JP4495572B2 - Stain film removal method - Google Patents

Description

  The present invention relates to a stain film removing method for removing a stain film generated after etching a semiconductor substrate or a ceramic substrate.

In a semiconductor element manufacturing process or a ceramic element manufacturing process, etching of a semiconductor substrate or a ceramic substrate is performed with a mixed solution of hydrofluoric acid and nitric acid as described in Patent Document 1, for example. That is, the semiconductor substrate or the ceramic substrate is etched by a solution made of nitric acid having oxidizing power, hydrofluoric acid or the like that dissolves the oxide film. When a silicon substrate or a ceramic substrate is etched with a high-concentration acid aqueous solution that allows an etching reaction to proceed, and taken out from the reaction solution, a black-brown porous silicon film called a stain film adheres to the surface of the silicon substrate or ceramic substrate. Hereinafter, it is referred to as a stain film. In order to remove the stain film, it is necessary to permeate with a strong alkaline aqueous solution such as a NaOH aqueous solution. The silicon substrate or ceramic substrate that has been infiltrated with a strong alkaline aqueous solution needs to be further neutralized with an aqueous acid solution (Non-Patent Document 1).
Further, Patent Document 2 discloses a method in which noble metal (platinum) is used for the protective film and etching is performed so that the stain film does not adhere.
Japanese Patent Laid-Open No. 2-1996 Japanese Patent Laid-Open No. 62-13033 A.Hauser and 5 others, “A SIMPLIFIED PROCESS FOR ISOTROPIC TEXTURING OF MC-SI”, 3rd World Conference on Photovoltaic Energy Conversion Proceedings, Osaka, 11-18, MAY 2003, 4PC433

However, as in Non-Patent Document 1, in the method of removing the stain film using strong alkali, NaOH contains alkali metal such as sodium which adversely affects the characteristics of the semiconductor, and NaOH remains as white powder in the device. Contaminate equipment. Furthermore, as shown in FIG. 5, the etching process immersed in the mixed acid solution tank 51, the cleaning process immersed in the water tank 52, the stain film removing process immersed in the strong alkaline solution tank 53, the cleaning process of the water tank 54, the acid neutralization liquid tank Like the neutralization process immersed in 55 and the washing | cleaning process of the water tank 56, since a process is passed, a process becomes long and the usage-amount of chemical | medical solution also increases.
In addition, the etching method in which the stain film does not adhere in Patent Document 2 uses a noble metal (platinum) as the protective film, resulting in high cost.
SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a method for removing a stain film at low cost and not contaminating the semiconductor substrate or the ceramic substrate itself after etching the silicon substrate or the ceramic substrate with a mixed acid solution. It is what.

In the method for removing a stain film of the present invention, in order to remove the stain film formed on the surface of the substrate when the silicon substrate is etched with hydrofluoric acid, the etching is followed by washing with pure water, and then ammonia water, alkaline ionized water, The problem is solved by washing with a stain film removing solution containing at least one hot water and having an OH concentration of 3 × 10 ⁻⁶ to 2 × 10 ⁻² mol / L.
In the stain film removing method of the present invention, the pH value of the aqueous ammonia is preferably 9.5 to 12.0.
In the stain film removing method of the present invention, the pH value of the alkaline ionized water is preferably 7.5 to 12.0.
In the stain film removing method of the present invention, it is further preferable that the alkaline ionized water is washed at 80 to 100 ° C.
In the stain film removing method of the present invention, it is desirable that the warm water is 80 ° C. to 100 ° C.

In the method for removing a stain film of the present invention, in order to remove the stain film formed on the surface of the substrate when the silicon substrate is etched with hydrofluoric acid, the etching is followed by washing with pure water, and then ammonia water, alkaline ionized water, Washing with a stain film removing solution containing at least one hot water and having an OH-concentration of 3 × 10 ⁻⁶ to 2 × 10 ⁻² mol / L allows the stain film to be almost completely removed. In the present invention, since the stain film removing liquid does not contain a metal element such as sodium or potassium, the semiconductor element or ceramic element is not contaminated by heat treatment in the subsequent semiconductor manufacturing process or ceramic element manufacturing process.
In addition, the acid neutralization treatment step after the stain film removal step is unnecessary, and the amount of chemical solution used and the amount of pure water used are small compared with the conventional step. As a result, it is possible to reduce the amount of waste liquid and waste water, and the processing cost can be reduced.
Furthermore, in the case of alkaline ionized water and hot water treatment, the waste liquid treatment cost can be further reduced, and the environmental load can be reduced.
In addition, since the present invention has no process change and no additional process other than the stain film removal process, it is possible to reduce the processing cost and the environmental load without increasing the manufacturing cost.

