TW201802059A - Cordierite-based sintered body, method for producing the same, and composite substrate - Google Patents

Abstract

A cordierite-based sintered body according to the present invention contains cordierite as a main component and silicon nitride or silicon carbide. The cordierite-based sintered body preferably has a thermal expansion coefficient less than 2.4 ppm/DEG C at 40 DEG C to 400 DEG C, an open porosity of 0.5% or less, and an average grain size of 1 [mu]m or less.

Description

Scopolite sintered body, manufacturing method thereof, and composite substrate

The present invention relates to a sapphire sintered body, a method for manufacturing the same, and a composite substrate.

Ocherite is known as a material having high heat resistance and a small thermal expansion coefficient, and can be used as a material having high thermal shock resistance. In order to increase the mechanical properties of ochreite, it is known that ocherite can be compounded with silicon nitride, silicon carbide, or the like having a high Young's modulus or strength (Patent Documents 1 and 2). Patent Document 1 discloses that a rare earth oxide and silicon nitride or silicon carbide are added to vermiculite having an average particle diameter of 1.2 μm, and firing is performed in the atmosphere to obtain a vermiculite sintered body having a relative density of 97 to 98%. Patent Document 2 discloses that silicon nitride or silicon carbide having an average particle diameter of 1 μm is added to the vermiculite having an average particle diameter of 3 μm, and firing is performed at normal pressure in a nitrogen atmosphere to obtain a sintered body of the vermiculite having a high Young's modulus. .

In addition, Patent Document 3 discloses a composite substrate formed by directly bonding a functional substrate composed of lithium tantalate, lithium niobate, and the like and a support substrate of a sintered sintered system as an acoustic wave device such as a surface acoustic wave device. In this acoustic wave device, since the thermal expansion coefficient of the sapphire sintered body supporting the substrate is 1.1 ppm / ° C (40-400 ° C) which is very small, the frequency temperature dependence is greatly improved.

[Previous Technical Literature]

[Patent Literature]

Patent Document 1: Patent No. 3574560

Patent Document 2: Patent Publication No. 4416191

Patent Document 3: International Patent Publication No. 2015/186571

When a functional substrate and a support substrate are bonded in the manner of Patent Document 3, the surfaces of both substrates are required to have a high flatness. However, the relative density of the sapphire sintered body of Patent Document 1 is 97 to 98%, and there are several percentages of pores. Even if the surface of the sintered body is polished, high flatness cannot be obtained. In addition, in the ocherite sintered body of Patent Document 2, the ocherite raw material particles are 3 μm, and the addition of a sintering aid causes the sintered particle size to be larger than that of the ocherite raw material particles. Therefore, even if a polishing process is performed, high flatness cannot be obtained. In addition, if this sapphire sintered body is used as an elastic wave device, the sintered particle size also becomes larger than the interval between the electrodes, and at this time, there may be a change in the sound velocity of the composite substrate, which changes the characteristics of the device.

In order to solve the above-mentioned problems, the main object of the present invention is to enable the scopolite sintered body to maintain the low thermal expansion coefficient of the scopolite, improve the rigidity, and increase the flatness of the polished surface.

The ocherite sintering system of the present invention is a ocherite sintered body mainly composed of ocherite and contains silicon nitride or silicon carbide. The thermal expansion coefficient of 40 to 400 ° C is less than 2.4 ppm / ° C, and the open porosity is 0.5%. Hereinafter, the average crystal particle diameter (average particle diameter of the sintered particles) is 1 μm or less. If using this ochre sinter Body, can maintain the low thermal expansion coefficient of ocherite, improve rigidity and increase the flatness of the polished surface.

