Ceramic substrates and composite substrates

Abstract

A ceramic substrate and a composite substrate capable of suppressing granulation are provided. According to an embodiment of the present invention, the ceramic substrate comprises an aluminum nitride sintered body. The aluminum nitride sintered body has a plurality of first pores. On the surface of the ceramic substrate, the maximum length of each of the plurality of first pores is less than 0.5 μm.

Description

Technical Field

[0001] This invention relates to ceramic substrates and composite substrates. Prior Technology

[0002] In recent years, researchers have been exploring the application of GaN and other group III nitride films in various industrial products, such as power devices. Group III nitrides are typically manufactured by crystallization and growth on composite substrates with specific multilayer structures. At the core of such a composite substrate, it is known to be a ceramic substrate containing an aluminum nitride sintered body, which has undergone appropriate surface processing. However, once the ceramic substrate is surface-processed, ceramic particles sometimes fall off the surface of the ceramic substrate (hereinafter referred to as granulation), forming multiple pores (voids) on the surface of the ceramic substrate. Therefore, a method has been proposed to bury and planarize the pores on the surface of a ceramic substrate (see Patent Document 1). In the method described in Patent Document 1, a barrier layer is first sealed into a ceramic substrate having the front side, and then a bonding layer is formed bonded to the barrier layer. Subsequently, a portion of the bonding layer is removed to expose at least a portion of the barrier layer to define a filling region, and a second bonding layer is deposited on the exposed barrier layer and at least a portion of the filling region. [Preliminary Technology Documents] [Patent Literature]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-524615 Summary of the Invention

[0004] [The problem the invention aims to solve] However, in the composite substrate manufactured using the method described in Patent Document 1, the bonding layer buries the pores existing on the surface of the ceramic substrate, resulting in a spot-like appearance with inconsistent bonding layer thickness. Since the thermal conductivity of the bonding layer material differs from that of the ceramic substrate, variations in the heat transfer coefficient from the interior of the composite substrate to its surface may occur due to the inconsistent bonding layer thickness. Consequently, in the manufacture of a Group III nitride film using this composite substrate, there is a risk of surface temperature variations in the composite substrate due to variations in the heat transfer coefficient. This variation in the composition of the manufactured Group III nitride film can lead to problems such as variations in the characteristics of devices using the Group III nitride film. To suppress such property variations, there is a strong desire for a ceramic substrate with suppressed grain removal. The main objective of this invention is to provide ceramic substrates and composite substrates that can suppress de-granulation. [Methods used to solve problems]

[0005] [1] According to an embodiment of the present invention, a ceramic substrate comprises an aluminum nitride sintered body. The aluminum nitride sintered body has a plurality of first pores. On the surface of the ceramic substrate, the maximum length of each of the plurality of first pores is less than 0.5 μm. [2] In the ceramic substrate described in [1] above, the aluminum nitride sintered body may also have a plurality of second pores. On the surface of the ceramic substrate, the maximum length of each of these plurality of second pores is 0.5 μm or more but less than 1.5 μm. [3] In the surface of the ceramic substrate described in [1] or [2] above, the proportion of the number of the first pores per unit area can also satisfy the following formula (1): 0.5≦N1 / (N1+N2+N3)...(1) (In formula (1), N1 represents the number of first pores with a maximum length of less than 0.5 μm per unit area, N2 represents the number of second pores with a maximum length of more than 0.5 μm but less than 1.5 μm per unit area, and N3 represents the number of third pores with a maximum length of more than 1.5 μm per unit area). [4] On the surface of the ceramic substrate described in [2] or [3] above, the proportion of the total number of the first pores and the second pores per unit area can also satisfy the following formula (2): 0.8≦(N1+N2) / (N1+N2+N3)...(2) (In formula (2), N1 represents the number of first pores with a maximum length of less than 0.5 μm per unit area, N2 represents the number of second pores with a maximum length of more than 0.5 μm but less than 1.5 μm per unit area, and N3 represents the number of third pores with a maximum length of more than 1.5 μm per unit area). [5] In the surface of the ceramic substrate described in any of [2] to [4] above, the proportion of the total area of ​​the plurality of first pores and the plurality of second pores may also be 0.0001% or more. [6] In the surface of the ceramic substrate described in any of [2] to [5] above, the proportion of the total area of ​​the plurality of first pores and the plurality of second pores may also be less than 5%. [7] In any of the ceramic substrates described in [1] to [6] above, the aluminum nitride sintered body may also contain a plurality of aluminum nitride grains. The average grain size of these plurality of aluminum nitride grains may also be 3 μm or less. [8] In any of the ceramic substrates described in [1] to [7] above, the aluminum nitride sintered body may also include a plurality of aluminum nitride grains, and more than 50% of the plurality of first pores may also be located inside the aluminum nitride grains. [9] In any of the ceramic substrates described in [2] to [6] above, the aluminum nitride sintered body may also include a plurality of aluminum nitride grains, and more than 50% of the plurality of second pores may also be located inside the aluminum nitride grains.

[10] In the surface of the ceramic substrate described in any of [1] to [9] above, the periphery of the first pore may also have a circular or elliptical shape.

[11] In the surface of the ceramic substrate described in any of [2] to [6] above, the periphery of the second pore may also have a circular or elliptical shape.

[12] In any of the above [1] to

[11] , the ceramic substrate in the above aluminum nitride sintered body may also have the following contents: the content of metal elements other than Al is less than 0.1% by mass when converted to oxides, and the content of carbon is less than 0.1% by mass.

[13] In any of the ceramic substrates described in [1] to

[12] above, the lightness of the aluminum nitride sintered body as specified in JIS Z8781 may also be L*40 or less.

