GAN substrate and surface processing method of GAN substrate

Abstract

This GaN substrate has a front surface and a rear surface. A processing-affected layer is formed on the rear surface. The surface roughness Ra of the rear surface is more than 0.23 μm but less than 0.45 μm. An in-plane standard deviation of a half-value width of (0006) plane diffraction that is obtained by X-ray rocking curve analysis correlated with a distortion amount of the processing-affected layer on the rear surface is less than 0.012, and an in-plane standard deviation of the surface roughness Ra of the rear surface is less than 0.030.

Description

GaN substrate and method for processing the surface of a GaN substrate

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

[0002] 2. Description of the Related Art There is known a technique for performing a grinding process on the surface of a semiconductor substrate having a front surface and a back surface (for example, Japanese Patent No. 7421470).

[0003] Japanese Patent No. 7421470 discloses a semiconductor substrate in which the difference between the depth of the processing strain layer on the front surface and the depth of the processing strain layer on the back surface is 0.3 μm or more and 4.0 μm or less, and the arithmetic mean roughness Ra of the back surface of the semiconductor substrate is less than 0.3 nm.

[0004] The surface roughness and processing distortion of a semiconductor substrate tend to affect parameters for evaluating the shape of the substrate, such as SORI and BOW. In other words, if the surface roughness and processing distortion of the back surface of a semiconductor substrate vary significantly, areas where the stress balance between the back surface and the front surface is locally disrupted will occur. This local stress imbalance can cause localized depressions or protrusions, potentially leading to a local decrease in flatness.

[0005] An object of the present disclosure is to provide a technique capable of suppressing a local decrease in flatness in a GaN substrate having a front surface and a back surface.

[0006] A GaN substrate according to one embodiment of the present disclosure is a GaN substrate having a front surface and a back surface, wherein a processing-affected layer is formed on the back surface, the back surface has a surface roughness Ra of more than 0.23 μm and less than 0.45 μm, the in-plane standard deviation of the half-width of (0006) plane diffraction obtained by X-ray rocking curve analysis, which correlates with the amount of strain in the processing-affected layer on the back surface, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of the back surface is less than 0.030.

[0007] A surface processing method for a GaN substrate according to another aspect of the present disclosure is a surface processing method for a GaN substrate having a front surface and a back surface, the method including a grinding step of grinding the front surface and the back surface using a grinding wheel, wherein the front surface and the back surface are ground in the grinding step so that the surface roughness Ra of each of the front surface and the back surface is greater than 0.23 μm and less than 0.45 μm, the in-plane standard deviation of the half-width of (0006) plane diffraction obtained by X-ray rocking curve analysis, which correlates with the amount of strain in each of the process-affected layers forming the front surface and the back surface, respectively, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of each of the front surface and the back surface is less than 0.030.

[0008] As described above, according to the present disclosure, it is possible to prevent a local decrease in flatness in a GaN substrate having a front surface and a back surface.

[0009] 1 is a cross-sectional view of a GaN substrate according to an embodiment of the present disclosure, FIG 2 is a cross-sectional view of a GaN substrate according to an embodiment of the present disclosure before grinding, and FIG 3 is a cross-sectional view of a GaN substrate according to an embodiment of the present disclosure before CMP.

[0010] Hereinafter, embodiments for carrying out the present disclosure will be described with reference to the drawings. Components indicated by the same reference numerals in each drawing are the same or similar components. Note that duplicated explanations and reference numerals may be omitted in the embodiments described below. Furthermore, all drawings used in the following description are schematic, and the dimensional relationships, ratios, etc. of each element shown in the drawings do not necessarily match those in reality. Furthermore, the dimensional relationships, ratios, etc. of each element between multiple drawings do not necessarily match.

[0011] [GaN Substrate] The GaN substrate 20 according to one embodiment of the present disclosure is a compound semiconductor substrate made of a compound of gallium (Ga) and nitrogen (N) and having a crystalline structure. The GaN substrate 20 according to this embodiment is disk-shaped and has a front surface 20a and a back surface 20b. Note that the front surface 20a of the GaN substrate 20 is the surface on which, for example, an epitaxial film, a semiconductor element, etc. are formed, and the back surface 20b is the surface opposite the front surface 20a.

[0012] A process-affected layer 30 is formed on the back surface 20b of the GaN substrate 20. Here, the term "process-affected layer" refers to a layered region formed by processing the surface of the GaN substrate 20, resulting in a change in the material that forms the surface (the material that constitutes the GaN substrate, also referred to as the base material). For example, the boundary between the base material 22 of the GaN substrate 20 and the process-affected layer 30 can be determined using a transmission electron microscope.

