TW201640687A - Method for producing solar cell and solar cell - Google Patents

Abstract

An objective of this invention is to obtain a method for producing solar cell having long carrier lifetime, which prevents migration of impurities in backside while conducting diffusion of impurities by thermal treatment after forming a film of solid diffusion source. The method for producing solar cell of this invention contains: forming a film of a solid diffusion source on a first surface of a first conductivity-type semiconductor substrate having first and second surfaces, and a thermal treatment step of diffusing impurities with second conductivity from the solid diffusion source by thermal treatment, to form a second conductivity -type diffusion layer, wherein the thermal treatment is conducted in the same furnace, and contains: a first step of heating at a first temperature T1 for a time t0 while supplying oxygen, a second step of supplying an inert gas and stopping the supply of oxygen, and heating at a second temperature T2 for a time t1 to diffuse the impurities, and a third step of heating at a third temperature T3 for a time t2 while supplying oxygen again.

Description

Solar cell manufacturing method and solar cell

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

In the solar cell, for example, as disclosed in Patent Document 1, a method of diffusing impurities on a light-receiving surface as a light incident surface or a back surface opposite to a light-receiving surface is disclosed, and after a diffusion source is formed by a CVD method, A method in which a film of a substrate and a diffusion source is heated in a nitrogen atmosphere to diffuse impurities into the substrate.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2004-247364

However, in the method for producing a solar cell described in Patent Document 1, after the phosphorous silicate glass or the boron silicate glass is formed on the substrate, A heat treatment for diffusion of impurities is performed in a nitrogen atmosphere. Therefore, impurities such as phosphorus (phosphorus) or boron are wound around the back surface of the substrate at the time of film formation, and at the same time, impurities are diffused from the adhered product to the back surface, so that an unexpected contamination of the back surface is caused. The incorporation of impurities causes a decrease in the carrier life of the solar cell.

The present invention has been made in view of the above circumstances, and an object of the invention is to provide a method for producing a solar cell, which is characterized in that when a solid phase diffusion source is formed into a film and then diffusion of impurities by heat treatment is performed, impurities are prevented from being mixed into the back surface. And it is a solar cell with a long carrier life.

In order to solve the above problems and achieve the object, the present invention includes a step of forming a solid phase diffusion source on a first main surface of a first conductivity type semiconductor substrate having first and second main faces, and a step of forming a film by heat treatment. A heat treatment step in which a conductive impurity is diffused by a solid phase diffusion source to form a diffusion layer of a second conductivity type. The heat treatment step is carried out in the same furnace, and includes a first step of heating while supplying oxygen at a first temperature, and a second step of stopping supply of oxygen to supply an inert gas to the second step. The temperature is heated and the impurities are diffused; and in the third step, oxygen is again supplied while being heated to the third temperature.

According to the present invention, it is possible to prevent the impurities on the back surface from being mixed when the impurities are diffused by the heat treatment after the solid phase diffusion source is formed, and the effect of the carrier life of the solar cell can be improved.

1‧‧‧n type single crystal germanium substrate

1A‧‧‧Stained surface

1B‧‧‧Back

2‧‧‧BSG film

3,6,8,11,24‧‧‧矽Oxide film

4‧‧‧Titanium-containing deposits

5‧‧‧Titanium oxide attachments

7‧‧‧p type diffusion layer

9‧‧‧Film formation

9a‧‧‧Ozone film formation failure

9b‧‧‧BSG formed a bad department

9c‧‧‧ Bad part of BSG film and tantalum oxide film

10‧‧‧p type diffusion layer formation defective part

10a, 10b‧‧‧p type diffusion layer shallowing

10c‧‧‧p type diffusion layer not formed

14, 18, 20, 22, 23‧‧‧n type diffusion layer

15‧‧‧Anti-reflective film

15a‧‧‧Glossy anti-reflection film

15b‧‧‧Back anti-reflection film

16‧‧‧Electrode

16a‧‧‧Photon surface electrode

16b‧‧‧Back electrode

17, 19‧‧‧ diffusion sources

21‧‧‧ openings

Fig. 1 is a flow chart showing a method of manufacturing a solar cell according to the first embodiment.

Fig. 2 (a) to (d) are cross-sectional views showing the steps of a method for manufacturing a solar cell according to the first embodiment.

Fig. 3 (a) to (d) are cross-sectional views showing the steps of a method for manufacturing a solar cell according to the first embodiment.

Fig. 4 is an explanatory view showing a time chart of the temperature in the furnace and the environmental state in the heat treatment step in the manufacturing process of the solar cell of the first embodiment.

Fig. 5 (a) and (b) are views showing a cross section of an n-type single crystal germanium substrate in a case where a film formation failure portion is partially formed when a BSG film and a tantalum oxide film are formed in the method of the first embodiment.

Fig. 6 is a flow chart showing a method of manufacturing the solar cell of the second embodiment.

Fig. 7 is a flow chart showing a method of manufacturing the solar cell of the third embodiment.

Fig. 8 (a) and (b) are views showing a step of forming an n-type diffusion layer on the back surface of the method for producing a solar cell according to the third embodiment.

(a) and (b) of FIG. 9 are views showing a step of forming an n-type diffusion layer on the back surface of a method for producing a solar cell according to a modification of the third embodiment.

Fig. 10 (a) and (b) are views showing a step of forming an n-type diffusion layer on the back surface of the method for producing a solar cell according to the fourth embodiment.

11 is a view showing a method of manufacturing a solar cell according to Embodiment 1. A graph of the average sheet resistance of the back surface of the solar cell substrate when the oxygen flow rate during the heat treatment is changed.

Fig. 12 is a flow chart showing the steps of manufacturing the solar cell of the fifth embodiment.

Fig. 13 is an explanatory view showing a time chart of temperature and environmental conditions in the furnace in the method for manufacturing a solar cell according to the fifth embodiment.

Fig. 14 is a flow chart showing the steps of manufacturing the solar cell of the sixth embodiment.

Fig. 15 is a cross-sectional view showing an essential part of a solar cell in the manufacturing process of the solar cell of the sixth embodiment.

Fig. 16 is a flow chart showing an important part of the manufacturing process of the solar cell of the seventh embodiment.

Fig. 17 (a) and (b) are cross-sectional views showing important parts of a solar cell in the manufacturing process of the solar cell of the seventh embodiment.

(Example)

Hereinafter, embodiments of the solar cell manufacturing method and solar cell of the present invention will be described in detail based on the drawings. In addition, the present invention is not limited to the embodiment, and can be appropriately modified without departing from the scope of the invention. Further, in the drawings shown below, in order to facilitate understanding, the scale of each layer or each member may be different from the actual one, and the same applies to each drawing.

Embodiment 1.

Fig. 1 is a flow chart showing the manufacturing procedure of the first embodiment of the method for producing a solar cell of the present invention, and Figs. 2(a) to (d) and Figs. 3(a) to 3(d) show the first embodiment. A cross-sectional view of a step of a method of manufacturing a solar cell. Fig. 2 (a) to (d) are cross-sectional views showing changes in the solar cell substrate during continuous processing in the furnace shown in Fig. 1 in the method for producing a solar cell of the present invention. Fig. 3 (a) to (d) are views showing changes in the cross section of the solar cell in the step of heat treatment shown in Fig. 2 in the manufacturing step of the first embodiment. Fig. 4 is an explanatory view showing a time chart of temperature and environmental conditions in the furnace.

