JP2011222682A - Forming method of silicone oxide film and manufacturing method of solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a silicone oxide film on the surface of silicon substrates by heat treatment in a furnace; these substrates, closely attached without an air gap in-between, are stacked in upright position in the length direction of the furnace, or laminated in the height direction of the furnace for heat treatment.SOLUTION: This forming method of silicon oxide film improves productivity remarkably because the number of silicon substrates loaded in a thermal oxidation furnace increases dramatically. This method is quite effective for manufacturing a crystal-based solar cells that requires a texture structure. It also produces no degradation in solar cell performance. Therefore it can provide an efficient manufacturing method of solar cell.

Description

  The present invention relates to a silicon oxide film forming method for forming an oxide film simultaneously and uniformly on a large number of silicon substrates, and a solar cell manufacturing method using the same.

In the manufacture of semiconductor devices, the silicon oxide film plays a major role. The silicon oxide film is a field oxide film in an element isolation region that electrically isolates elements such as individual active elements and passive elements, and a MOS transistor. It is widely used as a gate oxide film. Therefore, a technique for forming a silicon oxide film is an important technique.
As a technique for forming a silicon oxide film, there is a thermal oxidation method in which a silicon substrate surface is directly oxidized. In a conventional method for forming a silicon oxide film, a horizontal or vertical furnace is used, and a silicon substrate to be heat-treated is a silicon substrate with respect to one boat support groove by a boat made of SiC or quartz material. It is not easy to process a large number of silicon substrates simultaneously with a single thermal oxidation.

As countermeasures, conventional silicon substrate thermal oxidation methods and apparatuses for mass silicon substrates are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 57-97622 and 53-25351 (Patent Documents 1 and 2). . In this state, a plurality of silicon substrates are stacked vertically and placed on a silicon boat, and the presser plates are pressed and supported on both sides of the stacked silicon substrates. The silicon substrate pressed and supported above is heat-treated in a heat treatment furnace.
However, in general, with a mirror-finished silicon substrate, if multiple silicon substrates are stacked and thermally oxidized, the silicon substrates adhere to each other, making it difficult to peel off the silicon substrates after thermal oxidation. It has not been.

By the way, the role of the silicon oxide film in the manufacturing process of solar cells, particularly crystalline solar cells, is diverse, not only as an antireflection film, but also as a passivation film on the light-receiving surface and non-light-receiving surface, and also as a mask for diffusion, etc. Has been.
FIG. 1 shows an overview of a cross section of a high-efficiency solar cell using a single crystal or polycrystalline silicon substrate. A thin diffusion layer 102 having a conductivity type opposite to that of the substrate is provided on the light receiving surface of the substrate 101 of the solar cell 1, and a silicon oxide film (SiO ₂ film) is formed thereon as the antireflection film 103. On the light receiving surface side, electrodes 105 for collecting photoexcited carriers are provided at intervals of several mm. A passivation film 104 made of a silicon oxide film is formed on the non-light-receiving surface, and a metal such as silver or aluminum is deposited partially or entirely on the electrode 106 for current collection. In order to improve the productivity of a solar cell having such a structure, it is necessary to process a large number of substrates in the same thermal oxidation process, and a method for forming a silicon oxide film more efficiently has been demanded.

JP-A-57-97622 Japanese Patent Laid-Open No. 53-25351

  The present invention has been made in view of the above problems, and a silicon oxide capable of simultaneously thermally oxidizing a large number of silicon substrates and forming a silicon oxide film uniformly on the silicon substrate surface in a single heat treatment step. It aims at providing the formation method of a film | membrane, and the manufacturing method of a solar cell using the same.

