JP2014212231A - Forming method of silicon substrate having porous surface - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treatment method of a silicon substrate having an ant nest-like porous surface in gas phase etching using a ClF3 gas of atmospheric pressure or less.SOLUTION: A silicon substrate is exposed to a gas mixture containing ClF3 gas the dilution ratio of which is 1-20% with respect to O2 gas and N2 gas. An uneven state of 3 μm or more in depth from the surface can be controlled in 50% or more area within the substrate. A textured structure of low reflection ratio of 10% or less can be formed.

Description

  The present invention relates to, for example, a technique for treating the surface of a silicon substrate, and relates to the production of a silicon substrate for solar cells in which a concavo-convex texture that reflects sunlight is formed on the surface of the silicon substrate.

  In a silicon solar cell (photoelectric conversion element), when sunlight reaches a silicon substrate serving as a cell, the light is separated into light entering the substrate and light reflected from the substrate surface. Of these lights, only the light that enters the substrate contributes to the photovoltaic effect. Therefore, in order to reduce the reflectance on the substrate surface, a large number of uneven portions are formed in a continuous texture shape on the substrate surface. I have to.

  As conventional texturing of the silicon substrate surface, as disclosed in Patent Documents 1 and 2, when the substrate has a single crystal structure, it is formed by anisotropic etching using an alkaline aqueous solution. Anisotropic etching utilizes the difference in etching rate due to the crystal plane of silicon.

  After removing a damaged layer having a depth of about 10 to 20 μm from the surface of silicon sliced by a wire saw or the like by wet etching or the like, it is continuously wet etched with an alkaline aqueous solution such as potassium hydroxide or sodium hydroxide aqueous solution. This is a method of forming a texture by performing uneven processing on the surface of a silicon substrate.

  This etching method has become the mainstream of mass production by anisotropic processing utilizing the fact that the etching rate of single crystal silicon is higher in the crystal orientation plane (100) than in the (111) plane. However, there are disadvantages in terms of cost, such as the need for a surface cleaning process as a pretreatment and an increase in equipment size. Moreover, the silicon substrate that can be formed by the wet process is limited to a single crystal silicon substrate having a crystal plane orientation of (100), and a silicon substrate having another plane orientation cannot form a texture by the wet process.

  On the other hand, a method for forming a texture on the surface of a silicon substrate by a dry process has also been proposed.

For example, 1) a method using a method called reactive ion etching using plasma, 2) a gas of ClF ₃ , XeF ₂ , BrF _3, or BrF ₅ is introduced into a silicon substrate in a reaction chamber Thus, a method of etching the silicon substrate surface has been proposed.

  In the dry etching method such as the reactive ion etching method of 1) above, for example, a gas such as nitrogen trifluoride gas or chlorine is allowed to flow in a decompressed reaction vessel, and ions dissociated by the plasma are applied to the silicon surface. There is an RIE (reactive ion etching) method for performing etching (see Patent Documents 3 and 4).

  Further, as a method for forming a texture by dry etching without using plasma of 2) above, for example, there is a dry etching method using chlorine trifluoride gas (see Patent Documents 5 and 6). In this dry etching method, a CLF3 gas having a predetermined flow rate is introduced into a silicon substrate placed on a stage in a decompression chamber including under atmospheric pressure from a gas cylinder through a mass flow controller, and a high-vapor pressure three-fluid. This is a method for forming a texture using an exothermic reaction in which chlorine chloride gas decomposes and is exposed to a silicon substrate to react explosively with silicon.

JP 2006-344765 A JP-A-2005-311060 JP 2000-12517 A JP 2000-164555 A JP 2000-101111 A International Publication No. 2012/132433

  Currently, what is required for solar cells is high efficiency and reduced manufacturing costs. Although the structure differs depending on the solar cell manufacturer, the texture structure is an indispensable process for improving efficiency in any structure.

  As in the inventions described in Patent Documents 1 and 2, the current mainstream texture mass production method is a wet process using an alkaline aqueous solution, but removes a natural oxide film, impurities, and damage layers attached in a previous process such as a slicing process. As a pretreatment until the texture treatment with alkaline aqueous solution, etc., we have to carry out wet texture continuously after going through many complicated processes such as cleaning the substrate surface with hydrofluoric acid or pure water etc. I must.

