JP2005303255A - Low-reflectance processing method of silicon substrate for solar cells - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-reflectance processing method of a silicon substrate for solar cells with high productivity capable of reducing a reflectance by stably forming minute recesses and projections on the surface of the silicon substrate of a single crystal or polycrystal used for solar cells. <P>SOLUTION: A method of processing the surface of a silicon substrate of a single crystal or polycrystal used for solar cells with a low reflectance has a first process of performing a primary etching treatment of forming recesses and projections on the surface of the silicon substrate by using an aqueous solution of acid or alkali, and a second process of performing a secondary etching treatment of forming more minute recesses and projections by plasma etching on the surface of the silicon substrate that has been treated with the primary etching. For plasma etching, radicals and ions formed by the plasma generated by dielectric barrier discharge in a reaction gas are used. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a low-reflectance processing method for forming irregularities by etching to reduce the reflectance of the surface of a silicon substrate for solar cells.

In a silicon substrate used for a solar cell, it is very important to improve the characteristics of the solar cell by reducing the reflectance of the surface of the silicon substrate that absorbs sunlight and increasing the light absorption efficiency. As a method for increasing the light absorption efficiency, for example, fine irregularities are formed (roughened) on the surface of a silicon substrate.
Therefore, in Patent Document 1, the surface of the silicon substrate is covered with a mask pattern, and immersed in a mixed acid aqueous solution of concentrated hydrofluoric acid, concentrated nitric acid, and concentrated acetic acid, and etched to form a cylindrical shape on the surface layer portion of the silicon substrate. A method for forming a recess is described. Patent Document 2 describes a method of forming fine irregularities on the surface of the silicon substrate by etching the surface of the silicon substrate using a mixed acid aqueous solution and an alkaline aqueous solution.
Furthermore, Patent Document 3 discloses that a mask fine particle is dispersed and attached to the entire surface of a polycrystalline silicon substrate, and an area where the mask fine particle is not attached is etched (for example, dry etching using fluorine gas or chlorine gas). Alternatively, a method of forming fine irregularities on the surface of the silicon substrate by removing the fine particles for masking from the surface of the silicon substrate after wet etching using a mixed acid aqueous solution of hydrofluoric acid and nitric acid) has been proposed.

Japanese Patent Laid-Open No. 1-111887 JP-A-10-303443 JP 2000-261008 A

However, in the methods described in Patent Documents 1 and 3, the mask forming process must be performed before etching the surface of the silicon substrate, and the mask removing process must be performed after the etching process, which makes the processing method complicated. There was a problem. Moreover, in the method described in Patent Document 2, since it is necessary to use a mixed acid aqueous solution and an alkaline aqueous solution continuously, there is a problem in that various restrictions are likely to occur in the storage of these etching solutions and the operation of the etching process. .
Furthermore, the formation of fine irregularities on the surface of the silicon substrate using the etching process described in Patent Documents 1 to 3 utilizes the fact that the dissolution rate of silicon in the etching solution varies depending on the crystal orientation of silicon. In addition to the fact that the interval between the formed irregularities is limited to about several to 100 μm, there is a problem that irregularities can be formed only partially on the surface of a polycrystalline silicon substrate having no uniform crystal axis orientation.

The present invention has been made in view of such circumstances, and is capable of reducing the reflectance by stably forming fine irregularities on the surface of a single crystal or polycrystalline silicon substrate used for solar cells. An object of the present invention is to provide a low reflectivity processing method for a silicon substrate for a solar cell with productivity.

The low reflectance processing method of the silicon substrate for solar cells according to claim 1, which meets the object, is a method of processing the surface of a single crystal or polycrystalline silicon substrate used for solar cells to a low reflectance.
A first step of performing a primary etching process for forming irregularities on the surface of the silicon substrate using an acid or alkali aqueous solution;
And a second step of performing a secondary etching process for forming finer irregularities by plasma etching on the surface of the silicon substrate that has been subjected to the primary etching process.

By a primary etching process using an aqueous solution of acid or alkali, irregularities (primary texture structure) depending on the crystal orientation of silicon can be formed on the surface of the silicon substrate.
In the secondary etching plasma etching, radicals and ions are generated by the plasma, and the radicals react with the silicon substrate to generate volatile substances, and the ions collide with the silicon substrate at an accelerated rate. Physical etching takes place utilizing the physical removal of the atoms that occur.
Thereby, it is possible to form finer unevenness (fine texture structure) on each uneven surface of the unevenness (primary texture structure) formed by the primary etching. As a result, by combining the primary etching process with the secondary etching process, it is possible to form a texture structure with a high reflectance reduction effect in which a fine texture structure is incorporated in the primary texture structure formed on the surface of the silicon substrate.

The low reflectance processing method for a solar cell silicon substrate according to claim 2 is the low reflectance processing method for a solar cell silicon substrate according to claim 1, wherein the plasma etching is performed by a dielectric barrier discharge in a reactive gas. Uses the plasma generated by
In dielectric barrier discharge, a dielectric material is disposed on one side or both sides of an opposing electrode to generate a discharge. At this time, a discharge column is generated between the electrodes due to the presence of the dielectric material. This discharge column becomes a plasma source, and by supplying a reaction gas between the electrodes, radicals and ions can be generated from the reaction gas. Therefore, when the silicon substrate subjected to the primary etching process is disposed on the ground electrode, the silicon substrate can be etched by the generated radicals and ions.

The low reflectance processing method for a solar cell silicon substrate according to claim 3 is the low reflectance processing method for a solar cell silicon substrate according to claim 2, wherein the pressure of the reaction gas is substantially atmospheric pressure.
By setting the pressure of the reaction gas to substantially atmospheric pressure, the operation of maintaining the inside of the chamber for generating the dielectric barrier discharge in a reduced pressure state can be omitted. Here, atmospheric pressure refers to, for example, 80 to 120 kPa, preferably 90 to 110 kPa.

The low reflectance processing method for a solar cell silicon substrate according to claim 4 is the low reflectance processing method for a solar cell silicon substrate according to claim 2 or 3, wherein the reactive gas is carbon tetrafluoride gas or hexafluorocarbon. It is sulfurized gas.
When carbon tetrafluoride (CF ₄ ) gas or sulfur hexafluoride (SF ₆ ) gas is used, fluorine radicals (F ^* ) are formed by plasma, and these fluorine radicals chemically react with silicon to produce SiF ₄ . . The produced SiF ₄ is vaporized and removed from the silicon substrate, and chemical etching proceeds on the surface of the silicon substrate. Moreover, the effect of the physical etching of the silicon substrate surface by the simultaneously generated ions can be expected.

The low reflectance processing method of the silicon substrate for solar cells according to claim 5 is the low reflectance processing method of the silicon substrate for solar cells according to claims 2 and 3, wherein the reaction gas is a mixture of carbon tetrafluoride and oxygen. It is a gas or a mixed gas of sulfur hexafluoride and oxygen. When oxygen is mixed in the reaction gas, the anisotropy of plasma etching can be increased.
The low reflectivity processing method for a solar cell silicon substrate according to claim 6 is the low reflectivity processing method for a solar cell silicon substrate according to claim 4 or 5, wherein the dielectric barrier discharge has a frequency of 5 kHz or more. This is performed under the condition that a voltage of 3.5 kV or more and 7.5 kV or less, which is 50 kHz or less, is applied for a time of 60 seconds or more and 600 seconds or less.
By performing dielectric barrier discharge under such conditions, it is possible to efficiently generate radicals and ions from the reaction gas with plasma, and to effectively perform the secondary etching process. Here, the reason why the frequency is set to 5 kHz or more is to stabilize the discharge and efficiently perform plasma etching, and the frequency is set to 50 kHz or less to suppress the temperature rise of the plasma. As a result, it is possible to prevent the life of the electrodes from being shortened and the structure of the apparatus from becoming complicated due to the temperature rise of the plasma. The voltage is set to 3.5 kV or higher because it is necessary for the dielectric to cause dielectric breakdown and discharge, and the voltage is set to 7.5 kV or lower to continuously generate stable discharge. is there. Furthermore, the time was set to 60 seconds or more to form fine unevenness on the entire silicon substrate, and the time was set to 600 seconds or less because the uneven shape formed by the primary etching disappeared by plasma etching. This is to prevent it from falling out.

The low reflectance processing method of the silicon substrate for solar cells according to claim 7 is the low reflectance processing method of the silicon substrate for solar cells according to claims 2 and 3, wherein the reaction gas is inert with carbon tetrafluoride gas. A mixed gas of gas, or a mixed gas of sulfur hexafluoride gas and inert gas.
The low reflectance processing method of the silicon substrate for solar cells according to claim 8 is the low reflectance processing method of the silicon substrate for solar cells according to claims 2 and 3, wherein the reaction gas is carbon tetrafluoride gas, oxygen gas. , And an inert gas, or a mixed gas of sulfur hexafluoride gas, oxygen gas, and inert gas.
Here, the inert gas refers to a gas of group 0 of the periodic table such as helium gas or argon gas. By making the reaction gas a mixed gas containing an inert gas, carbon tetrafluoride gas (carbon tetrafluoride gas and oxygen gas) or sulfur hexafluoride gas (sulfur hexafluoride gas and oxygen gas) is inert. Even if the gas can be diluted with gas and a flow velocity distribution exists in the reaction gas and stagnation occurs, the difference in distribution of fluorine radicals (fluorine radicals and oxygen) formed by plasma can be reduced.