  As a result of diligent studies to achieve the above object, the present inventors preferably heat-treat two or more silicon substrates in a furnace to form a silicon oxide film on the substrate surface. Using a substrate having a fine concavo-convex structure on the surface, closely contacting without providing a gap between the substrates, and stacking upright in the length direction of the furnace or stacking in the height direction of the furnace By performing the heat treatment, it has been found that a large number of silicon substrates can be processed at one time without sticking the substrates together, and a silicon oxide film can be uniformly formed on the substrate surface, and the present invention has been made. In the present invention, the surface of the substrate means a surface on which a silicon oxide film is to be formed, and may be either one or both of the light receiving surface and the non-light receiving surface of the substrate.

Accordingly, the present invention provides the following silicon oxide film forming method and solar cell manufacturing method.
Claim 1:
A method of heat-treating a plurality of silicon substrates in a furnace to form a silicon oxide film on the surface of the substrate, wherein the substrates are brought into close contact with each other without providing a gap between them and are erected in the length direction of the furnace. A method of forming a silicon oxide film, wherein the silicon oxide film is heat-treated by being superposed in a state or stacked in the height direction of the furnace.
Claim 2:
The method for forming a silicon oxide film according to claim 1, wherein the substrate has a fine uneven structure on a surface thereof.
Claim 3:
The method for forming a silicon oxide film according to claim 2, wherein the unevenness of the substrate surface is 1 to 50 μm.
Claim 4:
4. The method for forming a silicon oxide film according to claim 2, wherein the concavo-convex structure on the substrate surface is a texture structure for reducing the reflectance of the substrate surface.
Claim 5:
The atmosphere in the furnace is an atmosphere selected from oxygen, water vapor and a mixed gas thereof, a mixed gas of hydrogen and oxygen, and a mixed gas obtained by adding a gas containing chlorine atoms to the mixed gas as necessary. 5. The method for forming a silicon oxide film according to any one of claims 1 to 4.
Claim 6:
The method for forming a silicon oxide film according to any one of claims 1 to 5, wherein a heat treatment temperature is 700 to 1,100 ° C.
Claim 7:
The method for forming a silicon oxide film according to claim 1, wherein the substrate is placed on a heat treatment boat and heat treated.
Claim 8:
8. The method for forming a silicon oxide film according to claim 1, wherein a support holder is disposed at a terminal portion of the stacked or stacked substrates.
Claim 9:
9. A method for manufacturing a solar cell, comprising a step of forming a silicon oxide film as an antireflection film, a passivation film or a diffusion mask on a semiconductor substrate surface, wherein the silicon oxide film is silicon according to any one of claims 1 to 8. It forms with the formation method of an oxide film, The manufacturing method of the solar cell characterized by the above-mentioned.
Claim 10:
The method for manufacturing a solar cell according to claim 9, wherein the silicon oxide film has a thickness of 5 to 250 nm.

  By using the method for forming a silicon oxide film of the present invention, the number of sheets filled in the thermal oxidation furnace is remarkably increased, so that productivity is remarkably improved. ADVANTAGE OF THE INVENTION According to this invention, since it is very effective for manufacture of the crystal-type solar cell in which a texture structure is essential, and since the fall of solar cell performance does not arise, the manufacturing method of an efficient solar cell can be provided.

It is a schematic sectional drawing which shows an example of the structure of a general solar cell. It is an expansion perspective view which shows an example of the texture structure of a board | substrate surface. It is the schematic explaining an example of the formation method of the silicon oxide film of this invention. It is the schematic explaining the other example of the formation method of the silicon oxide film of this invention. It is explanatory drawing which shows an example of the manufacturing process of the solar cell of this invention. (A) shows a state where the substrate surface is etched, (B) shows a state where an emitter layer is formed, (C) shows a state where an antireflection film and a passivation film are formed, and (D) shows a state where an electrode is formed. It is explanatory drawing which shows the other example of the manufacturing process of the solar cell of this invention. (A) is a state where a texture is formed on the substrate surface, (B) is a state where an emitter layer is formed, (C) is a state where a silicon oxide film is formed, (D) is a state where an antireflection film is formed, (E ) Shows the state where the electrodes are formed. It is explanatory drawing which shows another example of the manufacturing process of the solar cell of this invention. (A) is a state where a texture is formed on the substrate surface, (B) is a state where a silicon oxide film is formed, (C) is a state where the silicon oxide film is removed only on the light receiving surface, and (D) is a state where an emitter layer is formed. , (E) shows a state where an antireflection film is formed, and (F) shows a state where an electrode is formed.