  The present invention removes a stain film without using a strong alkali such as sodium hydroxide or potassium hydroxide, and after etching a semiconductor substrate or a ceramic substrate with mixed acid, at least one of ammonia water, alkaline ionized water, and hot water. The present inventors have found that the stain film adhering to the wafer surface after the mixed acid etching can be easily removed by cleaning with a stain film removing solution containing, without contaminating the wafer or apparatus with metal contamination, shortening the process.

The present invention relates to a method for removing a stain film after a silicon etching step using a mixed acid in the production of a crystalline silicon solar battery cell for homes, in particular where cost reduction is required, and the silicon stain film generated in the etching step is treated with sodium hydroxide or hydroxide. By removing without using strong alkali such as potassium, the process can be shortened and the cost can be reduced at the mass production level as compared with the conventional process.
The present invention eliminates a stain film containing at least one of ammonia water, alkali ion water, and hot water after mixed acid etching of a semiconductor substrate without using a strong alkali aqueous solution containing an alkali metal element such as sodium hydroxide or potassium hydroxide. By washing with the liquid, the stain film adhering to the surface during the mixed acid etching can be easily removed without metal contamination of the semiconductor substrate or device. Ammonia water, alkaline ionized water, and warm water may be used singly or in combination of two or more. Although ammonia water, alkaline ionized water, and hot water are described in the embodiments, they may not be pure, may contain other aqueous solutions, and contain impurities that do not affect the operation of the semiconductor element. May be.
In addition, ammonia water, alkaline ionized water, and hot water used as a stain film removing solution by the present invention can be processed in a shorter time than that of an aqueous alkali solution such as sodium or potassium, and a small amount of pure water is used. become. As a result, the amount of drainage can be reduced, and the processing cost can be reduced. Furthermore, since the present invention does not change other accompanying processes, it is possible to reduce only the processing cost and the environmental load without increasing other manufacturing costs.

Examples of the semiconductor substrate in the present invention include single crystal silicon and polycrystalline silicon. The silicon substrate usually has N-type or P-type polarity, but in the present invention, it may have either polarity. In the silicon substrate, the P-type impurity includes Group 3 elements such as boron and aluminum, and the N-type impurity includes Group 5 elements such as phosphorus and arsenic. The impurity concentration of the substrate doped to give P-type or N-type polarity to the silicon substrate is preferably 10 ¹³ to 10 ¹⁸ / cm ³ , although it varies depending on the type of impurity.

In mixed acid etching to which a silicon stain film adheres, the most common mixed acid is a mixed acid of hydrofluoric acid and nitric acid. In general, the etching reaction is performed at a temperature of about 0 ° C. to 30 ° C. The silicon stain film is formed after the etching is completed and before the silicon wafer is put into the washing tank in the atmosphere when being transferred from the mixed acid solution tank to the washing tank for stopping the etching reaction. Depending on the mixing ratio of the mixed acid, the stain film formation reaction may proceed during the etching.
As the aqueous solution used for removing the stain film, an aqueous solution containing at least one of ammonia water, alkali ion water, and hot water is used, and it may be not strong alkali. For example, the OH ⁻ concentration is preferably 0.1 mol / L to 10 ⁻⁷ mol / L. When the OH ⁻ concentration is 0.1 mol / L or more, the stain film removal reaction rate is increased and the processing time is shortened, so that control in the mass production process becomes difficult, and not only the stain film but also the substrate etching reaction tends to proceed. Become. When the OH ⁻ concentration is 10 ⁻⁷ mol / L or less, the solution becomes acidic when the solution is at room temperature, and thus the stain film removal reaction hardly proceeds. Therefore, 3 × 10 ⁻⁶ mol / L to ² × 10 ⁻² mol / L is particularly preferable because of easy control of the reaction.

