JP2012238719A - Silicon substrate etching method and etching device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an etching method which can form high density texture on a silicon substrate stably at low cost.SOLUTION: A silicon etching method involves carrying out repeatedly in multiple batches an etching process of supplying to the silicon substrate surface an alkaline etchant containing an additive for blocking an etching reaction on the silicon substrate surface and water and then forming irregularities on the silicon substrate surface. To ensure that a ratio of an increment of silicon concentration in the etchant during the previous batch of the etching process after the first batch of the etching process is finished to an increment of alkaline concentration in the etchant after the previous batch of the etching process is finished will be constant, alkali is added to the etchant every batch while the etching process is carried out repeatedly in multiple batches.

Description

  The present invention relates to a silicon substrate etching method and an etching apparatus, and more particularly, to a silicon substrate etching method and an etching apparatus suitable for use in forming a texture on the surface of a silicon substrate used as a cell of a photovoltaic power generation apparatus.

  In order to efficiently absorb sunlight in a cell of a solar power generation device, it is desirable to absorb as much light as possible on a silicon substrate constituting the device, that is, to reduce the light reflectance as much as possible. For this reason, the silicon substrate is etched by using a high-temperature alkaline solution or a mixed solution of hydrofluoric acid and nitric acid (hereinafter referred to as hydrofluoric acid) as an etchant. A texture is formed on the surface, and in the case of hydrofluoric acid, the surface shape of the silicon substrate is controlled for isotropic etching. That is, if the texture can be formed with a high density and the surface shape can be optimally controlled, the light reflectance can be reduced, and as a result, sunlight can be efficiently absorbed into the silicon substrate. The texture referred to here is a general term for minute irregularities formed on the surface of the silicon substrate.

  Normally, when etching a silicon substrate, it is common to perform batch processing, but exchanging the etchant every time the silicon substrate is put in can reduce the amount of chemical used and the time for preparing the etchant. Since it is not efficient when considered, it is necessary to perform a plurality of batch processes (multiple batch processes) without exchanging the etching solution. In the case where a plurality of batch processes are performed, there is, for example, Patent Document 1 as a method for stably performing etching. In Patent Document 1, after etching, fluoric nitric acid of an etching promoter is added, and a part of the etching solution is drawn out to lower the concentration of hexafluorosilicic acid in the etching inhibitor to adjust the concentration of hexafluorosilicic acid. ing.

JP 2005-210144 A

  The manufacture of solar cells is predicated on mass production, and multiple batch processing is essential. However, since a part of the etching solution is extracted in Patent Document 1, there is a problem that the amount of waste water increases and the load of waste water treatment increases. Furthermore, by extracting a part of the etching solution, the etching promoter's hydrofluoric acid is also discharged out of the system, and it is necessary to add the additional amount of hydrofluoric nitric acid, which increases the chemical cost. There was a problem.

  The present invention has been made in view of the above, and an object of the present invention is to obtain a silicon substrate etching method capable of forming a high-density texture stably and inexpensively on a silicon substrate.

  In order to solve the above-described problems and achieve the object, an etching method of a silicon substrate according to the present invention is an alkaline etching solution containing an additive that inhibits an etching reaction on the surface of the silicon substrate and water. An etching method for a silicon substrate, wherein an etching process for supplying irregularities to the surface of the silicon substrate by wet etching and repeatedly performing the etching process over a plurality of batches is performed, and after the completion of the etching process of the first batch, For each batch, the ratio of the increase in the silicon concentration in the etching solution in the batch etching process and the added amount of the alkali concentration in the etching solution after the end of the previous batch etching process is constant. In addition, an alkali is added to the etching solution to duplicate the etching process. Be repeatedly performed over a batch, characterized by.

  According to the present invention, it is possible to stably form a texture on the surface of a silicon substrate stably with a smaller amount of additive added and with a smaller amount of alkaline solution used. And high density texture can be formed.

FIG. 1 is a characteristic diagram showing the relationship between the molar concentration of silicon and the molar concentration of NaOH in the etching solution according to Embodiment 1 of the present invention. FIG. 2 is a characteristic diagram showing a change in reflectance (%) of light having a wavelength of 700 nm on the surface of the silicon substrate with respect to the number of batch processes. FIG. 3 is a characteristic diagram showing a change in reflectance of light having a wavelength of 700 nm with respect to an additional NaOH / increased Si ratio. FIG. 4 is an electron micrograph of the surface of the silicon substrate when the additional NaOH / increased Si ratio is 1.6. FIG. 5 is an electron micrograph when the additional NaOH / increased Si ratio is 2.1. FIG. 6 is a diagram schematically illustrating an example of the configuration of the wet etching apparatus according to the second embodiment of the present invention. FIG. 7 is a diagram schematically illustrating an example of the configuration of the wet etching apparatus according to the third embodiment of the present invention. FIG. 8-1 is a diagram schematically showing an image of the reaction state at the start of the etching reaction when the silicon substrate is immersed in a 1,6-hexanediol solution before the etching treatment. FIG. 8-2 is a diagram schematically showing an image of the reaction state at the start of the etching reaction when the silicon substrate is not immersed in the 1,6-hexanediol solution before the etching treatment. FIG. 9 is a diagram showing a processing method in the case where the pre-etching treatment and the etching treatment in which the silicon substrate is immersed in the texture forming additive are successively performed. FIG. 10 is a characteristic diagram showing a change in reflectance of light having a wavelength of 700 nm after a plurality of batch processes depending on the presence or absence of the pre-etching process when impurities (copper ions) are mixed in the etching solution. FIG. 11 is a characteristic diagram showing a change in reflectance of light having a wavelength of 700 nm after a plurality of batch processes depending on the presence or absence of a pre-etching process on a silicon substrate subjected to damage etching.