Hereinafter, embodiments of a method for forming a silicon oxide film and a method for manufacturing a solar cell according to the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments.
The method for forming a silicon oxide film of the present invention is a method in which a plurality of silicon substrates are heat-treated in a furnace to form a silicon oxide film on the surfaces of the substrates, and the substrate is formed without providing a gap between the substrates. It is made to adhere, and it heat-processes by superimposing in the upright state in the length direction of the said furnace, or laminating | stacking in the height direction of the said furnace.

  The substrate used in the method for forming a silicon oxide film of the present invention is not particularly limited, and any substrate such as a single crystal silicon substrate or a polycrystalline silicon substrate can be used. In particular, a P-type or N-type silicon substrate having a texture formed on the surface thereof, which is used for manufacturing a solar cell as described later, is preferable.

  In particular, a silicon substrate used for manufacturing a solar cell usually has a fine concavo-convex shape called a texture on the surface in order to reduce the reflectance in the visible light region on the light receiving surface. FIG. 2 shows a texture structure formed on the silicon substrate surface. This is a (111) plane pyramid structure by selective etching on the Si (100) plane. Generally, the height difference d of the texture on the substrate surface used for solar cell production is preferably 1 to 50 μm, more preferably 1 to 10 μm, and further preferably 1 to 5 μm. If the thickness is less than 1 μm, a sufficient reflectance reduction effect may not be obtained. If the thickness exceeds 50 μm, the electrodes formed on the light-receiving surface and the non-light-receiving surface may be distorted, which may cause deterioration in solar cell characteristics. In the present invention, the height difference of the irregularities on the substrate surface can be measured by a scanning electron microscope (SEM). The method for forming the fine uneven structure on the surface such as the texture is as described later.

  Here, in the case of a silicon substrate that has been generally mirror-finished so far, if a plurality of silicon substrates are laminated and subjected to thermal oxidation, the substrates adhere to each other, and the substrate cannot be peeled off after thermal oxidation. A method of mounting one silicon substrate on one boat support groove using a boat made of SiC or quartz material has been used. For this reason, the filling rate is limited, and it is not easy to form a silicon oxide film on a large number of substrates by one thermal oxidation. On the other hand, in the present invention, in particular, by using a substrate having a fine uneven structure such as a texture structure on the surface, even if a plurality of substrates are stacked and subjected to thermal oxidation, the substrates do not stick to each other, A uniform oxide film can be formed on a large number of silicon substrates at once.

  Specifically, in the method for forming a silicon oxide film of the present invention, for example, as shown in FIG. 3, two or more, preferably 100 to 5, silicon substrates 301 having a textured surface and a fine concavo-convex structure on the surface are used. 1,000, especially 1,300 to 3,400, are superposed and aligned in the upright state along the length direction of the heat treatment furnace, placed on the heat treatment boat 302, and this boat is placed in the furnace 303 in a predetermined atmosphere. By inserting, an oxide film can be uniformly formed on all substrate surfaces. At this time, the substrates are overlapped without being provided with a gap. The heat treatment boat 302 is made of a material having excellent high-temperature strength such as SiC or quartz material, and is a flat boat having a wheel 304 at the lower portion, and the surface in contact with the silicon substrate preferably has an uneven structure similar to that of the substrate. . This is because a boat having a concavo-convex structure is easier for gas to enter between the substrates and does not hinder the formation of an oxide film. The support holder 305 plays a role of preventing the silicon substrate from falling down and laterally shifting, and preventing damage. The support holder 305 is made of the same material as that of the boat, and for the same reason as described above, the surface in contact with the silicon substrate preferably has a concavo-convex structure similar to that of the substrate. The heat treatment boat 302 is inserted into a furnace 303 having a predetermined atmosphere by using a rail 306 made of the same material as the boat and the holder and leading to the furnace. When thermal oxidation is performed by this method, it is possible to process a silicon substrate six times or more as compared with the conventional method at a time.

