WO2025205094A1 - Ceramic substrate and composite substrate - Google Patents

Abstract

To provide a ceramic substrate and a composite substrate capable of suppressing grain pull-out. A ceramic substrate according to an embodiment of the present invention contains 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

Ceramic and composite substrates

The present invention relates to ceramic substrates and composite substrates.

In recent years, the application of Group III nitride films such as GaN to various industrial products such as power devices has been studied. Group III nitride films are typically produced by growing crystals on composite substrates having specific layered structures.
It is known to employ, as the core of such a composite substrate, a ceramic substrate containing sintered aluminum nitride, which has been appropriately surface-treated.
However, when the surface of a ceramic substrate is processed, ceramic particles may fall off from the surface of the ceramic substrate (hereinafter referred to as "particle shedding"), resulting in the formation of multiple voids (air gaps) on the surface of the ceramic substrate.
Therefore, a method for filling voids on the surface of a ceramic substrate and planarizing it has been proposed (see Patent Document 1). In the method described in Patent Document 1, a ceramic substrate having a front surface is first encapsulated in a barrier layer, and then a bonding layer bonded to the barrier layer is formed. Then, a portion of the bonding layer is removed to expose at least a portion of the barrier layer and 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.

Special table 2019-524615 publication

However, in composite substrates manufactured by the method described in Patent Document 1, the bonding layer fills voids present on the surface of the ceramic substrate, resulting in variations in the thickness of the bonding layer, i.e., the presence of spot-like singularities. Because the thermal conductivity of the bonding layer material differs from that of the ceramic substrate, variations in the thickness of the bonding layer can cause variations in the heat transfer coefficient when viewed from the inside of the composite substrate. Consequently, when manufacturing a Group III nitride film using the composite substrate, variations in the heat transfer coefficient can cause variations in the surface temperature of the composite substrate. This can lead to variations in the composition of the manufactured Group III nitride film, resulting in variations in the characteristics of devices using the Group III nitride film. To suppress such variations in characteristics, a ceramic substrate with reduced grain shedding is highly desirable.
A primary object of the present invention is to provide a ceramic substrate and a composite substrate that can suppress grain shedding.

[1] A ceramic substrate according to an embodiment of the present invention includes an aluminum nitride sintered body having a plurality of first pores, each of which has a maximum length of less than 0.5 μm on the surface of the ceramic substrate.
[2] In the ceramic substrate according to the above [1], the aluminum nitride sintered body may further have a plurality of second pores, each of which has a maximum length of 0.5 μm or more and less than 1.5 μm on the surface of the ceramic substrate.
[3] In the surface of the ceramic substrate according to the above [1] or [2], the ratio of the number of the first pores per unit area may satisfy the following formula (1):
0.5≦N1/(N1+N2+N3)...(1)
(In formula (1), N1 represents the number of first pores per unit area having a maximum length of less than 0.5 μm; N2 represents the number of second pores per unit area having a maximum length of 0.5 μm or more but less than 1.5 μm; and N3 represents the number of third pores per unit area having a maximum length of 1.5 μm or more.)
[4] In the surface of the ceramic substrate according to [2] or [3] above, the ratio of the total number of the first pores and the second pores per unit area may satisfy 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 having a maximum length of less than 0.5 μm; N2 represents the number of second pores per unit area having a maximum length of 0.5 μm or more but less than 1.5 μm; and N3 represents the number of third pores per unit area having a maximum length of 1.5 μm or more.)
[5] In the surface of the ceramic substrate according to any one of [2] to [4] above, the ratio of the total area of the plurality of first pores and the plurality of second pores may be 0.0001% or more.
[6] In the surface of the ceramic substrate according to any one of [2] to [5] above, the proportion of the total area of the plurality of first pores and the plurality of second pores may be 5% or less.
[7] In the ceramic substrate according to any one of [1] to [6] above, the aluminum nitride sintered body may include a plurality of aluminum nitride crystal grains, and the average grain size of the plurality of aluminum nitride crystal grains may be 3 μm or less.
[8] In the ceramic substrate according to any one of [1] to [7] above, 50% or more of the plurality of first pores may be located inside the aluminum nitride crystal grains.
[9] In the ceramic substrate according to any one of [2] to [6] above, 50% or more of the plurality of second pores may be located inside the aluminum nitride crystal grains.
[10] In the surface of the ceramic substrate according to any one of [1] to [9] above, the periphery of the first pore may have a circular or elliptical shape.
[11] In the surface of the ceramic substrate according to any one of [2] to [6] above, the periphery of the second pore may have a circular or elliptical shape.
[12] In the ceramic substrate according to any one of [1] to [11] above, in the aluminum nitride sintered body, the content of metal elements other than Al may be 0.1 mass % or less, calculated as oxides, and the content of carbon may be 0.1 mass % or less.
[13] In the ceramic substrate according to any one of [1] to [12] above, the aluminum nitride sintered body may have a lightness, as defined in JIS Z8781, of L*40 or less.
[14] A composite substrate according to another aspect of the present invention includes the ceramic substrate according to any one of [1] to [13] above and an engineering layer. The engineering layer is laminated on a surface of the ceramic substrate. The engineering layer contains Si.

Embodiments of the present invention make it possible to realize a ceramic substrate in which grain shedding is suppressed, and a composite substrate using the same.

