Patent Publication Number: US-2012034395-A1

Title: Processing method

Description:
The present application is based on Japanese patent application No. 2010-176911 filed on Aug. 6, 2010, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a processing method. In particular, this invention relates to a processing method for processing a surface of a workpiece. 
     2. Description of the Related Art 
     Conventionally, as a method of processing a surface of a workpiece such as a semiconductor wafer and the like, a catalyst-aided chemical processing method is known, the processing method including a step of disposing a workpiece having a processed surface in a processing solution containing an oxidizing agent, a step of bringing a solid catalyst for decomposing the oxidizing agent into contact with or in close proximity to the processed surface, a step of generating active species having oxidizing force from the processing solution by using a catalyst action of the solid catalyst, so as to chemically react the active species with surface atoms of the processed surface and generate a chemical compound and a step of removing the chemical compound generated, and the processing method processing the processed surface by using either one or a combination of not less than two of a step of irradiating the processed surface with a light, a step of applying a voltage between the processed surface and the solid catalyst, and a step of controlling the temperatures of the solid catalyst, the workpiece and/or the processing solution, during the processing of the processed surface. The processing method is disclosed in, for example, JP-A-2008-136983. 
     The catalyst-aided chemical processing method disclosed in JP-A-2008-136983 chemically processes a processed surface of a workpiece, so that the processed surface can be processed so as to have a high-accuracy surface. 
     SUMMARY OF THE INVENTION 
     However, the catalyst-aided chemical processing method disclosed in JP-A-2008-136983 decomposes an oxidizing agent in a surface of a solid catalyst that becomes a processing reference surface and generates active species that are used for a chemical reaction with the processed surface, so that the active species do not exist in any place except for places on or in proximity to the surface of the solid catalyst. Consequently, the flatness of surface of the solid catalyst is transferred onto the surface of the workpiece, and it is difficult to allow the processed surface to surpass the surface of the solid catalyst in the flatness. 
     Therefore, it is an object of the invention to solve the above-mentioned problem and provide a processing method that is capable of forming a processed surface of a workpiece having a high flatness property and not having a processing-degenerated layer by processing the processed surface of the workpiece. 
     (1) According to one embodiment of the invention, a processing method comprises: 
     disposing a workpiece having a processed surface in a processing solution, 
     disposing a photocatalyst film in the processing solution opposite the processed surface, 
     irradiating the photocatalyst film with a light, so as to generate active species from the processing solution by a photocatalytic action of the photocatalyst film, 
     controlling a diffusion distance of the active species in the processing solution by a radical scavenger added to the processing solution, and 
     chemically reacting the active species with surface atoms of the processed surface and generating a chemical compound to be eluted in the processing solution, so as to process the workpiece. 
     In the above embodiment (1) of the invention, the following modifications and changes can be made. 
     (i) The workpiece is processed by controlling a temperature of at least one member selected from the group consisting of the photocatalyst film, the workpiece and the processing solution. 
     (ii) The radical scavenger comprises a protic organic compound. 
     (iii) The protic organic compound comprises one of methanol, ethanol, propanol and butanol, or a mixture liquid of not less than two selected therefrom. 
     (iv) The photocatalyst film comprises a film of TiO 2 , and the TiO 2  comprises an anatase-type crystal or a rutile-type crystal, or a mixed crystal of the anatase-type crystal and the rutile-type crystal. 
     (v) The light has a wavelength of not more than 420 nm. 
     (vi) The photocatalyst film is disposed on a substrate formed of quartz or glass. 
     (vii) The light is irradiated from a side of the substrate toward the photocatalyst film, so as to generate active species. 
     (viii) The workpiece disposed in the processing solution comprises at least one material selected from the group consisting of SiC, GaN, sapphire, ruby and diamond. 
     Points of the Invention 
     According to one embodiment of the invention, a processing method is conducted such that the surface of a workpiece is sequentially processed in the increasing order of distance (i.e., from nearest to farthest) from a photocatalyst film that is disposed opposite the processed surface (i.e., the surface subjected to the processing) of the workpiece. Thereby, a substrate with a good flatness property can be produced. In other words, the processed surface of the workpiece can be processed with anisotropy to provide a substrate with a good flatness property. For example, the diffusion distance of an active species in a processing solution can be controlled by adding a radical scavenger into the processing solution, and the controlled diffusion distance of the active species allows an oxidation reaction to be sequentially conducted in the increasing order of distance (i.e., from nearest to farthest) of the processed surface from the photocatalyst film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The preferred embodiments according to the invention will be explained below referring to the drawings, wherein: 
         FIG. 1A  is a conceptual view schematically showing a processing method according to one embodiment of the invention; 
         FIG. 1B  is a conceptual view schematically showing a case that a workpiece is processed by the processing method according to the embodiment of the invention; 
         FIG. 2  is an explanatory view schematically showing an oxidation-reduction process of a photocatalytic reaction that is a process principle of the processing method according to the embodiment of the invention; 
         FIG. 3  is a conceptual view schematically showing a processing method according to Example 1; 
         FIG. 4  is a conceptual view schematically showing a processing method according to Example 2; 
         FIG. 5  is a graph showing an oxygen atom concentration in the respective surfaces of SiC substrates of Comparative Example 2 corresponding to a case before the processing method of the invention is applied thereto, Example 1 corresponding to a case that an aqueous solution of water and ethanol is used as a processing solution and Comparative Example 1 corresponding to a case that only water is used as a processing solution; and 
         FIG. 6  is a graph showing a root-mean-square surface roughness (Rms) of the processed surface of the SiC substrate relative to the respective reaction times according to Example 1, Example 2 and Comparative Example 1. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments 
     Outline of Processing Method 
       FIG. 1A  is a conceptual view schematically showing a processing method according to one embodiment of the invention. In addition,  FIG. 2  is an explanatory view schematically showing an oxidation-reduction process of a photocatalytic reaction that is a processing principle of the processing method according to the embodiment of the invention. 