[0013] The front surface 20a of the GaN substrate 20 may be formed from a base material 22, or may be formed from a process-affected layer (not shown). When the front surface 20a is formed from a process-affected layer, the thickness (layer thickness) of the process-affected layer forming the front surface 20a is made thinner than the thickness of the process-affected layer 30 forming the back surface 20b. In the GaN substrate 20 of this embodiment, as shown in FIG. 1 , the front surface 20a is formed from the base material 22.

[0014] Furthermore, the rear surface 20b of the GaN substrate 20 has a surface roughness Ra of more than 0.23 μm and less than 0.45 μm due to surface processing, and the in-plane standard deviation of the full width at half maximum (FWHM) of the (0006) plane diffraction obtained by X-ray rocking curve analysis (XRC), which correlates with the amount of strain in the process-affected layer 30, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra is less than 0.030. Note that the "amount of strain" referred to here is a physical quantity indicating the degree of disorder in the crystal structure in the process-affected layer, and is a numerical value that tends to correlate with the thickness of the process-affected layer.

[0015] It is more preferable that the rear surface 20b of the GaN substrate 20 has an in-plane standard deviation of the half-width, which correlates with the amount of strain in the damaged layer 30, set to less than 0.006 and an in-plane standard deviation of the surface roughness Ra set to less than 0.015.

[0016] The surface roughness Ra can be measured using, for example, a laser microscope, an atomic force microscope, or the like.

[0017] In this embodiment, the GaN substrate 20 has a disk-like shape (a circular shape when viewed from above), but the present disclosure is not limited to this configuration. For example, the GaN substrate 20 may have a polygonal shape when viewed from above.

[0018] [GaN substrate surface processing method] Next, a GaN substrate surface processing method according to an embodiment of the present disclosure will be described. The GaN substrate surface processing method according to this embodiment is a method for performing surface processing on a GaN substrate 18 before surface processing to obtain the above-described GaN substrate 20. Details of the GaN substrate surface processing method will be described below.

[0019] First, an ingot of semiconductor material is sliced ​​or otherwise processed into a disk-like GaN substrate 18 to prepare it.

[0020] (Grinding Step) Next, a grindstone is used to grind the front surface 18 a and the back surface 18 b of the GaN substrate 18 shown in Fig. 2. Specifically, the front surface 18 a and the back surface 18 b of the GaN substrate 18 are ground so that the surface roughness Ra of each of the front surface 18 a and the back surface 18 b is greater than 0.23 µm and less than 0.45 µm, the in-plane standard deviation of the half-width correlated with the amount of strain in each of the process-affected layers 30, 32 forming the front surface 18 a and the back surface 18 b is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of each of the front surface 18 a and the back surface 18 b is less than 0.030.

[0021] Alternatively, the model number of a grinding wheel, the number of grinding passes of the grinding target surface of the grinding wheel, and other information may be determined in advance so that the surface roughness Ra of each of the front surface 18 a and the back surface 18 b of the GaN substrate 18 is greater than 0.23 μm and less than 0.45 μm, the in-plane standard deviation of the half-width correlated with the amount of distortion of each of the process-affected layers 30, 32 forming the front surface 18 a and the back surface 18 b, respectively, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of each of the front surface 18 a and the back surface 18 b is less than 0.030, and the front surface 18 a and the back surface 18 b of the GaN substrate 18 may be ground based on the determined information.

[0022] As shown in FIG. 3, when the front surface 18a and the back surface 18b of the GaN substrate 18 are ground, the front surface 18a of the GaN substrate 18 becomes an affected layer 32, and the back surface 18b becomes an affected layer 30.

[0023] When the GaN substrate 18 satisfies the following conditions: the surface roughness Ra of each of the front surface 18a and the back surface 18b is greater than 0.23 μm and less than 0.45 μm; the in-plane standard deviation of the half-width, which correlates with the amount of strain in each of the process-affected layers 30, 32 forming the front surface 18a and the back surface 18b, is less than 0.012; and the in-plane standard deviation of the surface roughness Ra of each of the front surface 18a and the back surface 18b is less than 0.030, the stress and stress distribution occurring in the front surface 18a and the stress and stress distribution occurring in the back surface 18b are substantially the same. By suppressing the difference in stress occurring between the front surface 18a and the back surface 18b in this way, warping such as convex or concave portions is less likely to occur in part or all of the GaN substrate 18. This suppresses localized deterioration in the flatness of the GaN substrate 18.