The method for preparing a sapphire sintered body of the present invention includes (a) mixing 60 to 90% by volume of scopolite powder having an average particle size of 0.1 to 1 μm and 10 to 40% by volume of sapphire silicon powder having an average particle size of 0.1 to 1 μm. 100% by volume to obtain mixed raw material powder, or 70 to 90% by volume of vermiculite powder with an average particle size of 0.1 to 1 μm and 10 to 30% by volume of silicon carbide powder with an average particle size of 0.1 to 1 μm, totaling 100% by volume, A step of obtaining a mixed raw material powder; and (b) forming the above mixed raw material powder into a shaped body having a predetermined shape, and the shaped body is hot-pressed at a pressure of 20 to 300 kgf / cm ² and a firing temperature (highest temperature) of 1350 to 1450 ° C. A step of firing to obtain a sapphire sintered body. This method is suitable for producing the sapphire sintered body of the present invention. The average particle diameter of the powder was measured by a laser diffraction method (the same applies hereinafter).

The composite substrate of the present invention is a composite substrate in which a functional substrate and a support substrate are bonded together, and the support substrate is the above-mentioned sintered apatite sintered body. Since the polished surface of the sapphire sintered body serving as a support substrate in this composite substrate has high flatness, it can be well bonded to a functional substrate. In addition, when this composite substrate is used in a surface acoustic wave device, frequency-temperature dependence can be greatly improved. Furthermore, even in optical waveguide devices, LED devices, and switching devices, the thermal expansion coefficient of the supporting substrate is small, and performance can be improved.

10‧‧‧ composite substrate

12‧‧‧ Piezo substrate

14‧‧‧ support substrate

30‧‧‧Electronic components

32, 34‧‧‧IDT electrode

36‧‧‧Reflective electrode

Fig. 1 is a manufacturing process drawing of a sapphire sintered body.

FIG. 2 is a perspective view of the composite substrate 10.

FIG. 3 is a perspective view of an electronic component 30 manufactured using the composite substrate 10.

Fig. 4 is a SEM image of a polished surface of a sapphire sintered body in Experimental Example 3, (a) raw data, and (b) data after binarization.

Hereinafter, the embodiments of the present invention will be described, but it cannot be used to limit the present invention. Without departing from the spirit and scope of the present invention defined by the appended claims, those skilled in the art can make various changes based on common knowledge. And improvement.

The ocherite sintering system of this embodiment mainly includes ocherite, and includes silicon nitride or silicon carbide. The main component refers to the component having the largest volume in the sintered body. It is preferred that the scopolite sintered body has a thermal expansion coefficient of less than 2.4 ppm / ° C at 40 to 400 ° C, an open porosity of 0.5% or less, and an average crystal grain size of 1 µm or less. Although this scopolite sintered body contains silicon nitride or silicon carbide having a higher thermal expansion coefficient than scopolite, it can maintain a low thermal expansion coefficient. In addition, since the sapphire sintered body contains silicon nitride or silicon carbide having a higher Young's modulus than that of sapphire, the rigidity is higher than that of sapphire alone. In addition, this ocherite sintered body has an open porosity of 0.5% or less, contains almost no pores, has an average particle diameter of 1 μm or less, and improves the flatness of the polished surface (polished surface).

It is preferable that the number of pores having a maximum length of 1 μm or more per 100 μm × 100 μm area of the ocherite sintered body of this embodiment is 10 or less. When the number of pores is 10 or less, the flatness of the polished surface becomes higher. The number of pores is preferably 3 or less, and more preferably 0.

The sapphire sintered body of this embodiment preferably has a Young's modulus of 160 GPa or more and a 4-point bending strength of 220 MPa or more. Due to silicon nitride or carbide The Young's modulus or strength of silicon is higher than that of ocherite. By adjusting the addition ratio of ocherite, the Young's modulus of the ocherite sintered body can be 160 GPa or more, and the 4-point bending strength is 220 MPa or more.

The average roughness Ra of the center of the polished surface of the sapphire sintered body of this embodiment is preferably 1.5 nm or less. As a composite substrate used for an elastic wave device, it is known that a functional substrate and a support substrate are bonded. Therefore, a sapphire sintered body having a polished surface Ra of 1.5 nm or less is used as the support substrate, so that the support substrate and the functional substrate can be used. Good bonding. For example, the area ratio (joining area ratio) of actual joining in the joining interface is 80% or more (preferably 90% or more). The center average roughness Ra of the polished surface is 1.1 nm or less, more preferably 1.0 nm or less, and most preferably 0.8 nm or less.

