JP2008218993A - Method of recycling scrap wafer, and method for producing silicon substrate for solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of recycling scrap wafer which can reduce waste in resources for recycling, and regenerate it under conditions similar to those of a single crystal, by making a silicon wafer into a regenerated ingot without melting the silicon wafer, and to provide a manufacturing method of a silicon substrate for use in a solar cell. <P>SOLUTION: The method of recycling the scrap wafer includes procedures of a film removal step S3 for removing a film formed on the scrap wafer W; a mirror-finishing step S5 for mirror-finishing front and rear faces having the scrap wafer film removed; an arranging step S8 for overlapping the mirror-finished scrap wafers, with their crystal orientations aligned and arranged in a cylindrical form; and a heating step S9 for putting the scrap wafers, arranged in the cylindrical form into a heating furnace and heating in the range of 400-1,350°C and making the wafers mutually bond by diffusion and producing a regenerated ingot IG. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for reusing a scrap wafer that has been used for testing when producing semiconductor devices, or a scrap wafer that becomes defective and cannot be used as a product, and a method for recycling a scrap wafer and The present invention relates to a method for manufacturing a silicon substrate for a solar cell as a silicon substrate for a battery.

  In general, a silicon wafer has a plurality of electronic circuits formed in a single effective area, cut out for each electronic circuit, and mounted as a semiconductor device such as an IC or LSI on a home appliance or a machine control mechanism. in use. In the manufacturing process of a desired semiconductor device such as an IC or LSI, a used test wafer or a scrap wafer that cannot be used such as a defective wafer is generated.

  Therefore, conventionally generated scrap wafers are reused as test wafers through each process of film removal, polishing, cleaning, etc., or are dissolved after film removal of scrap wafers, or raw materials for polycrystalline solar cells, or It is reused as a ceramic raw material. In addition, an example (for example, refer patent document 1, 2) of the reuse method of the silicon wafer for solar cells which cannot be used as a product is demonstrated below.

  The silicon wafer recycling method involves sorting out non-standard silicon wafers that have been generated and discarded in the semiconductor device manufacturing process by a sorting process, and shaving the surface in a chemical processing process, and a physical processing process. The surface is shaved by. Then, the surface of the silicon wafer is activated by the chemical activation process, and then dried by the microwave drying process, thereby reusing the non-standard silicon wafer as a silicon substrate for solar cells (Patent Document). 1).

  Another example of a method for reusing a silicon wafer includes a silicon wafer including a solar battery cell exhibiting a characteristic defect, an impurity layer formed on a surface layer of the silicon wafer, and a surface silver electrode film formed on the impurity layer And a step of removing the antireflection film and the front and back silver electrode films by hydrofluoric acid treatment in a solar cell comprising an antireflection film, and a backside silver electrode film and an aluminum electrode layer formed on the backside of the silicon wafer; After the step of removing the aluminum electrode layer by the hydrochloric acid treatment is performed in this order or the reverse order, the step of removing the impurity layer by the mixed solution treatment of nitric acid and hydrofluoric acid or the sodium hydroxide treatment is performed.

Then, the silicon wafer from which the impurity layer on the surface has been removed is reused after the above-described solar cells are formed again, or it is dissolved and regenerated as a polycrystalline ingot and reused again as a silicon substrate for solar cells. (Patent Document 2).
JP 2003-101056 A JP 2005-166814 A

However, the conventional method for reusing a silicon wafer has the following problems.
In the method of reusing a silicon wafer that is used after removing the surface for each sheet, for example, when reusing from a thickness of 700 μm, if the required thickness of the reclaimed wafer is 300 μm, the thickness of 400 μm is used. As a result, a lot of waste was generated.

  In addition, in the method of reusing silicon wafers that are regenerated and used in ingots, each film is removed and then melted to create and reuse a polycrystalline ingot, so it can be reused as a single crystal. There wasn't.

  The present invention has been devised in view of the above-mentioned problems, and can be reused in a state close to a single crystal by reducing the waste of resources when reusing and making a recycled ingot without melting the silicon wafer. It is an object of the present invention to provide a method for recycling a scrap wafer and a method for manufacturing a silicon substrate for a solar cell.

  In order to solve the above problems, the present invention employs the following scrap wafer recycling method. That is, the scrap wafer reuse method is a scrap wafer reuse method in which a scrap wafer that cannot be used as a regular product silicon wafer is reused as a cylindrical recycle ingot, and the film formed on the scrap wafer is removed. A film removal step, a mirror polishing step for polishing the front and back surfaces of the scrap wafer removed to a mirror surface, an alignment step for aligning the crystal orientations of the polished scrap wafer on the mirror surface and aligning them in a columnar shape, and A heating step in which the scrap wafers arranged in a columnar shape are placed in a heating furnace and heated in a range of 400 ° C. to 1350 ° C. to diffuse and bond the scrap wafers together to produce a regenerated ingot. did.

