JP2013017925A - Method and device of photocatalyst reaction type chemical processing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of photocatalyst reaction type chemical processing that can improve the processing speed of a hard-to-work material such as SiC and GaN.SOLUTION: In the method of photocatalyst reaction type chemical processing that brings a photocatalyst thin film 1 into contact with or close proximity to a surface 14 of an object 3 to be processed while interposing a reaction process liquid 5 made of an acid aqueous solution and processes the surface 14 of the object 3 to be processed using an active species generated by irradiating the photocatalyst thin film 1 with light, the surface 14 of the object 3 to be processed is processed under the condition such that a pH of the reaction process liquid 5 is 4.88 or less.

Description

  The present invention relates to a light irradiation type chemical processing method, and more particularly to a photocatalytic reaction type chemical processing method and apparatus for processing a workpiece using active species generated by light irradiation.

  Conventionally, silicon power semiconductor devices (power devices) have been used for high-frequency and high-power control, but the power device performance is approaching theoretical limits calculated from the physical properties of silicon. In order to further improve the characteristics, power devices using new materials are being studied.

  Silicon carbide (SiC), gallium nitride (GaN), and the like are attracting attention as semiconductor materials for such power devices. SiC and GaN have a dielectric breakdown electric field that is an order of magnitude higher than that of silicon, so they are considered to be applicable to high-voltage devices, and because they have excellent heat resistance, they have much superior semiconductor element characteristics than silicon. It is expected.

  GaN has been attracting attention as a substrate material for blue light-emitting diodes, blue-violet lasers, solid-state lighting sources, etc. If mass production technology with high quality and low cost is established, next-generation large-capacity recording devices A wide range of applications such as reading devices, illumination, and light sources are expected.

  In order to fabricate a device from a single crystal substrate of SiC or GaN, it is necessary to polish the surface as smoothly as possible. However, SiC and GaN single crystal substrates are chemically extremely stable, have hardness next to diamond, and are difficult to process, which is an important issue in device development.

  In general, the surface of an object to be polished such as a semiconductor wafer is mechanically flattened by pressing the polishing member against the surface to be polished of the wafer while rotating both the wafer and the polishing member. Particularly in the case of difficult-to-work materials such as SiC and GaN, the mechanical polishing method mainly uses diamond abrasive grains, so that defects are introduced into the material by mechanical action and damage the crystal lattice. Therefore, in order to produce a highly accurate surface without a work-modified layer, it is necessary to use chemical processing that can be processed without generating lattice defects.

  As a chemical processing method, an etching method using a liquid chemical having a property of corroding a workpiece is generally used. Since it is a pure chemical process, it causes less damage to the workpiece than mechanical processing methods, and it is easy to obtain a highly accurate surface because there is no work-modified layer due to polishing. However, since chemical etching proceeds isotropically, if there are defects in the crystal, defective portions that are incomplete in the crystal are preferentially etched, so it is difficult to improve the flatness. is there.

  On the other hand, chemical mechanical polishing (CMP), which has characteristics of both mechanical processing and chemical processing, applies an abrasive (slurry) to the contact surface between the polishing pad and the wafer. It is possible to provide a polishing process with relatively high flatness by synergistic effect of chemical polishing with a liquid component and mechanical polishing with abrasive particles (see Patent Document 1). Here, the slurry is a suspension in which abrasive particles are dispersed in a chemical solution such as an alkaline aqueous solution.

  On the other hand, a workpiece is placed in a processing solution containing an oxidant, and a solid catalyst that decomposes the oxidant is used on the processing reference surface. There has been proposed a catalyst-assisted chemical processing method for processing a workpiece by removing or eluting a generated compound by a chemical reaction between a surface atom and an active species of the workpiece that is approaching (see Patent Document 2). ).

  In addition, by reducing the surface area of the polishing surface of the polishing member relative to the surface area of the surface to be polished, it is possible to directly supply an abrasive containing abrasive particles to the surface to be polished, and the material is directly supplied to the surface to be polished. By directly irradiating the abrasive and the surface to be polished with light, the abrasive particles that have absorbed the energy of light and acted as a photocatalyst can be quickly removed from the surface to be polished before the photocatalytic activity is lost. A method has been proposed in which the surface to be polished is oxidized by using a photocatalytic action and a photochemical reaction, which are sent to a region facing the polishing surface (see Patent Document 3).