The ocherite sintered body of this embodiment preferably has a thermal expansion coefficient of 40 to 400 ° C of 2.0 ppm / ° C or less. A composite substrate formed using a sapphire sintered body as a support substrate is applied to an elastic wave element. When the temperature of the elastic wave element rises, the functional substrate has a smaller thermal expansion than the original thermal expansion, which can improve the frequency-temperature dependence of the elastic wave element. The thermal expansion coefficient at 40 to 400 ° C is preferably 1.8 ppm / ° C or lower.

When the ocherite sintered body of this embodiment contains silicon nitride, the chertite phase is preferably 60 to 90% by volume, and the silicon nitride phase is 10 to 40% by volume. When the silicon carbide is included, the chertite phase is preferred. It is 70 to 90% by volume, and the silicon carbide phase is 10 to 30% by volume. If this combination ratio is used, characteristics such as the number of pores or Young's modulus, the center average roughness Ra of the polished surface, and the thermal expansion coefficient of 40 to 400 ° C are good. The volume% of each phase was obtained in the following manner. In other words, the polished surface of the sapphire sintered body according to this embodiment is image-observed and assembled with a SEM reflection electron microscope. In the analysis, the area ratio of each phase is obtained by the ratio of the ocherite in the image, and the volume ratio (volume%) of each phase is expediently used.

Next, an embodiment of a method for producing a sapphire sintered body according to the present invention will be described. Referring to FIG. 1, the production of the chertite sintered body includes (a) a step of preparing a mixed raw material powder, and (b) a step of making the chertite sintered body.

Step (a): Preparation of mixed raw material powder

The ocherite raw material is preferably a powder having a high purity and a small average particle size. The purity is preferably 99.0% or more, more preferably 99.5% or more, and most preferably 99.8% or more. The unit of purity is mass%. The average particle diameter (D50) is preferably 1 μm or less, and more preferably 0.1 to 1 μm. Obsidian raw materials can be commercially available, and can also be made using high-purity magnesium oxide, aluminum oxide, and silicon dioxide powder. The method for preparing the ocherite raw material can be referred to the method described in Patent Document 3. As the silicon nitride raw material or the silicon carbide raw material, a powder having a small average particle diameter is preferably used. The average particle diameter is preferably 1 μm or less, and more preferably 0.1 to 1 μm. When preparing the mixed raw material powder of the vermiculite raw material and the silicon nitride raw material, for example, weighing 60 to 90% by volume of the vermiculite raw material and 10 to 40% by volume of the silicon nitride raw material, totaling 100% by volume, and mixing them with a mixer such as a ball mill. , If necessary, drying with a spray dryer to obtain mixed raw material powder. In addition, when preparing the mixed raw material powder of the vermiculite raw material and the silicon carbide raw material, for example, weighing 70-90% by volume of the vermiculite raw material and 10-30% by volume of the silicon carbide raw material, totaling 100% by volume, and mixing them with a mixer such as a ball mill , If necessary, drying with a spray dryer to obtain mixed raw material powder.

Step (b): Fabrication of scopolite sintered body

From the mixed raw material powder obtained in step (a), a shaped body having a predetermined shape is formed. The molding method is not particularly limited, and a general molding method can be used. E.g, The mixed raw material powder is directly pressed and formed by a mold. At the time of press molding, the pellets can be formed by spray drying, and the moldability is improved. In addition, an organic adhesive is added to produce bad soil, extruded and formed, and a slurry is formed to form a sheet. These procedures must remove the organic adhesive before or during the firing step. Alternatively, CIP (cold hydrostatic pressure) high-pressure molding can also be used.