FIG. 1 is a schematic cross-sectional view of a ceramic substrate according to one embodiment of the present invention. FIG. 2 is a schematic plan view of the ceramic substrate of FIG. FIG. 3 is a schematic diagram of a composite substrate including the ceramic substrate of FIG. FIG. 4 is a scanning electron microscope (SEM) photograph of the polished surface of the ceramic substrate of Example 1, the SEM photograph being magnified 2000 times. FIG. 5 is a scanning electron microscope (SEM) photograph of the polished surface of the ceramic substrate of Example 1, the SEM photograph being magnified 10,000 times. FIG. 6 is an SEM photograph of the polished surface of the ceramic substrate of Comparative Example 1, the SEM photograph being taken at a magnification of 2000 times. FIG. 7 is a scanning electron microscope (SEM) photograph of the polished surface of the ceramic substrate of Comparative Example 1, the SEM photograph being magnified 10,000 times.

The following describes embodiments of the present invention, but the present invention is not limited to these embodiments. Furthermore, to clarify the explanation, the drawings may show the width, thickness, shape, etc. of each part more schematically than in the embodiments, but these are merely examples and do not limit the interpretation of the present invention.

A. Overview of Ceramic Substrate Fig. 1 is a schematic cross-sectional view of a ceramic substrate according to one embodiment of the present invention, and Fig. 2 is a schematic plan view of the ceramic substrate of Fig. 1. Note that Fig. 2 shows an enlarged view of a portion (encircled portion) of the surface of the ceramic substrate.

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 typically has a polycrystalline structure including a plurality of aluminum nitride crystal grains (hereinafter referred to as AlN crystal grains) 14. Adjacent AlN crystal grains 14 among the plurality of AlN crystal grains 14 are bonded to each other to form grain boundaries.
The AlN sintered body includes a plurality of first pores 11. When the AlN sintered body is processed into the ceramic substrate 1, the plurality of first pores 11 are exposed on the surface (i.e., the polished surface 1a) of the ceramic substrate 1. On the surface (polished surface 1a) of the ceramic substrate 1, the maximum length of each of the plurality of first pores 11 is less than 0.5 μm.
The present inventors have discovered that pores present on the surface of a ceramic substrate affect grain shedding in the ceramic substrate. As a result of extensive research into the arrangement and size of pores, they have found that grain shedding in a ceramic substrate can be suppressed by having pores of a specific size present on the surface of the ceramic substrate.
Specifically, the presence of multiple first pores with a maximum length of less than 0.5 μm on the surface of the ceramic substrate can alleviate micro-thermal strain that occurs at the grain boundaries due to heat generated during surface processing of the ceramic substrate (typically, lapping and precision polishing), and can suppress the occurrence and progression of cracks at the grain boundaries of AlN crystal grains.
The maximum length of the pores is measured, for example, by observing the surface of the ceramic substrate using a scanning electron microscope (SEM).

In one embodiment, the AlN sintered body further has a plurality of second pores 12. On the surface (polished surface 1a) of the ceramic substrate 1, the maximum length of each of the plurality of second pores 12 is not less than 0.5 μm and less than 1.5 μm.
The presence of a plurality of second pores having a maximum length of 0.5 μm or more and less than 1.5 μm on the surface of the ceramic substrate reduces the Young's modulus without reducing the strength of the sintered body, thereby enabling a reduction in the processing pressure during surface processing of the ceramic substrate (typically, lapping and precision polishing). As a result, the reduction in processing pressure also reduces the pressure applied to the grain boundaries, making it possible to suppress the occurrence and propagation of cracks at the grain boundaries of AlN crystal grains.

In addition to the first pores 11 and second pores 12, the AlN sintered body may further include third pores 13 that do not fall under the category of the first pores 11 or second pores 12. On the surface (polished surface 1a) of the ceramic substrate 1, the maximum length of the third pores 13 is 1.5 μm or more.

In one embodiment, the ratio 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 per unit area having a maximum length of less than 0.5 μm; N2 represents the number of second pores per unit area having a maximum length of 0.5 μm or more but less than 1.5 μm; and N3 represents the number of third pores per unit area having a maximum length of 1.5 μm or more.)
When the ratio of the number of first pores per unit area satisfies the above formula (1), microscopic thermal strains occurring at grain boundaries due to heat generation during surface processing of the ceramic substrate can be stably alleviated.
Furthermore, N1/(N1+N2+N3) is preferably 0.4 or more, more preferably 0.5 or more, even more preferably 0.6 or more, and particularly preferably 0.8 or more. On the other hand, N1/(N1+N2+N3) is, for example, 1.0 or less, or, for example, 0.99 or less.
In particular, when 0.5≦N1/(N1+N2+N3), microscopic thermal strains occurring at grain boundaries can be more stably alleviated.

Furthermore, on the surface (polished surface 1a) of the ceramic substrate 1, the ratio of the total number of the first pores 11 and the second pores 12 per unit area preferably 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 formula (1) above.)
When the ratio of the total number of first pores 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 AlN sintered body, which allows the processing pressure during surface processing of the ceramic substrate to be reduced and the occurrence and propagation of cracks at the grain boundaries of the AlN crystal grains to be stably suppressed.
Furthermore, (N1+N2)/(N1+N2+N3) is preferably 0.6 or more, more preferably 0.8 or more, and even more preferably 0.9 or more. On the other hand, N1/(N1+N2+N3) is, for example, 1.0 or less, or, for example, 0.99 or less.
In particular, when 0.8≦(N1+N2)/(N1+N2+N3), the occurrence and propagation of cracks at grain boundaries can be more stably alleviated.