  Furthermore, a processing method using a photocatalytic reaction in which an object to be processed is oxidized and removed using an active species (hydroxyl radical) generated on a photocatalyst film by light irradiation in an electrolyte solution (a photocatalytic type is proposed). (See Patent Document 4).

JP 2004-281865 A JP 2008-136983 A JP 2006-224252 A JP 2002-334856 A JP 2010-251699 A

  However, since the CMP of Patent Document 1 includes mechanical polishing, it is impossible in principle to completely remove a polishing modified layer (damage layer) such as a polishing mark. In addition, since SiC and GaN single crystals are chemically very stable, the chemical action is hardly effective and a sufficient processing speed cannot be obtained.

  On the other hand, the catalyst-assisted chemical processing method of Patent Document 2 decomposes the oxidizing agent on the surface of the solid catalyst serving as a processing reference surface and generates active species used for the chemical reaction. Therefore, the surface flatness of the solid catalyst is transferred to the workpiece surface. Since it is chemically processed, it is possible to produce a highly accurate surface, but the flatness of the surface of the solid catalyst is not exceeded on the processing principle.

  Further, even in the method using the photocatalytic action and the photochemical reaction of Patent Document 3, it is difficult to eliminate the work-modified layer as long as the polishing member and the abrasive grains are used.

  Furthermore, the method of simply bringing the photocatalytic film into contact with the workpiece as in Patent Document 4 does not consider the warpage of the workpiece substrate and the diffusion distance of hydroxyl radicals, resulting in isotropic processing. It is difficult to obtain highly accurate surface flatness.

  That is, in recent years, the importance as an electronic device material has been recognized, and difficult-to-work materials such as SiC and GaN, which are expected to be applied to power devices and light-emitting devices, do not introduce lattice defects crystallographically. In addition, when processing with high flatness, there is a problem that the processing speed is extremely slow compared with the polishing type processing method.

  The present invention has been made in view of the above situation, and an object thereof is to provide a photocatalytic reaction type chemical processing method and apparatus capable of improving the processing speed of difficult-to-process materials such as SiC and GaN.

  The present invention devised to achieve the above object is to bring the photocatalyst thin film into contact with or close to the surface of the workpiece, while interposing a reaction treatment solution comprising an acidic aqueous solution, and irradiating the photocatalyst thin film with light. In the photocatalytic reaction type chemical processing method in which the surface of the workpiece is processed using the active species generated in this way, the surface of the workpiece is processed under the condition that the pH of the reaction treatment liquid is 4.88 or less. This is a photocatalytic reaction type chemical processing method.

As the photocatalyst constituting the photocatalytic thin film, at least one metal oxide selected from the group consisting of TiO ₂ , KTaO ₃ , SrTiO ₃ , ZrO ₂ , NbO ₃ , ZnO, WO ₃ , and SnO ₂ may be used.

  The workpiece may be made of any one of SiC, GaN, sapphire, ruby, and diamond.

  The reaction treatment liquid may be an acidic aqueous solution containing at least one of sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, and hydrogen peroxide.

  The present invention also has a photocatalytic thin film formed on a flat surface, a rotating platen that rotates the photocatalytic thin film, and a rotating shaft that is eccentric with respect to the rotating shaft of the rotating platen, and holds the workpiece. A holder capable of moving up and down to bring the workpiece into contact with or close to the surface of the photocatalytic thin film, and a processing liquid for supplying a reaction processing liquid comprising an acidic aqueous solution between the photocatalytic thin film and the surface of the workpiece In the photocatalytic reaction type chemical processing apparatus comprising a supply means and a light irradiation means for irradiating the photocatalyst thin film with light, the treatment liquid supply means has a pH of 4.88 or less. This is a photocatalytic reaction type chemical processing apparatus for supplying the reaction treatment liquid.

  According to the present invention, the processing speed of difficult-to-process materials such as SiC and GaN can be improved.

It is a figure which shows the structure of the photocatalytic reaction type chemical processing apparatus which concerns on this invention. It is a schematic diagram explaining the processing principle of this invention.

  Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings.