A method of producing a solar cell according to the first aspect, characterized by a heat treatment step for forming a diffusion layer, wherein the heat treatment step is carried out in the same furnace before the diffusion of the impurities, and the oxygen is supplied at the first temperature. While heating, an oxide film is formed in advance, and the impurity is diffused at the second temperature, and then oxidized again at the third temperature. In the first step, heating is performed while supplying oxygen at the first temperature. In the second step, the supply of oxygen is stopped, the inert gas is supplied, and the second temperature is heated to diffuse the impurities. In the third step, oxygen is supplied again, and heating is performed at the third temperature to make the oxide film a dense oxide film, thereby improving barrier properties.

In the first embodiment, in the first step of performing impurity diffusion by a heating step in an environment containing no oxygen, the first step of heating the semiconductor substrate in an atmosphere containing oxygen is performed. An oxide film is formed on the surface opposite to the surface of the solid phase diffusion source of the semiconductor substrate. When the solid phase diffusion source is formed on the surface opposite to the film formation surface, it will surround The solid phase diffusion material is attached, but since the oxide film is formed, impurities from the deposit do not diffuse to the substrate. Therefore, the effect of the film formation process and the heat treatment of the solid phase diffusion source can be continuously performed.

In the solar cell of the first embodiment, an n-type single crystal silicon substrate 1 as a first conductivity type semiconductor substrate having a first main surface of the light receiving surface 1A and a second main surface of the back surface 1B is used. The manufacturing method will be described using Figs. 1 and 2 (a) to (d), Figs. 3 (a) to (d) and Fig. 4 . First, in step S101, the contamination or damage generated at the wafer slice of the surface is immersed in, for example, alkali which is dissolved in 1 wt% or more and less than 10 wt% of sodium (alkali). After the solution is removed and removed, a textured surface is formed on the light-receiving surface 1A of the n-type single crystal germanium substrate 1, wherein the texture of the unevenness is added to an alkaline solution of, for example, 0.1% or more and less than 10%. Additives such as isopropyl alcohol or caprylic acid are immersed in a solution to obtain an antireflective structure. In addition, the formation of texture can be performed simultaneously or separately with the removal of slicing and damage. Not only the light receiving surface, the formation of the texture can also be formed on the back side. In the second and third figures, the texture is not displayed, but the light-receiving surface and the back surface are shown as flat surfaces.

Next, the surface of the n-type single crystal germanium substrate 1 is washed through step S102. The washing step uses, for example, a combination of sulfuric acid and hydrogen peroxide, a hydrofluoric acid aqueous solution, a mixed solution of ammonia and hydrogen peroxide, and hydrochloric acid. A step of removing an organic substance, a metal, and an oxide film from a mixed solution of hydrogen peroxide. Or, depending on the method of texture formation, only hydrogen is used. An oxide film removal step of an aqueous solution of hydrofluoric acid.

Following the above-described cleaning step, a solid phase diffusion source is formed on the light-receiving surface 1A of the n-type single crystal germanium substrate 1 via the step S103, for example, as a borosilicate glass containing an oxide film of boron, that is, the BSG film 2. The film formation system uses, for example, reduced pressure CVD (Chemical Vapor Deposition) or atmospheric pressure CVD. Further, in the film forming step described above, the deposit 4 of boron containing the product due to the surrounding of the film forming gas adheres to the back surface 1B of the n-type single crystal germanium substrate 1. Next, a film of a cap, such as the tantalum oxide film 3, is formed in the upper half of the BSG film 2 during heat treatment. Similarly to the BSG film 2, the tantalum oxide film 3 is formed by a film forming step such as reduced pressure CVD or normal pressure CVD, and is preferable from the viewpoint of the continuity of the steps. When the tantalum oxide film 3 is formed, as in the case of forming the film of the BSG film 2, the deposit 5 containing the ruthenium oxide which is a product due to the surrounding of the film forming gas adheres to the back surface 1B.

After the film of the BSG film 2 and the tantalum oxide film 3 is formed, the n-type single crystal germanium substrate 1 is continuously subjected to heat treatment. This heat treatment uses a heat treatment furnace. First, in the preheating heat treatment furnace, when the first temperature T1 = 800 ° C, the n-type single crystal germanium substrate 1 is put into the heat treatment furnace at time t ₀₁ , and the furnace is replaced with nitrogen gas, and the time point t _{02 is the} nitrogen atmosphere. The heat treatment furnace supplies oxygen. Oxygen supply, while the first temperature preserving time t ₀ = 1 minutes to minutes until 20 is reached until the time t _11. In the above oxidation step, the n-type single crystal germanium substrate 1 introduced into the heat treatment furnace is oxidized by the surface contained by oxygen contained in the environment. Since the light-receiving side is covered by the BSG film 2 and the tantalum oxide film 3, the oxidation system selectivity is performed on the back side which is not covered with the film. The deposit 4 containing the boron adhered to the back side and the deposit 5 of the ruthenium oxide are not formed as a film but adhered as a particulate deposit. Therefore, oxygen easily reaches the interface between the deposit on the back side and the n-type single crystal germanium substrate 1, and forms an interface between the deposit 4 of boron-containing boron and the deposit 5 of the tantalum oxide and the n-type single crystal germanium substrate 1. Oxide film 6.

Further, by replacing the inside of the furnace with nitrogen gas at the time point t ₀₁ , oxidation of the wafer formed of the n-type single crystal germanium substrate 1 into the furnace can be suppressed. Then, purged with nitrogen end oxygen supply start time point t _02, whereby the entire surface to be n-type single crystal silicon substrate 1, silicon oxide film is uniformly formed.

Then at time _{t. 11} stops supply of oxygen, the furnace was replaced with nitrogen, a heat treatment furnace while the temperature was raised to reach the second temperature T ₂ = 1050 ℃ time point t of up to _12. While the nitrogen gas is supplied, the length of the second temperature is maintained for the time t ₁ to the time point t ₂₁ , and as shown in FIG. 2( c ), boron is diffused from the solid phase diffusion source 2 to form the p-type diffusion layer 7 .

Thereafter, the oxygen supply is started at time t ₂₁ and the oxygen supply is stopped at time t ₂₂ . Here, the third temperature T ₃ is set to the same temperature as the second temperature T ₂ , and oxygen supply is performed from the time point t ₂₁ to the time point t ₂₂ , and as shown in FIG. 2( d ), the dense tantalum oxide film 8 is formed. As described above, after the diffusion of the impurities is completed, oxygen is again introduced, whereby the tantalum oxide film 8 is formed on the entire surface of the n-type single crystal germanium substrate 1.

After the heating step after composed of three or more steps, while supplying nitrogen gas while the temperature was lowered, at time t ₃₀ be removed from the wafer in the heat treatment furnace, the back surface oxide film removing embodiment step S107, a back surface side of the silicon when necessary Removal of the oxide film 6. At this time, the deposit 4 containing boron and the deposit 5 of cerium oxide are also removed. As shown in Fig. 3(a), the tantalum oxide film 6 is removed to expose the back surface 1B. Further, when the tantalum oxide film 6 formed on the back surface 1B is thin, it is not removed, and the diffusion of impurities to the back surface 1B is continued.