     A processing method according to the embodiment is a photocatalytic reaction type chemical processing method that processes a workpiece  20  by using active species  40  generated from a processing solution  30  due to a light irradiation. 
     An outline of the processing method according to the embodiment will be explained with reference to  FIG. 1A . The processing method according to the embodiment includes a workpiece disposition step of disposing a workpiece  20  having a surface  20   a  to be processed in a processing solution  30 , a photocatalyst film disposition step of disposing a photocatalyst film  12  in the processing solution  30  opposite the surface  20   a  to be processed, a active species generation step of irradiating the photocatalyst film  12  with a light  60 , so as to generate active species  40  being radicals from the processing solution  30  by a photocatalytic action of the photocatalyst film  12 , and a processing step of chemically reacting the active species  40  with surface atoms  22  of the surface  20   a  to be processed, generating a chemical compound  50  to be eluted in the processing solution  30  and eluting the chemical compound  50  in the processing solution  30 , so as to process the workpiece  20 . 
     Further, the processing step can also have a feature that the workpiece  20  is processed by controlling a temperature of at least one member selected from the group consisting of the photocatalyst film  12 , the workpiece  20  and the processing solution  30 . In the processing step, the temperature of at least one member selected from the group consisting of the photocatalyst film  12 , the workpiece  20  and the processing solution  30  is controlled, so that a chemical reaction speed between the workpiece  20  and the active species  40  can be controlled in an increasing direction or a decreasing direction. For example, if the temperature is raised, the chemical reaction speed is increased, and if the temperature is lowered, the chemical reaction speed is decreased 
     In the embodiment, the photocatalyst film  12  is formed on a front surface  10   b  of a substrate  10  that transmits a light  60 . Consequently, the light  60  that enters a rear surface  10   a  of the substrate  10  transmits the substrate  10  and enters the photocatalyst film  12  formed on the front surface  10   b . In addition, the processing solution  30  contains a radical scavenger  42  capable of capturing the active species  40  being radicals. The radical scavenger  42  captures the active species  40  and becomes a chemical compound  52 . With regard to the active species  40 , some are captured by the radical scavenger  42 , and some are not be captured, react with surface atoms  22  of the surface  20   a  to be processed and generate a chemical compound  50 . 
     Details of Processing Method 
     First, as shown in  FIG. 1A , in the processing solution  30  containing the radical scavenger  42 , the photocatalyst film  12  is brought into contact with or in close proximity to the workpiece  20 . 
     Further, in case that the photocatalyst film  12  is brought in close proximity to the workpiece  20 , a distance between the front surface  12   a  of the photocatalyst film  12  and the workpiece  20  is within a range of the maximum diffusion distance of the active species  40  in the processing solution  30  that is determined by life-span of the active species  40  generated from the processing solution  30 . 
     For example, in case that the processing solution  30  is composed of only water, the active species  40  generated are hydroxyl radicals that have strong oxidation power, and the maximum diffusion distance in this case is approximately 1 μm. In case that the processing solution  30  is composed of water and the radical scavenger  42  added to water, the hydroxyl radical diffused in the processing solution  30  reacts with the radical scavenger  42  and becomes the chemical compound  52 , so that the maximum diffusion distance becomes shorter than that of the above-mentioned case, namely becomes less than 1 μm. Consequently, it is only necessary to control the distance brought in close proximity to be less than 1 μm. 
     Next, the light  60  is irradiated from a side of the rear surface  10   a  of the substrate  10 , and then the light  60  that passes through the substrate  10  reaches the photocatalyst film  12 . When the photocatalyst film  12  is irradiated with the light  60 , the active species  40  generated from the processing solution  30  by a photocatalytic action of the photocatalyst film  12  adhere to the front surface  12   a  of the photocatalyst film  12 . And, the active species  40  generated are diffused in the processing solution  30  toward the surface  20   a  to be processed. A part of the active species  40  chemically reacts with the radical scavenger  42  before it reaches the surface  20   a  to be processed of the workpiece  20 , so as to generate the chemical compound  52 . Consequently, as the diffusion distance is increased in the processing solution  30 , probability that a part of the plurality of active species  40  generated from the processing solution  30  is deactivated until it reaches the surface  20   a  to be processed is increased. Due to this, the workpiece  20  is processed sequentially from the part thereof that has the shortest distance from the photocatalyst film  12  to the workpiece  20  (for example, in case that there are concavity and convexity on the surface  20   a  to be processed, from the fore-end part of the convexity). As just described, the active species  40  that reach the surface  20   a  to be processed before they react with the radical scavenger  42  chemically react with the surface atoms  22  of the surface  20   a  to be processed, so as to generate the chemical compound  50  to be eluted in the processing solution  30 . Subsequently, the chemical compound  50  generated is eluted and diffused from the surface  20   a  to be processed into the processing solution  30 . 
     Due to this, the surface  20   a  to be processed of the workpiece  20  is processed. In particular, for example, as shown in  FIG. 1B , the surface of the workpiece  20  is processed and flat surfaces  20   b  are exposed in the processing solution  30 . 
