JP2012035188A - Processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a processing method which can form a processed surface of high flatness without having an affected layer by processing a surface to be processed of a workpiece.SOLUTION: The processing method includes: a workpiece disposing step for disposing a workpiece 20 having a processing surface 20a in a processing solution 30; a photocatalyst film disposing step for disposing a photocatalyst film 12 in the processing solution 30 opposite the processing surface 20a; an active species creating step for irradiating the photocatalyst film 12 with light to generate active species 40 from the processing solution 30 by a photocatalytic action of the photocatalyst film 12, an active species diffusion distance control step for controlling a diffusion distance of the active species 40 in the processing solution 30 by a radical scavenger 42 added to the processing solution 30; and a processing step for chemically reacting the active species 40 with surface atoms 22 of the processing surface 20a and generating a chemical compound 50 to be eluted in the processing solution 30 to process the workpiece 20.

Description

  The present invention relates to a processing method. In particular, the present invention relates to a processing method for processing the surface of a workpiece.

  Conventionally, as a method of processing the surface of a workpiece such as a semiconductor wafer, a step of installing a workpiece having a processing surface in a processing solution containing an oxidant and a solid catalyst for decomposing the oxidant are processed. A step of bringing the active species having oxidizing power into the processing solution by utilizing the catalytic action of the solid catalyst, and a chemical reaction with surface atoms of the surface to be processed to generate a compound. A step of irradiating the processing surface with light during processing of the processing surface, a step of applying a voltage between the processing surface and the solid catalyst, a catalyst, a workpiece, In addition, a catalyst-assisted chemical processing method is known in which a surface to be processed is processed by combining one or more of the steps for controlling the temperature of the treatment solution (see, for example, Patent Document 1).

  Since the catalyst-assisted chemical processing method described in Patent Document 1 chemically processes the workpiece surface of the workpiece, the workpiece surface can be processed into a highly accurate surface.

JP 2008-136983 A

  However, the catalyst-assisted chemical processing method described in Patent Document 1 generates active species used for a chemical reaction with a surface to be processed by decomposing an oxidant on the surface of a solid catalyst serving as a processing reference surface. Exists only on or near the surface of the solid catalyst. Therefore, the flatness of the surface of the solid catalyst is transferred to the surface of the workpiece, and it is difficult to improve the flatness of the surface to be processed over the flatness of the surface of the solid catalyst.

  An object of the present invention is to provide a processing method capable of forming a processing surface with high flatness and without a work-affected layer by processing the processing surface of a workpiece.

  In order to achieve the above object, the present invention provides a workpiece installation step in which a workpiece having a workpiece surface is placed in the treatment solution, and a photocatalyst that is placed in the treatment solution with the photocatalyst film facing the workpiece surface. Activity in the processing solution by the film installation process, the active species generation process in which the photocatalytic film is irradiated with light to generate active species from the processing solution by the photocatalytic action of the photocatalytic film, and the radical scavenger added to the processing solution Active species diffusion distance control process for controlling the diffusion distance of the seeds, and processing processes for processing the workpiece by chemically reacting the active species with the surface atoms of the surface to be processed to generate compounds that elute in the processing solution. Is provided.

  In the processing method, the processing step may process the workpiece by controlling the temperature of at least one member selected from the group consisting of a photocatalytic film, a workpiece, and a processing solution.

  Moreover, in the said processing method, it is preferable that a radical scavenger is a protic organic compound.

  In the above processing method, the protic organic compound is preferably one of methanol, ethanol, propanol, and butanol, or a mixture of two or more selected from these.

In the processing method, the photocatalytic film is a TiO ₂ film, and TiO ₂ is preferably anatase type, rutile type, or a mixed crystal of anatase type and rutile type.

  Moreover, in the said processing method, it is preferable that the wavelength of light is 420 nm or less.

  Moreover, in the said processing method, it is preferable that a photocatalyst film | membrane is provided on the base material consisting of quartz or the base material consisting of glass.

  Moreover, in the said processing method, it is preferable that an active species production | generation process irradiates the said light toward a photocatalyst film | membrane from the base material side.

  Moreover, in the said processing method, it is preferable that the workpiece installation process installs at least one workpiece selected from the group consisting of SiC, GaN, sapphire, ruby, and diamond.

  According to the processing method according to the present invention, it is possible to provide a processing method capable of forming a processing surface having high flatness and having no work-affected layer by processing the processing surface of the workpiece.

It is a conceptual diagram of the processing method which concerns on embodiment of this invention. It is the schematic which processed the workpiece by the processing method concerning an embodiment of the invention. It is a schematic diagram of the oxidation / reduction process of the photocatalytic reaction that is the processing principle of the processing method according to the present embodiment. 1 is a schematic diagram of a processing method according to Example 1. FIG. 6 is a schematic diagram of a processing method according to Embodiment 2. FIG. Oxygen atom concentration on each SiC substrate surface of Comparative Example 2 before applying the processing method of the present invention, Example 1 using an aqueous solution of water and ethanol as the treatment solution, and Comparative Example 1 using only water as the treatment solution FIG. It is a figure which shows the mean square roughness (Rms) of the to-be-processed surface of a SiC substrate with respect to each reaction time which concerns on Example 1, Example 2, and Comparative Example 1. FIG.

[Embodiment]
(Outline of processing method)
FIG. 1A shows a concept of a processing method according to an embodiment of the present invention. Moreover, FIG. 1B shows the outline | summary by which the to-be-processed object was processed by the processing method which concerns on embodiment of this invention.

