JP2007149843A - Method of forming passivation film, and manufacturing method of solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that thermal oxidizing for an extended period or a process at a high temperature is required for obtaining a thick dry oxide film on the surface of a silicon substrate, however the process at a high temperature shortens lifetime of the silicon substrate, and if the temperature for thermal oxidizing is lowered, the dry oxide film becomes relatively thin as film-forming requires time, and in case of a thin dry oxide film, its protective film is required to be formed, but the thin dry oxide film is damaged when the silicon substrate is moved to an APCVD device after taken out of an oxidizing furnace, resulting in low passivation. <P>SOLUTION: The forming method of a passivation film includes a process in which a dry oxide film and a steam oxide film are sequentially formed on a silicon substrate in the same furnace. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for forming a passivation film on a silicon substrate and a method for manufacturing a solar cell using the passivation film.

Many elements that constitute an electronic device using a silicon substrate have been developed. In particular, solar cells have attracted attention as clean energy sources, and are being actively developed. And, in order to improve the characteristics of solar cells using a silicon substrate as a semiconductor, not only the loss of minority carriers inside the semiconductor bulk but also the prevention of minority carrier loss on the semiconductor surface has been attempted. . In particular, with respect to prevention of minority carrier loss on the semiconductor surface, a silicon surface passivation technique has been developed, and the oxidation passivation film modification method (Patent Document 1) and the passivation effect are fully exhibited and have an antireflection effect. A silicon nitride film (Patent Document 2) is known.
Japanese Patent No. 3225268 JP 2005-159171 A

  When using a passivation film for the purpose of preventing carrier recombination to prevent minority carrier loss on the semiconductor surface, the use of a silicon nitride film as a passivation film has an effect as an antireflection film and is highly useful. It has been known. However, since the passivation effect is lower than that of the dry oxide film, and particularly the p + layer has a small passivation effect, it is necessary to use a dry oxide film formed by thermal oxidation in order to obtain a higher passivation effect. The deposition rate of the dry oxide film by thermal oxidation is small, and in order to obtain a thick film, long-time thermal oxidation or high-temperature treatment is required. Processing at a high temperature reduces the lifetime of the silicon substrate. On the other hand, if the temperature of thermal oxidation is lowered, it takes a long time to form a film, so that the dry oxide film becomes a relatively thin film. In the case of a thin film, it is necessary to form a protective film separately.

  As the protective film, a CVD oxide film by an APCVD (atmospheric pressure chemical vapor deposition) method has been deposited so far. A CVD oxide film has a small passivation effect, but is suitable as a protective film because of its high film formation rate. However, when a silicon substrate having a thin dry oxide film is taken out of the oxidation furnace and the silicon substrate is moved to the APCVD apparatus, the thin dry oxide film may be damaged, and high passivation may not be obtained.

  Accordingly, the present invention has been made in view of such conventional problems, and an object thereof is to provide a method for forming a passivation film and a method for manufacturing a solar cell that can sufficiently exhibit a passivation effect. .

  The present invention is a method for forming a passivation film including a step of sequentially forming a dry oxide film and a steam oxide film on a silicon substrate in the same furnace.

  In the present invention, it is preferable to form a CVD oxide film after the formation of the steam oxide film.

  In the present invention, an n-type diffusion layer and / or a p-type diffusion layer is preferably formed on the surface of the silicon substrate.

  In the present invention, the dry oxide film and the steam oxide film are preferably formed at a temperature of 700 to 1000 ° C.

  Further, according to the present invention, the step of sequentially forming the dry oxide film and the steam oxide film in the same furnace may include forming the dry oxide film, flowing nitrogen into the furnace, stopping the nitrogen, and then forming the steam oxide film. preferable.

  The present invention also relates to a method for manufacturing a solar cell using a silicon substrate in which a dry oxide film and a steam oxide film are formed in the same furnace as a passivation film.

  The present invention also provides a solar cell that sequentially performs a step of performing n-type diffusion and p-type diffusion on a part of the same surface of a silicon substrate, a step of performing dry oxidation and steam oxidation in the same furnace, and a step of depositing a CVD oxide film. It relates to the manufacturing method.

