JP2013131723A - Semiconductor substrate reforming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor substrate reforming method for obtaining a semiconductor substrate which is highly clean both inside and out by removing a metal impurity inside and out of the semiconductor substrate.SOLUTION: A semiconductor substrate reforming method comprises: a first step of performing an oxide film formation step of forming an oxide film by oxidizing a surface of a semiconductor substrate and an oxide film removal step of removing the oxide film in this order to adjust a total abundance of a metal impurity on the surface of the semiconductor substrate at not less than 1×10atoms/cmand not more than 1×10atoms/cm; a second step of forming a plurality of pits on the surface of the semiconductor substrate by wet etching the surface of the semiconductor substrate catalyzed by the metal impurity of the adjusted total abundance on the surface of the semiconductor substrate; a third step of diffusing impurities inside the semiconductor substrate and collecting the impurity adjacent to the surface of the semiconductor substrate and adjacent to the pits; and a fourth step of removing the impurities collected to the surface of the semiconductor substrate.

Description

  The present invention relates to a method for modifying a semiconductor substrate, and more particularly to a method for modifying a semiconductor substrate for removing metal impurities on the surface and inside.

  In the solar cell manufacturing process, if metal impurities adhere to the surface of the solar cell substrate (hereinafter referred to as solar cell wafer) before the diffusion (gettering) treatment step, the metal impurities on the surface of the solar cell wafer diffuse into the solar cell wafer. Thus, it becomes difficult to remove the internal metal impurities. As a result, in the solar cell using this solar cell wafer, the bulk lifetime is lowered and the photoelectric conversion efficiency is lowered. Therefore, it is necessary that the metal impurities on the surface of the solar cell wafer are not diffused into the solar cell wafer, or the impurities inside the solar cell wafer are easily removed by diffusion (gettering) treatment.

Solar cell wafer is about 10 ⁶ times metals and organic equivalents impurity semiconductor wafer compared when the wafer surface 10 ^twice for making a device such as many metal impurities content present within the wafer even 10 to 10 ⁵ About twice as many. Microcracks or wafer damage may upon further sliced wafer is from the wafer surface to 5~50μm about depth, defects of the wafer surface be 10 ⁵ to 10 ²⁰ times higher. For this reason, a high-purity solar cell wafer cannot be produced simply by cleaning the wafer surface, such as a semiconductor wafer used in a device or the like.

  In the solar cell manufacturing process, the solar cell wafer is subjected to an etching process in order to remove the damaged layer. At this time, the damaged layer and the metal impurities existing inside the wafer are dissolved in the etching solution and reattached to the wafer surface, so that the surface of the solar cell wafer is contaminated. Therefore, by removing and facilitating the removal of metal impurities on the surface of the solar cell wafer and the interior thereof, the heat treatment for diffusing elements such as phosphorus (P) and boron (B) to the surface of the solar cell wafer is performed next. The problem is to improve the gettering effect (the effect of modifying the solar cell wafer by collecting impurities inside the solar cell wafer on the surface).

  The gettering mechanism by heat treatment such as diffusion treatment is performed by diffusing elements such as phosphorus (P) and boron (B) into the solar cell wafer at a high temperature of about 800 ° C. to a depth of 0.2 to 2 μm near the wafer surface. Defects are formed in the region (inside), and metal impurities inside the wafer collect in these defects. Thereby, the solar cell wafer (silicon etc.) inside the vicinity of the wafer surface is highly purified.

Conventionally, as a semiconductor substrate cleaning method, there is a method of removing metal contamination and organic contamination on a semiconductor substrate by performing ozone water cleaning processing and DHF (dilute hydrofluoric acid) processing (see, for example, Patent Document 1). . Further, as a method for cleaning silicon semiconductors and silicon oxides, there is a method for performing high cleaning of a wafer by performing DHF (dilute hydrofluoric acid) treatment and FPM (HF + H ₂ O ₂ ) treatment (see, for example, Patent Document 2). ).

JP-A-10-256211 Japanese Patent Laid-Open No. 7-115077

However, according to the technique of the above-mentioned patent document 1, when only the ozone purification process and the DHF (dilute hydrofluoric acid) process are performed on the surface of the solar cell wafer, and the FPM (HF + H ₂ O ₂ ) process is not performed, the wafer surface Fine pits (porous layer and pores) are not formed. For this reason, even if heat treatment such as diffusion treatment is subsequently performed, the effect of gettering is low, and the bulk lifetime of the wafer is not improved. Furthermore, even if the FPM process is performed after the ozone water cleaning process and the DHF (dilute hydrofluoric acid) cleaning process, if there are no metal impurities on the wafer surface or the amount is very small, fine pits are formed on the wafer surface. It is difficult to form (porous layer and pores). For this reason, even if the solar cell wafer is subsequently subjected to a heat treatment such as a diffusion treatment, the metal impurities inside the wafer hardly collect on the wafer surface, and the inside of the wafer cannot be highly purified (modified) efficiently. For this reason, the bulk lifetime of the wafer is not improved.

  Here, the pits are a porous layer and its pores formed on the wafer surface.

Further, according to the technique of Patent Document 2, the solar cell wafer surface is not cleaned only by performing DHF (dilute hydrofluoric acid) cleaning processing and FPM (HF + H ₂ O ₂ ) processing on the solar cell wafer surface. Therefore, the amount of impurities attached to the wafer surface is large. In this case, since impurities such as metals are likely to enter the wafer by heat treatment such as diffusion treatment, the inside of the solar cell wafer cannot be highly purified (modified) efficiently. For this reason, the bulk lifetime of the wafer is not improved.

  Moreover, even if the semiconductor substrate is cleaned by combining the cleaning processes described in Patent Document 1 and Patent Document 2, many metal impurities remain inside the solar cell wafer, and the inside of the solar cell wafer is efficiently purified. (Reformation) is not possible. For this reason, the bulk lifetime of the wafer is not improved.

  That is, in the cleaning of the solar cell wafer before the heat treatment using the conventional method, even if the surface of the wafer can be cleaned, the effect of gettering by the subsequent heat treatment cannot be improved. As a result, metal impurities contained in the wafer cannot be removed efficiently, and the bulk lifetime cannot be improved efficiently.

  The present invention has been made in view of the above, and it is possible to obtain a method for modifying a semiconductor substrate by which metal impurities on the surface and inside of the semiconductor substrate are removed to obtain a highly clean semiconductor substrate on both the surface and inside. With the goal.

In order to solve the above-described problems and achieve the object, a method for modifying a semiconductor substrate according to the present invention includes an oxide film forming step of forming an oxide film by oxidizing a surface of a semiconductor substrate, and removing the oxide film. The oxide film removal step is performed in this order, and the total amount of metal impurities on the surface of the semiconductor substrate is adjusted to 1 × 10 ⁹ atoms / cm ² or more and 1 × 10 ¹⁴ atoms / cm ² or less. A plurality of pits (porous layers and holes) on the surface of the semiconductor substrate by performing wet etching on the surface of the semiconductor substrate using a metal impurity having a regulated amount of the surface of the semiconductor substrate as a catalyst; A second step of forming, a third step of diffusing impurities inside the semiconductor substrate and collecting the impurities in the vicinity of the surface of the semiconductor substrate and in the vicinity of the pits, and the semiconductor substrate A fourth step of removing the impurities collected on the surface, characterized in that it comprises a.

  According to the present invention, metal impurities on the surface and inside of a semiconductor substrate can be effectively removed. As a result, there is an effect that a highly clean semiconductor substrate can be obtained both on the surface and inside.