Thereafter, diffusion of impurities to the back surface 1B is performed as necessary. Here, a case of a phosphorus diffusion step performed using POCl ₃ gas for forming an n-type diffusion layer will be exemplified. The phosphorus in the phosphorus-dispersed POCl ₃ gas rapidly diffuses to the exposed back surface 1B through the step S108 of performing back diffusion in the POCl ₃ environment, and the tantalum oxide film which forms the barrier diffusion on the light-receiving surface 1A side where the p-type diffusion layer 7 is formed is formed. 8. The BSG film 2 and the tantalum oxide film 3 prevent the incorporation of phosphorus.

That is, as shown in Fig. 3(b), the phosphorus diffusion selectivity is performed on the back surface 1B, and the n-type diffusion layer 14 is formed on the back surface 1B.

Further, after the formation of the n-type diffusion layer 14, the ruthenium oxide film 8 which exhibits the functions of the BSG film 2, the ruthenium oxide film 3, and the barrier is removed using, for example, a 5 to 25% aqueous solution of hydrofluoric acid. Then, in the pn junction separation step S109, in order to separate the p-type diffusion layer 7 and the n-type diffusion layer 14, the end surface of the substrate is cut or etched, and as shown in FIG. 3(c), the light-receiving surface is formed. The 1A side is provided with a p-type diffusion layer 7 and a solar cell substrate having an n-type diffusion layer 14 on the back surface 1B side.

Then, in the anti-reflection film forming step S110, the light-receiving surface anti-reflection film 15a and the back surface anti-reflection film 15b are formed on the light-receiving surface 1A and the back surface 1B, respectively, and a tantalum nitride film is formed, for example, by plasma CVD. The light-receiving surface 1A and the back surface 1B can form a silicon oxide, for example, on the p-type diffusion layer 7, respectively. A passivation film such as a film or an alumina is formed on the n-type diffusion layer 14 to form a passivation film such as a tantalum oxide film, but is omitted here for easy understanding. Then, via the electrode forming step S111, the light-receiving surface electrode 16a and the back surface electrode 16b are formed on the light-receiving surface 1A side and the back surface 1B side, respectively. The electrode material can be formed by, for example, copper, silver, aluminum, or a mixture thereof, after the electrode paste is applied, and then subjected to a firing step, whereby the light-receiving surface 1A and the back surface 1B are formed to penetrate the light-receiving surface anti-reflection film 15a and the back surface anti-reflection film, respectively. The light-receiving surface electrode 16a and the back surface electrode 16b of 15b. This completes the solar cell.

As described above, according to the method of the first embodiment, it is possible to obtain a solar cell having high photoelectric conversion efficiency while preventing the incorporation of impurities on the back surface and having excellent workability and long carrier life.

Further, as described above, in the diffusion step of the first embodiment, as shown in the fourth diagram, the time chart indicated by the solid line a is used to increase the temperature, heat, and temperature while switching the temperature and the environment. First, the heat treatment furnace is preheated to an oxidation temperature T ₁ . The film formation environment of the tantalum oxide film 6 on the back surface 1B side is the oxidation temperature T at the first temperature after the n-type single crystal germanium substrate 1 on which the solid phase diffusion source 2 is formed on the light receiving surface 1A side is placed in the heat treatment furnace. ₁ oxygen is supplied while maintaining a certain time t ₀ to carry out the oxidation step of the first step. In the first embodiment, the oxidation temperature T _{1 is} set to 800 ° C, but the oxidation temperature T _{1 is} a temperature band of 500 ° C to 950 ° C, and the time t _{0 is} set to about 1 minute to 20 minutes. Preferably, the oxidation temperature T ₁ can be set to 700 ° C to 850 ° C. When the oxidation temperature T _{1 is} less than 700 ° C, the oxidation rate is slow. When the oxidation temperature exceeds 850 ° C, the back surface is continuously diffused before being coated with the oxide film, and the deposition on the back surface cannot be avoided. Further, by performing the oxidation temperature T ₁ at a constant temperature, the tantalum oxide film 6 covering the back surface 1B can be stably and surely formed.

The n-type single crystal germanium substrate 1 introduced into the heat treatment furnace oxidizes the surface due to oxygen contained in the environment. Since the light-receiving surface 1A side is covered by the BSG film 2 and the tantalum oxide film 3 of the solid phase diffusion source, the oxidation selectivity is performed on the back surface 1B side which is not covered with the film. The deposit 4 containing boron and the deposit 5 of ruthenium oxide attached to the back surface 1B side are not formed as a film but are adhered to a particulate deposit. Therefore, oxygen easily reaches the interface between the deposit on the back surface 1B side and the n-type single crystal germanium substrate 1, and the tantalum oxide is formed at the interface between the deposit 4 containing the boron and the deposit 5 of the tantalum oxide and the n-type single crystal germanium substrate 1. Membrane 6. In the first embodiment, the step of supplying oxygen to the heat treatment furnace before the diffusion of the impurities from the solid phase diffusion source 2 is generated in the heat treatment furnace to form the tantalum oxide film 6 for preventing the diffusion of undesired impurities toward the back surface 1B is performed. .

Further, when the oxidation step is performed at the first temperature T ₁ , the gas in which oxygen has been mixed can flow from the time of wafer loading. At this time, it is possible to omit the time in which the inside of the furnace is replaced with nitrogen, and the step of shortening can be shortened, but the unevenness of the oxidation treatment due to the gas mixed in the wafer or the temperature distribution in the furnace may occur.

On the other hand, as described above, when the first temperature T ₁ is a temperature exceeding 850 ° C, the oxidation rate is increased, so that the aforementioned unevenness is increased, and thus it is not preferable.

Further, although the oxidation step is maintained at the same temperature in the first temperature T ₁ , the temperature may be raised after the start of the oxygen supply, and the temperature may be raised by the second temperature T ₂ of the diffusion temperature. The modified example is a time chart shown by a broken line b in Fig. 4 . At this time, although the step of shortening can be performed, in the middle of the oxidation step, the back surface 1B is not completely covered by the tantalum oxide film 6, and diffusion starts.

Next, the diffusion temperature T ₂ which is raised to the second temperature is heated in an atmosphere containing, for example, an inert gas such as nitrogen or argon, and maintained for a predetermined time t ₁ to carry out the diffusion step of the second step. In the first embodiment, the diffusion temperature T ₂ is set to 1050 ° C, but the diffusion temperature T2 is a temperature band of 800 ° C to 1100 ° C, and the time t _{1 is} set to about 1 minute to 120 minutes.

Alternatively, the gas in the furnace may be replaced with an inert gas such as nitrogen or argon before the temperature is raised to the second temperature T ₂ . By replacing with an inert gas, oxidation at the time of temperature rise due to residual oxygen can be suppressed, and unevenness in film quality or film thickness of the oxide film can be suppressed.

As described above, after the tantalum oxide film 6 is selectively formed on the back surface 1B, the inflow of oxygen is stopped to reach the temperature at which the diffusion of impurities from the BSG film 2 is performed, for example, 900 ° C to 1100 ° C until the desired p type When the formation of the diffusion layer 7 is completed, the inflow of oxygen is stopped first. The second temperature T ₂ at this time is determined depending on the type of the impurity.

Then, after the diffusion of the impurities is completed, in the third step, oxygen is again introduced at the third temperature T ₃ to form a dense tantalum oxide film 8 on the entire surface of the n-type single crystal germanium substrate 1. The third temperature T ₃ can be set higher than the second temperature T ₂ . Thereby, the dense tantalum oxide film 8 is formed efficiently and in a short time.