The low reflectivity processing method for a solar cell silicon substrate according to claim 9 is the low reflectivity processing method for a solar cell silicon substrate according to claim 7 or 8, wherein the dielectric barrier discharge has a frequency of 5 kHz or more. It is performed under the condition that a voltage of 1 kV or more and 7.5 kV or less, which is 50 kHz or less, is applied for a period of 60 seconds or more and 600 seconds or less. By containing an inert gas in the reaction gas, a dielectric discharge can be caused to occur at a low voltage and a stable discharge can be continuously generated. Here, the voltage is set to 1 kV or more, which is necessary for the dielectric to cause dielectric breakdown and discharge, and the voltage is set to 7.5 kV or less to continuously generate stable discharge. . The reason why the time was set to 60 seconds or more is to form fine irregularities on the entire silicon substrate, and the time was set to 600 seconds or less because the irregularities formed by the primary etching disappeared by plasma etching. This is to prevent it from falling out.

In the low reflectance processing method for a solar cell silicon substrate according to the present invention, the plasma etching can also use plasma generated by discharge in a reaction gas in a pressure range of 1 Pa to 200 Pa. . By setting the pressure of the reaction gas to 1 Pa or more and 200 Pa or less, the discharge can be generated stably and uniformly. Here, the pressure is set to 1 Pa or more is necessary to generate discharge, and the pressure is set to 200 Pa or less to continuously generate stable discharge.

Moreover, in the low reflectance processing method of the silicon substrate for solar cells of the present invention, before performing the plasma etching, dry etching is performed by oxygen plasma generated by discharge in an oxygen gas having a pressure of 1 Pa to 200 Pa. Also good. Thereby, fine irregularities can be formed on the surface of the silicon substrate formed by the primary etching before the secondary etching process. Note that the pressure is set to 1 Pa or more in order to generate oxygen plasma by discharge, and the pressure is set to 200 Pa or less in order to maintain stable discharge and maintain the generation of oxygen plasma.

Furthermore, in the low reflectance processing method of the silicon substrate for solar cells of the present invention, it is preferable to use carbon tetrafluoride gas or sulfur hexafluoride gas as the reaction gas. As a result, fluorine radicals and carbon tetrafluoride or sulfur hexafluoride ions can be generated, and SiF ₄ generated by the chemical reaction between the fluorine radicals and silicon is vaporized, thereby chemically etching the surface of the silicon substrate. It can be expected to cause physical etching of the silicon substrate surface by ions.
Moreover, in the low reflectance processing method of the silicon substrate for solar cells of the present invention, the reaction gas may be a mixed gas of carbon tetrafluoride and oxygen, or a mixed gas of sulfur hexafluoride and oxygen. Thus, when oxygen is mixed in the reaction gas, the anisotropy of plasma etching can be increased.

And in the low-reflectance processing method of the silicon substrate for solar cells of this invention, as mentioned above, when the pressure of the reactive gas is lowered, the frequency is 10 MHz or more and 50 MHz or less, and the power is 50 watts or more and 500 watts. It is recommended to use the following high frequency power supply.
Thus, the density of plasma generated can be kept high by setting the frequency to 10 MHz to 50 MHz and the power to 50 watts to 500 watts, and high-speed etching can be performed. Here, if the frequency is less than 10 MHz, the density of the generated plasma is low and the etching rate is low, which is not preferable. On the other hand, if the frequency exceeds 50 MHz, it is difficult to uniformly generate a discharge over a wide area, and the manufacturing cost of the device increases, which is not preferable. If the power of the high frequency power source is less than 50 watts, the density of the generated plasma is low and the etching rate is low, which is not preferable. On the other hand, if the power exceeds 500 watts, the self-bias increases, and the influence of ions in etching becomes relatively large, making it difficult to control etching. For this reason, electric power was 50 watts or more and 500 watts or less.

In the low reflectance processing method of the silicon substrate for solar cells according to claim 1, since the secondary etching process by plasma etching is performed on the surface of the silicon substrate subjected to the primary etching process using an acid or alkali aqueous solution. A texture structure with a high reflectivity reduction effect can be formed on the surface of the silicon substrate by incorporating a fine texture structure formed by the secondary etching process in the primary texture structure formed by the primary etching process. A single crystal or polycrystalline silicon substrate with greatly reduced mass production is high and it is possible to manufacture it stably. Therefore, by using this silicon substrate, a solar cell with higher efficiency than before can be provided, which is economical.

In particular, in the low reflectance processing method for a solar cell silicon substrate according to claim 2, plasma etching uses plasma generated by dielectric barrier discharge in a reaction gas, and therefore is generated by dielectric barrier discharge. Radicals and ions, which are reactive species, are formed from the reaction gas by the plasma, and the surface of the silicon substrate is chemically etched by the radicals, and the surface of the silicon substrate is physically etched by the ions. Unevenness can be formed.

In the low reflectance processing method for a silicon substrate for a solar cell according to claim 3, since the pressure of the reaction gas is substantially atmospheric pressure, the operation of maintaining the inside of the chamber for generating the dielectric barrier discharge in a reduced pressure state is performed. It can be omitted, and productivity can be improved. In particular, when processing a large amount of silicon substrates, it is necessary to increase the volume of the chamber, but it is not necessary to keep the inside of the chamber in a reduced pressure state, so that the processing time can be greatly shortened. Furthermore, since the chamber does not need to have a reduced pressure durability specification, the manufacturing cost of the chamber can be reduced.

In the low reflectance processing method of the silicon substrate for solar cells according to claim 4, since the reaction gas is a carbon tetrafluoride gas or a sulfur hexafluoride gas, the fluorine radical and silicon react to produce SiF ₄ . Therefore, it is possible to add chemical etching and promote the formation of fine irregularities.

In the low reflectance processing method for a solar cell silicon substrate according to claim 5, since the reaction gas is a mixed gas of carbon tetrafluoride and oxygen or a mixed gas of sulfur hexafluoride and oxygen, It is possible to increase the directivity, and it is possible to promote the formation of finer irregularities by adjusting the mixing ratio of oxygen.
In the low reflectance processing method of the silicon substrate for solar cells according to claim 6, the dielectric barrier discharge is performed at a voltage of 3.5 kV or more and 7.5 kV or less having a frequency of 5 kHz or more and 50 kHz or less for 60 seconds or more. Since it is performed under the condition of applying a time of 600 seconds or less, it is possible to efficiently generate radicals and ions from the reaction gas by plasma and effectively perform the secondary etching process, and to form fine irregularities. It becomes possible to promote.

In the low reflectance processing method of the silicon substrate for solar cells according to claim 7, the reaction gas is a mixed gas of carbon tetrafluoride gas and an inert gas, or a mixed gas of sulfur hexafluoride gas and an inert gas. Therefore, carbon tetrafluoride gas or sulfur hexafluoride gas can be diluted with an inert gas to reduce the difference in the distribution of fluorine radicals, and chemical etching progresses uniformly on the surface of the silicon substrate. Thus, it is possible to promote the uniform formation of fine irregularities.

In the low reflectance processing method of the silicon substrate for solar cells according to claim 8, the reaction gas is a mixed gas of carbon tetrafluoride gas, oxygen gas, and inert gas, or sulfur hexafluoride gas, oxygen gas, and Since it is a mixed gas of inert gas, carbon tetrafluoride gas and oxygen gas or sulfur hexafluoride gas and oxygen gas can be diluted with inert gas to reduce the difference in distribution of fluorine radicals and oxygen, It becomes possible to promote the uniform formation of fine irregularities due to the anisotropy of plasma etching on the surface of the silicon substrate.

10. The low reflectance processing method for a silicon substrate for a solar cell according to claim 9, wherein the dielectric barrier discharge is performed at a frequency of 1 kHz to 7.5 kV with a frequency of 5 kHz or more and 50 kHz or less for 60 seconds to 600 seconds. Since it is performed under the conditions of applying for the following time, dielectric breakdown can be caused in the dielectric at low voltage, and stable discharge can be continuously generated, and fine irregularities can be easily formed by secondary etching treatment Is possible.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.
Here, FIG. 1 is an explanatory diagram of a low-reflectivity processing facility used in the low-reflectance processing method for a solar cell silicon substrate according to the first embodiment of the present invention, and FIG. 2 is an illustration of the solar cell silicon substrate. FIG. 3 is an explanatory view of a primary etching processing apparatus that performs a primary etching process of a low reflectance processing method, FIG. 3 is an explanatory view of a dielectric barrier discharge apparatus that performs a secondary etching process of the low reflectance processing method of the silicon substrate for the solar cell, FIG. 4 is an explanatory view showing the structure of the electrode portion of the dielectric barrier discharge device, and FIG. 5 shows the secondary etching process of the low reflectance processing method for the solar cell silicon substrate according to the second embodiment of the present invention. FIG. 6 is an explanatory view of a dielectric barrier discharge device to be performed, FIG. 6 is an explanatory view of a low reflectivity processing facility used for a low reflectivity processing method of a silicon substrate for solar cells according to a third embodiment of the present invention, and FIG. Low resistance of silicon substrate for solar cell It is an explanatory view of a discharge device for performing secondary etching rate processing methods.