  In the present invention, not only a method in which the silicon substrates are stacked in an upright state in the length direction of the furnace as shown in FIG. 3, but also stacked in the height direction of the furnace as shown in FIG. An oxide film can be formed on a large amount of silicon substrate. FIG. 4 shows a stack of two or more, preferably 50 to 1,000, particularly 50 to 100, stacked silicon substrates 401 as a group. A support holder 405 made of a material having excellent high-temperature strength is loaded, transferred to a heat treatment boat 402, and one or a plurality of laminates of 2 to 6 are thermally oxidized in a furnace 403 at a time. is there. Reference numeral 404 denotes a wheel, and 406 denotes a boat rail.

  The film thickness of the silicon oxide film formed by the method of the present invention cannot be generally specified depending on the purpose of film formation and the function of the film. For example, in the solar cell manufacturing process, about 5 to 250 nm, It is preferable that a silicon oxide film of about 10 to 150 nm can be formed uniformly with good controllability. Accordingly, the thermal oxidation treatment temperature (furnace temperature) in the method for forming a silicon oxide film of the present invention is preferably 700 to 1,100 ° C., more preferably 850 to 1,050 ° C. If the temperature is lower than 700 ° C., the amount of heat energy required for the reaction is small, and a sufficient oxide film may not be formed. If it exceeds 1,100 ° C., the thermal reaction effect becomes prominent, and it may be difficult to control the oxide film thickness. If the temperature is high, the diffusion rate of heavy metals will increase, so the lifetime will be reduced and the solar It may cause deterioration of battery characteristics. The treatment time is preferably 10 to 360 minutes, more preferably 10 to 300 minutes, and particularly preferably 30 to 100 minutes.

Gas types used for oxidation are O ₂ (oxygen gas), H ₂ O (water vapor), O ₂ —H ₂ O (mixed gas of oxygen and water vapor), H ₂ —O ₂ (mixed gas of hydrogen and oxygen), etc. Or a gas obtained by adding a gas containing a halogen atom (chlorine atom) such as HCl (hydrogen chloride gas) or Cl ₂ (chlorine gas) to these atmospheres, and performing thermal oxidation in a furnace of these atmospheres Thus, it is possible to form an oxide film on the substrate surface. In terms of equipment, it is easy to use O ₂ (dry oxidation), and using H ₂ —O ₂ (pyrogenic oxidation) increases the growth rate of the oxide film, which is preferable for shortening the process time. . When the H ₂ —O ₂ gas is used, the mixing ratio of H ₂ gas and O ₂ gas is preferably 1: 0.5 to 1: 2.

Next, with reference to FIGS. 5-7, the manufacturing method (I)-(III) of the solar cell using the formation method of the silicon oxide film of this invention is demonstrated.
(I) Manufacturing method of solar cell having silicon oxide film as antireflection film As shown in FIG. 5, high purity silicon is doped with a group III element such as boron and gallium, and a specific resistance of 0.1 to 5 Ω · cm The slicing damage on the surface of the as-cut single crystal {100} P-type silicon substrate 501 is a high concentration alkali such as sodium hydroxide or potassium hydroxide having a concentration of 5 to 60% by mass or a mixed acid of hydrofluoric acid and nitric acid. And etching (FIG. 5A). An uneven structure having a height difference of 1 to 50 μm, particularly 1 to 10 μm, especially 1 to 5 μm can be formed on the substrate surface by etching such as high concentration alkali treatment. The single crystal silicon substrate may be manufactured by either the CZ method or the FZ method. The conductivity type of the substrate may be an N type doped with a group V element such as phosphorus or arsenic. In the present invention, the case of a P type substrate will be described below.

