JP2014179360A - Composition for n-type diffusion layer formation, method for manufacturing semiconductor substrate having n-type diffusion layer, and method for manufacturing solar battery device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a composition for n-type diffusion layer formation which allows an n-type diffusion layer to be formed in a particular portion while suppressing the occurrence of out-diffusion; a method for manufacturing a semiconductor substrate having an n-type diffusion layer; and a method for manufacturing a solar battery device.SOLUTION: A composition 11 for n-type diffusion layer formation comprises glass powder including at least a donor element, and a dispersant, and has a free phosphoric acid concentration of 50,000 ppm or less. A method for manufacturing a semiconductor substrate having an n-type diffusion layer comprises the steps of: applying the composition 11 for n-type diffusion layer formation to a surface of a p-type semiconductor substrate 10, namely a face forming a light-receiving surface of a solar battery device; and performing a thermal treatment on the p-type semiconductor substrate 10 with the composition 11 for n-type diffusion layer formation applied thereto at a temperature of 600-1,200°C for 5-90 minutes. As a result of the thermal treatment, the donor element is diffused into the p-type semiconductor substrate, whereby an n-type diffusion layer 12 is formed.

Description

  The present invention relates to an n-type diffusion layer forming composition for a solar cell element, a method for manufacturing a semiconductor substrate having an n-type diffusion layer, and a method for manufacturing a solar cell element.

A manufacturing process of a conventional crystalline silicon solar cell element will be described. First, a semiconductor substrate such as p-type silicon having a texture structure formed on the light receiving surface is prepared so as to promote the light confinement effect and increase the efficiency. Subsequently, several tens of minutes of treatment is performed at 800 to 900 ° C. in a mixed gas atmosphere of phosphorus oxychloride (POCl ₃ ), nitrogen and oxygen to form an n-type diffusion layer uniformly. However, when forming an n-type diffusion layer, in the gas phase reaction using phosphorus oxychloride, not only one surface (usually the light-receiving surface, the surface) that originally requires the n-type diffusion layer but also the other surface (non-light-receiving surface) , The back surface) and the side surfaces also form n-type diffusion layers.

On the other hand, in the field of manufacturing semiconductor devices, a solution containing a phosphate such as phosphorus pentoxide (P ₂ O ₅ ) or ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) is applied to a semiconductor substrate and heated. A method of forming an n-type diffusion layer by diffusing has been proposed (see, for example, Patent Document 1).

JP2002-75894

  However, even in a method in which a solution containing a phosphate is applied and thermally diffused, the phosphorous-containing compound is volatilized out of the region where the n-type diffusion layer is formed, and other than the surface where the n-type diffusion layer of the semiconductor substrate is required An n-type diffusion layer is also formed on this surface.

  In order to obtain an element having a pn junction structure by the above method, a step of performing side etching to remove the n-type diffusion layer formed on the side surface of the substrate, and an n-type diffusion layer formed on the back surface are formed by p. A process of converting to a mold diffusion layer is required. In addition, when the n-type diffusion layer is formed in a specific region rather than the entire surface of the substrate, the above method is not suitable.

  If the n-type diffusion layer can be formed only in a desired region of the semiconductor substrate, a step of performing side etching to remove the n-type diffusion layer formed on the side surface of the substrate, and an n-type diffusion formed on the back surface The step of converting the layer into a p-type diffusion layer is not necessary. In addition, when the n-type diffusion layer is formed in a specific region of the substrate surface, the volatilization of the phosphorus-containing compound outside the region where the n-type diffusion layer is formed (this phenomenon is referred to as out diffusion) is suppressed. Is effective.

  The present invention has been made in view of the above-described conventional problems, and an n-type diffusion layer forming composition capable of forming an n-type diffusion layer only in a specific portion while suppressing the occurrence of out-diffusion, An object is to provide a method for manufacturing a semiconductor substrate having an n-type diffusion layer and a method for manufacturing a solar cell element.

Means for solving the problems are as follows.
<1> An n-type diffusion layer forming composition containing a glass powder containing at least a donor element and a dispersion medium, and having a free phosphoric acid concentration of 50000 ppm or less.
<2> The composition for forming an n-type diffusion layer according to <1>, wherein the donor element contains P (phosphorus).
<3> At least one donor element-containing material selected from the group consisting of P ₂ O ₃ and P ₂ O ₅ , wherein the glass powder containing at least the donor element, SiO ₂ , K ₂ O, Na ₂ O, Containing at least one glass component material selected from the group consisting of Li ₂ O, BaO, SrO, CaO, MgO, BeO, ZnO, PbO, CdO, V ₂ O ₅ , SnO, ZrO ₂ and MoO ₃ The n-type diffusion layer forming composition according to <1> or <2>.
<4> A step of applying the n-type diffusion layer forming composition according to any one of <1> to <3> on a semiconductor substrate and a heat treatment to form an n-type diffusion layer in the semiconductor substrate A method of manufacturing a semiconductor substrate having an n-type diffusion layer.
<5> A step of applying the n-type diffusion layer forming composition according to any one of <1> to <3> on a semiconductor substrate and a heat treatment to form an n-type diffusion layer in the semiconductor substrate The manufacturing method of the solar cell element which has the process to form and the process of forming an electrode on the said n type diffused layer.

  ADVANTAGE OF THE INVENTION According to this invention, in the manufacturing process of the solar cell element using a crystalline silicon substrate, the n type diffused layer formation composition which can form an n type diffused layer only in a specific part, suppressing generation | occurrence | production of out diffusion. The manufacturing method of the semiconductor substrate which has a thing and an n type diffused layer, and the manufacturing method of a solar cell element are provided.

It is sectional drawing which shows notionally an example of the manufacturing process of the solar cell element of this invention. (A) is the top view which looked at the solar cell element from the surface, (B) is the perspective view which expands and shows a part of (A). It is sectional drawing which shows notionally an example of a back contact type solar cell element.