  In the photocatalytic reaction type chemical processing method according to the present embodiment, the photocatalyst thin film is brought into contact with or in close proximity to the surface of the workpiece, and a reaction treatment liquid composed of an acidic aqueous solution is interposed therebetween, and the photocatalytic thin film is irradiated with light. When the surface of the workpiece is processed using the active species generated in this manner, the surface of the workpiece is processed under such a condition that the pH of the reaction treatment liquid is 4.88 or less.

First, the processing principle of this processing method will be described with reference to FIG. 2 taking as an example the case of using TiO ₂ as a photocatalyst.

The photocatalytic reaction type chemical processing method is to place a work piece in a reaction treatment solution, and to contact or bring a thin film having photocatalytic activity such as titanium dioxide (TiO ₂ ) into contact with or close to the work surface (surface) of the work piece. In addition, a reaction treatment solution consisting of an acidic aqueous solution is interposed between the active species having a strong oxidizing power generated on the thin film by a generally known photocatalytic reaction and the surface atoms of the workpiece. In this method, the surface of the workpiece is processed by sequentially removing the compound after changing it to an eluting compound.

When TiO ₂ having photocatalytic activity is irradiated with light (ultraviolet UV) having a band gap energy or more (400 nm or less), the following reaction is performed: TiO ₂ + light (UV) → h ⁺ + e ⁻
Accordingly, electrons existing in the valence band (E _v ) are excited to the conduction band (E _c ), and hole (h ⁺ ) and excited electron (e ⁻ ) pairs are generated.

Holes are the reactions shown below H ₂ O → H ⁺ + OH ⁻
h ^{+ +} OH ^- → · OH
Accordingly, an electron (e ⁻ ) is extracted from a hydroxide ion (OH ⁻ ) generated by ionization of water (H ₂ O) to generate a hydroxyl radical (hydroxyl radical; .OH).

The generated hydroxyl radical has a very strong oxidizing power, and the following formula SiC + 4 · OH + O ₂ → SiO ₂ + CO ₂ + 2H ₂ O is applied to chemically stable materials such as SiC, GaN and diamond.
2GaN + 6.OH → Ga ₂ O ₃ + N ₂ + 3H ₂ O
It is considered that an oxide film is formed as a result of the reaction.

On the other hand, unless the special oxidizable substance (sacrificial agent) is added to the excited electrons, the following reaction O ₂ + e ⁻ → O ₂ ⁻
According to the above, the oxygen gas is dissolved in the treatment liquid (dissolved oxygen) to reduce oxygen.

  Here, oxygen molecules (superoxide anion) reduced by one electron by the reaction are sequentially reduced and finally reduced by four electrons to become water. Therefore, it is expected that the processing speed can be improved by utilizing the superoxide anion for the reaction of a workpiece such as SiC or GaN.

The superoxide anion has the following formula in an aqueous solution: O ₂ ⁻ + H ⁺ ⇔ HOO.
In equilibrium as shown in, equilibrium constant pK _a of the reaction is 4.88.

Here, HOO. Is a hydroperoxy radical, which is a powerful reactive species together with the hydroxyl radical, and both SiC and GaN are represented by the following formula: SiC + 4HOO. → SiO ₂ + CO ₂ + 2H ₂ O + O ₂
2GaN + 6HOO · → Ga ₂ O ₃ + N ₂ + 3H ₂ O + 3O ₂
It is considered that an oxide film is formed by reacting as shown in FIG.

  Therefore, by setting the pH of the treatment solution to 4.88 or less, it can be expected that the superoxide anion is sequentially converted into a hydroperoxy radical and promotes an oxidation reaction that is a processing reaction of the workpiece.

The oxide films formed on the surface to be processed of SiC or GaN by the oxidation reaction are each expressed by the following formula SiO ₂ + 6HF → H ₂ SiF ₆ + 2H ₂ O in an appropriate treatment solution.
Ga ₂ O ₃ + 3H ₂ SO ₄ → Ga ₂ (SO ₄ ) ₃ + 3H ₂ O
The processing proceeds by being removed according to the above.

  Next, the configuration of a processing apparatus that performs the processing method according to the present embodiment will be described with reference to FIG.