  By sequentially forming the dry oxide film and the steam oxide film in the same furnace, it is possible to form a passivation film that is highly effective in preventing carrier recombination. In addition, a high passivation effect can be maintained by strengthening the passivation film with a steam oxide film having a higher deposition rate than the dry oxide film. Moreover, solar cell characteristics can be improved by using a highly effective passivation film.

<Method for manufacturing solar cell>
The solar cell obtained by the production method of the present invention is a solar cell in which a passivation film having a high passivation effect is formed on a silicon substrate. As shown in the plan view of the solar cell in FIG. 1A, the n-electrode 11 and the p-electrode 12 are provided on the silicon substrate 17.

  The substrate is crystalline silicon and may be either p-type or n-type. Since there is no difference in structure, an embodiment using an n-type substrate will be described below. The silicon substrate has a thickness of 100 to 300 μm, and the outer shape of the solar cell is a pseudo rectangle having a size of 10 cm × 10 cm to 15 cm × 15 cm. Hereinafter, a method for manufacturing a solar cell will be described with reference to FIG.

First, in order to remove the slice damage of the silicon substrate 17 sliced to a desired thickness, the surface is etched by about 10 to 20 μm with an alkaline solution such as a mixed acid of hydrofluoric acid and nitric acid or sodium hydroxide. Next, an oxide film as a dopant diffusion control mask (hereinafter referred to as “diffusion mask”) is formed on the entire surface of the silicon substrate 17 by 1000 to 4000 mm by steam oxidation or APCVD. Then, an acid-resistant resist is patterned into a desired shape by printing or a photo process on the back surface of the solar cell, and then the diffusion mask is etched with hydrofluoric acid. As the acid-resistant resist, for example, a positive resist generally used when etching an oxide film is used. Next, p + layer 14 can be formed in a desired shape by vapor-phase diffusion of BBr ₃ at 700 to 1000 ° C. for several tens of minutes. Then, the acid resistant resist is removed by dipping in an acetone solution. In addition to BBr ₃ vapor phase diffusion, it is also possible to use a method in which a chemical solution containing a B (boron) compound is spin coated on the silicon substrate 17 and then annealed at 700 to 1000 ° C.

  In addition, the p + layer 14 can be separately formed by patterning and baking an aluminum paste, forming a p + layer, and then dipping in a hydrochloric acid solution or the like to remove a metal portion such as Al. .

Thereafter, the diffusion mask is removed, and an oxide film as a diffusion mask is formed again on the entire surface of the silicon substrate 17 by steam oxidation or APCVD, and is applied to the back surface of the solar cell as in the case of the p + layer 14. Then, the diffusion mask is etched into a desired shape, and POCl ₃ is vapor-phase diffused for several tens of minutes at 700 to 1000 ° C., thereby forming the n + layer 13 in the desired shape. In addition to POCl ₃ vapor phase diffusion, it is also possible to use a method of annealing at 700 to 1000 ° C. after spin coating a chemical solution containing a P compound on the Si substrate.

  The shape of the p + layer 14 and the n + layer 13 may be a line or a dot shape. In the present invention, a shape in which the comb-shaped diffusion layers face each other is shown as an example in FIG. Moreover, it is necessary to pattern so that each diffusion layer may not overlap.

  Further, although the diffusion layers are formed in the order of the p + layer 14 and the n + layer 13, this order is not particularly limited, and any order and method may be used as long as a desired sheet resistance and diffusion profile can be obtained.

  Thereafter, the diffusion mask is removed with hydrofluoric acid, dry oxidation is performed at 700 to 1000 ° C. for several minutes to several hours in an oxygen atmosphere, the atmosphere is changed to steam in the same furnace, and steam is further performed for several minutes to several tens of minutes. Oxidize. Thereafter, a CVD oxide film can be deposited on the back surface of the solar cell by a CVD method if necessary. By this operation, the passivation film 15 is formed. A method for forming the passivation film 15 will be described in detail later.