<N-type diffusion layer forming composition>
The n-type diffusion layer forming composition of the present invention contains at least a glass powder containing a donor element and a dispersion medium, and the concentration of free phosphoric acid is 50000 ppm or less. In the n-type diffusion layer forming composition of the present invention, the donor element forming the n-type diffusion layer is contained in the glass powder. Since glass powder does not evaporate during heat treatment like a solvent, diffusion of donor elements to the outside of the semiconductor substrate is suppressed. For this reason, an n-type diffusion layer can be formed only in the portion to which the n-type diffusion layer forming composition is applied. Furthermore, since the density | concentration of the free phosphoric acid in an n type diffused layer formation composition is 50000 ppm or less, volatilization of a phosphorus containing compound is further suppressed and generation | occurrence | production of out diffusion can be suppressed. For this reason, the spreading | diffusion of the phosphorus containing compound to the part other than the part which provided the n type diffused layer formation composition can further be suppressed.

  In the present invention, free phosphoric acid means phosphoric acid present in the n-type diffusion layer forming composition in a state where it is not confined in the glass powder. The cause of the presence of free phosphoric acid in the n-type diffusion layer forming composition is that the phosphorus-containing compound contained in the glass powder absorbs moisture in the air and elutes out as phosphoric acid outside the glass powder, It is conceivable that the composition of the glass powder is not optimal. The free phosphoric acid in the n-type diffusion layer-forming product volatilizes around the portion where the n-type diffusion layer-forming composition is applied during thermal diffusion, thereby preventing the formation of a selective n-type diffusion layer. Accordingly, the concentration of free phosphoric acid in the n-type diffusion layer forming composition is preferably low.

  Examples of the method for reducing the concentration of free phosphoric acid in the n-type diffusion layer forming composition include management of storage humidity of the material or sample, management of storage time, improvement of the composition of glass powder, and the like. That is, for example, by strictly controlling the humidity at the time of glass powder production or material storage and suppressing the elution of phosphoric acid, the concentration of free phosphoric acid in the n-type diffusion layer forming composition can be lowered, Outdiffusion can be suppressed.

The concentration of free phosphoric acid in the n-type diffusion layer formation can be measured as follows. The n-type diffusion layer forming composition is diluted with ethanol and centrifuged under conditions of 3500 rpm (min ⁻¹ ) and 10 min. The obtained supernatant is appropriately diluted with ethanol to prepare a measurement solution. Using an ion chromatography system (Dionex, ICS-2000) and using Na ₂ CO ₃ / NaHCO ₃ as the eluent, multiply the numerical value of phosphoric acid by the dilution factor with ethanol to obtain n-type The concentration of free phosphoric acid in the diffusion layer forming composition can be calculated.

  The concentration of free phosphoric acid in the n-type diffusion layer formed product is 50000 ppm or less, preferably 10,000 ppm or less, more preferably 5000 ppm or less, and still more preferably 2000 ppm or less.

  If the concentration of the free phosphoric acid in the n-type diffusion layer formation exceeds 50000 ppm, the free phosphoric acid spreads around the area where the pattern is selectively printed during thermal diffusion, and the coating selectivity is lost. . This phenomenon is called out diffusion. Accordingly, the phosphorus compound in the n-type diffusion layer forming composition is preferably confined in the glass powder, and the concentration of free phosphoric acid is preferably as small as possible.

  Moreover, when the density | concentration of the free phosphoric acid of n type diffused layer formation exceeds 50000 ppm, the color of n type diffused layer formation may change. The change in color is a change that can be easily seen with the naked eye, and is generally not preferred. Even if there are no major changes in other characteristics, there are cases where changes in color are not preferred. Therefore, reducing the concentration of free phosphoric acid is also effective in suppressing unnecessary changes in color of the n-type diffusion layer formed product, and is effective in improving product stability.

  Whether or not out-diffusion has occurred can be evaluated by secondary ion mass spectrometry (SIMS). Specifically, after the n-type diffusion layer forming composition is applied onto the semiconductor substrate and heat-treated to form the n-type diffusion layer, the portion to which the n-type diffusion layer forming composition is applied and the end of the applied portion The phosphorus concentration at a point 2 mm away from the outside can be calculated by SIMS analysis and evaluated. In the present invention, when the phosphorus concentration at a point 2 mm from the end of the portion to which the n-type diffusion layer forming composition is applied is less than 1/10 of the phosphorus concentration in the portion to which the n-type diffusion layer forming composition is applied, it is out. Evaluate that no diffusion has occurred. As the SIMS device, for example, IMS-6 manufactured by Kameka Instruments Co., Ltd. can be used.

(Glass powder)
The n-type diffusion layer forming composition of the present invention contains a glass powder containing at least a donor element. A donor element is an element that can form an n-type diffusion layer by doping into a silicon substrate. Group 15 elements can be used as donor elements, and specific examples include P (phosphorus), Sb (antimony), As (arsenic), and the like. From the viewpoints of safety and ease of vitrification, P or Sb is preferred.

The donor element-containing substance used for introducing the donor element into the glass powder includes at least a substance containing phosphorus as a donor element, and may include a substance containing a donor element other than phosphorus as necessary. The substance containing phosphorus as a donor _element, include P ₂ O _3, P 2 _{O 5} or the _like, at least one selected from the group consisting of P 2 _{O 3} and _P 2 _{O 5} is preferred. Examples of the substance containing a donor element other than phosphorus include Sb ₂ O ₃ , Bi ₂ O ₃ , As ₂ O ₃ , and Sb ₂ O ₃ is preferable. For example, at least a donor element-containing material may be only at least one selected from the group consisting of _P 2 _{O 3} and _P 2 _{O 5,} it is selected from the group consisting of P 2 _{O 3} and _P 2 _{O 5} A combination of one kind and a substance containing a donor element other than phosphorus such as Sb ₂ O ₃ may be used.

As glass component substances contained in the glass powder, SiO ₂ , K ₂ O, Na ₂ O, Li ₂ O, BaO, SrO, CaO, MgO, BeO, ZnO, PbO, CdO, V ₂ O ₅ , SnO, ZrO ₂ , WO ₃ , MoO ₃ , MnO, La ₂ O ₃ , Nb ₂ O ₅ , Ta ₂ O ₅ , Y ₂ O ₃ , TiO ₂ , ZrO ₂ , GeO ₂ , TeO ₂ , Lu ₂ O _{3 and the} like. , SiO ₂ , K ₂ O, Na ₂ O, Li ₂ O, BaO, SrO, CaO, MgO, BeO, ZnO, PbO, CdO, V ₂ O ₅ , SnO, ZrO ₂ , and MoO ₃ It is preferable to use at least one selected from the above.

