JP2017028162A - Single crystal SiC substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a single crystal SiC substrate whose front surface is a mirror surface and rear surface is a surface with a large degree of roughness with a small warpage owing to a Twyman's effect generated by a difference in residual stress of the front surface and the rear surface.SOLUTION: The single crystal SiC substrate has a small circular plate shape and a thickness of less than 1 mm. One surface is a mirror surface. The other surface has Ra of greater than 1 nm. The SORI is less than 30 μm.SELECTED DRAWING: None

Description

  The present invention relates to a single crystal SiC substrate.

  SiC (silicon carbide), which is a semiconductor material, has a wider band gap than Si (silicon), which is currently widely used as a substrate for devices. Therefore, power devices, high-frequency devices, and high temperatures are obtained using single-crystal SiC substrates. Research is underway to produce operating devices.

  A single crystal SiC substrate is formed by, for example, cutting a substrate (parts to become) from a single crystal SiC ingot manufactured using a sublimation method, and mirror-treating the surface of the cut substrate (for example, Patent Documents). 1).

  Since the cut substrate has warpage, undulation, and processing distortion, the surface of the substrate is subjected to CMP (Chemical Mechanical Polishing) after reducing it by, for example, grinding using diamond abrasive grains. It has been done to mirror. Since the processing speed of CMP is low, it is anxious to make the depth of the work-affected layer as small as possible before performing CMP (see, for example, Non-Patent Document 1).

  In a single crystal substrate such as SiC, in the process of removing a work-affected layer, a phenomenon occurs in which the substrate warps due to the Twiman effect based on the residual stress balance between the front surface and the back surface (see, for example, Non-Patent Document 2). Since the substrate is advanced to the next process in a mirror-finished state by CMP, the shape at this time (e.g., evaluated by SORI which is a parameter indicating the degree of warpage of the substrate) is an object of interest.

  By the way, in Patent Document 2, by performing electrolytic etching while being immersed in an aqueous solution containing hydrofluoric acid, at least the work-affected layer remaining on the non-specular surface while maintaining the surface roughness of the non-specular surface (back surface) is maintained. There has been proposed a technique for eliminating the warp of the substrate due to the Twiman effect by removing a part. In this technique, since a deeply affected layer usually remains on a non-mirror surface having a large surface roughness, a difference in residual stress occurs between the mirror-finished surface and the substrate warps. The purpose of this is to solve the problem of the occurrence of problems.

According to Patent Document 2, when electrolysis is performed using a SiC work as a positive electrode and platinum as a negative electrode, reactions shown by the following formulas (1) and (2) occur. Here, when hydrofluoric acid is added to the electrolytic solution, hydrofluoric acid acts on the silicon dioxide SiO ₂ generated by the reaction of the formulas (1) and (2), and this is used as the electrolytic solution. Etching progresses by dissolving and removing the solution.
Positive electrode: SiC + 4H ₂ O → SiO ₂ + CO ₂ + 8H ⁺ + 8e ⁻ (1)
Negative electrode: 8H ⁺ + 8e ⁻ → 4H ₂ (2)

Japanese Patent No. 4499698 Japanese Patent No. 5560774

Takanori Kido, Kazutoshi Hotta, Kenji Kawata, Masatake Nagaya, Hiroto Maeda, Yoshihiro Deguchi, Shogo Matsuda, Atsunori Takeda, Ryuichi Takanabe, Tomohiro Nakayama, Tomohisa Kato: 21st Lecture Meeting on SiC and Related Wide Gap Semiconductors , P.M. 72-73 Masatake Nagaya, Takanori Kido, Tomohiro Nakayama, Kenji Kawada, Tomohisa Kato: Proceedings of the 1st Lecture of Advanced Power Semiconductor Subcommittee 86-87

  The front surface of the single crystal SiC substrate needs to be a mirror surface for epitaxial growth in the next step, but the back surface is preferably a surface having a large surface roughness from various viewpoints such as handling. However, the surface with a large surface roughness is usually deeply affected by processing, and the difference in residual stress between the front and back surfaces causes the substrate to warp due to the Twiman effect. there were. Therefore, the processing state of both sides is different, and it is difficult to reduce the warp with a single crystal SiC substrate where one side is a mirror surface and the back surface is not a mirror surface, especially when the substrate diameter is large or the substrate is greatly affected by the Twiman effect. When it is thin, it is difficult to obtain a substrate with small warpage.

  If the substrate has a mirror surface and the back surface has a large surface roughness, it can be put into practical use if the warping of the substrate due to the Twiman effect can be reduced in a way that reduces the burden on safety and waste disposal. Dissemination can be greatly accelerated.

  As a conventional technique, Patent Document 2 discloses a technique for controlling the shape of a single crystal SiC substrate by the Twiman effect while removing the work-affected layer of the substrate and maintaining the surface roughness of one side of the substrate in a large state. Yes.

  Patent Document 2 discloses a technique for controlling the shape of a single crystal SiC substrate by the Twiman effect. However, since electrolytic etching using hydrofluoric acid that is difficult to handle is performed, it is safer than when CMP is performed. It is necessary to pay careful attention to waste disposal, it is difficult to uniformly remove the work-affected layer with the disclosed substrate and cathode arrangement, and it can only be applied to n-type substrates. This is a major obstacle to the spread.

  The present invention has been made in view of such circumstances, and uses a hydrofluoric acid that is a single crystal SiC substrate having a mirror surface on the front surface and a large surface roughness on the back surface, which is difficult to handle. A single-crystal SiC substrate that can be obtained by reducing the warping of the substrate due to the Twiman effect in a method that places little burden on safety and waste disposal, regardless of whether the substrate is an n-type substrate or a p-type substrate. It is a first object to provide

  The second object of the present invention is to obtain a single-crystal SiC substrate having a small warpage in a state in which the processing state on both sides is different, one side is a mirror surface, and Ra on the other side is as large as 1 nm or more.