     Substrate  10   
     The substrate  10  according to the embodiment is formed of a transparent material that transmits the light  60 . In particular, in case that the light  60  is an ultraviolet light, the substrate  10  can be formed of a material that transmits the ultraviolet light. For example, as the substrate  10 , a glass substrate, a quartz substrate, a substrate formed of a synthetic resin such as an acrylic resin and the like can be used. The substrate  10  is formed of the transparent material that transmits the light  60 , so that the photocatalyst film  12  can be irradiated with the light  60  from a side of the rear surface  10   a  of the substrate  10 . Further, in case that the substrate  10  is formed of the synthetic resin, the substrate  10  is used, the substrate  10  having a transmittance of the light  60  of the degree that the synthetic resin is hard to be deteriorated against long-term use and simultaneously having the front surface  10   b  that has a flatness property not less than the flatness required for the surface  20   a  to be processed of the workpiece  20 . 
     Photocatalyst Film  12   
     The photocatalyst film  12  according to the embodiment is formed on the front surface  10   b  of the substrate  10  opposite the surface  20   a  to be processed of the workpiece  20 . And, the photocatalyst film  12  is formed of a film composed of a photocatalyst or a film including the photocatalyst. As the photocatalyst constituting the photocatalyst film  12 , at least one chemical compound selected from the group consisting of the metal oxides such as TiO 2 , KTaO 3 , SrTiO 3 , ZrO 2 , NbO 3 , ZnO, WO 3 , SnO 2  and the like that are metal oxides which have energy of not less than approximately 2.8 eV in the superior end of the valence band can be used. In addition, these chemical compounds can be doped with impurities. For example, the photocatalyst film  12  can be also formed of a nitrogen doped photocatalyst (for example, N-doped TiO 2 ) that is doped with nitrogen (N). 
     Here, in case that TiO 2  is used as the photocatalyst, it is preferred to use TiO 2  that has a crystal structure of an anatase-type. Further, TiO 2  of a rutile-type crystal, or a mixed crystal of TiO 2  of an anatase-type crystal and TiO 2  of a rutile-type crystal can be also used. 
     Method of Manufacturing the Photocatalyst Film  12   
     The photocatalyst film  12  according to the embodiment can be manufactured by using a sputtering method, a vapor-deposition method, a molecular beam epitaxy method (a MBE method), a laser ablation method, an ion plating method, a thermal CVD method, a plasma CVD method, a metal organic chemical vapor deposition method (a MOCVD method), a liquid phase epitaxy method, an aerosol deposition method (an AD method), a Langmuir-Blodgett method (a LB method), a sol-gel method, a plating method, a coating method or the like. Here, in the embodiment, it is preferred to use the sputtering method from the standpoint that the film formation is easily controlled and the like. 
     In case that the photocatalyst film  12  is manufactured by using the sputtering method, it can be manufactured as follows. For example, the sputtering is carried out by using a target formed of TiO 2  under an Ar atmosphere, so that the photocatalyst film  12  that is formed by being directly deposited as TiO 2  can be formed on the substrate  10 . In addition, the sputtering is carried out by using a target formed of Ti under an O 2  and Ar mixture atmosphere (hereinafter, may be referred to as O 2 /Ar atmosphere), so that the photocatalyst film  12  composed of TiO 2  formed by that Ti and O 2  in the atmosphere are reacted can be formed on the substrate  10 . Further, as a sputtering device for carrying out the sputtering method, a direct current sputtering device, a high-frequency sputtering device, a magnetron sputtering device, an ion beam sputtering device, an electron cyclotron resonance (ECR) sputtering device and the like can be used. 
     In addition, in case that the photocatalyst film  12  is formed by using the sputtering method, for the purpose of preventing mean free path of plasma such as Ar and the like from being increased so as to reduce a damage to the photocatalyst film  12  during the film formation, it is preferable that an output of the plasma is set to not more than 400 W and total pressure of gas in a chamber is set to not less than 1.0 Pa. 
     Further, for the purpose of increasing a generation amount of the active species  40  that chemically react with the surface atoms  22  of the surface  20   a  to be processed so as to carry out the processing of the workpiece  20  at a sufficient speed, it is preferable that the photocatalyst film  12  is formed to have a film thickness of not less than 150 nm that is a thickness capable of sufficiently absorbing the light  60 . In addition, it is more preferable that the film thickness of the photocatalyst film  12  is not less than 200 nm. Furthermore, in order that an amount of the active species  40  generated in accordance with an amount of the light that is irradiated from a side of the rear surface  10   a  of the substrate  10  toward the photocatalyst film  12  and reaches the photocatalyst film  12  becomes an amount sufficient for the processing of the workpiece  20 , it is preferable that the film thickness of the photocatalyst film  12  is not more than 1 μm. 
     In addition, as the light  60  in the embodiment, a light that has energy of not less than a band gap energy of the photocatalyst constituting the photocatalyst film  12  is used. For example, since the band gap energy of TiO 2  is 3.0 eV, TiO 2  fulfills a photocatalytic function to a light having a wavelength of not more than 420 nm. Consequently, in case that TiO 2  is used as the photocatalyst, as the light  60 , a light having a wavelength of preferably not less than 200 nm and not more than 420 nm, and more preferably not less than 200 nm and not more than 400 nm is used. Further, in case that the photocatalyst constituting the photocatalyst film  12  fulfills a photocatalytic function to a visible light, as the light  60 , the visible light can be also used. 
     Workpiece  20   
     The workpiece  20  according to the embodiment is formed of, for example, crystal materials such as a semiconductor material, an oxide material and the like that are used for an electronic device such as a power device, a light emitting device and the like. In particular, the workpiece  20  is a substrate formed of a crystal material such as SiC, GaN, sapphire, ruby, diamond or the like that has poor processability. 