Next, the obtained formed body is fired to produce a sapphire sintered body. At this time, the sintered particles are maintained in a fine state, and the discharge of pores during sintering can improve the flatness of the surface of the sapphire sintered body. This method is very effective for hot pressing. Compared with sintering at normal temperature, using this hot pressing method can improve the densification of the fine particle state at low temperature, and suppress the coarse atmospheric pores common in sintering at normal temperature. The firing temperature during hot pressing is preferably 1350 to 1450 ° C, and more preferably 1375 to 1425 ° C. The pressure during hot pressing is preferably 20 to 300 kgf / cm ² . In particular, low pressure can miniaturize the hot press fixture and prolong its life. The holding time of the firing temperature (highest temperature) can be determined by considering the shape or size of the compact, the characteristics of the heating furnace, and the like, and selecting a suitable and appropriate time. Specifically, the preferred holding time is, for example, 1 to 12 hours, and more preferably 2 to 8 hours. The firing environment is not particularly limited, and the environment during hot pressing may be a general inert gas environment such as nitrogen or argon. The heating rate or cooling rate can be appropriately and appropriately set in consideration of the shape or size of the formed body, the characteristics of the heating furnace, and the like, and can be set to, for example, a range of 50 to 300 ° C / hr.

Next, an embodiment of the composite substrate of the present invention will be described. The composite substrate of this embodiment is formed by bonding a functional substrate and a support substrate made of the above-mentioned sintered sintered body. This composite substrate can increase the bonding area of the two substrates to have good bonding characteristics. The functional substrate is not particularly limited. For example, it is selected from lithium tantalate, lithium niobate, gallium nitride, and silicon dioxide. Compared with the joining method Preferably, it is directly joined. In the case of direct bonding, the bonding surfaces of the functional substrate and the support substrate are separately polished, and then activated, and the two bonding surfaces are pressed against each other in a state where they face each other. For activation of the bonding surface, for example, the bonding surface is irradiated with an inert gas (such as argon), an ion beam, a plasma, or a neutral atom beam. The thickness ratio (functional substrate thickness / support substrate thickness) of the functional substrate to the support substrate is preferably 0.1 or less. Figure 2 shows an example of a composite substrate. The composite substrate 10 is formed by directly bonding a piezoelectric substrate 12 and a support substrate 14 as a functional substrate.

The composite substrate of this embodiment can be used as an electronic component. This electronic component can be selected from LED components, optical waveguide components, switching components, etc. on the acoustic wave component (surface acoustic wave device or Lamb wave, thin film resonator (FBAR)). When using the above-mentioned composite substrate for an elastic wave device, the thermal expansion coefficient of the sapphire sintered body as a support substrate is very small and less than 2.4 ppm / ° C (40 to 400 ° C), which can significantly improve frequency temperature dependence. FIG. 3 shows an example of an electronic component 30 produced using the composite substrate 10. The electronic component 30 is a One Port SAW resonator, that is, a surface acoustic wave device. First, a pattern of a plurality of electronic components 30 is formed on the piezoelectric substrate 12 of the composite substrate 10 using a general photo-etching, and then one electronic component 30 is cut out. The electronic component 30 uses ID technology to form IDT (Interdigital Transducer) electrodes 32 and 34 and a reflective electrode 36 on the surface of the piezoelectric substrate 12.

The above embodiments cannot be used to limit the present invention, and the technical scope of the present invention describes the present invention, and is not limited to the present invention.