  The present inventor considered that it would be effective to remove the work-affected layer that contributes to the Twiman effect in order to solve the above problems, and was interested in the technique disclosed in Patent Document 2. However, this technique has been found to have some major drawbacks. The biggest drawback is the need for hydrofluoric acid to remove the silicon dioxide that forms.

  In Patent Document 2, there is a description that the environmental load is small because chemicals that are difficult to handle are not used, but as is well known, hydrofluoric acid has the property of violently corroding the body when touched. It is a representative of chemicals that you want to avoid using. The difficulty of handling this chemical is detailed in, for example, a product safety data sheet created by Morita Chemical Co., Ltd.

  Moreover, in the electrolytic etching conditions disclosed in the only example described in Patent Document 2, 3% hydrofluoric acid is used. However, when this is to be discarded, the product safety data sheet includes As a precaution, when using this product, care must be taken to prevent environmental pollution. Neutralization with a slaked lime slurry solution (precaution for exotherm), the supernatant is pH 5.8 to 8.6, F: Drain as regulated value or less (Regulated value of F: 8 mg / L for public water areas other than sea areas, 15 mg / L for sea areas) (However, if there are restrictions imposed by local regulations, follow the values) As it is done, the burden on the environment is large, so it takes time-consuming disposal.

  Further, in the positional relationship between the SiC work and the negative electrode made of platinum disclosed in the drawing of Patent Document 2, the distance from the negative electrode differs from position to position on the surface of the SiC work subjected to electrolytic etching. In this case, when electrolytic etching is performed, the magnitude of the applied electric field differs depending on the position of the surface to be electrolytically etched, so it is estimated that it is difficult to perform uniform electrolytic etching within the surface.

  Further, in Patent Document 2, the n-type silicon carbide single crystal substrate processed into a mirror surface cannot be etched in-plane uniformly, and a work-affected layer is introduced as a non-mirror surface having a surface roughness Ra of greater than 3 nm. Thus, it is disclosed that etching can be performed uniformly. That is, in Patent Document 2, a depth of a work-affected layer corresponding to a case where Ra is 3 nm or more, that is, a machining allowance in CMP is assumed to be several microns or more. When the work-affected layer depth is submicron or subhalf micron, it is estimated that the technique disclosed in Patent Document 2 cannot be applied.

  It is unknown whether the above-described electrolytic etching can be performed on the p-type silicon carbide single crystal substrate in the same manner as the n-type silicon carbide single crystal substrate. In the case of single crystal SiC semiconductors, the current mainstream is n-type, so it can be said that the impact is somewhat minor compared to other defects, but there is a technology that can be applied to p-type in the future as well. preferable.

The present inventor has intensively studied together with the knowledge obtained in the efforts related to the processing of the single crystal SiC substrate so far, and the so-called anodic oxidation represented by the chemical reaction formula (1) described above is a process alteration of the single crystal SiC substrate. Judging from the fact that it is effective in removing residual stress caused by the layer, it was first conceived that the aqueous electrolyte solution does not contain a difficult-to-handle chemical such as hydrofluoric acid. When a component that dissolves silicon dioxide is not included, the reaction that occurs when a voltage is applied does not correspond to electrolytic etching, and corresponds to a reaction called anodization although it is a superordinate concept. If this reaction is expressed more accurately, it is anodization without dissolution of the product. In this case, since the product is not dissolved and removed, it remains on the surface of the single crystal SiC substrate. It is derived from hydrogen bonds or van der Waals forces on the anodized surface of a single crystal SiC substrate as an amorphous fragile amorphous composition represented by composition formula SiO ₂ · mSiC · nH ₂ O As a macroscopic shape, it remains in a film-like form.

  Since the conventional technique is electrolytic etching using hydrofluoric acid, whether the residual stress due to the work-affected layer of the single-crystal SiC substrate is effective when the product of anodic oxidation remains in this way is It is completely unknown and does not give any guidance to the present invention. For the first time in the present invention, it is possible to effectively remove the residual stress caused by the work-affected layer of the single crystal SiC substrate by anodic oxidation without dissolution of the product. The present inventor has further studied diligently, and came up with a method of applying an electric field uniformly between the surface to be anodized and the cathode, and if the single crystal SiC substrate has conductivity, the mirror surface of the n-type substrate The inventors came up with a technique that can uniformly anodize the processed surface and the p-type substrate without any problem, and completed the present invention.

The present invention provides the following means.
[1] A disk-shaped single crystal SiC substrate having a thickness of less than 1 mm, one surface being a mirror surface, Ra on the other surface being a value greater than 1 nm, and SORI being a value smaller than 30 μm A single crystal SiC substrate, characterized in that
[2] The single crystal SiC substrate according to [1], which has an orientation flat or a notch serving as an index of crystal orientation.
Even in the case of manufacturing a single crystal SiC substrate on which these indexes are formed, there is no trouble in forming the indexes in the process, and the manufactured single crystal SiC substrate has the indexes formed. Therefore, there is an advantage that the direction during the subsequent processing can be correctly controlled.
[3] The single crystal SiC substrate according to any one of [1] or [2], wherein the diameter is a value larger than 70 mm.
[4] The single crystal SiC substrate according to any one of [1] to [3], wherein the thickness is a value smaller than 0.6 mm.
[5] The single crystal SiC substrate according to any one of [1] to [4], wherein the thickness is less than 0.4 mm.
[6] The single crystal SiC substrate according to any one of [1] to [5], wherein Ra is a value larger than 10 nm.

