JPWO2017158691A1 - Manufacturing method of solar cell - Google Patents

Abstract

  In order to obtain a method for manufacturing a solar cell having a long carrier life by suppressing the contamination of impurities on the back surface when impurities are diffused by heat treatment after the solid phase diffusion source is formed, the light receiving surface (1A ) And a back surface (1B), and a step of forming a BSG film (2) as a solid phase diffusion source on the light receiving surface (1A) of the n-type single crystal silicon substrate (1), and an n-type single crystal silicon substrate (1) ) To heat the BSG film (2) to diffuse boron, which is an impurity of the second conductivity type, to form a p-type diffusion layer (7). Prior to the pn separation process, the BSG film ( 2) is formed and heated to form a p-type diffusion layer (7), and the BSG film (2) is removed. Further, prior to the heat treatment step for forming the p-type diffusion layer (7), a step of removing a solid phase diffusion source such as a boron-containing product and a silicon oxide-containing product formed on the back surface (1B) is included.

Description

  The present invention relates to a method for manufacturing a solar cell, and more particularly to improvement of photoelectric conversion efficiency.

  Conventionally, in a solar cell, as shown in Patent Document 1, a CVD (Chemical Vapor Deposition) method or the like is used as a method for diffusing impurities to a light receiving surface that is a light incident surface or a back surface that is a surface facing the light receiving surface. A method is disclosed in which after a diffusion source is formed using the substrate, the substrate and the film serving as the diffusion source are heated in a nitrogen atmosphere to diffuse impurities into the substrate.

JP 2004-247364 A

  However, in the method for manufacturing a solar cell disclosed in Patent Document 1, a phosphorous silicate glass (PSG) film or a boron silicate glass (BSG) film is formed on a substrate. Later, a heat treatment for impurity diffusion is performed in a nitrogen atmosphere. For this reason, impurities such as phosphorus or boron circulate on the back surface of the substrate during film formation, and impurity diffusion to the back surface simultaneously occurs from the attached product, which may cause unintentional contamination of the back surface. Impurity contamination leads to a reduction in the carrier life of the solar cell. In addition, the film has a problem that it is difficult to process due to a large film thickness and easily causes an electrical short circuit in combination with the mixing of the impurities.

  The present invention has been made in view of the above, and after forming a solid phase diffusion source into a film, it is possible to suppress impurities from being mixed into the back surface when performing impurity diffusion by heat treatment, and to form a film. The object is to obtain a solar cell having a long carrier life by eliminating the processing hindrance caused by the above, improving the electrical insulation.

  In order to solve the above-described problems and achieve the object, the present invention forms a solid phase diffusion source on a first main surface of a first conductivity type semiconductor substrate having first and second main surfaces facing each other. A step of removing the product formed on the second main surface in the step of forming a film, and heating the semiconductor substrate from which the product has been removed, from the solid phase diffusion source to the first main surface side. Forming a two-conductivity-type first diffusion layer; forming a second diffusion layer having the first conductivity-type on the second main surface of the semiconductor substrate; removing a solid-phase diffusion source; Electrically separating the second diffusion layer and the first diffusion layer. The step of removing the solid phase diffusion source is performed before the step of electrically separating the second diffusion layer and the first diffusion layer.

  According to the present invention, after the solid phase diffusion source is formed, when impurities are diffused by heat treatment, impurities are prevented from being mixed into the second main surface, which is the back surface, and the processing hindrance due to the film is inhibited. This eliminates the effect of improving the electrical insulation and obtaining a solar cell with a long carrier life.

1 is a flowchart showing a method for manufacturing a solar cell according to a first embodiment. (A) to (d) are process cross-sectional views illustrating a method for manufacturing a solar cell according to the first embodiment. (A) to (d) are process cross-sectional views illustrating a method for manufacturing a solar cell according to the first embodiment. Explanatory drawing which shows the time chart about the temperature in a furnace and environmental state of the heat treatment process in the manufacturing process of the solar cell concerning Embodiment 1. FIG. Explanatory drawing which shows the principal part of the manufacturing process of the solar cell concerning Embodiment 1. FIG. A flowchart which shows the manufacturing method of the solar cell concerning Embodiment 2. FIG. Process sectional drawing which shows the principal part of the manufacturing process of the solar cell concerning Embodiment 2 A flowchart showing a method of manufacturing a solar cell according to the third embodiment. Process sectional drawing which shows the principal part of the manufacturing process of the solar cell concerning Embodiment 3.

  Below, the manufacturing method of the solar cell concerning this invention and embodiment of a solar cell are described in detail based on drawing. In addition, this invention is not limited by this embodiment, In the range which does not deviate from the summary, it can change suitably. In the drawings shown below, the scale of each layer or each member may be different from the actual for easy understanding, and the same applies to the drawings.