As shown in FIG. 1, a low reflectance processing facility 10 used in a low reflectance processing method for a solar cell silicon substrate according to a first embodiment of the present invention is applied to a solar cell loaded on a substrate holder 11. A primary etching processing apparatus 12 for performing a primary etching process on a plurality of unprocessed single crystal or polycrystalline silicon substrates A to be used, and further fine irregularities are formed on the surface of the silicon substrate C after the primary etching process by plasma etching. It has a dielectric barrier discharge device 13 that performs the secondary etching process to be formed. These will be described in detail below.

As shown in FIG. 2, the primary etching processing apparatus 12 includes a degreasing tank 14 for storing an organic solvent used for a degreasing process for immersing the silicon substrate A loaded on the substrate holder 11 to remove oils and fats attached to the surface. And a dryer 15 that dries the silicon substrate A pulled up from the degreasing tank 14. As the dryer 15, for example, a type that dries the silicon substrate A using infrared rays or vacuum can be used.
Here, as the substrate holder 11, for example, a cage-like container made of ethylene tetrafluoride that shows corrosion resistance against acid or alkali without using a silicon substrate can be used.

The primary etching processing apparatus 12 is an example of a processing solution used for a surface layer removing process for immersing the degreased silicon substrate A to dissolve and remove the altered layer and the contaminated layer formed on the surface layer portion. An alkali tank 16 for storing a sodium hydroxide aqueous solution or a potassium hydroxide aqueous solution and a silicon substrate B pulled up from the alkali tank 16 and subjected to surface layer removal treatment are immersed to remove the surface residue (alkaline aqueous solution) by washing with water. A washing tank 17 and a dryer 18 for drying the silicon substrate B pulled up from the washing tank 17 are provided.
The alkaline tank 16 and the washing tank 17 are provided with a stored alkaline aqueous solution and a heater for heating water. Thereby, the surface layer removal process and the water washing process can be performed efficiently, and the silicon substrate B after the water washing process can be easily dried. Here, as the dryer 18, for example, a type that dries the silicon substrate B using infrared rays or vacuum can be used.

Further, the primary etching processing apparatus 12 is a mixture of isopropyl alcohol and an aqueous alkali solution such as an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution that immerses the dried silicon substrate B from the washing bath 17 to form irregularities on the surface thereof. A primary etching treatment tank 19 for performing a primary etching process using a liquid, and a water washing for removing residues (alkaline aqueous solution) on the surface of the silicon substrate C that has been pulled up from the primary etching treatment tank 19 and subjected to the primary etching process (alkali etching process) The tank 20 and the dryer 21 which dries the water adhering to the surface of the silicon substrate C pulled up from the washing tank 20 are provided.
Here, the primary etching treatment tank 19 and the rinsing tank 20 are provided with a stored alkaline aqueous solution and a heater for heating water. Thereby, a primary etching process and a water washing process can be performed efficiently. Moreover, the silicon substrate C after washing with water can be easily dried. Furthermore, the dryer 21 can be of a type that dries the silicon substrate C using infrared rays or vacuum, for example.

As shown in FIG. 3, the dielectric barrier discharge device 13 is provided so as to be opposed to the fixedly arranged high voltage electrode portion 22 and the lower portion of the high voltage electrode portion 22, and moves up and down with the silicon substrate C mounted thereon. A ground electrode portion 23 and a chamber 24 surrounding and sealing the high voltage electrode portion 22 and the ground electrode portion 23 are provided. In addition, the dielectric barrier discharge device 13 includes a high-frequency high-voltage power supply 25 that applies a high-frequency high voltage to the high-voltage electrode unit 22, and a reaction gas supply port 26 that is a mixed gas of sulfur hexafluoride and oxygen, which is an example of a reaction gas. A reaction gas supply means 27 for supplying the gas into the chamber 24 via the exhaust, an exhaust means 29 for exhausting the gas in the chamber 24 via the exhaust port 28, and an elevating means 30 for raising and lowering the ground electrode portion 23. .
With such a configuration, the ground electrode portion 23 on which the silicon substrate C is placed can be brought close to the high voltage electrode portion 22 to form a discharge space.

As shown in FIG. 4, the high-voltage electrode portion 22 is in contact with the introduction electrode 33 made of metal (for example, aluminum or stainless steel) whose base portion 32 is covered with an insulator 31 and the tip of the introduction electrode 33. It has a plate-like discharge electrode 34 formed of (for example, quartz glass or alumina). Further, for example, a stainless steel ring-shaped reaction gas flow passage 35 is formed around the introduction electrode 33, and the upstream side of the introduction passage 35 a branched from the reaction gas flow passage 35 is supplied with a reaction gas. It is connected to the mouth 26. Here, a plurality of gas discharge ports 36 are provided in the reaction gas flow passage 35, and a water cooling jacket 37 is provided on the proximal end side of the introduction electrode 33.
With such a configuration, the reaction gas supplied from the reaction gas supply means 27 can be caused to flow into the chamber 24 from the plurality of gas discharge ports 36 and be uniformly discharged into the discharge space.

The ground electrode portion 23 is provided on a metal (for example, aluminum) ground electrode 38 whose base portion 32a is covered with an insulator 31, and is provided on the outer peripheral side of the ground electrode 38. The upper end of the silicon substrate C and the high voltage electrode portion The spacer 39 has a spacer 39 for adjusting the distance (gap length) to the lower end of the 22 discharge electrodes 34 within a range of 0.1 to 2 mm. A water cooling jacket 40 is provided on the base end side of the ground electrode 38, and the ground electrode 38 is connected to the lifting / lowering means 30 via the water cooling jacket 40.
With such a configuration, by driving the elevating means 30, the ground electrode portion 23 is raised, and the upper end of the spacer 39 provided on the outer peripheral side of the ground electrode 38 is the discharge electrode 34 of the high voltage electrode portion 22. The rise of the ground electrode portion 23 can be stopped at the time when it comes into contact with the lower surface of the electrode. As a result, the height of the discharge space can always be made constant.

The reaction gas supply means 27 is connected to a sulfur hexafluoride cylinder to extract sulfur hexafluoride gas at a predetermined pressure at a predetermined flow rate, for example, a first gas regulator including a pressure setting device and a flow meter, Connected to an oxygen cylinder to extract oxygen gas at a predetermined pressure at a predetermined flow rate, for example, a second gas regulator having a pressure setter and a flow meter, and exhausted from the first and second gas regulators It has a gas mixer that mixes each gas. By setting it as such a structure, the mixed gas of various pressures which sulfur hexafluoride and oxygen mixed by arbitrary ratios can be obtained easily.
Further, for example, a vacuum pump can be used as the exhaust means 29 connected to the exhaust port 28, and a moving mechanism using a ball screw can be adopted as the elevating means 30.

Next, the low-reflectance processing method of the silicon substrate for solar cells according to the first embodiment of the present invention will be described. Here, the low reflectance processing method of the silicon substrate for solar cells includes a first step of performing a primary etching process for forming irregularities on the surface of the untreated silicon substrate A using an alkaline aqueous solution, and silicon subjected to the primary etching process. There is a second step of performing a secondary etching process for forming finer irregularities by plasma etching on the surface of the substrate C. This will be described in detail below.
First, the first step is a degreasing process for removing oils and fats adhering to the untreated silicon substrate A, a surface layer removing process for obtaining a silicon substrate B from which the altered layer and the contaminated layer existing in the surface layer part are uniformly removed, silicon An alkali etching process for obtaining a silicon substrate C having irregularities formed on the surface from the substrate B, and a water washing process for removing an alkaline aqueous solution attached on the silicon substrate C are provided.

First, as a degreasing process, the silicon substrate A set in the substrate holder 11 is immersed in a degreasing tank 14 in which an organic solvent is stored.
Here, examples of the organic solvent include ketone solvents such as acetone and methyl ethyl ketone, alcohol solvents such as ethanol and methanol, and halogen solvents such as dichloromethane and chloroform. Of these, acetone is particularly preferred.
After the degreasing process, the organic solvent adhering to the surface of the silicon substrate A in the state of being set on the substrate holder 11 is charged into the dryer 15 and dried to be removed, followed by a surface layer removing process.

In the surface layer removal process, a plurality of silicon substrates A mounted on the substrate holder 11 are immersed in, for example, an alkaline tank 16 in which 3 to 5% by weight of an aqueous sodium hydroxide solution is stored. As a result, the altered layer and the contaminated layer in the surface layer portion of the silicon substrate A are eluted in the sodium hydroxide aqueous solution. In addition, the temperature of the sodium hydroxide aqueous solution to be used is 70-90 degreeC, Preferably it is 75-85 degreeC. The processing time is 30 to 300 seconds, preferably 60 to 120 seconds.
Then, the substrate holder 11 mounted with the silicon substrate B subjected to the surface layer removal treatment is pulled up from the alkali tank 16 and immediately immersed in the water washing tank 17 to remove the sodium hydroxide aqueous solution on the surface of the silicon substrate B. Next, the substrate holder 11 is pulled up from the rinsing tank 17 and inserted into the dryer 18 to dry the substrate holder 11 and the silicon substrate B.