  According to the present invention, the front surface is a mirror surface and the back surface is a single crystal SiC substrate having a large surface roughness, and without using hydrofluoric acid that is difficult to handle, safety and waste treatment can be achieved. A single crystal SiC substrate in which the warping of the substrate due to the Twiman effect is reduced can be obtained by a method with a small burden on consideration.

  In addition, according to the present invention, it is possible to uniformly remove the work-affected layer over the entire surface without careful consideration for safety and waste disposal, regardless of whether the substrate is an n-type substrate or a p-type substrate. A single crystal SiC substrate can be obtained.

  In addition, according to the present invention, it is possible to obtain a single-crystal SiC substrate having a small warpage in a state in which both sides are processed differently, one side is a mirror surface, and Ra on the other side is as large as 1 nm or more.

(A) It is a figure which shows the shape of the board | substrate in Example 1 of this invention. (B) It is a figure which shows the shape of the board | substrate in the comparative example 1 of this invention. (A) It is a figure which shows the shape of the board | substrate in Example 2 of this invention. (B) It is a figure which shows the shape of the board | substrate in the comparative example 2 of this invention. (A), (b) It is an optical microscope photograph of the board | substrate in Example 3 of this invention.

  Hereinafter, a single crystal SiC substrate which is an embodiment to which the present invention is applied will be described in detail.

  The single-crystal SiC substrate of the present invention is a conductive single-crystal SiC substrate in which at least one surface is mechanically flattened to form a work-affected layer, and the single-crystal SiC substrate is added to the aqueous electrolyte solution contained in a container. Only the machined surface of the SiC substrate is in contact with the liquid, and the single crystal SiC substrate serves as an anode and a cathode is disposed so that the distance is constant, and then a DC voltage is applied between the anode and the cathode. This can be realized by a method for controlling the shape of a single crystal SiC substrate, which is characterized in that the wetted surface is anodized to remove the work-affected layer.

  Preferably, the mechanical processing can be realized by a method for controlling the shape of a single crystal SiC substrate, which includes processing using diamond abrasive grains.

  More preferably, the mechanical processing can be realized by a method for controlling the shape of a single crystal SiC substrate, characterized in that it includes at least one of lapping, polishing, and grinding.

  The electric field serving as the driving force in the anodic oxidation needs to be a uniform electric field that acts with a uniform strength in the plane in order to uniformly anodize one side of the single crystal SiC substrate. As applied to the parallel plate capacitor, the anode and the cathode are arranged in parallel. If the area of the plate is sufficiently large with respect to the distance between the two electrodes, the electric field between the two electrodes becomes a uniform equal electric field. In the present invention, a configuration in which a uniform equal electric field is obtained when a voltage is applied is adopted. On the other hand, the method is not particularly limited as long as a uniform equal electric field can be obtained when a voltage is applied.

  The electrolyte aqueous solution used in the anodic oxidation may be any water as long as the electrolyte is dissolved as defined. Also, the amount of electrolyte contributing to the present invention can be grasped by measuring conductivity. The greater the conductivity, the smaller the specific resistance and the easier the current flows. General water (water obtained from waterworks) usually has a conductivity of 100 to 300 microsiemens / cm (10 to 30 millisiemens / meter), which is also an aqueous electrolyte solution.

  The conductivity of the aqueous electrolyte solution is 1 millisiemens / meter (10 microsiemens / centimeter) or more and 10 siemens / meter (100 millisiemens / centimeter) or less. If it is less than 1 milliSiemens / meter (10 microsiemens / centimeter), the current required for the anodic oxidation of the present invention may not be obtained, and it exceeds 10 siemens / meter (100 millisiemens / centimeter). This is because there is a high possibility that the single crystal SiC substrate is broken by stress due to a rapid reaction corresponding to a large current.

  The conductivity of the aqueous electrolyte solution is preferably 1 millisiemens / meter (10 microsiemens / centimeter) or more and 1 siemens / meter (10 millisiemens / centimeter) or less. This is because if it exceeds 1 siemens / meter (10 millisiemens / centimeter), the single crystal SiC substrate may be broken by stress due to a rapid reaction corresponding to a large current.

  The conductivity of the aqueous electrolyte solution is more preferably 1 millisiemens / meter (10 microsiemens / centimeter) or more and 100 millisiemens / meter (1 millisiemens / centimeter) or less. This is because if it exceeds 100 millisiemens / meter (1 millisiemens / centimeter), there is a possibility that the single crystal SiC substrate may be broken by stress due to a rapid reaction corresponding to a large current.

  The pH of the aqueous electrolyte solution used in the present invention is 4 or more and 10 or less. If it is out of this range, touching the skin or the like is likely to be harmful and there is a problem in terms of safety. Moreover, it is preferable that pH of the electrolyte aqueous solution used by this invention is 5-9. This is because if it is out of this range, it may be harmful if it touches the skin or the like. In addition, the pH of the aqueous electrolyte solution used in the present invention is more preferably 5.8 or more and 8.6 or less. This is because pH adjustment is required at the time of drainage when it is out of this range.

  A method for controlling the shape of a single crystal SiC substrate and a method for manufacturing the same will be described using the anodization described above.

[Method for controlling shape of single-crystal SiC substrate]
The method for controlling the shape of a single-crystal SiC substrate according to the present invention is a method in which a part of SiC constituting a work-affected layer on at least one surface of a single-crystal SiC substrate is converted into SiO ₂ (modified) by anodizing And a step of removing the remaining portion (first step) and a step of measuring the shape of the single crystal SiC substrate after the anodizing treatment (second step).

