GaN substrate surface processing method and GaN substrate manufacturing method

Abstract

To provide a GaN substrate surface processing method that can perform surface processing of a GaN substrate in a short time, and a GaN substrate manufacturing method.SOLUTION: A GaN substrate surface processing method performs surface processing of a GaN substrate by grinding and polishing. The method comprises: a high grit size grinding step in which the GaN substrate surface is ground by a grindstone with a grit size of #6000 or more; and a CMP polishing step in which after the GaN substrate surface has been ground by the high grit size grinding step, the GaN substrate surface is polished by a CMP.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for processing the surface of a GaN substrate and a method for manufacturing a GaN substrate. [Background technology]

[0002] GaN substrates are expected to see market expansion as substrates for next-generation semiconductor devices. To use GaN substrates in devices, crystals obtained through various bulk crystal growth processes must be cut out and formed into the substrate shape, and finally the substrate surface must be finished to an atomically flat, damage-free, and undisturbed mirror finish. GaN is a highly brittle material that is mechanically hard and chemically stable, making it difficult to polish, and is therefore known as a difficult-to-process material.

[0003] Recent research and development has led to the development of a method for mirror-finishing GaN substrate surfaces, which is commonly performed using chemical mechanical polishing (hereinafter simply referred to as "CMP") with colloidal silica polishing solutions, followed by mechanical polishing such as lapping and polishing. However, lapping and polishing require time-consuming adjustments to the spacing and parallelism between the wafer and the surface plate, as polishing conditions change over time due to changes in the polishing solution. Furthermore, CMP still only achieves polishing efficiencies of a few to several tens of nanometers per hour. For this reason, new surface processing processes with higher efficiency are desired.

[0004] For example, Patent Document 1 discloses a grinding device that performs three grinding steps for substrates to be subjected to CMP: rough grinding, medium grinding, and finish grinding, in order to shorten the surface processing time. Specifically, paragraph 0070 of the document describes that grinding wheels with grit sizes of #250 to #500 are used for rough grinding, grinding wheels with grit sizes of #1200 to #1800 are used for medium grinding, and grinding wheels with grit sizes of #2500 to #3500 are used for finish grinding. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2014-65082 Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 describes an apparatus that can handle various substrates, such as sapphire, SiC, and GaN. However, because the substrate polishing conditions depend heavily on the substrate's material, the surface processing process must be optimized depending on the material. The grinding apparatus described in Patent Document 1 is said to be capable of performing a series of grinding processes for substrates of various materials with a single apparatus in order to shorten processing time. In other words, Patent Document 1 describes highly versatile processing conditions regardless of the material. In recent years, the miniaturization of electronic components has led to demands for higher surface quality for substrates. Meeting this demand requires an optimal surface processing process for each material, but Patent Document 1 has difficulty meeting this demand.

[0007] Furthermore, Patent Document 1 describes the three-stage grinding process, followed by CMP polishing. However, even in the third stage of finish grinding, a grinding wheel with a grit size of #3500 or less is used, and it is difficult to say that the substrate surface is in a state suitable for CMP. Therefore, if GaN, a known difficult-to-process material, is surface-processed by CMP after grinding with a low-grit grinding wheel, the CMP polishing time becomes long, resulting in a long total surface processing time. Furthermore, if the CMP polishing time is long, the CMP polishing fluid will easily seep between the substrate and the substrate mounting plate, and the polishing fluid will easily adhere to the backside of the substrate. If the polishing fluid adheres, it will be difficult to remove even in the cleaning process after CMP, raising concerns that a clean substrate may not be obtained.

[0008] To shorten the polishing time of CMP, it is generally considered necessary to reduce the surface roughness of the substrate used for CMP polishing. One way to achieve this is to use a grinding wheel with a larger grit size. However, to reduce the surface roughness using the method described in Patent Document 1, the grit size must be gradually increased. Therefore, to perform grinding with a grit size larger than #3500, the number of grinding steps must be increased to four or more, which requires a significant amount of time for grinding. Even if the CMP processing time is shortened by increasing the number of grinding steps, the total surface processing time until the CMP polishing is completed increases. In particular, the surface processing time for GaN is long, and optimizing the grit size and number of steps using the method described in Patent Document 1 requires a significant amount of time and effort, making it difficult to optimize the surface processing process.

[0009] In addition to the above, it is also possible to shorten the polishing time of CMP by optimizing the polishing solution used in CMP. For example, the material of the free abrasive grains and the chemical composition of the polishing solution can be adjusted taking into account the surface condition of the substrate before CMP. However, because the surface condition of the substrate changes depending on the processing conditions in the pre-CMP process, finding the optimal conditions requires endless investigation. As mentioned above, since the surface processing time for GaN is long, it is difficult to find the optimal conditions, and simply changing the conditions to achieve optimization is not realistic. As such, it is difficult to optimize the GaN surface processing process by randomly adjusting grinding and polishing conditions, and processing processes for substrates other than GaN cannot be applied. For this reason, the optimal final surface processing conditions specific to GaN have not yet been found.

[0010] Therefore, an object of the present invention is to provide a method for processing the surface of a GaN substrate, which allows the surface of the GaN substrate to be processed in a short time, and a method for manufacturing a GaN substrate. [Means for solving the problem]

[0011] The inventors conducted a study aimed at shortening the total surface processing time by reducing the load on CMP, even if it meant a longer grinding time. To optimize the processing method before CMP, they investigated the problems with the mechanical polishing conventionally performed in the pre-CMP process. Specifically, they conducted the study using cathodoluminescence (CL) analysis, which allows non-destructive observation of processed substrates. Observation of the substrate using CL analysis revealed that the processing-affected layer is composed of a primary-affected layer and latent scratches. Therefore, the inventors investigated the relationship between the thickness of the primary-affected layer and the depth of latent scratches formed on the surface of a GaN substrate and the processing method.

[0012] It is generally known that the thickness of a work-affected layer varies depending on the particle size of the abrasive grains. Therefore, it is thought that the larger the particle size of the abrasive grains, the thicker the work-affected layer. The inventors performed polishing using loose abrasive grains with an average particle size of 0.5 μm, which is commonly used for mechanical polishing, and grinding using a #3000 grinding wheel with an average particle size of 3 μm, as described in Patent Document 1, and investigated the behavior of the primary affected layer and latent scratches. Unexpectedly, GaN substrates ground with fixed abrasive grains with an average particle size of 3 μm exhibited a thicker primary affected layer, but shallower latent scratches, and a thinner work-affected layer, compared to GaN substrates polished with free abrasive grains with an average particle size of 0.5 μm. Furthermore, it was also found that grinding with fixed abrasive grains caused less damage to the primary affected layer.