Further, in the third step, while maintaining only the system temperature T ₃ is the third predetermined time T ₂ is performed, but also the third temperature T ₃ is the temperature of the diffusion of the second temperature T _2. Alternatively, the heating may be stopped and the cooling step may be used to oxidize. This is indicated by a broken line on the right side of Fig. 4, and is a state in which the temperature at t ₂₁ starts to decrease at the time of oxygen supply. At this time, the heating time can be further shortened. Since the third temperature T ₃ is set to be the same as the second temperature T ₂ , the third temperature T ₃ is not shown in FIG. 4 .

FIGS. 5(a) and 5(b) are cross-sectional views showing the n-type single crystal germanium substrate 1 when a film formation failure portion is partially formed when the BSG film 2 and the tantalum oxide film 3 are formed, and correspond to the aforementioned Figure 2 (b) and Figure 2 (d) of the manufacturing steps. The film formation failure portion 9 is divided into a tantalum oxide film formation failure portion 9a, a BSG formation failure portion 9b, and a formation failure portion 9c of both the BSG film and the tantalum oxide film. The film formation failure portion 9 forms a p-type diffusion layer defective portion 10 composed of the p-type diffusion layer thinning portions 10a and 10b and the p-type diffusion layer unformed portion 10c.

In the first embodiment, the oxidation treatment is performed in an atmosphere containing oxygen before the impurity diffusion step, so that oxygen reaches a position at which the film of the film defect portion 9 has been thinned and the impurity diffusion is not completed, for example, both the BSG film and the tantalum oxide film. In the light-receiving surface 1A of the n-type single crystal germanium substrate 1 directly under the defective portion 9c, the tantalum oxide film 11 is formed in the same manner as the back surface 1B. Since the tantalum oxide film 11 prevents intrusion of contaminants from the furnace body or the environment and functions as a barrier, the contamination of the light-receiving surface 1A in the heat treatment is prevented. In other words, by introducing oxygen before the diffusion of the impurities, an oxide film is formed at the film formation failure portion 9 or at a position where the film is not formed, for example, the back surface 1B, and it is possible to prevent the intrusion of the contaminant and the other position where the impurity is diffused. The tantalum oxide film 8 is formed by introducing oxygen after diffusion of impurities.

In addition, oxygen-containing environments use 10% to 100% oxygen. The flow rate ratio is mixed into an inert gas typified by nitrogen or argon. When the oxygen content is 10% or less, the oxidation rate of the surface of the n-type single crystal germanium substrate 1 is slow, and it is difficult to obtain an effect, and unevenness of the oxide film due to the input position in the furnace of the n-type single crystal germanium substrate 1 is generated or The oxide film on the surface of the substrate is uneven, which is not preferable. The oxygen can be set to 100%, but since the oxidation rate is limited by the diffusion of oxygen into the n-type single crystal germanium substrate 1, and as the flow ratio of oxygen becomes larger, the oxidation rate also increases, so it is necessary to oxidize. The time of the steps is limited to a short time. Therefore, it is preferred to carry out heating from an environment of 15% to excess oxygen, for example, 40% to avoid uneven distribution of oxygen in the furnace.

Further, for reference, Fig. 11 shows an average value of the sheet resistance of the back surface 1B of the n-type single crystal germanium substrate 1 after the above treatment is carried out with a flow rate ratio of 10% of oxygen to 20% of oxygen with respect to nitrogen gas. With respect to 10% of oxygen, an increase in sheet resistance was suppressed in a sample of 20% oxygen, which was equivalent to a value of 90 Ω/□ before and after the sheet resistance of the n-type single crystal germanium substrate itself used for the test. Further, the magnitude of the suppression effect is affected by the film thickness or density of the tantalum oxide film 6 formed on the back surface 1B side. In addition, by reducing the amount of boron diffused to the back surface 1B of the n-type single crystal germanium substrate 1 to the entire back surface and suppressing it to 1.6 × 10 ¹⁷ /cm ³ or less, it is possible to suppress the deterioration of the photoelectric conversion efficiency. degree. Boron is supplied to the light-receiving surface 1A of the second main surface of the n-type single crystal germanium substrate 1 as an impurity of the second conductivity type impurity.

In addition, the step of removing the slice damage, the step of forming the texture, and the step of washing the process are examples of the method for producing a solar cell according to the first embodiment, and are not limited thereto, and any steps may be used. The invention is not limited. Similarly, the step of forming the n-type diffusion layer 14 on the back surface 1B, the step of separating the pn junction, the step of forming the light-receiving surface anti-reflection film 15a and the back surface anti-reflection film 15b, and the step of forming the light-receiving surface electrode 16a and the back surface electrode 16b There is no direct relationship with the present invention, so any steps may be used, and the present invention is not limited. Further, the step of forming the n-type diffusion layer 14 up to the step of forming the electrode 16 can appropriately replace the order as long as the function of the solar cell can be exhibited, and the order of description is not limited to the present invention.

Further, although the n-type single crystal germanium substrate 1 is used for the description, the BSG film 2 is used for the solid phase diffusion source, and the phosphorus diffusion layer 14 is used for the back surface 1B, but it is not limited thereto. As long as the function of the solar cell is exhibited, a substrate of a polycrystalline germanium substrate or a tantalum-based crystal substrate such as tantalum carbide may be used as the substrate, and a p-type substrate may be used as the conductive type. Further, as the solid phase diffusion source, those having an n-type diffusion layer such as phosphonium silicate glass (PSG) may be used. An impurity which forms a p-type diffusion layer such as boron can be used for diffusion on the reverse side of the solid phase diffusion source.

According to the method for producing a solar cell of the first embodiment, even when the BSG film 2 and the tantalum oxide film 3 which continue the solid phase diffusion source are subjected to heat treatment, the tantalum oxide film 6 is formed on the back surface 1B before the impurity is diffused. The diffusion of impurities to the back surface 1B due to the boron-containing deposit 4 surrounding the product of the back surface 1B is prevented. Further, on the light-receiving surface 1A side on which the BSG film 2 and the tantalum oxide film 3 are formed, even if the film formation failure portion 9 is formed, the tantalum oxide film 11 is formed at a position where the impurity diffusion is not completed, so that the intrusion of the contaminant is prevented.

Next, a tantalum oxide film 8 as a barrier for impurity diffusion to be subsequently formed is formed on the light-receiving surface 1A. Therefore, on the light receiving surface 1A and back In addition to the target impurities, 1B suppresses the occurrence of the incorporation of impurities of opposite conductivity type or the incorporation of contaminants, and realizes a solar cell having a high photoelectric conversion efficiency with a long carrier life.

Further, since the film formation and heat treatment of the solid phase diffusion source can be continuously performed, it is not necessary to remove the boron-containing deposit 4 on the back surface 1B without using, for example, a hydrofluoric acid aqueous solution or a vapor thereof. In other words, since the removal process of the boron-containing deposit 4 on the back surface 1B side is not required, the tantalum oxide film 3 or the BSG film 2 on the light-receiving surface 1A side is not thinned.