(First step)
First, a single crystal SiC substrate to be anodized is prepared. Since at least one surface of the single crystal SiC substrate to be anodized is a stage prior to proceeding to the CMP process, a flattened one by mechanical processing is used.

  A single crystal SiC substrate (substrate) that performs mechanical processing usually has unevenness at the stage of being cut from an ingot, and is not flat, and further has a work-affected layer resulting from cutting. . The above-described mechanical processing is performed for the purpose of flattening the single crystal SiC substrate by eliminating the unevenness and reducing the depth of the work-affected layer.

  In the present invention, a flatly processed substrate means a substrate having a processed surface height variation excluding an area from the edge of the substrate up to 2 mm or a substrate thickness variation of 10 microns or less. When fixing with wax etc. and processing to flatten only one side by single-sided lapping or grinding, it is inappropriate to define a flatly processed substrate with thickness variation, so processing It shall be defined by the height variation of the processed surface in the received state. When processing for flattening the substrate is performed on both sides, it is defined by the thickness variation of the substrate.

  For example, as described in Non-Patent Document 2, the process before proceeding to the CMP process has been studied for high efficiency, high quality, and low cost by combining various processing processes. In Non-Patent Document 2, double-sided lapping using B4C abrasive grains is performed, but this is only an intermediate process, and the process is controlled to the extent that a single crystal SiC substrate can be machined and advanced to the CMP process. In order to obtain a deteriorated layer, it is necessary to perform processing using fine diamond abrasive grains. As processing using fine diamond abrasive grains, there are options of lapping, polishing, and grinding, and an optimum combination of processes is selected.

  Lapping, polishing, and grinding are names of widely used mechanical processing methods for workpieces, but their definitions are not universal. For example, polishing in the present invention is referred to as soft lapping, polishing, or softening. Sometimes called polishing. Therefore, the terms lapping, polishing, and grinding used in the present invention are defined as follows.

  Lapping is a mechanical processing method of a workpiece, also called lapping, and is roughly divided into single-sided wrapping and double-sided wrapping. Single-sided lapping places a workpiece on a metal table called a lapping platen, sandwiches abrasive grains between the lapping platen and the bottom surface of the workpiece, applies pressure from above to the workpiece, and slides the workpiece. This is a processing method performed by moving. Double-sided wrap is mainly intended for thin substrate-like workpieces, puts the workpieces into holes provided in a workpiece holding jig called a carrier thinner than the workpieces, and also has two sheets called lap surface plates. It is arranged so as to be sandwiched between plate-shaped metal bases, and further, abrasive grains are sandwiched between both surfaces of the two lap surface plates and the workpiece, pressure is applied to the workpiece held by the carrier, and revolution is performed by the gear mechanism. It is a processing method that is performed by sliding by giving a rotation motion.

  In both cases of single-sided wrapping and double-sided wrapping, there are wet laps in which processing liquid is added to the abrasive grains and dry wraps in which abrasive grains are embedded in the wrap surface plate and processing is performed without adding the processing liquid. . There are two types of wet lap working fluids, oil-based and water-based. In the case of processing a single crystal SiC substrate, a wet lap using hard abrasive grains such as diamond and a water-based or oil-based processing liquid is common, and both single-sided wrapping and double-sided wrapping are studied. It is being advanced.

  Polishing is a processing method in which a soft sheet-like tool called an abrasive cloth, a polisher, or a pad is attached to the above-described lapping surface plate, and processing is performed in the same manner as the above-described wet lapping. In the case of processing a single crystal SiC substrate, it is a processing method mainly used as a finishing process in mechanical processing using an aqueous processing liquid in which fine diamond abrasive grains are dispersed. After all, single-sided polish and double-sided polish are being studied.

  Grinding is a mechanical processing method that removes the surface of a workpiece by bringing a grindstone that rotates at high speed into contact with a workpiece that is fixed to a stage called a stage or a table. When the method of fixing the workpiece is vacuum suction using a vacuum chuck, the platform for fixing the workpiece may be simply referred to as a vacuum chuck or a chuck. The workpiece fixed to the table is usually given a rotational motion or a reciprocating motion according to the shape of the workpiece so that the machining surface is uniformly flattened. Since the abrasive grains are fixed to the grindstone and generate a large amount of heat at the processing point, processing is usually performed while cooling by supplying an aqueous coolant called a coolant. In the case of processing a single crystal SiC substrate, a grindstone formed by binding and fixing diamond abrasive grains with a resin or metal called a binder or a bond material or a glassy material is used.

  By these mechanical processing, a flat surface is obtained, and the surface roughness mainly depends on the grain size of the abrasive grains used in the processing. That is, the surface roughness increases when coarse abrasive grains are used, and the surface roughness decreases when fine abrasive grains are used.

  In the present invention, the work-affected layer is an altered region mainly composed of crystal strain remaining on the workpiece surface as a result of mechanical processing, and when the workpiece is a single crystal flat workpiece, This means something that affects the shape of the workpiece. In Patent Document 2, if a difference occurs in the processing strain remaining on the front surface and the back surface of the single crystal SiC substrate, the residual stress also differs, and the substrate warps to compensate for the difference. Occurs, which is commonly referred to as the Twiman effect.