  With such a procedure, in the scrap wafer recycling method, the film of the scrap wafer is removed by mechanical or chemical treatment in the film removal process from the scrap wafer classified according to a preset standard. In the scrap wafer recycling method, the front and back surfaces of the scrap wafer are formed into mirror surfaces by a mirror polishing process, and are aligned and aligned in a cylindrical shape for each crystal orientation by an alignment process. Next, in the scrap wafer recycling method, the contact surfaces of the stacked scrap wafers are diffusion bonded to each other by putting the aligned scrap wafers in a heating furnace and heating them in the range of 400 ° C. to 1350 ° C. by a heating process. Thus, a regenerated ingot is formed to obtain a similar single crystal ingot in a state close to a single crystal.

Further, in the scrap wafer recycling method, the procedure is to apply pressure in the stacking direction of the scrap wafers between the alignment step and the heating step or during the heating step.
By such a procedure, the crystal defect of the reproduction | regeneration ingot after joining can be reduced, and it can be set as the similar single crystal ingot of the state nearer to a single crystal.

  Further, in the scrap wafer recycling method, a surface adjustment step of cleaning the surface of the scrap wafer between the mirror polishing step and the alignment step and forming an oxide film on the surface or imparting hydrophilicity to the surface. The procedure was to be performed.

  Giving hydrophilicity to the surface is done, for example, by introducing hydroxyl groups on the surface. Since the hydroxyl groups on the surface bring the silicon surfaces closer together by hydrogen bonding, silicon diffusion bonding during overheating is effective. This is effective in increasing the bonding strength. When the bonding strength increases, the state after the diffusion bonding becomes closer to a pure single crystal with few defects at the bonding interface. Further, when an oxide film is formed on the surface by a method such as thermal oxidation, it is considered that the bonding strength increases because the diffusion bonding between the silicon oxides on the surface starts at a lower temperature than the diffusion bonding of silicon. Furthermore, an effect that the oxide film absorbs impurities and reduces the impurity concentration in silicon can be expected.

The method for imparting hydrophilicity can be achieved, for example, by performing SCl cleaning. The hydrophilicity here means a state in which a functional group having a polarity such as a hydroxyl group is introduced into the surface of the scrap wafer W.
The thermal oxide film formation is a process in which a thermal oxide film is formed by heating at a predetermined temperature on the surface from which the film is removed from the scrap wafer. For example, by exposing the scrap wafer W to an atmosphere at a temperature of 850 to 1000 ° C. for a predetermined time, a thermal oxide film having a thickness of 50 nm to 200 nm (preferably 70 nm to 150 nm, more preferably 85 nm to 125 nm) is formed on the surface of the scrap wafer. (Front and back) can be formed. Furthermore, after forming the thermal oxide film, hydrophilicity can be imparted to the surface of the thermal oxide film.

Further, in the scrap wafer recycling method, before the film removal step, the size of the scrap wafer is measured and classified according to size, and whether or not a film is formed on the scrap wafer is classified. It was set as the procedure which performs the shape classification | category process to perform. Here, the size refers to the diameter of the scrap wafer.
By such a procedure, in the scrap wafer recycling method, the shape is classified according to the size of the scrap wafer and the presence / absence of a film in the shape classification process, so that it is possible to cope with the variety of scrap wafer states.

And in the scrap wafer recycling method, between the film removal step and the mirror polishing step, the scrap wafer from which the film has been removed and the mirror surface on the front and back surfaces of the scrap wafer classified as having no film in the shape classification step The mirror surface classification step of detecting the presence or absence of the mirror surface and performing classification according to the presence or absence of the mirror surface is performed, and the mirror surface polishing step is a procedure for polishing the front and back surfaces of the scrap wafer classified as having no mirror surface in the mirror surface classification step to a mirror surface .
By such a procedure, in the scrap wafer recycling method, the scrap wafer is classified according to the presence / absence of the mirror surface by the mirror surface classification process, so that the variety of scrap wafer states can be dealt with.

Further, in the scrap wafer recycling method, between the mirror polishing step and the alignment step, the scrap wafer polished to a mirror surface in the mirror polishing step and the scrap already polished to a mirror surface in the mirror surface classification step The wafer is classified according to whether it is P-type or N-type, and a performance classification step is performed in which a specific resistance is detected and classified according to a predetermined specific resistance range. The procedure for aligning the scrap wafers in a columnar shape by the alignment process was used.
By such a procedure, in the scrap wafer recycling method, the scrap wafer state can be classified by detecting and classifying whether the scrap wafer is P-type or N-type by the performance classification process and the specific resistance. Can cope with gender.

  Giving hydrophilicity to the surface is done, for example, by introducing hydroxyl groups on the surface. Since the hydroxyl groups on the surface bring the silicon surfaces closer together by hydrogen bonding, silicon diffusion bonding during overheating is effective. This is effective in increasing the bonding strength. When the bonding strength increases, the state after the diffusion bonding becomes closer to a pure single crystal with few defects at the bonding interface. Further, when an oxide film is formed on the surface by a method such as thermal oxidation, it is considered that the bonding strength increases because the diffusion bonding between the silicon oxides on the surface starts at a lower temperature than the diffusion bonding of silicon. Furthermore, an effect that the oxide film absorbs impurities and reduces the impurity concentration in silicon can be expected.

