compound (C) produced as a by-product by immersing the silicon substrate in alkali hydroxide (B) reached a predetermined concentration in terms of Si.

<Silicon substrate etching>
A single crystal silicon substrate (surface crystal plane: (100) plane) having a practical size of 50 × 50 mm and a thickness of about 180 μm prepared by cutting a single crystal silicon ingot was used as an etching solution having the composition shown in Table 1. Immersion and texture etching were performed under the conditions shown in Table 1. However, for Example 3, macro etching was performed prior to texture etching. That is, the silicon substrate was immersed in a 25% by weight sodium hydroxide solution heated to 80 ° C. for about 15 minutes to remove deposits on the surface of the silicon substrate and the work-modified layer. Thereafter, it was washed with water and subjected to texture etching. In Example 13, a single crystal silicon substrate (surface crystal plane: (100) plane) having a size of 156 × 156 mm and thickness of 180 μm prepared by cutting a single crystal silicon ingot was used.

  The etching amount shown in the etching conditions in the table (μm) is the etching thickness per one side of the substrate obtained by measuring the weight of the silicon substrate before and after the texture etching, and the etching rate (μm / min) Represents an etching rate obtained by dividing the etching amount by the etching time.

<Evaluation of texture etching>
The foaming property of the etching solution at the time of etching and the silicon substrate after the etching were evaluated by the following methods.

(1) Presence or absence of foaming property of etching solution during texture etching:
Visual observation shows that bubbles generated by the hydrogen gas generated from the surface of the silicon substrate do not disappear at the interface of the etching solution and become bubbles and overflow from the etching tank. The case where bubbles due to disappeared at the interface of the etching solution and did not become bubbles was evaluated as “no foam”.

(2) Appearance evaluation of silicon substrate:
Visual evaluation was performed according to the following criteria.
○: The entire surface of the substrate is uniformly etched.
Δ: Slight spots or unevenness exists, but the etching uniformity is high over the entire surface of the substrate.
X: Spots or unevenness is confirmed.

(3) Texture average size:
The texture average size means the average side length of the pyramid obtained from the number of pyramids per unit area. Specifically, it is calculated by the following method. That is, first, an arbitrary 90 × 120 μm region of the textured substrate is enlarged and observed, and the number of vertices of the pyramid existing in the region is counted. Next, the area of the counting area is divided by the number of pyramid vertices, and the average side length of the pyramid obtained from the square is defined as the texture average size. The calculation formula is shown below.

[Equation 1]
Texture average size = √A / N
A (μm ² ): Area of measurement region (90 × 120 μm) N (pieces): Number of vertices of pyramids in measurement region (90 × 120 μm)

(4) Light reflectance:
An ultraviolet / visible / near infrared spectrophotometer (manufactured by Shimadzu Corporation, UV-3150) was used to measure the surface reflectance at a measurement wavelength of 600 nm.

(5) Electron microscope observation:
This was carried out using a scanning electron microscope (JSM-6510, manufactured by JEOL Ltd.).

  The evaluation results of texture etching are shown in Table 2 and FIGS.

  The uniformity of the appearance of the silicon substrate etched using the etching solutions of Examples 1 to 13 was equal to or higher than the etching solution using the conventional IPA (Reference Example 1). Furthermore, it was confirmed by electron microscope observation that a fine texture structure having an average size of 2.0 to 4.0 μm was formed on the surfaces of these silicon substrates. Moreover, the reflectance was also a sufficient value that can be used as a solar cell. On the other hand, the etching solution of Comparative Example 1 using sodium 4-octylbenzenesulfonate as the compound (A) has foamability during etching, and the etching rate is very slow, and the substrate appearance after etching is white. Unevenness occurred strongly.

  According to the present invention, fine irregularities having a texture average size of 2.0 to 4.0 μm can be formed on the surface of a silicon substrate, and high efficiency of a solar cell using the silicon substrate can be realized. It is industrially promising because it can reduce the environmental burden and reduce costs in terms of exhaust gas and wastewater treatment.

Claims (10)

An etching solution for immersing a silicon substrate to form pyramidal irregularities on the surface of the substrate, containing a compound (A) represented by the following general formula (1) and an alkali hydroxide (B) Etching solution characterized.
(In the formula (1), X is a sulfonic acid group or an alkali salt thereof, R ¹ and R ² are the same or different, and each represents a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group (all having 1 or 2), provided that R ¹ and R ² have 0 to 2 carbon atoms in total.   The etching solution according to claim 1, wherein the compound (A) is a compound having a substituent at least at the 4-position.   The etching solution according to claim 1 or 2, wherein the concentration of the compound (A) is 0.01 to 30% by weight.   The etching solution according to any one of claims 1 to 3, wherein the alkali hydroxide (B) is sodium hydroxide and / or potassium hydroxide.   The etching solution according to any one of claims 1 to 4, wherein the concentration of the alkali hydroxide (B) is 0.1 to 50% by weight.   Furthermore, the etching liquid in any one of Claims 1-5 containing a silicate compound (C).   The etching solution according to claim 6, wherein the silicate compound (C) is sodium or potassium silicate.   The etching solution according to claim 6 or 7, wherein the concentration of the silicate compound (C) is 10 wt% or less as the Si equivalent concentration.   A method for processing a surface of a silicon substrate, comprising the step of immersing the silicon substrate in the etching solution according to claim 1 to form pyramidal irregularities on the surface of the substrate. The surface processing method according to claim 9, wherein the average size of the pyramidal irregularities is 2.0 to 4.0 μm.