[0013] This finding can only be obtained with GaN substrates, which allow for the observation of latent scratches using the CL method. Conventionally, the cross section of the substrate surface was observed by cutting, but cutting applied stress to the substrate surface, resulting in a difference in the surface state from before cutting. Therefore, the conventional processing-affected layers and microcracks are different from what should be observed, and are not the main affected layers or latent scratches observed in this study. In particular, with the conventional method of cutting and then observing GaN substrates, which are difficult to process, the large stresses applied by processing make it impossible to observe the main affected layers and latent scratches that are observed before cutting for cross-sectional observation.

[0014] The inventors of the present invention have focused on the fact that the thickness of the work-affected layer can be reduced by shallowing latent scratches, even if the primary affected layer is thick, and have investigated the optimal grinding conditions for GaN substrates. They have found that in substrates ground using the #3000 grinding wheel described in Patent Document 1, latent scratches are shallower than in substrates mechanically polished using free abrasive grains with an average grain size of 0.5 μm, but this is not enough to enable polishing by CMP to be performed in a short time.

[0015] The #3000 abrasive grain size (average grain size) used in the above study was 3 μm. The above study showed that the thickness of the work-affected layer does not necessarily depend on the abrasive grain size. However, the inventors deliberately performed grinding using a #6000 abrasive grain size (average grain size) of 1.5 μm, and observed the surface condition of the substrate. As a result, it was found that with a #6000 abrasive grain, the primary affected layer became thinner and the latent scratches became significantly shallower. It was also found that damage to the primary affected layer was reduced. It was also found that the CMP processing time was shortened, ultimately resulting in a reduction in the total surface processing time. Furthermore, it was found that depending on the grit size, the difference between the thickness of the primary affected layer and the depth of latent scratches became smaller, further shortening the CMP processing time. The present invention, which was completed based on these findings, is as follows.

[0016] (1) A surface processing method for a GaN substrate, which processes the surface of the GaN substrate by grinding and polishing, comprising a high-grit grinding step in which the surface of the GaN substrate is ground with a grinding wheel having a grit size of #6000 or more, and a CMP polishing step in which the surface of the GaN substrate is polished by CMP after grinding the surface of the GaN substrate in the high-grit grinding step.

[0017] (2) A surface processing method for a GaN substrate according to (1) above, comprising, before the high-grit grinding step, a rough grinding step of grinding the GaN substrate with a grinding wheel having a grit size of less than #6000, or a mechanical polishing step of polishing the GaN substrate with free abrasive grains having a grain size of more than 0.5 μm. (3) The surface processing method of a GaN substrate according to (1) or (2) above, further comprising a cleaning step of cleaning the GaN substrate after the CMP polishing step. (4) The surface processing method for a GaN substrate according to any one of (1) to (3) above, wherein the grinding stone used in the high-grit grinding step has a grit size of greater than #8000. (5) The surface processing method for a GaN substrate according to any one of (1) to (4) above, wherein the grinding wheel is bonded with vitrified.

[0018] (6) A method for manufacturing a GaN substrate, comprising the method for processing the surface of a GaN substrate according to any one of (1) to (5) above. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a process diagram showing an example of a manufacturing process for a GaN substrate, where FIG. 1(a) is a process diagram showing an example of a manufacturing process for a conventional GaN substrate, and FIG. 1(b) is a process diagram showing an example of a manufacturing process for a GaN substrate to which the surface processing method for a GaN substrate according to this embodiment is applied. [Figure 2] FIG. 2 is a diagram showing the processing method and evaluation items of the substrate surface before CMP, and the slurry conditions for CMP and evaluation items of the substrate surface. [Figure 3] FIG. 3 is a schematic diagram showing a photographing device for a cathodoluminescence image (hereinafter referred to as a "CL image" where appropriate). [Figure 4] FIG. 4 shows the polishing time of CMP and CL images of the substrate surface at each time for a substrate that has been mechanically polished using 0.5 μm diamond abrasive grains and then CMPed. [Figure 5] FIG. 5 is a graph showing the relationship between the polishing time of CMP and the black line density. [Figure 6] FIG. 6 is a diagram showing an example of a means for widening the field of view of the CL image and confirming the presence of a black line after the CMP polishing time of 420 minutes in FIG. [Figure 7] FIG. 7 is a table showing the relationship between the grit size and the surface roughness in the high-grit grinding process before CMP. [Figure 8]FIG. 8 shows the polishing time of the CMP for a substrate that has been ground using a grindstone with a grit size of #8000 and then CMPed, and the CL images of the substrate surface at that time. [Figure 9] FIG. 9 shows the polishing time of the CMP for a substrate that has been ground using a grindstone with a grit size of #30000 and then CMPed, and the CL images of the substrate surface at that time. [Figure 10] Figure 10 is an image showing the degree of deterioration of the main altered layer and the depth of latent scratches, where Figure 10(a) is an image showing the damage to the substrate surface after mechanical polishing using 0.5 μm diamond abrasive grains as described in Comparative Example 1, Figure 10(b) is an image showing the damage to the substrate surface after grinding using a grinding stone with a grit size of #30000 as described in Example 3, and Figure 10(c) is an image showing the damage to the substrate surface after grinding using a grinding stone with a grit size of #8000 as described in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0020] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the accompanying drawings, but the present invention is not limited to the following embodiments. 1. Overview of the GaN substrate manufacturing method according to the present invention FIG. 1 is a process diagram showing an example of a manufacturing process for a GaN substrate, where FIG. 1(a) is a process diagram showing an example of a manufacturing process for a conventional GaN substrate, and FIG. 1(b) is a process diagram showing an example of a manufacturing process for a GaN substrate to which the surface processing method for a GaN substrate according to this embodiment is applied.

[0021] As shown in FIG. 1(a), a conventional method for manufacturing a GaN substrate (hereinafter sometimes simply referred to as a "substrate") involves first growing a GaN crystal on a GaN seed crystal, for example, by vapor phase epitaxial growth or liquid phase growth (S11). Next, the grown crystal is fixed to a jig and outer diameter ground to remove protrusions (S12). After outer diameter grinding, the grown crystal is sliced ​​to a thickness of, for example, 100 to 3000 μm (S13), thereby obtaining a GaN substrate. Next, chamfering is performed to remove edges created during slicing (S14), and rough grinding is performed to remove surface irregularities on the GaN substrate (S15).