FIG. 1 is a cross-sectional view of a solar cell wafer schematically showing a part indicated by terms used in the description of an embodiment of the present invention. FIG. 2 is a flowchart showing a flow of a method for modifying a solar cell wafer according to an embodiment of the present invention. FIG. 3A is a cross-sectional view schematically showing the state of the surface state of the solar cell wafer in each step of the flowchart shown in FIG. 3-2 is a cross-sectional view schematically showing the state of the surface state of the solar cell wafer in each step of the flowchart shown in FIG. FIG. 3C is a cross-sectional view schematically showing the surface state of the solar cell wafer in each step of the flowchart shown in FIG. 3-4 is a cross-sectional view schematically showing the state of the surface state of the solar cell wafer in each step of the flowchart shown in FIG. 3-5 is sectional drawing which shows typically the mode of the surface state of the solar cell wafer in each process of the flowchart shown in FIG. 3-6 is a cross-sectional view schematically showing the state of the surface state of the solar cell wafer in each step of the flowchart shown in FIG. 3-7 is a cross-sectional view schematically showing the state of the surface state of the solar cell wafer in each step of the flowchart shown in FIG. FIG. 4-1 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 1. FIG. 4-2 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 1. FIG. 4-3 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 1. 4-4 is sectional drawing which shows typically the modification | reformation method of the solar cell wafer based on the washing | cleaning process described in patent document 1. FIG. 4-5 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 1. FIG. 4-6 is sectional drawing which shows typically the modification | reformation method of the solar cell wafer based on the washing | cleaning process described in patent document 1. FIG. FIG. 5-1 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 2. FIG. 5-2 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 2. FIG. 5-3 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 2. FIG. 5-4 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 2. 5-5 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 2. FIG. FIG. 5-6 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 2. FIG. 6A is a cross-sectional view schematically illustrating a solar cell wafer modification method based on the cleaning process described in Patent Document 1 and Patent Document 2. FIG. FIG. 6B is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 1 and Patent Document 2. 6-3 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 1 and Patent Document 2. FIG. FIG. 6-4 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 1 and Patent Document 2. 6-5 is a cross-sectional view schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Literature 1 and Patent Literature 2. FIG. 6-6 is sectional drawing which shows typically the modification | reformation method of the solar cell wafer based on the washing | cleaning process described in patent document 1 and patent document 2. FIG. FIGS. 6-7 is sectional drawing which shows typically the modification | reformation method of the solar cell wafer based on the washing | cleaning process described in patent document 1 and patent document 2. FIG. FIG. 7-1 is a cross-sectional view schematically showing the method for modifying the semiconductor substrate of Comparative Example 1. FIG. 7-2 is a cross-sectional view schematically showing the method for modifying the semiconductor substrate of Comparative Example 1. FIG. 7C is a cross-sectional view schematically showing the method for modifying the semiconductor substrate of Comparative Example 1. FIG. 7-4 is a cross-sectional view schematically showing the method for modifying the semiconductor substrate of Comparative Example 1. 7-5 is a cross-sectional view schematically showing the method for modifying the semiconductor substrate of Comparative Example 1. FIG. 7-6 is a cross-sectional view schematically showing the semiconductor substrate reforming method of Comparative Example 1. FIG. 7-7 is a cross-sectional view schematically showing the semiconductor substrate reforming method of Comparative Example 1. FIG. FIG. 8 is a characteristic diagram showing the relationship between the amount of metal impurities on the silicon wafer surface and the bulk lifetime. FIG. 9 is a characteristic diagram showing the relationship between the amount of metal impurities on the surface of the silicon wafer and the photoelectric conversion efficiency of the solar battery cell.

  Embodiments of a method for modifying a semiconductor substrate according to the present invention will be described below in detail with reference to the drawings. In addition, this invention is not limited to the following description, In the range which does not deviate from the summary of this invention, it can change suitably. In the drawings shown below, the scale of each member may be different from the actual scale for easy understanding.

Embodiment A gettering process in which impurities inside a solar cell wafer (hereinafter sometimes referred to as a wafer) made of a semiconductor and used as a solar cell substrate is collected on the wafer surface is performed by applying heat to the wafer to phosphorus (P) or boron ( B) Elements such as B are diffused, and metal impurities inside the wafer are collected in defects of several ohms strong (Å) level formed inside the wafer by the treatment. Due to the presence of defects on the wafer surface, metal impurities tend to collect on the wafer surface. Therefore, the present inventor considered that metal impurities can be efficiently collected on the wafer surface by forming fine pits (porous layer and holes) on the wafer surface.

As a method of forming pits on the wafer surface, there are a physical method and a chemical method. As a physical method, there is a blast treatment in which a metal oxide such as aluminum oxide (Al ₂ O ₃ ) is sprayed. However, since a large amount of metal impurities adheres to the wafer surface in this method, the metal impurities attached to the wafer surface enter the inside of the wafer when a heat treatment such as diffusion treatment is performed thereafter. Therefore, when blasting is used as a method for forming pits on the wafer surface, the wafer cannot be effectively purified (modified).

When fine pits are formed on the wafer surface by a chemical method, it is necessary that metal impurities adhere to the wafer surface. That is, a metal impurity that is a starting point of pits is required on the wafer surface. The mechanism for forming pits on the surface of the wafer by a chemical method is that the wafer is FPM (HF + H ₂ ) with fine metal impurities acting as a catalyst for the etching reaction of the solar cell wafer (silicon, etc.) attached to the wafer surface. O _2, etc. oxidizing agent) was immersed in the solution, through the metal impurities adhering to the wafer surface to accelerate etching (dissolution) reaction of the solar cell wafer (silicon), is that.

  In this case, the more metal impurities exist on the wafer surface, the more fine pits are formed on the wafer surface. However, since the amount of metal impurities on the surface of the solar cell wafer increases, when heat treatment such as diffusion treatment is performed, the metal impurities on the wafer surface diffuse into the solar cell wafer. Therefore, such a method cannot effectively improve (modify) the solar cell wafer.

  On the other hand, when impurities on the wafer surface are removed by the cleaning process and no impurities exist on the wafer surface, no pits are formed on the wafer surface even when the FPM process is performed. Therefore, the gettering effect is low even when heat treatment such as diffusion treatment is performed.

  Therefore, when forming fine pits on the wafer surface by a chemical method, the present inventor must adjust the amount of metal impurities on the wafer surface before pit formation within a range that does not affect the gettering effect. In view of the above, the present inventors have found a method for modifying a semiconductor substrate capable of enhancing the gettering effect by heat treatment such as diffusion treatment.

  In order to solve the above-described problems, a method for modifying a semiconductor substrate according to the present invention includes an oxide film forming step for oxidizing the surface of a semiconductor substrate to form an oxide film, and an oxide film removing step for removing the oxide film. And performing wet etching on the surface of the semiconductor substrate using the metal impurity on the surface of the semiconductor substrate as a catalyst, and a first step of adjusting the abundance of metal impurities on the surface of the semiconductor substrate. A second step of forming a plurality of pits on the surface of the semiconductor substrate; a third step of diffusing impurities inside the semiconductor substrate and collecting the impurities in the vicinity of the surface of the semiconductor substrate and in the vicinity of the pits; And a fourth step of removing the impurities collected on the surface of the semiconductor substrate.

  In the following, a method for modifying a solar cell wafer will be described as an embodiment of a method for modifying a semiconductor substrate. FIG. 1 is a cross-sectional view of a solar cell wafer schematically showing a part indicated by terms used in the description of an embodiment of the present invention. In FIG. 1, the surface of a solar cell wafer 5 made of a semiconductor such as silicon, which is a solar cell substrate to be modified, is a solar cell wafer surface 1, and impurities 2 such as metal and organic matter adhere to the solar cell wafer surface 1. It shows the state. Further, the damage layer 4 means the unevenness of the solar cell wafer surface 1 formed when the wafer is sliced, and the solar cell wafer interior 3 means an inner part from the solar cell wafer surface 1, and more particularly than the damage layer 4. Means the inner part. Below, the case where the solar cell wafer 5 is a silicon wafer is demonstrated to an example.

  The process of the solar cell wafer modification method according to the present embodiment will be described with reference to FIG. 2 and FIGS. 3-1 to 3-7. FIG. 2 is a flowchart illustrating a flow of the method for modifying a solar cell wafer according to the embodiment. In the solar cell wafer modification method according to the present embodiment, a first etching step (step S101), a first cleaning step (step S102), a second cleaning step (step S103), and a pit formation step (step S104) are sequentially performed. ), Diffusion (gettering) step (step S105), second etching step (step S106), and third cleaning step (gettering step) (step S107), including the first cleaning step (step S102) and the second cleaning step. The process (step S103) is repeated.

  3-1 to 3-7 are cross-sectional views schematically showing the state of the surface state of the solar cell wafer 5 in each step of the flowchart shown in FIG. Specifically, FIG. 3A is a cross-sectional view schematically showing the state of the solar cell wafer 5 before performing the first etching step (step S101) shown in FIG. 3-2 is a cross-sectional view schematically showing the state of the solar cell wafer 5 after performing the first cleaning step (step S102) shown in FIG. FIG. 3C is a cross-sectional view schematically showing the state of the solar cell wafer 5 after performing the first cleaning process to the second cleaning process (steps S102 to S103) shown in FIG.

  3-4 is a cross-sectional view schematically showing the state of the solar cell wafer 5 after the pit formation step (step S104) shown in FIG. 2 is performed. 3-5 is a cross-sectional view schematically showing the state of the solar cell wafer 5 after performing the diffusion step (step S105) shown in FIG. 3-6 is sectional drawing which shows typically the mode of the solar cell wafer 5 after implementing the 2nd etching process (step S106) shown in FIG. 3-7 is a cross-sectional view schematically showing the state of the solar cell wafer 5 after performing the third cleaning step (step S107) shown in FIG. 3-1 to 3-7 show one solar cell wafer 5, but in the solar cell wafer modification method according to the present embodiment, a plurality of solar cell wafers 5 are simultaneously processed. For example, a carrier that can be transported by loading a plurality of solar cell wafers 5 may be used.

In the solar cell wafer modification method according to the present embodiment, first, in the first etching step (step S101), the solar cell wafer surface 1 is etched using an etchant. In the first etching step, an etchant containing an alkali such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), an anisotropic containing these alkalis and isopropyl alcohol (IPA), a surfactant, an organic substance such as a chelate. Etch the solar cell wafer surface 1 using an etching solution, ammonia hydrogen peroxide solution (APM), ammonia (NH ₃ ) water, mixed acid (HNO ₃ + HF), etc. as an etchant to remove the damage layer, Impurities 2 such as metals and organic substances existing in the vicinity of the battery wafer surface 1 are removed.