     Radical Scavenger  42   
     As the radical scavenger  42  according to the embodiment, a protic organic compound can be used. As the radical scavenger  42  of the protic organic compound, for example, any one of methanol, ethanol, propanol and butanol can be used. In addition, as the radical scavenger  42 , a mixture liquid prepared by selecting at least two from methanol, ethanol, propanol and butanol and mixing the selected at least two protic organic compounds can be also used. 
     Process Principle of Processing Method: Generation Reaction of Active Species 
       FIG. 2  is an explanatory view schematically showing an outline of oxidation-reduction process of a photocatalytic reaction that is a process principle of the processing method according to the embodiment. 
     With reference to  FIG. 2 , a principle that the active species  40  are generated from the processing solution  30  (for example, water) in the place on or adjacent to the surface of the photocatalyst film  12  will be explained. Here, as an example, a case that a TiO 2  is used as the photocatalyst will be explained. First, TiO 2  is irradiated with a light that has energy of not less than the band gap energy of TiO 2  and a wavelength of not more than 420 nm, so that in accordance with Reaction formula (1) described below, electrons existing in the valence band are excited to the conduction band so as to generate positive holes in the valence band, and simultaneously excited electrons are generated in the conduction band so as to generate a pair of positive hole and electron. Further, in the Reaction formula (1), “UV” represents an ultraviolet light for short. 
       TiO 2 +light (UV)→ h   +   +e   −   (Reaction formula (1))
 
     The positive holes extract electrons (e − ) from hydroxyl ions (OH − ) generated due to ionization of water (H 2 O) so as to generate hydroxyl radicals (.OH) in accordance with reactions shown in Reaction formula (2) and Reaction formula (3) described below. 
       H 2 O→H + +OH −   (Reaction formula (2))
 
         h   + +OH − →OH  (Reaction formula (3))
 
     The hydroxyl radicals generated in accordance with the reaction shown in Reaction formula (3) have extremely strong oxidation power. Consequently, the hydroxyl radicals are able to react with chemically stable materials such as SiC, GaN, diamond and the like, and process the chemically stable materials. 
     On the other hand, the electrons excited move toward oxygen gas (dissolved oxygen) dissolved in the processing solution  30  in accordance with Reaction formula (4) described below so as to reduce the oxygen, unless specific substances (sacrificial reagents) that are readily oxidized are added into the processing solution  30 . Further, it can be also adopted to enhance reaction efficiency by adding the sacrificial reagents instead of the dissolved oxygen into the processing solution  30 . 
       O 2 +e − →O 2   −   (Reaction formula (4))
 
     Process Principle of Processing Method: Reaction of Active Species and Workpiece and Processing Step [in Case of SiC] 
     Next, a processing step of SiC as the workpiece  20  in case that the workpiece is SiC will be explained. First, it can be considered that the surface of SiC is oxidized due to the active species  40  (as an example, hydroxyl radicals in case that the processing solution  30  is water) generated from the processing solution  30  by a light irradiation to the photocatalyst film  12  in accordance with Reaction formula (5) described below. 
       SiC+4.OH+O 2 →SiO 2 +CO 2 +2H 2 O  (Reaction formula (5))
 
     Here, in case that the radical scavenger  42  is added into the processing solution  30 , the hydroxyl radicals as the active species  40  react with the radical scavenger  42  before they reach the surface  20   a  to be processed that is the surface of the workpiece  20 , so that they are deactivated. Consequently, the diffusion distance of the hydroxyl radicals in the processing solution  30  is determined by reaction speed between the hydroxyl radicals and the radical scavenger  42 . As just described, by adding the radical scavenger  42  into the processing solution  30  and controlling the diffusion distance of the active species  40  in the processing solution  30 , the diffusion distance can advance an oxidation reaction sequentially from the surface  20   a  to be processed that is located at the nearest distance from the photocatalyst film  12  (for example, TiO 2  thin film). 
     It can be considered that after the oxidation reaction of the surface of the workpiece  20 , by applying a removal treatment of an oxide layer generated by the oxidation reaction to the surface  20   a  to be processed, the oxidized regions of the surface  20   a  to be processed are preferentially processed. Further, in case that the oxide layer generated by the oxidation reaction is SiO 2 , hydrofluoric acid can be used for the removal treatment of the oxide layer. In this case, the oxidized regions of the surface  20   a  to be processed are preferentially processed in accordance with Reaction formula (7) described below. 
       SiO 2 +6HF→H 2 SiF 6 +2H 2 O  (Reaction formula (6))
 
     Process Principle of Processing Method: Reaction of Active Species and Workpiece and Processing Step [in Case of GaN] 
     In addition, a processing step of GaN as the workpiece  20  in case that the workpiece is GaN will be explained. First, it can be considered that the surface of GaN is oxidized due to the active species  40  (as an example, hydroxyl radicals in case that the processing solution  30  is water) generated from the processing solution  30  by a light irradiation to the photocatalyst film  12  in accordance with Reaction formula (7) described below. 
       2GaN+7.OH+7/4O 2 →Ga 2 O 3 +2NO 2 +7/2H 2 O  (Reaction formula (7))
 
     In this case, similarly to the explanation about the case that the workpiece  20  is SiC, by adding the radical scavenger  42  into the processing solution  30 , the diffusion distance of hydroxyl radicals as the active species  40  can be controlled. And, it can be considered that after the oxidation reaction of the surface of GaN as the workpiece  20 , by applying a removal treatment of an oxide layer generated by the oxidation reaction to the surface  20   a  to be processed, the oxidized regions of the surface  20   a  to be processed are preferentially processed. Further, in case that the oxide layer generated by the oxidation reaction is Ga 2 O 3 , sulfuric acid can be used for the removal treatment of the oxide layer. In this case, the oxidized regions of the surface  20   a  to be processed are preferentially processed in accordance with Reaction formula (8) described below. 