[0022] Thereafter, mechanical lapping using free abrasive grains is performed (S16), followed by precision polishing using free abrasive grains such as diamond (S17). After that, CMP is performed to remove the process-affected layer formed on the surface of the substrate (S18), and finally the substrate is cleaned (S19).

[0023] In the lapping (S16) and precision polishing (S17) steps shown in Figure 1(a), the polishing fluid wears the surface plate, and the polishing fluid can change over time. Increasing the polishing pressure and rotation speed of the surface plate shortens the polishing time, but this increases the damage to the surface plate, requiring more frequent surface plate maintenance and reducing productivity. Meanwhile, to prevent the formation of a processing-affected layer, it is necessary to reduce the abrasive grain size used in lapping (S16) and precision polishing (S17). However, the smaller the abrasive grain size, the longer the polishing time, and it becomes more difficult to control grain size variation. Thus, it has been difficult to shorten the total surface processing time with conventional surface processing methods.

[0024] In contrast, as shown in FIG. 1(b), a GaN substrate manufacturing method incorporating the GaN substrate surface processing method of the present invention performs high-grit grinding (S27) using a grinding wheel with abrasive grains of #6000 or greater (average grain size of the abrasive grains of 1.5 μm or less) instead of the lapping (S16) and precision polishing (S17) of FIG. 1(a). This reduces the total surface processing time. Note that in FIG. 1(b), the processes other than the high-grit grinding (S27) and the CMP polishing process (S28) can generally be performed under the same conditions as in FIG. 1(a) for conventional GaN substrate manufacturing. The GaN substrate surface processing method of the present invention is described in detail below.

[0025] 2. Surface processing method of GaN substrate according to the present invention The surface processing method according to the present invention is a method specialized for GaN substrates, which are difficult-to-process materials, and includes a high-grit grinding step (S27) in which the surface of the GaN substrate is ground with a grinding wheel having a grit size of #6000 or more (average grain size of abrasive grains of 1.5 μm or less), and a CMP polishing step (S28) in which the surface of the GaN substrate is polished by CMP after the surface has been ground by the high-grit grinding step (S27). Each step will be described in detail.

[0026] 2-1.High-grit grinding process In the high-grit grinding step (S27) of the present invention, the surface of the GaN substrate is ground using a grinding stone of #6000 or larger (average grain size of abrasive grains of 1.5 μm or smaller). The high-grit grinding step (S27) of the present invention is performed using a conventional grinding device. The substrate fixed to a surface plate is ground with a grinding wheel attached to a motor. Grinding conditions such as the rotation speed of the grinding wheel and the pressure applied to the substrate are not particularly limited, but as long as the grinding wheel grit falls within the ranges described below, the thickness of the work-affected layer and the depth of latent scratches can be reduced regardless of the rotation speed, pressure, or feed rate. In this embodiment, the grinding time for the high-grit grinding step is preferably 1 to 20 minutes, and more preferably 1 to 5 minutes. The grinding rate may be 5 to 60 μm / min, and may be 5 to 20 μm / min.

[0027] The grit size of the abrasive grains is #6000 or higher (average grain size of the abrasive grains is 1.5 μm or less). A grit size of #6000 or higher causes less damage to the substrate, thinner primary altered layers, and less deep latent scratches, shortening the CMP polishing time and reducing the total surface processing time. Furthermore, replacing mechanical polishing with mechanical grinding facilitates automation of the high-grit grinding process, thereby reducing the total surface processing time. Furthermore, in the present invention, the grain size of the abrasive grains can be measured using a particle size distribution analyzer using laser diffraction, dynamic light scattering, image analysis, gravitational sedimentation, or the like.

[0028] The material of the abrasive grains is not particularly limited, and various abrasive grains such as alumina, alumina zirconia, silicon carbide, CBN, and diamond can be used.

[0029] If the grinding wheel used in the high-grit grinding step (S27) has a small grit size, the grinding is completed quickly due to the large abrasive grain size, but the damage to the substrate is significant and the latent scratches are deep. Therefore, the CMP polishing step (S28) must be extended to remove the latent scratches. On the other hand, if the grit size is large, the grinding time is slightly longer, but the damage to the primary altered layer is small and the latent scratches are shallow, so the CMP polishing step (S28) time is shortened. Since the polishing time by CMP is significantly longer than the time required for mechanical grinding, shortening the CMP polishing time can shorten the total surface processing time. The grit size of the grinding wheel is preferably greater than #8000 (average abrasive grain size less than 1.0 μm), more preferably #10000 or more (average abrasive grain size 0.7 μm or less), and most preferably #13000 or more (abrasive grain size 0.5 μm or less). In particular, when using a high grit size of #13000 or more, the polishing time for GaN becomes long due to the relationship between the grain size of the abrasive grains and the hardness of GaN, and this has traditionally been avoided. However, in this invention, the knowledge that the polishing time of the CMP polishing process described below can be shortened was first obtained through observations of GaN using the CL method. As a result, the total processing time was shortened.

[0030] Although there is no particular upper limit, increasing the grit size can reduce the thickness of the primary altered layer and the depth of latent scratches during the grinding process, further shortening the time for the CMP polishing process. On the other hand, if the grit size is too high, the increased grinding time will be longer than the shortened CMP polishing time. Therefore, simply increasing the grit size does not necessarily shorten the total surface processing time from grinding to polishing. From the perspective of shortening this total time, it is preferably #50000 or less (average grain size of abrasive grains of 0.1 μm or more), and more preferably #30000 or less (average grain size of abrasive grains of 0.2 μm or more).

[0031] The binder for the grinding wheel may be any commonly used one, and must be able to withstand grinding resistance and the like, and must be able to break down due to increased grinding resistance when the abrasive grains are worn down, allowing the abrasive grains to self-regenerate to an appropriate degree. Examples of binders include resinoid, rubber, metal, shellac, and vitrified, with vitrified being preferred because it changes little over time and has excellent durability.