[14] According to another aspect of the present invention, a composite substrate comprises: a ceramic substrate as described in any one of [1] to

[13] above; and a design layer. The design layer is deposited on the surface of the ceramic substrate. The design layer contains Si. [Invention Benefits]

[0006] According to an embodiment of the present invention, a ceramic substrate in which threshing is suppressed and a composite substrate using the same can be realized. Simple Explanation of the Diagram

[0007] Figure 1 is a schematic cross-sectional view of a ceramic substrate according to an embodiment of the present invention. Figure 2 is a schematic plan view of the ceramic substrate in Figure 1. Figure 3 is a schematic diagram of a composite substrate having the ceramic substrate shown in Figure 1. Figure 4 is a scanning electron microscope (SEM) photograph of the polished surface of the ceramic substrate of Example 1, at a magnification of 2000x. Figure 5 is a scanning electron microscope (SEM) photograph of the polished surface of the ceramic substrate of Example 1, at a magnification of 10,000. Figure 6 is a scanning electron microscope (SEM) photograph of the polished surface of the ceramic substrate of Comparative Example 1, at a magnification of 2000x. Figure 7 is a scanning electron microscope (SEM) photograph of the polished surface of the ceramic substrate of Comparative Example 1, at a magnification of 10,000. Implementation

[0008] [The form in which the invention is carried out] The following description focuses on embodiments of the present invention, but the present invention is not limited to these embodiments. Furthermore, in order to make the explanation clearer, the drawings may sometimes schematically represent the width, thickness, shape, etc., of various parts compared to the embodiments; however, this is merely an example and should not limit the interpretation of the present invention.

[0009] A. Overview of Ceramic Substrates Figure 1 is a schematic cross-sectional view of a ceramic substrate according to an embodiment of the present invention; Figure 2 is a schematic plan view of the ceramic substrate in Figure 1. Furthermore, Figure 2 shows an enlarged view of a portion of the surface of the ceramic substrate (the portion enclosed in a circle).

[0010] As shown in Figures 1 and 2, in one embodiment, the ceramic substrate 1 includes an aluminum nitride sintered body (hereinafter referred to as an AlN sintered body). The AlN sintered body is typically characterized by having a polycrystalline structure and containing a plurality of aluminum nitride grains (hereinafter referred to as AlN grains) 14. Adjacent AlN grains 14 among the plurality of AlN grains 14 form interlocking grain boundaries. The AlN sintered body contains a plurality of first pores 11. When the AlN sintered body is processed into a ceramic substrate 1, the plurality of first pores 11 are exposed on the surface of the ceramic substrate 1 (i.e., the polished surface 1a). On the surface of the ceramic substrate 1 (polished surface 1a), the maximum length of each of the plurality of first pores 11 is less than 0.5 μm. The inventors of this invention discovered that pores present on the surface of a ceramic substrate can affect granulation in the ceramic substrate. Therefore, through careful study of the configuration and size of the pores, they found that granulation in the ceramic substrate can be suppressed by having pores of a specific size present beneath the surface of the ceramic substrate. Specifically, multiple first pores with a maximum length of less than 0.5 μm exist below the surface of the ceramic substrate. This can mitigate the micro-thermal strain that occurs at the grain boundaries due to heat generated during surface processing of the ceramic substrate (represented by polishing and precision grinding), and can suppress the occurrence and growth of cracks in the grain boundaries of AlN grains. In addition, the maximum length of the pores is determined, for example, by observing the surface of the ceramic substrate using a scanning electron microscope (SEM).

[0011] In one embodiment, the AlN sintered body also has a plurality of second pores 12. On the surface of the ceramic substrate 1 (polished surface 1a), the maximum length of each of the plurality of second pores 12 is more than 0.5 μm but less than 1.5 μm. Multiple secondary pores with a maximum length of 0.5 μm to less than 1.5 μm exist beneath the surface of the ceramic substrate. These pores reduce the Young's modulus without decreasing the strength of the sintered body, thereby reducing the processing pressure during surface finishing of the ceramic substrate (such as polishing and precision grinding). Therefore, the pressure applied to the grain boundaries is also reduced by lowering the processing pressure, which can suppress the occurrence and growth of cracks at the grain boundaries of AlN grains.

[0012] In addition to the first pores 11 and the second pores 12, the AlN sintered body may also include a third pore 13 that does not conform to the first pores 11 and the second pores 12. On the surface of the ceramic substrate 1 (polished surface 1a), the maximum length of the third pore 13 is 1.5 μm or more.

[0013] In one embodiment, the proportion of the number of first pores 11 per unit area on the surface (polished surface 1a) of the ceramic substrate 1 satisfies the following formula (1). 0.3≦N1 / (N1+N2+N3)...(1) (In formula (1), N1 represents the number of first pores with a maximum length of less than 0.5 μm per unit area, N2 represents the number of second pores with a maximum length of more than 0.5 μm but less than 1.5 μm per unit area, and N3 represents the number of third pores with a maximum length of more than 1.5 μm per unit area.) Once the proportion of the number of first pores per unit area satisfies the above formula (1), the micro thermal strain that occurs at the grain boundary due to the heat generated during the surface processing of the ceramic substrate can be stabilized and mitigated. Furthermore, N1 / (N1+N2+N3) is preferably 0.4 or higher, more preferably 0.5 or higher, even better 0.6 or higher, and exceptionally preferably 0.8 or higher. On the other hand, N1 / (N1+N2+N3) is, for example, 1.0 or lower, or for example, 0.99 or lower. In particular, once the value is 0.5≦N1 / (N1+N2+N3), it can stabilize and mitigate the micro-thermal strain that occurs at the grain boundaries.

[0014] Furthermore, on the surface of the ceramic substrate 1 (polished surface 1a), the proportion of the total number of first pores 11 and second pores 12 per unit area is preferably such that it satisfies the following formula (2). 0.5≦(N1+N2) / (N1+N2+N3)...(2) (In formula (2), N1, N2 and N3 are the same as N1, N2 and N3 in (1) above.) Once the ratio of the total number of first and second pores per unit area satisfies the above formula (2), the Young's modulus can be sufficiently reduced without reducing the strength of the sintered body. Therefore, the processing pressure during surface processing of ceramic substrates can be reduced, and the occurrence and growth of cracks in the grain boundaries of AlN grains can be stabilized and suppressed. Furthermore, (N1+N2) / (N1+N2+N3) is preferably 0.6 or higher, more preferably 0.8 or higher, and even more preferably 0.9 or higher. On the other hand, (N1+N2) / (N1+N2+N3) is, for example, 1.0 or lower, or for example, 0.99 or lower. In particular, once 0.8≦(N1+N2) / (N1+N2+N3), the occurrence and growth of cracks in the grain boundaries can be stabilized and mitigated.