Embodiment 1 FIG.
FIG. 1 is a flowchart of a manufacturing process showing Embodiment 1 of a method for manufacturing a solar cell according to the present invention. FIGS. 2 (a) to (d) and FIGS. 3 (a) to (d) FIG. 10 is a process cross-sectional view illustrating the solar cell manufacturing method according to the first embodiment. FIGS. 2A to 2D are cross-sectional views showing changes in the solar cell substrate in the continuous process in the furnace shown in FIG. 1 in the method for manufacturing the solar cell according to the first embodiment. FIGS. 3A to 3D are schematic views showing changes in the cross section of the solar cell in the process following the heat treatment shown in FIGS. 2A to 2D during the manufacturing process of the first embodiment. FIG. 4 is a time chart for the temperature in the furnace and the environmental state.

  In the method for manufacturing a solar cell according to the first embodiment, the solid phase diffusion source is formed and heated prior to the pn separation step of electrically separating the first diffusion layer and the second diffusion layer. A diffusion layer is formed and the solid phase diffusion source is removed. Further, prior to the heat treatment step for forming the first diffusion layer, the solid phase diffusion source formed on the second main surface is removed.

  In the manufacturing method of the solar cell according to the first embodiment, the solid phase diffusion source is formed and heated to form the diffusion layer, and then the solid phase diffusion source is removed and electrical separation is performed. Insulation performance at the location where the material is subjected is improved. Furthermore, since the step of removing the solid phase diffusion source formed on the second main surface is included prior to the step of performing impurity diffusion from the solid phase diffusion source by the heating step, the solid phase diffusion source of the semiconductor substrate is formed. The solid phase diffusion material wraps around and adheres to the opposite side of the surface, but heat treatment is performed after the solid phase diffusion source is removed, so that impurities from the deposits are prevented from diffusing into the substrate. It is a thing.

  The solar cell according to the first embodiment includes an n-type single crystal silicon substrate 1 as a first conductivity type semiconductor substrate having a first main surface serving as a light receiving surface 1A and a second main surface serving as a back surface 1B. Use. The manufacturing method will be described with reference to FIGS. 1, 2A to 2D, FIGS. 3A to 3D, and FIG. First, in damage layer removal step S101, contamination or damage that occurs during wafer slicing on the surface is removed. For example, the n-type single crystal silicon substrate 1 is immersed in an alkaline solution in which 1 wt% or more and less than 10 wt% of sodium hydroxide is dissolved to remove slice contamination and damage. Thereafter, the light-receiving surface 1A of the n-type single crystal silicon substrate 1 is immersed in a solution obtained by adding an additive such as isopropyl alcohol or caprylic acid in an alkaline solution of 0.1 wt% or more and less than 10 wt%, for example, to prevent reflection. The texture which is the unevenness | corrugation for obtaining is formed. The removal of slice contamination and damage and the formation of texture may be performed simultaneously or individually. The texture may be formed not only on the light receiving surface 1A but also on the back surface 1B. 2 and 3, the texture is not shown for easy understanding, and both the light receiving surface 1A and the back surface 1B are shown as flat surfaces.

  Next, the surface of the n-type single crystal silicon substrate 1 is cleaned in a pre-deposition cleaning step S102. In the cleaning step, for example, a mixed solution of sulfuric acid and hydrogen peroxide, a hydrofluoric acid aqueous solution, a mixed solution of ammonia and hydrogen peroxide, a mixed solution of hydrochloric acid and hydrogen peroxide, called RCA cleaning, A step of removing an organic material, a metal, and an oxide film combined with each other, or a step of removing an oxide film using only a hydrofluoric acid aqueous solution is used depending on, for example, a texture forming method. In addition, the cleaning liquid may be selected from one or a plurality of types of cleaning liquids, a mixed solution of hydrofluoric acid and hydrogen peroxide, or water containing ozone.

  In addition, in order to prevent each processing solution itself from causing contamination to others or unintentional reactions, and to ensure safety after taking it out of the device, pure water is used in the middle or before drying. Wash with water.

  Subsequent to the cleaning step, in the film forming step S103 on the light receiving surface 1A side of the solid phase diffusion source, as shown in FIG. For example, a BSG (Boron Silicate Glass) film 2 which is an oxide film containing boron is formed. For film formation, for example, low pressure CVD or normal pressure CVD is used. Note that the boron-containing product 4 adheres to the back surface 1B of the n-type single crystal silicon substrate 1 by the wrapping of the film forming gas during the film forming process described above. Subsequently, a film that becomes a cap, that is, a protective film, for example, a silicon oxide film 3 at the time of heat treatment is formed on the BSG film 2. The silicon oxide film 3 is preferably formed by a film forming process such as low pressure CVD or atmospheric pressure CVD like the BSG film 2 in view of the continuity of the process. When the silicon oxide film 3 is formed, as in the case of forming the BSG film 2, the film forming gas flows around the back surface 1B and the silicon oxide-containing product 5 adheres.