On the surface (polished surface 1a) of the ceramic substrate 1, the number N1 of the first pores 11 per unit area is, for example, 1×10 ⁵ pores/cm ² to 1×10 ⁸ pores/cm ² , and preferably 1×10 ⁶ pores/cm ² to 1×10 ⁷ pores/cm ² .
On the surface (polished surface 1a) of the ceramic substrate 1, the number N2 of second pores 12 per unit area is, for example, 1×10 ⁴ pores/cm ² to 1×10 ⁷ pores/cm ² , and preferably 1×10 ⁵ pores/cm ² to 1×10 ⁶ pores/cm ² .
On the surface (polished surface 1a) of the ceramic substrate 1, the number N3 of the third pores 13 per unit area is, for example, 0/cm ² to 1×10 ⁶ /cm ² , and preferably 0/cm ² to 1×10 ⁵ /cm ² .

In the surface (polished surface 1a) of such ceramic substrate 1, the proportion of the total area of the plurality of first pores 11 and the plurality of second pores 12 is 0.0001% or more, preferably 0.001% or more.
When the ratio of the total area of the first pores and the second pores on the surface of the ceramic substrate is equal to or greater than this lower limit, the propagation of cracks at the grain boundaries of the AlN crystal grains can be more stably suppressed and the Young's modulus of the ceramic substrate can be reduced. Reducing the Young's modulus of the ceramic substrate can reduce the stress required for processing the ceramic substrate, thereby sufficiently reducing the occurrence of grain shedding during processing of the ceramic substrate.
On the other hand, the ratio of the total area of the plurality of first pores 11 and second pores 12 to the surface of the ceramic substrate 1 is, for example, 5% or less, preferably 1% or less. When the ratio of the total area of the first pores and second pores to the surface of the ceramic substrate is not more than this upper limit, the rigidity of the ceramic substrate can be sufficiently ensured.
The ratio 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 analyzing an image obtained by observing the surface of the ceramic substrate using an electron microscope (SEM: Scanning Electron Microscope).

B. Details of the Ceramic Substrate Next, the details of the ceramic substrate will be described.
The ceramic substrate 1 may have any appropriate shape depending on the application. In the illustrated example, the ceramic substrate 1 has a disk 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, for example, 125 mm to 350 mm, or, further, for example, 250 mm to 350 mm.

B-1. AlN Sintered Body As described above, the ceramic substrate 1 is made of an AlN sintered body containing a plurality of AlN crystal grains 14. The AlN sintered body contains an AlN crystal phase.
The average grain size of the plurality of AlN crystal grains 14 is, for example, 10 μm or less, preferably 3 μm or less, whereas the average grain size of the plurality of AlN crystal grains is, for example, 0.5 μm or more, preferably 1 μm or more.

The AlN sintered body does not substantially contain any metal elements other than Al. The AlN content in the AlN sintered body is, for example, 99.5 mass% or more, preferably 99.8 mass% or more. On the other hand, the upper limit of the AlN content in the AlN sintered body is typically 100 mass%.
The content of metal elements other than Al in the AlN sintered body is, for example, 1 mass % or less, preferably 0.1 mass % or less, and more preferably 0.05 mass % or less, calculated as oxides. On the other hand, the lower limit of the content of metal elements other than Al in the AlN sintered body is typically 0.0001 mass % or more.
When the content ratio of metal elements other than Al in the AlN sintered body is within this range, when the ceramic substrate is used as the core of a Group III nitride film-formed composite substrate, diffusion of metal elements other than Al into the engineering layer during the formation of the Group III nitride film can be suppressed, thereby reducing adverse effects on the Group III nitride film to be produced.
The composition of the AlN sintered body is measured, for example, by an X-ray diffraction (XRD) device.

The AlN crystalline phase may contain carbon as a solid solution. The carbon content in the AlN sintered body is, for example, 0.5 mass% or less, preferably 0.1 mass% or less. On the other hand, the carbon content in the AlN sintered body is, for example, 0.01 mass% or more, preferably 0.03 mass% or more.
When the carbon content in the AlN sintered body is within this range, the first pores and the second pores can be stably formed in the ceramic substrate, grain shedding in the ceramic substrate can be more stably suppressed, and the lightness of the AlN sintered body can be stably adjusted to the range described below.

The brightness of the AlN sintered body as defined in JIS Z8781 is, for example, L*50 or less, preferably L*40 or less. Meanwhile, the lower limit of the brightness of the AlN sintered body as defined in JIS Z8781 is typically L*10. When the brightness of the AlN sintered body is within this range, when the ceramic substrate is used in a composite substrate for producing a Group III nitride film, the ceramic substrate can efficiently absorb light and be uniformly heated. This reduces the effective temperature distribution on the surface of the ceramic substrate, thereby reducing the compositional variation of the formed Group III nitride film. As a result, the variation in the characteristics of devices to which the Group III nitride film is applied can be reduced.
The brightness of the AlN sintered body is determined, for example, by a spectrophotometer.

The thermal conductivity of an AlN sintered body at 20°C is calculated by measuring thermal diffusivity using, for example, the laser flash method. The physical property value of aluminum nitride, 753 J/kg·K, is used for the specific heat. The thermal conductivity of an AlN sintered body is, for example, 70 W/m·K to 120 W/m·K, and preferably 80 W/m·K to 110 W/m·K.

The thermal expansion coefficient of the AlN sintered body at 1000°C is, for example, 5.0 ppm/°C to 6.0 ppm/°C, and preferably 5.5 ppm/°C to 5.8 ppm/°C. The thermal expansion coefficient of the AlN sintered body is measured, for example, in accordance with JIS R1618.

The porosity of an AlN sintered body is, for example, 0.0001% to 5%, and preferably 0.01% to 1%. The porosity of an AlN sintered body can be calculated by subtracting the relative density from the theoretical density of AlN.