[Example]

1. Production of mixed raw material powder

3mm)，在使用純水之球磨機中進行70小時粉碎，以製作平均粒徑0.5~0.6μm的菫青石粉碎物。將所獲得的泥漿在大氣下，以110℃進行乾燥，藉由篩選乾燥物獲得菫青石粉末。依表一中實驗例1~9的原料粉末組成比例，秤量菫青石原料與氮化矽原料或碳化矽原料，藉由使用

5mm氧化鋁圓石的球磨機混合，利用噴霧乾燥製作混合原料粉末。此外，氮化矽原料可為市售平均粒徑0.8μm，純度97%以上的粉末，碳化矽原料可為市售平均粒徑0.5μm，純度97%以上的粉末。 First, a high-purity magnesium oxide, alumina, and silicon dioxide powder having a commercially available average particle diameter of 1 μm or less and a purity of 99.9% or more is used to prepare the ocherite raw material. In other words, each powder is weighed, mixed for the purpose of forming the aragonite composition, and heated at 1400 ° C. for 5 hours to obtain coarse aragonite particles. For the obtained obsidian coarse particles, alumina was used as cobblestone (

3 mm), and pulverized in a ball mill using pure water for 70 hours to produce a pulverized lapisite with an average particle diameter of 0.5 to 0.6 μm. The obtained slurry was dried at 110 ° C. in the air, and the dried product was screened to obtain a vermiculite powder. According to the composition ratios of the raw material powders in Experimental Examples 1 to 9 in Table 1, weigh the gangue raw material and the silicon nitride raw material or the silicon carbide raw material.

A 5 mm alumina cobblestone ball mill was mixed, and a mixed raw material powder was produced by spray drying. In addition, the silicon nitride raw material may be a powder having a commercially available average particle size of 0.8 μm and a purity of 97% or more, and the silicon carbide raw material may be a commercially available powder having an average particle size of 0.5 μm and a purity of 97% or more.

2. Production of scopolite sintered body

100mm厚度25mm的成形體。將各成形體置於石墨製的模具中，使用熱壓爐，在200kgf/cm²壓力下，以1375~1425℃燒成溫度(最高溫度)進行5小時燒成，以製作菫青石質燒結體。各實驗例的燒成溫度如表一所示。燒成環境為氬氣環境，升溫速度為100℃/hr，降溫速度為200℃/hr，降溫時從1200℃以下退火。此外，實驗例10僅使用菫青石粉末，以相同條件製作成形體，使用熱壓爐，在壓力200kgf/cm²下，以1425℃燒成溫度(最高溫度)，進行5小時燒成，以製作單獨菫青石的燒結體。 The mixed raw material powders of Experimental Examples 1 to 9 were pressure-formed in a 50 kgf / cm ² uniaxial mold to obtain

100mm thickness 25mm formed body. Each formed body was placed in a graphite mold, and fired at 200 kgf / cm ² under a pressure of 200 kgf / cm ² at a firing temperature (highest temperature) of 1375 to 1425 ° C. for 5 hours to prepare a sapphire sintered body. . The firing temperature of each experimental example is shown in Table 1. The firing environment is an argon atmosphere, the heating rate is 100 ° C / hr, the cooling rate is 200 ° C / hr, and the temperature is annealed from 1200 ° C or lower when the temperature is lowered. In addition, in Experimental Example 10, only a vermiculite powder was used, and a compact was produced under the same conditions. A hot-pressing furnace was used at a firing temperature (maximum temperature) of 1425 ° C. at a pressure of 200 kgf / cm ² to produce Sintered body of ochreite alone.

3. Characteristic evaluation

A test piece (4 × 3 × 40 mm anti-fold bar) was cut out from the sapphire sintered body of Experimental Examples 1 to 10 to perform an evaluation test. In addition, the polished surface of the sintered body can be processed into a mirror-like shape by polishing one side of a 4 × 3 × 10 mm test piece. 3 μm diamond particles, 0.5 μm diamond particles are used for grinding in order, and finally, diamond particles less than 0.1 μm are used for final grinding modification. The evaluation of the characteristics is as follows.

(1) Crystal phase

The sintered body was pulverized, and the crystalline phase was measured by an X-ray diffractometer. The measurement conditions were CuKα, 50 kV, 300 mA, and 2θ = 5-70 °. A rotating counter-cathode type X-ray diffractometer (RINT manufactured by Rigaku Electrode) was used.