Subsequently, texture formation is performed on the substrate surface. The texture is immersed in an alkali solution (concentration: 1 to 10% by mass, temperature: 60 to 100 ° C.) such as heated sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, and sodium hydrogencarbonate for about 10 to 30 minutes. It is easy to make. In many cases, a predetermined amount of 2-propanol is dissolved in the solution to promote the reaction. As a result, a concavo-convex structure having a height difference of 1 to 50 μm, particularly 1 to 10 μm, especially 1 to 5 μm is formed on the substrate surface.
After texture formation, washing is performed in an acidic aqueous solution of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, or the like or a mixture thereof. From an economic and efficient standpoint, washing in hydrochloric acid is preferred. In order to improve the cleanliness, 0.5 to 5% by mass of hydrogen peroxide may be mixed in a hydrochloric acid solution and heated to 60 to 90 ° C. for washing.

Over this substrate, an emitter layer 502 is formed by a vapor phase diffusion method using phosphorus oxychloride (FIG. 5B). In general silicon solar cells, it is necessary to form a PN junction only on the light-receiving surface, and in order to achieve this, diffusion is performed in a state where two substrates are overlapped with each other, or silicon is oxidized on the non-light-receiving surface before diffusion. A film (SiO ₂ film) or the like may be formed as a diffusion mask so that a PN junction cannot be formed on the non-light-receiving surface. After diffusion, the glass on the surface is removed with hydrofluoric acid.

Next, the antireflection film 503 on the light receiving surface is formed by the silicon oxide film forming method (thermal oxidation) of the present invention (FIG. 5C). Further, by using the thermal oxide film (silicon oxide film) 504 also on the non-light-receiving surface, the passivation effect of the light-receiving surface and the non-light-receiving surface is increased, which contributes to improvement in conversion efficiency. Thermal oxidation is performed in a state where the substrates are overlapped by the method of the present invention. Dry oxidation (O ₂ ), wet oxidation (O ₂ —H ₂ O), pyrogenic at 950 to 1,100 ° C., especially 1,000 to 1,050 ° C. for about 5 to 120 minutes, particularly 90 to 120 minutes In addition to oxidation (H ₂ —O ₂ ), any method such as introducing a gas such as HCl or Cl ₂ may be used. By this method, a silicon oxide film having a thickness of 90 to 150 nm, particularly 100 to 120 nm is formed on the light receiving surface. If it is out of this range, the reflectance becomes high, and there may be a problem that the short-circuit current is lowered.

Subsequently, electrodes on the light receiving surface and the non-light receiving surface are formed by a method such as vapor deposition, sputtering, screen printing, or ink jet. In the case of the screen printing method, a silver paste in which silver powder and glass frit are mixed with an organic binder is screen-printed and then baked at a temperature of 700 to 850 ° C. for 5 to 30 minutes to form electrodes 505 and 506 (see FIG. 5 (D)). By baking, silver powder is passed through the silicon oxide films (SiO ₂ films) 503 and 504 (fire through), and the electrode and silicon are made conductive. The firing of the light-receiving surface electrode and the non-light-receiving surface electrode can be performed at one time, but may be performed separately.

  When an evaporation method or a sputtering method is used for forming an electrode, it is necessary to provide an opening for forming an electrode in the oxide film. For forming the opening, there are a method of applying thermal energy by a laser, a method of physically forming with a dicer or the like, and a method of chemically forming with an etching paste. If the aluminum film is formed on the entire surface after the opening is formed only on the non-light-receiving surface, the passivation effect on the non-light-receiving surface is dramatically improved, and the conversion efficiency of the solar cell is improved.

  Needless to say, the electrode formation by the screen printing method and the electrode formation by the vapor deposition method after opening can be used in combination with the light receiving surface and the non-light receiving surface. In the above method, an oxide film is formed on both the light-receiving surface and the non-light-receiving surface, but only the oxide film on the light-receiving surface is removed with hydrofluoric acid or the like, and a SiNx film is formed to reflect the reflectance. Alternatively, aluminum may be formed on the entire surface by removing only the oxide film on the non-light-receiving surface.