  The processing method according to the present embodiment is a photocatalytic reaction type chemical processing method in which the workpiece 20 is processed using the active species 40 generated from the processing solution 30 by light irradiation.

  An outline of the processing method according to the present embodiment will be described with reference to FIG. 1A. In the processing method according to the present embodiment, the processing solution 30 is disposed by placing the processing object 20 having the processing surface 20a in the processing solution 30 and the photocatalyst film 12 facing the processing surface 20a. A photocatalyst film installation step installed inside, an active species generation step of irradiating the photocatalyst film 12 with light 60 to generate active species 40 as radicals from the treatment solution 30 by the photocatalytic action of the photocatalyst film 12, and active species 40 And a surface atom 22 of the processing surface 20a are chemically reacted to generate a compound 50 that elutes in the processing solution 30, and a processing step of processing the workpiece 20 by eluting the compound 50 into the processing solution 30; Is provided.

  In the processing step, the workpiece 20 can be processed by controlling the temperature of at least one member selected from the group consisting of the photocatalytic film 12, the workpiece 20, and the processing solution 30. By controlling the temperature of at least one member selected from the group consisting of the photocatalytic film 12, the workpiece 20, and the treatment solution 30 in the processing step, the chemical reaction rate between the workpiece 20 and the active species 40 is increased. It is possible to control in the direction of decreasing or decreasing. For example, increasing the temperature increases the chemical reaction rate, and decreasing the temperature decreases the chemical reaction rate.

  In the present embodiment, the photocatalytic film 12 is formed on the surface 10 b of the substrate 10 that transmits the light 60. Therefore, the light 60 incident on the back surface 10a of the base material 10 passes through the base material 10 and enters the photocatalytic film 12 formed on the front surface 10b. Further, the treatment solution 30 includes a radical scavenger 42 that can trap the active species 40 that are radicals. The radical scavenger 42 captures the active species 40 and becomes a compound 52. The active species 40 include those that are captured by the radical scavenger 42 and those that are not captured but chemically react with the surface atoms 22 of the processed surface 20 a to generate the compound 50.

(Details of processing method)
First, as shown in FIG. 1A, the photocatalytic film 12 is brought into contact with the workpiece 20 or the photocatalytic film 12 is brought close to the workpiece 20 in the treatment solution 30 containing the radical scavenger 42.
When the photocatalyst film 12 is brought close to the workpiece 20, the distance between the surface 12 a of the photocatalyst film 12 and the workpiece 20 is the active species 40 determined by the lifetime of the active species 40 generated from the treatment solution 30. This is within the range of the maximum diffusion distance in the treatment solution 30.
For example, when the treatment solution 30 is only water, the generated active species 40 are hydroxyl radicals having a strong oxidizing power, and the maximum diffusion distance in this case is about 1 μm. When the treatment solution 30 is a solution obtained by adding a radical scavenger 42 to water, the hydroxyl radical diffusing in the treatment solution 30 reacts with the radical scavenger 42 to become a compound 52. Shorter than that of 1 m. Therefore, it is sufficient that the close distance is less than 1 μm.

  Next, when the light 60 is irradiated from the back surface 10 a side of the base material 10, the light 60 that has passed through the base material 10 reaches the photocatalyst film 12. When the photocatalyst film 12 is irradiated with light 60, the active species 40 generated from the treatment solution 30 by the photocatalytic action of the photocatalyst film 12 adheres to the surface 12a of the photocatalyst film 12. The generated active species 40 diffuses in the processing solution 30 toward the processing surface 20a. A part of the active species 40 chemically reacts with the radical scavenger 42 before reaching the processing surface 20 a of the workpiece 20 to generate a compound 52. Therefore, the probability that some of the plurality of active species 40 generated from the processing solution 30 will be deactivated before reaching the processing surface 20a increases as the diffusion distance in the processing solution 30 increases. Thus, the processing is sequentially performed from a portion where the distance from the photocatalyst film 12 to the workpiece 20 is short (for example, from the tip portion of the convex portion when the processing surface 20a is uneven). In this way, the active species 40 that has reached the processing surface 20a before reacting with the radical scavenger 42 chemically reacts with the surface atoms 22 of the processing surface 20a and elutes into the processing solution 30. Is generated. Subsequently, the generated compound 50 is eluted and diffused into the processing solution 30 from the processing surface 20a.

  Thereby, the processing surface 20a of the workpiece 20 is processed. Specifically, for example, as shown in FIG. 1B, the surface of the workpiece 20 is processed, and the flat surface 20 b is exposed in the processing solution 30.

(Substrate 10)
The substrate 10 according to the present embodiment is formed from a transparent material that transmits light 60. Specifically, when the light 60 is ultraviolet light, the substrate 10 can be formed of a material that transmits ultraviolet light. For example, the substrate 10 can be a glass substrate, a quartz substrate, a substrate made of a synthetic resin such as acrylic, or the like. By forming the base material 10 from a transparent material that transmits the light 60, the photocatalyst film 12 can be irradiated with the light 60 from the back surface 10a side of the base material 10. In addition, when comprising the base material 10 from a synthetic resin, it has the transmittance | permeability of the light 60 of the grade which a synthetic resin does not deteriorate easily by long-term use, and more than the flatness requested | required of the to-be-processed surface 20a of the to-be-processed object 20 A substrate 10 having a flat surface 10b is used.