The method for imparting hydrophilicity can be achieved, for example, by performing SCl cleaning. The hydrophilicity here means a state in which a functional group having a polarity such as a hydroxyl group is introduced into the surface of the scrap wafer W.
The thermal oxide film formation is a process in which a thermal oxide film is formed by heating at a predetermined temperature on the surface from which the film is removed from the scrap wafer. For example, by exposing the scrap wafer W to an atmosphere at a temperature of 850 to 1000 ° C. for a predetermined time, a thermal oxide film having a thickness of 50 nm to 200 nm (preferably 70 nm to 150 nm, more preferably 85 nm to 125 nm) is formed on the surface of the scrap wafer. (Front and back) can be formed. Furthermore, after forming the thermal oxide film, hydrophilicity can be imparted to the surface of the thermal oxide film.

In the scrap wafer recycling method, after the heating step, a cutting step of cutting out a plate-like plate-like wafer from the recycled ingot, and flattening the plate-like wafer cut out by the cutting step, the plate And a deteriorated layer removing step of removing the work damaged layer of the wafer.
With such a procedure, the scrap wafer recycling method can cut and use a plate-like wafer from a recycled ingot by a cutting process, so that less material is wasted than when scrap wafers are reused alone. That's it.

In the scrap wafer recycling method, in the cutting step of cutting out the recycled ingot, a procedure may be used in which a plate-like wafer is cut out in a direction parallel to the axial direction of the recycled ingot.
According to such a procedure, when it is reused for an electronic device in which a current flows in a direction perpendicular to the surface of the plate-like wafer, a crystal interface and a bonding interface where an oxide film is likely to be generated are parallel between the front and back surfaces of the plate-like wafer. It is possible to prevent the characteristics of the semiconductor device from being deteriorated by being sandwiched.

  In addition, as a method for manufacturing a silicon substrate for a solar cell, a solar cell is manufactured by manufacturing a cylindrical regenerated ingot for a scrap wafer that cannot be used as a product silicon wafer, and cutting the regenerated ingot to manufacture a silicon substrate for a solar cell. In the silicon substrate manufacturing method, a film removal step for removing the film formed on the scrap wafer, a mirror polishing step for polishing the front and back surfaces from which the film of the scrap wafer was removed to a mirror surface, and the mirror surface were polished Alignment process of aligning crystal orientations of scrap wafers and aligning them in a columnar shape, and putting the scrap wafers aligned in a columnar shape into a heating furnace and heating them in a range of 400 ° C. to 1350 ° C. A heating step of manufacturing a recycled ingot by diffusion bonding the wafers; A cutting step of cutting out a plate-like plate-like wafer corresponding to the thickness of the silicon substrate for the solar cell from the base plate, and an altered layer for flattening the plate-like wafer and removing a work-affected layer of the plate-like wafer And a removal step.

  By such a procedure, in the method of manufacturing a silicon substrate for solar cells, the film is removed from the scrap wafer, polished to a mirror surface, aligned in a cylindrical shape with the crystal orientation aligned, and heated, similar to a single crystal. A single crystal regenerated ingot is manufactured, a plate-like wafer is cut out from the regenerated ingot, and the silicon substrate for solar cells is efficiently manufactured by flattening and removing the altered layer.

The scrap wafer recycling method and the solar cell silicon substrate manufacturing method according to the present invention have excellent effects as described below.
Since the scrap wafer recycling method can remove the scrap wafer film, make it a mirror surface, align it and heat it, and can create a regenerated ingot in a state close to a single crystal. It is possible to efficiently reuse the (silicon substrate for solar cell) with less waste.

  The scrap wafer recycling method can be reused as a silicon wafer in a state close to a single crystal cut out from a recycled ingot to a predetermined thickness, so it has a wide range of applications and can be reused efficiently as a large-area wafer material depending on the cutting direction. It becomes possible.

  In the scrap wafer recycling method, the surface adjustment process is further performed, so that the state becomes closer to the single crystal when the recycled ingot is manufactured. For example, the single wafer can be used as a more efficient solar cell material similar to the single crystal. . In addition, the scrap wafer recycling method can cope with the variety of scrap wafer states by performing at least one of a shape classification process, a mirror surface classification process, or a performance classification process. Furthermore, the scrap wafer recycling method reduces the crystal defects of the recycled ingot after bonding by applying pressure in the stacking direction of the scrap wafer before or during the heating process, and is closer to a single crystal. A similar single crystal ingot.

  Since the manufacturing method of the silicon substrate for solar cells manufactures a regenerated ingot, it can be effectively reused except that it corresponds to a cutting allowance for cutting the regenerated ingot, so that the recycle efficiency is increased. Further, since it can be reused almost as a single crystal, an efficient silicon substrate for solar cells can be provided.

The best mode for carrying out the present invention will be described below with reference to the drawings.
FIG. 1 is a flowchart showing the procedure of the scrap wafer reuse method, and FIG. 2 is a schematic diagram showing the procedure of the scrap wafer reuse method.