In particular, a region within 5 mm from the outer side of the end of the n-type single crystal germanium substrate 1 is a region where the thinning is increased by the removal process. However, in the first embodiment, the removal process for removing the deposit on the back surface 1B is not required. Therefore, the tantalum oxide film 3 or the BSG film 2 is not thinned. The film thickness of the tantalum oxide film 3 or the BSG film 2 is uniform even in an end region within 5 mm from the outer side of the n-type single crystal germanium substrate 1 end. Therefore, as the impurity concentration at the end portion of the n-type single crystal germanium substrate 1 approaches the end portion from the impurity concentration of the region of the adjacent n-type single crystal germanium substrate 1, the nonlinear reduction can be suppressed, and the uniformity across the light receiving surface can be formed. The diffusion layer realizes a solar cell excellent in photoelectric conversion efficiency with long carrier life. Further, SIMS analysis (Secondary Ion Mass spectrometry) can be used for the measurement of the impurity concentration.

Further, since the thin film of the tantalum oxide film does not occur, and the portion where the oxygen is formed at the interface between the BSG film 2 and the tantalum oxide film 3 in the defective portion of the tantalum oxide film 3, the tantalum oxide film 11 is formed, so that the p-type is formed. The diffusion layer 7 is adjacent to the n-type diffusion layer 14 to prevent the generated current leakage path Formation. The solar cell manufactured by the method for producing a solar cell of the first embodiment can realize a solar cell having excellent diode characteristics and high photoelectric conversion efficiency, since a leak path is not formed.

As described above, in the first embodiment, the first step of heating the first period is performed at the first temperature while supplying oxygen, the supply of oxygen is stopped, the inert gas is supplied, and the second period is heated at the second temperature. And a third step of heating the third period by the third temperature, and forming a heat treatment for diffusing the second conductivity type diffusion source by the solid phase diffusion source to form the diffusion layer of the second conductivity type. step. Therefore, an oxide film can be formed at the interface between the product adhered to the back surface of the substrate and the substrate by the oxygen introduced at the initial stage of the heat treatment, and the diffusion of impurities due to the product can be prevented. Further, since the step of removing the deposit to the back side is not required, the solid phase diffusion source is not thinned. Therefore, a uniform diffusion layer can be formed throughout the entire surface of the solar cell substrate. Further, since the oxide film formed during the heat treatment is also formed in the film formation failure portion of the solid phase diffusion source, it is possible to prevent the intrusion of the contaminant into the film formation defect portion.

Further, the impurity concentration in the diffusion layer of 5 mm at the end of the solar cell substrate from which the impurity diffusion has been carried out from the solid phase diffusion source and the impurity concentration in the diffusion layer of the adjacent solar cell substrate change only linearly. Therefore, a uniform diffusion layer is formed over the entire surface of the solar cell substrate, thereby realizing a solar cell having a long carrier life.

In addition, the impurities on the back surface of the solar cell substrate after the diffusion of the impurity are diffused from the solid phase diffusion source to an average of 1.6 × 10 ¹⁷ /cm ³ or less in the entire back surface, so that the impurities on the back surface are suppressed from affecting the semiconductor substrate. The amount is below. Therefore, a solar cell having excellent photoelectric conversion efficiency with long carrier life is realized.

Further, the amount of impurities can be obtained by performing the aforementioned SIMS analysis on the back surface of the solar cell substrate and averaging the area of the solar cell substrate.

Further, in the first embodiment, after the heat treatment step consisting of three steps, the chemical liquid treatment is performed to remove the deposit on the back surface. The solid phase diffusion source or the tantalum oxide film subjected to the heat treatment can be changed into a film having high chemical resistance after the film formation is instantaneous, thereby reducing the film formation caused by the chemical solution treatment, and removing the deposit on the back surface. After that, impurity diffusion can be performed, and a uniform diffusion layer can be formed on the back surface.

Embodiment 2.

With respect to the method for producing a solar cell according to the first embodiment, the method for producing a solar cell according to the second embodiment is the same as the method of forming a solid phase diffusion source and the heat treatment step continuously performed. The way to omit the details.

Fig. 6 is a flow chart showing an important part of a method of manufacturing a solar cell according to the second embodiment. In the second embodiment, the substrate cleaning step S103 is performed on the light-receiving surface side of the solid phase diffusion source, and the substrate cleaning step S102S of the cleaning substrate is performed between the heat treatment steps including the step S104, the step S105, and the step S106. The heat treatment step is a first step of the oxidation step, a second step of the diffusion step, and a third step of the third step of the oxidation step. The first step is a heat treatment step in an environment containing oxygen. In S104, the second step is a step S105 of heat treatment in an inert gas atmosphere, and the third step is a step S106 of heat treatment in an atmosphere containing oxygen.

In the method of manufacturing a solar cell according to the second embodiment, the substrate cleaning step S102S is inserted between the film formation step S103 of the solid phase diffusion source and the subsequent heat treatment step in the flowchart shown in the first embodiment. In the second embodiment, after the BSG film 2 as a solid phase diffusion source is formed, a washing step such as ultrasonic washing or damage to the BSG film 2 or the tantalum oxide film 3 is inserted, but the coating step is not canceled. The effect described in the form 1. In the washing step, when the chemical solution of the BSG film 2 or the tantalum oxide film 3 is dissolved by using a hydrofluoric acid aqueous solution or the like, the end portion of the n-type single crystal germanium substrate 1 and the film of the BSG film 2 or the tantalum oxide film 3 in the surface are generated. Or partial peeling. Therefore, it is necessary to obtain a state in which the back surface deposit is not completely removed, and the balance of the thinning of the BSG film 2 or the tantalum oxide film 3 is necessary.

However, the first step that is continuously performed, that is, the thinning region or the peeling portion of the BSG film 2 or the tantalum oxide film 3 by heating in an oxygen atmosphere at the initial stage of heat treatment, and FIG. 5(a) and b) Similarly, it is possible to prevent the intrusion of contaminants from the furnace body of the heat treatment furnace in which the tantalum oxide film formed by heat is formed.

Further, in the second embodiment, most of the deposits on the side of the back surface 1B can be removed by the above-described cleaning treatment. By the removal step, after the n-type single crystal germanium substrate 1 is placed in the heat treatment furnace, the initial oxidation step, that is, the time t _{0 of the} oxygen introduction step, can be shortened, and the processing per unit time can be increased. number.

In other words, according to the method for producing a solar cell of the second embodiment, the impurity distribution in one end portion or the substrate surface of the n-type single crystal germanium substrate is uneven, and the tantalum oxide film 11 formed by heat is formed on the one hand. In this part, the intrusion of pollutants from the furnace body of the heat treatment furnace does not occur, and the solar battery having a long carrier life can be realized, and the number of production per unit time can be increased.

As described above, in the second embodiment, the heat treatment step is performed after the step of forming the solid phase diffusion source to form a film, and then the step of washing the semiconductor substrate. Since the chemical solution before the heat treatment step can remove most of the deposits caused by the product surrounding the back side when the solid phase diffusion source is formed, the inflow time of oxygen in the heat treatment can be shortened by this method. And can increase the number of production per unit time. In addition, even if a position where the solid phase diffusion source has been thinned in the cleaning process is present, the oxide film is formed at the thinned position by the introduction of oxygen during the heat treatment, so that the intrusion of the contaminant can be prevented.

Embodiment 3.

In the method for producing a solar cell according to the first embodiment or the second embodiment, the method for producing a solar cell according to the third embodiment is the same as the method of removing the oxide film on the back side and the phosphorus diffusion step. 1 or the embodiment 2 is omitted.