[0024] (Polishing Step) After grinding the front surface 18a and the back surface 18b of the GaN substrate 18, the front surface 18a is subjected to CMP (Chemical Mechanical Polishing). This CMP removes the damaged layer 32 from the GaN substrate 18, thereby forming the GaN substrate 20 (see FIG. 1). Note that in the CMP of the front surface 18a, the damaged layer 32 may not be completely removed from the GaN substrate 18, and a portion of it may remain.

[0025] In this embodiment, the front surface 18 a of the GaN substrate 18 is subjected to CMP after the grinding process using a grindstone, but the present disclosure is not limited to this configuration. For example, another grinding or polishing process may be performed between the grinding process using a grindstone and the CMP process.

[0026] Next, the effects of this embodiment will be described. In GaN substrate 20 of this embodiment, rear surface 20b formed by process-affected layer 30 has a surface roughness Ra of more than 0.23 μm and less than 0.45 μm, an in-plane standard deviation of the half-width correlated with the amount of strain in process-affected layer 30 of less than 0.012, and an in-plane standard deviation of the surface roughness Ra of less than 0.030. By setting the surface roughness Ra, the in-plane standard deviation of the half-width correlated with the amount of strain, and the in-plane standard deviation of the surface roughness Ra to satisfy the above-mentioned respective numerical ranges, the surface roughness and the amount of strain in process-affected layer 30 become closer to uniform within rear surface 20b of GaN substrate 20. By making the surface roughness and the strain amount of the process-affected layer 30 more uniform within the back surface 20b of the GaN substrate 20 in this manner, a stress balance can be maintained between the front surface 20a and the back surface 20b of the GaN substrate 20, compared to, for example, a configuration in which the surface roughness and the strain amount of the process-affected layer 30 vary within the back surface 20b. This makes it possible to suppress localized decreases in flatness in the GaN substrate 20. As a result, it is possible to improve the quality of films (e.g., epitaxial films) stacked on the front surface 20a of the GaN substrate 20. It is also possible to improve the yield of wiring (rewiring layers) and the like stacked on the front surface 20a of the GaN substrate 20.

[0027] In the GaN substrate 20 of this embodiment, the front surface 20a is formed of a process-affected layer that is thinner than the base material 22 or the process-affected layer 30 that forms the back surface 20b. Therefore, the GaN substrate 20 is less susceptible to the influence of the front surface 20a on the stress balance, and it is easy to maintain the stress balance between the front surface 20a and the back surface 20b of the GaN substrate 20. This makes it easier to prevent the flatness of the GaN substrate 20 from decreasing locally.

[0028] In the GaN substrate surface processing method of this embodiment, the front surface 18a and the back surface 18b of the GaN substrate 18 are ground using a grindstone as described above. In this grinding process, the front surface 18a and the back surface 18b are ground so that the surface roughness Ra of each of the front surface 18a and the back surface 18b is greater than 0.23 μm and less than 0.45 μm, the in-plane standard deviation of the half-width, which correlates with the amount of strain in each of the work-affected layers forming the front surface 18a and the back surface 18b, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of each of the front surface 18a and the back surface 18b is less than 0.030. By performing this grinding process on the front surface 18a and the back surface 18b of the GaN substrate, the surface roughness and the amount of strain in the work-affected layers within the front surface 18a and the back surface 18b of the GaN substrate become closer to uniform. In the GaN substrate 20 whose front surface 18 a and back surface 18 b have been processed in this manner, the surface roughness and the amount of strain in the process-affected layer are closer to uniform within the front surface 18 a and back surface 18 b, and therefore, compared to a configuration in which the surface roughness and the amount of strain in the process-affected layer vary within the front surface 18 a and back surface 18 b, for example, a stress balance is maintained between the front surface 18 a and back surface 18 b of the GaN substrate 20. As a result, with the method for processing the surface of the GaN substrate 20, it is possible to suppress a local decrease in the flatness of the GaN substrate 20 even when the GaN substrate 20 is subjected to a grinding process.

[0029] Furthermore, in the GaN substrate surface processing method of the present embodiment, after grinding in the grinding step, CMP is performed on front surface 18a of GaN substrate 18, thereby reducing the thickness of or removing damage layer 32 that forms front surface 18a of GaN substrate 18. By performing CMP on front surface 18a of GaN substrate 18 in this manner, the quality of device films fabricated on GaN substrate 20 can be improved.