  Furthermore, since complicated and many process managements such as strong acid and pure water are necessary continuously, there is a problem that the initial cost rises because the equipment becomes larger and more than one manufacturing equipment is required for mass production. .

  In addition, it is necessary to immerse in chemicals for a long time, and in order to maintain process reproducibility, a constant concentration is maintained, so that the running costs associated with chemical replacement costs increase and the manufacturing costs increase. Is a factor.

  In addition, the etching amount of the substrate is often required to be 20 μm or more for the texture formation by the wet. Therefore, the thickness of the substrate has to be, for example, 100 μm or more from the viewpoint of yield such as quality assurance such as cracking.

  On the other hand, in the texture forming method by reactive ion etching as in the inventions described in Patent Documents 3 and 4, high-cost members such as a vacuum pump for forming a vacuum atmosphere and a high-frequency power source for generating plasma Had a problem that required it.

  In addition, since the silicon substrate is etched with ions from which the source gas has been dissociated by the plasma process, there is no crystal plane orientation of the processed surface, and damage to the processed surface due to plasma, such as bond defects, on the surface, or When it is generated inside the substrate, there is a problem that sufficient current cannot be taken out and the efficiency is lowered.

  Further, for example, as in the invention described in Patent Document 5, the method using ClF3 gas has a small texture size by etching, and the wet process is continuously performed after dry etching, so that the visible light region of sunlight is about. It is difficult to ensure a texture size of several μm or more in order to effectively absorb the wavelength of. In addition, as in the invention described in Patent Document 6, it is difficult to control the processing of unevenness of a certain size or more in a wide range within the substrate surface, and there is a problem that the light reflectance does not decrease.

  Therefore, the present invention eliminates a natural oxide film, an impurity, and a damaged layer at the same time by optimizing the dilution ratio with the gas added to the ClF 3 gas without requiring an etching mask, and at the same time, removes a deep recess of 3 μm or more in the silicon substrate Can be formed in a wide area of 50% or more.

  This simplifies the number of processes before and after the texture process, reduces manufacturing costs without lowering production capacity, and minimizes the etching amount of the substrate to reduce material costs by making the silicon substrate thinner. It becomes possible. Another object of the present invention is to provide a silicon substrate having a concavo-convex shape that is low-reflective with respect to sunlight by securing concave portions having a depth of 3 μm or more in the silicon substrate surface at a certain density or higher. Moreover, it aims at providing the solar cell containing it.

  In order to achieve the above object, the present invention relates to a silicon substrate having a textured surface shown below and a solar cell including the same.

  The texture forming method of the present invention is a plasmaless dry etching method using ClF3 gas under a reduced pressure including atmospheric pressure, and uses an exothermic reaction between ClF3 gas and silicon to add N2 gas and O2 gas. Provided is a textured silicon substrate having a concavo-convex shape having low reflection with respect to sunlight on the surface of the processed portion of the silicon substrate by optimizing the mixing ratio.

  That is, this invention relates to the board | substrate which has a texture formation surface shown below, and a solar cell including the same.

  [1] By irradiating a silicon substrate with a mixed gas of ClF3 gas diluted to a concentration of 1% or more and 20% or less with respect to nitrogen and oxygen gas at atmospheric pressure or less, the surface of the silicon substrate is porous. The surface is roughened.

  [2] In the above [1], the roughened uneven portion having a depth of 3 μm or more has 50% or more of the processed surface.

  [3] In the above [1] or [2], the mixed gas further includes a gas containing in the molecule at least one of a nitrogen atom, an oxygen atom, and an argon atom.

  [4] In any one of the above [1] to [3], a porous uneven shape is formed on the surface by etching a plurality of times with the ClF3 gas.

  [5] In any one of the above [1] to [4], an uneven shape having a smooth surface to be processed is formed by immersing the silicon substrate in an alkaline aqueous solution.

  [6] In any one of the above [1] to [5], the silicon substrate is annealed by heating to 100 ° C. or higher in a mixed gas atmosphere containing hydrogen atoms.

  [7] In any one of the above [1] to [6], the plane orientation of the silicon substrate is a (111) plane.

  [8] In any one of the above [1] to [6], the plane orientation of the silicon substrate is a (100) plane.