  As a result, the impurities 2 such as metal and organic matter existing in the vicinity of the solar cell wafer surface 1 shown in FIG. 3A are once removed, but the metal impurities on the surface of the solar cell wafer 5 as shown in FIG. 6 reattaches. That is, from the state where impurities 2 such as metal and organic matter adhere to the solar cell wafer surface 1 as shown in FIG. 3A, the impurities such as metal and organic matter on the solar cell wafer surface 1 as shown in FIG. 2 is removed, and the metal impurities 6 are reattached to the surface of the solar cell wafer 5. Note that metal impurities 11 exist in the solar cell wafer inside 3.

In the first etching step, a depth range of 0.2 μm to 50 μm from the solar cell wafer surface 1 is etched. The etching process having such a depth is performed by etching for about 0.1 to 60 minutes with the above-described etching solution. As an etching solution in the first etching step, an etching solution containing an alkali such as potassium hydroxide (KOH), sodium hydroxide (NaOH), aqueous ammonia (NH ₄ OH), or an acid such as a mixed acid (HNO ₃ + HF) is included. In the case of using an etching solution, the chemical concentration is preferably about 0.01 wt% to 96 wt%, and the concentration of the additive used for anisotropic etching is preferably 0.01 wt% to 50 wt%. The treatment temperature is preferably about 25 ° C to 95 ° C.

  Before the first etching step, a cleaning process for forming an oxide film on the solar cell wafer surface 1 using an oxidant such as ozone water, and then a solar cell wafer surface 1 using a cleaning solution containing hydrofluoric acid. A cleaning process for cleaning to remove the oxide film and impurities contained in the oxide film may be repeated. Thereby, since impurities 2 such as metal and organic matter on the solar cell wafer surface 1 can be removed, anisotropic etching due to the effect of metal and organic matter is suppressed in the first etching step, the solar cell wafer surface 1 becomes flat, and pits It becomes easy to form pits uniformly on the surface of the solar cell wafer in the forming process.

After the first etching step, the solar cell wafer 5 is preferably subjected to a rinsing process such as a water washing process. The rinse treatment is preferably a rinse treatment with ultrapure water, pure water, anode water, cathode water, hydrogen water or the like. The treatment time for the water washing treatment is about 0.1 to 60 minutes. The water washing treatment is preferably performed at a temperature of 90 ° C. or less at which water does not evaporate. Furthermore, the alkali on the surface of the solar cell wafer 5 may be neutralized with hydrofluoric acid (HF), hydrochloric acid (HCl), sulfuric acid (H ₂ SO ₄ ), nitric acid (HNO ₃ ) HNO ₃ or the like after the rinsing treatment.

Next, in the first cleaning step (step S <b> 102), the solar cell wafer 5 is cleaned using a cleaning liquid that oxidizes the surface of the solar cell wafer 5 to form an oxide film, and an oxide film is formed on the surface of the solar cell wafer 5. Form. The cleaning liquid used in the cleaning process in the first cleaning step, cleaning liquid containing ozone _{(O 3)} and hydrogen peroxide _(H 2 _{O 2)} as an oxidizing agent (ozone _{(O 3)} water, hydrogen peroxide _(H 2 _{O 2} ) Water)), ammonia (NH ₃ ) water, ammonia hydrogen peroxide water (APM), hydrochloric acid (HCl), nitric acid (HNO ₃ ), sulfuric acid (H ₂ SO ₄ ), phosphoric acid (H ₃ PO ₄ ), etc. Cleaning liquid containing acid, cleaning liquid containing hydrochloric acid (HCl), nitric acid (HNO ₃ ), sulfuric acid (H ₂ SO ₄ ) and other acids and hydrogen peroxide (H ₂ O ₂ ) (hydrochloric acid hydrogen peroxide (HPM) , Sulfuric acid hydrogen peroxide solution (SPM), etc.) can be used. Among them, ozone (O ₃ ) water, hydrogen peroxide (H ₂ O ₂ ) water, and hydrochloric acid are common materials and are easily available.

  The ozone water cleaning treatment time when ozone water is used as the cleaning liquid in the first cleaning step is about 1 second to 60 minutes, and the temperature is preferably 50 ° C. or lower. If the ozone water cleaning process is performed for 1 second to 60 minutes and the ozone water cleaning process and the second cleaning process (step S103) (hydrofluoric acid cleaning process) are repeatedly performed, the solar cell wafer can be used regardless of the processing time. No. 5 surface cleaning effect is obtained.

  In the first cleaning step, an oxide film having a thickness of 1 nm or less is formed on the surface of the solar cell wafer 5. This oxide film is formed by cleaning the solar cell wafer 5 with a cleaning liquid for forming an oxide film such as ozone water for about 0.1 seconds to 600 seconds. However, although the thickness of the oxide film depends on the cleaning conditions, even if the treatment is continued for 10 seconds to 60 minutes and further processing is continued, no further oxide film growth occurs. For this reason, it is not necessary to perform the cleaning with the cleaning liquid for forming the oxide film for a time longer than the time for the oxide film to grow a certain amount. However, when adjusting the amount of metal impurities on the surface of the solar cell wafer 5, the cleaning time must be 10 seconds or less.

  When ozone water is used as the cleaning liquid in the first cleaning process, if the ozone water cleaning process and the hydrofluoric acid cleaning process are repeatedly performed even if the ozone water cleaning process time is changed, the cleaning effect hardly changes. Even if the concentration is increased, it is considered that the cleaning effect does not change in the same manner as the change in the processing time. However, when the ozone concentration is actually increased, the cleaning effect by the ozone water cleaning process is increased. When the ozone water cleaning treatment is performed using ozone water having an ozone concentration of 10 mg / L or more, the cleaning effect is further enhanced. For this reason, it is preferable that the ozone concentration of the ozone water used for the cleaning liquid is in the range of 10 mg / L to 150 mg / L. However, the cleaning effect of ozone water can be obtained even when the ozone concentration is in the range of 0.1 to 10 mg / L.

  As described above, when the ozone concentration of the ozone water is increased, the cleaning effect is improved. The ozone concentration is increased, so that ozone in pure water or ultrapure water and impurities such as metal and organic matter on the surface of the solar cell wafer 5 are obtained. 2 is easily contacted, and the metal impurities 6 on the surface of the solar cell wafer 5 are easily ionized when the solar cell wafer 5 and the ozone water come into contact with each other. In the ozone water cleaning process, an oxide film is formed on the surface of the solar cell wafer 5, and this oxide film suppresses ionization of metal impurities. Therefore, in order to efficiently remove the metal impurities 6 on the surface of the solar cell wafer 5 before the oxide film is formed on the surface of the solar cell wafer 5 in the ozone water cleaning, it is effective that the ozone concentration of ozone water is high. It is.

When hydrochloric acid (HCl), sulfuric acid (H ₂ SO ₄ ), hydrogen peroxide solution (H ₂ O ₂ ), ammonia (NH ₃ ) water, nitric acid (HNO ₃ ) or the like is used as the cleaning liquid in the first cleaning step The chemical concentration is preferably about 0.0001 wt% to 96 wt%. Even if the chemical concentration is higher than 96 wt%, the thickness of the oxide film to be formed is saturated. Therefore, even if the treatment is performed at a concentration higher than this, an additional effect cannot be expected.

  Prior to the first cleaning step, the solar cell wafer 5 cleaned in the first etching step may be cleaned using a cleaning liquid containing hydrofluoric acid (HF) in pure water, ultrapure water, or the like.

  In addition, after the first cleaning step (step S102), it is preferable to perform a rinsing process such as a water cleaning process on the solar cell wafer 5 if necessary. The rinse treatment is preferably a rinse treatment with ultrapure water, pure water, anode water, cathode water, hydrogen water or the like.

Next, in the second cleaning step (step S103), the surface of the solar cell wafer 5 is cleaned for about 1 second to 60 minutes using a cleaning liquid containing hydrofluoric acid (HF), and the solar cell formed in the first cleaning step An oxide film (for example, silicon oxide (SiO ₂ )) on the surface of the wafer 5 and metal impurities contained in the oxide film are removed. The cleaning liquid used in the second cleaning step is preferably an aqueous solution having a hydrofluoric acid (HF) concentration of 0.01 wt% to 50 wt%.

  In this embodiment, the first cleaning process (step S102) and the second cleaning process (step S103) are used as a cleaning process unit, and this cleaning process unit is repeated a plurality of times. By repeating the first cleaning step (step S102) and the second cleaning step (step S103) a plurality of times, the surface of the solar cell wafer 5 in the first etching step (step S101) as shown in FIG. The metal impurities 6 adhering to are gradually removed. The metal impurities 6 that have not been removed remain on the surface of the solar cell wafer 5 as shown in FIG.

As a result, as shown in FIG. 3C, the amount of metal impurities on the surface of the solar cell wafer 5 (the total amount of the metal impurities 6) can be adjusted. In this embodiment, the solar cell wafer before the diffusion step (step S105) is performed by repeatedly performing the first cleaning step and the second cleaning step to adjust the amount of metal impurities on the surface of the solar cell wafer 5. 5 is adjusted to 1 × 10 ⁹ atoms / cm ² or more and 1 × 10 ¹⁴ atoms / cm ² or less. Here, the metal impurities whose total abundance is adjusted include calcium (Ca), manganese (Mn), chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), Zinc (Zn), titanium (Ti), silver (Ag), indium (In), tin (Sn), cadmium (Cd), molybdenum (Mo), aluminum (Al), magnesium (Mg), gold (Au), Platinum (Pt) and palladium (Pd) are included.