As the alkali etching treatment, the silicon substrate B set in the substrate holder 11 is immersed in a primary etching treatment tank 19 in which an alkaline aqueous solution is stored. Here, as the alkaline aqueous solution, for example, 1 to 3 wt% aqueous sodium hydroxide solution heated to 75 to 85 ° C., 1 to 3 wt% potassium hydroxide aqueous solution heated to 75 to 85 ° C., Alternatively, a mixed solution in which isopropyl alcohol is added to an alkaline aqueous solution (the mixing volume ratio is, for example, 1 to 10 isopropyl alcohol with respect to the alkaline aqueous solution 100) can be used. The treatment time is 10 to 60 minutes, preferably 20 to 40 minutes.

The substrate holder 11 on which the alkali-etched silicon substrate C is mounted is pulled up from the primary etching treatment tank 19 and immediately immersed in the washing tank 20 to be washed with water, and the aqueous alkali solution on the surface of the silicon substrate C is removed. Here, the reason why the silicon substrate C is washed with water promptly after the alkali etching treatment is to prevent non-uniform etching from proceeding to the surface layer portion of the silicon substrate C. For this reason, the water washing treatment is preferably performed within 30 seconds, preferably within 10 seconds after the alkali etching treatment.
Next, the substrate holder 11 is pulled up from the washing tank 20 and inserted into the dryer 21, and water attached to the surface of the silicon substrate C is dried and removed.

Before the silicon substrate C is dried, the silicon substrate C is cleaned using, for example, hot water of 50 to 100 ° C., preferably 60 to 80 ° C., in order to improve the drainage of water adhering to the silicon substrate C. Alternatively, it may be washed with an organic solvent. Here, as the organic solvent, those similar to those used in the above-described degreasing treatment can be used, but it is preferable to use acetone or methanol.
Thus, the first step of performing the primary etching process of the silicon substrate A is completed, and the surface layer portion of the obtained silicon substrate C is in a state in which the unevenness (primary texture structure) depending on the crystal orientation of silicon is formed. Yes.

Subsequently, the silicon substrate C that has completed the first step is transferred to the second step.
First, the raising / lowering means 30 of the dielectric barrier discharge device 13 is driven to lower the ground electrode portion 23 to the lowest lowered position to open the chamber 24, and the silicon substrate C is placed at the center of the upper surface of the ground electrode 38 of the ground electrode portion 23. Is placed. And after closing the chamber 24, the raising / lowering means 30 is driven and the ground electrode part 23 is raised. Here, since the spacer 39 is provided on the outer peripheral side of the ground electrode 38 so that the gap length can be adjusted within a range of 0.1 to 2 mm, the upper end of the spacer 39 is the discharge electrode 34 of the high voltage electrode portion 22. The rise of the ground electrode part 23 is stopped when it contacts the lower surface. Thereby, the distance of the discharge space formed between the discharge electrode 34 of the high voltage electrode part 22 and the ground electrode 38 of the ground electrode part 23 can be kept constant.

Next, the inside of the chamber 24 is evacuated by the evacuation unit 29 through the evacuation port 28, the pressure in the chamber 24 is evacuated to, for example, 10 to 50 kPa, the evacuation port 28 is closed, and the evacuation unit 29 is stopped. Then, a mixed gas of sulfur hexafluoride and oxygen is supplied into the chamber 24 from the reaction gas supply means 27 through the reaction gas supply port 26. When the pressure in the chamber 24 reaches 90 to 110 kPa, the exhaust port 28 is opened, and the mixed gas is discharged to the outside through the discharge portion 43 provided in the exhaust means 29. As a result, the pressure in the chamber 24 can be substantially maintained at atmospheric pressure (90 to 110 kPa) while allowing the mixed gas to flow through the chamber 24.
Here, the purity of the sulfur hexafluoride gas is preferably 99.5% or more, and the purity of the oxygen gas is preferably 99.0% or more, and the ratio of the sulfur hexafluoride gas flow rate P to the oxygen gas flow rate Q in the mixed gas. P / Q is preferably set to 1/4 to 4/1.

After confirming that a stable mixed gas flow was formed in the chamber 24, the gap length was adjusted to 2 mm or less, preferably 0.1 to 1 mm, more preferably 0.2 to 0.8 mm, and high frequency high voltage The frequency between the high voltage electrode portion 22 and the ground electrode portion 23 from the power source 25 is 3.5 to 7.5 kV, preferably 4.0 to 7.5 kV, more preferably 4.0 to 7 with a frequency of 5 to 50 kHz. A voltage of 0.0 kV is applied for a period of 60 to 600 seconds, preferably 150 to 600 seconds, more preferably 300 to 600 seconds. When the voltage applied to the discharge electrode 34 reaches the dielectric breakdown voltage of the dielectric of the discharge electrode 34, a dielectric barrier discharge occurs, and a discharge column is generated between the discharge electrode 34 and the ground electrode 38. It becomes a plasma source. This plasma ionizes the sulfur hexafluoride gas to form ions and fluorine radicals F ^* in the discharge space.

The generated radicals and ions are accelerated by the voltage applied to the discharge electrode 34 and move in the discharge space toward the ground electrode 38 and collide with the surface of the silicon substrate C placed on the ground electrode 38. To do. Thereby, the surface of the silicon substrate C is plasma etched. As a result, finer unevenness (fine texture structure) is formed on each uneven surface constituting the primary texture structure formed on the silicon substrate C.

Furthermore, since the anisotropy of plasma etching is increased by the oxygen gas in the mixed gas, it is possible to form unevenness with a finer shape by adjusting the oxygen ratio.
As a result, when the second step is completed, the silicon substrate C is a silicon substrate D having a texture structure with a high reflectance reduction effect in which a fine texture structure is incorporated in the primary texture structure.
Thereafter, a pn junction is formed on the silicon substrate D, and then electrodes are formed on the front and back surfaces of the silicon substrate D to manufacture a solar cell.

The low-reflectivity processing equipment used in the low-reflectance processing method for a silicon substrate for solar cells according to the second embodiment of the present invention is a plurality of unprocessed sheets used for solar cells loaded on the substrate holder 11. A primary etching apparatus 12 for performing a primary etching process on the single crystal or polycrystalline silicon substrate A and a secondary etching process for forming finer irregularities on the surface of the silicon substrate C after the primary etching process by plasma etching. A dielectric barrier discharge device 44 is provided. Here, the dielectric barrier discharge device 44 is compared with the dielectric barrier discharge device 13 of the low reflectivity processing equipment 10 used in the low reflectivity processing method of the solar cell silicon substrate according to the first embodiment. The reaction gas supply means 45 has a different configuration. For this reason, the same components are denoted by the same reference numerals, and detailed description thereof is omitted. Only the configuration of the reactive gas supply means 45 will be described.

The reactive gas supply means 45 is connected to a carbon tetrafluoride cylinder and takes out carbon tetrafluoride gas at a predetermined pressure at a predetermined flow rate, for example, a first gas regulator equipped with a pressure setting device and a flow meter, Connect to an oxygen cylinder to extract oxygen gas at a predetermined pressure at a predetermined flow rate, for example, connect to a second gas regulator equipped with a pressure setter and a flow meter, and a helium cylinder as an example of an inert gas. For example, a third gas regulator equipped with a pressure setting device and a flow meter, and a gas that mixes each gas discharged from the first to third gas regulators. Has a mixer. By setting it as such a structure, the mixed gas of various pressures which carbon tetrafluoride gas, oxygen gas, and helium gas mixed at arbitrary ratios can be obtained easily.

Next, a low reflectance processing method for a solar cell silicon substrate according to a second embodiment of the present invention will be described. Here, the low reflectance processing method of the solar cell silicon substrate according to the second embodiment is the low reflectance processing method of the solar cell silicon substrate according to the first embodiment. Compared to the reflectance processing method, the second etching process is performed using a mixed gas of carbon tetrafluoride gas, oxygen gas, and helium gas as a reaction gas. For this reason, only the method of the secondary etching process will be described.

First, as shown in FIG. 5, the lifting / lowering means 30 of the dielectric barrier discharge device 44 is driven to lower the contact electrode portion 23 to the lowest lowered position to open the chamber 24, and the upper surface of the ground electrode 38 of the ground electrode portion 23. After the silicon substrate C is placed on the center of the substrate and the chamber 24 is closed, the elevating means 30 is driven to raise the ground electrode portion 23. Here, since the spacer 39 is provided on the outer peripheral side of the ground electrode 38 so that the gap length can be adjusted within a range of 0.1 to 2 mm, the upper end of the spacer 39 is the discharge electrode 34 of the high voltage electrode portion 22. The rise of the ground electrode part 23 is stopped when it contacts the lower surface. Thereby, the distance of the discharge space formed between the discharge electrode 34 of the high voltage electrode part 22 and the ground electrode 38 of the ground electrode part 23 can be kept constant.

Next, the inside of the chamber 24 is evacuated by the evacuation unit 29 through the evacuation port 28, the pressure in the chamber 24 is evacuated to, for example, 10 to 50 kPa, the evacuation port 28 is closed, and the evacuation unit 29 is stopped. Then, a mixed gas of carbon tetrafluoride gas, oxygen gas, and helium gas is supplied into the chamber 24 from the reaction gas supply means 45 through the reaction gas supply port 26. When the pressure in the chamber 24 reaches 90 to 110 kPa, the exhaust port 28 is opened, and the mixed gas is discharged to the outside through the discharge portion 43 provided in the exhaust means 29. As a result, the pressure in the chamber 24 can be substantially maintained at atmospheric pressure (90 to 110 kPa) while allowing the mixed gas to flow through the chamber 24.