[0032] In the surface-processed substrate, a primary alteration layer and latent scratches are formed on the surface due to stress during processing. Substrates ground in the high-grit grinding step (S27) of the present invention are conventionally ground using a high-grit grinding wheel to reduce the degree of alteration in the primary alteration layer (hereinafter referred to as "damage" as appropriate). In the case of GaN substrates, when a large-grit grinding wheel is used as in the high-grit grinding step (S27), the thickness of the primary alteration layer is approximately 700 to 1000 nm. However, when grinding with a high-grit grinding wheel, damage to the primary alteration layer is minimal, so the degree of alteration is moderate and latent scratches do not become deep. The depth of the latent scratches is approximately 1.0 to 2.0 μm from the surface of the substrate.

[0033] The thickness of the primary altered layer can be determined by multiplying the polishing time and polishing rate at which the brightness of the CL image in the area other than the latent scratches becomes equivalent to that of the as-grown crystal during CMP polishing, as described below. The as-grown crystal in the present invention is obtained by crystal growth (S11) shown in Figure 1(b). Therefore, in the present invention, a CL image of the obtained GaN surface is taken using a SEM (described below), and the average brightness data (average pixel value) of the taken CL image is preliminarily obtained as the brightness of the as-grown crystal using image analysis software (sm-300 Series) attached to the SEM. The thickness of the primary altered layer can be determined by multiplying the polishing time and polishing rate at which the brightness becomes equivalent to that of the as-grown crystal during CMP polishing. The depth of the latent scratches can be determined by multiplying the polishing time and polishing rate of the CMP polishing. The polishing rate of CMP will be described later.

[0034] Furthermore, the surface roughness of the substrate ground in the high-grit grinding step (S27) is preferably 2.0 nm or less, more preferably 1.0 nm or less, even more preferably 0.5 nm, particularly preferably 0.1 nm or less, and most preferably 0.05 nm or less. When performing surface processing of a GaN substrate as in the present invention, if the surface roughness after the high-grit grinding step (S27) is preferably 2.0 nm or less, damage is minimal and latent scratches do not become deep. Therefore, it is easy to achieve a surface roughness of approximately 0.1 nm or less after surface processing with a short CMP polishing time (S28).

[0035] GaN substrates processed by the GaN substrate surface processing method according to the present invention have a depth of latent scratches formed on the surface that is reduced by 30% or more, preferably by 40% or more, and more preferably by 60% or more, compared to GaN substrates processed by the conventional processing method shown in Figure 1(a). Since there is no significant difference in the polishing ability of the CMP polishing step (S28) described below, the polishing time of the CMP polishing step (S28) becomes shorter as the latent scratches become shallower. Since the majority of the surface polishing time is spent in the CMP polishing step (S28), shortening this polishing time also shortens the total surface processing time.

[0036] Figure 2 shows the processing method and evaluation items for the substrate surface before CMP, as well as the CMP slurry conditions and evaluation items for the substrate surface. As shown in Figure 2, the surface roughness of a substrate polished by conventional mechanical polishing can be observed using a scanning electron microscope (SEM), atomic force microscope (AFM), transmission electron microscope (TEM), cathodoluminescence (CL), etc. The thickness of the primary affected layer and the depth of latent scratches can be observed using the CL method.

[0037] 2-2.CMP polishing process The CMP polishing step (S28) of the present invention is carried out after the surface of the substrate is ground in the high grit grinding step (S27). A typical method for the CMP polishing step (S28) is to attach the substrate to a carrier, press the substrate against a polishing pad, and rotate both the substrate and the polishing pad while supplying a polishing liquid between the substrate and the polishing pad.

[0038] The polishing solution used in the CMP polishing step (S28) generally contains abrasive grains such as alumina or silica, and is acidified with an oxidizing agent such as hydrogen peroxide or persulfuric acid. The oxidizing agent oxidizes the surface of the substrate, and the resulting oxide film is then removed with the abrasive grains to achieve polishing. The abrasive grain size is 100 nm or less, and the abrasive grain concentration should be 20 to 60 mass % of the total mass of the polishing solution. The abrasive grain size can be measured using the same method as for the abrasive grains in grinding wheels.

[0039] The conditions for the CMP polishing step (S28) are not particularly limited, but the rotation speed of the carrier is 20 to 1000 rpm and the supply rate of the polishing liquid is 50 to 1000 ml / h. Under these conditions, the polishing rate falls within a predetermined range. For example, the polishing rate may be 50 to 3000 nm / h. Latent scratches remain on the obtained substrate, but as shown in FIG. 2, the presence or absence of latent scratches can be confirmed by a cathode luminescence image. The polishing time is set to a value when the density of the black lines, which can be confirmed in a cathode luminescence image described later, reaches, for example, 1 cm -2 The time should be as long as it is expected to be less than the above.

[0040] FIG. 3 is a schematic diagram showing a photographing device for a cathodoluminescence image (hereinafter referred to as a "CL image") . The CL device 10 includes an electron beam irradiation device (electron beam generator) 20, a sample stage 30, a (CL light) detector 40, and a computing device (controller) 50. The electron beam irradiation device 20 (electron beam generator) irradiates an electron beam 21 onto a substrate 60 placed on the sample stage 30. The (CL light) detector 40 detects CL light 22 emitted from the substrate 60. The computing device (controller) 50 performs various processes based on the data detected by the (CL light) detector 40. The electron beam irradiation device (electron beam generator) 20, sample stage 30, and computing device (controller) 50 may be attached to an SEM, and therefore the CL device 10 may be built into the SEM. The arithmetic device (control device) 50 includes a CPU (Central Processing Unit) that performs various processes, a memory, a non-volatile storage device, input devices such as a keyboard and a microphone, a monitor, an input / output interface, etc. These pieces of hardware can perform the following calculations using various functions (not shown) according to programs stored in the storage device.

[0041] In cathodoluminescence (CL) spectroscopy, the surface of a GaN crystal is irradiated with an electron beam, and CL light, which is generated by the radiative recombination process near the GaN crystal surface, is observed. CL evaluations typically utilize the electron beam irradiation function of a scanning electron microscope (SEM). Similar to SEM images, CL intensity mapping images (hereafter simply referred to as "CL images") can be obtained. These CL images can visualize subsurface defects as black lines, rather than surface scratches. Therefore, they are an extremely useful method for determining the presence or absence of processing damage within the GaN substrate surface. This is reported, for example, in the following literature: Hideo Aida, Hidetoshi Takeda, Koji Koyama, Haruji Katakura, Kazuhiko Sunakawa, and Toshiro Doi, "Chemical Mechanical Polishing of Gallium Nitride with Colloidal Silica," Journal of the Electrochemical Society, 158 (12) H1206-H1212 (2011)). An example of calculating the polishing time of the CMP polishing step (S28) from the time change of the CL image will be described in detail.