Fig. 7 is a flow chart showing a method of manufacturing a solar cell according to a third embodiment, showing a separation step from heat treatment to pn bonding. Fig. 8 (a) and (b) are schematic views showing changes in the cross section of the n-type single crystal germanium substrate 1 in the n-type impurity diffusion step. Hereinafter, description will be made using FIGS. 7 and 8. In the method for producing a solar cell according to the third embodiment, after the steps S104, 105, and 106 of the heat treatment step for forming the p-type diffusion layer 7 are performed, as shown in Fig. 8(a), the solid phase diffusion source is continuously applied. The film formation step S108a on the back side and the back surface diffusion step S108b in the heat treatment step. Here, a diffusion source 17 containing a high concentration of an impurity of a conductivity type (for example, 1 × 10 ²⁰ /cm ³ or more) which exhibits an n-type conductivity is formed on the tantalum oxide film 6 of the back surface 1B. Thereafter, after the diffusion source 17 is formed, the n-type single crystal germanium substrate 1 is subjected to heat treatment through the back surface diffusion step S108b. The impurity diffusion from the diffusion source 17 is carried out at a temperature of, for example, 800 ° C to 1000 ° C. The tantalum oxide film 6 formed on the back surface 1B exists directly under the diffusion source. However, since the impurity concentration of the diffusion source 17 is high, the impurity is diffused in the n-type single crystal germanium substrate 1 in contact with the diffusion source 17. And an n-type diffusion layer 18 is formed. Then, through the pn junction separation step S109, the anti-reflection film forming step S110 and the electrode forming step S111 shown in Fig. 1 are carried out. Further, the removal of the solid phase diffusion source is desirably performed between steps S108b and S109 or after S109. The removal of the solid phase diffusion source uses, for example, an aqueous solution of hydrofluoric acid.

On the other hand, the n-type single crystal germanium substrate 1 in the region other than the diffusion source 17 adheres to impurities which have been separated from the diffusion source 17 into the environment, but is lower than the impurity concentration of the diffusion source 17 itself. The concentration does not pass through the oxide film formed on the surface of the n-type single crystal germanium substrate 1.

Fig. 9 (a) and (b) show a modification of the shape in which the diffusion source 17 is formed. The diffusion source 19 shown in Fig. 9(a) is implemented by, for example, After the film formation, a photolithography step using photoresist or a printing step or the like is used to pattern in an arbitrary region. The n-type diffusion layer 20 is formed directly under the diffusion source 19 which has been formed at an arbitrary position. Further, the n-type diffusion layer 20 has the same pattern shape as the comb electrode, a shape in which a straight pattern is arranged, a dot shape in which dots are scattered, and the like, and the position of the back electrode formed in the subsequent step is the same. Made from shape.

The step of forming a solid phase diffusion source selectively forms a diffusion source 19 on the back surface 1B of the second main surface. The step of forming the first conductivity type n-type diffusion layer 20 is performed by diffusion of the PSG film from the solid phase diffusion source, that is, the diffusion source 19.

Therefore, according to the third embodiment and its modifications, the oxide film removing step of the back surface 1B can be removed from the manufacturing method without affecting the tantalum oxide films 6, 8, and 11 formed on the n-type single crystal germanium substrate 1. Next, the diffusion step to the n-type impurity can be completed.

Further, the step of forming the n-type diffusion layer of the first conductivity type diffusion layer is such that the surface of the second main surface of the n-type single crystal germanium substrate 1 has an impurity of 1 × 10 ²⁰ /cm ³ or more. The source of diffusion. According to this method, even if the tantalum oxide film is present in the portion contacting the diffusion source, the impurity diffusion layer can be formed, and the step of removing the tantalum oxide film 6 on the back surface 1B of the n-type single crystal germanium substrate 1 can be omitted. Further, since the diffusion from the diffusion source is directly performed in the entire surface of the surface of the n-type single crystal germanium substrate 1 of the solar cell substrate, even if impurities released from the diffusion source to the environment adhere to The n-type single crystal germanium substrate 1 does not diffuse into the inside of the substrate.

As described above, according to the method for manufacturing a solar cell of the third embodiment, since the oxide film removal step on the back surface is not required, the formation of a leakage path adjacent to the p-type and n-type impurities is prevented, and solar energy having excellent diode characteristics is realized. battery.

Embodiment 4.

In the method for producing a solar cell according to the first to third embodiments, the method for producing a solar cell according to the fourth embodiment is the same as the method of removing the back side oxide film and the phosphorus diffusion step. The details of the embodiment are omitted.

Fig. 10 (a) and (b) are views showing changes in the cross section of the solar cell in the step of forming the n-type diffusion layer on the back surface in the method for producing a solar cell according to the fourth embodiment. In the method for producing a solar cell according to the fourth embodiment, the step S106 of the first embodiment is the same as the three-stage heat treatment step, and the back surface oxide film at any position is formed in the step of forming the n-type diffusion on the back surface. Selective removal. As a method of selective removal, for example, a method of etching removal using an etching paste, a method of removing using a laser, or the like can be employed. On the back surface after the removal, an opening 21 is formed in the tantalum oxide film 6, and a position where the n-type single crystal germanium substrate 1 is exposed and a position where the tantalum oxide film 6 is covered are present. Since the exposed position easily enters impurities, the n-type diffusion layer 22 can be selectively formed by using, for example, a simple POCl ₃ gas.

That is, according to the method of manufacturing a solar cell of the fourth embodiment, the oxide film at the position of the n-type diffusion layer on the back surface is selected. Sexually removed, an n-type diffusion layer can be formed at any position even without using a local diffusion step. Further, since the removal of the oxide film is performed locally, the tantalum oxide film in other regions is not thinned, impurities in other regions are prevented from diffusing, and leakage channels are prevented from being formed, thereby realizing a solar cell having excellent diode characteristics.

As described above, in the fourth embodiment, in the step of forming the conductive diffusion layer different from the diffusion layer formed after the heat treatment step, a part of the tantalum oxide film as the protective film is removed. Therefore, it is possible to use the diffusion of impurities by a simple gas, and the film system remains, except for the oxide film removing portion, so that impurities can be prevented from entering and a leak path can be formed.

Embodiment 5.

The method for producing a solar cell according to the fifth embodiment is characterized in that the inflow ratio of oxygen in the heat treatment step is changed to sequentially pass through a different heat treatment environment. In the method for producing a solar cell according to the first embodiment and the second embodiment, the method for producing a solar cell according to the fifth embodiment is a step S104S in which heat treatment is performed in an environment containing O ₂ , which is different from the first embodiment, and oxygen is used. The rest of the steps are the same except for the portion where the inflow ratio is changed. Therefore, the detailed description will be omitted with reference to the first embodiment to the second embodiment. Fig. 12 is a flow chart showing the steps of manufacturing the solar cell of the fifth embodiment. Fig. 13 is an explanatory view showing a time chart of temperature and environmental conditions in the furnace in the method for producing a solar cell according to the fifth embodiment.

As shown in the explanatory diagram of the furnace environment in Fig. 4, in the method of manufacturing a solar cell according to the fifth embodiment, the flow rate of oxygen flowing in the timing of t ₀₂ to t ₁₁ is changed. Through step S104S, as shown in Fig. 13, the flow ratio of oxygen in the total gas is changed between t ₀₂ and t ₁₁ . In Fig. 13, the vertical axis represents the oxygen gas flow ratio, and the horizontal axis represents the elapsed time. In the method for producing a solar cell according to the first embodiment, when the tantalum oxide film 6 shown in Fig. 2(b) is formed, the flow rate of oxygen is changed, and the timing of the oxidation and the timing of removing the supplied oxygen are combined. The oxidation step of a plurality of times forms a film thickness of the tantalum oxide film 6 of the desired thickness.