  The presence of the work-affected layer can be confirmed, for example, by preparing the sample for cross-sectional observation and observing the cross-section with a transmission electron microscope. However, even if there is a difference in the work-affected layer on the front surface and the back surface, the Twiman effect does not occur if it is sufficiently small. The object of the present invention is not to remove all of the work-affected layer by mechanical processing, but to remove the work-affected layer that contributes to the Twiman effect. This is because one of the two substrates having the same history applies the present invention to the front surface and the back surface, and the other removes all processing distortions of the front surface and the back surface by, for example, CMP, and shapes the both. By comparing, it can be confirmed whether or not the work-affected layer contributing to the Twiman effect has been removed. Alternatively, for example, a substrate is prepared by removing all processing strains on the front and back surfaces by CMP, and the shape is measured. For example, the back surface is ground to introduce a work-affected layer, and the shape at this stage is changed. It can also be confirmed by measuring the Twiman effect and applying the present invention to the back surface and comparing the shape of each stage.

  The relationship between the work-affected layer and the surface roughness is not necessarily one-to-one, but the processed surface that has a large amount of work-affected layer, that is, requires a large machining allowance in CMP, tends to be rough. . As one criterion, when the surface roughness Ra of the surface obtained by mechanical processing exceeds 10 nm, the depth of the work-affected layer is expected to be several microns or more. It is not practical to proceed to a small CMP process. Therefore, for the purpose of the present invention, the value of the surface roughness Ra on one or both sides of the single crystal SiC substrate needs to be 10 nm or less, and 5 nm or less, which is expected to have a smaller work-affected layer depth. Is preferably 3 nm or less, and most preferably 2 nm or less.

  The single crystal SiC substrate 5 only needs to have conductivity, and may be either an n-type single crystal SiC substrate or a p-type single crystal SiC substrate.

  Next, one main surface (one surface) of the prepared single crystal SiC substrate is brought into contact with the aqueous electrolyte solution accommodated in the container, and the surface of the single crystal SiC substrate that is not in contact with the liquid surface (a surface other than the liquid contact surface). And the cathode are electrically connected with a direct current power supply interposed therebetween. Then, a voltage is applied between the single crystal SiC substrate and the cathode by using a DC power supply device, and anodization treatment is uniformly performed over one surface of the single crystal SiC substrate throughout the surface. When there is a work-affected layer on both main surfaces of the single crystal SiC substrate, the anodic oxidation treatment is performed uniformly over each surface, one surface at a time. There is no restriction | limiting regarding the temperature of an anodizing process, and it can carry out at room temperature.

  When both surfaces of the single crystal SiC substrate are processed mechanically flat, one surface is a mirror surface without a work-affected layer, and the other surface is a surface having a work-affected layer, the other surface Anodizing treatment is performed only on the surface. It is assumed that the surface roughness Ra of the other surface before the anodizing treatment is 0.7 nm or more.

At least a part of the work-affected layer is converted (modified) into SiO ₂ by anodic oxidation, and this, unreacted SiC, and H ₂ O contained in the aqueous solution are mixed in an amorphous state. However, it originates from hydrogen bonding or van der Waals forces and adheres to the bulk substrate, and the remainder is removed. The inventor has confirmed that the product adhering to the bulk substrate does not have a compressive stress that causes warping. The work-affected layer is anodized to become the above product, so that it is not in a crystalline state continuous with the bulk substrate, so it is considered that the warpage due to volume expansion has been eliminated.

  In the anodic oxidation treatment, the voltage applied between the single crystal SiC substrate and the cathode is preferably 5 V or more and 30 V or less. This is because when the applied voltage is less than 5 V, the current required for the anodic oxidation of the present invention may not be obtained. When the applied voltage exceeds 30 V, the stress due to rapid reaction corresponding to the large current is This is because the crystal SiC substrate is likely to break.

  The applied voltage is more preferably 8 V or more and 24 V or less. If the applied voltage is less than 8V, the anodization may take too much time, and if the applied voltage exceeds 24V, the single crystal SiC substrate may be broken by stress due to a rapid reaction corresponding to a large current. Because there is.

  The applied voltage is most preferably 12 V or more and 20 V or less. When the applied voltage is less than 12V, it may take time for anodization. When the applied voltage exceeds 20V, there is a possibility that the single crystal SiC substrate may be broken due to stress due to a rapid reaction corresponding to a large current. Because.

  The distance between the liquid contact surface of the single crystal SiC substrate and the cathode during the anodizing treatment is preferably 1 mm or more and 100 mm or less. If the thickness is less than 1 mm, there is a high possibility that the single crystal SiC substrate 5 is cracked by stress due to a rapid reaction corresponding to a large current. If the thickness exceeds 100 mm, the current necessary for the anodic oxidation of the present invention is obtained. This is because it may disappear.

  More preferably, the distance between the liquid contact surface of the single crystal SiC substrate and the cathode is 3 mm or more and 50 mm or less. This is because if the thickness is less than 3 mm, the single crystal SiC substrate 5 may be broken due to stress due to a rapid reaction corresponding to a large current, and if it exceeds 50 mm, it may take too much time for anodization. .

  The distance between the liquid contact surface 6 of the single crystal SiC substrate and the cathode is most preferably 5 mm or more and 20 mm or less. This is because if the thickness is less than 5 mm, there is a possibility that the single crystal SiC substrate 5 is broken due to stress due to a rapid reaction corresponding to a large current, and if it exceeds 20 mm, it may take a long time for anodization.

(Second step)
Subsequently, the shape of the single crystal SiC substrate after the anodizing treatment (for example, SORI which is a parameter indicating the degree of warpage of the substrate) is measured. The substrate to be measured is not subjected to CMP, and a part of the work-affected layer that has been modified remains on the surface. However, as described above, the residual stress due to this work-affected layer has been removed. The warpage is equivalent to the case of performing. Therefore, it is possible to realize a warped state of the substrate when CMP processing is performed from the measurement result here. That is, the shape of the single crystal SiC substrate after CMP can be evaluated without performing CMP.