Specific examples of the glass powder containing a donor _element, P ₂ O 5 -SiO _{2 system,} P ₂ O 5 -K ₂ O _based, P ₂ O 5 -Na _{2 O-based,} P ₂ O 5 -Li ₂ O system , _P 2 _O 5 _-BaO-based, P ₂ O 5 -SrO _based, P ₂ O 5 _-CaO-based, P ₂ O 5 _-MgO-based, P ₂ O 5 -BeO _based, P ₂ O 5 -ZnO-based, P ₂ O ₅ -CdO _based, P ₂ O 5 -PbO _{based, P 2 O 5 -V 2 O} 5 _system, P ₂ O 5 _-SnO-based, P ₂ O 5 -GeO _{2 system,} P ₂ O 5 -Sb ₂ O _{3 type,} P ₂ O 5 -TeO _{2 system,} P ₂ O 5 _-As 2 _{O 3} system _P 2 _{O 5} system, such as, include glass powder _Sb 2 _{O 3} system, and the like.
In the above has been illustrated a composite glass powder containing 2 _components, may be three or more types of composite glass powder if necessary, such as _{P 2 O 5 -SiO 2 -V 2} O 5.

  The glass powder preferably has a small content of lifetime killer element that lowers the lifetime of the carrier, more preferably zero. Specifically, the total amount of lifetime killer elements in the glass powder is preferably 1000 ppm or less, more preferably 500 ppm or less, and even more preferably 100 ppm or less. Specific examples of the lifetime killer element include Fe, Cu, Ni, Mn, Cr, W, and Au.

  The glass powder can control the melting temperature, softening temperature, glass transition temperature, chemical durability, and the like by adjusting the component ratio as necessary. The content ratio of the glass component substance in the glass powder is preferably set as appropriate in consideration of the melting temperature, the softening temperature, the glass transition temperature, and the chemical durability, and is generally 0.1% by mass to 95% by mass. It is preferable that it is 0.5 mass% or more and 90 mass% or less.

Specifically, in the case of P ₂ O ₅ —V ₂ O ₅ glass, the content ratio of V ₂ O ₅ is preferably 1% by mass or more and 50% by mass or less, and preferably 3% by mass or more and 40% by mass. The following is more preferable.

  The softening point of the glass powder is preferably 200 ° C. to 1000 ° C., more preferably 300 ° C. to 900 ° C., from the viewpoints of diffusibility during the diffusion treatment and dripping.

  The particle size of the glass powder is desirably 100 μm or less. When glass powder having a particle size of 100 μm or less is used, a smooth coating film is easily obtained. Furthermore, the particle size of the glass powder is more desirably 50 μm or less.

The glass powder containing a donor element is produced by the following procedure. First, weigh the ingredients and fill the crucible. Examples of the material for the crucible include platinum, platinum-rhodium, iridium, alumina, quartz, carbon, and the like, and are appropriately selected in consideration of the melting temperature, atmosphere, reactivity with the molten material, and the like. Next, it heats with the temperature according to a glass composition with an electric furnace, and is set as a melt. At this time, it is desirable to stir the melt uniformly. Subsequently, the obtained melt is poured onto a zirconia substrate, a carbon substrate or the like to vitrify the melt. Finally, the glass is crushed into powder. A known method such as a jet mill, a bead mill, or a ball mill can be applied to the pulverization.

  The content ratio of the glass powder containing the donor element in the n-type diffusion layer forming composition is determined in consideration of the coating property, the diffusibility of the donor element, and the like. Generally, the content ratio of the glass powder in the n-type diffusion layer forming composition is preferably 0.1% by mass or more and 95% by mass or less, and more preferably 1% by mass or more and 90% by mass or less.

(Dispersion medium)
The n-type diffusion layer forming composition contains a dispersion medium. The dispersion medium means a medium in which the glass powder is dispersed in the composition. Specifically, a binder, a solvent, a combination thereof, or the like can be used.

  As the binder, either an organic binder or an inorganic binder can be used. Specific examples of the organic binder include dimethylaminoethyl (meth) acrylate polymer, polyvinyl alcohol, polyacrylamide, polyvinylamide, polyvinylpyrrolidone, poly (meth) acrylic acid, polyethylene oxide, polysulfone, acrylamide alkyl sulfonic acid, cellulose ether, carboxy Cellulose derivatives such as methyl cellulose, hydroxyethyl cellulose, ethyl cellulose, gelatin, starch and starch derivatives, sodium alginate, xanthan, gua and gua derivatives, scleroglucan, tragacanth, dextrin and dextrin derivatives, acrylic resin, acrylic ester resin, butadiene resin , Styrene resins, and copolymers thereof. Specific examples of the inorganic binder include silicon dioxide. These binders are used alone or in combination of two or more.

  The molecular weight of the binder is not particularly limited, and it is desirable to adjust appropriately in view of the desired viscosity of the composition.