The timing of the oxidation of the back surface 1B is performed from t ₀₂ to t ₀₂₁ in FIG. 13 , from t ₀₂₄ to t ₀₂₅ , and from t ₀₂₈ to t ₁₁ . In the period from t ₀₂₂ to t ₀₂₃ in Fig. 1 and from t ₀₂₆ to t ₀₂₇ , oxygen is reduced, and the timing of oxygen intrusion into the BSG film 2 and the tantalum oxide film 3 is continuously removed. By periodically performing a period in which the oxygen concentration is set to a high concentration and a period in which the concentration is set to a low concentration or no oxygen, as described above, oxygen entering the BSG film 2 and the tantalum oxide film 3 at the time of oxidation can be removed, and Oxygen is prevented from reaching the light receiving surface 1A of the n-type single crystal germanium substrate 1. On the other hand, since the back surface 1B is open or there is only a rough back surface attachment, it is rapidly contacted with oxygen when oxygen is supplied, and is rapidly oxidized.

That is, in the oxidation timing described above, since the oxidation of the back surface 1B is successively performed, the tantalum oxide film 6 of the back surface 1B can be selectively formed thickly by switching the oxygen concentration plural times. As shown in step S104S of Fig. 12, the oxidation step of increasing the oxygen concentration and promoting the oxidation time tA of the back surface oxidation, and the reduction of the oxygen concentration are carried out alternately and immersed in the BSG film 2 and the tantalum oxide film 3 on the light-receiving side. Oxygen removal and return to the furnace The soaking back step of returning time tB. In the fifth embodiment, the oxidation first step, the permeation return first step, the oxidation second step, and the permeation return second step are sequentially performed. The oxidation time tA and the permeation return time tB differ depending on the oxidizing environment, but are as large as above, and the oxidation time tA per one time is set to be less than 1 minute, and the permeation return time tB per time is set to about 1 to 2 minutes. Expected. Also, the saturation return time tB may be shorter as the temperature is lower.

According to the method of manufacturing a solar cell of the fifth embodiment, the oxide film of the back surface 1B can be increased without forming an oxide film on the light-receiving surface 1A, and the diffusion suppressing effect of the tantalum oxide film 6 can be increased, and the photoelectricity with long carrier life can be realized. A solar cell with high conversion efficiency.

Embodiment 6.

The method for producing a solar cell according to the sixth embodiment is characterized by a method of forming an inflow of oxygen and an impurity diffusion layer on the back surface 1B. In the method for producing a solar cell according to the first embodiment and the second embodiment, the method for producing a solar cell according to the sixth embodiment includes the step S104SS of performing heat treatment in an environment containing POCl ₃ and O ₂ , and the impurity of the back surface 1B. The method of forming the diffusion layer is the same, and the rest are the same. Therefore, the details of the first to second embodiments are omitted. Fig. 14 is a flow chart showing the steps of manufacturing the solar cell of the sixth embodiment. Fig. 15 is a cross-sectional view showing an essential part of a solar cell in the manufacturing process of the solar cell of the sixth embodiment.

In the method of manufacturing the solar cell of the sixth embodiment, in the timing of t ₀₂ to t ₁₁ shown in the diagram of the furnace environment shown in Fig. 4, not only oxygen but also the first conductivity type impurity layer is supplied. A source of diffusion, such as POCl ₃ gas. Specifically, the supply of POCl ₃ is started from t ₀₂ until the supply of POCl ₃ is stopped before t ₁₁ . During this period, as shown in Fig. 15, the back surface 1B is in direct contact with POCl ₃ , and the n-type diffusion layer 23 is formed on the back surface 1B. On the other hand, since the light-receiving surface 1A is covered with the BSG film 2 and the tantalum oxide film 3, the n-type diffusion layer is not formed. Since gas is stopped supplying POCl ₃ up to ₁₁ t, the oxygen flows, silicon oxide film 24 is formed on the back surface 1B. The tantalum oxide film 24 suppresses the diffusion of impurities of the first conductivity type derived from POCl ₃ adhering to the back surface 1B. Therefore, after the formation of the tantalum oxide film 24, the amount of impurities in the n-type diffusion layer can be kept constant.

In the method for producing a solar cell according to the first embodiment, the heat treatment step 104S for performing heat treatment in an environment containing POC ₁₃ and O ₂ is used instead of the first embodiment. The heat treatment step 104 of heat treatment is performed in an atmosphere containing O ₂ , and the step S108 of performing back diffusion is omitted. Further, in the sixth embodiment, in the same manner as in the fifth embodiment, the step S104SS can perform oxygen introduction in plural times.

Therefore, even after the high temperature treatment for forming the p-type diffusion layer, the concentration of the n-type diffusion layer can be suppressed from increasing. In other words, according to the method for manufacturing a solar cell of the sixth embodiment, the n-type diffusion layer 24 can be formed in the same heat treatment as the formation of the p-type diffusion layer 7, and the number of steps can be reduced. Further, since the tantalum oxide film 24 formed on the back surface 1B can suppress the concentration of the n-type diffusion layer, a low-concentration n-type diffusion layer can be formed, and a solar cell having excellent photo-electric conversion efficiency with long carrier lifetime can be realized.

Embodiment 7.

In the seventh embodiment, in addition to the method for producing a solar cell according to the sixth embodiment, a high-concentration n-type diffusion layer 20 from the solid phase diffusion source 19 is formed from the back surface. In other words, in the method for producing a solar cell according to the sixth embodiment, the method for producing a solar cell according to the seventh embodiment is the step S104SS of performing heat treatment in an environment containing POCl ₃ and O ₂ , but further connected to the back surface. The 1B side is the same as the part in which the high-concentration n-type diffusion layer 20 is formed from the solid phase diffusion source 19, and the rest is the same. Therefore, the details of the method of the sixth embodiment and the first to second embodiments are omitted. Fig. 16 is a flow chart showing an important part of the manufacturing steps of the solar cell of the seventh embodiment. Fig. 17 (a) and (b) are cross-sectional views showing important parts of a solar cell in the manufacturing process of the solar cell of the seventh embodiment.

In the method for producing a solar cell according to the seventh embodiment, as shown in Fig. 16, a heat treatment step 104SS for performing heat treatment in an environment containing POCl ₃ and O ₂ is carried out. In the timing of t ₀₂ to t ₁₁ shown in the diagram of the furnace in Fig. 4, not only oxygen but also a diffusion source for forming the first conductivity type impurity layer, for example, POCl ₃ gas, is supplied. Specifically, the supply of POC _l3 is started from t ₀₂ , and the supply of POCl ₃ is stopped before t ₁₁ . During this period, as shown in Fig. 15 of the sixth embodiment, the back surface 1B is in direct contact with POCl ₃ , and the n-type diffusion layer 23 is formed on the back surface 1B. On the other hand, since the light-receiving surface 1A is covered by the BSG film 2 and the tantalum oxide film 3, an n-type diffusion layer is not formed. POCl ₃ gas is supplied from the stop to t ₁₁ until the inflow of oxygen, the silicon oxide film 24 is formed on the back surface 1B. Since the silicon oxide film 24 is suppressed from adhering to represent the conductivity type impurity diffusion back surface 1B of the first POCl _3, so that the silicon oxide film 24 is formed, impurities may remain in the n-type diffusion layer to a constant.