  As shown in FIG. 1, a processing apparatus (photocatalytic reaction type chemical processing apparatus) 10 according to the present embodiment includes a rotating platen 11 that has a photocatalytic thin film 1 formed on a flat surface and rotates the photocatalytic thin film 1. The rotating table 13 has a rotating shaft 13 that is eccentric with respect to the rotating shaft 9 of the rotating platen 11 and can be moved up and down to hold the workpiece 3 and to bring the workpiece 3 into contact with or close to the surface of the photocatalytic thin film 1. A treatment liquid supply means 6 for supplying a reaction treatment liquid 5 made of an acidic aqueous solution between the holder 4, the photocatalytic thin film 1 and the surface 14 of the workpiece 3, and a light irradiation means 8 for irradiating the photocatalytic thin film 1 with light 7. With. Further, when the processing apparatus 10 planarizes the surface 14 of the workpiece 3, the processing liquid supply means 6 supplies the reaction processing liquid 5 having a pH of 4.88 or less.

  In this processing apparatus 10, the thin film supporting substrate 2 is preferably made of a transparent material that transmits light 7 and has a flatness that is equal to or higher than the flatness required for the surface 14 of the workpiece 3. The thin film supporting substrate 2 includes a rotating shaft 9 that rotates integrally. The rotating shaft 9 and the photocatalytic thin film 1 form a rotating platen 11. The rotating shaft 9 is inserted into a processing tank 12 that prevents the reaction processing solution 5 from scattering or leaking, and the thin film support base 2 is disposed in the processing tank 12. The photocatalytic thin film 1 is rotated by rotating the rotating platen 11 by the rotating shaft 9.

  The holder 4 that holds the workpiece 3 includes a rotating shaft 13 that is eccentric with respect to the rotating shaft 9 of the rotating surface plate 11, and the holder 4 rotates integrally with the rotating shaft 13. The holder 4 holds the workpiece 3 with the surface (surface to be processed) 14 facing the photocatalytic thin film 1, and brings the photocatalytic thin film 1 into contact with or in close proximity to the surface 14.

  The holder 4 can move up and down, and when the workpiece 3 is fixed to the holder 4, the rotating surface plate 11 and the holder 4 are separated from each other to facilitate handling of the workpiece 3, or when the workpiece 3 is processed. A predetermined pressing force, for example, a pressure (55 Pa) at which the workpiece 3 is pressed against the rotating surface plate 11 by its own weight is applied to the surface 14 which is the processing surface.

  The processing liquid supply means 6 supplies the reaction processing liquid 5 by dropping it onto the surface of the rotating platen 11. In this embodiment, a dropping nozzle is used as the processing liquid supply means 6. The dripping nozzle as the processing liquid supply means 6 is provided above the rotary platen 11 and on the side of the holder 4. The reaction treatment liquid 5 dropped from the treatment liquid supply means 6 forms a thin treatment liquid layer 5 a on the surface of the photocatalytic thin film 1 by the rotation of the rotating platen 11, and the photocatalytic thin film 1 and the surface 14 of the workpiece 3 The reaction processing liquid 5 is interposed therebetween. In the present invention, the treatment liquid supply means 6 is not limited to the dropping nozzle.

  The light irradiation means 8 is installed on the back surface side of the thin film supporting substrate 2 and below the workpiece 3, and the light 7 is irradiated upward from the light irradiation means 8. The light 7 irradiated from the light irradiation means 8 passes through the thin film supporting substrate 2 made of a transparent material, and reaches the photocatalytic thin film 1 around the workpiece 3 through this.

  The light 7 that has reached the photocatalytic thin film 1 forms a compound layer 15 on the surface of the workpiece 3 according to the processing principle described above. By sequentially removing or eluting the compound layer 15 by reaction with the reaction treatment liquid 5, the surface 14 of the workpiece 3 is processed. At this time, the surface 14 of the workpiece 3 is uniformly processed by simultaneously rotating the rotary shaft 13 provided in the holder 4.

  Next, the main part of the processing apparatus 10 will be described in detail.

  In the present embodiment, the workpiece 3 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. More specifically, the workpiece 3 is a substrate made of a difficult-to-work crystal material such as SiC, GaN, sapphire, ruby, diamond or the like.

The photocatalytic thin film 1 is formed from a thin film made of a photocatalyst or a thin film containing a photocatalyst. Examples of the photocatalyst constituting the photocatalytic thin film 1 include TiO ₂ , KTaO ₃ , SrTiO ₃ , ZrO ₂ , NbO ₃ , ZnO, WO ₃ , and SnO ₂ having an energy at the upper end of the valence band of about 2.8 eV or more. At least one compound selected from the group consisting of metal oxides can be used. Further, these compounds can be doped with impurities. For example, the photocatalytic thin film 1 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. When using a mixed crystal, it is desirable that the content of anatase type is 75% or more.