In addition, the amount of metal impurities of Ca, Mn, Cr, Fe, Co, Ni, Cu, Zn, Ti, Ag, In, Sn, Cd, Mo, Al, Mg, Au, Pt, and Pd is 1 × 10 ⁹ atoms / When it is less than cm ^2, metal impurities serving as a catalyst for the etching reaction of the solar cell wafer 5 are insufficient, so that FPM by hydrofluoric acid hydrogen peroxide water (FPM: HF / H ₂ O ₂ / H ₂ O) is used in the next step. Even if the processing is performed, the pits 7 are hardly formed on the surface of the solar cell wafer 5. For this reason, even if it heat-processes after that, such as a diffusion process, the metal impurity inside the solar cell wafer 5 is hard to collect on the solar cell wafer surface. Therefore, the gettering effect in the heat treatment such as diffusion treatment is low, and the solar cell wafer cannot be highly purified (modified) effectively.

Further, the amount of metal impurities of Ca, Mn, Cr, Fe, Co, Ni, Cu, Zn, Ti, Ag, In, Sn, Cd, Mo, Al, Mg, Au, Pt, and Pd is 1 × 10 ¹⁴ atoms / If it is greater than cm ² , there are sufficient metal impurities to serve as a catalyst for the etching reaction of the solar cell wafer 5, so that hydrogen peroxide hydrofluoric acid (FPM: HF / H ₂ O ₂ / H ₂ O) is used in the next step. A large amount of pits 7 are formed on the surface of the solar cell wafer 5 by performing the FPM process. However, in this case, a large amount of pits 7 are formed, but the amount of metal impurities diffused into the solar cell wafer 5 in the subsequent heat treatment such as diffusion treatment also increases. Therefore, the solar cell wafer cannot be highly purified (modified) effectively.

It should be noted that metals such as Na (sodium) and K (potassium) are not involved in the etching catalytic reaction in the FPM treatment for solar cell wafers 5 such as silicon. However, when these metals are present on the surface of the solar cell wafer 5, there is a possibility of deteriorating the characteristics of the solar cell. Therefore, the amount of metal impurities such as Na (sodium) and K (potassium) on the surface of the solar cell wafer 5 is preferably 10 ⁹ atoms / cm ² or less.

Next, in the pit formation step (step S104), a solution containing hydrofluoric acid (HF) and an oxidizing agent such as hydrogen peroxide (H ₂ O ₂ ) water, hydrochloric acid (HCl), ozone (O ₃ ), etc. The solar cell wafer 5 is processed to form a plurality of fine pits 7 on the surface of the solar cell wafer 5 as shown in FIGS. 3-4. The treatment with a solution containing an oxidizing agent such as hydrofluoric acid (HF), hydrogen peroxide (H ₂ O ₂ ) water, hydrochloric acid (HCl), ozone (O ₃ ) has an effect of etching the surface of the solar cell wafer 5. is there. Thereafter, the surface of the solar cell wafer 5 is subjected to a drying process such as nitrogen (N ₂ ) blow drying, IPA drying, spin drying or the like to remove moisture on the surface of the solar cell wafer 5.

  Here, when impurities such as metal adhere to the surface of the solar cell wafer 5, fine pits 7 can be effectively formed on the surface of the solar cell wafer 5. That is, the metal impurity 6 shown in FIG. 3C accelerates the etching reaction with the solar cell wafer 5 such as silicon by the etching solution described above. As a result, as shown in FIG. 3-4, a plurality of fine pits 7 are formed on the surface of the solar cell wafer 5, and a porous layer is formed on the surface of the solar cell wafer 5. In this case, the more metal impurities 6 on the surface of the solar cell wafer 5, the easier it is to form a porous layer on the surface of the solar cell wafer 5, but on the other hand, it diffuses from the surface of the solar cell wafer 5 into the solar cell wafer interior 3. The amount of metal impurities also increases. However, if the amount of metal impurities adhering to the surface of the solar cell wafer 5 can be adjusted, a high purity solar cell wafer can be obtained.

Therefore, before the pit formation process, the first cleaning process (step S102) and the second cleaning process (step S103) described above are repeated to adjust the amount of metal impurities on the surface of the solar cell wafer 5 to thereby adjust the surface of the solar cell wafer 5. In this state, a predetermined amount of the metal impurity 6 is intentionally attached to the surface. Then, using the catalytic effect of the metal impurity 6 on the surface of the solar cell wafer 5, an oxidizing agent such as hydrofluoric acid (HF), hydrogen peroxide (H ₂ O ₂ ), hydrochloric acid (HCl), ozone (O ₃ ), etc. A pit formation process is performed using a solution containing, and the gettering effect of the diffusion process to be performed next is enhanced.

In the pit formation step, the solar cell wafer 5 is cleaned and etched for about 1 second to 60 minutes using the above-described etching solution. As a result, the etching reaction of the solar cell wafer 5 is accelerated by the catalytic effect of the metal impurities 6 such as Ag, Cu, Ni, Fe, Au, Pt, and Pd adhering to the surface of the solar cell wafer 5. Pits 7 are formed on the surface of the solar cell wafer 5 together with the metal impurities 6 on the surface of the solar cell wafer 5 shown in FIG. 3 (FIG. 3-4). The etching solution used in the pit formation process is, for example, 0.1 wt% to 50 wt% of hydrofluoric acid (HF), 50 wt% to 99.9 wt% of pure water, and hydrogen peroxide solution (H ₂ O ₂ ) as an oxidizing agent. Is preferably an aqueous solution containing 0.1 wt% to 30 wt%. Further, in the above aqueous solution, instead of hydrogen peroxide (H ₂ O ₂ ) as an oxidizing agent, ozone (O ₃ ) contains ozone (O ₃ ), and if it contains HCl, the concentration is 0.1 to 30 mg / L. An aqueous solution of 0.1 wt% to 30 wt% is preferable.

  Next, in the diffusion step (step S <b> 105), the solar cell wafer 5 is heated to diffuse an impurity element such as P (phosphorus) or B (boron) into the solar cell wafer interior 3 on the surface of the solar cell wafer 5. In the diffusion step, for example, the solar cell wafer 5 is exposed to a gas containing phosphorus (P) or boron (B), or a compound containing an element such as phosphorus (P) or boron (B) is exposed to the surface of the solar cell wafer 5. Then, heat treatment is performed at a temperature of about 100 ° C. to 950 ° C. for about 1 minute to 120 minutes.

  As a result, the metal impurities 11 inside the solar cell wafer 3 gather in the vicinity of the surface of the solar cell wafer 5, and further the metal impurities 11 aggregate in the vicinity of the porous layer formed in the pit formation step (step S104). As a result, as shown in FIG. 3-5, the metal impurities 8 gather near the surface of the solar cell wafer 5 and around the porous layer.

  In addition, since the oxide film may be formed on the surface of the solar cell wafer 5 after the diffusion step, the solar cell wafer 5 is made using a cleaning liquid in which hydrofluoric acid is contained in pure water or ultrapure water. It is preferable to remove the oxide film by washing.

Next, in the second etching step (step S106), the solar cell wafer 5 is etched using an etching solution, so that it aggregates in the vicinity of the surface of the solar cell wafer 5 and around the porous layer in the diffusion step (step S105). The metal impurity 8 is removed. That is, in the second etching step, the high concentration impurity layer in which the metal impurities 8 are collected in the diffusion step is removed. In the second etching step, an etchant containing an alkali such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), an anisotropy containing these alkalis and isopropyl alcohol (IPA), a surfactant, an organic substance such as a chelate, etc. The solar cell wafer 5 is cleaned and etched using an etching solution, ammonia hydrogen peroxide solution (APM), ammonia (NH ₃ ) water, mixed acid (HNO ₃ + HF) or the like. Thereby, as shown in FIG. 3-6, the metal impurities 8 aggregated in the vicinity of the surface of the solar cell wafer 5 and the periphery of the porous layer in the diffusion step (step S105) are removed, and the solar cell wafer 5 with high purity is obtained. can get.

In the second etching step, a depth range of 0.2 μm to 50 μm from the surface of the solar cell wafer 5 is etched. The etching process at such a depth is performed by etching for about 0.1 second to 60 minutes with the above-described etching solution. As an etching solution in the first etching step, an etching solution containing an alkali such as potassium hydroxide (KOH), sodium hydroxide (NaOH), aqueous ammonia (NH ₄ OH), or an acid such as a mixed acid (HNO ₃ + HF) is included. In the case of using an etching solution, the chemical concentration is preferably about 0.01 wt% to 96 wt%, and the concentration of the additive used for anisotropic etching is preferably 0.01 wt% to 50 wt%. The treatment temperature is preferably about 25 ° C to 95 ° C.

  After the second etching step, it is preferable to carry out a rinsing process such as a water washing process on the solar cell wafer 5 if necessary. The rinse treatment is preferably a rinse treatment with ultrapure water, pure water, anode water, cathode water, hydrogen water or the like. The treatment time for the water washing treatment is about 0.1 to 60 minutes. The water washing treatment is preferably performed at a temperature of 90 ° C. or less at which water does not evaporate.