  In the product removal step S104 on the back surface, as shown in FIG. 2B, the product formed on the back surface 1B side is removed. Here, the product formed on the back surface 1B side of the n-type single crystal silicon substrate 1 is removed. That is, after the formation of the BSG film 2 and the silicon oxide film 3, the boron-containing product 4 and the silicon oxide-containing product 5 on the back surface 1B side are removed. The removal is performed, for example, by dissolution using a hydrofluoric acid aqueous solution. However, since the boron-containing product 4 is essentially the same material as the BSG film 2 and the silicon oxide-containing product 5 is, for example, single-sided etching. It is desirable to remove the boron-containing product 4 and the silicon oxide-containing product 5 by bringing the hydrofluoric acid aqueous solution into contact with only the back surface 1B side using an apparatus. As an example of a single-sided etching apparatus, single-sided etching can be realized by using an apparatus in which an etching solution is sprayed from below with an etching surface down or an etching device having a structure in which only one surface is immersed in an etching solution.

  Next, in step S105 in which heat treatment is performed, the n-type single crystal silicon substrate 1 is subjected to heat treatment continuously. A heat treatment furnace is used for the heat treatment. First, the heat treatment furnace is preheated, and as shown in FIG. 2C, heat treatment is performed in an inert gas atmosphere to form a diffusion layer. By the heat treatment, boron diffuses from the BSG film 2 and the p-type diffusion layer 7 is formed.

Next, in the step of performing heat treatment continuously in an atmosphere containing oxygen O ₂ , the heat treatment is performed while supplying oxygen O ₂ . In the heat treatment, the temperature is raised, heated, and lowered while the temperature and the film formation atmosphere are switched. The atmosphere during heating is divided into an atmosphere containing an inert gas such as nitrogen or argon and an atmosphere containing oxygen. Both are heated for an arbitrary time in a temperature range of 800 ° C. to 1100 ° C., but in an atmosphere containing oxygen, heating is performed for a time of 1 minute to 20 minutes or less.

  First, in an atmosphere containing, for example, an inert gas such as nitrogen or argon, a temperature T at which impurity diffusion from the BSG film 2 proceeds, for example, 800 ° C. to 1100 ° C. is reached, and a desired p-type diffusion layer 7 is formed. Form. After the formation of the p-type diffusion layer 7 is completed, by introducing oxygen, silicon is deposited on the entire surface of the n-type single crystal silicon substrate 1 on which the p-type diffusion layer 7 is formed, as shown in FIG. An oxide film 8 is formed.

The temperature profile of this heat treatment is shown by curve a in FIG. When the inside of the furnace is replaced with nitrogen and the furnace is preheated to become a nitrogen atmosphere and the temperature T = 900 ° C., the n-type single crystal silicon substrate 1 is charged into the heat treatment furnace at time t ₀₁ , and time is reached until time t _02. t ₁ = 1 to 30 minutes. At the time t _02, supplying oxygen to the heat treatment furnace. While supplying oxygen, the temperature T is maintained for a time t ₂ = 1 minute to 20 minutes until the time t ₀₃ . In the oxidation step, the surface of the n-type single crystal silicon substrate 1 put in the heat treatment furnace is oxidized by oxygen contained in the atmosphere. The oxidation proceeds selectively on the back surface 1B side not covered with the film because the light receiving surface 1A side is covered with the BSG film 2 and the silicon oxide film 3, but also on the surface of the p-type diffusion layer 7. Partial oxidation proceeds, and a silicon oxide film 8 is formed as shown in FIG. At time _t03 , supply of oxygen is stopped, nitrogen gas is supplied, and nitrogen substitution is performed. The silicon oxide film 8 functions as a part of a barrier that prevents the entry of n-type impurities when forming an n-type diffusion layer described later. The thickness is desirably 5 nm or more and 10 nm or less. If it is less than 5 nm, the function as a barrier is poor in a later step, and if it is 10 nm or more, the barrier function is increased, which increases the risk that the n-type diffusion layer on the back side is not formed well.

After the above heating process, the temperature is lowered while supplying nitrogen, and the n-type single crystal silicon substrate 1 is taken out from the heat treatment furnace at time t ₀₄ , and if necessary, the silicon oxide film 8 on the back surface 1 B side. Remove. After the silicon oxide film 8 is removed, the back surface 1B is exposed. If the silicon oxide film 8 formed on the back surface 1B is thin, the subsequent impurity diffusion to the back surface 1B may be performed without performing removal.

Thereafter, impurity diffusion to the back surface 1B is performed in step S106. Here, the case of using a phosphorus diffusion step with POCl ₃ gas for forming the n-type diffusion layer as an example. In this process, the POCl ₃ gas is thermally decomposed on the entire surface of the n-type single crystal silicon substrate 1 to form a phosphorous silicate glass (PSG) film first, and this is used as a diffusion source and subsequently heated in the subsequent heating process. Penetrates or diffuses. In this way, in step S106 in which the back surface diffusion is performed in the POCl ₃ gas atmosphere, the phosphor in the POCl ₃ gas for phosphorus diffusion is quickly diffused to the exposed back surface 1B and the p-type diffusion layer 7 is formed. Since the silicon oxide film 8, the BSG film 2, and the silicon oxide film 3 serving as diffusion barriers are formed on the 1A side, the mixing of phosphorus is prevented. At this time, the back surface 1B side is disposed so as to be directly exposed to the furnace atmosphere by overlapping two sheets, and the PSG film is formed in a desired thickness.