[0030] [Example] Next, the following test was conducted to verify the effects of the GaN substrate of this embodiment. GaN substrates sliced ​​from the same ingot were used in the test. In the test, the front and back surfaces of each GaN substrate were ground using grindstones with the model numbers shown in Table 1. After grinding, the front surface of each GaN substrate was subjected to CMP. Thereafter, the full width at half maximum (FWHM), which correlates with the amount of strain in the process-affected layer forming the back surface of the GaN substrate, and the surface roughness of the back surface were measured.

[0031] The amount of strain in the process-affected layer forming the back surface of the GaN substrate was determined by measuring the half-width of the (0006) plane diffraction in the X-ray rocking curve method, which correlates with the amount of strain. A thin film X-ray diffractometer manufactured by Panalytical was used to measure the half-width correlated with the amount of strain. <Thin film X-ray diffractometer (device name: X-Pert MRD)> Optical system: Ge (440) 4-crystal monochromator Slit width: 10 mm square X-ray tube: Current / voltage value: 40 mA 45 kV X-ray wavelength: 1.54 Å Measured crystal plane: 0006 Measured axis: ω scan

[0032] The surface roughness of the rear surface of the GaN substrate was measured using a laser microscope manufactured by Lasertec Corporation. <Laser microscope (device name: OPTELICS HYBRID+)> Measurement area: 300 μm square Measurement item: arithmetic mean roughness Ra Microscope magnification: 50x

[0033] The half-width and surface roughness, which correlate with the amount of strain, were measured at the center of the GaN substrate (including the center) and at any point on a concentric circle that is concentric with the GaN substrate and has a radius that is 80% of the radius of the GaN substrate. The substrate center in Table 1 is the center of the GaN substrate, and the substrate periphery is any point on the concentric circle. In this test, the measurement value at the substrate periphery is the average value of the values ​​measured at four points on the concentric circle. Here, the four points on the concentric circle are equidistant points.

[0034] After measuring the full width at half maximum, which correlates with the amount of strain in the process-affected layer forming the back surface of the GaN substrate, and the surface roughness of the back surface, the standard deviation of the full width at half maximum, which correlates with the amount of strain, and the standard deviation of the surface roughness were determined, as shown in Table 1. In the following, the standard deviation of the full width at half maximum, which correlates with the amount of strain, will be abbreviated to simply "standard deviation of strain amount."

[0035]

[0036] As shown in Table 1, in Examples 1 to 4, the standard deviation of the strain amount and the standard deviation of the surface roughness are smaller on the rear surface of the GaN substrate than in Comparative Examples 1 and 2. In this way, in Examples 1 to 4, a balance of stress is maintained between the front and rear surfaces, and therefore, a local decrease in the flatness of the GaN substrate is suppressed.

[0037] Although the embodiments of the present disclosure have been described above, these embodiments are merely examples and can be modified in various ways without departing from the spirit of the present disclosure. It goes without saying that the scope of the present disclosure is not limited to these embodiments.

[0038] The following additional notes are provided regarding the above-described embodiments.

[0039] (Supplementary Note 1) A GaN substrate having a front surface and a back surface, wherein a processing-affected layer is formed on the back surface, the back surface has a surface roughness Ra of more than 0.23 μm and less than 0.45 μm, the in-plane standard deviation of the half-width of (0006) plane diffraction obtained by X-ray rocking curve analysis, which correlates with the amount of strain in the processing-affected layer on the back surface, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of the back surface is less than 0.030.

[0040] In the GaN substrate of Supplementary Note 1, the rear surface of the GaN substrate formed by the process-affected layer has a surface roughness Ra of more than 0.23 μm and less than 0.45 μm, an in-plane standard deviation of the half-width of the (0006) plane diffraction obtained by X-ray rocking curve analysis, which correlates with the amount of strain in the process-affected layer, is less than 0.012, and an in-plane standard deviation of the surface roughness Ra is less than 0.030. By setting the surface roughness Ra, the in-plane standard deviation of the half-width correlated with the amount of strain in the process-affected layer, and the in-plane standard deviation of the surface roughness Ra to satisfy the above-mentioned respective numerical ranges, the surface roughness and the amount of strain in the process-affected layer are each closer to uniform within the rear surface of the GaN substrate. By making the surface roughness and the amount of strain in the process-affected layer closer to uniform within the rear surface of the GaN substrate in this way, it is possible to maintain a stress balance between the front and rear surfaces of the GaN substrate, compared to, for example, a configuration in which the surface roughness and the amount of strain in the process-affected layer vary within the rear surface. This makes it possible to prevent the flatness of the GaN substrate from being locally reduced.