Then, similarly to the steps shown in Fig. 9 (a) and (b) of the third embodiment, as shown in Fig. 17 (a) of the solid phase diffusion source forming step 108a, it is high in any region. The diffusion source 19 of the concentration of n-type impurities is patterned by a printing step. Thereafter, heat treatment is performed by the back diffusion step 108b, whereby the n-type diffusion layer 20 is formed directly under the diffusion source 19 formed at an arbitrary position as shown in Fig. 17(b). Further, before the solid phase diffusion source forming step 108a, it is desirable to perform a back surface oxide film removing step, and if necessary, to remove the tantalum oxide film 6 on the back side. At this time, the deposit 4 containing boron and the deposit 5 of cerium oxide are also removed. As in the third embodiment, the removal of the solid phase diffusion source is performed between steps S108b and S109 or after S109. In the seventh embodiment, for example, a hydrofluoric acid aqueous solution is used for the removal of the solid phase diffusion source.

Therefore, in the seventh embodiment, the solar cell having the n-type diffusion layer 23 having a low concentration and the n-type diffusion layer 20 having a high concentration can be formed on the back surface 1B side without increasing the number of steps, and the carrier life can be realized without increasing the number of steps. A solar cell that is further elongated and has excellent photoelectric conversion efficiency.

As described above, in the above-described first to seventh embodiments, when a film containing impurities as a solid phase diffusion source is formed on one surface of the light-receiving surface side, and heat treatment is continuously performed, the product from the back surface is prevented. The manufacturing step of impurity diffusion. Specifically, in the case of heat treatment, In the heat treatment of the general example, when the treatment is to be carried out using an inert gas such as nitrogen or argon, the heat treatment is performed in an environment in which oxygen flows in before and after, and a three-stage heat treatment is performed. The supply of oxygen is carried out before and after the heat treatment in an environment containing no oxygen which diffuses impurities from the film of the solid phase diffusion source. In other words, when an oxide film as a diffusion barrier is formed on the substrate on the back surface of the substrate and the substrate interface due to the oxygen that is contacted after the furnace is charged, and the supply of oxygen is stopped, the impurity is diffused from the film formation, and the impurities are only Spread to the film forming surface. Then, by heat-treating the last inflow of oxygen, an oxide film is formed on the film formation surface of the solid phase diffusion source, and a function as a barrier to other types of diffusion which is successively performed is added. By this method, impurities can be diffused only to the film formation surface.

Further, the step for performing oxidation is desirably heated at a temperature of 700 ° C to 850 ° C for 1 to 20 minutes. The oxidation step is desirably a temperature at which the impurity is not diffused, and an oxide film excellent in film quality can be formed. Therefore, depending on the type of impurities and the composition of the substrate, the temperature conditions are also different. Further, when the impurity is phosphorus, it is set to 700 ° C to 760 ° C, and when the impurity is boron, it is set to about 740 ° C to 800 ° C. When the upper limit is higher, the diffusion of impurities starts, and when the lower limit is lower, the oxidation rate becomes slower and the film quality also decreases.

Further, in the above-described first to seventh embodiments, a semiconductor substrate using germanium as a material is described, but a compound semiconductor substrate such as GaAs or GaN may be applied.

Further, in the first to seventh embodiments, the temperature in the second step for diffusing impurities is determined depending on the type of impurities to be diffused, and can be appropriately changed. And about the species of the diffusion environment Since the diffusion rate is controlled, it can be set as a reducing environment such as a hydrogen environment, and can be appropriately adjusted.

The configuration shown in the above embodiment is an example of the present invention, and may be combined with other known techniques, and a part of the configuration may be omitted or changed without departing from the gist of the present invention.

Claims (15)

A method for producing a solar cell, comprising: forming a solid phase diffusion source on the first main surface of a first or second conductivity type semiconductor substrate having first and second main surfaces; and heat treatment a heat treatment step of diffusing impurities of the second conductivity type by the solid phase diffusion source to form a diffusion layer of the second conductivity type, wherein the heat treatment step is performed in the same furnace, and includes: a first step While supplying oxygen, the first oxide film is heated at the first temperature to form a first oxide film, the second step is to stop the supply of oxygen, the second temperature is heated at the second temperature, and the impurity is diffused by the solid phase diffusion source; and the third step The oxygen is supplied again, and the third period is heated at the third temperature, and a second oxide film is formed on the interface between the solid phase diffusion source and the semiconductor substrate. The method for producing a solar cell according to claim 1, wherein the heat treatment step is carried out after the step of forming the solid phase diffusion source into a film. The method for producing a solar cell according to the first aspect of the invention, wherein the heat treatment step is performed after the step of forming the solid phase diffusion source to form a film, and then washing the semiconductor substrate. The method for producing a solar cell according to any one of claims 1 to 3, comprising: performing the medicine after the heat treatment step The step of liquid treatment to remove the attachment on the back side. The method for producing a solar cell according to any one of claims 1 to 3, further comprising, after the heat treatment step, forming the second main surface of the semiconductor substrate on which an oxide film is formed The step of the first conductivity type diffusion layer. The method for producing a solar cell according to claim 5, wherein the step of forming the diffusion layer of the first conductivity type includes forming a solid phase containing impurities of the first conductivity type on the second main surface. a step of diffusing the source; and a heat treatment step of diffusing the impurity of the first conductivity type by the solid phase diffusion source. The method for producing a solar cell according to claim 6, wherein the step of forming the solid phase diffusion source includes the step of selectively forming the solid phase diffusion source on the second main surface. The method for producing a solar cell according to claim 1, comprising the step of selectively removing the first oxide film formed on the second main surface and forming an opening after the third step; The method includes the step of diffusing the first conductivity type impurity from the opening to the second main surface to form a first conductivity type diffusion layer. The application of the method of manufacturing a solar cell patentable scope of item 8, wherein the first conductivity type formed in step diffusion layers lines comprising: forming on the second main surface of the semiconductor substrate containing 1 × 10 ²⁰ pieces / The step of diffusing the impurity of cm ³ or more. The manufacturer of the solar cell as described in claim 9 In the method of the second conductivity type diffusion layer, the impurity concentration in the diffusion layer of 5 mm at the peripheral end portion of the semiconductor substrate is uniform. The method for producing a solar cell according to claim 1, wherein after the heat treatment step, the amount of the second conductivity type impurity of the second main surface of the semiconductor substrate is 1.6×10 ^{17 on} average. /cm ³ or less. The method for producing a solar cell according to claim 1, wherein the first step includes a step of different oxygen concentrations. The method of manufacturing a solar cell according to claim 8 or claim 12, wherein the first step includes: a step of supplying a diffusion source forming a semiconductor layer of the first conductivity type; and stopping the diffusion source Supply, a step of forming an oxide film by flowing oxygen. The method for producing a solar cell according to claim 1, wherein the first step is a step of heating at a temperature of 700 ° C to 850 ° C for 1 to 20 minutes. A solar cell comprising a first conductivity type formed on a first main surface of a first or second conductivity type semiconductor substrate having first and second main surfaces, and diffusion of a second conductivity type impurity diffused with a second conductivity type impurity In the layer, the amount of the impurities of the second conductivity type on the second main surface is 1.6 × 10 ¹⁷ /cm ³ or less on average.