(II) Method for Manufacturing Solar Cell Having Silicon Oxide Film as Passivation Film (Surface Stabilization or Protection Film) In the above method, the silicon oxide film is used as an antireflection film, but the silicon oxide film has a refractive index of 1. Since it is as low as about .5, an effective antireflection effect may not be obtained. For this reason, there is a method in which the oxide film on the light-receiving surface is thinned so that only the passivation effect is exhibited, and an antireflection film is formed from another material. Below, an example is shown using FIG.

A texture is formed on the surface of the silicon substrate 601 by the same method as in (I) (FIG. 6A), a diffusion layer (emitter layer) 602 is formed (FIG. 6B), and then the diffusion layer 602 is formed. Then, thin silicon oxide films 603 and 604 are formed on the non-light-receiving surface side (FIG. 6C). The oxide film thickness is preferably 5 to 50 nm, particularly preferably 10 to 20 nm. If it exceeds 50 nm, the reflectance becomes high, and a problem such as a short circuit current being reduced may occur. On the other hand, if the thickness is less than 5 nm, the passivation effect does not appear and the open circuit voltage may be lowered.
In forming the silicon oxide film, thermal oxidation is performed in a state where a plurality of substrates are stacked by the method of the present invention. 700 to 1,000 ° C., especially 850 to 950 ° C. for about 5 to 60 minutes, especially 10 to 30 minutes. In addition to dry oxidation, wet oxidation, pyrogenic oxidation, and other gases such as HCl and Cl ₂ are introduced. The method may be used.

After the formation of the thin silicon oxide films 603 and 604, antireflection films 607 and 608 such as a SiNx film are formed on the light receiving surface and the non-light receiving surface by a method such as CVD (FIG. 6D). In the SiNx formation by the CVD method, monosilane (SiH ₄ ) and ammonia (NH ₃ ) are often mixed and used as the reaction gas, but nitrogen can be used instead of NH ₃ , and the process In some cases, hydrogen is mixed into the reaction gas in order to adjust the pressure, dilute the reaction gas, and further promote the bulk passivation effect of the substrate when polycrystalline silicon is used for the substrate.

Next, electrodes 605 and 606 on the light-receiving surface and the non-light-receiving surface are formed by a method such as vapor deposition, sputtering, screen printing, or ink jet (FIG. 6E). In the case of the screen printing method, a silver paste in which silver powder and glass frit are mixed with an organic binder is screen-printed and then baked at a temperature of 700 to 850 ° C. for 5 to 30 minutes after printing to make the electrode and silicon conductive. The electrode is formed. The firing of the light-receiving surface electrode and the non-light-receiving surface electrode can be performed at one time, but may be performed separately.
In the above method, aluminum may be formed on the entire surface without forming the SiNx film on the silicon oxide film 604 on the non-light-receiving surface.

(III) Manufacturing method of solar cell having silicon oxide film as diffusion mask As in (I) and (II), the silicon oxide film is not only used as an antireflection film or a passivation film, but also as a diffusion mask. Can also be used. Hereinafter, an example is shown using FIG.
After the texture is formed on the surface of the substrate 701 by the same method as in (I) (FIG. 7A), thick silicon oxide films 709 and 704 are formed on the light-receiving surface and the non-light-receiving surface of this substrate (FIG. 7 ( B)). This oxide film is used later as a mask for removing impurities on the non-light-receiving surface side in the vapor phase diffusion method while removing the light-receiving surface side and leaving the non-light-receiving surface side. The oxide film thickness is preferably 50 to 250 nm, particularly preferably 100 to 150 nm. If it is less than 50 nm, the function as a diffusion mask cannot be achieved, and a PN junction is formed on the non-light-receiving surface side, which may deteriorate the solar cell characteristics. Although there is no problem with the thicker portion, an oxide film exceeding 250 nm may be difficult to form because the oxidation reaction of silicon becomes diffusion-limited. Thermal oxidation is performed in a state where a plurality of substrates are stacked by the method of the present invention. In addition to dry oxidation, wet oxidation, pyrogenic oxidation, and gases such as HCl and Cl ₂ at 950 to 1,100 ° C., especially 1,000 to 1,050 ° C. for about 5 to 360 minutes, particularly 120 to 300 minutes. Any method such as introduction may be used.