[0042] The CL method makes it possible to visually evaluate the presence or absence of latent scratches. Since the observation area of ​​a CL image is generally several tens of micrometers square, it is difficult to guarantee the absence of latent scratches over a wide area, such as the entire surface of the substrate. Furthermore, latent scratches are observed in CL images as extremely small dotted lines less than 0.5 micrometers wide.

[0043] Thus, it is necessary to make it possible to observe with a realistic field of view that can be achieved with conventional devices, and to make it possible to accurately observe even extremely slight altered layers. For example, a typical CL image size that allows efficient image acquisition by temporarily pausing processing during processing is assumed to be approximately 35 μm x 50 μm. Immediately after the start of CMP, black lines cannot be recognized, but as processing approaches the middle, the number of black lines confirmed in the CL observation area decreases to a level where they can be counted. When processing has progressed to the point where not a single black line can be observed in this observation area, the black line density will be approximately 10 4 cm -2 It will be about that level.

[0044] Subsequent observations showed that the black line density was 10 4 cm -2 Since the CL observation area needs to be expanded when the image density falls below this level, the observation area can be expanded by moving the measurement stage with the substrate placed on it vertically and horizontally to determine the black line density. The black line density is the number of black lines observed in the observation area divided by the area of ​​the observation area.

[0045] In the present invention, the black line density is plotted for each CMP polishing time from the middle to the end of the processing. 4 cm -2 Draw an approximate line based on the black line density information within the range below, and the black line density is 1 cm -2 The polishing time is estimated as the time it takes for this to occur as the CMP polishing time, and the depth of the latent scratches can be calculated by multiplying the CMP polishing rate by the polishing time.

[0046] In the present invention, the black line density is 10 6 cm -2 From the polishing time below, the black line density observable in a general observation area was measured, and the black line density was measured to be 10 4 cm -2 1cm from -2 The decrease in black line density with the passage of polishing time is grasped during the polishing time up to the polishing time. In this way, the CMP polishing time can be estimated with high accuracy.

[0047] If the polishing time by CMP is short, the CMP polishing liquid will not get into the gap between the substrate and the substrate mounting plate, and the polishing liquid can be prevented from adhering to the back surface of the substrate. In this embodiment, a high-grit grinding step suitable for processing GaN substrates is performed before the CMP polishing step, so the CMP polishing time can be shortened. In this embodiment, the CMP polishing time is preferably 1 to 30 hours. This is a processing time that is significantly shorter than that of conventional processing methods.

[0048] In this way, by understanding the decreasing trend of black line density, it is possible to -2 The polishing time and amount required to achieve this can be estimated, and this amount corresponds to the depth of the latent scratches. By performing the high-grit grinding (S27) shown in Figure 1(b) using various grits in a similar manner, the grit size and depth of latent scratches in the GaN substrate can be accurately predicted.

[0049] The thickness of the primary altered layer can be measured by multiplying the polishing rate of CMP polishing by the polishing time required to obtain a CL image of the area other than the latent scratches obtained during CMP polishing until the brightness of the CL image is equivalent to that of the as-grown crystal obtained as described above. However, when the grinding wheel used in high-grit grinding (S27) is #50000 or smaller (average abrasive grain size of 0.1 μm or larger), the depth of the latent scratches is equal to or greater than the thickness of the primary altered layer. Therefore, when the depth of the latent scratches can be estimated using the CL method as described above, the primary altered layer can be removed by removing the latent scratches, without measuring the thickness of the primary altered layer.

[0050] 2-3. Rough grinding process or mechanical polishing process The surface processing method for a GaN substrate according to the present invention may include a rough grinding step (S25) of grinding the surface of the GaN substrate with a grinding stone having a grit size of less than #6000 (average grain size of abrasive grains exceeding 1.5 μm) before the high-grit grinding step (S27). The rough grinding step (S25) can be performed with a grinding stone having a grit size smaller than that used in the high-grit grinding step (S27). Alternatively, instead of the rough grinding step (S25), a mechanical polishing step (S25) of polishing the GaN substrate with free abrasive grains having an average grain size exceeding 0.5 μm may be included.

[0051] The rough grinding step (S25) is performed using a conventional grinding device, similar to the high-grit grinding step (S27). In the rough grinding step (S25), the thickness of the primary altered layer and the depth of latent scratches do not need to be considered. The grinding conditions are not particularly limited, but the rotation speed of the surface plate is 100 to 500 rpm, the rotation speed of the grinding wheel is 200 to 1000 rpm, and the feed rate is 5 to 50 μm / min. The grinding time may be 1 to 5 minutes for a substrate with a diameter of 2 inches, for example, and may be determined appropriately depending on the size of the substrate.

[0052] The upper limit of the grit size of the abrasive grains used in the grinding stone in the rough grinding step (S25) is preferably less than #6000 (average grain size of the abrasive grains exceeds 1.5 μm), more preferably #4000 or less (average grain size of the abrasive grains is 2.0 μm or more), and even more preferably #3000 or less (average grain size of the abrasive grains is 3.0 μm or more). The grain size of the abrasive grains is the same as in the high-grit grinding step (S27). There is no particular restriction on the lower limit of the grit size of the abrasive grains, but #240 or more (average grain size of the abrasive grains is 127 μm or less), more preferably #400 or more (average grain size of the abrasive grains is 75 μm or less), and even more preferably #600 or more (average grain size of the abrasive grains is 30 μm or less). When the rough grinding step (S25) is performed, the grinding time of the high-grit grinding step (S27) is shortened, and ultimately the total surface processing time may be shortened. The material of the abrasive grains, the binder of the grinding wheel, and the concentration of the abrasive grains in the grinding wheel are the same as those in the high-grit grinding step (S27).