       Ga 2 O 3 +3H 2 SO 4 →Ga 2 (SO 4 ) 3 +3H 2 O  (Reaction formula (8))
 
     Advantages of the Embodiment 
     The processing method according to the embodiment includes disposing the workpiece  20  in the processing solution  30  and simultaneously disposing the photocatalyst film  12  in the processing solution  30  opposite the surface  20   a  to be processed, and then irradiating the photocatalyst film  12  with the light  60 , so as to generate active species  40  from the processing solution  30 , controlling a diffusion distance of the active species  40  in the processing solution  30  by the radical scavenger  42  added to the processing solution  30 , and chemically reacting the active species  40  with the surface atoms  22  of the surface  20   a  to be processed and generating the chemical compound  50  to be eluted in the processing solution  30 , so as to process the workpiece  20 , so that mechanical defects generated in the workpiece  20  when abrasive grain or abrasive compound is used are not generated. Due to this, in accordance with the processing method according to the embodiment, a workpiece not having a processing-degenerated layer (for example, a damaged layer generated on the surface of workpiece  20  when polishing is carried out by using the abrasive compound or the like) and having a surface with high accuracy (namely, a surface with a high flatness property) can be manufactured without generating crystal defects due to the processing. 
     In addition, the processing method according to the embodiment does not need the abrasive grain or abrasive compound to be used, so that for example, disposal of industrial waste such as disposal of used slurry required in a polishing method such as chemical mechanical polishing (CMP) method or the like is not needed. Consequently, the processing method according to the embodiment can reduce a cost of disposal, and does not discharge the industrial waste, so that the processing method is a preferable processing method from the viewpoint of conservation of environment. 
     Further, the processing method according to the embodiment has a step that the processing is carried out sequentially from the surface  20   a  to be processed that is located at near distance from the photocatalyst film  12 , so that a substrate having a good flatness property can be manufactured. In other words, the processing method according to the embodiment can process the surface of the workpiece  20  with anisotropic aspect, so that a substrate having a good flatness property can be manufactured. 
     Example 1 
     Hereinafter, Examples will be explained. 
       FIG. 3  is a conceptual view schematically showing an outline of processing method according to Example 1. 
     Fabrication of Photocatalyst Film  12   
     First, a quartz substrate  14  using quartz as a base material was disposed in a chamber of a high-frequency magnetron sputtering device. And, a TiO 2  thin film as the photocatalyst film  12  was formed on the surface of quartz substrate  14  under an Ar gas 100% atmosphere under the condition that an output of the plasma was 300 W, the substrate temperature was 300 degrees C., the total pressure was 3 Pa, the film formation time was 24 minutes. Further, as the target of high-frequency magnetron sputtering device, a target formed of a TiO 2  fired body was used. 
     Evaluation of Photocatalyst Film 
     Properties of the TiO 2  thin film formed on the surface of quartz substrate  14  were evaluated. The TiO 2  thin film had a film thickness of 200 nm. In addition, the TiO 2  thin film had a crystal structure of a mixed crystal of an anatase type crystal 76% and a rutile-type crystal 24%. 
     Photocatalytic Reaction Type Chemical Processing of SiC Substrate 
     In Example 1, a SiC substrate was used as the workpiece  20 . In particular, the SiC substrate used in Example 1 is a SiC substrate of a single crystal, and an n-type 4H—SiC that has a diameter of 50 mm and has (0001) Si face inclined by 8 degrees in a [11-20] direction was used. In addition, an electrical resistivity of the SiC substrate was 0.017 Ωcm. Further, the SiC single crystal substrate of {0001} face has polarity, one is a Si face formed of which outmost face is composed of Si atoms and another is a C face formed of which outmost face is composed of C atoms. In Example 1, the Si face was used as the surface  20   a  to be processed. In addition, in Example 1, the surface  20   a  (Si face) to be processed before the processing is a surface to which the CMP treatment was applied after a mechanical mirror-polishing was applied thereto. Further, in case that the C face was used as the surface  20   a  to be processed, although the case is somewhat different in a processing speed from the case that the Si face was used, a mechanism of advancement of the processing is similar to the case of the Si face. 
     In particular, the processing method according to Example 1 is as follows. First, the surface of SiC substrate was cleaned with a 10% HF aqueous solution. Next, after the SiC substrate was put into a glass beaker  70 , the quartz substrate  14  on which the photocatalyst film  12  was formed was introduced. In this case, the processed surface of the SiC substrate and the photocatalyst film  12  were brought into contact with each other. Next, an aqueous solution  32  prepared by adding methanol as a radical scavenger to water was introduced into the glass beaker  70  as a processing solution. Here, as the aqueous solution  32 , an aqueous solution prepared by adjusting the concentration of methanol to 50% (volume/volume %: v/v %) was used. In this case, the aqueous solution  32  was introduced into the glass beaker  70  so that the surface of aqueous solution  32  was located at an upper side than a contact surface (hereinafter, may be referred to as “interface”) of the photocatalyst film  12  and the processed surface of the SiC substrate (namely, a position that at least the contact surface exists in the aqueous solution  32 ). Due to this, the aqueous solution  32  entered the interface of the photocatalyst film  12  and the processed surface of the SiC substrate by capillarity so that an aqueous solution film layer  34  (hereinafter, may be referred to as “radical transport layer”) was formed. 