  As described above, according to the present invention, processing alteration that affects the shape of a single crystal SiC substrate without performing CMP, which is a process with a large load from any viewpoint, such as cost, time, labor, and environment. Residual stress due to the layer can be effectively removed. Therefore, the shape of the single crystal SiC substrate can be controlled simply, quickly, accurately, inexpensively and safely so as to have a shape equivalent to that obtained when CMP is performed.

  As a result, a single crystal SiC substrate having a mirror surface on the front surface and a large surface roughness on the back surface does not use hydrofluoric acid which is difficult to handle, and the burden of consideration for safety and waste disposal is reduced. With a small method, it is possible to reduce the warping of the substrate due to the Twiman effect.

  In addition, the effect of the present invention can be obtained regardless of whether the single crystal SiC substrate is an n-type substrate or a p-type substrate. Therefore, it is possible to remove the work-affected layer uniformly over the surface without giving careful consideration to safety and waste disposal in both the n-type single crystal SiC substrate and the p-type single crystal SiC substrate. It becomes.

[Method for producing single-crystal SiC substrate]
The method for producing a single crystal SiC substrate of the present invention includes a step of performing the anodic oxidation treatment in the first step described above and removing a product (impurities) generated therewith. The removal of the product can be performed using, for example, hydrofluoric acid. The product removal using hydrofluoric acid corresponds to a cleaning process performed by immersing the substrate in hydrofluoric acid, and unlike electrolytic etching, there is no safety problem.

  As a result, it is possible to obtain a single crystal SiC substrate in which the warping of the substrate due to the Twiman effect is reduced and the film formation process can be performed in the next step.

  Hereinafter, the effects of the present invention will be made clearer by examples. In addition, this invention is not limited to a following example, In the range which does not change the summary, it can change suitably and can implement.

Example 1
A commercially available 100 mm diameter n-type (0001) 4H—SiC (4 ° off) single crystal substrate in an as-sliced state was ground by attaching a diamond wheel to a grinding device HRG300 manufactured by Tokyo Seimitsu Co., Ltd. As a final finish, use a # 8000 diamond wheel on both the Si and C surfaces, remove about 10 μm under grinding conditions with a wheel rotation speed of 1250 rpm, a workpiece rotation speed of 300 rpm, and a wheel feed speed of 0.4 μm / s. As a result, the mirror surface was finished.

  The value of the surface roughness Ra was about 0.8 nm for both the Si surface and the C surface when measured using a non-contact surface shape measuring instrument NewView 7300 manufactured by zygo. When the Si surface was measured using a flat master MSP150 DL, a flatness / parallelism / height measuring machine manufactured by CorningTropel, the thickness was 351.1 μm and the SORI was 95.0 μm.

  The present invention uses a DC stabilized power supply AD-8723D with a laboratory water of pH 7.7, conductivity 25.7 millisiemens / meter (0.257 millisiemens / centimeter) and temperature 25 ° C. as an electrolyte aqueous solution. Wiring for anodic oxidation was performed.

  The C surface of the substrate was gently brought into contact with the aqueous electrolyte solution, and a voltage was applied at 15 V for 1 minute. The current value gradually decreased from about 0.2 A and became about 0.15 A after 1 minute.

The anodized C-face is covered with an amorphous brittle amorphous composition represented by SiO ₂ · mSiC · nH ₂ O as a product composition formula, and interference color due to light is observed. It was.

  Next, with the same setup, the Si surface was anodized to clean the substrate. When the shape of the substrate was measured on the Si surface in this state, the value of SORI was 83.3 μm.

  The anodic oxidation test was completed in 30 minutes including preparation and cleaning of the substrate shape. The water in the container after the anodic oxidation was in a state where it could be drained as it was because there was no change in pH and there was no problem with the transparency. The cost of the anodic oxidation test itself was less than 20,000 yen for the procurement of DC stabilized power supplies and containers.

(Comparative Example 1)
In the same manner as in the first example, a # 8000 diamond wheel is used for both the Si surface and the C surface in the same manner as in the first example, and the wheel rotation speed is set so that the single crystal SiC substrate used in the first example is used. Under a grinding condition of 1250 rpm, a workpiece rotation speed of 300 rpm, and a wheel feed speed of 0.4 μm / s, approximately 10 microns were removed to finish the mirror surface. The single crystal SiC substrate used may be considered to have the same warpage after CMP if the processing lot and processing conditions are the same.

  The thickness of the Si surface measured was 351.9 μm, and the value of SORI was 103.2 μm. A CMP apparatus RDP-500T manufactured by Fujikoshi Machine Industry Co., Ltd. was used, and a CMP process was carried out using a colloidal silica-based slurry to remove the C surface and the Si surface in order of 0.5 microns. The numerical value of 0.5 micron here corresponds to the machining allowance in which it has been confirmed that a surface without a work-affected layer can be obtained. When the shape of the substrate was measured on the Si surface, the value of SORI was 82.2 μm.

  FIGS. 1A and 1B show measurement data representing the shape of the substrate. Fig.1 (a) is a perspective view of the board | substrate by Example 1 accompanied by the anodizing process. FIG.1 (b) is a perspective view of the board | substrate by the comparative example 1 accompanied by CMP process. 1A and 1B shows the distribution of unevenness in the thickness direction with respect to the reference plane.