Here, the purity of carbon tetrafluoride gas is preferably 99.5% or more, the purity of oxygen gas is preferably 99.0% or more, and the purity of helium gas is preferably 99.0% or more. The ratio R / S between the carbon tetrafluoride gas flow rate R and the oxygen gas flow rate S is preferably 1/4 to 4/1, and the helium gas flow rate T is the carbon tetrafluoride gas flow rate R and oxygen. The ratio T / U should be adjusted to 4 to 20 with respect to the total flow rate U with the gas flow rate S. Furthermore, the flow rate T of helium gas is in the range of 100 sccm to 3000 sccm, preferably 300 sccm to 2000 sccm, more preferably 500 sccm to 1000 sccm.
Thereby, carbon tetrafluoride gas and oxygen gas can be diluted with helium gas, and even if distribution occurs in the flow velocity of the mixed gas passing through the chamber 24, It is possible to prevent a large difference from occurring in the oxygen supply amount.

After confirming that a stable mixed gas flow was formed in the chamber 24, the gap length was adjusted to 2 mm or less, preferably 0.1 to 1.2 mm, more preferably 0.2 to 1.0 mm. The frequency between the high voltage power supply 25 and the high voltage electrode portion 22 and the ground electrode portion 23 is 5 to 50 kHz, the lower limit is 1 kV, preferably 1.5 kV, and the upper limit is 7.5 kV, preferably 7.0 kV. Is applied for 60 to 600 seconds, preferably 150 to 600 seconds, and more preferably 200 to 600 seconds. When the voltage applied to the discharge electrode 34 reaches the dielectric breakdown voltage of the dielectric of the discharge electrode 34, a dielectric barrier discharge occurs, and a discharge column is generated between the discharge electrode 34 and the ground electrode 38. It becomes a plasma source. This plasma ionizes the carbon tetrafluoride gas to form ions and fluorine radicals F ^* in the discharge space.

Here, since the carbon tetrafluoride gas and the oxygen gas are diluted with helium gas, there is no great difference in the supply amount of carbon tetrafluoride and oxygen at each part in the chamber 24, and the gas is formed by plasma. Differences in the distribution of fluorine radicals and ions can be reduced. The generated radicals and ions are accelerated by the voltage applied to the discharge electrode 34 and move in the discharge space toward the ground electrode 38, and the surface of the silicon substrate C placed on the ground electrode 38. Collide with. Thereby, the surface of the silicon substrate C is uniformly plasma etched. As a result, finer unevenness (fine texture structure) is formed more uniformly on each uneven surface constituting the primary texture structure formed on the silicon substrate C. Furthermore, since the anisotropy of plasma etching is increased by the oxygen gas in the mixed gas, it is possible to form unevenness with a finer shape by adjusting the oxygen ratio.

The low reflectance processing equipment 47 used in the low reflectance processing method for a solar cell silicon substrate according to the third embodiment of the present invention is applied to a solar cell mounted on the substrate holder 11 as shown in FIG. A primary etching processing apparatus 12 that performs a primary etching process on a plurality of unprocessed single crystal or polycrystalline silicon substrates A to be used, and dry etching the surface of the silicon substrate C after the primary etching process with oxygen plasma. It has a discharge device 48 that performs a secondary etching process for forming finer irregularities by plasma etching using a reactive gas. Here, the low-reflectance processing equipment 47 is compared with the low-reflectance processing equipment 10 used in the low-reflectance processing method for the silicon substrate for solar cells according to the first embodiment, and the dielectric barrier discharge device 13. It is characterized in that a discharge device 48 is installed instead of the above. For this reason, only the discharge device 48 will be described.

As shown in FIG. 7, the discharge device 48 includes an upper electrode portion 49, a lower electrode portion 51 provided below the upper electrode portion 49 and provided with a lower electrode 50 on which the silicon substrate C is placed, A chamber 52 that surrounds and seals the upper electrode portion 49 and the lower electrode portion 51 is provided. Here, the upper electrode portion 49 includes a cylindrical body 54 whose one end is fixed to the top plate 53 of the chamber 52 and an upper electrode 56 which is fixed to the other end of the cylindrical body 54 and has a plurality of holes 55 formed therein. doing. By adopting such a configuration, a space 57 constituted by the top plate 53, the cylindrical body 54, and the upper electrode 56 can be formed in the upper electrode portion 49. The discharge device 48 includes a high-frequency power source 58 that applies a high-frequency voltage to the lower electrode unit 51, a matching box 59 provided between the lower electrode unit 51 and the high-frequency power source 58, and a chamber 52. Exhaust means 61 for exhausting the air through the exhaust port 60. Furthermore, the discharge device 48 includes oxygen gas supply means 63 for supplying oxygen gas to the space portion 57 via the valve 62, and sulfur hexafluoride and oxygen, which are examples of reaction gases, to the space portion 57 via the valve 64. Reactive gas supply means 65 for supplying a mixed gas is provided.

The upper electrode portion 49 and the lower electrode 50 are each made of metal (for example, aluminum or stainless steel), an insulating member 66 is provided between the lower electrode 50 and the chamber 52, and the chamber 52 side is grounded. . With such a configuration, a discharge space can be formed between the lower electrode 50 and the upper electrode portion 49 on which the silicon substrate C is placed.
Then, by operating the valves 62 and 64, oxygen gas is supplied from the oxygen gas supply means 63 or reaction gas is supplied from the reaction gas supply means 65 to the space 57 in the upper electrode portion 49, and is provided on the upper electrode 56. It can be discharged into the chamber 52 through the formed hole 55.

Next, a low reflectance processing method for a solar cell silicon substrate according to a third embodiment of the present invention will be described. Here, the low reflectance processing method of the solar cell silicon substrate according to the third embodiment is the low reflectance processing method of the solar cell silicon substrate according to the first embodiment. Compared with the reflectance processing method, the surface of the silicon substrate C after the primary etching treatment is dry-etched by oxygen plasma generated by discharge in oxygen gas having a pressure of 1 Pa or more and 200 Pa or less, and then the pressure as a reaction gas Is characterized in that the secondary etching process is performed using a mixed gas of sulfur hexafluoride and oxygen having a pressure of 1 Pa or more and 200 Pa or less. Therefore, only dry etching and secondary etching processing methods using oxygen plasma will be described.
First, the chamber 52 is opened, and the silicon substrate C is placed on the center of the upper surface of the lower electrode 50 of the lower electrode portion 51. Then, after the distance of the discharge space formed between the upper electrode 56 of the upper electrode portion 49 and the lower electrode 50 of the lower electrode portion 51 is set to 10 to 70 mm, the chamber 52 is closed.

Next, the inside of the chamber 52 is exhausted by the exhaust means 61 through the exhaust port 60, and the pressure in the chamber 52 is exhausted to, for example, 1.0 × 10 ^{−3 to} 5.0 × 10 ⁻³ Pa, and the upper electrode Oxygen gas is supplied into the chamber 52 from the oxygen gas supply means 63 via the part 49. Then, the exhaust amount of the exhaust means 61 is adjusted so that the pressure in the chamber 52 is 1 to 200 Pa, preferably 40 to 100 Pa. Here, the purity of the oxygen gas is preferably 99.0% or more. After confirming that a stable flow of oxygen gas is formed in the chamber 52, the frequency is 10 MHz or more and 50 MHz or less between the lower electrode unit 51 and the upper electrode unit 49 from the high frequency power source 58, preferably the lower limit value is High frequency power of 10 MHz and upper limit of 15 MHz, power of 50 watts to 500 watts, preferably lower limit of 100 W and upper limit of 300 W, 30 to 600 seconds, preferably lower limit of 60 seconds and upper limit of 300 Apply for seconds. A discharge is generated between the lower electrode 50 and the upper electrode 56, the oxygen gas is ionized, and oxygen plasma is formed in the discharge space.

The generated oxygen plasma is accelerated by the voltage applied between the upper electrode 56 and the lower electrode 50 and moves in the discharge space toward the lower electrode 50, and the silicon substrate placed on the lower electrode 50. Collides with C surface. Thereby, the surface of the silicon substrate C is dry etched by oxygen plasma. As a result, surface modification is performed so that finer unevenness (fine texture structure) can be formed by secondary etching on each uneven surface constituting the primary texture structure formed on the silicon substrate C.
When dry etching is completed by the oxygen plasma, the driving of the high frequency power supply 58 is stopped, and the supply of oxygen gas from the oxygen gas supply means 63 is stopped. Next, a mixed gas of sulfur hexafluoride and oxygen is supplied into the chamber 52 from the reaction gas supply means 65 through the upper electrode portion 49 so that the pressure in the chamber 52 becomes 1 to 200 Pa, preferably 40 to 100 Pa. The exhaust amount of the exhaust means 61 is adjusted. Here, the purity of the sulfur hexafluoride gas is preferably 99.5% or more, and the purity of the oxygen gas is preferably 99.0% or more. When the sulfur hexafluoride gas flow rate is P and the oxygen gas flow rate is Q, the ratio P / (P + Q) of the sulfur hexafluoride gas in the mixed gas is 0.1-10.