  As shown in FIGS. 1 and 2, the scrap wafer W used in the scrap wafer recycling method S is a defective silicon wafer generated in the manufacturing process or a test silicon wafer used in a test before being handled as a product. A silicon wafer that cannot be used as a regular product.

  The scrap wafer reusing method S includes a shape classification process SA (classification process by size S1, film presence / absence classification process S2), a film removal process S3, and a mirror surface classification. A similar single crystal ingot (regenerated ingot) is manufactured by the step S4, the mirror polishing step S5, the performance classification step S6, the surface adjustment step S7, the alignment step S8, and the heating step S9. In the scrap wafer recycling method S, the manufactured similar single crystal ingot is further subjected to a cutting step S10 and a deteriorated layer removing step SB (lapping step [= planarization step] S11, etching step S12), and scrap The wafer W is reused. Hereinafter, each step will be described.

  As shown in FIGS. 1 and 2, the shape classification step SA includes a film (oxide film, oxide film) formed on the surface of the scrap wafer W together with a size-specific classification step S <b> 1 that measures the outer shape of the scrap wafer W and classifies it according to size. This is a step of performing the presence / absence classification step S2 of the film classified according to the presence / absence of the surface electrode film). More specifically, the film formed on the scrap wafer W is a metal film such as Al, W, Ti, or Cu, a silicon oxide film, a silicon nitride film, a polysilicon film, or the like.

  In this shape classification step SA, either the size-specific classification step S1 for classification according to the size of the scrap wafer W or the film presence / absence classification step S2 for classification based on the presence / absence of a film formed on the surface of the scrap wafer W. You can go.

  In the shape classification process SA, when the scrap wafers W are classified by size in the size classification process S1, for example, the scrap wafer W has a diameter of 300 mm or 200 mm in most cases. In addition, 150 mm, 125 mm, Some have a diameter of 100 mm, but the number is small. Therefore, the size-specific classification step S1 is performed using the measuring means 1 in the conveyance path or at the measurement position. The measuring means 1 is, for example, for determining by measuring the size with a plurality of styluses 1a that are in contact with the side surface of the scrap wafer W.

  In the size-specific classification step S1, any means capable of measuring and classifying the outer size (diameter) of the scrap wafer W may be used. For example, the size can be determined by measuring the size of the scrap wafer W, For example, the scrap wafer W may be classified by size by scanning light (laser light) and measuring the outer diameter. In the size-specific classification step S1, the size measurement (measurement) may be performed in the conveyance path of the scrap wafer W or in the measurement location of the conveyance destination.

  The film presence / absence classification step S2 for classifying the scrap wafer W according to the presence / absence of a film in the shape classification step SA is a step of classifying the scrap wafer W based on whether or not there is a film on the front or back surface of the scrap wafer W. In the film presence / absence classification step, the detection unit 2 detects and classifies the presence / absence of a film. The detection means 2 includes, for example, a window portion 2a that can contact the placement surface of the scrap wafer W in the conveyance path or at a detection position, and the surface of the scrap wafer W from above the window portion 2a and from above the window portion. The surface resistance is measured and determined by bringing a probe (not shown) into contact with the back surface. That is, if the resistance on the front surface side of the scrap wafer W and the resistance on the rear surface side of the scrap wafer W match, it is determined that there is no film, and if the resistance does not match, it is determined that there is a film. Thus, the presence or absence of a film on the front and back surfaces of the scrap wafer W is determined. Note that light may be applied to the front and back surfaces of the scrap wafer W, and the presence or absence of a film may be determined based on a reflection state of the applied light. Further, in the film presence / absence classification step S2, the surface resistance of one surface of the scrap wafer W may be measured, and then reversed to measure the surface resistance of the other surface.

  The film removal step S3 is a step of removing the film formed on the scrap wafer W. In this film removing step S3, the film of the scrap wafer W is removed by the film removing means 3 which is mechanical means or chemical means. The film removing means 3 is, for example, a film on the surface by removing the surface film by mechanical means such as grinding, lapping (flat surface polishing), blasting, or by performing chemical etching as a chemical means. Remove. The film removal step S3 may be performed using both mechanical means and chemical means which are the film removal means 3.

Further, the removal of the film by chemical etching is, for example, hydrofluoric acid, diluted hydrofluoric acid (for example, HF: H ₂ O = 1: 100), a mixture of hydrofluoric acid and nitric acid, a mixture of nitric acid, sulfuric acid and hydrogen peroxide, hydrochloric acid Inorganic acids such as a mixture of hydrofluoric acid and hydrochloric acid, and alkaline solutions such as aqueous potassium hydroxide (sodium) solution and aqueous quaternary alkylammonium hydroxide are mainly used. Further, when the film of the scrap wafer W is an organic film such as a resist, organic amines represented by alkanolamine are also used. When removing the film, the scrap wafer W is immersed in these chemicals to dissolve or peel off various films. Note that whether or not the resist is an organic film is determined by visual appearance observation, a dissolution test in an organic solvent, and performing infrared spectroscopic analysis by collecting a part of the film.