  [9] In any one of [1] to [8] above, surface impurities are removed by immersing in pure water.

  [10] In the above [1], the silicon substrate is a polycrystalline silicon substrate.

  As described above, when the dry etching method using the ClF3 gas of the present invention at a reduced pressure including the atmospheric pressure is used, the deep fine texture of 3 μm or more in the region of 50% or more in the silicon substrate surface causes sunlight. A low-reflection uneven shape can be realized, and a highly efficient solar cell can be realized. In addition, since the processing on the silicon surface can be realized, the silicon substrate can be thinned in advance. In addition, since the impurity components can be removed only by washing with pure water after texturing, the number of processes can be reduced, the number of processes can be reduced without introducing expensive equipment, and the manufacturing cost can be reduced.

The figure which shows the etching flow in embodiment of this invention Device diagram showing an embodiment of the present invention Electron micrograph showing the texture structure according to the present invention Electron micrograph showing a texture structure according to a conventional example The figure which shows the relationship between the convex part density of 3 micrometers or more implemented by this invention, and a reflectance. The figure which shows the relationship between the wavelength and reflectance in this invention and a prior art example The figure which shows the effect of this invention

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing an etching flow in the embodiment of the present invention.

  FIG. 2 is a diagram of a device configuration for implementing FIG. Based on the process flow of FIG. 1, it demonstrates using FIG.

  A substrate having a plane orientation (100) plane orientation was used as the single crystal silicon to be etched. In the production of a solar cell, it is a step [1] of slicing a single crystal silicon ingot produced by a pulling method to produce a silicon wafer having a thickness of about 200 μm.

  Next, the slicing step [2] is performed. By this step, in order to remove the mechanical damage layer generated by the wire saw from the substrate surface to a depth of about 10 to 20 μm, the damage layer of about 20 μm is removed by dipping in an alkaline solution such as NaOH.

  Next, an impurity diffusion step [3] is performed. In this step [3], for example, in the case of an N-type silicon substrate, N-type P (phosphorus) is diffused into the silicon substrate using a thermal diffusion method using phosphorus oxychloride (POCl3). Next, after performing the process of removing the natural oxide film adhering to the surface of the silicon substrate that has been subjected to the impurity diffusion process, phosphorous oxide generated in the silicon substrate in the process of diffusing phosphorus into the silicon substrate or potentially existing metal Impurities such as components and organic substances are removed.

  Next, the texture process [4] is performed. The silicon substrate that has been subjected to the impurity diffusion and removal step [3] is placed on a stage with a temperature control mechanism in a vacuum vessel, and the vacuum vessel is held in vacuum. The surface of the silicon substrate is adjusted to a predetermined pressure by injecting a mixed process gas of ClF 3 gas diluted with N 2 gas and O 2 gas while maintaining the substrate temperature from room temperature to 100 ° C. or less.

  Note that an ESC (Electro Static chuck) stage equipped with a heating mechanism was used as the stage equipped with this temperature control mechanism. The reaction by this etching is isotropic etching, and the surface to be processed is made porous by irradiating the silicon substrate with a mixed gas of at least 1% or more ClF3 gas diluted to 20% or less with O2 gas and N2 gas. A texture having an uneven shape is formed.

  By making the isotropic etching process proceed dominantly, the in-plane processed area can be widened, and the result of 20% dilution rate is a porous texture having uneven portions with a depth of 3 μm or more. The region can be formed at 50% or more of the substrate surface.

  The purpose of adding O2 gas is to form an atomic silicon oxide on the surface to be processed, and the oxide bonded to the silicon itself serves as an etching mask and serves as a starting point for etching.

  Next, a silicon substrate cleaning step [5] is performed.

  This silicon substrate cleaning step is to prevent reaction with moisture in the atmosphere by removing fluorine and chlorine components remaining on the silicon surface by etching with ClF 3 gas. The removal method is performed by performing ultrasonic cleaning on pure water for several tens of minutes.

  Using a silicon substrate from which mechanical damage and impurities have been removed by etching and removing the single crystal silicon substrate sliced up from the ingot with a caustic soda (NaOH) solution, mixed with O2 gas and ClF3 gas diluted in N2 gas A texture having a porous uneven shape was formed on the surface of the silicon substrate by gas.