After confirming that a stable mixed gas flow is formed in the chamber 52, the frequency is 10 MHz or more and 50 MHz or less, preferably the lower limit value between the lower electrode unit 51 and the upper electrode unit 49 from the high frequency power source 58. High frequency power of 10 MHz and upper limit of 15 MHz, power of 50 watts to 500 watts, preferably lower limit of 100 W and upper limit of 300 W, 30 to 600 seconds, preferably lower limit of 60 seconds and upper limit of 300 Apply for seconds. Discharge occurs between the lower electrode 50 and the upper electrode 56, which becomes a plasma source, and the sulfur hexafluoride gas is ionized to form ions and fluorine radicals F ^* in the discharge space. The generated radicals and ions are accelerated by the voltage applied between the upper electrode 56 and the lower electrode 50 and move in the discharge space toward the lower electrode 50, and the silicon placed on the lower electrode 50. It collides with the surface of the substrate C. Thereby, the surface of the silicon substrate C is plasma etched. Here, since the surface modification is performed by dry etching with oxygen plasma on each uneven surface constituting the primary texture structure formed on the silicon substrate C, fine unevenness is obtained by the secondary etching process. Can be formed uniformly and efficiently. As a result, when the second step is completed, the silicon substrate C is a silicon substrate D having a texture structure with a high reflectance reduction effect in which a fine texture structure is incorporated in the primary texture structure.

Moreover, the low reflectivity processing method of the silicon substrate for solar cells according to the fourth embodiment of the present invention is a hexafluoride whose pressure is 1 Pa or more and 200 Pa or less as a reaction gas to the silicon substrate C after the primary etching treatment. A secondary etching process is performed using a mixed gas of sulfur and oxygen. Therefore, the low reflectance processing method of the solar cell silicon substrate according to the fourth embodiment is the same as the low reflectance processing method of the solar cell silicon substrate according to the third embodiment, in which dry etching using oxygen plasma is performed. It is substantially the same as omitting. For this reason, the description regarding the low-reflectance processing method of the silicon substrate for solar cells which concerns on 4th Embodiment is abbreviate | omitted.

Next, examples carried out for confirming the effects of the present invention will be described. Here, FIG. 8 is an electron micrograph showing the crystal structure of the surface of the silicon substrate after the primary etching process in the low reflectance processing method for the silicon substrate for solar cells according to Example 1 of the present invention, and FIG. FIG. 10 is an electron micrograph showing the crystal structure of the surface of the silicon substrate after the secondary etching process in the low reflectance processing method for a solar cell silicon substrate according to Example 3, and FIG. 10 is the sun according to Examples 1 and 2 of the present invention. FIG. 11 is a graph showing the wavelength dependence of the reflectance of a silicon substrate obtained by the low reflectance processing method for a battery silicon substrate. FIG. 11 is a low reflectance processing of a solar cell silicon substrate according to Examples 3 and 4 of the present invention. The graph which shows the wavelength dependence of the reflectance of the silicon substrate obtained by the method, FIG. 12 is after the primary etching process in the low reflectance processing method of the silicon substrate for solar cells which concerns on Example 5 of this invention. FIG. 13 is an electron micrograph showing the crystal structure of the surface of the silicon substrate. FIG. 13 shows the crystal structure of the surface of the silicon substrate after the secondary etching process in the low reflectance processing method for the silicon substrate for solar cells according to Example 5 of the present invention. FIG. 14 is a graph showing the wavelength dependence of the reflectance of a silicon substrate obtained by the low reflectance processing method for a solar cell silicon substrate according to Examples 5 to 7 of the present invention, and FIG. 16 is an electron micrograph showing the crystal structure of the surface of the silicon substrate after the secondary etching in the low reflectance processing method for a solar cell silicon substrate according to Example 8 of the invention. FIG. 16 shows Examples 8 and 9 of the present invention. FIG. 17 is a graph showing the wavelength dependence of the reflectance of a silicon substrate obtained by the low reflectance processing method for a silicon substrate for solar cells, and FIG. 17 shows the silicon substrate for solar cells according to Example 9 of the present invention. Is an electron micrograph showing the crystal structure of the surface of the silicon substrate after the secondary etching treatment in the low reflectivity processing method.

(Example 1)
First, a single crystal silicon substrate was immersed in a degreasing tank in which acetone was stored for 300 seconds for degreasing treatment, and then dried in a dryer.
Next, the silicon substrate was immersed for 60 seconds in an alkali bath in which a 3 wt% sodium hydroxide aqueous solution heated to 80 ° C. was stored, and the silicon substrate subjected to the surface layer removal treatment was pulled up from the alkali bath, and 10 The solution was immersed in a water washing tank within 2 seconds to remove the aqueous sodium hydroxide solution, pulled up from the water washing tank, charged in a dryer and dried.
Then, after immersing the silicon substrate in a primary etching treatment tank in which a mixed solution of 1% by weight sodium hydroxide aqueous solution and 5% by volume isopropyl alcohol heated to 75 ° C. is stored for 30 seconds, within 10 seconds. A sodium hydroxide aqueous solution was removed from the silicon substrate having irregularities formed on the surface by immersing it in a water rinsing tank, and it was pulled up from the water rinsing tank and charged into a dryer to be dried. The state of the surface of the silicon substrate obtained at this time is shown in FIG.

Subsequently, the lifting and lowering means of the dielectric barrier discharge device is driven to lower the ground electrode portion to the lowest lowered position to open the chamber, and the first etching process is performed on the center portion of the upper surface of the ground electrode of the ground side electrode portion. Place the substrate. Further, the height of the spacer is adjusted so that the gap length is 0.2 mm. Then, the ground electrode portion is raised and stopped when the upper end of the spacer comes into contact with the lower surface of the discharge electrode of the high voltage electrode portion.
Next, the inside of the chamber is evacuated by the evacuation unit through the evacuation port, and when the pressure in the chamber reaches 10 kPa, the evacuation port is closed and the evacuation unit is stopped. Then, sulfur hexafluoride gas is supplied into the chamber from the reaction gas supply means through the reaction gas supply port, and when the pressure in the chamber reaches 100 kPa, the exhaust port is opened and the discharge provided in the exhaust means is provided. The sulfur hexafluoride gas is released to the outside through the section.
Subsequently, a voltage of 8 kHz and a voltage of 7 kV is applied for 300 seconds between the high voltage electrode unit and the ground electrode unit from the high frequency high voltage power source, and between the discharge electrode of the high voltage electrode unit and the ground electrode of the ground electrode unit. A dielectric barrier discharge was generated in the silicon substrate, and the silicon substrate was subjected to a secondary etching process. Further, FIG. 10 shows the wavelength dependence of the reflectance of the obtained silicon substrate.

(Example 2)
The silicon substrate obtained by performing the same primary etching process as in Example 1 was placed on the ground electrode of the ground electrode portion of the dielectric barrier discharge device, and carbon tetrafluoride gas was used instead of sulfur hexafluoride gas. Discharge in the high voltage electrode part by circulating in the chamber and holding the inside of the chamber at 100 kPa, applying a voltage of 7 kHz at a frequency of 8 kHz between the high voltage electrode part and the ground electrode part from the high frequency high voltage power source for 300 seconds. A dielectric barrier discharge was generated between the electrode and the ground electrode of the ground electrode portion, and the silicon substrate was subjected to a secondary etching process.
FIG. 10 shows the wavelength dependence of the reflectance of the silicon substrate obtained by the secondary etching process.

(Comparative Example 1)
A silicon substrate obtained by performing the same primary etching process as in Example 1 was set in a high-frequency parallel plate type reactive ion etching apparatus, and a mixed gas of 7 Pa (at a ratio of oxygen 1 to sulfur hexafluoride 4). Etching was performed in a mixed atmosphere at a high frequency output of 200 W.
FIG. 10 shows the wavelength dependence of the reflectance of a silicon substrate obtained by etching using a reactive ion etching apparatus.

(Comparative Example 2)
The silicon substrate obtained by performing the same primary etching process as in Example 1 was set in a high-frequency parallel plate type reactive ion etching apparatus and etched in a 9 Pa carbon tetrafluoride gas atmosphere under the condition of a high-frequency output of 150 W. .
FIG. 10 shows the wavelength dependence of the reflectance of a silicon substrate obtained by etching using a reactive ion etching apparatus. FIG. 10 also shows the reflectance of the silicon substrate C after the primary etching process.

(Example 3)
The silicon substrate obtained by performing the same primary etching process as in Example 1 was placed on the ground electrode of the ground electrode part of the dielectric barrier discharge device, and mixed with sulfur hexafluoride 1 at a ratio of 4 oxygen. The mixed gas is circulated in the chamber, the chamber is maintained at 100 kPa, and a high voltage electrode unit is applied by applying a 6 kV voltage at a frequency of 8 kHz between the high voltage electrode unit and the ground electrode unit for 180 seconds from the high frequency high voltage power source. The silicon substrate was subjected to a secondary etching process by generating a dielectric barrier discharge between the discharge electrode and the ground electrode of the ground electrode portion. The height of the spacer provided on the outer peripheral side of the ground electrode of the ground electrode portion was adjusted so that the gap length was 0.8 mm. The state of the surface of the silicon substrate obtained by the secondary etching process is shown in FIG. FIG. 11 shows the wavelength dependence of the reflectance of the silicon substrate obtained by the secondary etching process.