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

Embodiment 1 FIG.
The present inventor examined multiple batch processing of wet etching under alkaline conditions using an additive in wet etching processing for forming a texture on the surface of the silicon substrate in order to reduce the surface light reflectance of the silicon substrate. . As a result, it was found that by adding an alkaline solution in accordance with the silicon concentration in the etching solution, a texture can be satisfactorily formed on the surface of the silicon substrate by a plurality of batch processes.

Silicon (Si) on the silicon substrate reacts with an alkali such as sodium hydroxide (NaOH) in the etching solution and is eluted into the etching solution, whereby a texture is formed on the surface of the silicon substrate. This etching reaction is expressed by the following formula (1), and hydrogen gas is generated at that time. In order to accelerate this reaction, wet etching is usually performed under conditions of high temperature and high concentration alkali. In the formula (1), OH ⁻ is an etching promoter and Si (OH) ₆ ²⁻ is an etching inhibitor.

  The additive (texture forming additive) added to the etching solution for texture formation does not chemically participate in the etching reaction, but moderately inhibits the etching reaction on the silicon substrate surface, Contributes to formation. In order to form a texture, it is necessary to increase anisotropy, that is, to cause a difference in etching rate due to a difference in crystal plane (for example, (100), (111), etc.). It is better to make the concentration of as low as possible at the level at which the etching reaction proceeds. On the other hand, when the alkali concentration is high, for example, when the NaOH concentration exceeds 0.8 (mol: M), the anisotropy is weakened, and even when silicon is not dissolved, texture cannot be formed efficiently.

On the other hand, when the etching reaction proceeds, silicon of the silicon substrate is dissolved in the etching solution, and Si (OH) ₆ ²⁻ is accumulated in the etching solution. Si (OH) ₆ ²⁻ is an etching inhibitor and inhibits the etching reaction of the above formula (1). That is, the etching reaction is difficult to proceed.

The present inventor used an alkaline solution, which is an etching promoter, as a method capable of performing multiple batch processing of wet etching under alkaline conditions using a texture forming additive in wet etching processing for forming a texture on the surface of a silicon substrate. By adding each batch process according to the concentration of Si (OH) ₆ ²⁻ which is an etching inhibitor, even if the concentration of the alkaline solution (for example, NaOH) is increased, the anisotropy is maintained and the silicon substrate is efficiently formed. It has been found that a texture can be formed. That is, silicon Si (OH) ₆ ²⁻ which is an etching inhibitor dissolved in the etching solution suppresses the etching reaction, but does not cancel the anisotropic action of the alkaline solution. Also, the texture forming additive is not affected by the etching inhibitor silicon.

  In addition, since the etching reaction of the silicon substrate using an alkaline solution is carried out at a high temperature, the amount of the etching solution increases because water evaporates even if an alkaline solution (or solid) is added to the alkaline solution for each batch process. Therefore, it is not necessary to discharge a part of the etching solution, and the load on the wastewater treatment is suppressed.

  FIG. 1 is a characteristic showing the relationship between the molar concentration of silicon and the molar concentration of NaOH in the case of performing 20 batches of wet etching processing for forming a texture on the surface of a silicon substrate using an NaOH solution as an etching solution. FIG. In the formula shown in FIG. 1, x is the molar concentration of silicon (mol: M), and y is the molar concentration of NaOH (mol: M). Here, a NaOH solution having a predetermined concentration (0.2968M) is prepared at the start of etching.

  After the completion of the first batch etching process, the amount of increase in the silicon concentration in the etching solution in the previous batch etching process and the addition of the alkali concentration in the etching liquid after the last batch etching process is completed. Ratio (addition amount of NaOH molar concentration Δy / increase amount of silicon molar concentration Δx, (mol / mol), hereinafter referred to as additional NaOH / increase Si ratio) is constant (predetermined ratio) As described above, NaOH is added to the etching solution without replacing the etching solution to re-prepare the etching solution. That is, in FIG. 1, x and y are in a proportional relationship of a predetermined ratio except for the molar concentration of NaOH (offset value, here, 0.2968) at the start of etching. Here, the alkaline solution is added so that the predetermined ratio of the additional NaOH / increased Si ratio is 1.6 times.

  FIG. 2 is a characteristic diagram showing a change in reflectance (%) of light having a wavelength of 700 nm on the surface of the silicon substrate with respect to the number of batch processes. As shown in FIG. 1, NaOH was additionally added for each batch process depending on the molar concentration of silicon. At this time, as shown in FIG. 2, the reflectance of light having a wavelength of 700 nm is stabilized at about 10%, and it is considered that a good texture was formed on the surface of the silicon substrate.

  Moreover, it is preferable to add NaOH so that the said additional NaOH / increase Si ratio after an etching start may be 1.0-3.0. By setting such conditions, the alkali corresponding to the silicon concentration in the etching solution can be replenished. Therefore, even when the alkali concentration is higher than the initial batch concentration, the anisotropy of etching can be secured, and the density is high. The texture can be formed continuously and stably.

Note that the range of the additional NaOH / increased Si ratio is effective as long as the conditions allow the texture to be uniformly formed on the surface of the silicon substrate in the first batch. Further, in the first batch after the etching solution is replaced, a part of the etching solution before the solution replacement remains, that is, Si (OH) ₆ ²⁻ is one in the remaining etching solution such as in a pipe or a pump. In some cases, the additional NaOH / increased Si ratio may be set within the above range.