  On the other hand, the light receiving surface 1A side is arranged so as not to be directly exposed to the atmosphere of the furnace by overlapping two sheets, and the formation of phosphorus glass is greatly limited. Further, a silicon oxide film 8, a BSG film 2, and a silicon oxide film 3 are formed on the surface, and these function as a diffusion barrier, thereby preventing phosphorus from entering the silicon. That is, phosphorus is diffused selectively on the back surface 1B, and the n-type diffusion layer 14 is formed on the back surface.

  That is, as shown in FIG. 3A, phosphorus is diffused selectively on the back surface 1B, and the n-type diffusion layer 14 is formed on the back surface 1B.

  After the formation of the n-type diffusion layer 14, in the solid phase diffusion source removal step S107, as shown in FIG. 3B, the BSG film 2 as the solid phase diffusion source is removed. The BSG film 2, the silicon oxide film 3, and the silicon oxide film 8 functioning as a barrier are removed using, for example, 5 to 25% hydrofluoric acid aqueous solution. At this time, an oxide film formed by washing with water, generally called a natural oxide film, may be used as a passivation layer described later or a part thereof. Alternatively, for the same purpose, an oxide film obtained by cleaning with water containing ozone may be used.

Subsequently, in the pn junction isolation step S108, the p-type diffusion layer 7 on the side surface of the substrate is removed, and the p-type diffusion layer 7 and the n-type diffusion layer 14 are separated. Specifically, for example, as shown in FIG. 5, several tens to several hundreds of n-type single crystal silicon substrates 1 that have undergone the above steps are stacked, sandwiched between holders H, and plasma excitation gas PG is generated by plasma discharge. Then, end face etching for etching the side surface of the substrate is performed. As the plasma excitation gas PG, fluorine plasma gas obtained by converting CF ₄ gas into a plasma, which is stable, has low toxicity, and has a high etching rate is used. Alternatively, laser separation may be used in which laser is irradiated to the vicinity of the side edge of the front surface or back surface of the substrate or the side surface of the substrate. For example, the p-type diffusion layer 7 may be removed at any location near the side surface of the substrate, the side edge of the substrate surface, and the side edge of the back surface of the substrate. FIG. 5 is an explanatory view showing an end face etching process which is a main part of the manufacturing process of the solar cell according to the first embodiment.

  The essence of electrical isolation is to insulate parts and regions that cause electrical shorts. Specifically, it means that the portion where the p-type diffusion layer and the n-type diffusion layer are in contact with or close to each other in the silicon substrate is removed or processed to greatly reduce the electrical conductivity and electrically insulate. That is, any process including a step of electrically separating the first diffusion layer formed on the first main surface side and the reverse conductivity type region on the second main surface side of the semiconductor substrate may be used. For example, prior to the step of electrically separating the second main surface side of the semiconductor substrate and the first diffusion layer, a step of removing the first diffusion layer on the side surface portion of the semiconductor substrate may be performed.

  Therefore, it is a precondition that removal or processing is applied to the n-type single crystal silicon substrate. However, the BSG film 2 and the silicon oxide film 3 according to the present invention have high resistance to plasma discharge processing, and other diffusions. Since the thickness is larger than that of the diffusion source of the method, if end face etching is performed with these remaining, the processing becomes insufficient and good separation cannot be performed.

  Therefore, after the BSG film 2 and the silicon oxide film 3 are removed, it is possible to perform sufficient separation by end face etching by performing end face etching.

  Further, since it is necessary to remove the BSG film 2 and the silicon oxide film 3 before the formation of an antireflection film or a back surface insulating film, which is a subsequent process, the entire BSG film 2 and the silicon oxide film 3 are subjected to end face etching. The removal method is the simplest and effective for improving productivity or reducing manufacturing costs.

  As described above, the end face of the substrate is cut or etched, and as shown in FIG. 3C, the p-type diffusion layer 7 is provided on the light receiving surface 1A side and the n-type diffusion layer 14 is provided on the back surface 1B side. A solar cell substrate is formed.

  Note that this separation step may be omitted depending on the state of separation, that is, the magnitude of leakage current, or the cell arrangement in the module that will be the final power generation product.

  Thereafter, in the antireflection film formation and back surface insulating film forming step S109, the back surface insulating film 15b made of a silicon nitride film is formed on the back surface 1B by using, for example, plasma CVD. Note that a passivation layer may be formed between the silicon nitride film and the n-type diffusion layer. In this case, the passivation layer is preferably a silicon oxide film, and in addition to general thermal oxidation, an oxide film obtained by washing with water or washing with ozone-containing water as described above may be used.

  Subsequently, the light receiving surface antireflection film 15a is similarly formed on the light receiving surface 1A side by using, for example, a silicon nitride film using plasma CVD. A passivation layer may be formed between the silicon nitride film constituting the light-receiving surface antireflection film 15 a and the n-type diffusion layer 14.