  Next, in the third cleaning step (step S107), the surface of the solar cell wafer 5 is cleaned with a cleaning liquid, and further, the cleaning step is performed a plurality of times by combining the water washing steps as necessary. Remove metal impurities. After the second etching step, the metal impurities 8 inside the solar cell wafer 3 are removed, but there are metal impurities 9 attached to the surface of the solar cell wafer 5 in the second etching step.

Therefore, in the third cleaning step, the solar cell wafer 5 is cleaned to remove the metal impurities 9 on the surface of the solar cell wafer 5 to obtain a highly clean solar cell wafer surface 10. As a result, the solar cell wafer 5 with high cleanliness and improved bulk lifetime is obtained on both the solar cell wafer interior 3 and the surface of the solar cell wafer 5. The cleaning liquid used for the cleaning process in the third cleaning step includes a cleaning liquid containing an acid such as hydrochloric acid (HCl), sulfuric acid (H ₂ SO ₄ ), and nitric acid (HNO ₃ ), ozone (O ₃ ), hydrogen peroxide (H ₂ ). Examples thereof include a cleaning liquid containing an oxidizing agent such as O ₂ ), ammonia (NH ₃ ) water, and the like.

  After that, by forming, for example, amorphous silicon (a-Si), silicon nitride (SiN) or the like as a passivation film on the solar cell wafer surface 10, the recombination speed inside the solar cell wafer 5 can be kept lower than before. And the bulk lifetime of the solar cell wafer 5 is improved.

  In addition, as the solvent of the solution used in the first etching process to the third cleaning process (steps S101 to S107), functional water such as pure water, ultrapure water, anode water, cathode water, and hydrogen water may be used. it can. In the first etching process to the third cleaning process (steps S101 to 107), ultrasonic cleaning, electrolytic treatment, drying treatment, and the like can be performed as necessary.

  In addition, you may implement a water washing process etc. before and after each wet process (step S101-step S107) of embodiment.

  Next, a conventional solar cell wafer cleaning method will be described for comparison. First, a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 1 will be described with reference to FIGS. FIGS. 4-1 to 4-6 are cross-sectional views schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 1. FIGS.

  When the solar cell wafer modification method based on the cleaning process of Patent Document 1 is performed, the damaged layer 4 is first removed by etching the solar cell wafer surface 1 shown in FIG. Impurities 2 such as metals and organic substances existing in the vicinity of 1 are removed (FIG. 4-2). At this time, the metal impurities present in the damaged layer 4 and the solar cell wafer inside 3 are dissolved in the etching solution and reattached to the surface of the solar cell wafer 5 as the metal impurities 6. Note that metal impurities 11 exist in the solar cell wafer inside 3.

  When this solar cell wafer 5 is subjected to ozone water cleaning processing and DHF (dilute hydrofluoric acid) cleaning processing to adjust the amount of metal impurities on the surface of the solar cell wafer 5, as shown in FIG. 4-3. The metal impurities 6 attached to the surface of the solar cell wafer 5 are removed, and the metal impurities 6 that have not been removed remain on the surface of the solar cell wafer 5.

Next, after performing a pure water cleaning process and a drying process, diffusion to the solar cell wafer 5 is performed without performing an FPM process using hydrogen peroxide hydrofluoric acid (FPM: HF / H ₂ O ₂ / H ₂ O). When the (gettering) process is performed, as shown in FIG. 4-4, metal impurities 8 gather near the surface of the solar cell wafer 5 and around the porous layer, and the metal impurities 6 diffuse into the solar cell wafer interior 3. .

  When the etching process is performed on the surface of the solar cell wafer 5 in this state, the metal impurities 8 collected near the surface of the solar cell wafer 5 are removed, but the metal inside the solar cell wafer 3 is low because the gettering effect is low. Impurities are hardly removed, and a large amount of metal impurities 11 remain in the solar cell wafer interior 3. Further, metal impurities 9 adhere to the surface of the solar cell wafer 5 (FIGS. 4-5).

  Thereafter, when a cleaning process is performed on the surface of the solar cell wafer 5, the metal impurities 9 on the surface of the solar cell wafer 5 are removed, so that a highly clean solar cell wafer surface 10 is obtained. However, a large amount of the metal impurities 11 and the diffused metal impurities 6 that remain in the solar cell wafer interior 3 remain, and the solar cell wafer interior 3 cannot be highly purified (modified) (FIG. 4-6).

  Next, a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 2 will be described with reference to FIGS. FIGS. 5-1 to 5-6 are cross-sectional views schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Document 2. FIGS.

  When the solar cell wafer modification method based on the cleaning process of Patent Document 2 is implemented, the damaged layer 4 is first removed by etching the solar cell wafer surface 1 shown in FIG. Impurities 2 such as metals and organic substances existing in the vicinity of 1 are removed (FIG. 5-2). At this time, the metal impurities present in the damaged layer 4 and the solar cell wafer inside 3 are dissolved in the etching solution and reattached to the surface of the solar cell wafer 5 as the metal impurities 6. Note that metal impurities 11 exist in the solar cell wafer inside 3.

Without performing the ozone water cleaning process and the DHF (dilute hydrofluoric acid) cleaning process on the solar cell wafer 5, that is, without adjusting the metal impurity amount on the surface of the solar cell wafer 5 by the cleaning, the FPM is performed. (HF + H ₂ O ₂ ) treatment is performed. In this case, since the surface of the solar cell wafer 5 is in a state where a large amount of metal impurities 6 are adhered, a large amount of pits 7 are formed on the surface of the solar cell wafer 5, and the surface of the solar cell wafer 5 is formed. The solar cell wafer 5 shown in FIG. 5-3 having a porous layer is obtained.

  When the diffusion (gettering) process is performed on the solar cell wafer 5 after performing the pure water cleaning process and the drying process, as shown in FIG. As metal impurities 8 gather, metal impurities 6 diffuse into the solar cell wafer interior 3.

  When the etching process is performed on the surface of the solar cell wafer 5 in this state, the metal impurities 8 collected in the vicinity of the surface of the solar cell wafer 5 are removed, but the metal impurities 9 adhere to the surface of the solar cell wafer 5. (Fig. 5-5). Thereafter, when a cleaning process is performed on the surface of the solar cell wafer 5, the metal impurities 9 on the surface of the solar cell wafer 5 are removed, so that a highly clean solar cell wafer surface 10 is obtained. However, a large amount of metal impurities 6 remain in the solar cell wafer interior 3, and the solar cell wafer interior 3 cannot be highly purified (modified) (FIGS. 5-6).

  As described above, when the method for modifying a solar cell wafer based on the cleaning process of Patent Document 1 is performed, the surface of the solar cell wafer 5 is cleaned (ozone water cleaning and DHF (dilute hydrofluoric acid) cleaning). ) But does not implement FPM processing. For this reason, fine pits 7 are not formed on the surface of the solar cell wafer 5, and even if diffusion (gettering) processing is performed, the gettering effect is low, and the bulk lifetime of the solar cell wafer 5 is not improved.

  In addition, when the solar cell wafer modification method based on the cleaning process of Patent Document 2 is performed, the surface cleaning process (ozone water cleaning process and DHF (dilute hydrofluoric acid) cleaning process) of the solar cell wafer 5 is performed. Therefore, the amount of metal impurities on the surface of the solar cell wafer 5 cannot be adjusted, and a large amount of metal impurities 6 diffuse into the solar cell wafer interior 3 during the diffusion (gettering) process. The metal impurities 6 diffused into the solar cell wafer 5 cannot be removed by subsequent etching and cleaning. For this reason, the bulk lifetime is not improved in the solar cell wafer 5.

  Therefore, in any of these modification methods, the modification effect of the solar cell wafer cannot be sufficiently obtained, and the bulk lifetime of the solar cell wafer 5 is not improved.

  Next, a method for modifying a solar cell wafer by combining the cleaning processes described in Patent Document 1 and Patent Document 2 will be described with reference to FIGS. FIGS. 6-1 to 6-7 are cross-sectional views schematically showing a method for modifying a solar cell wafer based on the cleaning process described in Patent Literature 1 and Patent Literature 2. FIGS.

  When the solar cell wafer modification method combined with the cleaning processes described in Patent Document 1 and Patent Document 2 is performed, the damaged layer 4 is removed by etching the solar cell wafer surface 1 shown in FIG. Then, impurities 2 such as metals and organic substances existing in the vicinity of the solar cell wafer surface 1 are removed (FIG. 6-2). At this time, the metal impurities present in the damaged layer 4 and the solar cell wafer inside 3 are dissolved in the etching solution and reattached to the surface of the solar cell wafer 5 as the metal impurities 6. Note that metal impurities 11 exist in the solar cell wafer inside 3.

When the ozone water cleaning process and the DHF (dilute hydrofluoric acid) cleaning process are repeatedly performed on the solar cell wafer 5, the metal impurities 6 attached to the surface of the solar cell wafer 5 are removed as shown in FIG. 6-3. Then, the metal impurities 6 that have not been removed remain on the surface of the solar cell wafer 5. However, since the amount of metal impurities on the surface of the solar cell wafer 5 is not adjusted, the amount of metal impurities on the surface of the solar cell wafer 5 is, for example, less than 1 × 10 ⁹ atoms / cm ² .