  FIG. 3 is a characteristic diagram showing a change in reflectance (%) of light having a wavelength of 700 nm with respect to an additional NaOH / increased Si ratio. Here, the change in the light reflectance at a wavelength of 700 nm of the 20th batch of single crystal silicon substrate with respect to the additional NaOH / increased Si ratio is shown. Etching conditions are 50 mM 1,6-hexanediol as a texture forming additive, NaOH concentration before etching: 1.6 wt%, etching solution temperature: 90 ° C., etching time: 30 minutes, number of silicon substrates in each batch: 30 It was a sheet. Since 20 batches were processed, 600 wafers were used.

  As can be seen from FIG. 3, when the additional NaOH / increased Si ratio is smaller than 1.0, NaOH is insufficient with respect to silicon, the etching reaction does not proceed, and the reflectance increases. On the other hand, when the additional NaOH / increased Si ratio is larger than 3.0, NaOH becomes excessive and the etching anisotropy becomes weak, texture is not formed, and the reflectance increases.

  FIG. 4 is an electron micrograph (magnification: 2000 times) of the silicon substrate surface when the additional NaOH / increased Si ratio is 1.6. FIG. 5 is an electron micrograph (magnification: 2000 times) when the additional NaOH / increased Si ratio is 4.1. As shown in FIG. 4, when the additional NaOH / increased Si ratio was 1.6, it was confirmed that a pyramidal high-density texture was formed. On the other hand, as shown in FIG. 5, when the additional NaOH / increased Si ratio is 4.1, a gap is generated between the pyramidal textures. In this case, it is considered that the light reflectance on the surface of the silicon substrate is lowered.

  From the above, in the etching method of the silicon substrate that constitutes the cell of the solar power generation device, the amount of increase in the concentration of silicon in the etching solution, which is an etching inhibitor, and etching using an alkaline solution containing water and a texture forming additive By adding an alkaline solution for each batch process and performing etching so that the ratio of additional NaOH / increased Si with the additional amount of alkali concentration as a promoter is constant, etching of the silicon substrate can be stably performed over multiple batches. The effect that it can implement and a texture can be efficiently formed in the silicon substrate surface is acquired. Furthermore, the amount of waste water of the etching solution can be reduced, and the effect of reducing the load on waste water treatment can be obtained.

  Note that 1,6-hexanediol, which is a texture-forming additive used in the etching of the present invention, is also called hexamethylene glycol, but in this specification, the name 1,6-hexanediol is used.

  In addition, as the alkaline solution used in the etching of the present invention, at least one of strong bases such as a sodium hydroxide solution, a potassium hydroxide solution, a sodium carbonate solution and a tetramethylammonium hydroxide (TMAH) solution is used. It is preferable.

  In addition, the additive (texture forming additive) used in the etching of the present invention is not limited to 1,6-hexanediol, and for example, isopropanol, 1-butanol, 2-butanol, 1,4-butanediol, 1,2 -Butanediol, 2-methyl-1-propanol, 2-methyl-2-propanol, 2-butene-1,4-diol, 2-methyl-1,3-propanediol, 1-pentanol, 2-pentanol ( sec-amyl alcohol), 3-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol (isoamyl alcohol), 2-methyl-2-butanol (tert-amyl alcohol), 3-methyl-2 -Butanol, 2,2-dimethyl-1-propanol (neopentyl alcohol), 1,5-pentanediol 3-methyl-1,3-butanediol, 2-methyl-1,4-butanediol, 1-hexanol, 2-hexanol, 3-hexanol, 1,2-hexanediol, 1,3-hexanediol, 1, 4-hexanediol, 1,5-hexanediol, 2,3-hexanediol, 2,4-hexanediol, 2,5-hexanediol, 3,4-hexanediol, 1,2,3-hexanetriol, 1 , 2,4-hexanetriol, 1,2,5-hexanetriol, 1,2,6-hexanetriol, 1,3,6-hexanetriol, 2-methyl-1-pentanol, 3-methyl-1- Pentanol, 4-methyl-2-pentanol, 3-methyl-2-pentanol, 2-methyl-3-pentanol, 2-methyl-2,4- Nthanediol, 2-methyl-1,5-pentanediol, 1,2,6-hexanetriol, 1-heptanol, 2-heptanol, 3-heptanol, 4-heptanol, 5-heptanol, 6-heptanol, 7-heptanol 8-heptanol, 2,4-dimethyl-3-pentanol 1,2-heptanediol, 1,3-heptanediol, 1,4-heptanediol, 1,5-heptanediol, 1,6-heptanediol, 1,7-heptanediol, 2,3-heptanediol, 2,4-heptanediol, 2,5-heptanediol, 2,6-heptanediol, 2,7-heptanediol, 3,4-heptanediol, 3 , 5-heptanediol, 3,6-heptanediol, 3,7-heptanediol, and the like Isomers other than the substances listed in (1), those having the same carbon number and linear or side chains and / or those having different OH group positions. Further, ketones such as acetone and methyl ethyl ketone, organic acids such as formic acid, propionic acid, heptanoic acid, hexanoic acid, caproic acid, valeric acid and lactic acid, and salts thereof may be mentioned.

  Such texture formation by etching can be applied to a solar power generation device configured using a semiconductor wafer such as a silicon substrate or a method for manufacturing a solar power generation device having a semiconductor thin film such as a silicon thin film. For example, a texture forming step of forming a texture on the first main surface of the first conductivity type semiconductor wafer, a diffusion layer forming step of forming a second conductivity type diffusion layer on the surface of the textured semiconductor wafer, A diffusion layer removing step of removing a diffusion layer formed on an end surface of the semiconductor wafer; an antireflection film forming step of forming an antireflection film on the first main surface of the semiconductor wafer; and a first main surface of the semiconductor wafer And an electrode forming step of printing and baking an electrode having a predetermined shape on the second main surface opposite to the first main surface, and the texture forming step in the method for manufacturing a solar power generation apparatus, the method described above A uniform texture can be formed by etching the first main surface of the semiconductor wafer using.