  In this case, the passivation layer is preferably a silicon oxide film, an aluminum oxide film, or a laminate of both. When a silicon oxide film is used for the passivation layer, an oxide film obtained by washing with water or washing with ozone-containing water as described above may be used in addition to general thermal oxidation. When an aluminum oxide film is used, it is formed, for example, by plasma CVD or ALD (Atomic Layer Deposition). In this case, since the fixed charge included at the time of film-forming has the effect of improving the passivation capability, it is more preferable.

  In particular, when the end face etching process is performed after the hydrofluoric acid aqueous solution treatment in the step of removing the BSG film 2 and the silicon oxide film 3, the substrate surface cleaned by the hydrofluoric acid aqueous solution treatment is subjected to the end face etching. By laminating at the time of processing, when the substrate surfaces come into contact with each other and the fluorine plasma gas is used, the surface state becomes unstable, and the passivation effect due to the formation of the passivation layer in the next step may be reduced.

In particular, when a fluorine-based etching solution such as CF ₄ is used for etching and etching is performed with a fluorine plasma gas, fluorine ions may remain adsorbed on the surface of the silicon substrate after etching. When a silicon nitride film is formed on the surface of the substrate by plasma CVD, a silicon nitride film is formed on the fluorine ions, and fluorine atoms remain between the silicon substrate and the silicon nitride film, thereby reducing the surface passivation effect. .

On the other hand, ALD is a method of forming an aluminum oxide film on the surface of a silicon substrate by repeating a plurality of cycles with water introduction, nitrogen N ₂ purge, TMA (trimethylaluminum) introduction, and N ₂ purge as one cycle. In ALD, since the cycle including N ₂ purge is repeated, fluorine adsorbed on the substrate surface is discharged during N ₂ purge, and a clean surface state can be formed.

  Therefore, by using ALD for forming the passivation layer, a sufficient passivation effect can be obtained even when the fluorine plasma gas treatment is performed after the hydrofluoric acid aqueous solution treatment.

  Note that the order of formation of the light-receiving surface antireflection film 15a, the back surface insulating film 15b, and both passivation layers is not necessarily limited to the above-described order, and the order other than the above may be appropriately selected and formed. good.

  Thereafter, as shown in FIG. 3D, after the light receiving surface antireflection film 15a and the back surface insulating film 15b are formed on the light receiving surface 1A and the back surface 1B, respectively, in the electrode forming step S110, the light receiving surface 1A side and the back surface 1B side. The light receiving surface electrode 16a and the back surface electrode 16b are formed respectively. As the electrode material, for example, copper, silver, aluminum, or a mixture thereof is used. For example, a paste made by mixing a metal powder of copper, silver, aluminum or a mixture thereof, glass, a ceramic component powder, and an organic solvent is formed into a pattern of a desired shape by, for example, screen printing, and then dried. And formed by firing. In this way, the solar cell is completed.

  As described above, according to the method of manufacturing the solar cell of the first embodiment, the solid phase diffusion source is formed and heated to form the diffusion layer, and after removing the solid phase diffusion source, electrical separation is performed. Therefore, the insulation performance of the processed part is improved and the leakage current of the solar cell can be reduced. In addition, mixing of impurities other than the target impurities on the light-receiving surface and the back surface, or contamination of contaminants forming an opposite conductivity type, is suppressed, and a solar cell with a long carrier life and high photoelectric conversion efficiency is realized.

  Further, when the BSG film 2 and the silicon oxide film 3 of the solid phase diffusion source are formed, even if a product containing boron is formed around the back surface 1B, it is removed before heating for diffusion. Impurity diffusion to the back surface can also be prevented by subsequent heating.

  Therefore, it is possible to obtain a solar cell having a long carrier life and high photoelectric conversion efficiency by suppressing the mixing of impurities forming opposite conductivity types or contaminants other than the target impurity on the light receiving surface and the back surface. .

  Note that the slice damage removing process, the texture forming process, and the cleaning process are examples used to describe the process of the first embodiment, and are not limited to these. A process may be used and is not limited to the above process. Similarly, the back surface n-type diffusion layer 14 forming step, the pn junction separation step, the light receiving surface antireflection film 15a and the back surface insulating film 15b forming step, and the light receiving surface electrode 16a and the back electrode 16b forming step. However, any process may be used and is not limited to the above process. In addition, the order from the step of forming the n-type diffusion layer 14 to the step of forming the electrode 16 may be appropriately changed as long as it functions as a solar cell, and is not limited to the order described.

  Further, for the sake of explanation, the n-type single crystal silicon substrate 1, the BSG film 2 as the solid phase diffusion source, and the n-type diffusion layer 14 by phosphorous diffusion on the back surface 1B are used. Absent. As long as it functions as a solar cell, another silicon-based crystal substrate such as a polycrystalline silicon substrate or silicon carbide may be used as the substrate, and a p-type substrate may be used as the conductive type. Further, the solid phase diffusion source may include an impurity containing an impurity for forming an n-type diffusion layer such as a PSG film. For the diffusion opposite to the solid phase diffusion source, an impurity forming a p-type diffusion layer such as boron may be used. As described above, for the diffusion layer formed on the light receiving surface and the back surface of the substrate as well, it is possible to appropriately select the p-type or n-type and the impurity element forming the diffusion layer.