(Photocatalytic film 12)
The photocatalyst film 12 according to the present embodiment is formed on the surface 10b of the substrate 10 facing the processed surface 20a of the workpiece 20. And the photocatalyst film | membrane 12 is formed from the film | membrane which consists of a photocatalyst, or the film | membrane containing a photocatalyst. As the photocatalyst constituting the photocatalyst film 12, TiO ₂ , KTaO ₃ , SrTiO ₃ , ZrO ₂ , NbO ₃ , ZnO, WO ₃ , which is an oxide having an energy at the upper end of the valence band of about 2.8 eV or more, and At least one compound selected from the group consisting of metal oxides such as SnO ₂ can be used. Further, these compounds can be doped with impurities. For example, the photocatalytic film 12 can also be formed from a nitrogen-doped photocatalyst doped with nitrogen (N) (for example, N-doped TiO ₂ ).

Here, when TiO ₂ is used as the photocatalyst, it is desirable to use TiO ₂ having a crystal structure of anatase type. Note that rutile TiO ₂ or a mixed crystal of anatase TiO ₂ and rutile TiO ₂ can also be used.

(Method for producing photocatalytic film 12)
The photocatalytic film 12 according to the present embodiment is formed by sputtering, vapor deposition, molecular beam epitaxy (Molecular Beam Epitaxy: MBE), laser ablation, ion plating, thermal CVD, plasma CVD, organic metal Vapor phase growth method (Metal Organic Chemical Deposition method: MOCVD method), liquid phase epitaxy method, aerosol deposition method (Aerosol Deposition method: AD method), Langmuir-Blodgett method (Lgmuir-Blodgett method, L method) It can be formed using a method, a plating method, a coating method, or the like. Here, in this embodiment mode, it is preferable to use a sputtering method from the viewpoint of easy control of film formation.

When the photocatalytic film 12 is formed using the sputtering method, it can be formed as follows. For example, by performing sputtering under an Ar atmosphere using a target composed of TiO _2, the photocatalyst film 12 formed by directly depositing a TiO ₂ can be formed on the substrate 10. Further, by performing sputtering in a mixed atmosphere of O ₂ and Ar (hereinafter sometimes referred to as “O ₂ / Ar atmosphere”) using a target made of Ti, Ti and O ₂ in the atmosphere can be reduced. A photocatalytic film 12 made of TiO ₂ formed by reaction can be formed on the substrate 10. Note that as a sputtering apparatus that performs sputtering, a DC sputtering apparatus, a high-frequency sputtering apparatus, a magnetron sputtering apparatus, an ion beam sputtering apparatus, an electron cyclotron resonance (ECR) sputtering apparatus, or the like can be used.

  Further, when the photocatalytic film 12 is formed by using the sputtering method, the plasma output is intended to suppress damage to the photocatalytic film 12 during film formation by suppressing an increase in the mean free path of plasma such as Ar. Is set to 400 W or less, and the total pressure of the gas in the chamber is preferably set to 1.0 Pa or more.

  Note that the film thickness of the photocatalytic film 12 is to increase the amount of active species 40 that chemically react with the surface atoms 22 of the processing surface 20a, and to process the workpiece 20 at a sufficient speed. The thickness is 150 nm or more, which is a thickness that can sufficiently absorb the light 60. The film thickness of the photocatalyst film 12 is more preferably 200 nm or more. Furthermore, the amount of the active species 40 generated according to the amount of light irradiated from the back surface 10a side of the base material 10 toward the photocatalyst film 12 and reaching the photocatalyst film 12 is an amount sufficient for processing the workpiece 20. Thus, the film thickness of the photocatalyst film 12 is preferably 1 μm or less.

The light 60 used in the present embodiment is light 60 having energy equal to or higher than the band gap energy of the photocatalyst constituting the photocatalyst film 12. For example, since the band gap energy of TiO ₂ is 3.0 eV, TiO ₂ exhibits a photocatalytic function for light having a wavelength of 420 nm or less. Therefore, when TiO ₂ is used as the photocatalyst, the light 60 is a light 60 having a wavelength of 200 nm to 420 nm, preferably 200 nm to 400 nm. In addition, when the photocatalyst which comprises the photocatalyst film | membrane 12 exhibits a photocatalytic function with respect to visible light, visible light can also be used as the light 60. FIG.

(Workpiece 20)
The workpiece 20 according to the present embodiment is a crystalline material such as a semiconductor material or an oxide material used for an electronic device such as a power device or a light emitting device. Specifically, the workpiece 20 is a substrate made of a difficult-to-work crystal material such as SiC, GaN, sapphire, ruby, diamond or the like.

(Radical scavenger 42)
As the radical scavenger 42 according to the present embodiment, a protic organic compound can be used. As the radical scavenger 42 as a protic organic compound, for example, methanol, ethanol, propanol, or butanol can be used. Here, as the radical scavenger 42, any one of methanol, ethanol, propanol, and butanol can be used. In addition, as the radical scavenger 42, a mixed solution in which two or more selected from methanol, ethanol, propanol, and butanol and a mixture of two or more selected protic organic compounds can be used.

(Processing principle of processing method: Generation reaction of active species)
FIG. 2 shows an outline of the oxidation / reduction process of the photocatalytic reaction, which is the processing principle of the processing method according to the present embodiment.