  Specific examples of the solvent include acetone, methyl ethyl ketone, methyl-n-propyl ketone, methyl-isopropyl ketone, methyl-n-butyl ketone, methyl isobutyl ketone, methyl-n-pentyl ketone, methyl-n-hexyl ketone, diethyl ketone, Ketone solvents such as dipropyl ketone, diisobutyl ketone, trimethylnonanone, cyclohexanone, cyclopentanone, methylcyclohexanone, 2,4-pentanedione, acetonylacetone; diethyl ether, methyl ethyl ether, methyl-n-propyl ether, diisopropyl Ether, tetrahydrofuran, methyltetrahydrofuran, dioxane, dimethyldioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol Di-n-propyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, diethylene glycol methyl n-propyl ether, diethylene glycol methyl n-butyl ether, diethylene glycol di-n-propyl ether, diethylene glycol di- n-butyl ether, diethylene glycol methyl-n-hexyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, triethylene glycol methyl-n-butyl ether, triethylene glycol di-n-butyl ether, triethylene Glycol meth -N-hexyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, tetraethylene glycol methyl-n-butyl ether, tetraethylene glycol di-n-butyl ether, tetraethylene glycol methyl-n-hexyl Ether, tetraethylene glycol di-n-butyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, propylene glycol di-n-propyl ether, propylene glycol dibutyl ether, dipropylene glycol dimethyl ether, dipropylene glycol diethyl ether, dipropylene glycol methyl ethyl Ether, dipropylene glycol methyl Ru-n-butyl ether, dipropylene glycol di-n-propyl ether, dipropylene glycol di-n-butyl ether, dipropylene glycol methyl-n-hexyl ether, tripropylene glycol dimethyl ether, tripropylene glycol diethyl ether, tripropylene glycol methyl Ethyl ether, tripropylene glycol methyl-n-butyl ether, tripropylene glycol di-n-butyl ether, tripropylene glycol methyl-n-hexyl ether, tetrapropylene glycol dimethyl ether, tetrapropylene glycol diethyl ether, tetradipropylene glycol methyl ethyl ether, Tetrapropylene glycol methyl-n-butyl ether, tetrapropylene glycol Ether solvents such as alkyl di-n-butyl ether, tetrapropylene glycol methyl-n-hexyl ether, tetrapropylene glycol di-n-butyl ether; methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate Sec-butyl acetate, n-pentyl acetate, sec-pentyl acetate, 3-methoxybutyl acetate, methylpentyl acetate, 2-ethylbutyl acetate, 2-ethylhexyl acetate, 2- (2-butoxyethoxy) ethyl acetate, benzyl acetate, Cyclohexyl acetate, methyl cyclohexyl acetate, nonyl acetate, methyl acetoacetate, ethyl acetoacetate, diethylene glycol methyl ether acetate, diethylene glycol monoethyl ether acetate, dipropylene glycol methyl ether acetate Dipropylene glycol ethyl ether, glycol diacetate, methoxy triglycol acetate, ethyl propionate, n-butyl propionate, isoamyl propionate, diethyl oxalate, di-n-butyl oxalate, methyl lactate, ethyl lactate, lactate n -Butyl, n-amyl lactate, ethylene glycol methyl ether propionate, ethylene glycol ethyl ether propionate, ethylene glycol methyl ether acetate, ethylene glycol ethyl ether acetate, propylene glycol methyl ether acetate, propylene glycol ethyl ether acetate, propylene glycol Ester solvents such as propyl ether acetate, γ-butyrolactone, γ-valerolactone; acetonitrile, N-methylpyrrole Non-protic polar solvents such as non, N-ethylpyrrolidinone, N-propylpyrrolidinone, N-butylpyrrolidinone, N-hexylpyrrolidinone, N-cyclohexylpyrrolidinone, N, N-dimethylformamide, N, N-dimethylacetamide, dimethylsulfoxide Methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, t-butanol, n-pentanol, isopentanol, 2-methylbutanol, sec-pentanol, t-pentanol, 3-methoxybutanol, n-hexanol, 2-methylpentanol, sec-hexanol, 2-ethylbutanol, sec-heptanol, n-octanol, 2-ethylhexanol, sec-octanol, n-nonyl alcohol, n-decanol, sec-undecyl alcohol, trimethylnonyl alcohol, sec-tetradecyl alcohol, sec-heptadecyl alcohol, phenol, cyclohexanol, methylcyclohexanol, benzyl alcohol, ethylene glycol, 1,2- Alcohol solvents such as propylene glycol, 1,3-butylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, isobornylcyclohexanol; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monophenyl Ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethyl Glycol mono-n-butyl ether, diethylene glycol mono-n-hexyl ether, ethoxytriglycol, tetraethylene glycol mono-n-butyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, tripropylene glycol Glycol monoether solvents such as monomethyl ether; terpinene such as α-terpinene, terpineol such as α-terpineol, myrcene, allocymene, limonene, dipentene, α-pinene, β-pinene, etc. Terpene solvent; water and the like. These are used singly or in combination of two or more.

  The content ratio of the dispersion medium in the n-type diffusion layer forming composition is determined in consideration of impartability, donor element concentration, and the like. Generally, the content ratio of the dispersion medium in the n-type diffusion layer forming composition is preferably 50% by mass to 99% by mass, and more preferably 60% by mass to 95% by mass.

  The viscosity of the n-type diffusion layer forming composition is preferably 10 mPa · s or more and 1000000 mPa · s or less, and more preferably 50 mPa · s or more and 500000 mPa · s or less in consideration of applicability. The viscosity can be measured with an E-type viscometer.

(Other ingredients)
The n-type diffusion layer forming composition may contain components other than the glass powder and the dispersion medium. Examples of other components include metal elements that react with the glass powder and crystallize, thixotropic agents, and the like.

  The n-type diffusion layer forming composition is applied on a semiconductor substrate and heat-treated at a high temperature to form an n-type diffusion layer. At that time, a glass layer is formed on the surface. This glass layer is removed by immersing the semiconductor substrate in a solution such as hydrofluoric acid, but it may be difficult to remove depending on the type of glass. In that case, the glass can be easily removed after the acid cleaning by adding a metal element such as Ag, Zn or Si that reacts with the glass to crystallize. Among these, it is preferable to use at least one selected from the group consisting of Ag, Si and Zn. The content ratio of the metal element that reacts with the glass powder and crystallizes is preferably adjusted as appropriate depending on the type of glass and the type of the metal element. It is preferable that it is below mass%.

  When the n-type diffusion layer forming composition contains a thixotropic agent, application characteristics such as applicability of the n-type diffusion layer forming composition may be improved. Specific examples of the thixotropic agent include silica fine particles, organic bentonite, montmorillonite, hydrotalcite, modified urea, polyethylene glycol, diethylene glycol, propylene glycol, and the like, but thixotropic agents other than these can also be used.

<Manufacturing method of semiconductor substrate having n-type diffusion layer>
The method of manufacturing a semiconductor substrate having an n-type diffusion layer according to the present invention includes a step of applying the n-type diffusion layer forming composition of the present invention on a semiconductor substrate, and a heat treatment to form the n-type diffusion layer in the semiconductor substrate. Forming.