  For example, ammonia water is used as a cleaning liquid having a high hydroxide ion concentration used for removing the stain film, and the ammonia water has a concentration of 0.005 to 5%, a pH of 9.5 to 12, and a treatment temperature of Performed at 30 ° C. to 0 ° C. Although the treatment time varies depending on the concentration and pH, it is immersed in aqueous ammonia for 5 to 200 seconds. When the concentration of ammonia water is 0.005% or less, the effect of removing the stain film decreases. When the concentration of ammonia water is 5% or more, not only the stain film but also the etching reaction of the substrate easily proceeds. That is, the aqueous ammonia solution is less effective at removing the stain film at pH 9.5 or lower. In addition, since the aqueous ammonia easily proceeds to the substrate etching reaction at pH 12 or higher, the pH is preferably 9.5 to 12.

Alkaline ion water can be used as a cleaning liquid having a high hydroxide ion used for removing the stain film. Alkaline ion water is generated on the negative electrode side by electrolysis of water. The water used for the electrolysis of water may be pure water or tap water, and additives such as potassium chloride and sodium chloride that promote electrolysis of water may or may not be added. At this time, the pH of the alkaline ionized water is 7.5 to 12, the redox potential is +100 mV to −1000 mV, and the treatment temperature is 20 ° C. to 100 ° C. Although the treatment time varies depending on the pH and treatment temperature, it is immersed in alkaline ionized water for 10 to 1000 seconds. Alkaline ion water has a less effective stain film removal at pH 7.5 or lower. In the case of alkaline ionized water, the pH may be 12 or more. However, since it takes a lot of time and energy to produce alkaline ionized water having a pH of 12 or more, the pH is preferably about 12 or less from the viewpoint of cost. When the treatment temperature of the alkaline ionized water is 20 ° C. or less, the effect of removing the stain film is reduced and a longer treatment time is required. Therefore, the treatment temperature is preferably 20 ° C. or more.
When alkaline ion water is generated by electrolysis of water, the acidic water that is simultaneously generated on the positive electrode side is used as wafer cleaning water after removal of the stain film, and during drainage, the alkaline ion used for removal of the stain film is used. Water and acid water used for washing can be mixed to neutralize and drain.

Moreover, the warm water used for stain film removal uses the water which heated the pure water to 80 to 100 degreeC. The hot water at this time is neutral and the pH is in the range of 6.0 to 6.5, but the hydroxide ion concentration [OH ⁻ ] was 10 ⁻⁷ mol / l at 25 ° C., but 80 ° C. Since it is 10 ⁻⁶ mol / l as described above, the stain film can be removed. Although processing time changes with temperature, it is immersed for 1 to 30 minutes. An additive such as potassium chloride may be added to raise the boiling point, and hot water at 100 ° C. or higher may be used. Further, in the state of warm water, the temperature at normal pressure does not become 100 ° C. or higher when there is no additive, but the stain film can be removed even by using heated steam heated to 100 ° C. or higher. .
The following examples further illustrate the present invention.

  A process schematic diagram of the stain film removal of Example 1 is shown in FIG. The as-sliced silicon substrate 1-A in FIG. 1 is put into the etching reaction tank A containing the mixed acid solution 1 and etched as shown in FIG. 3A (step S1 in FIG. 2). After completion of the etching, the etched silicon substrate 1-B is taken out from the tank A and transferred to the washing tank B containing the pure water 2 (step S2). Pure water is neither heated nor cooled, is at room temperature, and is 10 ° C to 30 ° C. The etching solution remaining on the surface of the silicon substrate during the transfer of the silicon substrate 1-B as shown in FIG. 3 (b) until the silicon substrate 1-B is taken out from the reaction vessel A and enters the washing bath B. In reaction, a product generally called a silicon stain film S is deposited on the surface of the silicon substrate. In order to prevent the stain film S from adhering, the exposure time to the atmosphere when transferring from the etching reaction tank A to the rinsing tank B should be less than 2 seconds. When mass transport is used, it is difficult to transport within 2 seconds. Further, depending on the mixing ratio of the mixed acid aqueous solution, the stain film may adhere even in the aqueous solution. In this case, the stain film removal process is necessary in any case.

  In the present invention, the chemical reaction (etching reaction) by the acid on the surface of the silicon substrate is stopped in the water rinsing tank B, and the silicon substrate 1-C having the stain film S attached to the surface after the water washing is removed from the stain film removing tank. Transfer to C (step S3). The stain film removal tank C contains ammonia water 3 as a stain film removal solution, and the stain film S is removed as shown in FIG. The silicon substrate 1-D from which the stain film has been removed is transferred to a water washing tank D containing pure water 3 and washed with water (step S4). Then, it dries and complete | finishes an etching process.