(2) Composition of sintered body

The finished polished surface of the sintered body was subjected to reflection electron development observation and composition analysis by SEM, and the area ratio of the ocherite phase to the other crystal phases was obtained from the image contrast ratio, and this was used as the volume ratio of the sintered body. Figure 4 shows the polished surface An example of a SEM image. FIG. 4 is an SEM image of the polished surface of the sapphire sintered body of Experimental Example 3. (a) is the original data, and (b) is the data after binarization. In Fig. 4 (a), the black part is the ocherite phase, and the white part is the silicon nitride phase.

(3) Bulk density, open porosity

The bulk density and the open porosity were measured by an Archimedes method using pure water using an anti-fold bar.

(4) Relative density

The calculated density of the sintered body was calculated from the composition of the sintered body and the density of each component, and the ratio of the measured bulk density to the calculated density was used as the relative density. In the present invention, pansy bluestone density 2.505g / cm ^3, a silicon nitride density of 3.20g / cm ^3, silicon carbide density of 3.21g / cm ^3. The density of silicon nitride and silicon carbide used in the present invention does not consider the influence of impurity oxygen and the like in the raw materials.

(5) Bending strength

The 4-point bending strength was measured in accordance with the JIS R1601 standard. The shape of the test piece was a 3 × 4 × 40 mm anti-fold bar or a half-length anti-fold bar.

(6) Young's modulus

The measurement was performed by the static twist method in accordance with the JIS R1602 standard. The shape of the test piece was a 3 × 4 × 40 mm anti-fold bar.

(7) Thermal expansion coefficient (40 ~ 400 ℃)

According to the JIS R1618 standard, the measurement was performed by a differential pressure method. The shape of the test piece was a 3 × 4 × 20 mm anti-fold bar.

(8) Number of stomata

The polished surface of the sintered body thus obtained was observed by SEM, and the number of pores having a maximum length of 1 μm or more per 100 μm × 100 μm was counted.

(9) Surface flatness (Ra)

About the polished surface of the sintered compact thus completed, the central average roughness Ra was measured using AFM. The measurement range is 10 μm × 10 μm.

(10) Average particle diameter of sintered particles

The polished surface of the sintered body thus completed was subjected to thermal etching at 1200 to 1400 ° C. for 2 hours, the size of 200 or more sintered particles was measured by SEM, and the average particle diameter was calculated using linear analysis. The linear analysis coefficient was 1.5, and the length measured by the SEM was multiplied by 1.5 as the average particle diameter.

(11) Jointability

A circular plate having a diameter of 100 mm and a thickness of 600 μm was cut from the sintered bodies of Experimental Examples 1 to 10. After grinding the circular plate according to the above method, the surface is cleaned to remove particles or pollutants. Next, using the circular plate as a support substrate, the support substrate and the functional substrate were directly bonded to obtain a composite substrate. In other words, first, the bonding surfaces of the support substrate and the functional substrate are respectively activated with an argon ion beam, and then the two bonding surfaces face each other and are pressed with 10 tonf to obtain a bonded composite substrate. As the functional substrate, a lithium tantalate (LT) substrate and a lithium niobate (LN) substrate can be used. The evaluation of jointability was judged by infrared images. The joint area of 90% was "best", 80% or more and 90% or less was "good", and 80% or less was "not good".