     Next, an ultraviolet light  62  having an ultraviolet light illuminance adjusted to 8 mW/cm 2  was irradiated from a side of rear surface  14   a  of the quartz substrate  14  by using a high pressure mercury vapor lamp. Due to this, a photocatalytic reaction between the photocatalyst film  12  and the aqueous solution  32  in the aqueous solution film layer  34  was initiated. With regard to a reaction time of the photocatalytic reaction between the photocatalyst film  12  and the aqueous solution  32  (namely, a reaction time of the active species and the workpiece), the processed surface was observed after every reaction for 1 hour and the reaction was carried out up to 5 hours as an accumulated time. 
     Due to the photocatalytic reaction, an oxide film was formed on the processed surface of the SiC substrate. After the photocatalytic reaction for 1 hour, the oxide film was removed by a 10% HF aqueous solution. And, the processed surface after the processing was observed by an atomic force microscope (AFM). After that, for further 1 hour, the photocatalytic reaction was similarly carried out, and the oxide film was removed and the processed surface was observed. This was repeated and the photocatalytic reaction was carried out for 5 hours as an accumulated time. As a result, as shown in  FIG. 6 , when a root-mean-square surface roughness (Rms) of the processed surface before the processing was 0.295 nm, the Rms of the processed surface after the processing was, 0.179 nm in case of the reaction for 1 hour, 0.136 nm in case of the reaction for 2 hour as an accumulated time, 0.125 nm in case of the reaction for 3 hour as an accumulated time, 0.121 nm in case of the reaction for 4 hour as an accumulated time, and 0.119 nm in case of the reaction for 5 hour as an accumulated time. As just described above, in accordance with the processing method according to Example 1, the flatness of the surface of SiC single crystal substrate can be improved. In addition, when a measurement whether the processing-degenerated layer was generated on the processed surface after the processing or not was carried out, the processing-degenerated layer did not exist. 
     Also in case that the processing similar to Example 1 was carried out by using KTaO 3 , SrTiO 3 , ZrO 2 , NbO 3 , ZnO, WO 3 , and SnO 2  as the photocatalyst film  12 , the root-mean-square surface roughness (Rms) was improved similarly to Example 1. In particular, in case that the reaction time was controlled to not less than 2 hours, the Rms of the processed surface after the processing could be flattened within a range of 0.100 nm to 0.150 nm in all the cases. It is understood that metal oxides other than TiO 2 , if they have a photocatalytic function, can also improve the flatness of the surface of SiC single crystal substrate as the photocatalyst film  12 . 
     Example 2 
       FIG. 4  is a conceptual view schematically showing an outline of processing method according to Example 2. 
     In Example 2, the photocatalyst film  12  and the aqueous solution  32  as the processing solution were heated so that a SiC substrate as the workpiece  20  was processed. 
     In particular, in Example 2, a SiC substrate of a single crystal that is similar to the SiC substrate used in Example 1 was used. Namely, as the SiC substrate used in Example 2, an n-type 4H—SiC that has a diameter of 50 mm and has (0001) Si face inclined by 8 degrees in a [11-20] direction was used. In addition, an electrical resistivity of the SiC substrate was 0.017 Ωcm. In Example 2, similarly, the Si face was used as the processed surface, and as the surface (Si face) to be processed before the processing, a surface to which the CMP treatment was applied after a mechanical mirror-polishing was applied thereto was used. 
     First, the surface of SiC substrate was cleaned with a 10% HF aqueous solution. Next, a quartz substrate  14  on which the photocatalyst film  12  was formed and the SiC substrate as the workpiece  20  were introduced into a glass beaker  70 . In this case, the processed surface of the SiC substrate and the photocatalyst film  12  were brought into contact with each other. Next, an aqueous solution  32  prepared by adding methanol as a radical scavenger to water was introduced into the glass beaker  70  as a processing solution. Here, as the aqueous solution  32 , an aqueous solution prepared by adjusting the concentration of methanol to 50% (v/v %) was used. In this case, the aqueous solution  32  was introduced into the glass beaker  70  so that the surface of aqueous solution  32  was located at an upper side than an interface of the photocatalyst film  12  and the processed surface of the SiC substrate (namely, a position that a least the interface exists in the aqueous solution  32 ). Due to this, the aqueous solution  32  entered the interface of the photocatalyst film  12  and the processed surface of the SiC substrate by capillarity so that an aqueous solution film layer  34  was formed. 
     Subsequently, an electric current applied to a heater  80  was adjusted by a heater controller  82 , and the temperature of aqueous solution  32  was heated up to 60 degrees C. Due to this, both of the aqueous solution  32  as the processing solution and the photocatalyst film  12  was heated to 60 degrees C. Next, an ultraviolet light  62  having an ultraviolet light illuminance adjusted to 8 mW/cm 2  was irradiated from a side of rear surface  14   a  of the quartz substrate  14  by using a high pressure mercury vapor lamp. Due to this, a photocatalytic reaction between the photocatalyst film  12  and the aqueous solution  32  was initiated. With regard to a reaction time of the photocatalytic reaction between the photocatalyst film  12  and the aqueous solution  32  (namely, a reaction time of the active species and the workpiece), the processed surface was observed after every reaction forl hour and the reaction was carried out up to 5 hours as an accumulated time. 