Example 4
The silicon substrate obtained by performing the same primary etching process as in Example 1 was placed on the ground electrode of the ground electrode portion of the dielectric barrier discharge device, and mixed with sulfur hexafluoride 4 at a ratio of 1 oxygen. The mixed gas is circulated in the chamber, the chamber is maintained at 100 kPa, and a high-frequency electrode is applied with a voltage of 8 kHz and 7 kV for 300 seconds from a high-frequency high-voltage power source to discharge and ground electrodes of the high-voltage electrode. A dielectric barrier discharge was generated between the silicon substrate and the ground electrode, and the silicon substrate was subjected to a secondary etching process. The height of the spacer provided on the outer peripheral side of the ground electrode of the ground electrode portion was adjusted so that the gap length was 0.8 mm.
FIG. 11 shows the wavelength dependence of the reflectance of the silicon substrate obtained by the secondary etching process. FIG. 11 also shows the reflectance of the silicon substrate C after the primary etching process.

(Example 5)
First, a single crystal silicon substrate was immersed in a degreasing tank in which acetone was stored for 300 seconds for degreasing treatment, and then dried in a dryer. Next, the silicon substrate was immersed for 60 seconds in an alkali bath in which a 3 wt% sodium hydroxide aqueous solution heated to 80 ° C. was stored, and the silicon substrate subjected to the surface layer removal treatment was pulled up from the alkali bath, and 10 The solution was immersed in a water washing tank within 2 seconds to remove the aqueous sodium hydroxide solution, pulled up from the water washing tank, charged in a dryer and dried.
Then, after immersing the silicon substrate in a primary etching treatment tank in which a mixed solution of 1% by weight sodium hydroxide aqueous solution and 5% by volume isopropyl alcohol heated to 75 ° C. is stored for 30 seconds, within 10 seconds. A sodium hydroxide aqueous solution was removed from the silicon substrate having irregularities formed on the surface by immersing it in a water rinsing tank, and it was pulled up from the water rinsing tank and charged into a dryer to be dried. The state of the surface of the silicon substrate obtained at this time is shown in FIG. As shown in FIG. 12, the surface of this silicon substrate has irregularities with an apex angle of about 70.5 ° and a surface roughness (maximum protrusion height) of about 10 μm.

Subsequently, the lifting and lowering means of the dielectric barrier discharge device is driven to lower the ground electrode portion to the lowest lowered position to open the chamber, and the first etching process is performed on the center portion of the upper surface of the ground electrode of the ground side electrode portion. Place the substrate. Further, the height of the spacer is adjusted so that the gap length is 1 mm. Then, the ground electrode portion is raised and stopped when the upper end of the spacer comes into contact with the lower surface of the discharge electrode of the high voltage electrode portion.
Next, the inside of the chamber is evacuated by the evacuation unit through the evacuation port, and when the pressure in the chamber reaches 10 kPa, the evacuation port is closed and the evacuation unit is stopped. Then, a mixed gas obtained by mixing helium gas, carbon tetrafluoride gas, and oxygen gas at a ratio of 750: 50: 75 is supplied into the chamber from the reactive gas supply means through the reactive gas supply port, and the pressure in the chamber is increased. When the air pressure reaches 100 kPa, the exhaust port is opened, and the mixed gas is discharged to the outside through the discharge portion provided in the exhaust means.
Then, a voltage of 8 kHz and a voltage of 3.5 kV is applied between the high voltage electrode unit and the ground electrode unit for 300 seconds from the high frequency high voltage power source, and the discharge electrode of the high voltage electrode unit and the ground electrode of the ground electrode unit are connected. A dielectric barrier discharge was generated in the meantime, and the silicon substrate was subjected to a secondary etching process. Then, after cleaning the surface of the silicon substrate obtained by the secondary etching process with hydrogen fluoride, the surface state was observed with a scanning electron microscope, and the wavelength dependence of the light reflectance was examined. The surface state of the silicon substrate and the wavelength dependence of the reflectance are shown in FIGS. 13 and 14, respectively. In FIG. 14, the reflectance of the silicon substrate C obtained by performing the primary etching process of Example 5 is also shown.
While the reflectance at a wavelength of 800 nm of the silicon substrate C obtained by performing the primary etching process is about 10%, the reflectance is lowered to 6.6% by performing the secondary etching process of the silicon substrate. I was able to. The reflectivity decreased because fine unevenness (fine texture structure) was formed more uniformly on each uneven surface constituting the primary texture structure formed on the silicon substrate C. This is thought to be due to an improved confinement effect.

(Example 6)
The silicon substrate obtained by performing the same primary etching process as in Example 5 was placed on the ground electrode of the ground electrode portion of the dielectric barrier discharge device, and helium gas, carbon tetrafluoride gas, and oxygen gas were 750: A mixed gas mixed at a ratio of 50:75 is supplied into the chamber, the pressure in the chamber is maintained at 100 kPa, and a frequency of 8 kHz between the high-voltage electrode unit and the ground electrode unit from the high-frequency high-voltage power supply is 2 kV. Was applied for 300 seconds to generate a dielectric barrier discharge between the discharge electrode of the high-voltage electrode portion and the ground electrode of the ground electrode portion, and the silicon substrate was subjected to a secondary etching process. Then, after cleaning the surface of the silicon substrate obtained by the secondary etching process with hydrogen fluoride, the wavelength dependence of the light reflectance of the surface was examined. FIG. 14 shows the wavelength dependence of the reflectance of the silicon substrate. When the secondary etching process was performed at a voltage of 2 kV, the reflectance at a wavelength of 800 nm was 7%.

(Example 7)
The silicon substrate obtained by performing the same primary etching process as in Example 5 was placed on the ground electrode of the ground electrode portion of the dielectric barrier discharge device, and helium gas, carbon tetrafluoride gas, and oxygen gas were 750: A mixed gas mixed at a ratio of 50:75 is supplied into the chamber, the pressure in the chamber is maintained at 100 kPa, and a frequency between the high voltage electrode unit and the ground electrode unit from the high frequency high voltage power source is 1.7 kV at 8 kHz. Was applied for 300 seconds to generate a dielectric barrier discharge between the discharge electrode of the high-voltage electrode portion and the ground electrode of the ground electrode portion, and the silicon substrate was subjected to a secondary etching process. Then, after cleaning the surface of the silicon substrate obtained by the secondary etching process with hydrogen fluoride, the wavelength dependence of the light reflectance of the surface was examined. FIG. 14 shows the wavelength dependence of the reflectance of the silicon substrate. When the secondary etching process was performed at a voltage of 1.7 kV, the reflectance at a wavelength of 800 nm was 7.8%.

(Example 8)
The silicon substrate obtained by performing the same primary etching process as in Example 5 is placed on the lower electrode of the lower electrode part of the discharge device, and the distance between the lower electrode of the lower electrode part and the upper electrode of the upper electrode part is set. 45 mm. Then, a mixed gas in which sulfur hexafluoride gas and oxygen gas are mixed at a ratio of 4: 1 is supplied into the chamber, the pressure in the chamber is maintained at 66.5 Pa, and the lower electrode portion and the upper electrode are supplied from the high frequency power source. A high frequency power of 13.56 MHz and 200 W was applied between the portions for 180 seconds to generate a discharge between the lower electrode and the upper electrode, and the silicon substrate was subjected to a secondary etching process. Then, the surface of the silicon substrate obtained by the secondary etching treatment was washed with 5% by weight of hydrofluoric acid, and then the surface state was observed with a scanning electron microscope, and the wavelength dependence of the light reflectance was examined. . The surface state of the silicon substrate and the wavelength dependence of the reflectance are shown in FIGS. 15 and 16, respectively. When the secondary etching process was performed, the reflectance at a wavelength of 800 nm was 8.8%. In FIG. 16, the reflectance of the silicon substrate C obtained by performing the primary etching process of Example 5 is also shown.

Example 9
The silicon substrate obtained by performing the same primary etching process as in Example 5 is placed on the lower electrode of the lower electrode part of the discharge device, and the distance between the lower electrode of the lower electrode part and the upper electrode of the upper electrode part is set. 45 mm. Then, oxygen gas is supplied into the chamber, the pressure in the chamber is maintained at 66.5 Pa, and a high frequency power of 200 W at a frequency of 13.56 MHz is applied from the high frequency power source between the upper electrode portion and the lower electrode portion for 300 seconds. Then, a discharge was generated between the lower electrode and the upper electrode, and dry etching treatment was performed using oxygen plasma. Next, after performing the same secondary etching process as in Example 8, the surface of the silicon substrate was washed with 5% by weight of hydrofluoric acid, and then the surface state was observed with a scanning electron microscope to obtain a light reflectance. The wavelength dependence of was investigated. The surface state of the silicon substrate and the wavelength dependence of the reflectance are shown in FIGS. 17 and 16, respectively. When the secondary etching process was performed, the reflectance at a wavelength of 800 nm was 6.2%.

Thus, by forming irregularities (primary texture structure) depending on the crystal orientation of silicon on the surface of the silicon substrate by the primary etching process using an alkaline aqueous solution, by performing plasma etching on the surface of the silicon substrate, A texture structure with a high reflectance reduction effect in which a fine texture structure is incorporated in the primary texture structure formed on the surface of the silicon substrate can be formed. As shown in FIG. 11, the reflectance is 5% at a wavelength of 800 nm. It was confirmed that it could be reduced to the following.