[0053] The mechanical polishing step (S25) is performed in the same manner as in the conventional method, by attaching the substrate to a carrier, pressing the substrate against a polishing pad, and rotating both the substrate and the polishing pad while supplying a polishing liquid between the substrate and the polishing pad. The polishing liquid used in the mechanical polishing step (S25) contains free abrasive grains with a particle size exceeding 0.5 μm. The abrasive grains may be made of, for example, diamond, alumina, or silica. The abrasive grain concentration may be 1 to 20 mass% relative to the total mass of the polishing liquid. The particle size of the abrasive grains is preferably 1.0 μm or more, and more preferably 1.5 μm or more. There is no particular upper limit, but it is sufficient if it is 5 μm or less. The particle size of the abrasive grains is defined in the same way as the abrasive grains in a grinding wheel, with the sieve mesh corresponding to the maximum particle size. The components of the polishing liquid other than the abrasive grains may be the same as in the conventional method.

[0054] The conditions for the mechanical polishing step (S25) are not particularly limited, but the polishing rate should be 50 to 300 nm / h, the rotation speed of the carrier should be 20 to 2000 rpm, and the supply rate of the polishing liquid should be 1 to 20 ml / h. The polishing rate should be 1 to 30 μm / h.

[0055] 2-4. Cleaning process The surface processing method of the GaN substrate according to the present invention can include a cleaning step (S29) of cleaning the GaN substrate after the CMP polishing step (S28). In cleaning after the CMP polishing step (S28), this method is preferably used to prevent the polishing liquid used in CMP from remaining on the substrate and contaminating it. As a detergent to be used in the cleaning step (S29), an alkaline cleaning liquid that electrostatically repels the substrate and abrasive grains is generally considered to be effective, but is not limited to this.

[0056] The substrate cleaning method may be the same as conventional methods. For example, after the CMP polishing step (S28), the substrate is placed on a spinner and scrubbed for 20 to 60 seconds while a cleaning solution is supplied to the substrate at a flow rate of approximately 50 to 300 ml / min. This cleaning method can be performed using a commercially available cleaning machine. In this embodiment, the CMP polishing time is short, so the CMP polishing solution does not penetrate between the substrate and the substrate mounting plate, and the polishing solution is less likely to adhere to the backside of the substrate. This allows the polishing solution to be easily removed in this cleaning step, resulting in a clean substrate. If the polishing time in the CMP polishing step is long, the polishing solution will begin to adhere, making it impossible to clean within the above-mentioned cleaning time. In this embodiment, as described above, the high-grit grinding step, which is a step prior to the CMP polishing step, is suitable for GaN substrates, thereby shortening the CMP polishing step time and thereby shortening the cleaning time. [Example]

[0057] An example of a surface polishing method specifically for GaN substrates will be described. As an example, the total surface processing time required for the surface processing methods of the example and comparative example was investigated based on FIGS.

[0058] 1. Comparative Example 1 1) Preparation of GaN substrate As shown in Figure 1(a), GaN crystals were grown by vapor-phase epitaxial growth (S11). CL images of the grown GaN crystals were taken using a scanning electron microscope (SEM, Topcon Corporation, Model SM-300) equipped with a CL photodetector at an accelerating voltage of 10 kV, a probe current of 90°, a working distance (WD) of 22.5 mm, and a magnification of 2000x. The average brightness data (average pixel values) of the CL images was then calculated as the brightness of the as-grown crystal using the image analysis software (SM-300 Series) provided with the SEM. After outer diameter grinding (S12), the crystals were sliced ​​(S13), and the edges were chamfered (S14) to prepare circular GaN substrates with a thickness of 400 μm and a diameter of 2 inches.

[0059] 2)Rough grinding The GaN substrate was fixed to a surface plate and ground for 5 minutes using a grinding wheel with a grit size of #600 (average particle size: 30 μm) at a feed rate of 20 μm / min (S15). The average particle size of the abrasive grains used in the grinding wheel was measured using a dynamic light scattering particle size distribution analyzer (LB-500 manufactured by Horiba, Ltd.). The average particle size was calculated from the obtained particle size distribution.

[0060] 3) Lapping, precision polishing (mechanical polishing) Next, using a polishing solution in which the concentration of diamond abrasive grains with an average particle size of 3 μm is 10 mass% relative to the total mass of the polishing solution, the supply rate of the polishing solution is 10 ml / h, and the polishing rate is 20 μm / h, and the lapping process is carried out for 120 minutes (S16). After that, using a polishing solution in which the concentration of diamond abrasive grains with an average particle size of 0.5 μm is 10 mass% relative to the total mass of the polishing solution, the polishing rate is 1 μm / h, and the precision polishing is carried out for 180 minutes (S17). The average particle size of the abrasive grains used in the grinding wheel is the average particle size measured under the same conditions using the above-mentioned device.

[0061] 4) Observation of the substrate surface The substrate surface was observed before CMP polishing. An atomic force microscope (AFM) was used to measure the surface roughness (Ra), and the surface irregularities were expressed as shades of gray. Ra was 0.4 nm, indicating that the surface irregularities were minimal.

[0062] CL images of the substrate surface were also taken. CL images were taken using a scanning electron microscope (SEM, Topcon Corporation, Model SM-300) equipped with a CL photodetector at an accelerating voltage of 10 kV, a probe current of 90°, a working distance (WD) of 22.5 mm, and a magnification of 2000x. The results are shown in Figure 10(a). The short arrow indicates the thickness of the primary affected layer (110°), and the long arrow indicates the depth of the latent scratches (120°). The thickness of the primary affected layer was approximately 200 nm, while the depth of the latent scratches reached a maximum of 2.4 μm. The depth of the latent scratches corresponds to the amount of polishing by CMP, which is described below, and is obtained by multiplying the CMP polishing time and the polishing rate. The thickness of the primary affected layer was obtained by multiplying the polishing time at which the brightness of the CL image, excluding the latent scratches, became equivalent to the brightness of the as-grown crystal by the CMP polishing rate.

[0063] 5)CMP polishing Polishing was performed using a polishing solution containing 35-45% by mass of silica abrasive grains with an average particle size of 60 nm and acidified with hydrogen peroxide. The carrier rotation speed was 30 rpm, the polishing solution was supplied at a rate of 50 mL / h, and the polishing rate was 180 nm / h (S18). Subsequently, CL images of the substrate surface were taken at predetermined polishing intervals, as shown in Figure 4. CL images were observed using a scanning electron microscope (SEM, Topcon Corporation, model SM-300) equipped with a CL photodetector at an accelerating voltage of 10 kV, a probe current of 90°, a working distance (WD) of 22.5 mm, and a magnification of 2000x. The particle size of the silica abrasive grains was the average particle size measured under the same conditions using the above-mentioned equipment.