With reference to FIG. 2, the principle that the active species 40 is generated from the treatment solution 30 (for example, water) on and near the surface of the photocatalytic film 12 will be described. Here, a case where TiO ₂ is used as a photocatalyst will be described. First, when TiO ₂ is irradiated with light having a band gap energy of TiO ₂ or more (420 nm or less), electrons existing in the valence band are excited to the conduction band according to the following reaction formula (1), and the holes become valences. A hole-electron pair is generated by generating an excited electron in the conduction band as well as in the electron band. In the reaction formula (1), “UV” is an abbreviation for ultraviolet rays.

In accordance with the reaction shown in the following reaction formula (2) and reaction formula (3), the holes extract electrons (e ⁻ ) from hydroxide ions (OH ⁻ ) generated by ionization of water (H ₂ O), and water Generates acid radicals (hydroxyl radical “OH”).

  The hydroxyl radical generated in the reaction formula (3) has very strong oxidizing power. Therefore, hydroxyl radicals can react with chemically stable materials such as SiC, GaN, diamond, etc., and these chemically stable materials can be processed.

  On the other hand, the excited electrons are oxygen dissolved in the treatment solution 30 according to the following reaction formula (4) unless a specific substance (sacrificial agent) that is easily oxidized is added to the treatment solution 30. Move to gas (dissolved oxygen) to reduce oxygen. Note that a sacrificial agent may be added to the treatment solution 30 instead of dissolved oxygen to improve the reaction efficiency.

(Processing principle of processing method: reaction between active species and workpiece, and processing process [in the case of SiC])
Next, a process of processing SiC as the workpiece 20 when the workpiece is SiC will be described. First, the active species generated from the treatment solution 30 by light irradiation on the photocatalyst film 12 (for example, hydroxyl radical when the treatment solution 30 is water) causes the surface of SiC to follow the reaction formula (5) shown below. It is thought to be oxidized.

Here, when the radical scavenger 42 is added to the treatment solution 30, the hydroxyl radical as the active species 40 is combined with the radical scavenger 42 before reaching the processing surface 20 a that is the surface of the workpiece 20. It reacts and deactivates. Therefore, the diffusion distance of the hydroxyl radical in the treatment solution 30 is determined by the reaction rate between the hydroxyl radical and the radical scavenger 42. Thus, the diffusion distance of the active species 40 is controlled by adding the radical scavenger 42 to the treatment solution 30 and controlling the diffusion distance of the active species 40 in the treatment solution 30 (for example, a TiO ₂ thin film). The oxidation reaction can proceed sequentially from the work surface 20a located at the closest distance from the surface of).

After the oxidation reaction on the surface of the workpiece 20, the oxidized region of the workpiece surface 20a is preferentially processed by performing a process for removing the oxide layer generated by the oxidation reaction on the workpiece surface 20a. It is thought to go. Note that when the oxide layer generated by the oxidation reaction is SiO ₂ , hydrofluoric acid can be used to remove the oxide layer. In such a case, according to the following reaction formula (7), the oxidized region of the work surface 20a is preferentially processed.

(Processing principle of processing method: reaction between active species and workpiece, and processing process [in the case of GaN])
In addition, a process of processing GaN as the workpiece 20 when the workpiece is GaN will be described. First, the active species generated from the treatment solution 30 by light irradiation on the photocatalyst film 12 (for example, hydroxyl radical when the treatment solution 30 is water) causes the surface of GaN to have a surface structure according to the following reaction formula (7). It is thought to be oxidized.

In this case, the diffusion distance of hydroxyl radicals as the active species 40 can be controlled by adding the radical scavenger 42 to the treatment solution 30 in the same manner as described for the case where the workpiece 20 is SiC. Then, after the oxidation reaction of the surface of GaN as the workpiece 20, the oxidized region of the workpiece surface 20 a is processed by removing the oxide layer generated by the oxidation reaction on the workpiece surface 20 a. It is thought that it will be processed preferentially. Note that when the oxide layer generated by the oxidation reaction is Ga ₂ O ₃ , sulfuric acid can be used to remove the oxide layer. In such a case, according to the following reaction formula (8), the oxidized region of the work surface 20a is preferentially processed.

(Effect of embodiment)
In the processing method according to the present embodiment, the workpiece 20 is placed in the processing solution 30 and the photocatalyst film 12 is placed in the processing solution 30 so as to face the processing surface 20a. The active species 40 is generated from the treatment solution 30 and the diffusion distance of the active species 40 in the treatment solution is controlled by the radical scavenger 42 added to the treatment solution 30. Since the workpiece 20 is processed by chemically reacting with the surface atoms 22 to generate the compound 50 that elutes in the processing solution 30, it is generated in the workpiece 20 when abrasive grains and abrasives are used. There is no mechanical defect. As a result, according to the processing method according to the present embodiment, the surface of the workpiece 20 is not affected by the processing-affected layer (for example, when polished using an abrasive or the like) without causing crystal defects due to processing. A workpiece having a highly accurate surface (that is, a surface with high flatness) without a damage layer) can be manufactured.

  Further, since the processing method according to the present embodiment does not require the use of abrasive grains and a polishing agent, for example, disposal processing of used slurry required in a polishing method such as a CMP (Chemical Mechanical Polishing) method. No disposal of industrial waste. Therefore, the processing method according to the present embodiment is a preferable processing method from the viewpoint of environmental protection because it can reduce the cost of disposal and does not discharge industrial waste.