     Due to the photocatalytic reaction, an oxide film was formed on the processed surface of the SiC substrate. After the photocatalytic reaction for 1 hour, the oxide film was removed by a 10% HF aqueous solution. And, the processed surface after the processing was observed by the AFM. After that, for further 1 hour, the photocatalytic reaction was similarly carried out, and the oxide film was removed and the processed surface was observed. This was repeated and the photocatalytic reaction was carried out for 5 hours as an accumulated time. As a result, as shown in  FIG. 6 , when a root-mean-square surface roughness (Rms) of the processed surface before the processing was 0.497 nm, the Rms of the processed surface after the processing was, 0.210 nm in case of the reaction for 1 hour, 0.145 nm in case of the reaction for 2 hour as an accumulated time, 0.136 nm in case of the reaction for 3 hour as an accumulated time, 0.127 nm in case of the reaction for 4 hour as an accumulated time, and 0.124 nm in case of the reaction for 5 hour as an accumulated time. In addition, an oxidizing speed was calculated based on the thickness of oxide film formed on the processed surface. As a result, it was found that the oxidizing speed in case of heating the aqueous solution  32  as the processing solution was increased than in case of not heating it, so that the processing speed of the processed surface was increased. In addition, it was found that the heating of aqueous solution  32  in the process of processing the processed surface was more effective in enhancing the processing speed than in improving the surface roughness of the processed surface. In addition, when a measurement whether the processing-degenerated layer was generated on the processed surface after the processing or not was carried out, the processing-degenerated layer did not exist. 
     Example 3 
     In Example 3, the workpiece  20  was processed nearly in the same way as Example 1 except that a GaN substrate was used as the workpiece  20 . In particular, the GaN substrate used in Example 3 is a GaN substrate of a single crystal, and an n-type GaN that has a diameter of 50 mm and has (0001) Ga face was used. Further, the GaN single crystal substrate of {0001} face has polarity, one is a Ga face formed of which outmost face is composed of Ga atoms and another is a N face formed of which outmost face is composed of N atoms. In Example 3, the Ga face was used as the processed surface. In addition, in Example 3, the surface (Ga face) to be processed before the processing is a surface to which the CMP treatment was applied after a mechanical mirror-polishing was applied thereto. Further, in case that the N face was used as the processed surface, although the case is somewhat different in a processing speed from the case that the Ga face was used, a mechanism of advancement of the processing is similar to the case of the Ga face. 
     Photocatalytic Reaction Type Chemical Processing of GaN Substrate 
     In particular, the processing method according to Example 3 is as follows. First, the surface of GaN substrate was cleaned with a 10% HF aqueous solution. Next, a quartz substrate  14  on which the photocatalyst film  12  was formed and the GaN substrate were introduced into a glass beaker  70 . In this case, the processed surface of the GaN substrate and the photocatalyst film  12  were brought into contact with each other. Next, an aqueous solution  32  prepared by adding methanol as a radical scavenger to water was introduced into the glass beaker  70  as a processing solution. Here, as the aqueous solution  32 , an aqueous solution prepared by adjusting the concentration of methanol to 50% (v/v %) was used. In this case, the aqueous solution  32  was introduced into the glass beaker  70  so that the surface of aqueous solution  32  was located at an upper side than an interface of the photocatalyst film  12  and the processed surface of the GaN substrate (namely, a position that at least the interface exists in the aqueous solution  32 ). Due to this, the aqueous solution  32  entered the interface of the photocatalyst film  12  and the processed surface of the GaN substrate by capillarity so that an aqueous solution film layer  34  was formed. 
     Next, an ultraviolet light  62  having an ultraviolet light illuminance adjusted to 8 mW/cm 2  was irradiated from a side of rear surface  14   a  of the quartz substrate  14  by using a high pressure mercury vapor lamp. Due to this, a photocatalytic reaction between the photocatalyst film  12  and the aqueous solution  32  was initiated. A reaction time of the photocatalytic reaction between the photocatalyst film  12  and the aqueous solution  32  was set to 1 hour. 
     Due to the photocatalytic reaction, an oxide film was formed on the processed surface of the GaN substrate. After the photocatalytic reaction for 1 hour, the oxide film was removed by a concentrated sulfuric acid. As a result, when a root-mean-square surface roughness (Rms) of the processed surface before the processing was 0.261 nm, the Rms of the processed surface after the processing was 0.178 nm. As just described above, in accordance with the processing method according to Example 3, the flatness of the surface of GaN single crystal substrate can be improved. In addition, when a measurement whether the processing-degenerated layer was generated on the processed surface after the processing or not was carried out, the processing-degenerated layer did not exist. 
     Also in case that the processing similar to Example 3 was carried out by using sapphire, ruby, and diamond as the workpiece  20 , although there was difference in the processing speed, the root-mean-square surface roughness (Rms) of the surface of workpiece  20  was improved similarly to Example 3. This shows that even a poor processability substance other than SiC and GaN as the workpiece  20  can be improved on the flatness of surface thereof. 
     Comparative Example 1 
     In Comparative Example 1, a SiC substrate was processed under the same conditions that were used in Example 1 except that only water was used as a processing solution instead of using an aqueous solution prepared by adding methanol to water. 
     The SiC substrate used in Comparative Example 1 was a SiC substrate of a single crystal similarly to Example 1, and an n-type 4H—SiC that has a diameter of 50 mm and has (0001) Si face inclined by 8 degrees in a [11-20] direction was used. In addition, an electrical resistivity of the SiC substrate was 0.017 Ωcm. And, the processed surface was the Si face, and the surface (Si face) to be processed before the processing is a surface to which the CMP treatment was applied after a mechanical mirror polishing. 