As described above, an n-type diffusion layer can be formed only at a desired site by applying an n-type diffusion layer forming composition containing a glass powder containing a donor element to a semiconductor substrate and performing a heat treatment. Therefore, a side etching step for removing the n-type diffusion layer formed on the side surface of the semiconductor substrate and a step of converting the n-type diffusion layer formed on the back surface of the semiconductor substrate into a p ⁺ -type diffusion layer are unnecessary. The manufacturing process can be simplified.

Furthermore, in the conventional manufacturing method, when converting the n-type diffusion layer formed on the back surface into the p-type diffusion layer, a paste of aluminum which is a group 13 element is applied on the n-type diffusion layer, and is baked to make n A method is adopted in which aluminum is diffused into a p-type diffusion layer by diffusing aluminum into the p-type diffusion layer. In this method, since the conversion to the p-type diffusion layer is sufficient, and an aluminum amount of a certain level or more is required to form a high-concentration electric field layer of the p ⁺ -type diffusion layer, which will be described later, It was necessary to form it thickly. However, since the coefficient of thermal expansion of aluminum is significantly different from that of silicon or the like constituting the semiconductor substrate, a large internal stress is generated in the semiconductor substrate during firing and cooling, which causes warpage of the semiconductor substrate. It was. This internal stress damages the crystal grain boundaries and causes a large power loss. Further, the warpage of the semiconductor substrate easily damages the solar cell element in the transportation of the solar cell element in the module process and the connection with the copper wire called the tab wire. In recent years, the thickness of the semiconductor substrate is being reduced due to the improvement of the slice processing technique, and the solar cell element tends to be easily broken.

  In the method of the present invention, it is not necessary to convert the back surface of the semiconductor substrate from the n-type diffusion layer to the p-type diffusion layer. Therefore, it is not necessary to form a thick aluminum layer on the back surface of the semiconductor substrate, and the above problem does not occur.

  The glass powder contained in the n-type diffusion layer forming composition of the present invention is melted by a thermal diffusion process to form a glass layer on the n-type diffusion layer. However, since the glass layer is formed on the n-type diffusion layer in the conventional gas phase reaction method and the method using the phosphate-containing solution, it is necessary to remove the glass layer by etching as in the present invention. Therefore, the method of the present invention does not increase the number of new steps compared to the conventional method.

  In the method for producing a semiconductor substrate having an n-type diffusion layer of the present invention, the method for applying the n-type diffusion layer forming composition of the present invention on the semiconductor substrate is not particularly limited, and printing method, spin method, brush coating, spraying Method, doctor blade method, roll coater method, ink jet method and the like can be selected according to the application.

  In the method for producing a semiconductor substrate having an n-type diffusion layer of the present invention, there is no particular limitation on the method for performing a heat treatment on the n-type diffusion layer forming composition applied on the semiconductor substrate. For example, the heat treatment temperature can be 600 ° C. to 1200 ° C., and the heat treatment time can be 5 minutes to 90 minutes. The apparatus for performing the heat treatment may be a known continuous furnace, batch furnace or the like, and is not particularly limited.

  When the n-type diffusion layer forming composition is heat-treated, a glass layer such as phosphate glass is formed on the surface of the n-type diffusion layer at the same time as the n-type diffusion layer is formed. Therefore, the glass layer is removed by etching. The etching method is not particularly limited, and a known method such as a method of immersing in a solution such as hydrofluoric acid or a method of immersing in an alkali such as caustic soda can be applied.

<Method for producing solar cell element>
The method for producing a solar cell element of the present invention includes a step of applying the n-type diffusion layer forming composition of the present invention on a semiconductor substrate, a step of performing a heat treatment to form an n-type diffusion layer in the semiconductor substrate, Forming an electrode on the semiconductor substrate;

  As a method of forming an electrode, a metal paste for forming an electrode containing an electrode material is applied to a desired region on a semiconductor substrate, and if necessary dried and then heat-treated, and formed by plating an electrode material. And a method of forming an electrode material by electron beam heating in a high vacuum. Examples of the method for applying the electrode metal paste onto the semiconductor substrate include a printing method, a spin method, a brush coating, a spray method, a doctor blade method, a roll coater method, and an ink jet method. The composition of the electrode metal paste is not particularly limited. For example, it may contain metal particles and glass particles as essential components, and may contain a resin binder and other additives as necessary.

Since the n-type diffusion layer forming composition of the present invention has a free phosphoric acid concentration of 50000 ppm or less, the occurrence of out-diffusion can be suppressed when the n-type diffusion layer is formed in a pattern. Therefore, the n-type diffusion layer forming composition of the present invention can be suitably used for the production of a solar cell element in which the n-type diffusion layer is formed in a pattern. Specifically, there is a back contact type solar cell element in which all the electrodes are provided on the back surface to increase the area of the light receiving surface. In a back contact type solar electronic device, it is necessary to form both a n-type diffusion site and a p ⁺ -type diffusion site on the back surface to form a pn junction structure. When the n-type diffusion layer forming composition of the present invention is used, it is possible to form an n-type diffusion site only at a specific site while suppressing the occurrence of out-diffusion.

  Next, the manufacturing method of the semiconductor substrate which has an n type diffused layer of this invention, and a solar cell element is demonstrated, referring FIG. FIG. 1 is a schematic cross-sectional view conceptually showing an example of the manufacturing process of the solar cell element of the present invention. In the subsequent drawings, common constituent elements are denoted by the same reference numerals.

  First, a p-type semiconductor substrate 10 shown in FIG. The p-type semiconductor substrate 10 has a texture structure on the light receiving surface (surface) when the solar cell element is configured. Specifically, the damaged layer on the surface of the p-type semiconductor substrate 10 generated when slicing from the ingot is removed with 20% by mass caustic soda. Next, etching is performed with a mixed solution of 1% by mass caustic soda and 10% by mass isopropyl alcohol to form a texture structure (the description of the texture structure is omitted in the figure). In the solar cell element, by forming a texture structure on the light receiving surface (surface) side, a light confinement effect is promoted, and high efficiency is achieved.

  Next, as shown in FIG. 1 (2), an n-type diffusion layer forming composition 11 is applied to the surface of the p-type semiconductor substrate 10, that is, the surface that becomes the light-receiving surface of the solar cell element. Although there is no restriction | limiting in the method to provide the n type diffused layer formation composition 11, Printing method, a spin method, brush coating, a spray method, a doctor blade method, a roll coater method, an inkjet method etc. are mentioned.