[0015] On the surface of the ceramic substrate 1 (polished surface 1a), the number N1 of the first pores 11 per unit area is, for example, 1×10⁵ pores / cm² to 1×10⁸ pores / cm², preferably 1×10⁶ pores / cm² to 1×10⁷ pores / cm². On the surface of the ceramic substrate 1 (polished surface 1a), the number N2 of the second pores 12 per unit area is, for example, 1×10⁴ pores / cm² to 1×10⁷ pores / cm², preferably 1×10⁵ pores / cm² to 1×10⁶ pores / cm². On the surface of the ceramic substrate 1 (polished surface 1a), the number N3 of the third pores 13 per unit area is, for example, 0 pores / cm2 to 1×106 / cm2, preferably 0 pores / cm2 to 1×105 / cm2.

[0016] On the surface (polished surface 1a) of such a ceramic substrate 1, the total area of ​​a plurality of first air holes 11 and a plurality of second air holes 12 accounts for more than 0.0001%, preferably more than 0.001%. When the proportion of the total area of ​​the first and second pores on the surface of the ceramic substrate is above such a lower limit, the occurrence and growth of cracks in the grain boundaries of AlN grains can be more stabilized and suppressed, and the Young's modulus of the ceramic substrate can be reduced. Once the Young's modulus of the ceramic substrate is reduced, the stress required for processing the ceramic substrate can be reduced, thus significantly reducing the occurrence of delamination during the processing of the ceramic substrate. On the other hand, the percentage of the total area of ​​the plurality of first pores 11 and second pores 12 on the surface of the ceramic substrate 1 is, for example, 5% or less, preferably 1% or less. Once the percentage of the total area of ​​the first pores and second pores on the surface of the ceramic substrate is below such an upper limit, the rigidity of the ceramic substrate can be sufficiently ensured. Furthermore, the proportion of the total area of ​​the plurality of first pores and second pores on the surface of the ceramic substrate is calculated, for example, by resolving an image obtained from observing the surface of the ceramic substrate using a scanning electron microscope (SEM).

[0017] B. Details of the ceramic substrate Next, we will explain the details of the ceramic substrate. The ceramic substrate 1 has any suitable shape according to its application. In the example shown, the ceramic substrate 1 has a circular plate shape. The thickness of the ceramic substrate 1 is, for example, 0.5 mm to 1.5 mm. The diameter of the ceramic substrate 1 is, for example, 75 mm to 350 mm, or more specifically, 125 mm to 350 mm, or even more specifically, 250 mm to 350 mm.

[0018] B-1. AlN sintered body As described above, the ceramic substrate 1 is composed of an AlN sintered body containing a plurality of AlN grains 14. The AlN sintered body contains an AlN crystalline phase. The average grain size of the plurality of AlN grains 14 is, for example, 10 μm or less, preferably 3 μm or less. On the other hand, the average grain size of the plurality of AlN grains is, for example, 0.5 μm or more, preferably 1 μm or more.

[0019] AlN sintered bodies essentially contain no metallic elements other than Al. The AlN content in AlN sintered bodies is, for example, 99.5% by mass or more, preferably 99.8% by mass or more. On the other hand, the upper limit of the AlN content in AlN sintered bodies is represented by 100% by mass. The percentage of metallic elements other than Al in AlN sintered bodies, expressed in oxide form, is preferably 1% by mass or less, and more preferably 0.1% by mass or less, and even more preferably 0.05% by mass or less. On the other hand, the lower limit of the percentage of metallic elements other than Al in AlN sintered bodies is typically 0.0001% by mass or more. When the content of metal elements other than Al in the AlN sintered body falls within a certain range, and the ceramic substrate is used as the core of the group III nitride film-forming composite substrate, the diffusion of metal elements other than Al to the design layer can be suppressed during group III nitride film formation. Therefore, the adverse effects on the manufactured group III nitride film can be reduced. In addition, the composition of AlN sintered bodies can be determined, for example, by using an X-ray diffraction (XRD) device.

[0020] Carbon can also be dissolved in the AlN crystalline phase. The carbon content in the AlN sintered body is preferably 0.5% by mass or less, and more preferably 0.1% by mass or less. On the other hand, the carbon content in the AlN sintered body is preferably 0.01% by mass or more, and more preferably 0.03% by mass or more. When the carbon content in the AlN sintered body is within a certain range, the first and second pores can be stably formed in the ceramic substrate, which can relatively stably suppress grain delamination in the ceramic substrate. Furthermore, the brightness of the AlN sintered body can be stably adjusted within the range described later.

[0021] The lightness of AlN sintered bodies, as specified in JIS Z8781, is preferably L*50 or less, and more preferably L*40 or less. On the other hand, the lower limit of the lightness of AlN sintered bodies, as specified in JIS Z8781, is representatively L*10. Once the lightness of the AlN sintered body is within such a range, when the ceramic substrate is applied to a composite substrate used to manufacture group III nitride films, the ceramic substrate can efficiently absorb light and be uniformly heated. Therefore, the effective temperature distribution on the surface of the ceramic substrate is reduced, and the compositional variation of the resulting group III nitride film can be minimized. In addition, the brightness of AlN sintered bodies is determined, for example, by using a spectrophotometer.

[0022] The thermal conductivity of the AlN sintered body at 20°C is calculated, for example, by measuring the thermal diffusivity using a laser flash method. The specific heat is taken as the property value of aluminum nitride, 753 J / kg·K. The thermal conductivity of the AlN sintered body is, for example, 70 W / mK to 120 W / mK, with 80 W / mK to 110 W / mK being preferred.

[0023] The coefficient of thermal expansion of AlN sintered bodies at 1000℃ is, for example, 5.0 ppm / ℃ to 6.0 ppm / ℃, preferably 5.5 ppm / ℃ to 5.8 ppm / ℃. Alternatively, the coefficient of thermal expansion of AlN sintered bodies is determined according to, for example, JIS R1618.

[0024] The porosity of AlN sintered bodies is, for example, 0.0001% to 5%, preferably 0.01% to 1%. The porosity of AlN sintered bodies can be obtained by subtracting the relative density from the theoretical density of AlN.