The relative density of the AlN sintered body is, for example, 95% to 99.9999%, and preferably 99% to 99.99%. The relative density of the AlN sintered body is the bulk density relative to the theoretical density of the ceramic substrate. The bulk density of the ceramic substrate is measured, for example, in accordance with JIS R1634.

B-2. Polished Surface The ceramic substrate 1 is prepared from the above-described AlN sintered body by any appropriate processing method. More specifically, the ceramic substrate 1 is prepared by cutting out an AlN sintered body into a substantially plate shape, and then polishing the surface (one surface in the thickness direction) by the above-described polishing method.
The arithmetic mean roughness Ra of the polished surface 1a of the ceramic substrate 1 is, for example, 0.1 nm to 100 nm, and preferably 0.1 nm to 10 nm. The arithmetic mean roughness Ra is measured, for example, in accordance with ISO 25178 using a white light interferometer.

B-3. First Pores As described above, the polished surface 1a of the ceramic substrate 1 has a plurality of first pores 11 exposed thereon.

As described above, the maximum length of the plurality of first pores 11 is less than 0.5 μm. The plurality of first pores 11 typically includes first pores 11 having a maximum length of 0.1 μm to 0.3 μm.
The proportion of the first pores 11 having a maximum length of 0.1 μm to 0.3 μm is, for example, 10% to 95%, and preferably 30% to 90%, when the total number of the first pores 11 is taken as 100%.

The multiple first pores 11 are arranged at any appropriate positions on the polished surface 1a of the ceramic substrate 1. More specifically, the multiple first pores 11 are located inside the AlN crystal grains 14, at the grain boundaries between adjacent AlN crystal grains 14, and/or on the circular ridges of the grain boundaries.

In one embodiment, the proportion of the first pores 11 located inside the AlN crystal grains 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 crystal grains 14 among the plurality of first pores 11 is, for example, 95% or less, and preferably 90% or less.
When the proportion of the first pores located inside the AlN crystal grains is within this range, microscopic thermal strains that occur at the grain boundaries due to heat generation during surface processing of the ceramic substrate can be more stably alleviated.

The peripheries of the first pores 11 have any appropriate shape on the polished surface 1 a of the ceramic substrate 1. Examples of the periphery shape of the first pores 11 as viewed in the thickness direction of the ceramic substrate 1 include irregular shapes including arc portions, circular shapes whose entire peripheries are arc portions, and elliptical shapes.
In one embodiment, the plurality of first pores 11 include first pores 11 having circular peripheries and/or first pores 11 having elliptical peripheries on the polished surface 1 a of the ceramic substrate 1. When 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.

The multiple first pores 11 located on the polished surface 1a of the ceramic substrate 1 may have the same peripheral shape as each other, or may have different peripheral shapes as each other.

On the polished surface 1 a of the ceramic substrate 1, the average aspect ratio of the plurality of first pores 11 is, for example, not more than 3, preferably not more than 2, and more preferably not more than 1.5. When the polished surface 1 a of the ceramic substrate has first pores having such an average aspect ratio, the propagation of cracks in the ceramic substrate can be stably suppressed, and grain shedding in the ceramic substrate can be stably suppressed.
On the other hand, the lower limit of the average aspect ratio of the plurality of first pores 11 on the surface of the ceramic substrate 1 is typically 1.
The average aspect ratio of the plurality of pores is calculated, for example, by analyzing an image obtained by observing the surface of the ceramic substrate using a scanning electron microscope (SEM).

The average circularity of the first pores 11 on the polished surface 1a of the ceramic substrate 1 is, for example, 0.5 to 1.0, and preferably 0.8 to 1.0. When the average circularity of the first pores is in this range, the propagation of cracks in the ceramic substrate can be more stably suppressed.
The average circularity of the plurality of pores is calculated, for example, by analyzing an image obtained by observing the surface of the ceramic substrate using a scanning electron microscope (SEM).

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

As described above, the maximum length of the plurality of second pores 12 is 0.5 μm or more and less than 1.5 μm. The plurality of second pores 12 typically includes second pores 12 having a maximum length of 0.8 μm to 1.0 μm.
The proportion of the second pores 12 having a maximum length of 0.8 μm to 1.2 μm is, for example, 10% to 95%, and preferably 30% to 90%, when the total number of the second pores 12 is taken as 100%.

The multiple second pores 12 are arranged at any appropriate positions on the polished surface 1a of the ceramic substrate 1. More specifically, the multiple second pores 12 are located inside the AlN crystal grains 14, at the grain boundaries between adjacent AlN crystal grains 14, and/or on the circular ridges of the grain boundaries.

In one embodiment, the proportion of the second pores 12 located inside the AlN crystal grains 14 among the plurality of second pores 12 is, for example, 40% or more, preferably 50% or more, and more preferably 60% or more. On the other hand, the proportion of the second pores 12 located inside the AlN crystal grains 14 among the plurality of second pores 12 is, for example, 95% or less, and preferably 90% or less.
When the proportion of the second pores located inside the AlN crystal grains is within this range, the Young's modulus is reduced without reducing the strength of the sintered body, thereby making it possible to more stably reduce the processing pressure during surface processing of the ceramic substrate.

The peripheries of the second pores 12 may have any appropriate shape on the polished surface 1 a of the ceramic substrate 1. Examples of the periphery shape of the second pores 12 as viewed in the thickness direction of the ceramic substrate 1 include irregular shapes including arc portions, circular shapes whose entire peripheries are arc portions, and elliptical shapes.
In one embodiment, the plurality of second pores 12 include second pores 12 having circular peripheries and/or second pores 12 having elliptical peripheries on the polished surface 1 a of the ceramic substrate 1. When 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 more stably suppressed.