  Then, the dry oxide film and the steam oxide film on the light receiving surface on which no CVD oxide film is deposited are removed with hydrofluoric acid, and a silicon nitride film is formed on the light receiving surface as an antireflection film 16 by plasma CVD.

  On the back surface of the solar cell, an acid-resistant resist is patterned into a desired shape by printing or a photo process on the passivation film 15, and the passivation film is etched with hydrofluoric acid to form a contact hole. Next, by depositing, for example, Ti—Pd—Ag as an electrode material, the n-electrode 11 and the p-electrode 12 having a desired pattern can be obtained by a wrist-off method in which the acid-resistant resist is removed by immersion in acetone. it can. In addition, the electrode is printed with a silver paste in a comb shape on the p + layer 14 and dried, and similarly, the silver paste is printed in a comb shape on the n + layer 13, and at 400 to 750 ° C. for several minutes to several tens of minutes. It is also possible to obtain an electrode having a desired pattern by firing within a range.

Although a silicon substrate is used in this embodiment mode, this silicon substrate may be either a single crystal substrate or a polycrystalline substrate. In the case of a single crystal substrate, texture treatment may be applied to the light receiving surface as necessary in order to suppress reflection on the surface. In FIG. 1B of the present embodiment, the cell light receiving surface side is shown as a texture structure.
<Method for Forming Passivation Film>
The passivation film is a surface protection film of a semiconductor element, and is a film that isolates and protects the element from the external environment and mechanically and chemically protects the surface of the semiconductor element. FIG. 2A is a schematic cross-sectional view of a solar cell according to one embodiment of the present invention. A desirable passivation film in the present invention formed on the back side of the solar cell shown in FIG. 2A is a laminate in which a dry oxide film 22, a steam oxide film 23, and a CVD oxide film 24 are sequentially formed on a silicon substrate 21. .

  Here, the dry oxide film refers to an oxide film formed by thermal oxidation, and the steam oxide film refers to an oxide film formed by steam oxidation. The CVD oxide film refers to an oxide film formed by a CVD method such as an APCVD method. The three types of oxide films are silicon oxide films having a passivation function.

≪Dry oxide film formation≫
The dry oxide film is formed to 1 to 20 nm on the silicon substrate. Since passivation such as dangling bonds on the substrate surface is terminated by oxygen, the passivation function is high. The dry oxide film is formed by placing it in a furnace having a 100% oxygen atmosphere for 5 minutes to 30 hours. In order to stop the formation of the dry oxide film, nitrogen is introduced into the furnace and the oxygen atmosphere is switched to the nitrogen atmosphere.

≪Formation of steam oxide film≫
Since the steam oxide film is formed faster than the dry oxide film, a thicker passivation film as a stacked body can be formed. The steam oxide film is formed with a thickness of 10 to 150 nm. The passivation function uses pure water compared to the dry oxide film, so the impurities are mixed in compared to oxygen gas, so the passivation function is inferior, but the film formation speed is higher than the dry oxide film. Since the passivation film can be reinforced by covering with a fast steam oxide film, it is useful. The steam oxide film is formed by placing it in a furnace at 700 to 1000 ° C. switched from the nitrogen atmosphere to an oxygen atmosphere containing water vapor for 5 to 60 minutes.

  When steam oxidation is performed, the temperature is increased in the steam oxidation than in the dry oxidation so that the formation of the steam oxide film is accelerated. Switch to and perform steam oxidation. This is because if the temperature is changed in an oxygen atmosphere, it becomes difficult to control the thickness of the dry oxide film, and therefore it is necessary to switch to a nitrogen atmosphere when the temperature is changed.

≪Formation of CVD oxide film≫
In the present invention, a CVD oxide film can be formed as necessary.

  Although the CVD oxide film has a lower passivation function than the other two oxide films, it has a role of protecting itself because the film formation rate is high. In the present invention, it is formed with a thickness of 100 to 400 nm.