  When dissolving or peeling the film from the scrap wafer W, the agitation and circulation of the liquid, the container in which the scrap wafer W is immersed, or the operation of vibrating the placement surface on which the scrap wafer W is placed are used in combination. It doesn't matter. Further, the film may be removed by combining a plurality of types of chemical solutions. For example, after being treated with an aqueous sodium hydroxide solution, the film of the scrap wafer W is removed by dipping in hydrofluoric acid. Here, the film removal is usually attempted by chemical means, and mechanical means are employed when the film cannot be removed by itself. If there is a trace of the pattern on the wafer surface, mechanical means are essential. Also, when multiple films with extremely different chemical properties are stacked, it is not possible to remove all the films with a single chemical solution.For example, after immersing in an organic solvent and removing the resist film, The remaining metal film and dielectric film are removed by immersion. Alternatively, when a noble metal film such as copper or silver is present, the dielectric film is removed by immersing in hydrofluoric acid after immersing in nitric acid to remove them.

  The mirror surface classification step S4 is a step of classifying the scrap wafer W depending on whether or not the mirror surface is formed on the front and back surfaces of the scrap wafer W. In this mirror surface classification step S4, the mirror surface detection means 4 detects and classifies the mirror surface. The mirror surface detection means 4 determines and classifies the presence or absence of a mirror surface by, for example, the amount of light reflected by irradiating the scrap wafer W with light. In addition, since the presence or absence of the mirror surface is determined for the front and back surfaces of the scrap wafer W, the mirror surface detection unit 4 inverts the scrap wafer W or emits light from a window portion provided so that light can be irradiated to the front and back surfaces. It is comprised so that it may irradiate.

  The mirror polishing step S5 is a step of polishing the front and back surfaces of the scrap wafer W to a mirror surface. In this mirror polishing step S5, the unpolished front and back surfaces of the scrap wafer W are polished by the polishing means 5. The polishing means 5 is lapped by, for example, a lapping apparatus and polished to a mirror surface by a polishing apparatus (grinding stone polishing, buff polishing, etc.) (same means as that for polishing a silicon wafer). Note that the polished scrap wafer W is washed to remove polishing powder and the like.

  The performance classification step S6 is a step of classifying according to the performance of the scrap wafer W. In the performance classification step S6, the classification is performed based on the determination as to whether the type is P-type or N-type and whether the value of the specific resistance is within a preset value range. The P-type and N-type determinations are made by using, for example, the thermoelectromotive force method because the silicon wafer is doped with boron, aluminum, phosphorus, arsenic, and the like. ASTM F42 (American Society for Testing and Testing Society and The method of heat transfer is measured and performed according to Materials. And it classify | categorizes into N type and P type by the measured value.

  The specific resistance is measured using a four-probe method defined by ASTM F84 or an eddy current method defined by ASTM F673. This specific resistance range is divided into ranges that can be regarded as uniform to an appropriate value according to the reuse application. For example, when it is reused for a silicon substrate for a solar cell, it is desirable that it is P type and has a specific resistance in the range of about 0.3 to 30 Ω · cm.

  The surface adjustment step S7 is a step performed to remove fine particles and metal impurities adhering to the surface (front and back surfaces) of the scrap wafer W and to increase the bonding strength of silicon. In the surface adjustment step S7, the scrap wafer W is cleaned by, for example, RCA Standard Clean (RCA cleaning). A hydrophilic group is formed on the surface of the scrap wafer W that has been subjected to RCA cleaning, which is a preferable state for diffusion bonding the scrap wafer W. In the surface adjustment step S7, an oxide film may be formed. The scrap wafer W can form an oxide film on the surface by maintaining the state heated to a predetermined temperature for a predetermined time. Furthermore, when an oxide film is formed here, hydrophilicity may be imparted to the surface by RCA cleaning of the surface.

  The alignment step S8 is a step in which the crystal orientation is aligned and aligned in a cylindrical shape by aligning with the orientation flat W0 (or notch) formed on the scrap wafer W. In the alignment step S8, the scrap wafers W are aligned and aligned until a predetermined cylinder length is reached. In the alignment step S8, for example, alignment is performed by stacking the orientation flats W0 in an aligned state with a robot hand as alignment means. In addition, when the scrap wafer W has no orientation flat W0 or notch, the crystal orientation is detected by irradiating X-rays and detecting the diffracted light, and in the direction of the detected crystal orientation. To be aligned.

  The heating step S9 is a step of heating the scrap wafers W arranged in a cylindrical shape at a predetermined temperature for a predetermined time. In this heating step S9, for example, heating is performed in the range of 400 to 1350 ° C. (the appropriate range is 500 to 1100 ° C.) for 1 to 10 hours using an atmosphere adjustment furnace. In addition, in heating process S9, you may make it pressurize, heating. When pressurizing, a method of heating while pressing with a weight placed on the scrap wafer W, a hot press apparatus, or the like is used. Alternatively, the scrap wafer W may be preliminarily pressurized in the overlapping direction immediately before performing the heating step S9.