The second pores 12 located on the polished surface 1a of the ceramic substrate 1 may have the same peripheral shape as each other, or may have different peripheral shapes from each other.

On the polished surface 1a of the ceramic substrate 1, the second pores 12 have an average aspect ratio of, for example, 3 or less, preferably 2 or less, and more preferably 1.5 or less. The presence of second pores having such an average aspect ratio on the polished surface of the ceramic substrate can more stably suppress the growth of cracks in the ceramic substrate and can more stably suppress grain shedding in the ceramic substrate.
On the other hand, the lower limit of the average aspect ratio of the second pores 12 on the surface of the ceramic substrate 1 is typically 1.

On the polished surface 1a of the ceramic substrate 1, the average circularity of the multiple second pores 12 is, for example, 0.5 to 1.0, and preferably 0.8 to 1.0. When the average circularity of the second pores is within this range, the propagation of cracks in the ceramic substrate can be more stably suppressed.

B-5. Third Pores In addition to the plurality of first pores 11 and the plurality of second pores 12, third pores 13 may be present on the polished surface 1a of the ceramic substrate 1.

As described above, the maximum length of the third pores 13 is 1.5 μm or more. The third pores 13 are typically located at the grain boundaries between the AlN crystal grains 14 adjacent to each other.
The peripheral shape of the third pores 13 when viewed from the thickness direction of the ceramic substrate 1 may be, for example, an approximately elliptical shape with a high aspect ratio, a polygonal shape such as a triangle with a low degree of circularity, or an irregular shape including an arc portion with a radius of curvature of, for example, less than 0.2 μm.

C. Manufacturing Method of Ceramic Substrate Next, a manufacturing method of a ceramic substrate according to one embodiment will be described. In one embodiment, the manufacturing method of a ceramic substrate includes a mixing step of mixing an AlN source and a carbon source, a molding step of molding the raw material mixture obtained in the mixing step, a firing step of firing the molded body obtained in the molding step, a step of adjusting the sintered body obtained in the firing step to a desired thickness, and a polishing step of polishing the surface of the sintered body adjusted to the desired thickness.

C-1. Mixing Step In the mixing step, the AlN source and the carbon source are mixed to prepare a raw material mixture.

The AlN source is mainly composed of AlN, and the content of AlN in the AlN source is, for example, 99 mass % to 99.8 mass %.
In addition to AlN, the AlN source typically contains aluminum oxide (Al ₂ O ₃ ), which is formed, for example, by oxidation of the surface of AlN under the influence of oxygen and/or moisture in the atmosphere.
The content of aluminum oxide in the AlN source is, for example, 0.2 mass % to 2.0 mass %, and preferably 0.3 mass % to 1.0 mass %.

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

Carbon sources include, for example, resin materials such as phenolic resins and acrylic resins; and carbon materials such as carbon black. Carbon sources may be used alone or in combination. Of the carbon sources, carbon materials are preferred, and carbon black is more preferred.

The carbon source is typically in powder form.
The powdered carbon source has an average primary particle size of, for example, 0.02 μm to 0.5 μm, and preferably 0.03 μm to 0.1 μm.
When a carbon source that can be dissolved in a solvent is used, the particle size is not particularly limited.

The amount of carbon source added, converted into carbon components per 100 parts by mass of AlN source, is, for example, 0.01 parts by mass or 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, converted into carbon components per 100 parts by mass of AlN source, is, for example, 1 part by mass or less, preferably 0.5 parts by mass or less, and more preferably 0.3 parts by mass or less.

The mixing method may be dry mixing or wet mixing. In one embodiment, wet mixing is performed in the mixing step.

The environmental conditions for the mixing step are not particularly limited. The mixing step is typically carried out at room temperature (23° C.) and atmospheric pressure (0.1 MPa).
The mixing time is set arbitrarily and appropriately, for example, 1 hour to 10 hours. When wet mixing is employed and it is necessary to dry the mixing solvent, a spray drying method may be used, or after carrying out a vacuum drying method, the dried powder may be passed through a sieve to adjust its particle size.

C-2. Forming Step Next, in the forming step, the raw material mixture is formed into a desired plate shape by any appropriate forming method.
As a molding method, for example, known methods such as dry press molding, doctor blade method, extrusion, casting, tape casting, etc. can be applied, and dry press molding is preferred. The pressure in dry press molding is preferably, for example, 100 kgf/ ^cm2 , but is not particularly limited as long as the shape can be maintained. For example, it is also possible to fill a hot press die in a powder state.
In this way, a molded body having a desired shape is prepared.

C-3. Firing Step Next, in the firing step, the molded body is typically fired by any appropriate firing method.

The sintering temperature in the firing step is not particularly limited, but is, for example, 1700°C to 2200°C, and preferably 1750°C to 2050°C.
During the temperature rise process in the firing step, it is preferable to maintain the temperature at 300 to 1700° C., and during this maintenance step, the amount of carbon remaining in the sintered body can be controlled within a desired range.
The firing step is typically carried out in a nitrogen atmosphere, and the range of the atmospheric pressure is, 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).

In the firing step, any appropriate firing method can be used. As the firing method, a method of heating while applying pressure is preferred, such as hot press sintering or spark plasma sintering, with hot press firing being preferred.
In hot press firing, typically, the compact is placed in a hot press die (for example, a graphite die) and fired as described above while being pressed in the thickness direction at a predetermined pressure.
The pressure (pressing pressure) for obtaining a molded substrate is, for example, 50 kgf/cm2 or ^more , preferably 100 kgf/ ^cm2 or more, and more preferably 150 kgf/ ^cm2 or more, but if the shape can be maintained, it is also possible to fill the hot press die in a powder state.