  Furthermore, since the processing method according to the present embodiment is sequentially processed from the processing surface 20a having a short distance from the photocatalyst film 12, a substrate having a surface with good flatness can be manufactured. In other words, since the processing method according to the present embodiment can process the surface of the workpiece 20 with anisotropy, a substrate having a surface with good flatness can be manufactured.

  Examples will be described below.

  FIG. 3 shows an outline of the processing method according to the first embodiment.

(Preparation of photocatalytic film 12)
First, a quartz substrate 14 using quartz as a base material was placed inside a chamber of a high-frequency magnetron sputtering apparatus. Then, a TiO ₂ thin film as the photocatalyst film 12 was formed on the surface of the quartz substrate 14 under the conditions of an Ar gas 100% atmosphere, a plasma output of 300 W, a substrate temperature of 300 ° C., a total pressure of 3 Pa, and a film formation time of 24 minutes. In addition, as a target of the high-frequency magnetron sputtering apparatus, a target made of a TiO ₂ fired body was used.

(Evaluation of photocatalytic film)
The physical properties of the TiO ₂ thin film formed on the surface of the quartz substrate 14 were evaluated. The thickness of the TiO ₂ thin film was 200 nm. The crystal structure of the TiO ₂ thin film was a mixed crystal having 76% anatase crystals and 24% rutile crystals.

(Photocatalytic chemical processing of SiC substrates)
In Example 1, a SiC substrate was used as the workpiece 20. Specifically, the SiC substrate used in Example 1 is a single crystal SiC substrate, and uses n-type 4H—SiC with a diameter of 50 mm and a (0001) Si surface inclined 8 degrees in the [11-20] direction. It was. Moreover, the electrical resistivity of the SiC substrate was 0.017 Ωcm. The {0001} -plane SiC single crystal substrate has polarity, one is a Si surface whose outermost surface is made of Si atoms, and the other is a C-plane whose outermost surface is made of C atoms. In Example 1, a Si surface was used as the processing surface 20a. Moreover, in Example 1, the to-be-processed surface 20a (Si surface) before processing is a surface that has been subjected to CMP after mechanical mirror polishing. Note that when the C surface is used as the surface to be processed 20a, the processing speed is slightly different from that when the Si surface is used, but the mechanism of the processing is the same as that of the Si surface.

  The processing method according to Example 1 is specifically as follows. First, the surface of the SiC substrate was washed with a 10% HF aqueous solution. Next, after putting the SiC substrate into the glass beaker 70, the quartz substrate 14 on which the photocatalytic film 12 was formed was introduced. In such a case, the processed surface of the SiC substrate and the photocatalytic film 12 were brought into contact with each other. Next, an aqueous solution 32 in which methanol as a radical scavenger was added to water as a treatment solution was introduced into the glass beaker 70. Here, as the aqueous solution 32, an aqueous solution 32 prepared by adjusting the concentration of methanol to 50% (volume / volume%: v / v%) was used. In this case, the surface of the aqueous solution 32 is above the contact surface (hereinafter sometimes referred to as “interface”) between the photocatalyst film 12 and the processed surface of the SiC substrate (that is, at least the contact surface is in the aqueous solution 32). The aqueous solution 32 was introduced into the glass beaker 70 so as to be positioned at the position). As a result, the aqueous solution 32 entered the interface between the photocatalyst film 12 and the processed surface of the SiC substrate by capillary action, and an aqueous solution film layer 34 (hereinafter sometimes referred to as “radical transport layer”) was formed.

Next, using a high-pressure mercury lamp, ultraviolet rays 62 whose ultraviolet illuminance was adjusted to 8 mW / cm ² were irradiated from the back surface 14 a side of the quartz substrate 14. Thereby, the photocatalytic reaction between the photocatalyst film 12 and the aqueous solution 32 in the aqueous solution film layer 34 was started. Regarding the reaction time of the photocatalytic reaction between the photocatalyst film 12 and the aqueous solution 32, that is, the reaction time of the active species and the workpiece, the surface to be processed was observed each time the reaction was carried out for 1 hour, and the reaction was continued for a total of 5 hours.

  An oxide film was formed on the processed surface of the SiC substrate by the photocatalytic reaction. After the photocatalytic reaction for 1 hour, the oxide film was removed with a 10% HF aqueous solution. And the to-be-processed surface after a process was observed with the atomic force microscope (Atomic Force Microscope: AFM). Thereafter, a photocatalytic reaction was similarly performed for another hour, the oxide film was removed, and the processing surface was observed. This was repeated and the photocatalytic reaction was carried out for a cumulative period of 5 hours. As a result, as shown in FIG. 6, when the mean square roughness (Rms) of the processed surface before processing was 0.295 nm, the Rms of the processed surface after processing was reacted for 1 hour. 0.179 nm, reacted for 2 hours, 0.136 nm, reacted for 3 hours, 0.125 nm, reacted for 4 hours, 0.121 nm, reacted for 5 hours, 0.1. It was 119 nm. Thus, according to the processing method which concerns on Example 1, the flatness of the SiC single crystal substrate surface can be improved. Further, when it was measured whether there was a work-affected layer on the processed surface after processing, no work-affected layer was present.