[0025] The relative density of the AlN sintered body is, for example, 95% to 99.9999%, preferably 99% to 99.99%. Furthermore, the relative density of the AlN sintered body is the bulk density of the ceramic substrate relative to the theoretical density. The bulk density of the ceramic substrate is determined, for example, according to JIS R1634.

[0026] B-2. Grinding surface The ceramic substrate 1 is prepared from the AlN sintered body by any suitable cutting method. More specifically, the ceramic substrate 1 is prepared by cutting the AlN sintered body into a general plate shape and then grinding the surface (the side in the thickness direction) by the grinding method described above. The arithmetic mean roughness Ra of the polished surface 1a of the ceramic substrate 1 is, for example, 0.1 nm to 100 nm, preferably 0.1 nm to 10 nm. In addition, the arithmetic mean roughness Ra is measured by a white interferometer according to, for example, ISO 25178.

[0027] B-3. ​​First pore On the polished surface 1a of the ceramic substrate 1, a plurality of first pores 11 are exposed as described above.

[0028] The maximum length of the plurality of first pores 11 is less than 0.5 μm, as mentioned above. The plurality of first pores 11 is representative of those with a maximum length of 0.1 μm to 0.3 μm. The proportion of the first pores 11 with a maximum length of 0.1μm to 0.3μm is, for example, 10% to 95%, with 30% to 90% being preferred when the total number of the first pores 11 is set to 100%.

[0029] A plurality of first pores 11 are disposed at arbitrary appropriate positions on the polished surface 1a of the ceramic substrate 1. More specifically, the plurality of first pores 11 are located inside the AlN grains 14, at the grain boundaries and / or the rounded edges of the adjacent AlN grains 14.

[0030] In one embodiment, the proportion of the first pores 11 located inside the AlN grain 14 among the plurality of first pores 11 is, for example, 40% or more, preferably 50% or more, and more preferably 60% or more. On the other hand, the proportion of the first pores 11 located inside the AlN grain 14 among the plurality of first pores 11 is, for example, 95% or less, preferably 90% or less. Once the proportion of the primary pores located inside the AlN grains is within such a range, the micro-thermal strain that occurs at the grain boundaries due to heat generated during surface processing of the ceramic substrate can be more stable and mitigated.

[0031] On the polished surface 1a of the ceramic substrate 1, the periphery of the first pore 11 has an arbitrary fit shape. Examples of the periphery shape of the first pore 11 viewed from the thickness direction of the ceramic substrate 1 include irregular shapes including arc portions, circular shapes whose entire periphery is composed of arc portions, and elliptical shapes. In one embodiment, on the polished surface 1a of the ceramic substrate 1, a plurality of first pores 11 include first pores 11 with circular peripheries and / or first pores 11 with elliptical peripheries. Once circular and / or elliptical first pores are present on the surface of the ceramic substrate, the occurrence of cracks in the ceramic substrate can be stably suppressed.

[0032] The plurality of first pores 11 located on the polishing surface 1a of the ceramic substrate 1 may have the same peripheral shape or different peripheral shapes.

[0033] On the polished surface 1a of the ceramic substrate 1, the average aspect ratio of the plurality of first pores 11 is, for example, 3 or less, preferably 2 or less, and more preferably 1.5 or less. Once the first pores with such an average aspect ratio are present on the polished surface of the ceramic substrate, the growth of cracks in the ceramic substrate can be stabilized and suppressed, and the delamination in the ceramic substrate can be stabilized and suppressed. On the other hand, the lower limit of the average appearance ratio of the plurality of first pores 11 on the surface of the ceramic substrate 1 is represented by 1. In addition, the average appearance ratio of the plurality of pores is calculated, for example, by resolving an image obtained from observing the surface of a ceramic substrate using a scanning electron microscope (SEM).

[0034] On the polished surface 1a of the ceramic substrate 1, the average circularity of the plurality of first pores 11 is, for example, 0.5 to 1.0, preferably 0.8 to 1.0. Once the average circularity of the first pores is within such a range, the growth of cracks in the ceramic substrate can be more stably suppressed. Furthermore, the average roundness of the plurality of pores is calculated, for example, by resolving an image obtained from observing the surface of a ceramic substrate using a scanning electron microscope (SEM).

[0035] B-4. Second pore In one embodiment, in addition to the plurality of first pores 11, a plurality of second pores 12 are exposed on the polished surface 1a of the ceramic substrate 1.

[0036] The maximum length of the plurality of second pores 12, as described above, is 0.5 μm or more but less than 1.5 μm. A representative plurality of second pores 12 includes second pores 12 with a maximum length of 0.8 μm to 1.0 μm. The proportion of the second pores 12 with a maximum length of 0.8μm to 1.2μm is, for example, 10% to 95%, with 30% to 90% being preferred when the total number of the second pores 12 is set to 100%.

[0037] A plurality of second pores 12 are disposed at arbitrary appropriate positions on the polished surface 1a of the ceramic substrate 1. More specifically, the plurality of second pores 12 are located inside the AlN grains 14, at the grain boundaries and / or the rounded edges of the adjacent AlN grains 14.

[0038] In one embodiment, among the plurality of second pores 12, the proportion of the second pores 12 located inside the AlN grains 14 is, for example, 40% or more, preferably 50% or more, and more preferably 60% or more. On the other hand, among the plurality of second pores 12, the proportion of the second pores 12 located inside the AlN grains 14 is, for example, 95% or less, preferably 90% or less. Once the proportion of the second pores located inside the AlN grains is within such a range, the Young's modulus can be reduced without reducing the strength of the sintered body, thereby allowing for a more stable reduction in the processing pressure during surface processing of the ceramic substrate.

[0039] On the polished surface 1a of the ceramic substrate 1, the periphery of the second pore 12 has an arbitrary fit shape. Examples of the periphery shape of the second pore 12 viewed from the thickness direction of the ceramic substrate 1 include irregular shapes including arc portions, circular shapes whose entire periphery is composed of arc portions, and elliptical shapes. In one embodiment, on the polished surface 1a of the ceramic substrate 1, a plurality of second pores 12 include second pores 12 with circular peripheries and / or second pores 12 with elliptical peripheries. Once circular and / or elliptical second pores are present on the surface of the ceramic substrate, the occurrence of cracks in the ceramic substrate can be stably suppressed.