FIG. 1 will be described in more detail.
As a substrate used in FIG. 1, for example, a P-type polycrystalline silicon substrate having a size of 126 mm × 126 mm or 155 mm × 155 mm can be used. However, since the substrate size and the cell characteristics are not directly related, a substrate having any shape or size may be used. Further, not only a polycrystalline silicon substrate but also a single crystal silicon substrate may be used, and a substrate other than silicon such as PZT piezoelectric ceramics may be used.
In the case of a silicon substrate, a thin film silicon substrate can also be used. In the case of single crystal silicon, any substrate manufactured by CZ, MCZ, FZ method or the like may be used. In any case, the thickness of the substrate may be any thickness as long as the mechanical strength is maintained. Usually, it is several tens of micrometers to several hundreds of micrometers.
The polarity of the silicon substrate may be either P-type or N-type. In the case of P-type, a Group 3 element such as boron or aluminum is doped. In the case of N-type, a Group 5 element such as phosphorus or arsenic is doped. The The resistivity of the silicon substrate is not particularly limited and depends on the application, but is generally in the range of 0.5 to 1000 Ωcm.

  As the mixed acid solution 1 used in FIG. 1, for example, a mixed solution of hydrofluoric acid and nitric acid is used. Examples of acids having oxidizing power such as nitric acid include sulfuric acid and hydrogen peroxide, and any combination of acid having oxidizing power and hydrofluoric acid that etches an oxide film is not limited to hydrofluoric acid and nitric acid. Any mixed acid may be used, and acetic acid or the like may be added. The concentration of the mixed acid used here is such that the concentration of the acid in the solution is 5 to 50%. The mixing ratio of hydrofluoric acid and nitric acid is hydrofluoric acid: nitric acid = 3: 1 to 1:20 when a general 50% hydrofluoric acid and 60% nitric acid are used. However, this is not limited as long as chemical etching with acid proceeds. The temperature of the etching solution must be below room temperature because the hydrofluoric acid that is always included in the mixed acid under any conditions is an aqueous solution in which hydrogen fluoride, which is a gas at room temperature and normal pressure, is dissolved in water. The temperature of the mixed acid solution is preferably 0 to 30 ° C. The time required for etching depends on the concentration of the mixed acid solution, the temperature, the thickness of the as-sliced silicon substrate and the desired thickness, and is 0.5 to 30 minutes. Here, since the etching amount of the substrate may be more than the extent that the slice damage layer is removed, the etching amount is about 2 microns or more on one side and the thickness that can maintain the strength of the substrate. There is no upper limit.

Ammonia water is used as the stain film removing liquid used in FIG. Here, the concentration of ammonia water used is such that the ratio of ammonia is 0.005 to 5% of water, and the pH value at this time is 9.5 to 12.0. The temperature of the ammonia water is an aqueous solution in which ammonia, which is a gas at normal temperature and pressure, is dissolved in water, and since it is highly volatile, it must be at or below normal temperature. It is preferable that temperature is 0-30 degreeC. The time required for removing the stain film is determined depending on the concentration (pH value) of the ammonia water, the temperature, and the concentration and mixing ratio of the mixed acid that determines the thickness of the stain film, but is 2 to 200 seconds.
Through the above steps, the stain film was completely removed by visual and microscopic confirmation.

The process for removing the stain film of Example 2 can employ the same steps as the process for removing the stain film of Example 1 except that the stain film removal solution is different.
Alkaline ion water is used as the stain film removing solution used in the second embodiment. Here, the alkaline ionized water used is obtained by electrolyzing water. The water used for the production of alkaline ionized water may be either pure water or tap water, but tap water contains impurities such as chlorine, calcium, sodium and potassium, so the possibility of metal contamination of semiconductors is low. From the viewpoint of doing, it is more preferable to use pure water. However, if cleanliness at a level that allows the entry of sodium, etc., is sufficient, tap water is more efficient when water is mixed with ionic impurities such as chloride ions and sodium ions. Used. In water electrolysis, alkaline ionized water is generated on the negative electrode side and acidic water is generated on the positive electrode side. However, only alkaline ionized water is used to remove the stain film, and the acidic water is washed in the subsequent process. Used as water.