[0064] In each CL image taken as shown in Figure 4(a) to Figure 4(e), the black line density was calculated from the number of black lines, and as shown in Figure 5, the vertical axis was the black line density and the horizontal axis was the polishing time, and the black line density was calculated from the number of black lines. 6 cm -2In the above cases, it was difficult to measure the black line density in the 2000x CL image. It was possible to measure the black line density when the CMP polishing time was between 90 and 420 minutes. Then, when the polishing time was 420 minutes, the number of black lines in the observation area was reduced to one. When the CMP polishing time exceeded 420 minutes, almost no black lines were visible in the 2000x CL image, so the black line density was measured while enlarging the field of view.

[0065] To observe black lines on a CL image, a magnification of about 2000x is required. Therefore, it is difficult to observe a wide area with CL and accurately calculate the black line density. Therefore, while the CL image magnification was kept at 2000x, the observation area was moved and still images of the CL image were captured at any position. Based on the obtained images, the black line density over a wide area was confirmed and the polishing time was estimated. All of the CL images shown on the right side of Figure 6 were taken at 2000x, and the letters written on each CL image indicate the position where the CL image was taken within the field of view for each CMP polishing time shown on the left side of Figure 6. In a similar manner, the black line density for each polishing time was plotted as shown in Figure 5. When the black line density was 10 4 cm -2 The plot below was fitted with a straight line, and the black line density was 1cm -2 It was found that the polishing time in the following CMP polishing step (S18) was approximately 800 minutes (796 minutes). Therefore, it was found that the CMP polishing required polishing a thickness of 800 minutes x 180 nm / h ≒ 2400 nm. Therefore, the depth of the latent scratches can be estimated to be approximately 2400 nm.

[0066] The thickness of the primary affected layer was calculated by taking CL images using the same scanning electron microscope as in S11 at each CMP polishing time. Then, for the area other than the latent scratches in the CL images, the polishing time at which the brightness of the CL image became equal to the brightness of the as-grown crystal obtained after S11 was multiplied by the polishing rate of CMP polishing, 180 nm / h. As a result, the thickness of the primary affected layer in Comparative Example 1 was 200 nm.

[0067] 6) Cleaning After the CMP polishing was completed, the substrate was washed with an alkaline cleaning solution (S19).

[0068] 2. Example 2 1) Preparation of GaN substrate and rough grinding A roughly ground substrate was obtained through the same steps as in Comparative Example 1 (S21 to S25).

[0069] 2) High-grit grinding Next, the substrate after rough grinding was ground for 2 minutes at a grinding rate of 10 μm / min using a grinding wheel with a grit size of #8000 (average grain size: 1.0 μm) (S27). The average grain size of the abrasive grains used in the grinding wheel was the average grain size measured under the same conditions using the above-mentioned device.

[0070] 3) Observation of the substrate surface The substrate surface was observed before CMP polishing. A non-contact surface profiler, NewView7300, manufactured by Zygo, was used to observe the substrate surface, and the surface roughness (Ra) was measured. The results are shown in Figure 7. The surface roughness Ra was 1.3 nm, which was found to be greater than the surface roughness of Comparative Example 1.

[0071] Additionally, the thickness of the primary affected layer on the substrate surface and the depth of the latent scratches were measured in the same manner as in Comparative Example 1. The results are shown in Figure 10(c). The short arrow indicates the thickness 130 of the primary affected layer, and the long arrow indicates the depth 140 of the latent scratches. It was found that the thickness of the primary affected layer was approximately 900 nm, which was thicker than in Comparative Example 1. Meanwhile, the depth of the latent scratches was a maximum of 1500 nm, which was approximately 40% less than in Comparative Example 1. This is thought to result in a 40% reduction in the CMP polishing time.

[0072] 4)CMP polishing Polishing by CMP was carried out in the same process as in Comparative Example 1. As shown in Fig. 8(a) to Fig. 8(e), in each CL image taken at 2000x magnification as in Comparative Example 1, the black line density was calculated from the number of black lines, and the vertical axis was the black line density, and the horizontal axis was the polishing time, and the result was plotted. When the polishing time was 390 minutes, the number of black lines in the observation area was 1. In order to estimate the polishing time, CL images were taken with a wider observation field as in Comparative Example 1, and the black line density for each polishing time was plotted. When the black line density was 10 4 cm -2 The plot below was fitted with a straight line, and the black line density was 1cm -2 The polishing time was approximately 500 minutes, and it was found that the total surface processing time was significantly reduced compared to Comparative Example 1.

[0073] 5) Cleaning After the CMP polishing was completed, the substrate was cleaned in the same manner as in Comparative Example 1 (S29).

[0074] 3. Example 3 In Example 2, the surface of the substrate was processed through the same steps as in Example 1, except that the high-grit grinding stone was changed from #8000 to #30000 (average grain size: 0.2 μm). The average grain size of the abrasive grains used in the grinding stone was measured under the same conditions using the above-mentioned device. The substrate surface was observed and the surface roughness (Ra) was measured before CMP polishing in the same manner as in Example 2. The results are shown in Figure 7. The surface roughness Ra was 1.8 nm, which was found to be greater than the surface roughness in Example 2.

[0075] Additionally, the thickness of the primary affected layer on the substrate surface and the depth of the latent scratches were measured in the same manner as in Comparative Example 1. The results are shown in Figure 10(b). The short arrow indicates the thickness of the primary affected layer (150), and the long arrow indicates the depth of the latent scratches (160). It was found that the thickness of the primary affected layer was approximately 700 nm, which was thicker than in Comparative Example 1. Meanwhile, the depth of the latent scratches was a maximum of approximately 1000 nm, which was approximately 60% less than in Comparative Example 1. This also shortens the polishing time for CMP. It is therefore believed that the polishing time for CMP will be significantly shortened.

[0076] Polishing by CMP was carried out in the same process as in Example 2. As shown in Figures 9(a) to 9(d), in each CL image taken at 2000x magnification as in Comparative Example 1, the black line density was calculated from the number of black lines, and the vertical axis was the black line density, and the horizontal axis was the polishing time, and the result was plotted. When the polishing time was 300 minutes, the number of black lines in the observation area was one. In order to estimate the polishing time, CL images were taken with a wider observation field as in Comparative Example 1, and the black line density for each polishing time was plotted. When the black line density was 10 4 cm -2 The plot below was fitted with a straight line, and the black line density was 1cm -2 The polishing time was 330 minutes, which was found to be a significant reduction in the total surface processing time compared to Comparative Example 1. After the CMP polishing was completed, the substrate was cleaned in the same manner as in Comparative Example 1.