  In FIG. 1 (a), the position of the surface relative to the reference plane is 42.314 μm, 30.737 μm, 30.959 μm, 23.388 μm, 15.214 μm, 8.245 μm, 0.513 μm, −6.898 μm, −14. Focusing on the cases of .459 μm, −22.441 μm, −29.612 μm, −37.184 μm, and −40.969 μm, the degree of color shading is shown separately for each of these positions.

  In FIG. 1B, the positions of the surface with respect to the reference surface are 39.296 μm, 29.652 μm, 28.090 μm, 20.618 μm, 18.140 μm, 5.674 μm, −1.748 μm, −9.274 μm, − Attention is paid to the cases of 16.743 μm, −24.116 μm, −31.687 μm, −39.159 μm, and −42.895 μm, and the degree of color shading is shown separately for each of these positions.

  Each of the substrates of Example 1 and Comparative Example 1 has a shape in which the central portion is raised with respect to the outer peripheral portion. From the comparison of the numerical value of SORI and the distribution of unevenness, it can be seen that the substrate of Example 1 and the substrate of Comparative Example 1 have almost the same shape with respect to deviation from a perfect plane due to warpage, surface unevenness and the like. From these results, it is understood that the warped state after the CMP process can be realized even by anodic oxidation without dissolving the silicon oxide.

  Both of the two substrates are obtained in an as-sliced state and are the same ingot having a common substrate ID, but since both have almost the same shape, it can be inferred that they are the same cutting lot.

  The CMP process took 8 hours from preparation to cleaning up and evaluation of the shape of the substrate. Since the CMP process wastewater discharged from the CMP apparatus cannot be drained as it is, it was collected in a plastic tank. Further, not only is the CMP apparatus expensive, but also consumables such as slurry and pads are expensive, so both initial cost and running cost are expensive.

(Example 2)
A commercially available 100 mm diameter n-type (0001) 4H-SiC (4 ° off) single crystal substrate with both mirror surfaces processed, using # 8000 diamond wheel, wheel rotation speed 1250 rpm, workpiece rotation speed 300 rpm, wheel feed speed Was removed by about 10 microns each in the order of the C surface and the Si surface under the grinding conditions of 0.4 μm / s. The thickness measured on the Si surface of the substrate was 333.8 μm, and the value of SORI was 13.6 μm. When this substrate was anodized in the same manner as in the first example, the SORI value measured on the Si surface of the substrate was 9.2 μm.

(Comparative Example 2)
A substrate having a common substrate ID with the single crystal SiC substrate used in the second embodiment is used in the same manner as in the second embodiment, using a # 8000 diamond wheel, with a wheel rotation speed of 1250 rpm and a workpiece rotation speed of 300 rpm. Each of the C surface and the Si surface was removed in the order of 10 μm under a grinding condition of a wheel feed rate of 0.4 μ / s. The thickness measured on the Si surface of the substrate was 334.7 μm, and the value of SORI was 14.4 μm. When this substrate was processed by CMP in the same manner as in the first comparative example, the value of SORI measured on the Si surface was 9.3 μm.

  FIGS. 2A and 2B show measurement data representing the shape of the substrate. FIG. 2A is a perspective view of a substrate according to Example 2 accompanied by anodization. FIG. 2B is a perspective view of a substrate according to Comparative Example 2 accompanied by a CMP process. 2A and 2B show the distribution of unevenness in the thickness direction with respect to the reference plane.

  In FIG. 2A, the positions of the surface with respect to the reference plane are 42.314 μm, 30.737 μm, 30.959 μm, 23.388 μm, 15.214 μm, 8.245 μm, 0.513 μm, −6.898 μm, −14. Focusing on the cases of .459 μm, −22.441 μm, −29.612 μm, −37.184 μm, and −40.969 μm, the degree of color shading is shown separately for each of these positions.

  In FIG. 2B, the positions of the surface with respect to the reference plane are 39.296 μm, 29.652 μm, 28.090 μm, 20.618 μm, 18.140 μm, 5.674 μm, −1.748 μm, −9.274 μm, − Attention is paid to the cases of 16.743 μm, −24.116 μm, −31.687 μm, −39.159 μm, and −42.895 μm, and the degree of color shading is shown separately for each of these positions.

  Each of the substrates of Example 2 and Comparative Example 2 has a shape in which two end portions facing the center portion are warped up. From the comparison of the numerical values of SORI and the distribution of unevenness, it can be seen that the substrate of Example 2 and the substrate of Comparative Example 2 have almost the same shape with respect to deviation from a perfect plane due to warpage, surface unevenness, and the like. From these results, it is understood that the warped state after the CMP process can be realized even by anodic oxidation without dissolving the silicon oxide.

(Example 3)
By preparing a small piece of p-type (0001) 4H-SiC (4 ° off) single crystal substrate doped with Al and masking part of the liquid contact surface (Si surface, C surface) with the aqueous electrolyte solution with resin A test for confirming whether or not anodic oxidation occurs was performed in the same manner as in Example 1.

  FIGS. 3A and 3B show the results of observing the Si surface and the C surface of the substrate after the anodizing treatment with an optical microscope, respectively. On both the Si surface and the C surface, interference colors are observed in the portions that were not masked, and unclear processing marks are evident in the masked portions. This result means that in the portion where the work-affected layer remains on the substrate and is not masked, that portion is preferentially anodized. From this result, it was confirmed that even a p-type substrate can be anodized without problems.

  As in the case of the n-type SiC substrate shown in Example 1, even in the case of a p-type SiC substrate, it is sufficient to add 4 V for 1 minute with water as an aqueous electrolyte solution. Since it is clear that no current can be obtained, sufficient anodization cannot be performed. In addition, when deionized water having an electrical conductivity of 200 microsiemens / meter (2 microsiemens / centimeter) is used as an electrolyte aqueous solution and a voltage of 15 V for 1 minute is applied, a sufficient current cannot be obtained. Sufficient anodization cannot be performed.