  The present invention is not particularly limited with respect to the method for producing the photocatalytic thin film 1. For example, sputtering, vapor deposition, molecular beam epitaxy (Molecular Beam Epitaxy; MBE), laser ablation, ion plating, thermal CVD, Plasma CVD method, metal organic chemical vapor deposition method (Metal Organic Chemical Deposition method; MOCVD method), liquid phase epitaxy method, aerosol deposition method (Aerosol Deposition method; AD method), Langmuir-Blodgett method (Langmuir-Brodet method) LB method), sol-gel method, plating method, coating method and the like. In this embodiment mode, a sputtering method is used from the viewpoint of easy control of film formation.

When the photocatalytic thin film 1 is formed using a 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 photocatalytic film 1 formed by directly depositing a TiO ₂ can be formed on the thin film support substrate 2. 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. The photocatalytic thin film 1 made of TiO ₂ formed by reaction can be formed on the thin film supporting substrate 2.

  At this time, the plasma output is set to 400 W or less, more preferably 300 W or less for the purpose of suppressing an increase in the mean free path of plasma of Ar or the like and reducing damage to the photocatalytic thin film 1 during film formation. At the same time, the total pressure of the gas in the chamber may be set to 1.0 Pa or higher, preferably 1.2 Pa or higher, more preferably 3.0 Pa or higher.

  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.

  The film thickness of the photocatalytic thin film 1 is sufficient to increase the amount of active species that chemically react with the surface atoms of the workpiece 3 and to sufficiently process the light 7 for the purpose of processing the workpiece 3 at a sufficient speed. The thickness is 150 nm or more, which is a thickness that can be absorbed into the film. The film thickness of the photocatalytic thin film 1 is more preferably 200 nm or more. On the other hand, when the film thickness of the photocatalytic thin film 1 is too thick, the amount of light 7 reaching the surface of the photocatalytic thin film 1 is reduced, and the amount of active species generated and the processing speed of the workpiece 3 are reduced. Therefore, the film thickness of the photocatalytic thin film 1 is preferably 1 μm or less, more preferably 500 nm or less so that the amount of active species corresponding to the amount of light 7 is sufficient for processing the workpiece 3. .

As the light 7 irradiated to such a photocatalyst thin film 1, a light having an energy equal to or higher than the band gap energy of the photocatalyst constituting the photocatalyst thin film 1 is used. For example, the band gap energy of TiO ₂ is 3.0 eV, and 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 7 uses ultraviolet light having a wavelength of 200 nm to 420 nm, preferably 200 nm to 400 nm, and the light irradiation means 8 for irradiating the light 7 is, for example, a high-pressure mercury lamp. Use. In addition, when the photocatalyst which comprises the photocatalyst thin film 1 exhibits a photocatalytic function with respect to visible light, visible light can also be used as the light 7. FIG.

  The thin film supporting substrate 2 is formed from a transparent material that transmits light 7. More specifically, when the light 7 is ultraviolet light, the thin film support base 2 can be made of a material that transmits ultraviolet light, for example, a synthetic resin such as a glass substrate, a quartz substrate, or acrylic. By forming the thin film supporting substrate 2 from a transparent material that transmits the light 7, the light 7 can be irradiated to the photocatalytic thin film 1 from the back side of the thin film supporting substrate 2. In the case where the thin film supporting substrate 2 is made of a synthetic resin, it is preferable to use a resin material having a transmittance that is not easily deteriorated by the light 7. In particular, when ultraviolet rays are used as the light 7, the synthetic resin may be deteriorated by absorbing the ultraviolet rays. Therefore, the deterioration due to long-term use can be suppressed by using a resin material having a high transmittance.

  The reaction treatment liquid 5 is composed of an acidic aqueous solution whose pH is controlled to 4.88 or lower. Thereby, the superoxide anion produced | generated by the reduction reaction side, ie, the 1-electron reduction reaction to the oxygen molecule by an excited electron, can be sequentially converted into the hydroperoxy radical with higher oxidation reactivity. Moreover, the processing speed of the workpiece 3 is improved by promoting the oxidation reaction.