  Next, only the oxide film 709 on the light-receiving surface side is removed using hydrofluoric acid or the like (FIG. 7C). An emitter layer 702 is formed on the light-receiving surface of the substrate by a vapor phase diffusion method using phosphorus oxychloride (FIG. 7D). Since the oxide film 704 exists on the non-light-receiving surface, phosphorus is not diffused on the non-light-receiving surface side. After diffusion, the glass on the surface is removed with hydrofluoric acid.

Next, an antireflection film (SiO ₂ film, SiNx film, etc.) 703 on the light receiving surface is formed by thermal oxidation or CVD (FIG. 7E), and electrodes 705 and 706 on the light receiving surface and the non-light receiving surface are deposited by evaporation. A solar cell is manufactured by a method such as a sputtering method, a screen printing method, or an inkjet method (FIG. 7F). The formation method is as described above.
The mask oxide film 704 on the non-light-receiving surface finally functions as a passivation film on the non-light-receiving surface and contributes to improving the characteristics of the solar cell.

  EXAMPLES Hereinafter, although an experiment example and an Example are shown and this invention is demonstrated more concretely, this invention is not restrict | limited to the following Example.

[Experimental Example 1]
There were prepared 100 substrates each of three types of substrates: a silicon substrate with a mirror finish, a thickness of 155 × 155 mm, a silicon substrate after alkali etching, and a silicon substrate after texture formation. Removal of the damage layer by slicing is performed using hot concentrated potassium hydroxide having a concentration of 5 to 60% by mass, and then 60 to 100 in a potassium hydroxide / 2-propanol aqueous solution (potassium hydroxide concentration 1 to 10% by mass). A texture was formed by immersion at 10 ° C. for 10 to 30 minutes. The height difference of the surface irregularities after damage etching and texture formation was about 3 μm. Using these substrates, 100 sheets (301) are stacked on the heat treatment boat 302 using the support holder 305 by the method shown in FIG. 3, and are placed in the horizontal heat treatment furnace 303 to be 1,050 ° C. in an oxygen atmosphere. Thermal oxidation was performed for 6 hours.

As a result, some of the substrates with a mirror finish were bonded to each other, and some were difficult to peel, whereas the substrate after damage etching having a concavo-convex structure on the surface In addition, the substrates after texture formation do not adhere to each other, and all the substrate surfaces change to an orange color peculiar to the silicon oxide film, and all the substrates exhibit an in-plane uniform color and are uniform even in the stacked state. An oxide film was formed.
As described above, in the case of a substrate having a concavo-convex structure in which the substrate surface is not polished, even if a plurality of substrates are arranged in a stack and subjected to thermal oxidation, the substrates do not stick to each other at a time with respect to a large number of silicon substrates. It is possible to form a uniform oxide film.

[Example 1]
In the same manner as in Experimental Example 1, after removing the damaged layer by slicing with a hot concentrated potassium hydroxide aqueous solution on 100 boron-doped {100} P-type as-cut silicon substrates having a thickness of 250 μm and a specific resistance of 1 Ω · cm, Texture formation was performed by dipping in an aqueous potassium oxide / 2-propanol solution, followed by washing in a hydrochloric acid / hydrogen peroxide mixed solution. The height difference of the surface irregularities was 3 μm.
Next, heat treatment was performed in a phosphorus oxychloride atmosphere at 870 ° C. with the non-light-receiving surfaces overlapped to form an emitter layer. After diffusion, the glass was removed with hydrofluoric acid, washed and dried.
After the above processing, as shown in FIG. 4, in a state 401 in which 50 substrates are stacked, a pair of support holders 405 loaded on top is placed on two sets of heat treatment boats 402. It was put into a horizontal heat treatment furnace 403 and subjected to thermal oxidation. Oxidation was performed in a dry oxygen atmosphere (oxygen gas only) at a temperature of 1,050 ° C. for 60 minutes.