Embodiment 2. FIG.
In the solar cell manufacturing method according to the second embodiment, a partial high-concentration diffusion layer is formed on either or both of the light-receiving surface side and the back surface side as compared with the solar cell manufacturing method shown in the first embodiment. To do. Since it is the same except for the oxide film removal step on the back side and the phosphorus diffusion step, detailed description is omitted as referring to the first embodiment.

  FIG. 6 is a flowchart showing a process from the heat treatment to the pn junction separation process in the solar cell manufacturing method according to the second embodiment. FIG. 7A and FIG. 7B are schematic views showing changes in the cross section of the n-type single crystal silicon substrate 1 during the n-type impurity diffusion step. Hereinafter, a description will be given with reference to FIGS. 6 and 7.

In the method for manufacturing a solar cell according to the second embodiment, after performing step S105, which is a heat treatment step for forming the p-type diffusion layer 7, as shown in FIG. A film forming step S106a on the back surface side of the diffusion source and a back surface diffusion step S106b which is a heat treatment step are performed. Here, a diffusion source 17 containing an n-type conductivity impurity at a high concentration, for example, 1 × 10 ²⁰ / cm ³ or more of phosphorus is formed on the silicon oxide film 8 on the back surface 1B. Thereafter, after the diffusion source 17 is formed, in the back surface diffusion step S106b, the n-type single crystal silicon substrate 1 is subjected to heat treatment in a POCl ₃ gas atmosphere as in the back surface diffusion step S106 of the first embodiment. Impurity diffusion from the diffusion source 17 is performed at a temperature of 800 ° C. to 1000 ° C., for example.

Although the silicon oxide film 8 formed on the back surface 1B exists immediately under the diffusion source, its thickness is as thin as 5 to 10 nm, and the impurity concentration of the diffusion source 17 is high. It will not affect its formation. The diffusion source 17 in this portion is formed of a phosphorous silicate glass (PSG) film formed by thermal decomposition of POCl ₃ gas, and impurities are contained in the n-type single crystal silicon substrate 1 in contact with the diffusion source 17. Is diffused to form a high-concentration n-type diffusion layer 18. An n-type diffusion layer 20 having a lower concentration than the n-type diffusion layer 18 is formed in a region not covered with the diffusion source 17.

  Then, after the pn junction separation step S108, the antireflection film and back surface insulating film formation step S109 and the electrode formation step S110 shown in FIG. 1 are performed.

  On the other hand, the n-type single crystal silicon substrate 1 in a region other than directly under the diffusion source 17 is attached with impurities desorbed from the diffusion source 17 into the atmosphere, but its concentration or total amount is lower than the impurity concentration of the diffusion source 17 itself. The oxide film formed on the surface of the n-type single crystal silicon substrate 1 cannot pass through.

  Therefore, according to the second embodiment, a structure having two levels of concentration can be formed in the diffusion layer. If the distribution of the two is appropriately performed, the region other than the region directly below the diffusion source 17 can be suppressed to a lower concentration, so that a more efficient solar cell is realized.

  As described above, the step of forming the solid phase diffusion source is selectively formed on the back surface 1B which is the second main surface, and the first conductivity type n is formed by diffusion from the PSG film which is the diffusion source 17. This is a step of forming the mold diffusion layer 20.

  According to the second embodiment, the oxide film removing step on the back surface 1B can be removed from the manufacturing method, and the n-type is performed without affecting the silicon oxide film 8 formed on the n-type single crystal silicon substrate 1. The process up to the impurity diffusion process can be completed.

In the step of forming the n-type diffusion layer which is the first conductivity type diffusion layer, impurities of 1 × 10 ²⁰ / cm ³ or more are applied to the back surface 1B which is the second main surface of the n-type single crystal silicon substrate 1. It forms a diffusion source containing. By this method, an impurity diffusion layer can be formed even if a silicon oxide film is present at a portion in contact with the diffusion source, and the silicon oxide film 8 on the back surface 1B of the n-type single crystal silicon substrate 1 is removed. The process can be omitted. In addition, since the diffusion from the diffusion source is performed while the entire surface of the n-type single crystal silicon substrate 1 that is a solar cell substrate is covered with the silicon oxide film 8, impurities released from the diffusion source into the atmosphere are n. Even if it adheres to the type single crystal silicon substrate 1, it does not diffuse inside the substrate.

  As described above, according to the method for manufacturing the solar cell according to the second embodiment, the step of removing the oxide film on the back surface is not required, so that the formation of a leak path in which p-type and n-type impurities are adjacent to each other can be prevented. A solar cell with excellent characteristics is realized.

Embodiment 3 FIG.
As for the removal of the BSG film 2 and the silicon oxide film 3, the BSG film 2 and the silicon oxide film 3 may be removed only for the end face and the vicinity thereof instead of removing all of them. FIG. 8 is a flowchart showing a method for manufacturing the solar cell according to the third embodiment, and FIGS. 9A to 9D are process cross-sectional views illustrating the main part of the manufacturing process for the solar cell according to the third embodiment. is there.