Even when KTaO ₃ , SrTiO ₃ , ZrO ₂ , NbO ₃ , ZnO, WO ₃ , and SnO ₂ are used as the photocatalytic film 12 in the same manner as in Example 1, the mean square roughness is the same as in Example 1. Improved. Specifically, when the reaction time was 2 hours or longer, all the Rms of the processed surface after processing could be flattened in the range of 0.100 nm to 0.150 nm. It can be seen that even if a metal oxide other than TiO _{2 is used} as the photocatalytic film 12 as long as it has a photocatalytic function, the flatness of the SiC single crystal substrate surface can be improved.

  FIG. 4 shows an outline of a processing method according to the second embodiment.

  In Example 2, the photocatalyst film 12 and the aqueous solution 32 as the processing solution were heated to process the SiC substrate as the workpiece 20.

  Specifically, in Example 2, a single crystal SiC substrate similar to the SiC substrate used in Example 1 was used. In other words, the SiC substrate used in Example 2 was n-type 4H—SiC having a diameter of 50 mm and having a (0001) Si surface inclined by 8 degrees in the [11-20] direction. Moreover, the electrical resistivity of the SiC substrate was 0.017 Ωcm. Also in Example 2, a Si surface was used as a surface to be processed, and the surface to be processed (Si surface) before processing was a surface subjected to CMP after mechanical mirror polishing.

  First, the surface of the SiC substrate was washed with a 10% HF aqueous solution. Next, the quartz substrate 14 on which the photocatalytic film 12 was formed and the SiC substrate as the workpiece 20 were introduced into the glass beaker 70. In such a case, the processed surface of the SiC substrate and the photocatalytic film 12 were brought into contact with each other. Next, an aqueous solution 32 in which methanol as a radical scavenger was added to water as a treatment solution was introduced into the glass beaker 70. Here, the aqueous solution 32 prepared by adjusting the concentration of methanol to 50% (v / v%) was used. In this case, in the glass beaker 70, the surface of the aqueous solution 32 is positioned above the interface between the photocatalyst film 12 and the processed surface of the SiC substrate (that is, at least the position where the interface is in the aqueous solution 32). An aqueous solution 32 was introduced into the solution. As a result, the aqueous solution 32 entered the interface between the photocatalytic film 12 and the processed surface of the SiC substrate by capillary action, and an aqueous solution film layer 34 was formed.

Subsequently, the current flowing through the heater 80 was adjusted by the heater controller 82, and the temperature of the aqueous solution 32 was heated to 60 ° C. Thereby, both the aqueous solution 32 and the photocatalyst film | membrane 12 as a process solution were heated to 60 degreeC. Next, using a high-pressure mercury lamp, ultraviolet rays 62 whose ultraviolet illuminance was adjusted to 8 mW / cm ² were irradiated from the back surface 14 a side of the quartz substrate 14. Thereby, the photocatalytic reaction between the photocatalytic film 12 and the aqueous solution 32 was started. Regarding the reaction time of the photocatalytic reaction between the photocatalyst film 12 and the aqueous solution 32, that is, the reaction time of the active species and the workpiece, the surface to be processed was observed each time the reaction was carried out for 1 hour, and the reaction was continued for a total of 5 hours.

  An oxide film was formed on the processed surface of the SiC substrate by the photocatalytic reaction. After the photocatalytic reaction for 1 hour, the oxide film was removed with a 10% HF aqueous solution. And the to-be-processed surface after a process was observed by AFM. Thereafter, a photocatalytic reaction was similarly performed for another hour, the oxide film was removed, and the processing surface was observed. This was repeated and the photocatalytic reaction was carried out for a cumulative period of 5 hours. As a result, as shown in FIG. 6, when the mean square roughness (Rms) of the processed surface before processing was 0.497 nm, Rms of the processed surface after processing was reacted for 1 hour. 0.210 nm, reacted for 2 hours, 0.145 nm, reacted for 3 hours, 0.136 nm, reacted for 4 hours, 0.127 nm, reacted for 5 hours, 0.1. It was 124 nm. Further, the oxidation rate was calculated from the thickness of the oxide film formed on the work surface. As a result, it has been found that the oxidation rate when the aqueous solution 32 as the treatment solution is heated is higher than that when the aqueous solution 32 is not heated, and the processing speed of the surface to be processed is increased. Further, it was found that when the aqueous solution 32 is heated to process the processed surface, the effect of improving the processing speed is greater than the effect of improving the surface roughness of the processed surface. Further, when it was measured whether there was a work-affected layer on the processed surface after processing, no work-affected layer was present.

  In Example 3, the workpiece 20 was processed in substantially the same manner as in Example 1 except that a GaN substrate was used as the workpiece 20. Specifically, the GaN substrate used in Example 3 was a single crystal GaN substrate, and n-type GaN having a diameter of 50 mm and a (0001) Ga plane was used. The {0001} -plane GaN single crystal substrate has polarity, one is a Ga surface whose outermost surface is made of Ga atoms, and the other is an N surface whose outermost surface is made of N atoms. In Example 3, a Ga surface was used as the work surface. Moreover, in Example 3, the to-be-processed surface (Ga surface) before processing is a surface that has been subjected to CMP after mechanical mirror polishing. Note that when the N surface is used as the surface to be processed, the processing speed is slightly different from that when the Ga surface is used, but the mechanism of the processing is the same as that of the Ga surface.