Example 4
The C-plane of a 100 mm diameter n-type (0001) 4H—SiC (4 ° off) single crystal substrate that is both mirror-finished on the market is used a # 4000 diamond wheel, the wheel rotation speed is 1250 rpm, the workpiece rotation speed is 300 rpm, About 8 μm was removed in the thickness direction of the substrate under grinding conditions with a wheel feed rate of 1 μm / s. Further, using a # 8000 diamond wheel, about 8 μm was removed in the thickness direction of the substrate under grinding conditions in which the wheel rotation speed was 1250 rpm, the workpiece rotation speed was 300 rpm, and the wheel feed speed was 0.4 μm / s.

  The thickness measured on the Si surface of the substrate before grinding was 355.7 μm and the value of SORI was 11.5 μm, but the thickness measured on the Si surface of the substrate after grinding was 339.4 μm. The value of was 53.5 μm. The shape of the substrate was a bowl shape recessed on the Si surface side, and when the C surface was ground, a shape change due to the Twiman effect due to the introduced work-affected layer was observed.

  The surface roughness Ra of the substrate Si surface before grinding was 0.355 nm, and the value of the surface roughness Ra of C surface was 0.433 nm. The value of the surface roughness Ra increased to 0.945 nm. When the C surface of the substrate was anodized in the same manner as in the first example, the SORI value measured on the Si surface of the substrate was 11.0 μm, which was reduced to the value before grinding.

  Further, the value of the surface roughness Ra of the C-plane of the substrate was increased to 8.2 nm. The anodized substrate was immersed in a 50% aqueous hydrofluoric acid solution at room temperature for 1 minute to remove the product by anodization. As a result, the SORI value measured on the Si surface of the substrate was 11.9 μm, which was almost the same as that after anodic oxidation. Further, the value of the surface roughness Ra of the C-plane of the substrate was 2.4 nm, which was larger than the value after grinding and before anodic oxidation. In this example, a substrate having a small SORI value without the Twiman effect was obtained in which the Si surface was a mirror surface, the C surface was a surface having a large surface roughness.

(Fifth embodiment)
Commercially available both mirror-finished 100mm diameter n-type (0001) 4H-SiC (4 ° off) single crystal substrate C surface using # 4000 diamond wheel, wheel rotation speed 1250rpm, workpiece rotation speed 300rpm, wheel About 14 μm was removed under grinding conditions with a feed rate of 1 μm / s.

  The thickness measured on the Si surface of the substrate before grinding was 355.2 μm and the value of SORI was 12.7 μm, but the thickness measured on the substrate Si surface after grinding was 341.2 μm. The value of was 149.8 μm. The shape of the substrate was a bowl shape recessed on the Si surface side, and a shape change due to the Twiman effect due to the work-affected layer introduced by grinding the C surface was observed.

  Further, the surface roughness Ra of the Si surface of the substrate before grinding was 0.337 nm, and the value of the surface roughness Ra of the C surface was 0.490 nm. The value of roughness Ra increased to 10.0 nm. When the C surface of this substrate was anodized in the same manner as in the first example, the SORI value measured on the Si surface of the substrate was considerably reduced to 65.2 μm. Further, the value of the surface roughness Ra of the substrate C surface was increased to 11.4 nm.

  Further, the same anodic oxidation as in Example 1 was repeatedly performed on the C surface of the substrate, and a total of 6 times (6 minutes) of anodic oxidation was performed. It decreased to 4 μm, and the value of the surface roughness Ra of the C-plane of the substrate increased to 41.2 nm. The substrate which had been anodized a total of 6 times was immersed in a 50% aqueous hydrofluoric acid solution at room temperature for 1 minute to remove the product by anodization. As a result, the value of SORI measured on the Si surface of the substrate was 20.7 μm, which was a little smaller than that after anodic oxidation. Further, the value of the surface roughness Ra of the C-plane of the substrate was 16.2 nm, which was larger than the value before grinding and before anodic oxidation. In this example, the Si surface was a mirror surface, the C surface was a surface having a large surface roughness, the Twiman effect was reduced, and a substrate having a sufficiently small SORI value in terms of handling was obtained.

  Effective residual stress caused by work-affected layer that affects the shape of single-crystal SiC substrate without performing CMP, which is a very heavy process, from any viewpoint such as cost, time, labor, environment, etc. Can be removed. Therefore, it is possible to control the shape of the single crystal SiC substrate to be the same shape as after CMP without performing CMP, so that the shape can be equivalent to that after CMP. It is possible to greatly accelerate efforts for continuous improvement.

Claims (6)

A disc-shaped single crystal SiC substrate having a thickness smaller than 1 mm,
A single crystal SiC substrate, wherein one surface is a mirror surface, Ra of the other surface is a value larger than 1 nm, and SORI is a value smaller than 30 μm.   The single crystal SiC substrate according to claim 1, wherein the single crystal SiC substrate has an orientation flat or a notch that serves as an index of crystal orientation.   3. The single crystal SiC substrate according to claim 1, wherein the diameter is a value larger than 70 mm.   The single crystal SiC substrate according to any one of claims 1 to 3, wherein the thickness is a value smaller than 0.6 mm.   The single crystal SiC substrate according to any one of claims 1 to 4, wherein the thickness is a value smaller than 0.4 mm.   The single crystal SiC substrate according to claim 1, wherein Ra has a value larger than 10 nm.