This produces a sintered body with the desired shape.

In such a firing step, the firing conditions are appropriately adjusted depending on the amount of the carbon source used in the mixing step, thereby stably forming the first pores 11 described above in the sintered body, and preferably also forming the second pores 12.
When the AlN source used in the mixing step contains aluminum oxide, the chemical reactions represented by the following formulas (1) to (4), for example, proceed in the firing step.
Al ₂ O ₃ +3C+N ₂ →2AlN+3CO...(1)
_Al2O3 +C _{→ Al2O2} + _CO ...(2)
_Al2O3 + _2C → _Al2O +2CO...(3)
_Al2O3 + _3C → _Al2 +3CO...(4)

In particular, by adjusting the holding temperature and/or atmospheric pressure within the above-mentioned range during the temperature rise process, the chemical reaction represented by the above formula (1) proceeds stably, and aluminum oxide contained in the AlN source reacts with carbon and nitrogen derived from the carbon source to produce AlN. As a result, carbon derived from the carbon source is consumed, and the carbon content in the molded body can be appropriately controlled.
Furthermore, in the sintering process, the CO partial pressure can be reduced by adjusting the atmospheric pressure within the above-mentioned range. Therefore, the chemical reactions represented by the above formulas (2) to (4) can proceed smoothly. As a result, aluminum oxide contained in the AlN source reacts with carbon derived from the carbon source to smoothly generate CO gas. Since the sintering of the AlN particles is progressing in the sintering process, the generated CO gas can be prevented from being discharged from the sintered body. Therefore, CO gas can be left inside the sintered body, and the remaining CO gas can stably form the first pores described above, and preferably, can also form the second pores 12.
Furthermore, if hot press firing is used in the firing process and the pressing pressure is adjusted to the above-mentioned range, the sintering of the AlN particles can be promoted, and the first pores and second pores 12 can be more stably formed inside the sintered body.

C-4. Step of adjusting the sintered body to the desired thickness The obtained sintered body is machined to the desired thickness.
Examples of the machining method include a combination of cutting using a surface grinder or a machining center, hollowing by coring, and slicing by a wire saw.

C-5 Polishing Step Next, one or both surfaces of the fired body in the thickness direction are polished by any appropriate polishing method.
As a polishing method, for example, a combination of flattening by lapping and mirror finishing by polishing can be mentioned.
In this manner, the ceramic substrate 1 is manufactured.

D. Composite Substrate Such a ceramic substrate 1 can be used in any suitable industrial product. Examples of applications of the ceramic substrate 1 include composite substrates for forming Group III nitride films, particularly composite substrates for forming gallium nitride films.

As shown in FIG. 3, the ceramic substrate 1 is particularly suitable for use as a composite substrate for forming a gallium nitride film.
The composite substrate 100 for gallium nitride film formation (hereinafter referred to as composite substrate 100 ) includes the above-described ceramic substrate 1 and an engineering layer 2 .

The ceramic substrate 1 is used as a core of the composite substrate 100 .
The engineering layer 2 is laminated on at least the surface of the ceramic substrate 1, typically on the polished surface 1a of the ceramic substrate 1. In the illustrated example, the engineering layer 2 is provided so as to cover the entire ceramic substrate 1. The engineering layer 2 contains Si. The first pores 11 and second pores 12 present on the polished surface 1a of the ceramic substrate 1 may be filled with a material that constitutes the engineering layer 2.

In the illustrated example, the engineering layer 2 is a first adhesive layer 21. The first adhesive layer 21 is configured to bond the ceramic substrate 1 to a conductive layer 22, which will be described later. The first adhesive layer 21 is typically made of tetraethyl orthosilicate.

In one embodiment, the composite substrate 100 comprises a conductive layer 22 , a second adhesive layer 23 , a barrier layer 24 , a BOX layer 25 , and a crystalline layer 26 .
The conductive layer 22 is typically made of polycrystalline silicon. In the illustrated example, the conductive layer 22 is provided so as to cover the entire first adhesive layer 21.
The second adhesive layer 23 is configured to bond the conductive layer 22 and the barrier layer 24. The second adhesive layer 23 is typically made of tetraethyl orthosilicate. In the illustrated example, the second adhesive layer 23 is provided so as to cover the entire conductive layer 22.
The barrier layer 24 is typically made of silicon nitride. In the illustrated example, the barrier layer 24 is provided so as to cover the entire second adhesive layer 23.
The BOX layer 25 is provided on a portion of the barrier layer 24 located on the polished surface 1a of the ceramic substrate 1. The BOX layer 25 is typically made of silicon dioxide.
The crystal layer 26 is provided on the BOX layer 25. The crystal layer 26 is typically made of single crystal silicon.
Examples of such a composite substrate 100 include the substrate structure described in JP-A-2019-523994 and the processed substrate structure described in JP-A-2018-533845, the entire disclosure of which is incorporated herein by reference.

In such a composite substrate, a ceramic substrate 1 with significantly reduced grain shedding is used as the core, which makes it possible to improve the uniformity of the film thickness of the engineering layer (typically the first adhesive layer). This makes it possible to stabilize the warping behavior of the composite substrate when heated, and to improve the temperature uniformity on the substrate surface. As a result, it is possible to improve the uniformity of the composition and/or film thickness of the Group III nitride film produced using the composite substrate, and reduce variation in the characteristics of the Group III nitride film.