  In heating process S9, the range of heating temperature is performed in the range of 400-1350 degreeC from the relationship between heating temperature and joining strength. When the interface of the scrap wafer W is covered with a natural oxide film, if the temperature is lower than 400 ° C., the wafer interface remains only bonded by hydrogen bonding between hydroxyl groups on the surface of the natural oxide film, and sufficient bonding strength cannot be obtained. . Further, since the melting point of silicon is 1414 ° C., if the heating temperature exceeds 1350 ° C., temperature control becomes difficult.

  Therefore, the optimum range of the heating temperature in the heating step S9 is 400 ° C. or higher although it depends on the structure and thickness of the oxide film on the wafer surface, and the lower limit of the practical range is 500 ° C. or higher here. Further, the upper limit value of the heating temperature in the heating step S9 is 1350 ° C. or less, and here, it is 1100 ° C. or less because of the heat resistance of the quartz glass material usually used for the spacer during heating. Further, although the heating time varies depending on the type and size of the furnace to be used, the thickness of the wafer to be bonded, and the diameter, it is practically 1 hour to 10 hours. The furnace atmosphere is not particularly limited as long as it is a clean atmosphere in which dust adhesion does not cause a problem. By this heating step S9, the scrap wafer W is manufactured as a cylindrical regenerated ingot IG.

  The recycled ingot IG manufactured by the heating step S9 may have a crystal interface or an oxide film formed on the bonding surface of the scrap wafer W, but all other parts are configured as single crystals. Therefore, since the thickness of the oxide film at the crystal interface of the regenerated ingot IG is several μm or less, the ratio that can be used in a single crystal state is large, and it can be used as a single crystal compared to a complete polycrystalline wafer. For example, it is excellent when it is reused as a silicon substrate for solar cells.

  The cutting step S10 is a step of cutting the manufactured regenerated ingot IG into a plate-like plate-like wafer Ws. In the cutting step S10, as a cutting means, for example, a wire wafer (wire saw) or a diamond cutter (inner peripheral cutting machine) is used to cut out the regenerated ingot IG into a plate-like wafer having a predetermined thickness. In the cutting step S10, for example, if the scrap wafer W recycled as the silicon substrate for solar cells is reused, it is cut to a thickness of 300 μm as an example.

  In addition, the cutting direction may be a direction orthogonal to the axial direction of the cylinder that is the reproduction ingot IG, or a direction parallel to the axial direction of the cylinder. The plate-like wafer Ws is formed into a rectangle (not shown) by being cut out in a direction parallel to the axial direction of the cylinder, or formed into a predetermined rectangle (not shown) by further cutting. Can do. In addition, when reused for an electronic element in which a current flows in a direction perpendicular to the surface of a plate-like wafer such as a solar cell, it is cut out in a direction parallel to the axial direction of the cylindrical ingot (not shown), and the crystal interface or It is possible to prevent a device interface from being deteriorated due to a bonding interface in which an oxide layer is easily generated being sandwiched in parallel between the front and back surfaces of a plate-like wafer.

  The deteriorated layer removing step SB is a step of performing a lapping step S11 and an etching step S12 for performing chemical cleaning after the lapping step S11 in order to flatten the surface of the cut plate-like wafer Ws. In the lapping step S11, the plate-like wafer Ws is polished (lapped) to reduce the cut strain layer, thereby reducing variations in thickness and unevenness.

The etching step S12 is a step of chemically cleaning (etching) the surface of the lapped plate-like wafer Ws. In this etching step S12, the cutting distortion layer of the plate-like wafer Ws is completely removed by chemical cleaning (etching), and at the same time, the abrasive, wax, metal impurities, and particles (attached dust) adhering to the wafer surface are removed. Yes.
Through this deteriorated layer removal step SB, the plate-like wafer Ws can be used as a similar single crystal substrate close to a single crystal.

  When the plate-like wafer Ws is used as a silicon substrate for a solar cell, an antireflection film forming step in which an antireflection film is formed is performed after the deteriorated layer removing step SB, and an electrode forming step is further performed. Is called. As the antireflection film, a SiN film is formed by p-CVD. Thereafter, an Ag paste is formed as a light-receiving surface electrode, and an Al paste is printed as a back electrode, dried and baked to form an electrode. Further, inspection of characteristics and appearance is performed, and a module manufacturing process is performed. .

In the above-described steps, here, the film removal step S3, the mirror polishing step S5, the alignment step S8, and the heating step S9 are performed by classifying the scrap wafers W according to size. Thereby, the reproduction | regeneration ingot IG can be manufactured.
And by performing surface adjustment process S7 between mirror surface polishing process S5 and alignment process S8, reproduction | regeneration ingot IG manufactured by heating process S9 can be made into a state closer to a single crystal.

  Further, before the film removal step S3, the shape classification step SA, the mirror removal step S4 between the film removal step S3 and the mirror polishing step S5, or the performance classification step S6 between the mirror polishing step S5 and the alignment step S8. By performing each independently or in combination, it becomes possible to cope with various types of scrap wafers W.