  Since the BSG film 2 and the silicon oxide film 3 need to be removed before the formation of the antireflection film or the back surface insulating film, which is a subsequent process, as shown in the first embodiment, the BSG film 2 and the silicon oxide film 3 are removed. The method of removing the entire oxide film 3 before the separation process is the simplest and easily contributes to improvement of productivity or reduction of manufacturing cost. However, what is important to be removed before the separation processing is the portion to be processed, specifically, the end face and its very vicinity. Most of the light receiving surface 1A and the back surface 1B need not be removed before separation processing.

  Therefore, in the third embodiment, the BSG film 2 and the silicon oxide film 3 are removed only for the end face and the vicinity thereof by a method of dropping the etching solution while rotating. Other steps are the same as those in the first embodiment.

Up to the back surface diffusion step S106 in which the back surface diffusion is performed in the POCl ₃ gas atmosphere is performed in the same manner as in the first embodiment, and the back surface n-type diffusion layer 14 is formed as shown in FIG. FIG. 9A corresponds to FIG. 2D of the first embodiment.

  Thereafter, a solid phase diffusion source removal step S107S is performed on the end face, and the BSG is applied only to the end face and its immediate vicinity as shown in FIG. 9B by a method of dropping an etching solution while rotating the substrate. The film 2 and the silicon oxide film 3 are removed.

  Next, a pn junction separation step S108 is performed, and end face etching is performed on most portions of the light receiving surface and the back surface both surfaces using the remaining BSG film 2 and silicon oxide film 3 as protective films, as shown in FIG. 9C. Thus, the p-type diffusion layer 7 on the substrate end face is removed.

  Then, the remaining solid phase diffusion source removal step S107SS is performed, and the remaining BSG film 2 and silicon oxide film 3 are removed as shown in FIG. At this time, the silicon oxide film 8 on the front surface and the back surface is also removed. This corresponds to FIG. 3C in the first embodiment, and similarly to the first embodiment, the antireflection film 15a, the protective film 15b, the light receiving surface electrode 16a, and the back surface electrode 16b are formed to complete the solar cell. .

  With the above configuration, the BSG film 2 and the silicon oxide film 3 remaining on most of the light receiving surface and the back surface can be used as protective films during separation processing. Thereby, the characteristic of a solar cell can further be improved. In addition, leakage current can be suppressed.

  In the first to third embodiments, after the heat treatment step, a part of the silicon oxide film as the protective film is removed in a step of forming a conductive type diffusion layer different from the formed diffusion layer. Accordingly, it becomes possible to use impurity diffusion by a simple gas, and since the film remains except for the oxide film removal portion, it is possible to prevent the entry of impurities and the formation of a leak path.

  As described above, in the first to third embodiments, after forming a film containing an impurity serving as a solid phase diffusion source, the diffusion source on the back side is removed, and then the heat treatment is performed after removing the diffusion source on the back side. The manufacturing process for preventing the impurity diffusion from is shown. Specifically, in the case of the heat treatment, in the usual heat treatment, the treatment is performed using an inert gas such as nitrogen or argon, and the heat treatment is performed in an atmosphere in which oxygen is introduced in the middle of the heat treatment. To implement. The supply of oxygen is performed after a heat treatment in an atmosphere not containing oxygen for diffusing impurities from the film of the solid phase diffusion source. In other words, oxygen that touches the furnace after it is introduced into the furnace forms an oxide film as a diffusion barrier at the product and substrate interface on the backside of the substrate, and impurity diffusion is carried out from the film while the supply of oxygen is stopped. Impurities are diffused. An oxide film is also formed on the film-forming surface of the solid-phase diffusion source by oxygen introduced at the end of the heat treatment step, and a function as a barrier against another type of diffusion that is subsequently performed is added. By this method, impurities can be diffused only on the film formation surface.

  Note that the oxidation step after the diffusion step may be performed by introducing oxygen only for the last necessary time of the heat treatment step for diffusion, as described in Embodiment 1 with reference to FIG. In addition, oxygen may be introduced for a necessary time in the temperature lowering step after the heat treatment step. In addition, after the temperature of the diffusion furnace is once lowered to room temperature, an oxidation heat treatment step may be performed.

  In the first to third embodiments, the temperature in the heat treatment process for performing impurity diffusion is determined by the type of the impurity to be diffused and can be changed as appropriate. Also, the diffusion atmosphere can be a reducing atmosphere such as a hydrogen atmosphere in order to control the diffusion rate depending on the type of impurities, and can be adjusted as appropriate.

  In the first to third embodiments, the example in which the second diffusion layer having the same conductivity type as that of the semiconductor substrate is formed on the second main surface, that is, the back surface side of the semiconductor substrate has been described. It does not have to be formed. In this case, the solid phase diffusion source is removed prior to the pn separation between the semiconductor substrate and the first diffusion layer. In addition, although an n-type single crystal silicon substrate is used as a semiconductor substrate, other crystalline silicon substrates such as p-type single crystal silicon substrates, p-type and n-type polycrystalline silicon substrates, or silicon compounds such as silicon carbide. Needless to say, the present invention can also be applied to the formation of a diffusion layer using a compound semiconductor including the above. The impurities of the first and second conductivity types are determined in accordance with the conductivity type of the semiconductor substrate. The types of impurities are n-type impurities such as phosphorus, arsenic, and p-type impurities. Needless to say, usual impurities other than boron and gallium are applicable.

  Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope of the invention, and also included in the invention described in the claims and the equivalents thereof.

  1 n-type single crystal silicon substrate, 1A light-receiving surface, 1B back surface, 2 BSG film, 3 silicon oxide film, 4 boron-containing product, 5 silicon oxide-containing product, 7 p-type diffusion layer, 8 silicon oxide film, 14 n Type diffusion layer, 15a light receiving surface antireflection film, 15b back surface insulating film, 16 electrodes, 16a light receiving surface electrode, 16b back surface electrode, 17 diffusion source, 18 n type diffusion layer, 20 low concentration n type diffusion layer.

In order to solve the above-described problems and achieve the object, the present invention provides a solid phase diffusion source and a protective film on a first main surface of a first conductivity type semiconductor substrate having first and second main surfaces facing each other. And the step of removing the product formed on the second main surface in the step of forming the film, and heating the semiconductor substrate from which the product has been removed, from the solid phase diffusion source, Forming a first diffusion layer of the second conductivity type on the surface side, forming a second diffusion layer having the first conductivity type on the second main surface of the semiconductor substrate, solid phase diffusion source and protection Removing both the film and electrically separating the second diffusion layer and the first diffusion layer. The step of removing both the solid phase diffusion source and the protective film is performed before the step of electrically separating the second diffusion layer and the first diffusion layer.

Thereafter, in the antireflection film formation and back surface insulating film forming step S109, the back surface insulating film 15b made of the silicon nitride film 15 is formed on the back surface 1B by using, for example, plasma CVD. Note that a passivation layer may be formed between the silicon nitride film and the n-type diffusion layer. In this case, the passivation layer is preferably a silicon oxide film, and in addition to general thermal oxidation, an oxide film obtained by washing with water or washing with ozone-containing water as described above may be used.

Subsequently, a light-receiving surface antireflection film 15a is similarly formed on the light-receiving surface 1A side by using, for example, a silicon nitride film 15 using plasma CVD. A passivation layer may be formed between the silicon nitride film 15 constituting the light-receiving surface antireflection film 15a and the n-type diffusion layer 14.

Then, the remaining solid phase diffusion source removal step S107SS is performed, and the remaining BSG film 2 and silicon oxide film 3 are removed as shown in FIG. At this time, the silicon oxide film 8 on the front surface and the back surface is also removed. This corresponds to FIG. 3C in the first embodiment, and similarly to the first embodiment, the antireflection film 15a, the back surface insulating film 15b, the light receiving surface electrode 16a, and the back surface electrode 16b are formed, and the solar cell is completed. To do.

Claims (6)

Forming a solid phase diffusion source on the first main surface of the first conductive type semiconductor substrate having first and second main surfaces facing each other;
Removing the product formed on the second main surface in the film forming step;
Heating the semiconductor substrate from which the product has been removed, and forming a first diffusion layer of a second conductivity type on the first main surface side from the solid phase diffusion source;
Forming a second diffusion layer having a first conductivity type on the second main surface of the semiconductor substrate;
Removing the solid phase diffusion source;
Electrically separating the second diffusion layer and the first diffusion layer;
Including
Removing the solid phase diffusion source comprises:
The method for manufacturing a solar cell, which is performed before the step of electrically separating the second diffusion layer and the first diffusion layer. The step of removing the back surface product includes:
The method for manufacturing a solar cell according to claim 1, wherein the method is performed prior to the step of forming the first diffusion layer. Removing the solid phase diffusion source comprises:
3. The solar cell according to claim 1, wherein the solar cell is a step of selectively removing side surfaces of the semiconductor substrate and the first diffusion layer while leaving the first main surface and the second main surface. Manufacturing method. The step of forming a film of the solid phase diffusion source includes:
Forming a protective film together with the solid phase diffusion source,
Removing the solid phase diffusion source comprises:
The method for manufacturing a solar cell according to any one of claims 1 to 3, further comprising a step of removing a protective film together with the solid phase diffusion source.   2. The step of forming the second diffusion layer having the first conductivity type includes a step of selectively forming the solid phase diffusion source on a part of the second main surface. 5. The method for producing a solar cell according to any one of items 1 to 4. Forming a solid phase diffusion source on the first main surface of the first conductive type semiconductor substrate having first and second main surfaces facing each other;
Removing the product formed on the second main surface in the film forming step;
Heating the semiconductor substrate from which the product has been removed, and heating the semiconductor substrate from the solid phase diffusion source to form a second diffusion layer of the second conductivity type on the first main surface side; ,
Removing the solid phase diffusion source;
Electrically separating the semiconductor substrate and the first diffusion layer;
Including
Removing the solid phase diffusion source comprises:
A method for manufacturing a solar cell, which is performed before the step of electrically separating the semiconductor substrate and the first diffusion layer.

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