  When the n-type diffusion layer forming composition contains a solvent, a drying step for volatilizing the solvent contained in the applied n-type diffusion layer forming composition may be necessary. For example, it is dried at a temperature of about 80 ° C. to 300 ° C. for 1 minute to 10 minutes when using a hot plate, and about 10 to 30 minutes when using a dryer or the like. The drying conditions depend on the composition of the solvent contained in the n-type diffusion layer forming composition, and are not particularly limited to the above conditions in the present invention.

  Next, as shown in FIG. 1 (3), the p-type semiconductor substrate 10 provided with the n-type diffusion layer forming composition 11 is heat-treated at 600 to 1200 ° C. for 5 to 90 minutes. By this heat treatment, the donor element diffuses into the p-type semiconductor substrate, and the n-type diffusion layer 12 is formed. A known continuous furnace, batch furnace, or the like can be applied to the heat treatment. Since a glass layer (not shown) such as phosphate glass is formed on the surface of the n-type diffusion layer 12, it is removed by etching. As the etching, known methods such as a method of immersing in an acid such as hydrofluoric acid and a method of immersing in an alkali such as caustic soda can be applied.

  In the n-type diffusion layer forming method of the present invention in which the n-type diffusion layer 12 is formed using the n-type diffusion layer forming composition 11 of the present invention, the n-type diffusion layer 12 is formed only at a desired site, An unnecessary n-type diffusion layer is not formed on the side surface. Therefore, the side etching process is not necessary, and the process is simplified.

As shown in FIGS. 1 (2) and (3), a p-type diffusion layer forming composition 13 containing a Group 13 element such as boron is applied to the back surface of the p-type semiconductor substrate 10 and heat-treated to form p ^+. A mold diffusion layer (high-concentration electric field layer) 14 can be formed. In the method of the present invention, there is no need to convert the n-type diffusion layer formed on the back surface of the p-type semiconductor substrate 10 into a p-type diffusion layer. For this reason, the element contained in the p-type diffused layer formation composition 13 is not limited to aluminum, and the choice of a manufacturing method spreads.

As shown in FIG. 1 (4), an antireflection film 16 may be formed on the n-type diffusion layer 12. The antireflection film 16 can be formed by a known technique. For example, when the antireflection film 16 is a silicon nitride film, it is formed by a plasma CVD method using a mixed gas of SiH ₄ and NH ₃ as a raw material. At this time, hydrogen diffuses into the crystal, and orbits that do not contribute to the bonding of silicon atoms, that is, dangling bonds and hydrogen are combined to inactivate defects (hydrogen passivation). More specifically, the mixed gas flow rate ratio (NH ₃ / SiH ₄ ) is 0.05 to 1.0, the reaction chamber pressure is 13.3 Pa to 266.6 Pa (0.1 Torr to 2.0 Torr), The film is formed under conditions where the temperature during film formation is 300 ° C. to 550 ° C. and the frequency for plasma discharge is 100 kHz or more.

  Next, as shown in FIG. 1 (5), a surface electrode metal paste is applied by screen printing on the antireflection film 16 on the surface (light-receiving surface) of the p-type semiconductor substrate 10 and dried to form the surface electrode 18. (Before heat treatment) is formed. The composition of the metal paste for the surface electrode is not particularly limited. For example, it may contain metal particles and glass particles as essential components, and may contain a resin binder and other additives as necessary.

A back electrode 20 is also formed on the p ⁺ -type diffusion layer 14 on the back surface of the p-type semiconductor substrate 10. In the conventional technique, an aluminum layer formed to convert an n-type semiconductor layer formed on the back surface of the p-type semiconductor substrate 10 into a p-type semiconductor layer may also be used as the back electrode 20. However, in the present invention, the material and forming method of the back electrode 20 are not particularly limited. For example, the back electrode 20 may be formed using a back electrode paste containing a metal such as silver or copper. Also, the thickness of the back electrode 20 can be made thinner than before.

  Next, as shown in FIG. 1 (6), the surface electrode 18 (before heat treatment) is heat-treated to conduct with the p-type semiconductor substrate 10 to complete the solar cell element. When heat treatment is performed in the range of 600 ° C. to 900 ° C. for several seconds to several minutes, the antireflection film 16 that is an insulating film is melted by the glass particles contained in the surface electrode 18 (before heat treatment) on the surface side, and the surface of the p-type semiconductor substrate 10 Is partially melted, and metal particles (for example, silver particles) in the surface electrode 18 (before heat treatment) form a contact portion with the p-type semiconductor substrate 10 and solidify. Thereby, the surface electrode 18 electrically connected to the p-type semiconductor substrate 10 is formed. This is called fire-through.

In the method of the present invention, the order of forming the n-type diffusion layer on the front surface of the p-type semiconductor substrate, the p ⁺ -type diffusion layer on the back surface, the antireflection film, the front surface electrode, and the back surface electrode is the order shown in FIG. It is not necessary and not particularly limited.

  FIG. 2 is a view of the solar cell element as seen from the surface, and shows an example of the shape of the surface electrode 18. The surface electrode 18 includes a bus bar electrode 30 and finger electrodes 32 intersecting with the bus bar electrode 30. FIG. 2A is a plan view of a solar cell element in which the surface electrode 18 includes a bus bar electrode 30 and a finger electrode 32 intersecting with the bus bar electrode 30 as viewed from the surface. FIG. 2B is an enlarged perspective view illustrating a part of FIG.

  Such a surface electrode 18 can be formed, for example, by means such as application of the above-described electrode-forming metal paste by screen printing, plating of the electrode material, or evaporation of the electrode material by electron beam heating in a high vacuum. . The surface electrode 18 composed of the bus bar electrode 30 and the finger electrode 32 is generally used as an electrode on the light receiving surface side and is well known, and it is possible to apply known forming means for the bus bar electrode and finger electrode on the light receiving surface side. it can.