[0040] The plurality of second pores 12 located on the polishing surface 1a of the ceramic substrate 1 may have the same peripheral shape or different peripheral shapes.

[0041] On the polished surface 1a of the ceramic substrate 1, the average aspect ratio of the plurality of second pores 12 is, for example, 3 or less, preferably 2 or less, and more preferably 1.5 or less. Once second pores with such an average aspect ratio are present on the polished surface of the ceramic substrate, the growth of cracks in the ceramic substrate can be stabilized and suppressed, and the delamination in the ceramic substrate can be stabilized and suppressed. On the other hand, on the surface of the ceramic substrate 1, the lower limit of the average appearance ratio of the plurality of second pores 12 is represented by 1.

[0042] On the polished surface 1a of the ceramic substrate 1, the average circularity of the plurality of second pores 12 is, for example, 0.5 to 1.0, preferably 0.8 to 1.0. Once the average circularity of the second pores is within such a range, the growth of cracks in the ceramic substrate can be more stably suppressed.

[0043] B-5. Third pore In addition to the plurality of first pores 11 and the plurality of second pores 12, a third pore 13 may also exist on the polished surface 1a of the ceramic substrate 1.

[0044] The maximum length of the third pore 13 is 1.5 μm or more, as mentioned above. The third pore 13 is typically located at the grain boundaries of adjacent AlN grains 14. Examples of the peripheral shape of the third pore 13 as viewed from the thickness direction of the ceramic substrate 1 include, for example, a generally elliptical shape with a high aspect ratio, a polygonal shape such as a triangle with low roundness, and an irregular shape including an arc portion with a radius of curvature of, for example, less than 0.2 μm.

[0045] C. Manufacturing method of ceramic substrate Next, a method for manufacturing a ceramic substrate according to one embodiment will be described. In one embodiment, the method for manufacturing a ceramic substrate includes: a mixing step, mixing an AlN source and a carbon source; a forming step, forming the raw material mixture obtained in the mixing step; a firing step, firing the formed body obtained in the forming step; a step of adjusting the sintered body obtained in the firing step to a desired thickness; and a grinding step, grinding the surface of the sintered body that has been adjusted to the desired thickness.

[0046] C-1. Mixing Steps In the mixing step, an AlN source and a carbon source are mixed to prepare a raw material mixture.

[0047] The AlN source is mainly composed of AlN. The AlN content in the AlN source is, for example, 99% to 99.8% by mass. AlN source regions contain aluminum oxide (Al2O3) in addition to AlN. Aluminum oxide is generated, for example, through the oxidation of the surface of AlN by oxygen and / or moisture in the atmosphere. The alumina content in the AlN source is, for example, 0.2% to 2.0% by mass, with 0.3% to 1.0% by mass being preferred.

[0048] AlN sources are typically in powder form. The average primary particle size of the powdered AlN source is, for example, 0.2 μm to 2 μm, preferably 0.5 μm to 1.5 μm.

[0049] Examples of carbon sources include resin materials such as phenolic resin and acrylic resin; and carbon materials such as carbon black. Carbon sources can be used alone or in combination. Among carbon sources, carbon materials are preferred, with carbon black being a particularly desirable option.

[0050] Carbon source, typically in powder form. The average primary particle size of the powdered carbon source is, for example, 0.02 μm to 0.5 μm, preferably 0.03 μm to 0.1 μm. Furthermore, when using a carbon source that is soluble in a solvent, the particle size is not particularly limited.

[0051] The amount of carbon source added, when converted to carbon composition relative to 100 parts by mass of AlN source, is preferably 0.01 parts by mass or more, and more preferably 0.03 parts by mass or more, and more preferably 0.05 parts by mass or more. On the other hand, the amount of carbon source added, when converted to carbon composition relative to 100 parts by mass of AlN source, is preferably 1 part by mass or less, and more preferably 0.5 parts by mass or less, and more preferably 0.3 parts by mass or less.

[0052] The mixing method can be either dry mixing or wet mixing. In one embodiment, wet mixing is performed as the mixing step.

[0053] No special restrictions were set on the environmental conditions during the mixing process. The mixing process was typically carried out at room temperature (23°C) and normal pressure (0.1 MPa). The mixing time can be set arbitrarily and appropriately. For example, the mixing time can be 1 hour to 10 hours. In the case of wet mixing, if it is necessary to dry the solvent used for mixing, spray drying can be used, or the dried powder can be sieved to adjust its particle size after vacuum drying.

[0054] C-2. Forming Steps Next, in the forming step, the raw material mixture is shaped into the desired plate shape using any suitable forming method. As forming methods, well-known methods such as dry pressure forming, scraper forming, extrusion, casting, and strip forming can be used, with dry pressure forming being a preferred example. The pressure in dry pressure forming is preferably, for example, 100 kgf / cm², but there are no particular limitations as long as the shape can be maintained. For example, powder can also be filled into a hot-pressurized mold. In this way, a molded body with the desired shape can be prepared.

[0055] C-3. Firing Steps Next, in the firing step, the shaped body is fired by any suitable method.

[0056] The sintering temperature in the firing process is not specifically limited, but for example, it is 1700℃~2200℃, with 1750℃~2050℃ being preferred. During the heating process of the firing step, it is best to maintain a temperature between 300 and 1700°C. This holding step allows the residual carbon content in the sintered body to be controlled within the desired range. The firing process is typically carried out in a nitrogen atmosphere, with an atmosphere pressure ranging from, for example, 0.11 MPa (absolute pressure) to 1.0 MPa (absolute pressure), preferably 0.15 MPa (absolute pressure) to 0.80 MPa (absolute pressure).

[0057] Any suitable firing method can be used in the firing step. As a firing method, the method of applying pressure and heating at the same time is preferred, such as hot pressure sintering and electric discharge plasma sintering. Hot pressure sintering is the preferred method. In hot pressurization firing, the formed body is typically placed in a hot pressurization mold (e.g., a graphite mold) and pressed under a predetermined pressure in the thickness direction, as described above. The pressure (applied pressure) used to obtain the molded substrate is, for example, 50 kgf / cm2 or more, preferably 100 kgf / cm2 or more, and more preferably 150 kgf / cm2 or more. However, as long as the shape can be maintained, it can also be filled into the hot pressurized mold in the form of powder.