  In Embodiment 1 described above, when etching the surface of a silicon substrate by a plurality of batch processes using an alkaline solution containing water and a texture forming additive as an etching solution, an alkaline solution such as NaOH according to the silicon concentration. In other words, the additional NaOH / increased Si ratio between the increased amount of silicon in the etching solution, which is an etching inhibitor, and the additional amount of alkali, which is an etching promoter, is made constant. Since etching is performed by adding an alkaline solution for each batch process, a texture can be efficiently and stably formed on the surface of the silicon substrate even in a process of a plurality of batches.

  Thereby, according to Embodiment 1, it is possible to stably form a high-density texture on the surface of the silicon substrate stably as a process with a smaller amount of additive and a smaller amount of alkaline solution used. Mass processing is possible. As a result, the amount of light absorbed by the silicon substrate is increased, and in the solar power generation device manufactured using this silicon substrate, the current value is increased and the photoelectric conversion efficiency is improved. Moreover, it is not necessary to discharge a part of the etching solution, and the load on wastewater treatment can be reduced. Furthermore, since the number of batches can be etched a plurality of times without exchanging the etching solution, the number of exchanges of the etching solution is reduced, and a long production time can be secured.

  Therefore, according to Embodiment 1 mentioned above, mass production of the silicon substrate which has a low reflectance can be implemented efficiently, and the silicon substrate which has a texture suitable for a solar power generation device can be provided in large quantities. For example, mass production of a silicon substrate having a light reflectance of 10% or less can be efficiently performed.

  Note that although a single crystal silicon substrate is shown as an object to be etched in the first embodiment, it has been confirmed that the same effect as described above can be obtained even with a polycrystalline silicon substrate.

  In Embodiment 1 described above, an efficient texture forming method using multiple batch processing has been described. In the following second and third embodiments, an etching apparatus that realizes the silicon substrate etching method described in the first embodiment will be described.

Embodiment 2. FIG.
FIG. 6 is a diagram schematically illustrating an example of the configuration of the wet etching apparatus according to the second embodiment of the present invention. An alkali supply unit 10, a texture forming additive supply unit 20, a pure water supply pipe 40, and a gas supply unit 50 are connected to the etching tank 1 that performs wet etching.

  The alkali supply unit 10 includes an alkali supply pipe 11 connected to the etching tank 1, and includes an alkali storage tank 12 at the end of the alkali supply pipe 11. The alkali supply unit 10 includes an alkali supply pump 13 that supplies alkali from the alkali storage tank 12 to the etching tank 1 on the alkali supply pipe 11 between the alkali storage tank 12 and the etching tank 1. The alkaline storage tank 12 stores an alkaline solution such as an aqueous sodium hydroxide solution.

  The texture-forming additive supply unit 20 has an additive supply pipe 21 connected to the etching tank 1, and includes an additive storage tank 22 at the end of the additive supply pipe 21. The texture forming additive supply unit 20 supplies the additive from the additive reservoir 22 to the etching reservoir 1 on the additive supply pipe 21 between the additive reservoir 22 and the etching reservoir 1. A supply pump 23 is provided. The additive storage tank 22 stores the texture-forming additive described in the first embodiment (hereinafter may be referred to as an additive).

  The pure water supply pipe 40 supplies ion exchange water, distilled water, or pure water to the etching tank 1. The gas supply unit 50 includes a gas discharge unit 51 provided near the bottom of the etching tank 1 and a gas supply pipe 52 connected to the gas discharge unit 51. The gas discharge part 51 should just be arrange | positioned so that it may be located lower than the etching process target object 72. FIG. Note that it is desirable that the gas discharge unit 51 is a device that can supply small bubbles.

  The etching tank 1 is provided with a temperature control unit 60. The temperature controller 60 includes a heater 61 provided in the etching tank 1 for heating the etching solution in the etching tank 1, a heater heat source 62 provided outside the etching tank 1, a heater heat source 62, and the heater 61. Heater wiring 63 for connecting between the two. Further, the temperature control unit 60 includes a temperature measurement unit 64 put into the etching tank 1 and a heater heat source so that the etching solution becomes a desired temperature based on the temperature of the etching solution measured by the temperature measurement unit 64. And a temperature controller 65 with a thermometer for performing temperature control 62. The heater heat source 62 and the thermometer-equipped temperature regulator 65 are connected via a temperature adjustment signal line 66.

  Furthermore, a carrier support base 71 is provided in the etching tank 1 so as to be above the gas discharge part 51. A carrier 73 that holds a plurality of etching objects 72 such as a silicon substrate is placed on the carrier support base 71. For example, one carrier 73 can hold 10 to 100 silicon substrates. On the carrier support 71, for example, 1 to 10 carriers 73 can be placed.

  The etching tank 1 is also provided with an overflow tank 81. The overflow tank 81 is connected to the etching tank 1 by an etchant circulation pipe 82. An etching solution circulation pump 83 is provided on the etching solution circulation pipe 82 to circulate the etching solution overflowed from the etching vessel 1 to the overflow vessel 81 to the etching vessel 1 through the etching solution supply pipe 85. . Further, a drain 84 for discharging the etching solution is connected to the etching tank 1 and the overflow tank 81.

  In addition, the etching apparatus is provided with a silicon concentration measuring unit 101, a control unit 102, a silicon concentration signal line 103, and a pump signal line 104. Here, the number of carriers 73 is two. The silicon concentration measurement unit 101 is installed on the etching solution circulation pipe 82 and is connected to the control unit 102 via the silicon concentration signal line 103. The control unit 102 is connected to the alkali supply pump 13 via the pump signal line 104. The silicon concentration measuring unit 101 can continuously monitor the silicon concentration.