Adhesion between the substrates after the treatment was not confirmed at all, the appearance was all blue, and no in-plane color unevenness was confirmed on any of the substrates. The oxide film thickness was determined to be about 110 nm from the color.
Next, 10 sheets were randomly selected from the obtained samples, and silver paste was screen-printed as an electrode layer on the light-receiving surface and then dried. Then, it baked in 780 degreeC air atmosphere. Next, an opening was formed in the oxide film on the non-light-receiving surface using a dicer, and an aluminum electrode was formed on the entire surface of the non-light-receiving surface by vapor deposition to produce a solar cell.
Table 1 shows the measurement results of electric characteristics (average value of 10 sheets) at the time of simulated sunlight irradiation of 25 ° C., 100 mW / cm ² , spectrum AM1.5 global using the produced solar cell.

  From the above results, it was found that a highly efficient solar cell can be produced by using the oxidation method according to the present invention.

DESCRIPTION OF SYMBOLS 1 Solar cell 101 Substrate 102 Diffusion layer 103 Antireflection film (silicon oxide film)
104 Passivation film 105 Light-receiving surface electrode 106 Non-light-receiving surface electrode 301, 401 Substrate 302, 402 Heat treatment boat 303, 403 Heat treatment furnace 304, 404 Wheel 305, 405 Support holder 306, 406 Boat rail 501, 601, 701 Substrate 502, 602 702 Emitter layer (diffusion layer)
503,703 Antireflection film (silicon oxide film)
504, 603, 604 Passivation film (silicon oxide film)
505, 605, 705 Light-receiving surface electrode 506, 606, 706 Non-light-receiving surface electrode 607, 608 Antireflection film (SiNx film)
704, 709 Diffusion mask (silicon oxide film)
d Surface unevenness difference

Claims (10)

  A method of heat-treating a plurality of silicon substrates in a furnace to form a silicon oxide film on the surface of the substrate, wherein the substrates are brought into close contact with each other without providing a gap between them and are erected in the length direction of the furnace. A method of forming a silicon oxide film, wherein the silicon oxide film is heat-treated by being superposed in a state or stacked in the height direction of the furnace.   The method for forming a silicon oxide film according to claim 1, wherein the substrate has a fine uneven structure on a surface thereof.   The method for forming a silicon oxide film according to claim 2, wherein the unevenness of the substrate surface is 1 to 50 μm.   4. The method for forming a silicon oxide film according to claim 2, wherein the concavo-convex structure on the substrate surface is a texture structure for reducing the reflectance of the substrate surface.   The atmosphere in the furnace is an atmosphere selected from oxygen, water vapor and a mixed gas thereof, a mixed gas of hydrogen and oxygen, and a mixed gas obtained by adding a gas containing chlorine atoms to the mixed gas as necessary. 5. The method for forming a silicon oxide film according to any one of claims 1 to 4.   The method for forming a silicon oxide film according to any one of claims 1 to 5, wherein a heat treatment temperature is 700 to 1,100 ° C.   The method for forming a silicon oxide film according to claim 1, wherein the substrate is placed on a heat treatment boat and heat treated.   8. The method for forming a silicon oxide film according to claim 1, wherein a support holder is disposed at a terminal portion of the stacked or stacked substrates.   9. A method for manufacturing a solar cell, comprising a step of forming a silicon oxide film as an antireflection film, a passivation film or a diffusion mask on a semiconductor substrate surface, wherein the silicon oxide film is silicon according to any one of claims 1 to 8. It forms with the formation method of an oxide film, The manufacturing method of the solar cell characterized by the above-mentioned.   The method for manufacturing a solar cell according to claim 9, wherein the silicon oxide film has a thickness of 5 to 250 nm.

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