After forming the steam oxide film, a CVD oxide film is formed by the APCVD method or the like. Specifically, for example, a method of reacting a source gas of SiH ₄ and O ₂ at 300 to 600 ° C. to form a SiO ₂ film is used.
Example 1
In this example, an n-type silicon substrate was used. As the substrate shape, a pseudo rectangle having a thickness of 200 μm and a solar cell having an outer shape of 10 cm × 10 cm was used.

Hereinafter, the cross-sectional view of the solar cell in FIG. 1B will be described as an example.
First, in order to remove the slice damage of the silicon substrate 17 sliced to a thickness of 270 μm, the surface was etched by an alkaline solution such as a mixed acid of hydrofluoric acid and nitric acid or sodium hydroxide to about 70 μm. Next, an oxide film as a diffusion mask was formed to 200 nm on the entire surface of the silicon substrate 17 by the APCVD method. On the back surface of the solar cell, a positive resist as an acid resistant resist was patterned on the diffusion mask into a desired shape by a photo process, and then the diffusion mask was etched with hydrofluoric acid. Next, p + layer 14 could be formed in a desired shape by gas phase diffusion of BBr ₃ at 950 ° C. for 30 minutes. Then, the diffusion mask was removed by immersing in hydrofluoric acid.

Thereafter, another diffusion mask is formed on the entire surface of the silicon substrate 17 in the same manner as described above, and the diffusion mask is etched into a desired shape on the back surface of the solar cell, and POCl ₃ is vapor-phase diffused at 900 ° C. for 30 minutes. Thus, the n + layer 13 was formed in a desired shape.

  In this embodiment, the p + layer 14 and the n + layer 13 have a comb-shaped diffusion layer facing each other. Moreover, it patterned so that each diffusion layer might not overlap.

  Thereafter, the diffusion mask was removed with hydrofluoric acid, and the surface of the silicon substrate 17 was dry oxidized at 800 ° C. for 30 minutes in an oxygen atmosphere to form a 2 nm dry oxide film. Then, the inside of the furnace is replaced with a nitrogen atmosphere, the temperature inside the furnace is increased, and then the inside of the furnace is replaced with an oxygen atmosphere containing water vapor, steam oxidation is performed at 900 ° C. for 10 minutes, and a steam oxide film is deposited to a thickness of 70 nm. did. Thereafter, a 200 nm CVD oxide film was deposited only on the back surface of the solar cell by the APCVD method. Thereby, the passivation film 15 was formed. Thereafter, the passivation film on the light receiving surface was removed with hydrofluoric acid, and a silicon nitride film was formed as an antireflection film on the light receiving surface to a thickness of 70 nm by plasma CVD. As the antireflection film 16 on the light receiving surface, a film having a refractive index of 2.2 and a thickness of 70 nm was used.

  Then, an acid-resistant resist was patterned into a desired shape on the passivation film 15 on the back surface of the solar cell, and the passivation film was etched with hydrofluoric acid to form a contact hole. Next, after depositing Ti—Pd—Ag as an electrode material, the p-electrode 12 and the n-electrode 11 having a desired pattern could be obtained by a lift-off method (immersion in an acetone solution to remove the resist). In this example, a solar cell having the same form as the cross-sectional view schematically showing the solar cell shown in FIG. That is, a passivation film made of a laminate in which a dry oxide film 22, a steam oxide film 23, and a CVD oxide film 24 are sequentially formed is formed on one surface of the silicon substrate 21, and an antireflection film made of a silicon nitride film is formed on the other surface. A solar cell having 26 laminated thereon was completed.