(Photocatalytic chemical processing of GaN substrates)
The processing method according to Example 3 is specifically as follows. First, the surface of the GaN substrate was washed with a 10% HF aqueous solution. Next, the quartz substrate 14 on which the photocatalytic film 12 was formed and the GaN substrate were introduced into the glass beaker 70. In such a case, the processed surface of the GaN substrate and the photocatalytic film 12 were brought into contact with each other. Next, an aqueous solution 32 in which methanol as a radical scavenger was added to water as a treatment solution was introduced into the glass beaker 70. Here, the aqueous solution 32 prepared by adjusting the concentration of methanol to 50% (v / v%) was used. In this case, the inside of the glass beaker 70 is positioned so that the surface of the aqueous solution 32 is located above the interface between the photocatalytic film 12 and the surface to be processed of the GaN substrate (that is, at least the interface is in the aqueous solution 32). An aqueous solution 32 was introduced into the solution. As a result, the aqueous solution 32 entered the interface between the photocatalyst film 12 and the surface to be processed of the GaN substrate by capillary action, and an aqueous solution film layer 34 was formed.

Next, using a high-pressure mercury lamp, ultraviolet rays 62 whose ultraviolet illuminance was adjusted to 8 mW / cm ² were irradiated from the back surface 14 a side of the quartz substrate 14. Thereby, the photocatalytic reaction between the photocatalytic film 12 and the aqueous solution 32 was started. The reaction time of the photocatalytic reaction between the photocatalytic film 12 and the aqueous solution 32 was set to 1 hour.

  An oxide film was formed on the processed surface of the GaN substrate by the photocatalytic reaction. After the photocatalytic reaction for 1 hour, the oxide film was removed with concentrated sulfuric acid. And the to-be-processed surface after a process was observed by AFM. As a result, when the mean square roughness (Rms) of the processed surface before processing was 0.261 nm, Rms of the processed surface after processing was 0.178 nm. Thus, according to the processing method according to Example 3, the flatness of the surface of the GaN single crystal substrate can be improved. Further, when it was measured whether there was a work-affected layer on the processed surface after processing, no work-affected layer was present.

  Even when processing similar to that in Example 3 is performed using sapphire, ruby, and diamond as the workpiece 20, there is a difference in processing speed, but the mean square roughness of the surface of the workpiece 20 is similar to that in Example 3. Was improved. This indicates that even if the workpiece 20 is a difficult-to-process material other than SiC and GaN, the flatness of the surface can be improved.

[Comparative Example 1]
In Comparative Example 1, the SiC substrate was processed under the same conditions as in Example 1 except that only water was used instead of an aqueous solution obtained by adding methanol to water as the treatment solution.
The SiC substrate used in Comparative Example 1 is a single crystal SiC substrate as in Example 1, and has a diameter of 50 mm and an n-type 4H—SiC having a (0001) Si surface inclined in the [11-20] direction by 8 degrees. Was used. Moreover, the electrical resistivity of the SiC substrate was 0.017 Ωcm. And the to-be-processed surface is a Si surface, and the to-be-processed surface (Si surface) before a process is a surface which performed the CMP process, after giving mechanical mirror polishing.

  In the processing method according to Comparative Example 1, first, the surface of the SiC substrate was cleaned with a 10% HF aqueous solution. Next, the quartz substrate 14 on which the photocatalytic film 12 was formed and the SiC substrate were introduced into the glass beaker 70. In such a case, the processed surface of the SiC substrate and the photocatalytic film 12 were brought into contact with each other. Next, water as a treatment solution was introduced into the glass beaker 70. In this case, the water is put into the glass beaker 70 so that the surface of the water is located above the interface between the photocatalytic film 12 and the processed surface of the SiC substrate (that is, at least the interface is in the water). Introduced. As a result, water entered the interface between the photocatalyst film 12 and the processed surface of the SiC substrate by capillary action, and a water film layer was formed.

Next, using a high-pressure mercury lamp, ultraviolet rays 62 whose ultraviolet illuminance was adjusted to 8 mW / cm ² were irradiated from the back surface 14 a side of the quartz substrate 14. Thereby, the photocatalytic reaction of the photocatalyst film | membrane 12 and water was started. Regarding the reaction time of the photocatalytic reaction between the photocatalyst film 12 and water, that is, the reaction time of the active species and the workpiece, the surface to be processed was observed each time the reaction was performed for 1 hour, and the reaction was continued for a total of 5 hours.
An oxide film was formed on the processed surface of the SiC substrate by the photocatalytic reaction. After the photocatalytic reaction for 1 hour, the oxide film was removed with a 10% HF aqueous solution. And the to-be-processed surface after a process was observed with the atomic force microscope (Atomic Force Microscope: AFM). Thereafter, a photocatalytic reaction was similarly performed for another hour, the oxide film was removed, and the processing surface was observed. This was repeated and the photocatalytic reaction was carried out for a cumulative period of 5 hours. As a result, as shown in FIG. 6, when the mean square roughness (Rms) of the processed surface before processing was 0.354 nm, Rms of the processed surface after processing was reacted for 1 hour. What was reacted for 0.348 nm, accumulated 2 hours was 0.340 nm, what was reacted for 3 hours was 0.331 nm, what was reacted for 4 hours was 0.335 nm, and what was reacted for 5 hours was 0. It was 328 nm. In the processing method according to Comparative Example 1, the flatness of the SiC single crystal substrate surface was hardly improved. In Comparative Example 1, since the diffusion distance of the active species 40 is not controlled, it is considered that the active species 40 has reached the concave portion of the surface 20a to be processed as the processing proceeds, and has switched to an isotropic processing mode. . As described above, the Rms of the processed surface 20a of Comparative Example 1 is inferior to the case where the radical scavenger 42 is added.