In the above description, the solar cell element in which the n-type diffusion layer is formed on the front surface, the p ⁺ -type diffusion layer is formed on the back surface, and the front surface electrode and the back surface electrode are further provided on the respective layers has been described. If a layer formation composition is used, it is also possible to produce a back contact type solar cell element. The n-type diffusion layer forming composition of the present invention can form an n-type diffusion site only at a specific site, and therefore can be suitably applied to the production of a back contact type solar cell element.

FIG. 3 shows an example of the structure of a back contact solar cell element. The back contact solar cell element shown in FIG. 3 has a back electrode 21 provided in a pattern on the n-type diffusion layer 12 formed in a pattern on the back side of the p-type semiconductor substrate 10, and p The back surface electrode 22 is provided in a pattern on the p ⁺ type diffusion layer 14 formed in a pattern on the back side of the mold semiconductor substrate 10. By adopting a configuration in which no electrode is provided on the light receiving surface, the area of the light receiving surface can be increased, and the conversion efficiency of the solar cell element can be improved.

  Examples of the present invention will be described more specifically below, but the present invention is not limited to these examples. Unless otherwise stated, all chemicals used reagents. “%” Means “% by mass” unless otherwise specified.

[Example 1]
20 g of P ₂ O ₅ —SiO ₂ glass (P ₂ O ₅ : 20%, SiO ₂ : 80%) powder, 3 g of ethyl cellulose, and 77 g of 2- (2-butoxyethoxy) ethyl acetate were mixed to make a paste, and n-type A diffusion layer forming composition was prepared.

The concentration of free phosphoric acid in the n-type diffusion layer forming composition of Example 1 was determined by anion chromatography. After diluting the n-type diffusion layer forming composition with ethanol, the mixture was centrifuged under conditions of 3500 rpm / 10 min, and the supernatant was appropriately diluted with ethanol to prepare a measurement solution. The concentration of phosphoric acid was measured using an ion chromatography system (Dionex, ICS-2000) and using Na ₂ CO ₃ / NaHCO ₃ as an eluent. The concentration of the obtained phosphoric acid was multiplied by the dilution factor, and the concentration of free phosphoric acid in the n-type diffusion layer forming composition was calculated. The concentration of free phosphoric acid in the n-type diffusion layer forming composition of Example 1 was 1700 ppm.

  Next, the prepared paste was applied to the surface of the p-type silicon substrate in a square shape with a side of 3.5 cm by screen printing and dried on a hot plate at 150 ° C. for 5 minutes. Subsequently, thermal diffusion treatment was performed for 10 minutes in an electric furnace set at 1000 ° C. Then, in order to remove the glass layer formed on the surface, the substrate was immersed in 5% by mass of hydrofluoric acid for 5 minutes, washed with running water, and then dried.

  The sheet resistance of the surface of the p-type silicon substrate on the side where the n-type diffusion layer forming composition was applied was 50Ω / □, and it was confirmed that P (phosphorus) diffused and an n-type diffusion layer was formed. The sheet resistance on the back surface was 1000000 Ω / □ or more, which was not measurable, and it was confirmed that no n-type diffusion layer was formed.

Next, out diffusion was evaluated by secondary ion mass spectrometry (SIMS). As the SIMS device, IMS-6 manufactured by Kameka Instruments Co., Ltd. was used. In the evaluation, the phosphorus concentration was calculated by SIMS analysis of the part where the n-type diffusion layer forming composition was applied and the point 2 mm away from the edge of the applied part, and the influence of out diffusion was confirmed. The surface of the portion where the n-type diffusion layer forming composition was applied had a phosphorus concentration of 10 ²¹ atoms / cc, but at a point 2 mm away from the edge of the applied portion, it was 10 ¹⁸ atoms / cc. The phosphorus concentration was 1/1000. From the above, it was confirmed that the out diffusion was considerably small and within an allowable range.

[Example 2]
In the same manner as in Example 1, an n-type diffusion layer was formed with an n-type diffusion layer forming composition having a free phosphoric acid concentration of 3700 ppm. The sheet resistance on the surface on which the n-type diffusion layer forming composition was applied was 45Ω / □, and it was confirmed that P (phosphorus) diffused and an n-type diffusion layer was formed. The sheet resistance on the back surface was 1000000 Ω / □ or more, which was not measurable, and it was confirmed that no n-type diffusion layer was formed.

Next, out diffusion was evaluated by secondary ion mass spectrometry (SIMS) in the same manner as in Example 1. The surface of the portion where the n-type diffusion layer forming composition was applied had a phosphorus concentration of 10 ²¹ atoms / cc, but at a point 2 mm away from the edge of the applied portion, it was 10 ¹⁸ atoms / cc. The phosphorus concentration was 1/1000. From the above, it was confirmed that the out diffusion was considerably small and within an allowable range.

[Example 3]
In the same manner as in Example 1, an n-type diffusion layer was formed with an n-type diffusion layer forming composition having a free phosphoric acid concentration of 5700 ppm. The sheet resistance on the surface on which the n-type diffusion layer forming composition was applied was 45Ω / □, and it was confirmed that P (phosphorus) diffused and an n-type diffusion layer was formed. The sheet resistance on the back surface was 1000000 Ω / □ or more, which was not measurable, and it was confirmed that no n-type diffusion layer was formed.

Next, out diffusion was evaluated by secondary ion mass spectrometry (SIMS) in the same manner as in Example 1. The phosphorus concentration of 10 ²¹ atoms / cc was applied to the surface of the portion where the n-type diffusion layer forming composition was applied, but at a point 2 mm away from the edge of the applied portion, 2 × 10 ¹⁸ atoms / cc. Yes, the phosphorus concentration was 1/500. From the above, it was confirmed that the out diffusion was considerably small and within an allowable range.

[Example 4]
An n-type diffusion layer was formed in the same manner as in Example 1 except that 5000 ppm of phosphoric acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the n-type diffusion layer forming composition of Example 3. The sheet resistance on the surface on which the n-type diffusion layer forming composition was applied was 45Ω / □, and it was confirmed that P (phosphorus) diffused and an n-type diffusion layer was formed. The sheet resistance on the back surface was 1000000 Ω / □ or more, which was not measurable, and it was confirmed that no n-type diffusion layer was formed. The concentration of free phosphoric acid in the n-type diffusion layer forming composition was 13000 ppm.