The present invention will be explained in detail below using examples and comparative examples, but the present invention is not limited to these examples.

<<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 aluminum oxide content in the AlN powder was 0.9% by mass.
The resulting mixture was then uniaxially pressed to obtain a disk-shaped compact. The pressure during uniaxial pressing was 200 kgf/ ^cm2 . The diameter of the compact was 380 mm, and the thickness of the compact was 42 mm.

The resulting molded body was then fired by hot pressing.
More specifically, the compact was first placed in a hot press die made of graphite and set in a hot press furnace. The pressure in the hot press furnace was then reduced to 8 Pa or less. Next, the compact was heated to 1550°C while being pressed in the thickness direction at a pressure of 15 kgf/ ^cm2 .
Next, the pressure inside the hot press furnace was increased to 0.25 MPa, and the compact was pressed in the thickness direction at a pressure of 200 kgf/cm ² for 2 hours.
The temperature was then raised to 1800°C and fired at 1800°C for 2 hours.
This resulted in a disk-shaped sintered body having a diameter of 380 mm and a thickness of 20 mm.

The obtained sintered body was cored using a coring machine to obtain a sintered body (intermediate processed body) having a diameter of 305 mm and a thickness of 20 mm.
The obtained sintered body (intermediate processed body) was sliced by a multi-wire processing machine to obtain a plurality of sintered body substrates each having a diameter of 305 mm and a thickness of about 1 mm.
The resulting sintered body, which had a thickness of approximately 1 mm, was subjected to thickness reduction using a grinding machine, flattening using a lapping machine, outer periphery trimming using an outer periphery processing machine, and mirror finishing using a polishing machine to obtain a ceramic substrate with a diameter of 300 mm and a thickness of 0.8 mm.

In this way, a ceramic substrate was manufactured.
After observing the ceramic substrate at a magnification of 10,000 times using a scanning electron microscope (SEM), three locations were selected from the area where the first pores were observed, and images of each location were obtained. Figures 4 and 5 show SEM photographs of the polished surface of the ceramic substrate of Example 1.
The resulting image was then binarized using image processing software (ImageJ). In the binarized image, a small number of first pores with a maximum length of less than 0.5 μm, second pores with a maximum length of 0.5 μm or more but less than 1.5 μm, and third pores with a maximum length of 1.5 μm or more were observed. When the number of first pores per unit area is N1, the number of second pores per unit area is N2, and the number of third pores per unit area is N3, the ratio of the number of first pores per unit area N1/(N1+N2+N3) was 0.8, and the ratio of the total number of first pores and second pores per unit area (N1+N2)/(N1+N2+N3) was 0.95. Table 1 shows N1/(N1+N2+N3) and (N1+N2)/(N1+N2+N3).
The average maximum length, average aspect ratio, and average circularity of the first pores were calculated from the binarized image. The average maximum length of the first pores was 0.2 μm, the average aspect ratio of the first pores was 1.4, and the average circularity of the first pores was 0.9.
Furthermore, the average maximum length, average aspect ratio, and average circularity of the second pores were calculated from the binarized image. The average maximum length of the second pores was 0.9 μm, the average aspect ratio of the second pores was 1.4, and the average circularity of the second pores was 0.75.
Here, the maximum length is the distance (Feret's Diameter) drawn in a straight line connecting the farthest pixels in the binarized region, the aspect ratio is the ratio of the lengths of the major and minor axes of the best-fit ellipse, and the circularity is a value that represents the proximity to a circle and is calculated as 4π × area / (perimeter × perimeter).

<<Example 2>>
A ceramic substrate was produced in the same manner as in Example 1, except that 0.1 parts by mass of yttrium oxide, which is generally known as a 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. First pores having a maximum length of less than 0.5 μm, second pores having a maximum length of 0.5 μm or more but less than 1.5 μm, and third pores having a maximum length of 1.5 μm or more were confirmed on the polished surface of the ceramic substrate produced in Example 2.
The ratio of the number of first pores per unit area N1/(N1+N2+N3) was 0.6, and the ratio of the total number of first pores and second pores per unit area (N1+N2)/(N1+N2+N3) was 0.8.

<<Example 3>>
A ceramic substrate was manufactured in the same manner as in Example 1, except that the pressure applied to the molded body was changed to 100 kgf/ ^cm2 and the pressing time for the molded body was changed to 1 hour while the pressure inside the hot press furnace was increased to 0.25 MPa.
The polished surface of the obtained ceramic substrate was analyzed in the same manner as in Example 1. First pores having a maximum length of less than 0.5 μm, second pores having a maximum length of 0.5 μm or more but less than 1.5 μm, and third pores having a maximum length of 1.5 μm or more were confirmed on the polished surface of the ceramic substrate produced in Example 3.
The ratio of the number of first pores per unit area N1/(N1+N2+N3) was 0.5, and the ratio of the total number of first pores and second pores per unit area (N1+N2)/(N1+N2+N3) was 0.6.

<<Example 4>>
A ceramic substrate was manufactured in the same manner as in Example 1, except that the pressure inside the hot press furnace was reduced to 8 Pa or less, and the compact was pressed at a pressure of 15 kgf/ ^cm2 in the thickness direction and heated at 1650°C.
The polished surface of the obtained ceramic substrate was analyzed in the same manner as in Example 1. First pores having a maximum length of less than 0.5 μm, second pores having a maximum length of 0.5 μm or more but less than 1.5 μm, and third pores having a maximum length of 1.5 μm or more were confirmed on the polished surface of the ceramic substrate produced in Example 4.
The ratio of the number of first pores per unit area N1/(N1+N2+N3) was 0.4, and the ratio of the total number of first pores and second pores per unit area (N1+N2)/(N1+N2+N3) was 0.8.