(Diffusion distance control of active species)
A method for controlling the diffusion distance of active species (hydroxyl radicals) in the treatment solution in the processing method of the present invention will be described.
FIG. 5 shows the SiC substrate surfaces of Comparative Example 2 before the processing method of the present invention, Example 1 using an aqueous solution of water and ethanol as the processing solution, and Comparative Example 1 using only water as the processing solution. It is the figure which showed the oxygen atom density | concentration in.

  Specifically, first, the oxygen atom concentration on the surface of the SiC substrate before processing by the processing method according to Example 1 was measured by Auger electron spectroscopy. In such a case, the oxygen atom concentration of the natural oxide film on the surface of the SiC substrate was measured (hereinafter referred to as “oxygen concentration in Comparative Example 2”). Further, the oxygen atom concentration on the surface of the SiC substrate subjected to the processing by the processing method according to Example 1 was measured by Auger electron spectroscopic analysis (hereinafter referred to as “oxygen concentration of Example 1”). Furthermore, the oxygen atom concentration on the surface of the SiC substrate that was processed by the processing method according to Comparative Example 1 was measured by Auger electron spectroscopic analysis (hereinafter referred to as “oxygen concentration in Comparative Example 1”).

  Referring to FIG. 5, the oxygen atom concentration on the SiC substrate surface increased in the order of oxygen concentration of Comparative Example 2 <oxygen concentration of Example 1 <oxygen concentration of Comparative Example 1. This has shown that the oxidation reaction in the SiC substrate surface was suppressed by adding methanol as the radical scavenger 42 to water. That is, when the radical scavenger 42 is added to water, the diffusion distance of hydroxyl radicals as the active species 40 in water can be controlled. Therefore, in Example 1, the oxidation reaction of the surface of the workpiece 20 can proceed sequentially from the portion of the workpiece 20 closest to the surface of the photocatalyst film 12, and the presence of the radical scavenger 42 is an active species. It can be seen that 40 diffusion distances can be controlled.

  Even when ethanol, propanol, or butanol was used as the radical scavenger 42 in the same processing as in Example 1, the oxygen atom concentration on the SiC substrate surface had the same tendency as in Example 1. This indicates that the diffusion distance of hydroxyl radical as the active species 40 in water can be controlled also by adding a protic organic compound other than methanol as the radical scavenger 42 to water.

  As described above, according to the processing method according to the embodiment, the active species 40 (for example, hydroxyl radical when the main component of the treatment solution 30 is water) generated from the treatment solution 30 by the photocatalytic action of the photocatalyst film 12 is in the treatment solution 30. The diffusion distance in can be controlled by adding the radical scavenger 42 to the treatment solution 30. Thereby, while being able to provide the to-be-processed object 20 which has the surface without an abrasion trace, the said surface can be planarized by an atomic level.

  As mentioned above, although embodiment of this invention was described, embodiment described above does not limit the invention which concerns on a claim. In addition, it should be noted that not all the combinations of features described in the embodiments are essential for the means for solving the problems of the invention.

DESCRIPTION OF SYMBOLS 10 Base material 10a Back surface 10b Surface 12 Photocatalyst film 12a Surface 14 Quartz substrate 14a Back surface 14b Surface 20 Processed object 20a Processed surface 20b Flat surface 22 Surface atom 30 Treatment solution 32 Aqueous solution 34 Aqueous membrane layer 40 Active species 42 Radical scavenger 50 Compound 52 Compound 60 Light 62 Ultraviolet light 70 Glass beaker 80 Heater 82 Heater controller

Claims (9)

A workpiece installation step of installing a workpiece having a workpiece surface in the processing solution;
A photocatalyst film installation step of placing the photocatalyst film in the processing solution so as to face the processing surface;
Irradiating the photocatalyst film with light, and generating active species from the treatment solution by the photocatalytic action of the photocatalyst film; and
An active species diffusion distance control step for controlling a diffusion distance of the active species in the treatment solution by a radical scavenger added to the treatment solution;
A processing method comprising: a processing step of processing the workpiece by chemically reacting the active species with surface atoms of the processing surface to generate a compound that elutes in the processing solution.   The processing method according to claim 1, wherein the processing step processes the workpiece by controlling a temperature of at least one member selected from the group consisting of the photocatalytic film, the workpiece, and the processing solution. .   The processing method according to claim 1, wherein the radical scavenger is a protic organic compound.   The processing method according to claim 3, wherein the protic organic compound is any one of methanol, ethanol, propanol, and butanol, or a mixture of two or more selected from these. The photocatalytic film is a TiO ₂ film,
_2. The processing method according to claim 1, wherein the TiO ₂ is either an anatase type, a rutile type, or a mixed crystal of an anatase type and a rutile type.   The processing method according to claim 1, wherein the light has a wavelength of 420 nm or less.   The processing method according to claim 1, wherein the photocatalytic film is provided on a base material made of quartz or a base material made of glass.   The said active species production | generation process is a processing method of Claim 7 which irradiates the said light toward the said photocatalyst film | membrane from the said base material side.   The processing method according to any one of claims 1 to 8, wherein the workpiece installation step installs at least one workpiece selected from the group consisting of SiC, GaN, sapphire, ruby, and diamond. .

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