  Next, the operation of this etching apparatus will be described. First, pure water is supplied to the etching tank 1 through the pure water supply pipe 40, an alkaline solution is supplied from the alkali supply unit 10, and a texture formation additive is supplied from the texture formation additive supply unit 20. Specifically, in the alkali supply unit 10, the alkaline solution is supplied from the alkali storage tank 12 to the etching tank 1 by the alkali supply pump 13 through the alkali supply pipe 11. In the texture forming additive supply unit 20, a predetermined amount of additive is supplied from the additive storage tank 22 to the etching tank 1 by the additive supply pump 23 through the additive supply pipe 21.

  Here, a predetermined amount of pure water, an alkaline solution, and an additive are supplied so that the etching solution filled in the etching tank 1 is in the concentration range described in the first embodiment. For example, there is a method in which the supply flow rates of pure water, alkaline solution, and additive supplied in advance are measured, and the supply time is set so as to be within the predetermined concentration range. In addition, a water level gauge (not shown) is installed in the etching tank 1, and after supplying an alkaline solution and an additive, pure water is supplied. Can be kept constant. Furthermore, since the etching solution is maintained at a high temperature, the water is evaporated and the amount of the etching solution is reduced. Therefore, it is preferable that pure water is automatically supplied appropriately based on the measured value of the water level meter.

  Next, gas is continuously supplied from the gas discharge part 51 into the etching tank 1 through the gas supply pipe 52. Here, the gas supplied into the etching tank 1 is preferably a gas containing at least one of air, oxygen, nitrogen, ozone, hydrogen, helium, argon, krypton, xenon, methane, and propane.

  Further, the etchant overflowed into the overflow tank 81 is sent from the etchant supply unit 90 to the etch tank 1 via the etchant circulation pipe 82 and circulated by the etchant circulation pump 83. In the etchant supply unit 90, holes of appropriate sizes are formed at appropriate intervals so that the etchant uniformly spreads into the etch tank 1. Further, the etching solution in the etching tank 1 is heated to a predetermined temperature by the heater 61.

  Next, a transfer mechanism such as a robot arm (not shown) puts one to a plurality of carriers 73 (two in FIG. 6) in which a single crystal silicon substrate, which is an object to be etched 72, is stored, into the etching tank 1. To be placed on the carrier support 71. Then, the silicon substrate is immersed for a predetermined time, and a wet etching process is performed on the silicon substrate. After a predetermined time has elapsed, the carrier 73 is pulled up from the etching tank 1 again by a robot arm or the like. The pulled first carrier 73 is sent to the next step.

  At that time, it is preferable that the silicon substrate is subjected to a damage etching process in advance. As the damage etching treatment, there is a method of immersing a silicon substrate in an alkaline solution (for example, 3 wt% to 10 wt% as NaOH concentration, temperature: 70 ° C. to 90 ° C., damage etching time: 1 min to 5 min) for a predetermined time. is there. However, it is possible even when there is no damage etching.

  During the etching process, the silicon concentration measurement unit 101 measures the silicon concentration in the etching solution flowing through the etching solution circulation pipe 82 by infrared spectroscopy (IR). The measured value of the silicon concentration in the etching solution is sent to the control unit 102 through the silicon concentration signal line 103 as needed.

  The control unit 102 calculates the amount of alkali that needs to be added based on the measured value of the silicon concentration in the etching solution when the batch process is completed or during the etching process. The calculated required alkali amount information (additional amount information) is sent from the control unit 102 to the alkali supply pump 13 via the pump signal line 104.

  The alkali supply pump 13 is based on the required alkali amount information (additional amount information) sent from the control unit 102, or when the batch is completed or during the etching process, the alkali storage tank 12 is collectively or continuously. Then, an alkaline solution is additionally supplied to the etching tank 1 through the alkali supply pipe 11.

  Then, the etching tank 1 is stirred by the etching liquid circulation pump 83, and the additionally supplied alkaline solution is mixed with the etching liquid in the etching tank 1.

  Subsequently, one to a plurality of carriers 73 in which different silicon substrates are similarly stored are put into the etching tank 1 by a robot arm or the like, and etching processes are sequentially performed. When the predetermined number of batches of wet etching processing are completed, the etching solution in the etching tank 1 is discharged through the drain 84. Thereby, a series of multiple batch processing is completed.

  Thereafter, an etching solution is newly prepared in the etching tank 1 as described above, and a plurality of batch processing of the next lot is performed in the same manner as described above. The silicon substrate after etching is washed with pure water or the like, neutralized with hydrofluoric acid, washed again with pure water or the like, and then dried.

  Note that the number of batches is determined from the etching rate and processing time of the silicon substrate. The etching process time can be changed for each batch number, but the etching process time can be made constant by adjusting the concentration and the amount of the alkaline solution added.

  Here, the alkali and additive supplied to the etching tank 1 are preferably liquid, but may be supplied by dissolving the solid in pure water or the like, and a predetermined amount of the solid is directly charged into the etching tank 1. Also good. When 1,6-hexanediol is used as an additive, 1,6-hexanediol is a solid at room temperature, and is thus dissolved in pure water or the like and supplied as a liquid. The concentration of the liquid 1,6-hexanediol is preferably about 10 wt% to 80 wt%, and the concentration does not solidify even at 0 ° C. within this concentration range. Alternatively, 1,6-hexanediol may be directly charged into the etching tank 1 in a solid state using manual work or a machine.

  Moreover, the alkali supplied before one batch and after batch processing may be supplied from the same alkali storage tank 12, or may be supplied from separate alkali storage tanks 12. In the case where the alkali storage tank 12 is provided separately, for example, an alkali supply facility to be supplied after batch processing is required.

