GAN substrate and surface processing method for GAN substrate

Abstract

To provide a technique capable of suppressing local reduction in flatness in a GaN substrate having a front surface and a rear surface.SOLUTION: A GaN substrate is a GaN substrate having a front surface and a back surface, wherein a work-affected layer is formed on the back surface, a surface roughness Ra of the back surface is more than 0.23 μm and less than 0.45 μ m, an in-plane standard deviation of a full width at half maximum of (0006) plane diffraction obtained by an X-ray rocking curve method, which correlates with a strain amount of the work-affected layer on the back surface, is less than 0.012, and an in-plane standard deviation of a surface roughness Ra of the back surface is less than 0.030.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] BACKGROUND 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, Patent Document 1).

[0003] Patent Document 1 discloses a semiconductor substrate in which the difference between the depth of the processing-induced strain layer on the front surface and the depth of the processing-induced 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. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 7421470 Summary of the Invention [Problem to be solved by the invention]

[0005] The surface roughness and processing distortion of a semiconductor substrate tend to affect parameters used to evaluate the substrate shape, such as SORI and BOW. In other words, if the surface roughness and processing distortion on the back surface of a semiconductor substrate vary significantly, areas will be created where the stress balance between the front surface and the back surface is locally disrupted. This local stress imbalance can create localized depressions or protrusions, potentially leading to a reduction in local flatness.

[0006] An object of the present disclosure is to provide a technique capable of suppressing local reduction in flatness in a GaN substrate having a front surface and a back surface. [Means for solving the problem]

[0007] A GaN substrate according to a first aspect 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.

[0008] In the GaN substrate of the first embodiment, 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 respective numerical ranges, the surface roughness and the amount of strain in the process-affected layer are each closer to uniformity 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 uniformity within the rear surface of the GaN substrate, for example, it is possible to maintain a stress balance between the front and rear surfaces of the GaN substrate compared to 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.

[0009] A GaN substrate according to a second aspect of the present disclosure is the GaN substrate according to the first aspect, wherein the front surface is formed by a process-affected layer that is thinner than the base material of the substrate or the process-affected layer that forms the back surface.

[0010] In the GaN substrate of the second embodiment, 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 influence of 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 localized reductions in flatness in the GaN substrate.

[0011] A surface processing method for a GaN substrate according to a third aspect of the present disclosure is 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 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.

[0012] In a third aspect of the surface processing method for a GaN substrate, 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 work-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 this grinding on the front and back surfaces of the GaN substrate, the surface roughness and the amount of strain in the work-affected layers within the front and back surfaces of the GaN substrate become closer to uniform. 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 work-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 work-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, the above-mentioned method for surface processing a GaN substrate makes it possible to suppress local decreases in the flatness of the GaN substrate even when the GaN substrate is subjected to a grinding process.

[0013] A surface processing method for a GaN substrate according to a fourth aspect of the present disclosure includes, in the GaN substrate according to the third aspect, performing a CMP process on the front surface after performing a grinding process in the grinding step.

[0014] In the fourth aspect of the surface processing method for a GaN substrate, after grinding in the grinding step, CMP is performed on the front surface, thereby reducing the thickness of the process-affected layer forming the front surface of the GaN substrate or removing the process-affected layer. 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. [Effects of the Invention]

[0015] 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. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a cross-sectional view of a GaN substrate according to one embodiment of the present disclosure. [Figure 2] FIG. 2 is a cross-sectional view of a GaN substrate before grinding according to one embodiment of the present disclosure. [Figure 3] FIG. 3 is a cross-sectional view of a GaN substrate before CMP processing according to one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] [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.

[0019] 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 called 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.

[0020] The front surface 20a of the GaN substrate 20 may be formed of a base material 22, or may be formed of a process-affected layer (not shown). When the front surface 20a is formed of 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, the front surface 20a is formed of a base material 22, as shown in FIG.

[0021] Furthermore, the back 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.

[0022] It is more preferable that the in-plane standard deviation of the half-width correlated with the amount of strain in the damaged layer 30 on the back surface 20b of the GaN substrate 20 is set to less than 0.006, and the in-plane standard deviation of the surface roughness Ra is set to less than 0.015.

[0023] The surface roughness Ra can be measured using, for example, a laser microscope, an atomic force microscope, or the like.

[0024] 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.

[0025] [GaN substrate surface processing method] Next, a method for processing the surface of a GaN substrate according to an embodiment of the present disclosure will be described. The method for processing the surface of a GaN substrate according to this embodiment is a method for performing surface processing on a GaN substrate 18 prior to surface processing to obtain the above-described GaN substrate 20. Details of the method for processing the surface of a GaN substrate will be described below.