  Moreover, after manufacturing reproduction | regeneration ingot IG, it can use for a silicon substrate for solar cells as a similar single crystal substrate by performing cutting-out process S10 and altered layer removal process SB (S11, S12).

  Each of the described steps does not need to be performed continuously, and each step may be performed as a single process, and the next process may be performed by a transport mechanism (not shown) and sequentially processed. In addition, each process is performed by forming a transport line so that the scrap wafer W or the recycled ingot IG processed in each process is continuously performed through a moving unit such as a handler or a transport unit such as a conveyor. It doesn't matter.

Next, a procedure for creating a silicon substrate for solar cells by the scrap wafer recycling method S will be described with reference to FIG.
As shown in FIG. 1, first, the scrap wafer W is classified according to the wafer size by the shape classification step SA, and after each size, the scrap wafer W is classified according to the presence or absence of a film. The scrap wafer W classified as having a film formed in the film presence / absence classification step is subjected to removal of the film by mechanical means or chemical means in the film removal step S3. The scrap wafer W from which the film has been removed or the scrap wafer W classified as having no film is classified according to the presence or absence of a mirror surface. The scrap wafer W classified as having no mirror surface is subjected to mirror polishing in the mirror polishing step S5 and sent to the next step.

  In the mirror polishing step S5, it is desirable to clean after polishing. The scrap wafer W polished to a mirror surface or having a mirror surface formed in advance is classified as P-type or N-type by the performance classification step S6, and whether or not it is within a preset range by measuring specific resistance. It is classified by. Each time the scrap wafers W are classified, the surface of the scrap wafers W is cleaned or adjusted in a surface adjustment step S7, and aligned in a stacked cylindrical shape in accordance with the orientation flat W0 or notch in an alignment step S8. . Subsequently, it is put into a heating furnace by heating process S9, and it heats, pressurizing in the range of 500-1100 degreeC for 1 hour to 10 hours. And reproduction | regeneration ingot IG is manufactured by this heating process S9.

The regenerated ingot IG is cut into a plate-like wafer Ws having a predetermined thickness by the cutting step S10, flattened by the lapping step S11 and the etching step S12, and the altered layer is removed to produce a silicon substrate for solar cells. And the silicon substrate for solar cells is formed with an antireflection film in the antireflection film forming step, and further, necessary electrodes are formed when used as a solar cell in the electrode forming step. Furthermore, a module manufacturing process is performed by inspecting characteristics and appearance, and the module is used as a solar cell.
In addition, when manufacturing the reproduction | regeneration ingot IG, when performing the pressurization process which pressurizes in the stacking direction of the scrap wafer W, it may be the case where it performs between the alignment process S8 and the heating process S9, in that case Is performed by applying a predetermined pressure by a hot press apparatus.

Next, examples of the present invention will be described. In addition, this invention is not limited to this Example.
Description of Experiment Contents First, the following three types of surface adjustment were performed on a silicon wafer having both surfaces mirror-polished.

(1) The wafer was immersed in dilute hydrofluoric acid (HF: H ₂ O = 1: 100) to remove the natural oxide film on the surface (first wafer).
(2) After the treatment for removing the natural oxide film, the surface was made hydrophilic (second wafer) by SCl cleaning (H ₂ O ₂ : NH ₄ OH: H ₂ O = 1: 1: 5).
(3) After the treatment for removing the natural oxide film, a thermal oxide film having a thickness of about 100 nm is formed on the surface, and further, SCl cleaning (H ₂ O ₂ : NH ₄ OH: H ₂ O = 1: 1: 5). Then, the same SCl cleaning as in (2) above was performed to change the surface to hydrophilic (third wafer).

Next, the first wafer, the second wafer, and the third wafer that have been subjected to the processes (1) to (3) are overlapped to align the crystal orientation, and then 12 kgf / cm ² (1.18 MPa) and 60 kgf. Heat treatment was performed by heating to 300 ° C., 450 ° C., 600 ° C., 900 ° C., 1100 ° C., and 1300 ° C. in a vacuum furnace under a pressure of / cm ² (5.88 MPa) and maintaining for 2 hours.
After the heat treatment, the first wafer ingot on which the first wafer is overlaid, the second wafer ingot on which the second wafer is overlaid, and the third wafer ingot on which the third wafer is overlaid are taken out and cut perpendicularly to the bonding surface with a wire saw. The state of the joint surface was examined with an optical microscope and SEM.
The results are summarized in Table 1 below.

Evaluation Results As shown in Table 1, the cut state, optical microscope observation, and SEM observation were determined as “×”, “◯”, and “◎” according to the following criteria.

<Cutting>
×: Wafer peels off and cannot be cut into a plate shape ○: Can be cut into a plate shape and no overall peeling is observed (with partial peeling)
◎: Can be cut into a plate shape and no peeling is observed visually. <Optical microscope (optical microscope observation)>
X: Peeling is observed on all bonded surfaces. ○: Overall peeling is not observed (with partial peeling).
A: No separation is observed

<SEM (SEM observation)>
X: Peeling is observed on all bonded surfaces. ○: Overall peeling is not observed (with partial peeling).
A: No separation is observed

  As shown in Table 1, the wafer ingot heat-treated in the range of 400 to 1350 ° C. is good with little peeling at the time of cutting. Further, the second wafer ingot whose surface has been changed to hydrophilic is less peeled and better than the first wafer ingot not subjected to this treatment. Further, the third ingot, in which an oxide film is formed on the surface and subjected to the hydrophilic treatment, is less peeled and better than the second ingot.