  Here, the reaction treatment liquid 5 is preferably composed of an acidic aqueous solution containing at least one inorganic acid such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, and hydrogen peroxide. Thereby, since organic matter and metal ions do not remain as contaminants on the surface 14 of the workpiece 3, the cleanliness of the workpiece 3 after processing can be maintained high.

  Next, the processing method according to the present embodiment will be described together with the operation of the processing apparatus 10.

  First, the workpiece 3 made of a difficult-to-process material such as SiC or GaN is held by the holder 4. At this time, the surface 14 of the workpiece 3 is held facing the photocatalytic thin film 1. The method for holding the workpiece 3 by the holder 4 is not particularly limited. For example, the workpiece 3 is attached to the holder 4 with wax, or the workpiece 3 is sucked and fixed to the holder 4 with a suction device. It ’s fine.

  After the workpiece 3 is held by the holder 4, the photocatalytic thin film 1 is brought into contact with or in close proximity to the surface 14 of the workpiece 3. Here, extremely close refers to a state in which the reactive species generated on the photocatalytic thin film 1 are approaching to the extent that they can reach the surface of the workpiece 3 (approximately 1 μm or less). By bringing the photocatalytic thin film 1 very close to the surface 14 of the workpiece 3, the generated active reactive species can be diffused and reacted with the surface 14 of the workpiece 3.

  Further, while rotating the rotating platen 11 (that is, the photocatalytic thin film 1), the reaction liquid 5 whose pH is adjusted to 4.88 or less from the processing liquid supply means 6 is dropped onto the surface of the photocatalytic thin film 1 and the processing liquid layer 5a. The reaction treatment liquid 5 is interposed between the photocatalytic thin film 1 and the surface 14 of the workpiece 3. At the same time, the holder 4 is rotated by the rotating shaft 13. Here, the interposition means that the dropped reaction treatment liquid 5 enters between the photocatalytic thin film 1 and the workpiece 3 by a capillary phenomenon to form a thin film of the treatment liquid. In addition, the thin film of the treatment liquid formed by capillary action is necessary for transport (diffusion) of reactive active species, and can be regarded as an active species transport layer.

  Thereafter, by irradiating light 7 from the light irradiation means 8, the hydroxyl radicals generated by light irradiation on the surface of the photocatalytic thin film 1 react with the surface atoms of the workpiece 3 to form the compound layer 15. The compound layer 15 is removed by treatment with the reaction treatment solution 5 made of an acidic aqueous solution to which sulfuric acid or the like is added, and as a result, a highly accurate flat surface is formed.

  As described above, in the present invention, the photocatalytic thin film 1 is brought into contact with or in close proximity to the surface 14 of the workpiece 3, and the reaction treatment liquid 5 made of an acidic aqueous solution is interposed therebetween to irradiate the photocatalytic thin film 1 with light. When the surface 14 of the workpiece 3 is processed using the activated species generated in this manner, the surface 14 of the workpiece 3 is processed under the condition that the pH of the reaction treatment liquid 5 is 4.88 or less. ing.

  In the present invention, since the work piece 3 is processed by removing or eluting the compound layer 15 generated by the chemical reaction with the surface atoms of the work piece 3, the work-affected layer of the work-affected layer is used without using abrasive grains or abrasives. A highly accurate surface can be produced.

  In addition, by setting the pH of the reaction treatment solution 5 used for processing to 4.88 or less, the superoxide anion generated by the reduction reaction is sequentially converted into hydroperoxy radicals, which are reactive species, and used for processing. Time can be shortened.

  Conventionally, the abrasive particles contained in the slurry are the most expensive in the polishing process by the CMP method, which is a general polishing method for difficult-to-process materials such as SiC and GaN. Further, since the used slurry is required to be treated as industrial waste, it is not preferable from the viewpoint of environmental protection in addition to cost which cannot be ignored. According to the present invention, since an abrasive is unnecessary, these problems can be solved simultaneously.

  The technical scope of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.

  For example, in the above embodiment, the reaction treatment liquid 5 is interposed between the photocatalytic thin film 1 and the workpiece 3 by the treatment liquid layer 5a. However, the reaction treatment liquid 5 is stored in the processing tank 12, and the storage is performed. The photocatalytic thin film 1 and the workpiece 3 may be disposed in the reaction treatment liquid 5 thus processed.