<<Example 5>>
A ceramic substrate was manufactured in the same manner as in Example 1, except that the pressure inside the hot press furnace was reduced to 8 Pa or less, and the molded body was pressed at a pressure of 15 kgf/ ^cm2 in the thickness direction, and then heated at 1650°C; and the pressure inside the hot press furnace was increased to 0.25 MPa, and the pressing 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. First pores having a maximum length of less than 0.5 μm, second pores having a maximum length of 0.5 μm or more but less than 1.5 μm, and third pores having a maximum length of 1.5 μm or more were confirmed on the polished surface of the ceramic substrate produced in Example 5.
The ratio of the number of first pores per unit area N1/(N1+N2+N3) was 0.4, and the ratio of the total number of first pores and second pores per unit area (N1+N2)/(N1+N2+N3) was 0.6.

<<Comparative Example 1>>
A ceramic substrate was produced in the same manner as in Example 1, except that 5 parts by mass of yttrium oxide 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. However, no primary pores having a maximum length of less than 0.5 μm were observed on the polished surface of the ceramic substrate manufactured in Comparative Example 1. Figures 6 and 7 show SEM photographs of the polished surface of the ceramic substrate of Comparative Example 1.
In Comparative Example 1, the ratio of the number of first pores per unit area N1/(N1+N2+N3) was 0, and the ratio of the total number of first pores and second pores per unit area (N1+N2)/(N1+N2+N3) was 0.3.

<Evaluation>
As shown in FIGS. 4 to 7, the ceramic substrates manufactured in the examples and comparative examples were subjected to precision polishing and then observed with a scanning electron microscope. The presence or absence of grain shedding in the ceramic substrates was evaluated according to the following criteria, with the grain shedding occurrence rate in Comparative Example 1 being set at 100%.
◎: Shedding rate less than 5% ◯: Shedding rate 5% or more but less than 20% △: Shedding rate 20% or more but less than 50% ×: Shedding rate 50% or more It can be seen that the ceramic substrates of Examples 1 to 5 are smoother than the ceramic substrate of Comparative Example 1, and shedding is significantly suppressed.
In particular, as is clear from Examples 1 to 3, it is found that when N1/(N1+N2+N3) is 0.5 or more, grain shedding can be effectively suppressed.
Furthermore, as is clear from Examples 1, 2 and 4, it can be seen that when (N1+N2)/(N1+N2+N3) is 0.8 or more, grain shedding can be effectively suppressed.

Ceramic substrates according to embodiments of the present invention can be used in a variety of industrial products, and are particularly suitable for use in composite substrates used in the production of Group III nitride films.

REFERENCE SIGNS LIST 1 ceramic substrate 11 first pore 12 second pore 13 third pore 2 engineering layer 100 composite substrate

Claims (14)

A ceramic substrate including an aluminum nitride sintered body,
the aluminum nitride sintered body has a plurality of first pores,
A ceramic substrate, wherein the maximum length of each of the plurality of first pores on the surface of the ceramic substrate is less than 0.5 μm. the aluminum nitride sintered body further has a plurality of second pores,
The ceramic substrate according to claim 1 , wherein the maximum length of each of the plurality of second pores on the surface of the ceramic substrate is not less than 0.5 μm and less than 1.5 μm. 3. The ceramic substrate according to claim 2, wherein a ratio of the number of the first pores per unit area on a 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 having a maximum length of less than 0.5 μm; N2 represents the number of second pores per unit area having a maximum length of 0.5 μm or more but less than 1.5 μm; and N3 represents the number of third pores per unit area having a maximum length of 1.5 μm or more). 3. The ceramic substrate according to claim 2, wherein a ratio of the total number of the first pores and the second pores per unit area on the surface of the ceramic substrate 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 having a maximum length of less than 0.5 μm; N2 represents the number of second pores per unit area having a maximum length of 0.5 μm or more but less than 1.5 μm; and N3 represents the number of third pores per unit area having a maximum length of 1.5 μm or more). The ceramic substrate of claim 2, wherein the ratio of the total area of the plurality of first pores and the plurality of second pores to the surface of the ceramic substrate is 0.0001% or more. The ceramic substrate of 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 is 5% or less. the aluminum nitride sintered body includes a plurality of aluminum nitride crystal grains,
2. The ceramic substrate according to claim 1, wherein the average grain size of the plurality of aluminum nitride crystal grains is 3 μm or less. The ceramic substrate of claim 1, wherein 50% or more of the plurality of first pores are located inside the aluminum nitride crystal grains. The ceramic substrate of claim 2, wherein 50% or more of the plurality of second pores are located inside the aluminum nitride crystal grains. The ceramic substrate of claim 1, wherein the periphery of the first pores on the surface of the ceramic substrate has a circular or elliptical shape. The ceramic substrate of claim 2, wherein the periphery of the second pores on the surface of the ceramic substrate has a circular or elliptical shape. The ceramic substrate of claim 1, wherein the aluminum nitride sintered body contains metal elements other than Al in an amount of 0.1 mass% or less, calculated as oxides, and the carbon content is 0.1 mass% or less. The ceramic substrate according to claim 1, wherein the aluminum nitride sintered body has a lightness, as defined in JIS Z8781, of L*40 or less. A ceramic substrate according to any one of claims 1 to 13;
A composite substrate comprising: an engineering layer laminated on a surface of the ceramic substrate, the engineering layer containing Si.