[0058] In this way, sintered bodies with desired shapes can be prepared.

[0059] In this firing step, the firing conditions are appropriately adjusted according to the amount of carbon source added in the mixing step. This allows for the stable formation of the first pore 11 in the sintered body, and preferably allows for the further formation of the second pore 12. In cases where the AlN source used in the mixing step contains alumina, the calcination step is, for example, carried out a chemical reaction as shown in the following formulas (1) to (4). Al₂O₃ + 3C + N₂ → 2AlN + 3CO...(1) Al2O3 + C → Al2O2 + CO...(2) Al₂O₃ + 2C → Al₂O + 2CO...(3) Al2O3 + 3C → Al2 + 3CO...(4)

[0060] In particular, during the heating process, by adjusting the holding temperature and / or atmospheric pressure to the above range, the chemical reaction shown in the above formula (1) proceeds stably. The alumina contained in the AlN source reacts with the carbon and nitrogen from the carbon source to generate AlN. Therefore, by consuming the carbon from the carbon source, the carbon content in the molded body can be appropriately controlled. Furthermore, during the firing step, by adjusting the atmospheric pressure to the aforementioned range, the partial pressure of CO can be reduced. Therefore, the chemical reactions shown in equations (2) to (4) above can proceed smoothly. As a result, the alumina contained in the AlN source reacts with the carbon from the carbon source to smoothly generate CO gas. During the firing step, the sintering of AlN particles is underway, thus suppressing the emission of the generated CO gas from the sintered body. Therefore, CO gas can remain inside the sintered body, and the residual CO gas can stably form the aforementioned first pore, preferably allowing the further formation of the second pore 12. Furthermore, in the firing step, once hot pressurization firing is adopted and the pressure is adjusted to the above range, the sintering of AlN particles can be promoted, thus the first pore and the second pore can be formed more stably inside the sintered body.

[0061] C-4. Steps to adjust the sintered body to the desired thickness The resulting sintered body is machined to achieve the desired thickness. Examples of machining methods include combinations of surface grinding, cutting machine cutting, coring drilling, and wire saw cutting.

[0062] C-5. Grinding Steps Next, one or both sides of the sintered body in the thickness direction are ground using any suitable grinding method. As a grinding method, a combination of leveling process caused by sintering and mirror finish caused by polishing can be cited as an example. The above-mentioned ceramic substrate 1 is manufactured through the above methods.

[0063] D. Composite substrate Such a ceramic substrate 1 can be applied to any suitable industrial product. Examples of applications for the ceramic substrate 1 include, for instance, composite substrates for forming group III nitride films, and particularly, composite substrates for forming gallium nitride films.

[0064] As shown in Figure 3, the ceramic substrate 1 is particularly suitable for composite substrates used in gallium nitride film formation. The composite substrate 100 for gallium nitride film formation (hereinafter referred to as composite substrate 100) includes the above-mentioned ceramic substrate 1 and design layer 2.

[0065] The ceramic substrate 1 is used as the core of the composite substrate 100. Design layer 2 is deposited on at least the surface of ceramic substrate 1, typically the polished surface 1a of ceramic substrate 1. In the example shown, design layer 2 is configured to cover the entire surface of ceramic substrate 1. Design layer 2 contains Si. The first pores 11 and the second pores 12 present on the polished surface 1a of ceramic substrate 1 may also be filled with the material constituting design layer 2.

[0066] In the example diagram, layer 2 is designed as a first bonding layer 21. The first bonding layer 21 is configured to bond the ceramic substrate 1 to the conductive layer 22 described later. The first bonding layer 21 is typically composed of tetraethoxysilane.

[0067] In one embodiment, the composite substrate 100 includes a conductive layer 22, a second adhesive layer 23, a barrier layer 24, an embedded oxide layer 25, and a crystalline layer 26. The conductive layer 22 is typically made of polycrystalline silicon. In the example shown, the conductive layer 22 is configured to cover the entire first bonding layer 21. The second bonding layer 23 is configured to bond the conductive layer 22 and the barrier layer 24. The second bonding layer 23 is typically composed of tetraethoxysilane. In the example shown, the second bonding layer 23 is configured to cover the entire conductive layer 22. Barrier layer 24 is typically made of silicon nitride. In the example shown, barrier layer 24 is configured to cover the entire second bonding layer 23. The embedded oxide layer 25 is a portion of the barrier layer 24 located on the polished surface 1a of the ceramic substrate 1. The embedded oxide layer 25 is typically composed of silicon dioxide. The crystalline layer 26 is disposed on the embedded oxide layer 25. The crystalline layer 26 is typically composed of monocrystalline silicon. Examples of such composite substrates 100 include, for instance, the substrate structure described in Japanese Patent Application Publication No. 2019-523994, or the processed substrate structure described in Japanese Patent Application Publication No. 2018-533845. All descriptions in the aforementioned publications are incorporated herein by reference.

[0068] In such a composite substrate, a composite substrate 100 that significantly suppresses delamination is used as the core, thus improving the uniformity of the film thickness of the design layer (represented by the first adhesive layer). Therefore, the warpage dynamics of the composite substrate during heating can be stabilized, and the temperature uniformity across the substrate surface can be improved. As a result, the uniformity of the composition and / or film thickness of the Group III nitride film manufactured using the composite substrate can be improved, and the characteristic variation of the Group III nitride film can be reduced. [Example]

[0069] The present invention will now be described in detail with reference to embodiments and comparative examples, but the present invention should not be limited to these embodiments.

[0070] <<Example 1>> A raw material mixture was obtained by dry mixing 99.9 parts by mass of AlN powder (average primary particle size: 1.0 μm) and 0.1 parts by mass of carbon black powder (carbon source). The AlN powder contained 0.9% by mass of alumina. Next, the obtained raw material mixture is uniaxially pressurized to form a molded body with a circular plate shape. The uniaxial pressure is 200 kgf / cm². The diameter of the molded body is 380 mm, and the thickness of the molded body is 42 mm.