  Further, the silicon in the etching solution dissolved by the etching reaction can be recovered separately by drying, coagulating precipitation, or the like. It is also possible to reuse the etching solution by adding alkali again to the etching solution excluding silicon. As a result, not only the amount of pure water and additives used can be suppressed, but also the load on wastewater treatment can be reduced. Furthermore, although the batch type etching process has been described in the above description, a single wafer type etching process is also possible.

  Here, with respect to the supply of the alkaline solution and the additive, the additive supply pump 23 and the alkali supply pump 13 may be omitted, and a mechanism capable of weighing the additive storage tank 22 and the alkali storage tank 12 may be provided. For example, a predetermined amount of additive or alkaline solution is supplied from the outside to the additive storage tank 22 or the alkaline storage tank 12 by a pump (not shown). This predetermined amount can be measured by the weighing function of the additive storage tank 22 or the alkali storage tank 12. Subsequently, the additive and the alkaline solution stored in the additive storage tank 22 and the alkaline storage tank 12 are opened as they are in the etching tank by opening a valve (not shown) set in the additive storage tank 22 and the alkaline storage tank 12. Drop to 1 and supply.

  The silicon concentration may be calculated indirectly from the measurement result of the amount of hydrogen gas generated during etching, the change in the weight of the silicon substrate before and after etching, or the silicon concentration in the etching solution may be directly measured. Further, the amount of alkali supply after the end of one batch may be estimated from the change in the weight of the silicon substrate in the etching process performed in advance.

  According to the second embodiment described above, the silicon substrate etching method described in the first embodiment can be realized, and the effect of the silicon substrate etching method according to the first embodiment can be obtained.

  Note that although a single crystal silicon substrate is shown as an etching target in the second embodiment, it has been confirmed that the same effect as described above can be obtained even with a polycrystalline silicon substrate.

Embodiment 3 FIG.
FIG. 7 is a diagram schematically illustrating an example of the configuration of the wet etching apparatus according to the third embodiment of the present invention. In this etching apparatus, in the configuration of the wet etching apparatus according to the second embodiment, a fine bubble generation unit 53 is provided outside the etching tank 1 instead of the gas release unit 51. Further, the fine bubble generating unit 53 is provided between the etching tank 1 on the etching solution supply pipe 85 and the etching solution circulation pump 83. As this fine bubble generating part 53, a bubble generating device such as a slit type, a porous plate type, an arrayed porous plate type, an ultra fine needle type, a membrane type, a pressure dissolution type, a venturi type or the like can be used. Further, the gas supply pipe 52 is connected to the fine bubble generating unit 53. In addition, the same code | symbol is attached | subjected to the component same as Embodiment 2, and the description is abbreviate | omitted.

  By adopting such a configuration, when the etching solution overflowed into the overflow tank 81 is returned to the etching tank 1 by the etching liquid circulation pump 83, the etching liquid containing the fine bubbles generated in the fine bubble generating unit 53 is contained. It will be returned to the etching tank 1. Note that air may be used as the gas, or another gas described in the first embodiment may be used. Further, the present invention can be applied not only to a batch type wet etching process but also to a single wafer type wet etching process.

  According to the third embodiment described above, the silicon substrate etching method described in the first embodiment can be realized in the same manner as the wet etching apparatus according to the second embodiment, and the silicon according to the first embodiment can be realized. The effect of the substrate etching method can be obtained.

  Furthermore, according to the third embodiment, the bubbles contained in the etching solution returned to the etching tank 1 are miniaturized, whereby the stirring of the etching solution is promoted, and the texture can be formed more uniformly in the surface of the silicon substrate. The effect of being able to be obtained is obtained.

Embodiment 4 FIG.
In the fourth embodiment, a method for forming a texture on a silicon substrate in a more uniform and stable manner in the silicon substrate etching method and the etching apparatus using the first to third embodiments will be described.

  As the etching reaction of the silicon substrate proceeds, dissolved silicon is accumulated in the etching solution. In addition, foreign substances such as metals and organic substances brought into the silicon substrate, foreign substances such as metals and organic substances that dissolve from the environment, and foreign substances such as metals and organic substances that are generated and eluted from the wet etching apparatus are also present in the etching solution. Dissolve in The presence of such silicon and foreign matter may not allow the texture formation to proceed well.

  Furthermore, the surface shape of silicon differs depending on the method of slicing the silicon substrate. In addition, the texture formation may not proceed well due to the change in the slicing method. In particular, the smoother the surface shape of the silicon substrate, the more difficult it is to form the texture uniformly.

  Therefore, the texture formation status varies depending on the state of the etchant (the amount of silicon dissolved and foreign matter) and the type of silicon substrate (difference in the silicon substrate slicing method and the amount of metal inside the silicon substrate), so it is stable. Therefore, there is a demand for a method capable of forming a texture.

  In the formation of texture, it is important to increase the anisotropy and advance the etching reaction in that state. In addition, the first reaction for forming the texture, that is, the surface state of the silicon substrate at the time when the etching reaction starts. The present inventor has further found out that is important. In other words, making the surface state of the silicon substrate easy to form a texture is the point at which a uniform texture can be formed on the surface of the silicon substrate, and the difference in the surface state of the silicon substrate immediately before the etching reaction is completed. Found out that it has an effect. By making the texture formation starting point uniformly in advance over the entire surface of the silicon substrate at the start of the etching reaction, the texture formation can be made easier.