Next, out diffusion was evaluated by secondary ion mass spectrometry (SIMS) in the same manner as in Example 1. The phosphorus concentration of 10 ²¹ atoms / cc was applied to the surface of the portion where the n-type diffusion layer forming composition was applied, but at a point 2 mm away from the end of the applied portion, 4 × 10 ¹⁸ atoms / cc. Yes, the phosphorus concentration was 1/250. From the above, it was confirmed that the out diffusion was considerably small and within an allowable range.

[Example 5]
An n-type diffusion layer was formed in the same manner as in Example 1 except that 25,000 ppm of phosphoric acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the n-type diffusion layer forming composition of Example 3. The sheet resistance on the surface on which the n-type diffusion layer forming composition was applied was 45Ω / □, and it was confirmed that P (phosphorus) diffused and an n-type diffusion layer was formed. The sheet resistance on the back surface was 1000000 Ω / □ or more, which was not measurable, and it was confirmed that no n-type diffusion layer was formed. The concentration of free phosphoric acid in the n-type diffusion layer forming composition was 34000 ppm.

Next, out diffusion was evaluated by secondary ion mass spectrometry (SIMS) in the same manner as in Example 1. The surface of the portion where the n-type diffusion layer forming composition was applied had a phosphorus concentration of 10 ²¹ atoms / cc, but at a point 2 mm away from the edge of the applied portion, it was 10 ¹⁹ atoms / cc. The phosphorus concentration was 1/100. From the above, it was confirmed that the out diffusion was considerably small and within an allowable range.

[Example 6]
An n-type diffusion layer was formed in the same manner as in Example 1 except that 42,000 ppm of phosphoric acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the n-type diffusion layer forming composition of Example 3. The sheet resistance on the surface on which the n-type diffusion layer forming composition was applied was 50 Ω / □, and it was confirmed that P (phosphorus) diffused to form an n-type diffusion layer. The sheet resistance on the back surface was 1000000 Ω / □ or more, which was not measurable, and it was confirmed that no n-type diffusion layer was formed. The concentration of free phosphoric acid in the n-type diffusion layer forming composition was 50000 ppm.

Next, out diffusion was evaluated by secondary ion mass spectrometry (SIMS) in the same manner as in Example 1. The phosphorus concentration of 10 ²¹ atoms / cc was applied to the surface of the portion where the n-type diffusion layer forming composition was applied, but at a point 2 mm away from the edge of the applied portion, 2 × 10 ¹⁹ atoms / cc. Yes, the phosphorus concentration was 1/50. From the above, it was confirmed that there was little out-diffusion and it was within the allowable range.

[Comparative Example 1]
An n-type diffusion layer was formed in the same manner as in Example 1 except that 50000 ppm of phosphoric acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the n-type diffusion layer forming composition of Example 3. The sheet resistance on the surface on which the n-type diffusion layer forming composition was applied was 60 Ω / □, and it was confirmed that P (phosphorus) diffused to form an n-type diffusion layer. The sheet resistance on the back surface was 1000000 Ω / □ or more, which was not measurable, and it was confirmed that no n-type diffusion layer was formed. The concentration of free phosphoric acid in the n-type diffusion layer forming composition was 61000 ppm.

Next, out diffusion was evaluated by secondary ion mass spectrometry (SIMS) in the same manner as in Example 1. The surface of the portion where the n-type diffusion layer forming composition was applied had a phosphorus concentration of 10 ²¹ atoms / cc, but at a point 2 mm away from the edge of the applied portion, it was 10 ²⁰ atoms / cc. The phosphorus concentration was 1/10. From the above, it was confirmed that out diffusion occurred.

[Comparative Example 2]
An n-type diffusion layer was formed in the same manner as in Example 1 except that 100000 ppm of phosphoric acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the n-type diffusion layer forming composition of Example 3. The sheet resistance on the surface on which the n-type diffusion layer forming composition was applied was 60 Ω / □, and it was confirmed that P (phosphorus) diffused to form an n-type diffusion layer. The sheet resistance on the back surface was 1000000 Ω / □ or more, which was not measurable, and it was confirmed that no n-type diffusion layer was formed. The concentration of free phosphoric acid in the n-type diffusion layer forming composition was 110,000 ppm.

Next, out diffusion was evaluated by secondary ion mass spectrometry (SIMS) in the same manner as in Example 1. The surface of the portion where the n-type diffusion layer forming composition was applied had a phosphorus concentration of 10 ²¹ atoms / cc, but at a point 2 mm away from the edge of the applied portion, it was 10 ²⁰ atoms / cc. The phosphorus concentration was 1/10. From the above, it was confirmed that out diffusion occurred.

10 p-type semiconductor substrate 11 n-type diffusion layer forming composition 12 n-type diffusion layer 13 p-type diffusion layer forming composition 14 p ⁺ -type diffusion layer (high concentration electric field layer)
16 Antireflection film 18 Surface electrode 20, 21, 22 Back electrode (electrode layer)
30 Busbar electrode 32 Finger electrode

Claims (5)

  An n-type diffusion layer forming composition containing glass powder containing at least a donor element and a dispersion medium, and having a free phosphoric acid concentration of 50000 ppm or less.   The n-type diffusion layer forming composition according to claim 1, wherein the donor element contains P (phosphorus). The glass powder containing at least a donor element is at least one donor element-containing material selected from the group consisting of P ₂ O ₃ and P ₂ O ₅ , SiO ₂ , K ₂ O, Na ₂ O, Li ₂ O And at least one glass component selected from the group consisting of BaO, SrO, CaO, MgO, BeO, ZnO, PbO, CdO, V ₂ O ₅ , SnO, ZrO ₂ and MoO _3. The n-type diffused layer formation composition of Claim 1 or Claim 2.   The process of providing the n type diffused layer formation composition of any one of Claims 1-3 on a semiconductor substrate, The process of giving heat processing and forming an n type diffused layer in the said semiconductor substrate, A method for manufacturing a semiconductor substrate having an n-type diffusion layer.   The process of providing the n type diffused layer formation composition of any one of Claims 1-3 on a semiconductor substrate, The process of giving heat processing and forming an n type diffused layer in the said semiconductor substrate, And a step of forming an electrode on the n-type diffusion layer.