4. Evaluation results

Compared with the sapphire sintered body alone of Experimental Example 10, the sapphire sintered body of Experimental Examples 1 to 9 containing silicon nitride or silicon carbide has increased bending strength and Young's modulus. The Young's modulus is increased above 160 GPa, and the 4-point bending strength is above 220 MPa. In addition, the thermal expansion coefficient of the ochrelite sintered bodies of Experimental Examples 1 to 4, and 6 to 8 is less than 2.4 ppm / ° C (the experimental examples 1 to 4 with silicon nitride added are 1.4 to 1.8 ppm / ° C) , Experimental Examples 6 to 8 in which silicon carbide was added were 1.8 to 2.3 ppm / ° C.) Although the sintered sintered body of scopolite alone in Experimental Example 10 increased slightly, the thermal expansion coefficient was kept low. In addition, in the ocherite sintered bodies of Experimental Examples 1 to 4, 6 to 8, the open porosity was less than 0.1%, the average crystal grain size was 1 μm or less, and the center average roughness Ra of the polished surface was 1.1 nm or less. Therefore, the direct bonding between the circular plate cut from the sapphire sintered body of Experimental Examples 1 to 3, 6, and 7 and the functional substrate is the "best" of 90% or more of the bonding area of the two. From Experimental Example 4, The bonding between the circular plate cut from the 8 菫 cyanite sintered body and the functional substrate is "good" when the bonding area is 80% to 90%. In addition, when the center average roughness Ra of the polished surface becomes such a low value, it contributes to reducing the number of pores to three or less. In addition, one of the reasons why the average particle diameter of Experimental Examples 1 to 9 was 1 μm or less was that sintering was not performed using a rare earth oxide co-sintering agent.

In addition, Experimental Examples 1 to 4, 6 to 8 correspond to Examples of the present invention, and Experimental Examples 5, 9, and 10 correspond to Comparative Examples. This experimental example cannot be used to limit the present invention.

The present invention is based on Japanese Patent Application No. 2016-058969 filed on March 23, 2016 and Japanese Patent Application No. 2017-033437 filed on February 24, 2017, the contents of which are incorporated into the specification of the present invention .

Claims (9)

A sapphire sintered body, which contains scopolite as the main component and contains silicon nitride or silicon carbide, which has a thermal expansion coefficient of less than 2.4 ppm / ° C at 40-400 ° C. The open porosity is 0.5% or less, and the average crystal grain size is 1 μm or less. The sapphire sintered body described in item 1 of the scope of patent application, wherein the number of pores having a maximum length of 1 μm or more per 100 μm × 100 μm polished surface is 10 or less. The sapphire sintered body described in item 1 or 2 of the scope of patent application, wherein the Young's modulus is 160 GPa or more. The ochreite sintered body according to any one of claims 1 to 3, wherein the 4-point bending strength is 220 MPa or more. The sapphire sintered body according to any one of claims 1 to 4, wherein the center surface roughness Ra of the polished surface is 1.5 nm or less. The sapphire sintered body according to any one of claims 1 to 5, which contains silicon nitride, is subjected to image observation and composition analysis of the polished surface with a SEM reflection electron microscope. Cyanite ratio When the area ratio of a cyanite phase to a silicon nitride phase is obtained, the osmite phase has 60 to 90% by volume, and the silicon nitride phase has 10 to 40% by volume. The sapphire sintered body according to any one of claims 1 to 5 in the patent application scope, which contains silicon carbide, is subjected to image observation and composition analysis on the polished surface by a SEM reflection electron microscope, and When the area ratio of a cyanite phase to a silicon carbide phase is obtained from the cyanite ratio, the cyanite phase has a range of 70 to 70%. 90% by volume, and the silicon nitride phase has 10-30% by volume. A method for preparing a sapphire sintered body, comprising: (a) mixing 60-90% by volume of cyanite powder with an average particle size of 0.1-1 μm and 10-40% by volume of silicon nitride powder with an average particle size of 0.1-1 μm, a total of 100 100% by volume to obtain mixed raw materials, or 70 to 90% by volume of ochreite powder with an average particle size of 0.1 to 1 μm and 10 to 30% by volume of silicon carbide powder with an average particle size of 0.1 to 1 μm. Step; and (b) forming the mixed raw material to form a shaped body having a predetermined shape, the shaped body being hot-pressed at a pressure of 20 to 300 kgf / cm ² and a firing temperature of 1350 to 1450 ° C. to obtain Step of bluestone sintered body. A composite substrate is a composite substrate formed by bonding a functional substrate and a supporting substrate, wherein the supporting substrate is a sapphire sintered body as described in any one of claims 1 to 7 of the scope of patent application.