An FPM (HF + H ₂ O ₂ ) process is performed on the solar cell wafer 5. In this case, since the metal impurities 6 are hardly present on the surface of the solar cell wafer 5, it is difficult to form pits 7 on the surface of the solar cell wafer 5, and the solar cell having few pits 7 as shown in FIG. 6-4. A wafer 5 is obtained.

  When the diffusion (gettering) process is performed on the solar cell wafer 5 after performing the pure water cleaning process and the drying process, as shown in FIG. The metal impurities 8 gather and the metal impurities 6 diffuse into the solar cell wafer 5. However, since the number of pits 7 is small, the gettering effect of the metal impurities inside the solar cell wafer 5 is low, and the metal impurities inside the solar cell wafer 5 hardly gather on the surface of the solar cell wafer 5.

  When the etching process is performed on the surface of the solar cell wafer 5 in this state, the metal impurities in the solar cell wafer inside 3 gathered on the surface of the solar cell wafer 5 are removed, but the solar cell wafer is low because the gettering effect is low. The metal impurities in the interior 3 are difficult to remove, and a large amount of metal impurities 11 remain in the interior 3 of the solar cell wafer. Further, metal impurities 9 adhere to the surface of the solar cell wafer 5 (FIGS. 6-6).

  Thereafter, when a cleaning process is performed on the surface of the solar cell wafer 5, the metal impurities 9 on the surface of the solar cell wafer 5 are removed, so that a highly clean solar cell wafer surface 10 is obtained. However, a large amount of the metal impurities 11 and the diffused metal impurities 6 that remain in the solar cell wafer interior 3 remain, and the solar cell wafer interior 3 cannot be highly purified (modified) (FIGS. 6-7).

  As described above, even in the solar cell wafer modification method combined with the cleaning processes described in Patent Document 1 and Patent Document 2, the solar cell wafer modification effect cannot be sufficiently obtained. The bulk lifetime is not improved.

In contrast, in the present embodiment described above, a first cleaning process (such as an ozone water cleaning process) that forms an oxide film on the solar cell wafer 5 and a second cleaning process (such as a DHF process) that removes this oxide film. Are repeated several times to adjust the amount of metal impurities on the surface of the solar cell wafer 5 to 1 × 10 ⁹ atoms / cm ² or more and 1 × 10 ¹⁴ atoms / cm ² or less to form pits on the surface of the solar cell wafer 5. After the formation, a heat treatment (gettering process) of a diffusion process is performed, and an etching process and cleaning of the surface of the solar cell wafer 5 are performed. Thereby, in this Embodiment, since the modification | reformation effect of the solar cell wafer 5 becomes high and the inside of the solar cell wafer 5 can be highly purified (modified) efficiently, the bulk lifetime is made longer than before. That is, the surface recombination rate can be kept low.

  Furthermore, by forming a passivation film on the surface of the modified solar cell wafer 5, the bulk lifetime can be made longer than before, that is, the surface recombination rate can be kept lower. It becomes possible to greatly improve the photoelectric conversion efficiency of the used solar cell.

  Next, the present invention will be described based on specific examples. In order to verify the effect of the method for modifying a semiconductor substrate according to the present embodiment, an example of the method for modifying a semiconductor substrate according to the present embodiment is compared with a comparative example using immersion cleaning. It was. Table 1 shows the processing conditions before the diffusion step in Examples and Comparative Examples. The solar cell wafer 5 used for the modified solar cell substrate in the examples and comparative examples is an n-type single crystal silicon wafer (hereinafter sometimes referred to as a silicon wafer) having a size of 100 mm square, and has a specific resistance of 3 Ωcm. It is.

  In the example, the wafer modification process was performed on the silicon wafer by using the semiconductor substrate modification method according to the present embodiment. The process of the semiconductor substrate modification method according to the example corresponds to the cross-sectional views shown in FIGS.

  First, as a first etching step, the silicon wafer shown in FIG. 3A is etched for 30 minutes using an etching solution containing 1.5 wt% sodium hydroxide (alkali) and 5 wt% isopropyl alcohol (IPA) and silicon. A texture was formed on the wafer surface. Thereafter, the silicon wafer was treated with a treatment solution containing 0.5 wt% hydrofluoric acid (HF) for 1 minute to neutralize the alkali on the surface of the silicon wafer (FIG. 3-2).

  Next, as a first cleaning step, the silicon wafer was cleaned for 1 minute using ozone water having an ozone concentration of 30 mg / L (25 ° C.) to form an oxide film on the surface of the silicon wafer. After washing the silicon wafer with water, the silicon wafer is washed for 1 minute using a cleaning solution (DHF) (25 ° C.) containing hydrofluoric acid having a hydrofluoric acid (HF) concentration of 0.5 wt% as a second cleaning step. The oxide film on the silicon wafer surface was removed.

Then, after the water washing process is performed on the silicon wafer, the unit cleaning process is repeated so that the unit cleaning process is performed 3 to 50 times by using the first cleaning process and the second cleaning process as the unit cleaning process. In practice, the amount of metal impurities on the silicon wafer surface was set to 1 × 10 ⁹ atoms / cm ² or more and 1 × 10 ¹⁴ atoms / cm ² or less (FIG. 3-3).

Next, as a pit formation process, a silicon wafer is formed using an FPM solution (25 ° C.) containing 0.5 wt% of hydrogen peroxide (H ₂ O ₂ ) having a hydrofluoric acid (HF) concentration of 0.5 wt%. By performing the treatment for a minute, pits 7 were formed on the silicon wafer surface to form a porous layer (FIGS. 3-4).

Next, the silicon wafer was dried, and a diffusion process (gettering process) was performed in a POCl ₃ gas at 800 ° C. as a diffusion process (FIG. 3-5).

  Next, as a second etching step, the silicon wafer is etched for 30 minutes using an etching solution containing 1.5 wt% sodium hydroxide (alkali) and 5 wt% isopropyl alcohol (IPA) and a texture is formed on the silicon wafer surface. Thereafter, the silicon wafer was treated with a treatment solution containing 0.5 wt% hydrofluoric acid (HF) for 1 minute to neutralize the alkali on the silicon wafer surface (FIGS. 3-6).

  Next, as a third cleaning step, the silicon wafer was cleaned for 10 minutes using ozone water having an ozone concentration of 30 mg / L (25 ° C.), and then a hydrofluoric acid (HF) 0.5 wt% treatment was performed for 1 minute. . This third washing step was repeated 20 times (FIGS. 3-7).

  And after performing the drying process of a silicon wafer, the amorphous silicon was formed into a film on the silicon wafer surface, and lifetime was evaluated. Furthermore, solar cells were produced using this silicon wafer, and the cell characteristics of the solar cells were evaluated.

In Comparative Example 1, using the same procedure as the method for modifying a semiconductor substrate according to this embodiment, the amount of metal impurities on the surface of the silicon wafer is increased to more than 1 × 10 ¹⁴ atoms / cm ² and the wafer is modified to a silicon wafer. Quality treatment was performed. The process of the semiconductor substrate modification method according to Comparative Example 1 corresponds to the cross-sectional views shown in FIGS.

  First, as a first etching step, the silicon wafer shown in FIG. 7-1 is etched for 30 minutes using an etchant containing 1.5 wt% sodium hydroxide (alkali) and 5 wt% isopropyl alcohol (IPA) and silicon. A texture was formed on the wafer surface. Thereafter, the silicon wafer was treated with a treatment solution containing 0.5 wt% hydrofluoric acid (HF) for 1 minute to neutralize the alkali on the surface of the silicon wafer (FIG. 7-2).

  Next, as a first cleaning step, the silicon wafer was cleaned for 1 minute using ozone water having an ozone concentration of 30 mg / L (25 ° C.) to form an oxide film on the surface of the silicon wafer. After washing the silicon wafer with water, the silicon wafer is washed for 1 minute using a cleaning solution (DHF) (25 ° C.) containing hydrofluoric acid having a hydrofluoric acid (HF) concentration of 0.5 wt% as a second cleaning step. The oxide film on the silicon wafer surface was removed.

Then, after the water washing process is performed on the silicon wafer, the unit cleaning process is repeated so that the unit cleaning process is performed once and twice by using the first cleaning process and the second cleaning process as unit cleaning processes. The amount of metal impurities on the surface of the silicon wafer was increased from 1 × 10 ¹⁴ atoms / cm ² (FIG. 7-3).

Next, as a pit formation process, a silicon wafer is formed using an FPM solution (25 ° C.) containing 0.5 wt% of hydrogen peroxide (H ₂ O ₂ ) having a hydrofluoric acid (HF) concentration of 0.5 wt%. By treating for a minute, pits were formed on the silicon wafer surface to form a porous layer (FIG. 7-4).

Next, the silicon wafer was dried, and a diffusion process (gettering process) was performed in a POCl ₃ gas at 800 ° C. as a diffusion process (FIG. 7-5).

  Next, as a second etching step, the silicon wafer is etched for 30 minutes using an etching solution containing 1.5 wt% sodium hydroxide (alkali) and 5 wt% isopropyl alcohol (IPA) and a texture is formed on the silicon wafer surface. Thereafter, the silicon wafer was treated with a treatment solution containing 0.5 wt% hydrofluoric acid (HF) for 1 minute to neutralize the alkali on the surface of the silicon wafer (FIGS. 7-6).