  Further, a sacrificial agent may be added to the reaction treatment liquid 5 in accordance with the material characteristics (oxidation resistance, etc.) of the workpiece 3 to control the rate at which active species are generated.

  Examples of the present invention will be described below.

[Example 1]
As the photocatalyst material, TiO ₂ produced by a high frequency magnetron sputtering method was used.

As shown in FIG. 1, after a GaN substrate is attached as a workpiece 3 to a holder 4 with wax, a processed surface (surface 14) of the GaN substrate is deposited on a thin film support base 2 made of quartz. _The photocatalytic thin film 1 made of ₂ was brought into contact. Here, the GaN substrate used in this example is a single crystal free-standing substrate, which has a diameter of 50 mm, an n-type (0001) Ga plane, and a carrier concentration of (1-3) × 10 ¹⁸ / cm ³ . 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 this example, the Ga surface was processed. When processing the N plane, the processing speed is somewhat different, but the mechanism of the processing is the same as in the Ga plane. Note that the processed surface (Ga surface) of the GaN substrate used in this example is mechanically polished in advance.

After bringing the GaN substrate into contact with the TiO ₂ film, a sulfuric acid aqueous solution adjusted to pH 1.5 as the reaction treatment solution 5 is dropped onto the rotating platen 11 at a flow rate of 5 ml per minute, and the rotating platen 11 and the holder 4 The rotating shafts 9 and 13 included in the are rotated. There are no particular restrictions on the number of rotations of the rotating platen 11 and the holder 4, but the rotating platen 11 was rotated at a speed of 5 rotations per minute in order to minimize scratches and damage to the GaN substrate and quartz. The holder 4 with the GaN substrate attached was also rotated at a speed of 10 revolutions per minute for the same reason.

Subsequently, ultraviolet rays (light 7) with an ultraviolet illuminance adjusted to 30 mW / cm ² were irradiated from the back side of the rotating platen 11 by the light irradiation means 8 comprising a high-pressure mercury lamp, and processing was started by initiating a photocatalytic reaction. The processing time was 1 hour.

  The surface shape of the processed surface (Ga surface) was observed with a phase shift interference microscope, and the maximum height difference (Peak-valley; PV) was determined. Since PV represents the difference between the highest point and the lowest point in the plane, when the reaction time is constant, the degree of reduction of the highest point is known, and can be used as an index for comparing the processing speed.

  As a result, it was confirmed that the surface 14 having an in-plane (700 μm × 500 μm) PV of 18.90 nm before processing had a PV of 4.375 nm after processing. When the pH of the reaction treatment liquid 5 is higher than 4.88, PV is about 8 to 16 nm, and it can be seen that the processing speed is improved about 2 to 7 times by setting the pH to 4.88 or less. The processing speed in this embodiment is 14.6 nm / h, but it can be controlled in the range of 2 to 150 nm / h depending on the rotation speed of the substrate and the surface plate, the applied pressure, and the pH of the reaction treatment solution 5.

  Thus, when the photocatalytic reaction type chemical processing method is performed, the processing speed of the surface of the GaN single crystal substrate can be improved by setting the pH of the reaction processing solution to 4.88 or less.

[Example 2]
Using the SiC substrate, the substrate surface was processed by the photocatalytic reaction type chemical processing method in the same manner as in Example 1. As the photocatalytic material, TiO ₂ produced by a high frequency magnetron sputtering method was used.

As shown in FIG. 1, a SiC substrate cleaned with a 25% HF aqueous solution as a workpiece 3 is affixed to a holder 4 with wax, and then a processed surface (surface 14) of the SiC substrate is made of a thin film supporting base made of quartz. 2 was brought into contact with the photocatalytic thin film 1 made of TiO ₂ deposited on the substrate ₂ . Here, the SiC substrate used in this example is a single crystal, and is a n-type 4H—SiC with a diameter of 50 mm and a (0001) Si plane inclined by 8 degrees in the [11-20] direction, and the resistivity is 0.017 Ωcm. It is. 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 this example, the Si surface was processed. When the C surface is processed, the processing speed is somewhat different, but the mechanism of the processing is the same as in the case of the Si surface. The processed surface (Si surface) of the SiC substrate used in this example is mechanically polished in advance.