[0071] Next, the obtained molded body is fired using a hot pressurization method. More specifically, the molded body is first placed in a hot pressurization mold made of graphite and set in a hot pressurization furnace. Then, the pressure inside the hot pressurization furnace is reduced to below 8 Pa. Next, the molded body is heated to 1550°C while being pressurized in the thickness direction at a pressure of 15 kgf / cm2. Next, the pressure inside the hot pressurization furnace is increased to 0.25 MPa, and the molded body is pressurized in the thickness direction at a pressure of 200 kgf / cm2 for 2 hours. The temperature was further increased to 1800℃ and fired at 1800℃ for 2 hours. This yields a sintered body with a circular plate shape. The diameter of the sintered body is 380 mm, and the thickness of the sintered body is 20 mm.

[0072] The obtained sintered body is drilled using a core-taking machine to obtain a sintered body (intermediate processed body) with a diameter of 305 mm and a thickness of 20 mm. By cutting the sintered body (intermediate processed body) with a multi-wire processing machine, a plurality of sintered body substrates with a diameter of 305 mm and a thickness of about 1 mm are obtained. For the obtained sintered substrate with a thickness of about 1 mm, thickness control by a surface grinder, leveling by a polishing grinder, peripheral finishing by a peripheral processing machine, and mirror finishing by a polishing grinder are applied to obtain a ceramic substrate with a diameter of 300 mm and a thickness of 0.8 mm.

[0073] The above methods were used to manufacture a ceramic substrate. After confirming the ceramic substrate with a scanning electron microscope (SEM) at 10,000x magnification, three locations were selected from the confirmed first pore area, and images of each location were obtained. Figures 4 and 5 show SEM images of the polished surface of the ceramic substrate of Example 1. Next, the acquired image was binarized using image processing software (ImageJ). In this binarized image, the first pore with a maximum length of less than 0.5 μm, the second pore with a maximum length of more than 0.5 μm but less than 1.5 μm, and the third pore with a maximum length of more than 1.5 μm were identified. When the number of first pores per unit area was set as N1, the number of second pores per unit area as N2, and the number of third pores per unit area as N3, the percentage of the number of first pores per unit area (N1 / (N1+N2+N3)) was 0.8, and the percentage of the total number of first and second pores per unit area (N1+N2) / (N1+N2+N3) was 0.95. The values ​​of N1 / (N1+N2+N3) and (N1+N2) / (N1+N2+N3) are shown in Table 1. From this binarized image, the average maximum length, average appearance ratio, and average roundness of the first pore were calculated. The average maximum length of the first pore was 0.2 μm, the average appearance ratio of the first pore was 1.4, and the average roundness of the first pore was 0.9. From this binarized image, the average maximum length, average appearance ratio, and average roundness of the second pore were calculated. The average maximum length of the second pore was 0.9 μm, the average appearance ratio was 1.4, and the average roundness was 0.75. Here, maximum length refers to the distance (Feret's Diameter) between the farthest pixels connected by a straight line in the binarized region; appearance ratio refers to the ratio of the major axis to the minor axis of the best-fitting ellipse; and roundness refers to the value representing the degree of approximation to a circle, calculated as 4π × area / (circumference × circumference).

[0074] <<Example 2>> A ceramic substrate was manufactured in the same manner as in Example 1, except that 0.1 parts by mass of yttrium oxide, a known sintering aid for aluminum nitride, was added to the raw material mixture. The polished surface of the obtained ceramic substrate was analyzed in the same manner as in Example 1. For the polished surface of the ceramic substrate manufactured in Example 2, a first pore with a maximum length of less than 0.5 μm, a second pore with a maximum length of more than 0.5 μm but less than 1.5 μm, and a third pore with a maximum length of more than 1.5 μm were identified. The percentage of the number of first pores per unit area, N1 / (N1+N2+N3), is 0.6, and the percentage of the total number of first and second pores per unit area, (N1+N2) / (N1+N2+N3), is 0.8.

[0075] <<Example 3>> After pressurizing the hot pressurization furnace to 0.25 MPa, the pressure of the molded body was changed to 100 kgf / cm2 and the pressurization time of the molded body was changed to 1 hour, and the ceramic substrate was manufactured in the same manner as in Example 1. The polished surface of the obtained ceramic substrate was analyzed in the same manner as in Example 1. For the polished surface of the ceramic substrate manufactured in Example 3, a first pore with a maximum length of less than 0.5 μm, a second pore with a maximum length of more than 0.5 μm but less than 1.5 μm, and a third pore with a maximum length of more than 1.5 μm were identified. The percentage of the number of first pores per unit area, N1 / (N1+N2+N3), is 0.5, and the percentage of the total number of first and second pores per unit area, (N1+N2) / (N1+N2+N3), is 0.6.

[0076] <<Example 4>> After the pressure in the hot pressurization furnace is reduced to below 8 Pa, the molded body, which has been pressurized at a pressure of 15 kgf / cm2 in the thickness direction, is heated at 1650°C, and a ceramic substrate is manufactured in the same manner as in Example 1. The polished surface of the obtained ceramic substrate was analyzed in the same manner as in Example 1. For the polished surface of the ceramic substrate manufactured in Example 4, a first pore with a maximum length of less than 0.5 μm, a second pore with a maximum length of more than 0.5 μm but less than 1.5 μm, and a third pore with a maximum length of more than 1.5 μm were identified. The percentage of the number of first pores per unit area, N1 / (N1+N2+N3), is 0.4, and the percentage of the total number of first and second pores per unit area, (N1+N2) / (N1+N2+N3), is 0.8.

[0077] <<Example 5>> The ceramic substrate was manufactured in the same manner as in Example 1, except that "after the pressure in the hot pressurization furnace was reduced to below 8 Pa, the molded body was heated at 1650°C with a pressure of 15 kgf / cm2 in the thickness direction" and "after the pressure in the hot pressurization furnace was increased to 0.25 MPa, the pressure of the molded body was changed to 100 kgf / cm2 and the pressing time of the molded body was changed to 1 hour". The polished surface of the obtained ceramic substrate was analyzed in the same manner as in Example 1. For the polished surface of the ceramic substrate manufactured in Example 5, a first pore with a maximum length of less than 0.5 μm, a second pore with a maximum length of more than 0.5 μm but less than 1.5 μm, and a third pore with a maximum length of more than 1.5 μm were identified. The percentage of the number of first pores per unit area, N1 / (N1+N2+N3), is 0.4, and the percentage of the total number of first and second pores per unit area, (N1+N2) / (N1+N2+N3), is 0.6.