[0026] First, an ingot of semiconductor material is sliced ​​or otherwise processed into a disk-like shape to prepare a GaN substrate 18.

[0027] (Grinding process) Next, a grindstone is used to grind front surface 18a and back surface 18b of GaN substrate 18 shown in Fig. 2. Specifically, front surface 18a and back surface 18b are ground so that the surface roughness Ra of each of front surface 18a and back surface 18b of 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 strain in each of the process-affected layers 30, 32 forming front surface 18a and back surface 18b, respectively, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of each of front surface 18a and back surface 18b is less than 0.030.

[0028] Alternatively, the model number of a grinding wheel, the number of grinding passes of the grinding target surface of the grinding wheel, and the like may be determined in advance so that the surface roughness Ra of each of front surface 18a and back surface 18b of 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 process-affected layers 30, 32 forming front surface 18a and back surface 18b, respectively, is less than 0.012, and the in-plane standard deviation of the surface roughness Ra of each of front surface 18a and back surface 18b is less than 0.030, and front surface 18a and back surface 18b of GaN substrate 18 may be ground based on the determined information.

[0029] 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 side of the GaN substrate 18 becomes a damaged layer 32, and the back surface 18b side becomes a damaged layer 30.

[0030] In the GaN substrate 18, when 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.

[0031] (polishing process) After grinding the front surface 18a and 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, 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.

[0032] In this embodiment, the front surface 18a 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.

[0033] 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 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 amount of strain in the damage-affected layer 30 more uniform across the back surface 20b of the GaN substrate 20, it is possible to maintain a balance of stress between the front surface 20a and the back surface 20b of the GaN substrate 20, compared to a configuration in which the surface roughness and the amount of strain in the damage-affected layer 30 vary across the back surface 20b. This makes it possible to suppress localized reductions 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 (redistribution layers) and the like stacked on the front surface 20a of the GaN substrate 20.

[0034] 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.

[0035] 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 GaN substrate 20 in which front surface 18a and back surface 18b have been processed in this manner, the surface roughness and the amount of strain in the process-affected layer are closer to uniform within front surface 18a and back surface 18b, and therefore, compared to a configuration in which the surface roughness and the amount of strain in the process-affected layer vary within front surface 18a and back surface 18b, a stress balance is maintained between front surface 18a and back surface 18b of GaN substrate 20. As a result, the method for processing the surface of GaN substrate 20 makes it possible to suppress localized reduction in flatness of GaN substrate 20 even when GaN substrate 20 is subjected to a grinding process.

[0036] 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 the device film formed on GaN substrate 20 can be improved.

[0037] [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 distortion in the process-affected layer forming the back surface of the GaN substrate, and the surface roughness of the back surface were measured.

[0038] The strain amount of the process-affected layer forming the backside 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 strain amount. Note that the half-width correlated with the strain amount was measured using a thin film X-ray diffractometer manufactured by Panalytical. <Thin film X-ray diffraction instrument (instrument name: X-Pert MRD)> Optical system: Ge (440) 4-crystal monochromator Slit width: 10mm square X-ray tube: Current / Voltage: 40mA 45kV X-ray wavelength: 1.54Å Measurement crystal face: 0006 Measured axis: ω scan

[0039] 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 range: 300 μm square Measurement item: Arithmetic mean roughness Ra Microscope magnification: 50x

[0040] 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.

[0041] 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 calculated, 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."

[0042] [Table 1]

[0043] 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 stress balance is maintained between the front and rear surfaces, and therefore, a local decrease in the flatness of the GaN substrate is suppressed.

[0044] 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. [Explanation of symbols]

[0045] 18 GaN substrate 18a Front 18b back side 20 GaN substrate 20a Front 20b back side 22 Base material 30 Processing-affected layer 32 Processing-affected layer

Claims

1. A GaN substrate having a front surface and a back surface, a processing-affected layer is formed on the rear surface, The surface roughness Ra of the rear surface is greater 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 an X-ray rocking curve method, which correlates with the amount of strain in the process-affected layer on the back surface, is less than 0.012; The in-plane standard deviation of the surface roughness Ra of the back surface is less than 0.030; GaN substrate.

2. 2. The GaN substrate according to claim 1, wherein the front surface is formed of 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 grindstone, In the grinding step, the front surface and the back surface are ground 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 the (0006) plane diffraction obtained by an X-ray rocking curve method, which correlates 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. A method for processing the surface of a GaN substrate.

4. 4. The method for processing a surface of a GaN substrate according to claim 3, further comprising the step of: subjecting the front surface to CMP processing after the grinding processing in the grinding step.

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