  The substrate used was a single crystal silicon wafer having a plane orientation plane (100) (area of substrate surface: 125 mm × 125 mm, thickness 200 μm). In order to remove the natural oxide film generated after the impurity diffusion step on the silicon surface, the natural oxide film is removed by dipping in advance in a hydrofluoric acid aqueous solution having a hydrogen fluoride concentration (1%). In this case, it is sufficient to carry out until the surface becomes hydrophobic in several tens of seconds in an aqueous solution at about room temperature.

  After placing the silicon substrate from which the natural oxide film has been removed in a reaction vessel that can be depressurized, the vacuum pump 10 is operated while the pressure gauge 8 is being monitored, and the pressure is maintained at 90 KPa with nitrogen gas. The silicon substrate 4 is transported from a mounting chamber or the like and placed on the stage 2 in the chamber 1 where the temperature can be adjusted.

  At this time, the transfer chamber is also held in advance by an inert gas. Note that nitrogen gas was selected as the inert gas. At this time, the temperature is set to room temperature by the heating mechanism provided in the stage, but it is preferable that the temperature does not exceed 100 ° C.

  When the inert gas can be sufficiently replaced with air in the reaction chamber, there is no moisture in the chamber. Therefore, after placing the silicon substrate 4 on the stage 2 of the chamber 1, the gas cylinder 5-1 , 2, 3, through the mass flow controllers 6-1, 2, 3, the flow rate is controlled at N2: 15 SLM, O2: 0.25 SLM, ClF3: 0.5 SLM, and the silicon substrate 4 from the gas introduction nozzle 7 in the chamber 1. Then, the pressure in the reaction vessel is controlled to 90 KPa by the pressure regulating valve 9 and the mixed gas is introduced for 60 seconds.

  Here, the mixture of N2 gas and ClF3 gas diluted in O2 gas is introduced from the nozzle 7 and exhausted from the vacuum pump 10 while maintaining a pressure of 90 KPa. The plurality of gas cylinders 5 are described because a plurality of types of process gases are assumed.

  The nozzle 7 has a double structure that diffuses in an inner chamber 13 that is further installed in the chamber 1, and has a structure that allows the process gas to uniformly contact the entire surface of the silicon substrate 4.

  With the flow rate of the mixed gas in the embodiment, a texture having porous irregularities is formed in the processing region of the silicon substrate having a sparse surface of 50% or more of the processing area as shown in FIG. About the mechanism which forms the texture containing ClF3 gas, it estimates as follows. ClF3 gas is very reactive with silicon and chemically reacts to become SiF4 and diffuses into the gas layer.

  At this time, it is considered that the uneven shape is caused by the etching rate ratio of the plane orientation, but we form porous unevenness on the surface when the ratio of ClF3 gas to O2 gas and N2 gas is 1% or more and 20% or less. be able to.

  Furthermore, it was confirmed that a recess having a depth of 3 μm or more from the processed surface can be formed in a region of 50% or more in the substrate surface. The light reflectance of this shape was 10% or less in the visible light region of 700 nm.

  On the other hand, when the dilution rate of the ClF 3 gas in the conventional example is 20% or more, the processing region is 50% or less in the substrate surface as shown in FIG. For example, the reflectance when the dilution rate is 25% is also 15%.

  FIG. 5 shows the results of the density within the substrate surface of the projections of 3 μm or more and the solar reflectance at a wavelength of 700 nm. When the dilution ratio of the conventional ClF3 gas is 20% or more, the in-plane density is 50% or less, and as a result of the insufficiently processed region increasing the reflectivity, the overall reflectivity in the substrate plane cannot be lowered sufficiently. The solar light absorption effect was not obtained, and it was difficult to obtain a sufficient effect for extracting current as a battery device.

  Under the processing conditions with a dilution rate of ClF3 gas of 20% or less in the present invention, as described above, the processing region of the uneven portion of 3 μm or more becomes 50% or more, and the overall reflectance within the substrate surface is suppressed to 10% or less. Can do. As a result, the effect of increasing the short-circuit current as a solar cell can be expected, and the improvement in power generation efficiency can be expected.