It is a flowchart which shows the scrap wafer reuse method which concerns on this invention. It is a schematic diagram which shows the scrap wafer reuse method which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Measuring means 1a Stylus 2 Detection means 2a Window part 3 Film | membrane removal means 4 Mirror surface detection means 5 Polishing means IG Recycle ingot S Scrap wafer reuse method S1 Size classification process S2 Presence / absence classification process S3 Film removal process S4 Mirror surface classification process S5 Mirror polishing process S6 Performance classification process S7 Surface adjustment process S8 Alignment process S9 Heating process S10 Cutting process S11 Lapping process S12 Etching process SA Shape classification process SB Altered layer removal process W0 Orientation flat W Scrap wafer Ws Plate wafer

Claims (10)

For scrap wafers that cannot be used as silicon wafers, which are regular products, in a scrap wafer recycling method in which a cylindrical recycle ingot is reused,
A film removal step of removing the film formed on the scrap wafer;
A mirror polishing step of polishing the front and back surfaces of the scrap wafer film to a mirror surface;
Aligning and aligning the crystal orientation of the scrap wafer polished on the mirror surface, aligning in a cylindrical shape,
A heating step of manufacturing the recycled ingot by diffusion bonding the scrap wafers by putting the scrap wafers arranged in a columnar shape in a heating furnace and heating in a range of 400 ° C. to 1350 ° C .;
A method for recycling a scrap wafer, comprising:   2. The scrap wafer recycling method according to claim 1, wherein pressure is applied between the alignment step and the heating step or in the overlapping direction of the scrap wafers in the heating step.   2. The surface adjustment step of cleaning the surface of the scrap wafer and forming an oxide film on the surface or imparting hydrophilicity between the mirror polishing step and the alignment step is performed. The scrap wafer recycling method according to claim 2.   Before the film removal step, the size of the scrap wafer is measured and classified according to size, and a shape classification step is performed to determine whether or not a film is formed on the scrap wafer. The scrap wafer recycling method according to claim 1 or 2.   Between the film removal step and the mirror polishing step, the scrap wafer from which the film has been removed and the presence or absence of a mirror surface are detected on the front and back surfaces of the scrap wafer classified as having no film in the shape classification step. 5. The scrap according to claim 4, wherein a mirror surface classification step is performed to classify according to presence / absence, and the mirror surface polishing step polishes the front and back surfaces of the scrap wafer classified as having no mirror surface in the mirror surface classification step to a mirror surface. Wafer reuse method.   Between the mirror polishing step and the alignment step, the scrap wafer polished to the mirror surface in the mirror polishing step and the scrap wafer already polished to the mirror surface in the mirror surface classification step are P-type or N-type. And performing a performance classification process for classifying according to whether there is a specific resistance range by detecting specific resistance, and for each scrap wafer classified by the performance classification process, a circle is formed by the alignment process. The scrap wafer recycling method according to claim 4, wherein the scrap wafers are aligned in a columnar shape.   6. The surface adjustment step of cleaning the surface of the scrap wafer and forming an oxide film on the surface or imparting hydrophilicity between the performance classification step and the alignment step is performed. The scrap wafer recycling method as described. A cutting step of cutting a plate-shaped plate-shaped wafer from the regenerated ingot after the heating step;
The flattened plate-like wafer cut out by the cutting step is flattened, and a deteriorated layer removing step of removing a work-affected layer of the plate-like wafer is performed. The method for recycling a scrap wafer according to the item.   9. The scrap wafer recycling method according to claim 8, wherein in the cutting step of cutting out the recycled ingot, a plate-like wafer is cut out in a direction parallel to the axial direction of the recycled ingot. For a scrap wafer that cannot be used as a silicon wafer, which is a product, in a manufacturing method of a solar cell silicon substrate, a cylindrical regenerated ingot is manufactured, and a silicon substrate for a solar cell is manufactured by cutting out from the regenerated ingot.
A film removal step of removing the film formed on the scrap wafer;
A mirror polishing step of polishing the front and back surfaces of the scrap wafer film to a mirror surface;
Aligning and aligning the crystal orientation of the scrap wafer polished on the mirror surface, aligning in a cylindrical shape,
A heating step of manufacturing the recycled ingot by diffusion bonding the scrap wafers by putting the scrap wafers arranged in a columnar shape in a heating furnace and heating in a range of 400 ° C. to 1350 ° C .;
A cutting step of cutting a plate-shaped plate-shaped wafer corresponding to the thickness of the silicon substrate for solar cells from the regenerated ingot;
While flattening the plate-like wafer, a deteriorated layer removing step of removing a work-affected layer of the plate-like wafer;
The manufacturing method of the silicon substrate for solar cells characterized by including.

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