[0078] <<Comparative Example 1>> A ceramic substrate was manufactured in the same manner as in Example 1, except that 5 parts by mass of yttrium oxide were added to the raw material mixture. The polished surface of the obtained ceramic substrate was analyzed in the same manner as in Example 1. However, no first pore with a maximum length of less than 0.5 μm was identified on the polished surface of the ceramic substrate manufactured in Comparative Example 1. Figures 6 and 7 show SEM images of the polished surface of the ceramic substrate of Comparative Example 1. In Comparative Example 1, the percentage of the number of first pores per unit area, N1 / (N1+N2+N3), is 0, and the percentage of the total number of first and second pores per unit area, (N1+N2) / (N1+N2+N3), is 0.3.

[0079] [Table 1]

[0080] <Evaluation> As shown in Figures 4 to 7, after the ceramic substrates manufactured in the Examples and Comparative Examples were precision ground, they were observed with a scanning electron microscope. The desquamation rate of Comparative Example 1 was set to 100%, and the presence or absence of desquamation in the ceramic substrate was evaluated according to the following criteria. ◎: Threshing incidence rate is less than 5% ○: Threshing incidence rate is over 5% but less than 20%. △: Threshing incidence rate is over 20% but less than 50%. ×: Threshing rate above 50% It was found that, compared with the ceramic substrate of Comparative Example 1, the ceramic substrates of Examples 1-5 were smoother and degranulation was significantly suppressed. In particular, it is understood that, as clearly shown in Examples 1-3, once N1 / (N1+N2+N3) is 0.5 or higher, threshing can be effectively suppressed. It was also learned, as clearly shown in Examples 1, 2 and 4, that once (N1+N2) / (N1+N2+N3) is 0.8 or higher, threshing can be effectively suppressed. [Industry availability]

[0081] The ceramic substrate according to embodiments of the present invention can be used in various industrial products, and is particularly suitable for composite substrates used in the manufacture of group III nitride films.

[0082] 1: Ceramic substrate 1a: Grinding surface 2: Design Layer 11: First pore 12: Second pore 13: Third pore 14: AlN grains 21: First connecting layer 22: Conductive layer 23: Second connecting layer 24: Barrier Layer 25: Embedded oxide layer 26: Crystalline layer 100: Composite substrate

Claims

1. A ceramic substrate comprising an aluminum nitride sintered body, wherein the aluminum nitride sintered body has a plurality of first pores; wherein the content of metal elements other than Al in the aluminum nitride sintered body is less than 0.1% by mass when converted to oxides; and on the surface of the ceramic substrate, the maximum length of each of the plurality of first pores is less than 0.5 μm.

2. The ceramic substrate as described in claim 1, wherein the aforementioned aluminum nitride sintered body further has a plurality of second pores; on the surface of the aforementioned ceramic substrate, the maximum length of each of the aforementioned plurality of second pores is more than 0.5 μm but less than 1.5 μm.

3. The ceramic substrate as described in claim 2, wherein the proportion of the number of the first pores per unit area on the surface of the ceramic substrate satisfies the following formula (1): 0.5≦N1 / (N1+N2+N3)... (1) (In formula (1), N1 represents the number of first pores per unit area with a maximum length of less than 0.5μm, N2 represents the number of second pores per unit area with a maximum length of more than 0.5μm but less than 1.5μm, and N3 represents the number of third pores per unit area with a maximum length of more than 1.5μm).

4. The ceramic substrate as described in claim 2, wherein on the surface of the aforementioned ceramic substrate, the proportion of the total number of the aforementioned first pores and the aforementioned second pores per unit area satisfies the following formula (2): 0.8≦(N1+N2) / (N1+N2+N3)...(2) (In formula (2), N1 represents the number of first pores per unit area with a maximum length of less than 0.5μm, N2 represents the number of second pores per unit area with a maximum length of more than 0.5μm but less than 1.5μm, and N3 represents the number of third pores per unit area with a maximum length of more than 1.5μm).

5. The ceramic substrate as described in claim 2, wherein the total area of ​​the plurality of first pores and the plurality of second pores on the surface of the ceramic substrate accounts for more than 0.0001%.

6. The ceramic substrate as described in claim 2, wherein the total area of ​​the plurality of first pores and the plurality of second pores on the surface of the ceramic substrate accounts for less than 5%.

7. The ceramic substrate as described in claim 1, wherein the aforementioned aluminum nitride sintered body comprises a plurality of aluminum nitride grains, wherein the average grain size of the plurality of aluminum nitride grains is 3 μm or less.

8. The ceramic substrate as described in claim 1, wherein the aforementioned aluminum nitride sintered body comprises a plurality of aluminum nitride grains, and more than 50% of the aforementioned plurality of first pores are located inside the aforementioned aluminum nitride grains.

9. The ceramic substrate as described in claim 2, wherein the aforementioned aluminum nitride sintered body comprises a plurality of aluminum nitride grains, and more than 50% of the aforementioned plurality of second pores are located inside the aforementioned aluminum nitride grains.

10. The ceramic substrate as described in claim 1, wherein the periphery of the first pore on the surface of the ceramic substrate has a circular or elliptical shape.

11. The ceramic substrate as described in claim 2, wherein the periphery of the second pore on the surface of the ceramic substrate has a circular or elliptical shape.

12. The ceramic substrate as described in claim 1, wherein the carbon content in the aforementioned aluminum nitride sintered body is less than 0.1% by mass.

13. The ceramic substrate as described in claim 1, wherein the lightness of the aforementioned aluminum nitride sintered body is specified in JIS Z8781 as L*40 or less.

14. A composite substrate comprising: a ceramic substrate as described in any one of claims 1 to 13; and a design layer deposited on the surface of the ceramic substrate, the design layer comprising Si.

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