[0077] 4. Example 1 In Example 2, the surface processing of the substrate was carried out through the same steps as in Example 2, except that the high-grit grinding stone was changed from #8000 to #6000 (average grain size: 1.5 μm). The average grain size of the abrasive grains used in the grinding stone was the average grain size measured under the same conditions using the above-mentioned device. As in Example 2, the substrate surface was observed before CMP polishing, and the surface roughness (Ra) was measured. The results are shown in Figure 7. The surface roughness Ra was 1.0 nm, which was found to be smaller than the surface roughness of Example 2. The thickness of the main altered layer was 1000 nm, and the depth of the latent scratches was 2000 nm. Compared to Comparative Example 1, the depth of the latent scratches was significantly shallower. After the CMP polishing was completed, the substrate was cleaned in the same manner as in Comparative Example 1.

[0078] 5. Comparative Example 2 In Example 2, the surface processing of the substrate was carried out through the same steps as in Example 2, except that the grinding stone for high grinding was changed from #8000 to #3000 (average grain size: 3.0 μm). The average grain size of the abrasive grains used in the grinding stone was the average grain size measured under the same conditions using the above-mentioned device. The substrate surface was observed before CMP polishing, and the surface roughness (Ra) was measured in the same manner as in Example 2. The surface roughness Ra was 5.0 nm, which was found to be greater than the surface roughness of Example 2. The thickness of the primary affected layer was 2500 nm, and the depth of the latent scratches was 3000 nm. Compared to either of the examples, the primary affected layer was found to be thicker and the latent scratches deeper. After the CMP polishing was completed, the substrate was cleaned in the same manner as in Comparative Example 1.

[0079] 10 is an image showing the degree of alteration of the primary alteration layer and the depth of latent scratches. FIG. 10(a) is an image showing the damage to the substrate surface 100 after mechanical polishing using diamond abrasive grains with an average particle size of 0.5 μm as described in Comparative Example 1. FIG. 10(b) is an image showing the damage to the substrate surface 100 after grinding using a grindstone with a grit size of #30000 as described in Example 3. FIG. 10(c) is an image showing the damage to the substrate surface 100 after grinding using a grindstone with a grit size of #8000 as described in Example 2. In these figures, the primary alteration layer and latent scratches are schematically shown in the near-surface region of the cross section of the substrate, and the magnitude of the damage to the primary alteration layer is expressed as a shade based on the brightness of the CL image. Furthermore, to make the degree of damage easier to recognize, the damage is quantified using a relative ratio to FIG. 10(a).

[0080] As shown in Figure 10(a), the substrate whose surface was processed using the conventional process had a thin primary affected layer, but the brightness of the CL image was low, indicating significant damage. Furthermore, for latent scratches, the brightness of the CL image was low and black lines were observed deep from the substrate surface 100, indicating significant damage reaching the deepest depths.

[0081] In contrast, as shown in Figure 10(c), the substrate whose surface was processed by the steps of Example 2 had a primary-affected layer about three times thicker than that of Comparative Example 1, but the brightness of the CL image was lower than that of Comparative Example 1, and the damage to the primary-affected layer was small. If we assume that the damage shown in Figure 10(a) is 1, the damage in Figure 10(c) was 0.4. It was also found that the depth of latent scratches was significantly reduced.

[0082] Furthermore, as shown in Figure 10(b), the substrate whose surface was processed by the steps of Example 3 had a thickness of the primary altered layer that was more than twice as thick as that of Comparative Example 1, but the brightness of the CL image was even lower than that of Example 2, and damage was significantly reduced. Assuming that the damage shown in Figure 10(b) is 1, the damage in Figure 10(b) was 0.1. It was also found that the depth of latent scratches was further reduced. [Explanation of symbols]

[0083] 10 Cathodoluminescence (CL) device 20 Electron beam irradiation device (electron beam generator) 21 Electron beam 22 CL light 30 Sample stage 40 (CL light) detector 50 Calculation device (control device) 60 boards 100 Substrate surface 110, 130 150 Thickness of the main altered layer 120, 140 160 Depth of latent wound

Claims

1. A method for processing the surface of a GaN substrate by grinding and polishing, comprising: a high-grit grinding step of grinding the surface of the GaN substrate with a grinding wheel having a grit size of #6000 or more; a CMP polishing step of polishing the surface of the GaN substrate by CMP after grinding the surface of the GaN substrate by the high-grit grinding step; Equipped with In the high-grit grinding step, a process-affected layer composed of a primary affected layer and latent scratches is formed on the surface of the GaN substrate, The process-affected layer has a thickness of the main affected layer of 700 nm or more and 1000 nm or less, and a depth of the latent scratches of 1000 nm or more and 2000 nm or less, and a surface roughness Ra of the GaN substrate after the high-grit grinding step and before the CMP polishing step is 1.0 nm or more and 1.8 nm or less; the primary affected layer is a layer having a thickness determined by multiplying a polishing time and a polishing rate at which the brightness of the cathode luminescence image of an area other than the latent scratches becomes equal to the brightness of an as-grown crystal measured in advance, using a cathode luminescence image, in the CMP polishing step; The latent scratches are scratches having a depth determined by multiplying the polishing time of the CMP polishing step by the polishing rate. GaN substrate surface processing method characterized by:

2. Before the high-grit grinding step, a rough grinding step of grinding the GaN substrate with a grinding wheel having a grit size of less than #6000; or 2. The method for processing the surface of a GaN substrate according to claim 1, further comprising a mechanical polishing step of polishing the GaN substrate with free abrasive grains having an average grain size exceeding 0.5 μm.

3. Furthermore, a cleaning step of cleaning the GaN substrate after the CMP polishing step.

3. The method for processing a surface of a GaN substrate according to claim 1, comprising:

4. 4. The method for processing the surface of a GaN substrate according to claim 1, wherein the grinding stone used in the high-grit grinding step has a grit size of greater than #8000.

5. 5. The method for processing the surface of a GaN substrate according to claim 1, wherein the grinding wheel is bonded with vitrified cement.

6. A method for manufacturing a GaN substrate, comprising the method for processing the surface of a GaN substrate according to any one of claims 1 to 5.

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