  Next, as a third cleaning step, the silicon wafer was cleaned for 10 minutes using ozone water having an ozone concentration of 30 mg / L (25 ° C.), and then a hydrofluoric acid (HF) 0.5 wt% treatment was performed for 1 minute. . This third washing step was repeated 20 times (FIGS. 7-7).

  And after performing the drying process of a silicon wafer, the amorphous silicon was formed into a film on the silicon wafer surface, and lifetime was evaluated. Furthermore, solar cells were produced using this silicon wafer, and the cell characteristics of the solar cells were evaluated.

In Comparative Example 2, using the same procedure as the method for modifying a semiconductor substrate according to the present embodiment, the amount of metal impurities on the surface of the silicon wafer is reduced to less than 1 × 10 ⁹ atoms / cm ² and the wafer is modified to a silicon wafer. Quality treatment was performed. The process of the semiconductor substrate modification method according to Comparative Example 2 corresponds to the cross-sectional views shown in FIGS.

  First, as a first etching step, the silicon wafer shown in FIG. 6A is etched for 30 minutes using an etchant containing 1.5 wt% sodium hydroxide (alkali) and 5 wt% isopropyl alcohol (IPA) and silicon. A texture was formed on the wafer surface. Thereafter, the silicon wafer was treated with a treatment solution containing 0.5 wt% hydrofluoric acid (HF) for 1 minute to neutralize the alkali on the surface of the silicon wafer (FIG. 6-2).

  Next, as a first cleaning step, the silicon wafer was cleaned for 1 minute using ozone water having an ozone concentration of 30 mg / L (25 ° C.) to form an oxide film on the surface of the silicon wafer. After washing the silicon wafer with water, the silicon wafer is washed for 1 minute using a cleaning solution (DHF) (25 ° C.) containing hydrofluoric acid having a hydrofluoric acid (HF) concentration of 0.5 wt% as a second cleaning step. The oxide film on the silicon wafer surface was removed.

Then, after performing the water washing process on the silicon wafer, the unit cleaning process is repeatedly performed so that the unit cleaning process is performed 100 times as the unit cleaning process of the first cleaning process and the second cleaning process, The amount of metal impurities on the silicon wafer surface was made smaller than 1 × 10 ⁹ atoms / cm ² (FIG. 6-3).

Next, as a pit formation process, a silicon wafer is formed using an FPM solution (25 ° C.) containing 0.5 wt% of hydrogen peroxide (H ₂ O ₂ ) having a hydrofluoric acid (HF) concentration of 0.5 wt%. By performing the treatment for 5 minutes, pits 7 were formed on the surface of the silicon wafer to form a porous layer (FIG. 6-4).

Next, the silicon wafer was dried, and a diffusion process (gettering process) was performed in a POCl ₃ gas at 800 ° C. as a diffusion process (FIG. 6-5).

  Next, as a second etching step, the silicon wafer is etched for 30 minutes using an etching solution containing 1.5 wt% sodium hydroxide (alkali) and 5 wt% isopropyl alcohol (IPA) and a texture is formed on the silicon wafer surface. Thereafter, the silicon wafer was treated with a treatment solution containing 0.5 wt% hydrofluoric acid (HF) for 1 minute to neutralize the alkali on the surface of the silicon wafer (FIGS. 6-6).

  Next, as a third cleaning step, the silicon wafer was cleaned for 10 minutes using ozone water having an ozone concentration of 30 mg / L (25 ° C.), and then a hydrofluoric acid (HF) 0.5 wt% treatment was performed for 1 minute. . This third washing step was repeated 20 times (FIGS. 6-7).

  And after performing the drying process of a silicon wafer, the amorphous silicon was formed into a film on the silicon wafer surface, and lifetime was evaluated. Furthermore, solar cells were produced using this silicon wafer, and the cell characteristics of the solar cells were evaluated.

  In Comparative Example 3, the wafer modifying process was performed on the silicon wafer without performing the pit forming step in the semiconductor substrate modifying method according to the present embodiment. The process of the semiconductor substrate modification method according to Comparative Example 3 corresponds to the cross-sectional views shown in FIGS.

  First, as a first etching step, the silicon wafer shown in FIG. 4A is etched for 30 minutes using an etchant containing 1.5 wt% sodium hydroxide (alkali) and 5 wt% isopropyl alcohol (IPA) and silicon. A texture was formed on the wafer surface. Thereafter, the silicon wafer was treated with a treatment solution containing 0.5 wt% hydrofluoric acid (HF) for 1 minute to neutralize the alkali on the surface of the silicon wafer (FIG. 4-2).

  Next, as a first cleaning step, the silicon wafer was cleaned for 1 minute using ozone water having an ozone concentration of 30 mg / L (25 ° C.) to form an oxide film on the surface of the silicon wafer. After washing the silicon wafer with water, the silicon wafer is washed for 1 minute using a cleaning solution (DHF) (25 ° C.) containing hydrofluoric acid having a hydrofluoric acid (HF) concentration of 0.5 wt% as a second cleaning step. The oxide film on the silicon wafer surface was removed.

Then, after the water washing process is performed on the silicon wafer, the unit cleaning process is repeated so that the unit cleaning process is performed 1 to 100 times by using the first cleaning process and the second cleaning process as the unit cleaning process. performed, the amount of metal impurities 1 × ¹⁰ 9 atoms / cm less than ² silicon wafer surface, 1 × ¹⁰ 9 atoms / cm ² or more and 1 × ¹⁰ 14 atoms / cm ² or less, from 1 × ¹⁰ 14 atoms / cm ² Many wafers were produced (FIG. 4-3).

Next, the silicon wafer was dried without performing the pit formation process, and a diffusion process (gettering process) was performed in a POCl ₃ gas at 800 ° C. as a diffusion process (FIG. 4-4).

  Next, as a second etching step, the silicon wafer is etched for 30 minutes using an etching solution containing 1.5 wt% sodium hydroxide (alkali) and 5 wt% isopropyl alcohol (IPA) and a texture is formed on the silicon wafer surface. Thereafter, the silicon wafer was treated with a treatment solution containing 0.5 wt% hydrofluoric acid (HF) for 1 minute to neutralize the alkali on the silicon wafer surface (FIGS. 4-5).

  Next, as a third cleaning step, the silicon wafer was cleaned for 10 minutes using ozone water having an ozone concentration of 30 mg / L (25 ° C.), and then a hydrofluoric acid (HF) 0.5 wt% treatment was performed for 1 minute. . This third washing step was repeated 20 times (FIGS. 4-6).

  And after performing the drying process of a silicon wafer, the amorphous silicon was formed into a film on the silicon wafer surface, and lifetime was evaluated. Furthermore, solar cells were produced using this silicon wafer, and the cell characteristics of the solar cells were evaluated.

  In Comparative Example 4, the wafer modification process was performed on the silicon wafer without performing the pit formation step in the semiconductor substrate modification method according to the present embodiment. The process of the semiconductor substrate reforming method according to Comparative Example 4 corresponds to the cross-sectional views shown in FIGS.

  First, as a first etching step, the silicon wafer shown in FIG. 5A is etched for 30 minutes using an etching solution containing 1.5 wt% sodium hydroxide (alkali) and 5 wt% isopropyl alcohol (IPA) and silicon. A texture was formed on the wafer surface. Thereafter, the silicon wafer was treated with a treatment solution containing 0.5 wt% hydrofluoric acid (HF) for 1 minute to neutralize the alkali on the surface of the silicon wafer (FIG. 5-2).

Next, a hydrofluoric acid (HF) concentration is used as a pit formation process in a state where the amount of metal impurities on the silicon wafer surface is set to 1 × 10 ¹⁴ atoms / cm ² or more without performing the first cleaning process and the second cleaning process. Pits 7 are formed on the surface of the silicon wafer by treating the silicon wafer for 20 minutes with an FPM solution (25 ° C.) containing 0.5 wt% of 0.5 wt% hydrogen peroxide (H ₂ O ₂ ). Thus, a porous layer was formed (FIG. 5-3).

Next, the silicon wafer was dried, and a diffusion process (gettering process) was performed in a POCl ₃ gas at 800 ° C. as a diffusion process (FIG. 5-4).

  Next, as a second etching step, the silicon wafer is etched for 30 minutes using an etching solution containing 1.5 wt% sodium hydroxide (alkali) and 5 wt% isopropyl alcohol (IPA) and a texture is formed on the silicon wafer surface. Thereafter, the silicon wafer was treated with a treatment solution containing 0.5 wt% hydrofluoric acid (HF) for 1 minute to neutralize the alkali on the surface of the silicon wafer (FIGS. 5-5).

  Next, as a third cleaning step, the silicon wafer was cleaned for 10 minutes using ozone water having an ozone concentration of 30 mg / L (25 ° C.), and then a hydrofluoric acid (HF) 0.5 wt% treatment was performed for 1 minute. . This third washing step was repeated 20 times (FIGS. 5-6).

  And after performing the drying process of a silicon wafer, the amorphous silicon was formed into a film on the silicon wafer surface, and lifetime was evaluated. Furthermore, solar cells were produced using this silicon wafer, and the cell characteristics of the solar cells were evaluated.