In addition, at any three locations in the plane of the silicon substrate obtained in Example 3 and Example 6 (for example, the central part, the outer peripheral part, and the intermediate part between the central part and the outer peripheral part), a constant wavelength (for example, The light reflectance for a wavelength of 800 nm was measured. Then, when the value M / m of the ratio between the maximum value M and the minimum value m obtained is obtained, the ratio value M / m is 1.15 in Example 3, and the ratio value M / m in Example 6. m was 1.05. Thus, the value M / m of the example 6 in which the treatment with the reaction gas mixed with helium gas is smaller than that in the example 3 in which the treatment with the reaction gas not mixed with the helium gas is smaller. It was confirmed that the chemical etching in the surface progressed more uniformly. Further, although not shown, the same result could be confirmed by observing the surface of the silicon substrate obtained in Example 3 and Example 6 with a scanning electron microscope.

As mentioned above, although embodiment of this invention was described, this invention is not limited to this embodiment, The change in the range which does not change the summary of invention is possible, Each above-mentioned embodiment is possible. The case where the silicon substrate low reflectivity processing method of the present invention is configured by combining some or all of the forms and modifications is also included in the scope of the present invention.
For example, although alkali etching is performed in the primary etching process, acid etching may be performed. In this case, for example, a mixed acid aqueous solution in which hydrofluoric acid and nitric acid are mixed can be used as the etching solution to be used. In the first embodiment, a mixed gas of carbon tetrafluoride and oxygen may be used as the reaction gas.
In the second embodiment, instead of a mixed gas of helium gas, carbon tetrafluoride gas, and oxygen gas as a reaction gas, a mixed gas of helium gas and carbon tetrafluoride gas, helium gas, sulfur hexafluoride gas, And a mixed gas of oxygen gas or a mixed gas of helium gas and sulfur hexafluoride gas may be used. Moreover, argon gas can be used as an inert gas instead of helium gas.
In the third and fourth embodiments, a mixed gas of sulfur hexafluoride gas and oxygen gas is used as a reaction gas. However, a mixed gas of carbon tetrafluoride gas and oxygen gas can also be used. Sulfur gas or carbon tetrafluoride gas can also be used alone.

It is explanatory drawing of the low reflectivity processing equipment used for the low reflectivity processing method of the silicon substrate for solar cells which concerns on the 1st Embodiment of this invention. It is explanatory drawing of the primary etching processing apparatus which performs the primary etching process of the low reflectance processing method of the silicon substrate for solar cells. It is explanatory drawing of the dielectric barrier discharge apparatus which performs the secondary etching process of the low reflectance processing method of the silicon substrate for solar cells. It is explanatory drawing which shows the structure of the electrode part of the same dielectric barrier discharge apparatus. It is explanatory drawing of the dielectric barrier discharge apparatus which performs the secondary etching process of the low-reflectance processing method of the silicon substrate for solar cells which concerns on the 2nd Embodiment of this invention. It is explanatory drawing of the low reflectance processing equipment used for the low reflectance processing method of the silicon substrate for solar cells which concerns on the 3rd Embodiment of this invention. It is explanatory drawing of the discharge device which performs the secondary etching process of the low-reflectance processing method of the silicon substrate for solar cells. It is an electron micrograph which shows the crystal structure of the surface of the silicon substrate after the primary etching process in the low reflectance processing method of the silicon substrate for solar cells which concerns on Example 1 of this invention. It is an electron micrograph which shows the crystal structure of the surface of the silicon substrate after the secondary etching process in the low reflectance processing method of the silicon substrate for solar cells which concerns on Example 3 of this invention. It is a graph which shows the wavelength dependence of the reflectance of the silicon substrate obtained by the low reflectance processing method of the silicon substrate for solar cells which concerns on Example 1 and 2 of this invention. It is a graph which shows the wavelength dependence of the reflectance of the silicon substrate obtained by the low reflectance processing method of the silicon substrate for solar cells which concerns on Example 3 and 4 of this invention. It is an electron micrograph which shows the crystal structure of the surface of the silicon substrate after the primary etching process in the low reflectance processing method of the silicon substrate for solar cells which concerns on Example 5 of this invention. It is an electron micrograph which shows the crystal structure of the surface of the silicon substrate after the secondary etching process in the low reflectance processing method of the silicon substrate for solar cells which concerns on Example 5 of this invention. It is a graph which shows the wavelength dependence of the reflectance of the silicon substrate obtained with the low-reflectance processing method of the silicon substrate for solar cells which concerns on Examples 5-7 of this invention. It is an electron micrograph which shows the crystal structure of the surface of the silicon substrate after the secondary etching process in the low reflectance processing method of the silicon substrate for solar cells which concerns on Example 8 of this invention. It is a graph which shows the wavelength dependence of the reflectance of the silicon substrate obtained with the low reflectance processing method of the silicon substrate for solar cells which concerns on Example 8 and 9 of this invention. It is an electron micrograph which shows the crystal structure of the surface of the silicon substrate after the secondary etching process in the low reflectance processing method of the silicon substrate for solar cells which concerns on Example 9 of this invention.

Explanation of symbols

10: Low reflectance processing equipment, 11: Substrate holder, 12: Primary etching processing device, 13: Dielectric barrier discharge device, 14: Degreasing bath, 15: Dryer, 16: Alkaline bath, 17: Washing bath, 18: Dryer, 19: primary etching tank, 20: washing tank, 21: dryer, 22: high voltage electrode section, 23: ground electrode section, 24: chamber, 25: high frequency high voltage power supply, 26: reaction gas supply port 27: reactive gas supply means, 28: exhaust port, 29: exhaust means, 30: lifting means, 31: insulator, 32, 32a: base, 33: introduction electrode, 34: discharge electrode, 35: reaction gas flow path 35a: introduction passage, 36: gas discharge port, 37: water cooling jacket, 38: ground electrode, 39: spacer, 40: water cooling jacket, 43: discharge section, 44: dielectric barrier discharge device, 45: reaction gas supply Step: 47: Low reflectance processing equipment, 48: Discharge device, 49: Upper electrode part, 50: Lower electrode, 51: Lower electrode part, 52: Chamber, 53: Top plate, 54: Cylindrical body, 55: Hole, 56: upper electrode, 57: space portion, 58: high frequency power supply, 59: matching box, 60: exhaust port, 61: exhaust means, 62: valve, 63: oxygen gas supply means, 64: valve, 65: reaction gas supply Means 66: Insulating member

Claims (9)

A method of processing the surface of a single crystal or polycrystalline silicon substrate used for solar cells with low reflectance,
A first step of performing a primary etching process for forming irregularities on the surface of the silicon substrate using an acid or alkali aqueous solution;
Low reflectivity processing of a silicon substrate for solar cells, comprising: a second step of performing a secondary etching process for forming finer irregularities by plasma etching on the surface of the silicon substrate subjected to the primary etching process Method. 2. The low reflectance processing method for a solar cell silicon substrate according to claim 1, wherein the plasma etching uses plasma generated by dielectric barrier discharge in a reaction gas. Low reflectivity processing method. 3. The low reflectance processing method for a solar cell silicon substrate according to claim 2, wherein the pressure of the reaction gas is substantially atmospheric pressure. 4. The solar cell silicon substrate low-reflectance processing method according to claim 2, wherein the reaction gas is a carbon tetrafluoride gas or a sulfur hexafluoride gas. 5. Low reflectivity processing method of silicon substrate for industrial use. 4. The method for processing a low-reflectance silicon substrate for a solar cell according to claim 2, wherein the reaction gas is a mixed gas of carbon tetrafluoride and oxygen, or a mixed gas of sulfur hexafluoride and oxygen. A low reflectivity processing method for a silicon substrate for a solar cell. 6. The method of processing low reflectance of a silicon substrate for a solar cell according to claim 4, wherein the dielectric barrier discharge has a frequency of 3.5 kV or more and 7.5 kV having a frequency of 5 kHz or more and 50 kHz or less. A low-reflectance processing method for a silicon substrate for a solar cell, wherein the following voltage is applied for 60 seconds or more and 600 seconds or less. 4. The low reflectance processing method for a solar cell silicon substrate according to claim 2, wherein the reaction gas is a mixed gas of carbon tetrafluoride gas and an inert gas, or sulfur hexafluoride gas. A low reflectivity processing method for a silicon substrate for a solar cell, which is a mixed gas of an inert gas. 4. The low reflectance processing method for a solar cell silicon substrate according to claim 2, wherein the reaction gas is a mixed gas of carbon tetrafluoride gas, oxygen gas, and inert gas, or six fluorine. A low reflectivity processing method for a silicon substrate for a solar cell, which is a mixed gas of sulfurized gas, oxygen gas, and inert gas. 9. The low-reflectance processing method for a solar cell silicon substrate according to claim 7, wherein the dielectric barrier discharge has a frequency of 5 kHz to 50 kHz and a frequency of 1 kV to 7.5 kV. A low-reflectance processing method for a silicon substrate for a solar cell, which is performed under a condition in which a voltage is applied for a time of 60 seconds or more and 600 seconds or less.