  Therefore, the present inventor pre-immerses the silicon substrate for about 1 second to 300 seconds in an aqueous solution (concentration is 0.1 wt% to 80 wt%) containing 1,6-hexanediol which is a texture forming additive as pre-etching treatment. Then, the silicon substrate etching method and the etching method using the etching apparatus shown in the first to third embodiments without washing with water and the like are effective for forming a texture on the silicon substrate uniformly and stably. I found out that

  By performing such treatment, the texture forming additive is adsorbed uniformly on the silicon substrate in advance in the surface to prevent the formation of the texture due to the foreign matter, and the texture formation adsorbed on the surface of the silicon substrate. An additive becomes a starting point of texture formation, and it becomes easy to form a texture.

  On the other hand, if the texture forming additive is not previously adsorbed uniformly on the silicon substrate, there is no origin of texture formation on the surface of the silicon substrate, and the formation of the texture is liable to be inhibited by foreign matter or dissolved silicon. . In addition, when a silicon substrate is immersed in the aqueous solution containing the 1,6-hexanediol, and then poured into an etching solution, it is considered that 1,6-hexanediol attached to the silicon substrate during the immersion gradually moves into the etching solution. . However, 1,6-hexanediol remains on the silicon substrate until the etching reaction is started, and texture formation by etching is started from there, and the dissolved silicon present in the above-described etching solution. In addition, it is possible to suppress the adsorption of the silicon substrate by the foreign material or foreign matter, that is, to prevent the formation of the texture and to easily form the texture.

  FIG. 8-1 is a diagram schematically showing an image of the reaction state at the start of the etching reaction when the silicon substrate is immersed in a 1,6-hexanediol solution before the etching treatment. FIG. 8-2 is a diagram schematically showing an image of the reaction state at the start of the etching reaction when the silicon substrate is not immersed in the 1,6-hexanediol solution before the etching treatment.

  As shown in FIG. 8A, when the silicon substrate is immersed in the 1,6-hexanediol solution before the etching process, 1,6-hexanediol 203 is adsorbed on the surface of the silicon substrate 205. Adsorption of foreign matter (organic matter) 201, foreign matter (metal) 202 and 1,6-hexanediol 203 on the surface of silicon substrate 205 is suppressed, and inhibition of etching due to NaOH caused by these impurities is suppressed.

  On the other hand, as shown in FIG. 8B, when the silicon substrate is not immersed in the 1,6-hexanediol solution before the etching process, 1,6-hexanediol 203 is adsorbed on the surface of the silicon substrate 205. Therefore, foreign matter (organic matter) 201, foreign matter (metal) 202, and 1,6-hexanediol 203 can be adsorbed on the surface of the silicon substrate 205, and etching by NaOH caused by these impurities is hindered.

  FIG. 9 is a diagram showing a processing method in the case where the pre-etching treatment and the etching treatment in which the silicon substrate is immersed in the texture forming additive are successively performed. When the pre-etching process and the etching process are performed continuously, as shown in FIG. 9, 50 silicon substrates to be etched 72 are set on a carrier 73 and 1,6-hexanediol having a concentration of 50 wt% is set. The carrier 73 is immersed for 30 seconds in the pretreatment tank 111 containing the aqueous solution containing. When the carrier 73 is immersed, the conveyor 112 may hold the carrier 73 or may be separated.

  After immersing the carrier 73 for a predetermined time, the carrier 73 is pulled up from the pretreatment tank 111 by using the conveyor 112. Next, the carrier 73 is immersed in the etching tank 1 using the transfer device 112, and etching is performed to form a texture. The etching solution in the etching tank 1 is adjusted in advance for NaOH concentration, 1,6-hexanediol concentration, and temperature. For example, the wet etching apparatus described in the second and third embodiments can be used for the etching in the etching tank 1.

  After performing the etching for forming the texture in the etching tank 1, the silicon substrate is washed with pure water, the oxide film on the surface is removed with hydrofluoric acid, and dried (not shown).

  FIG. 10 is a characteristic diagram showing a change in reflectance of light having a wavelength of 700 nm after a plurality of batch processes depending on the presence or absence of the pre-etching process when impurities (copper ions) are mixed in the etching solution. The result shown in FIG. 10 is a result of performing multiple batch processing 20 times on an as slice (sliced with a wire saw) silicon substrate. As the pre-etching treatment, the silicon substrate was previously immersed in a 50 wt% 1,6-hexanediol solution for 30 seconds before etching for texture formation. The etching conditions are the same when the pre-etching treatment is performed and when it is not performed.

  As shown in FIG. 10, when the pre-etching treatment was performed, the reflectance of light having a wavelength of 700 nm was on the order of 10% to 11% in all batches even when the batch treatment was repeated. On the other hand, when the pre-etching treatment was not performed, the light reflectance increased as the batch processing was repeated. As a result, the silicon substrate is preliminarily immersed in the 1,6-hexanediol solution before the texture forming etching process as a pre-etching process to form a texture starting point on the substrate surface and suppress the inhibition of the texture formation due to impurities. I was able to confirm that I was able to.

  Furthermore, the influence of the presence or absence of the pre-etching treatment was examined on a silicon substrate that had been etched with only NaOH to remove the damaged layer and smooth the surface. FIG. 11 is a characteristic diagram showing a change in reflectance of light having a wavelength of 700 nm after a plurality of batch processes depending on the presence or absence of a pre-etching process on a silicon substrate subjected to damage etching. The result shown in FIG. 11 is a result of performing multiple batch processing 20 times on a silicon substrate that has been subjected to damage etching. As the pre-etching treatment, the silicon substrate was previously immersed in a 50 wt% 1,6-hexanediol solution for 30 seconds before etching for texture formation. The etching conditions are the same when the pre-etching treatment is performed and when it is not performed.