After the SiC substrate is brought into contact with the TiO ₂ film, a sulfuric acid aqueous solution having a pH adjusted to 3.0 is dropped as a reaction treatment liquid 5 onto the rotating platen 11 at a flow rate of 5 ml per minute, and the rotating platen 11 and the holder 4 The rotating shafts 9 and 13 included in the are rotated. There are no particular restrictions on the number of rotations of the rotating platen 11 and the holder 4, but the rotating platen 11 was rotated at a speed of 5 rotations per minute in order to minimize scratches and damage to the SiC substrate and quartz. The holder 4 attached with the SiC substrate was also rotated at a speed of 10 revolutions per minute for the same reason.

Subsequently, ultraviolet rays (light 7) with an ultraviolet illuminance adjusted to 30 mW / cm ² were irradiated from the back side of the rotating platen 11 by the light irradiation means 8 comprising a high-pressure mercury lamp, and processing was started by initiating a photocatalytic reaction. The processing time was 1 hour.

  The surface shape of the processed surface (Si surface) was observed with an atomic force microscope (AFM), and the maximum height difference (PV) was obtained.

  As a result, it was confirmed that the surface 14 whose in-plane PV was 20.66 nm before processing had a PV of 4.696 nm after processing. Further, when the pH of the reaction treatment solution 5 is set to be higher than 4.88, the surface 14 having PV of 10.77 nm has a pH of 2.751 nm after processing by setting the pH to 4.88 or less. It was confirmed that From this, it can be seen that the processing speed was improved by about 2 times by setting the pH to 4.88 or less. The processing speed in this embodiment is 16.0 nm / h, but it can be controlled in the range of 2 to 150 nm / h depending on the rotation speed of the substrate and the surface plate, the applied pressure, and the pH of the reaction treatment liquid 5.

  Thus, the processing speed of the SiC single crystal substrate surface can be improved by setting the pH of the reaction processing solution to 4.88 or lower when carrying out the photocatalytic reaction type chemical processing method.

DESCRIPTION OF SYMBOLS 1 Photocatalyst thin film 2 Thin film support base material 3 Workpiece 4 Holder 5 Reaction processing liquid 5a Processing liquid layer 6 Processing liquid supply means 7 Light 8 Light irradiation means 9 Rotating shaft 10 Photocatalytic reaction type chemical processing apparatus 11 Rotating surface plate 12 Processing Tank 13 Rotating shaft 14 Surface 15 of workpiece 3 Compound layer

Claims (5)

The photocatalytic thin film is brought into contact with or very close to the surface of the work piece, and a reaction treatment liquid composed of an acidic aqueous solution is interposed therebetween, and the photocatalytic thin film is irradiated with light, and the surface of the work piece is formed using the active species generated by light irradiation. In the photocatalytic reaction type chemical processing method to be processed,
A photocatalytic reaction type chemical processing method, wherein the surface of a workpiece is processed under a condition such that the pH of the reaction treatment solution is 4.88 or less. The photocatalyst constituting the photocatalytic thin film is at least one metal oxide selected from the group consisting of TiO ₂ , KTaO ₃ , SrTiO ₃ , ZrO ₂ , NbO ₃ , ZnO, WO ₃ , and SnO _2. The photocatalytic reaction type chemical processing method described.   3. The photocatalytic reaction type chemical processing method according to claim 1, wherein the workpiece is made of any one of SiC, GaN, sapphire, ruby, and diamond.   The photocatalytic reaction type chemical processing method according to any one of claims 1 to 3, wherein the reaction treatment liquid is an acidic aqueous solution containing at least one of sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, and hydrogen peroxide. A photocatalytic thin film is formed on a flat surface, and has a rotating surface plate that rotates the photocatalytic thin film, and a rotation axis that is eccentric with respect to the rotation axis of the rotation surface plate. A holder capable of moving up and down to bring the workpiece into contact with or in close proximity to the surface; a treatment liquid supply means for supplying a reaction treatment liquid comprising an acidic aqueous solution between the photocatalytic thin film and the surface of the workpiece; In the photocatalytic reaction type chemical processing apparatus comprising a light irradiation means for irradiating the photocatalytic thin film with light, and planarizing the surface of the workpiece,
The photocatalytic reaction type chemical processing apparatus, wherein the treatment liquid supply means supplies the reaction treatment liquid having a pH of 4.88 or less.