The solar cell characteristics were measured by irradiating the solar cell with AM-1.5 light. The results are shown in Table 1. In this example, since a single crystal substrate is used, in order to suppress reflection of the surface of the light receiving surface, the light receiving surface is textured by performing alkali etching.
(Comparative Example 1)
A cross-sectional view showing an outline of the solar cell of Comparative Example 1 is shown in FIG. As in the solar cell production in Example 1, after forming a p + layer and an n + layer on the silicon substrate 21, the diffusion mask was removed with hydrofluoric acid, and oxidation was performed in an oxygen atmosphere at 800 ° C. for 30 minutes. The dry oxide film 22 was formed. Thereafter, a CVD oxide film 24 was deposited only on the back surface of the solar cell by the APCVD method. Thereafter, the dry oxide film on the light receiving surface was removed with hydrofluoric acid, and a silicon nitride film was formed on the light receiving surface as an antireflection film 26 by a plasma CVD method to a thickness of 70 nm. Then, the p electrode, the n electrode, etc. were produced like Example 1, and the solar cell was completed. The characteristic results of the solar cell are shown in Table 1.
(Comparative Example 2)
FIG. 2C is a cross-sectional view illustrating the outline of the solar cell of Comparative Example 2. As in the solar cell production in Example 1, after forming a p + layer and an n + layer on the silicon substrate 21, the diffusion mask is removed with hydrofluoric acid, and a silicon nitride film 25 is formed as a passivation film by plasma CVD method to a thickness of 70 nm. did. A silicon nitride film as an antireflection film 26 was formed to 70 nm on the light receiving surface by plasma CVD. Then, the p electrode, the n electrode, etc. were produced like Example 1, and the solar cell was completed. The characteristic results of the solar cell are shown in Table 1.
(Solar cell characteristics comparison)
Based on the results shown in Table 1, the characteristics of the solar cells of Example 1 and Comparative Examples 1 and 2 were compared. In the table, Isc is a short circuit current, Voc is an open circuit voltage, FF is a fill factor, and Pm is a maximum output.

  Since the solar cell of Example 1 had the largest open circuit voltage value, it was shown that it can generate power even with weak sunlight. This can be interpreted as the effect of suppressing surface recombination on the semiconductor function by the passivation film. It was also found that the solar cell of Example 1 had the highest maximum output and fill factor. From this, it was shown that the solar cell of Example 1 has high conversion efficiency. And it was shown that the short circuit current is also high.

  From the above results, it was shown that the solar cell characteristics can be improved by the present invention.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  By sequentially forming the dry oxide film and the steam oxide film in the same furnace, it is possible to form a passivation film that is highly effective in preventing carrier recombination. Moreover, a solar cell characteristic can be improved by using the passivation film of this invention.

(A) is the light-receiving surface side top view of the solar cell of this invention, (b) is A-A 'sectional drawing of Fig.1 (a). (A) is a schematic sectional drawing of the solar cell of Example 1, (b) is a schematic sectional drawing of the solar cell of the comparative example 1, (c) is a schematic sectional drawing of the solar cell of the comparative example 2.

Explanation of symbols

  11 n electrode, 12 p electrode, 13 n + layer, 14 p + layer, 15 passivation film, 16 antireflection film, 17 silicon substrate, 21 silicon substrate, 22 dry oxide film, 23 steam oxide film, 24 CVD oxide film, 25 silicon Nitride film, 26 Antireflection film.

Claims (7)

  A method for forming a passivation film, comprising a step of sequentially forming a dry oxide film and a steam oxide film on a silicon substrate in the same furnace.   The method of forming a passivation film according to claim 1, wherein a CVD oxide film is formed after the formation of the steam oxide film.   2. The method for forming a passivation film according to claim 1, wherein an n-type diffusion layer and / or a p-type diffusion layer are formed on the surface of the silicon substrate.   2. The method for forming a passivation film according to claim 1, wherein the formation temperature of the dry oxide film and the steam oxide film is 700 to 1000 [deg.] C.   2. The step of sequentially forming a dry oxide film and a steam oxide film in the same furnace comprises flowing nitrogen into the furnace after forming the dry oxide film, stopping the nitrogen, and then forming a steam oxide film. A method for forming a passivation film as described in 1. above.   A method of manufacturing a solar cell, comprising using a silicon substrate on which a dry oxide film and a steam oxide film are formed in the same furnace as a passivation film.   A solar cell characterized by sequentially performing a step of performing n-type diffusion and p-type diffusion on a part of the same surface of a silicon substrate, a step of performing dry oxidation and steam oxidation in the same furnace, and a step of depositing a CVD oxide film Production method.

Images