  The mainstream of the current mass production texture process is the wet method, but as shown in FIG. 6, in the gas etching using ClF3 gas, the reflectivity is about 11% in the wavelength region at 700 nm when the dilution rate is 20% or more. However, in this example, the effect of suppressing to 50% or less of the conventional value of 6% under the condition of the dilution rate: 10% was obtained.

  As shown in FIG. 7, the dilution rate of the ClF 3 gas was compared with 10% and 20% that were performed this time, and 25% and 30% of the conventional example. As a result, when the dilution rate is 20% or less, the processing region in the substrate surface is 50% or more, and when the dilution rate is 10%, an uneven region of 3 μm or more is obtained in 70% of the substrate surface. It was also found that the reflectance at a wavelength of 700 nm was a minimum of 6%. By processing at a dilution rate equal to or less than that of the present embodiment, it is possible to further maximize the processing area and further reduce the reflectance.

  Thus, by making ClF3 gas 1% or more and 20% or less with respect to N2 gas and O2 gas, the processing area by etching increases because the density of atomic oxide formation on the silicon processing surface increases. Therefore, a relatively shallow unevenness having a depth of 3 μm or more can be processed in 50% or more of the substrate surface.

  Here, the flow rate ratio between the ClF 3 gas and the oxygen gas to be added must be optimally controlled. When the atmospheric pressure is maintained in the chamber 1 with air, the amount of oxygen in the air is unstable, and the apparent flow rate ratio and the actual flow rate ratio are different, so that a desired texture may not be formed. Also in this sense, it is important to hold the inside of the chamber with an inert gas in advance. If ClF3 and moisture in the air chemically react during this process, the substrate surface temperature rises due to explosive reaction with ClF3, making it difficult to control the optimal texture shape. It is also conceivable that hydrofluoric acid (HF) harmful to the human body is generated and adheres to the members in the chamber, causing harm to the human body during apparatus maintenance.

  The texture formation method by dry etching using ClF3 gas of the present invention under atmospheric pressure not only generates a harmful substance to the human body such as hydrofluoric acid, but also etches with a mixed gas of ClF3 gas and oxygen. Can also be expected to easily control the gas ratio. This technique can be applied not only to the texture forming method but also to all processing applications using ClF3.

DESCRIPTION OF SYMBOLS 1 Chamber 2 Stage 4 Silicon substrate 5 Gas cylinder 6 Mass flow controller 7 Nozzle 8 Pressure gauge 9 Pressure control valve 10 Vacuum pump 13 Inner chamber

Claims (10)

By irradiating the silicon substrate with a mixed gas of ClF3 gas diluted to a concentration of 1% or more and 20% or less with respect to nitrogen and oxygen gas at atmospheric pressure or less, the surface of the silicon substrate is porously roughened A method for forming a silicon substrate having a porous shape on a surface thereof, wherein irregularities are formed. The method for forming a silicon substrate having a porous shape on a surface according to claim 1, wherein the roughened uneven portion having a depth of 3 µm or more has 50% or more of the processed surface. The mixed gas further includes a gas containing at least one of a nitrogen atom and an oxygen atom or an argon atom in the molecule.
A method for forming a silicon substrate having a porous shape on a surface according to claim 1. The method for forming a silicon substrate having a porous shape on a surface according to any one of claims 1 to 3, wherein a porous uneven shape is formed on the surface by etching a plurality of times with the ClF3 gas. The method for forming a silicon substrate having a porous shape on a surface according to any one of claims 1 to 4, wherein an uneven shape having a smooth surface to be processed is formed by immersing the substrate in an alkaline aqueous solution. . The method for forming a silicon substrate having a porous shape on a surface according to any one of claims 1 to 5, wherein the silicon substrate is annealed by heating to 100 ° C or higher in a mixed gas atmosphere containing hydrogen atoms. The method for forming a silicon substrate having a porous shape on a surface according to any one of claims 1 to 6, wherein a plane orientation of the silicon substrate is a (111) plane. The method for forming a silicon substrate having a porous shape on a surface according to any one of claims 1 to 6, wherein a plane orientation of the silicon substrate is a (100) plane. The method for forming a silicon substrate having a porous shape on a surface according to any one of claims 1 to 8, wherein impurities on the surface are removed by immersion in pure water. The method for forming a silicon substrate having a porous shape on a surface according to claim 1, wherein the silicon substrate is a polycrystalline silicon substrate.

Images