The pH value of the alkaline ionized water at this time is 7.5 to 12.0. The oxidation-reduction potential is +100 mV to −1000 mV. The temperature of the alkaline ionized water is not particularly limited as long as it is in a liquid state, and the temperature of the water when removing the stain film may be 0 to 100 ° C., but considering the cost, 20 It is preferable to set it as normal temperature of about 25 degreeC, without heating and cooling, especially 30 degreeC. In order to shorten the removal time of the stain film, since the ion concentration increases as the temperature increases, it is preferably heated to a high temperature of 80 ° C. to 100 ° C. The time required for removing the stain film is determined by the pH of the alkaline ionized water and the treatment temperature, but is preferably 1 to 20 minutes.
Through the above steps, the stain film was completely removed by visual and microscopic confirmation.

  The electrolysis of water shown in FIG. 4 will be described in more detail as a method for generating alkaline ionized water. Water 12 is put into the container 11, and a positive electrode 14 and a negative electrode 15 are placed on both sides partitioned by the diaphragm 13. Here, the diaphragm 13 is a film having a hole that allows water molecules and ions to pass therethrough, and examples thereof include a fluorine resin film such as Nafion and a porous film such as polysilicon. Examples of the electrode material of the positive electrode 14 and the negative electrode 15 include titanium and platinum or iridium coated. However, any material can be used as long as an electric current flows and the electrode material becomes ions and dissolves in water. Such an electrode material may be used. The materials of the positive electrode 14 and the negative electrode 15 may be the same or different. When an electric current is passed between both electrodes, on the positive electrode side, water molecules become hydrogen ions, oxygen molecules, and electrons, and the hydrogen ions increase, resulting in acidic water. On the other hand, on the negative electrode side, electrons act on water to increase hydroxide ions and hydrogen, so that it becomes alkaline water, and this alkaline water is used as alkaline ion water for removing the stain film.

The process for removing the stain film of Example 3 can employ the same steps as the process for removing the stain film of Example 1 except that the stain film removal solution is different.
Hot water is used as the stain film removal solution used in Example 3. Here, the hot water used may be either pure water or tap water, but tap water contains impurities such as chlorine, calcium, sodium, potassium, etc., thus reducing the possibility of metal contamination of semiconductors. From the viewpoint, it is more preferable to use pure water. However, tap water may be used from the viewpoint of cost when the cleanliness level at which mixing of sodium or the like is acceptable is sufficient. In addition, potassium chloride, sodium chloride, or the like may be added. In this case, since ions in the water promote ionization of the water, the time for removing the stain film is shortened.
The temperature of the hot water when removing the stain film is 80 to 100 ° C., and the pH value is 6.0 to 6.5. In order to shorten the removal time of the stain film, the higher the temperature, the higher the ion concentration. Therefore, a high temperature of 95 ° C. or higher is preferable. The time required for removing the stain film is determined by the treatment temperature of hot water and the additive concentration, but is preferably 3 to 60 minutes.
Through the above steps, the stain film was completely removed by visual and microscopic confirmation.

FIG. 2 is a schematic diagram of a process according to the invention. FIG. 4 is a process flow diagram according to the present invention. A sectional view of a silicon substrate is shown. It is the schematic of an electrolyzed water generating apparatus. It is the schematic of the conventional stain film removal process.

Explanation of symbols

1-A As-sliced silicon substrate 1-B Etched silicon substrate 1-C Silicon substrate 1-D with stain film attached 1-D Silicon substrate 1 with stain film removed 1 Mixed acid solution 2 Washing water 3 Ammonia water 4 Washing water

Claims (5)

In order to remove the stain film formed on the surface of the substrate when the silicon substrate is etched with hydrofluoric acid, the substrate is washed with pure water after the etching, and then contains at least one of ammonia water, alkaline ionized water, hot water, and OH. A method for removing a stain film, comprising washing with a stain film removing solution having a concentration of 3 × 10 ⁻⁶ to 2 × 10 ⁻² mol / L. The stain film removing method according to claim 1 , wherein the pH value of the ammonia water is 9.5 to 12.0. The stain film removing method according to claim 1 , wherein the pH value of the alkaline ionized water is 7.5 to 12.0. The stain film removing method according to claim 3 , wherein the washing with alkaline ionized water is performed at 80 to 100 ° C. The stain film removing method according to claim 1 , wherein the warm water is 80 ° C to 100 ° C.

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