  As shown in FIG. 11, when the pre-etching process was performed, the reflectance of light having a wavelength of 700 nm was in the range of 10% to 11% in all batches even when the batch process was repeated. On the other hand, when the pre-etching treatment was not performed, the light reflectance increased as the batch processing was repeated. As a result, even when the surface of the silicon substrate is smooth and the starting point of texture formation is small, the silicon substrate is stably immersed in the 1,6-hexanediol solution in advance as a pre-etching treatment before the texture forming etching treatment. It was confirmed that a texture could be formed.

  The texture formation etching conditions shown in FIGS. 10 and 11 are the same. The NaOH concentration is 2 wt%, the etching solution temperature is 87 ° C., the amount of 1,6-hexanediol charged in the texture formation tank is 2 wt%, and the etching time is 25. Minutes. Furthermore, the amount of NaOH to be replenished for each batch was set so that the additional NaOH / increased Si ratio was 2.0. Further, 50 batches of silicon substrates in one batch of the etching process and the amount of the etching solution are 35L.

  According to the above-described fourth embodiment, in the etching using the silicon substrate etching method and the etching apparatus described in the first to third embodiments, the texture forming additive that is the starting point of the texture formation in advance before the texture forming etching. By carrying out the pre-etching treatment in which the silicon substrate is brought into contact with and held, a high-density texture can be stably formed without being affected by impurities such as silicon, metal, and organic matter in the etching solution. As a result, the amount of light absorbed by the silicon substrate is increased, and in the solar power generation device manufactured using this silicon substrate, the current value is increased and the photoelectric conversion efficiency is improved.

  In addition, the chemical | medical agent which can be used by an etching pre-processing can use not only 1, 6- hexanediol but ketones, such as IPA (isopropyl alcohol) and acetone, and an organic acid or its salt, for example. However, 1,6-hexanediol is most effective, and when IPA or the like having a low boiling point is used, it is necessary to supplement the amount reduced by volatilization. Even if it does not volatilize like 1,6-hexanediol, the liquid in the pretreatment tank is taken out of the tank by the silicon substrate or the carrier, so it is necessary to replenish appropriately. Moreover, since the effect as a pre-etching treatment is sufficient if 1,6-hexanediol is attached to the surface of the silicon substrate, it is not limited to immersion, and for example, the silicon substrate surface contains 50 wt% of 1,6-hexanediol. An aqueous solution may be applied.

  As described above, the method for etching a silicon substrate according to the present invention is useful for forming a high-density texture stably and inexpensively on a silicon substrate.

DESCRIPTION OF SYMBOLS 1 Etching tank 10 Alkali supply part 11 Alkali supply pipe 12 Alkali storage tank 13 Alkali supply pump 20 Texture formation additive supply part 21 Additive supply pipe 22 Additive storage tank 23 Additive supply pump 25 Etching time 40 Pure water supply pipe 50 Gas supply section 51 Gas discharge section 52 Gas supply pipe 53 Fine bubble generation section 60 Temperature control section 61 Heater 62 Heater heat source 63 Heater wiring 64 Temperature measurement section 65 Temperature controller with thermometer 66 Temperature adjustment signal line 71 Carrier support base 72 Etching object 73 Carrier 81 Overflow tank 82 Etch solution circulation pipe 83 Etch solution circulation pump 84 Drain 85 Etch solution supply pipe 90 Etch solution supply unit 101 Silicon concentration measurement unit 102 Control unit 103 Silicon concentration signal line 04 pump signal line 111 pretreatment tank 112 conveyor 201 foreign substances (organic matter)
202 Foreign object (metal)
203 1,6-hexanediol 204 NaOH
205 Silicon substrate

Claims (6)

An etching process in which an alkaline etching solution containing an additive that inhibits the etching reaction on the silicon substrate surface and water is supplied to the surface of the silicon substrate to form irregularities on the surface of the silicon substrate by wet etching is repeated over a plurality of batches. A silicon substrate etching method to be performed,
After the completion of the etching process of the first batch, the amount of increase in the silicon concentration in the etching solution in the previous batch etching process and the addition of the alkali concentration in the etching solution after the end of the previous batch etching process Repeatedly performing the etching process over a plurality of batches by adding alkali to the etching solution for each batch so that the ratio of the amount is constant.
A method for etching a silicon substrate. Contacting and holding the additive on the surface of the silicon substrate as a pre-process for performing the wet etching;
The method for etching a silicon substrate according to claim 1. The additive is 1,6-hexanediol;
The method for etching a silicon substrate according to claim 1, wherein: The ratio of the additional amount of the alkali concentration to the increased amount of the silicon concentration is 1 to 3 in terms of molar ratio.
The method for etching a silicon substrate according to any one of claims 1 to 3. An etching tank for etching the silicon substrate by storing an etchant while supplying an additive that inhibits the etching reaction on the surface of the silicon substrate, an alkali, and water, and preparing the etchant;
Alkali supply means for supplying alkali to the etching tank;
Water supply means for supplying water to the etching bath;
An additive supply means for supplying the additive to the etching tank;
Heating control means for heating the etching solution to a predetermined temperature;
Arithmetic means for determining the supply amount of alkali to be additionally supplied to the etching solution according to the silicon concentration in the etching solution;
With
The calculation means includes an amount of increase in the silicon concentration in the etching solution in the previous etching process of the silicon substrate in the etching tank, and the etching solution after the previous etching process of the silicon substrate in the etching tank. Determining the supply amount of the alkali to be additionally supplied so that the ratio of the alkali concentration to the inside is constant,
An etching apparatus characterized by the above. Comprising a silicon concentration measuring means for measuring the silicon concentration in the etching solution overflowed from the etching tank;
The calculating means determines the supply amount of the alkali to be additionally supplied according to the silicon concentration measured by the silicon concentration measuring means;
The etching apparatus according to claim 5.