[0041] (Supplementary Note 2) The GaN substrate according to Supplementary Note 1, wherein the front surface is formed of a base material of the substrate or a process-affected layer that is thinner than the process-affected layer that forms the back surface.

[0042] In the GaN substrate of Supplementary Note 2, the front surface is formed of a process-affected layer that is thinner than the base material of the substrate or the process-affected layer that forms the back surface. Therefore, the GaN substrate is less susceptible to the stress balance from the front surface, and it is easy to maintain the stress balance between the front and back surfaces of the GaN substrate. This makes it easier to prevent the flatness of the GaN substrate from decreasing locally.

[0043] (Supplementary Note 3) A surface processing method for a GaN substrate having a front surface and a back surface, comprising: a grinding step of grinding the front surface and the back surface using a grinding wheel, wherein the front surface and the back surface are ground in such a way that each of the front surface and the back surface has a surface roughness Ra of more than 0.23 μm and less than 0.45 μm, the in-plane standard deviation of the half-width of (0006) plane diffraction obtained by X-ray rocking curve analysis correlated with the amount of strain in each of the process-affected layers forming the front surface and the back surface is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of each of the front surface and the back surface is less than 0.030.

[0044] In the GaN substrate surface processing method of Supplementary Note 3, the front and back surfaces of the GaN substrate are ground using a grinding wheel. In this grinding, the front and back surfaces are ground so that the surface roughness Ra of each of the front and back surfaces is greater than 0.23 μm and less than 0.45 μm, the in-plane standard deviation of the half-width of the (0006) plane diffraction obtained by X-ray rocking curve analysis, which correlates with the amount of strain in each of the process-affected layers forming the front and back surfaces, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of each of the front and back surfaces is less than 0.030. By performing such grinding on the front and back surfaces of the GaN substrate, the surface roughness and the amount of strain in the process-affected layers within the front and back surfaces of the GaN substrate approach uniformity. In a GaN substrate whose front and back surfaces have been processed in this manner, the surface roughness and the amount of strain in the process-affected layer are closer to uniform within the front and back surfaces, and therefore, compared to a configuration in which the surface roughness and the amount of strain in the process-affected layer vary within the front and back surfaces, a stress balance between the front and back surfaces of the GaN substrate is maintained. As a result, with the above-mentioned method for surface processing of a GaN substrate, it is possible to suppress a local decrease in the flatness of the GaN substrate even when the GaN substrate is subjected to a grinding process.

[0045] (Supplementary Note 4) The surface processing method for a GaN substrate according to Supplementary Note 3, wherein after the grinding process is performed in the grinding step, the front surface is subjected to CMP processing.

[0046] In the GaN substrate surface processing method of Supplementary Note 4, since the front surface is subjected to CMP after grinding in the grinding step, the thickness of the process-affected layer forming the front surface of the GaN substrate is reduced or the process-affected layer is removed. By performing CMP on the front surface of the GaN substrate in this manner, the quality of the device film fabricated on the GaN substrate can be improved.

[0047] The disclosure of Japanese Patent Application No. 2024-100754, filed on June 21, 2024, is incorporated herein by reference in its entirety.

[0048] All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A GaN substrate having a front surface and a back surface, wherein a processing-affected layer is formed on the back surface, the surface roughness Ra of the back surface is greater than 0.23 μm and less than 0.45 μm, the in-plane standard deviation of the half-width of (0006) plane diffraction obtained by X-ray rocking curve analysis, which correlates with the amount of strain in the processing-affected layer on the back surface, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of the back surface is less than 0.

030.

2. The GaN substrate according to claim 1, wherein the front surface is formed from a base material of the substrate or a process-affected layer thinner than the process-affected layer forming the back surface.

3. A surface processing method for a GaN substrate having a front surface and a back surface, comprising a grinding step of grinding the front surface and the back surface using a grinding wheel, wherein the front surface and the back surface are ground in such a way that the surface roughness Ra of each of the front surface and the back surface is greater than 0.23 μm and less than 0.45 μm, the in-plane standard deviation of the half-width of (0006) plane diffraction obtained by X-ray rocking curve analysis, which correlates with the amount of distortion of each of the process-affected layers forming the front surface and the back surface, respectively, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of each of the front surface and the back surface is less than 0.

030.

4. The surface processing method for a GaN substrate according to claim 3, wherein after the grinding process is performed in the grinding step, the front surface is subjected to CMP processing.

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