Diffusion pretreatment conditions, number of unit cleaning treatments (first cleaning step + first cleaning step), and amount of metal impurities on the silicon wafer surface before the pit forming step in the above-described examples and comparative examples 1, 2, 3, 4 (Atoms / cm ² ) and lifetime (μsec) are shown in Table 1 and FIG. FIG. 8 is a characteristic diagram showing the relationship between the amount of metal impurities on the silicon wafer surface and the bulk lifetime.

  In addition, in order to produce a solar battery cell, an amorphous silicon film was formed on the surface of the silicon wafers of Examples and Comparative Examples 1, 2, 3, and 4 by a CVD film forming apparatus, and an ITO electrode was formed thereon. did. Further, an Ag electrode was attached by opening with an ITO electrode laser. IV measurement was performed about each produced photovoltaic cell, and the cell characteristic, ie, the photoelectric conversion efficiency of a photovoltaic cell, was evaluated. The results are shown in Table 1 and FIG. FIG. 9 is a characteristic diagram showing the relationship between the amount of metal impurities on the surface of the silicon wafer and the photoelectric conversion efficiency of the solar battery cell.

  As can be seen from Table 1 and FIGS. 8 and 9, in the example in which the semiconductor substrate reforming method according to the embodiment was performed, the effect of suppressing recombination of charge carriers in the silicon wafer and the photoelectric conversion efficiency of the solar battery cell were It was significantly higher than Comparative Examples 1 to 4. That is, the bulk lifetime of the silicon wafer can be improved as compared with the conventional method by the method for modifying a semiconductor substrate according to the present embodiment. Furthermore, by forming a passivation film on the surface of the silicon wafer, the bulk lifetime can be extended, and the photoelectric conversion efficiency of the solar battery cell can be greatly improved.

  In Examples and Comparative Examples 1 to 3, the number of executions of the unit cleaning process (first cleaning process (ozone water cleaning) + second cleaning process (DHF process)) is changed to change the metal impurities on the silicon wafer surface. The amount is adjusted. However, the amount of metal impurities on the surface of the silicon wafer before performing the unit cleaning process varies depending on the type (manufacturer) of the silicon wafer to be used and pre-processing conditions (such as etching processing conditions).

  In addition, metal impurities on the surface of the silicon wafer may or may not be easily removed from the surface of the silicon wafer due to organic matter adhering to the surface of the silicon wafer or processing (such as etching processing conditions) before unit cleaning processing. Therefore, the amount of metal impurities on the surface of the silicon wafer is not determined by the number of executions of the first cleaning step for forming the oxide film and the second cleaning step for removing the oxide film. However, the amount of metal impurities on the silicon wafer surface tends to decrease as the number of executions of the first cleaning step (ozone water cleaning) for forming the oxide film and the second cleaning step (DHF treatment) for removing the oxide film increases. It is in.

As described above, in the method for modifying a semiconductor substrate according to the embodiment, the first cleaning step (such as an ozone water cleaning process) for forming an oxide film on the semiconductor substrate before the heat treatment (gettering process) of the diffusion process. ) And the second cleaning step (DHF treatment or the like) for removing the oxide film are repeated a plurality of times, and the number of treatments is adjusted to reduce the amount of metal impurities on the surface of the semiconductor substrate to 1 × 10 ⁹ atoms / cm ^2. Above, adjust to 1 × 10 ¹⁴ atoms / cm ² or less. That is, before the heat treatment (gettering process) such as diffusion, the amount of metal impurities on the surface of the semiconductor substrate is adjusted so that the metal impurities are intentionally attached to the surface of the semiconductor substrate.

  Next, a pit formation process (FPM process) is performed in which fine pits are chemically formed on the surface of the semiconductor substrate using the catalytic effect of metal impurities attached to the surface of the semiconductor substrate. The metal impurities make it easier for the surface of the semiconductor substrate to gather, and the gettering effect is enhanced.

  Next, phosphorus (P) or boron (B) or the like is diffused in the semiconductor substrate to make it difficult to diffuse metal impurities inside the semiconductor substrate, and a heat treatment (gettering treatment) such as diffusion is carried out on the surface of the semiconductor substrate. Collect metal impurities inside the semiconductor substrate. Then, metal impurities collected on the surface of the semiconductor substrate by gettering are removed by etching, and a cleaning process is performed on the surface of the semiconductor substrate. Thereby, the metal impurities on the surface and the inside of the semiconductor substrate can be efficiently removed, and a semiconductor substrate excellent in bulk lifetime that is highly clean on both the surface and the inside can be obtained.

  Further, by forming a passivation film on the surface of the semiconductor substrate thus obtained, the surface recombination rate can be kept lower than before and the bulk lifetime can be further increased.

  Also, during industrial use, the amount of metal and organic impurities on the surface of the solar cell wafer at the time of acceptance and purchase (raw) changes with each wafer ingot and slicing step. Therefore, it is preferable to extract and evaluate the amount of metal impurities on the wafer surface before manufacturing the solar cell, accept the number of repeated treatments with ozone water cleaning and hydrofluoric acid treatment, and appropriately determine each wafer ingot and slicing step at the time of purchase. . This makes it possible to adjust the amount of metal impurities on the wafer surface with good reproducibility by repeating the first cleaning process (ozone water cleaning, step S102) and the second cleaning process (hydrofluoric acid process, step S103). I can do it.

  And the photovoltaic conversion efficiency of a photovoltaic cell can be improved by producing a photovoltaic cell using the semiconductor substrate modified by the modification method of the semiconductor substrate concerning this embodiment, and photoelectric conversion efficiency is insufficient. The unnecessary generation | occurrence | production which originated can be suppressed and the yield of a solar cell can be improved.

  The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention when it is practiced. Further, the above embodiments include inventions at various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent requirements.

  For example, even if some constituent requirements are deleted from all the constituent requirements shown in the above embodiment, the problem described in the column of the problem to be solved by the invention can be solved, and is described in the column of the effect of the invention. In the case where a certain effect can be obtained, a configuration from which this configuration requirement is deleted can be extracted as an invention. Furthermore, the structural requirements over the above embodiments may be combined as appropriate.

  As described above, the method for modifying a semiconductor substrate according to the present invention is useful for realizing a highly clean semiconductor substrate, and is particularly suitable for modifying a solar cell wafer for a solar cell.

DESCRIPTION OF SYMBOLS 1 Solar cell wafer surface 2 Impurities, such as a metal and organic substance 3 Solar cell wafer inside 4 Damaged layer 5 Solar cell wafer 6 Metal impurity 7 Pit 8 Metal impurity 9 Metal impurity 10 Solar cell wafer surface 11 Metal impurity

Claims (10)

An oxide film forming step for forming an oxide film by oxidizing the surface of the semiconductor substrate and an oxide film removing step for removing the oxide film are performed in this order to reduce the amount of metal impurities present on the surface of the semiconductor substrate. A first step of adjusting the sum to 1 × 10 ⁹ atoms / cm ² or more and 1 × 10 ¹⁴ atoms / cm ² or less;
A second step of forming a plurality of pits on the surface of the semiconductor substrate by wet-etching the surface of the semiconductor substrate using a metal impurity whose amount of the surface of the semiconductor substrate is adjusted as a catalyst;
A third step of diffusing impurities inside the semiconductor substrate and collecting the impurities in the vicinity of the surface of the semiconductor substrate and in the vicinity of the pits;
A fourth step of removing the impurities collected on the surface of the semiconductor substrate;
A method for modifying a semiconductor substrate, comprising: Performing the second step after adjusting the amount of metal impurities on the surface of the semiconductor substrate by performing the first step a plurality of times;
The method for modifying a semiconductor substrate according to claim 1. Metal impurities on the surface of the semiconductor substrate are calcium (Ca), manganese (Mn), chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), Titanium (Ti), silver (Ag), indium (In), tin (Sn), cadmium (Cd), molybdenum (Mo), aluminum (Al), magnesium (Mg), gold (Au), platinum (Pt), Containing palladium (Pd),
The method for modifying a semiconductor substrate according to claim 1, wherein: In the oxide film forming step, the surface of the semiconductor substrate is treated with at least one of ozone water, hydrogen peroxide water, and hydrochloric acid;
The method for modifying a semiconductor substrate according to any one of claims 1 to 3. In the oxide film removing step, the surface of the semiconductor substrate on which the oxide film is formed is treated with a solution containing hydrofluoric acid,
The method for modifying a semiconductor substrate according to any one of claims 1 to 4, wherein: In the oxide film removing step, the surface of the semiconductor substrate on which the oxide film is formed is treated with pure water containing hydrofluoric acid,
The method for modifying a semiconductor substrate according to claim 5. In the second step, wet etching using a solution containing hydrofluoric acid and an oxidizing agent as an etchant;
The method for modifying a semiconductor substrate according to claim 1, wherein: In the second step, wet etching using pure water containing hydrofluoric acid and hydrogen peroxide or pure water containing hydrofluoric acid and ozone as an etchant;
The method for modifying a semiconductor substrate according to claim 7. Removing the damage layer formed on the surface layer of the semiconductor substrate before the first step;
The method for modifying a semiconductor substrate according to claim 1, wherein: After the fourth step, cleaning metal impurities attached to the surface of the semiconductor substrate in the fourth step,
The method for modifying a semiconductor substrate according to claim 1, wherein:

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