     In the processing method according to the Comparative Example 1, first, the surface of SiC substrate was cleaned with a 10% HF aqueous solution. Next, a quartz substrate  14  on which the photocatalyst film  12  was formed and the SiC substrate were introduced into a glass beaker  70 . In this case, the processed surface of the SiC substrate and the photocatalyst film  12  were brought into contact with each other. Next, water as the processing solution was introduced into the glass beaker  70 . In this case, water was introduced into the glass beaker  70  so that the surface of water was located at an upper side than an interface of the photocatalyst film  12  and the processed surface of the SiC substrate (namely, a position that at least the interface exists in water). Due to this, water entered the interface of the photocatalyst film  12  and the processed surface of the SiC substrate by capillarity so that a water film layer was formed. 
     Next, an ultraviolet light  62  having an ultraviolet light illuminance adjusted to 8 mW/cm 2  was irradiated from a side of rear surface  14   a  of the quartz substrate  14  by using a high pressure mercury vapor lamp. Due to this, a photocatalytic reaction between the photocatalyst film  12  and water was initiated. With regard to a reaction time of the photocatalytic reaction between the photocatalyst film  12  and water (namely, a reaction time of the active species and the workpiece), the processed surface was observed after every reaction fort hour and the reaction was carried out up to 5 hours as an accumulated time. 
     Due to the photocatalytic reaction, an oxide film was formed on the processed surface of the SiC substrate. After the photocatalytic reaction for 1 hour, the oxide film was removed by a 10% HF aqueous solution. And, the processed surface after the processing was observed by an atomic force microscope (AFM). After that, for further 1 hour, the photocatalytic reaction was similarly carried out, and the oxide film was removed and the processed surface was observed. This was repeated and the photocatalytic reaction was carried out for 5 hours as an accumulated time. As a result, as shown in  FIG. 6 , when a root-mean-square surface roughness (Rms) of the processed surface before the processing was 0.354 nm, the Rms of the processed surface after the processing was, 0.348 nm in case of the reaction for 1 hour, 0.340 nm in case of the reaction for 2 hour as an accumulated time, 0.331 nm in case of the reaction for 3 hour as an accumulated time, 0.335 nm in case of the reaction for 4 hour as an accumulated time, and 0.328 nm in case of the reaction for 5 hour as an accumulated time. In the processing method according to the Comparative Example 1, the flatness of the surface of SiC single crystal substrate was hardly improved. It is considered that the reason why the flatness was hardly improved is that in Comparative Example 1, the diffusion distance of the active species  40  was not controlled, so that the active species  40  reached concave portions in association with progression of the processing and the processing mode was shifted to an isotropic mode. As just described above, the RMS of surface  20   a  to be processed of Comparative Example 1 is inferior to the case that the radical scavenger  42  was added. 
     Control of Diffusion Distance of Active Species 
     Hereinafter, a method of controlling a diffusion distance of the active species (hydroxyl radicals) in the processing solution in the processing method according to the invention will be explained. 
       FIG. 5  is a graph showing an oxygen atom concentration in the respective surfaces of SIC substrates of Comparative Example 2 corresponding to a case before the processing method of the invention is applied thereto, Example 1 corresponding to a case that an aqueous solution of water and ethanol is used as a processing solution and Comparative Example 1 corresponding to a case that only water is used as a processing solution. 
     In particular, first, the oxygen atom concentration in the surface of SiC substrate before the processing method according to Example 1 is applied thereto was measured by Auger electron spectroscopy analysis. In this case, the measurement resulted in measuring the oxygen atom concentration of a natural oxide film in the surface of SiC substrates (hereinafter, referred to as “oxygen concentration of Comparative Example 2”). In addition, the oxygen atom concentration in the surface of SiC substrate to which the processing due to the processing method according to Example 1 was applied was measured by Auger electron spectroscopy analysis (hereinafter, referred to as “oxygen concentration of Example 1”). Further, the oxygen atom concentration in the surface of SiC substrate to which the processing due to the processing method according to Comparative Example 1 was applied was measured by Auger electron spectroscopy analysis (hereinafter, referred to as “oxygen concentration of Comparative Example 1”). 
     Referring to  FIG. 5 , the oxygen atom concentration in the surface of SiC substrate was increased in ascending order of the oxygen concentration of Comparative Example 2, the oxygen concentration of Example 1 and the oxygen concentration of Example 1. This shows that the oxidation reaction in the surface of SiC substrate was inhibited due to the fact that methanol as the radical scavenger  42  was added to water. Namely, this shows that the radical scavenger  42  was added to water, so that the diffusion distance of hydroxyl radicals as the active species  40  in water can be controlled. Consequently, it is understood that in Example 1, the oxidation reaction in the surface of the workpiece  20  can be advanced sequentially from a part of the workpiece  20  that has the shortest distance from the surface of photocatalyst film  12 , and the diffusion distance of the active species  40  can be controlled due to existing of the radical scavenger  42 . 
     Also in case that the processing was carried out similarly to Example 1 by using ethanol, propanol and butanol as the radical scavenger  42 , the oxygen atom concentration in the surface of SiC substrate showed a tendency similar to Example 1. This show that the diffusion distance of hydroxyl radicals as the active species  40  in water can be also controlled due to adding protic organic compounds other than methanol as the radical scavenger  42  to water. 
     From the above, in accordance with the processing methods according to Examples, the diffusion distance in the processing solution  30  of active species  40  (for example, in case that the main component of processing solution  30  is water, hydroxyl radicals) generated from the processing solution  30  by the photocatalytic action of photocatalyst film  12  can be controlled due to adding the radical scavenger  42  to the processing solution  30 . Due to this, the workpiece  20  that has a surface without polishing marks can be provided and simultaneously the surface can be flattened at the atom level. 
     Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.