Alumina sintered body and method for manufacturing the same

Abstract

To provide an alumina sintered body having small dielectric loss tangent (tanδ) and a manufacturing method thereof.SOLUTION: An alumina sintered body of the present invention is an alumina-based sintered body containing 99.50 mass% or over and 99.95 mass% or under of Al2O3 as well as Na and Si. With respect to a surface layer part A in a given cross section and a central part B in the cross section in a depth direction from the surface layer part A, a concentration ratio of Na and Si in the surface layer part A, (Na / Si ratio), is smaller than a concentration ratio of Na and Si in the central part B, (Na / Si ratio).SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to an alumina sintered body with low dielectric loss and a method for manufacturing the same, for example, a dielectric loss tangent (tanδ) of 1 × 10⁻¹⁰ -4 This relates to alumina sintered bodies of less than 100% and a method for manufacturing the same. [Background technology]

[0002] In recent years, alumina sintered bodies used in semiconductor manufacturing equipment and liquid crystal panel manufacturing equipment are required to have not only high corrosion resistance but also low dielectric loss tangent. One reason for this requirement for low dielectric loss tangent is to stabilize the plasma inside the semiconductor manufacturing equipment. In other words, to stabilize the plasma generated inside the semiconductor manufacturing equipment, it is necessary to use semiconductor manufacturing materials with a low dielectric loss tangent (tanδ), and in recent years, materials with a dielectric loss tangent (tanδ) of 1 × 10¹⁶ have been used for semiconductor manufacturing materials. -4 An alumina sintered body with a specific molecular weight (less than 10%) is required.

[0003] In response to such requirements, Patent Documents 1 and 2 have shown that the presence of Na oxides is a factor that increases the dielectric loss tangent. Furthermore, it has been proposed that by reducing the content of Na oxides, alumina sintered bodies with a low dielectric loss tangent (tanδ) can be manufactured.

[0004] Furthermore, Patent Document 3 describes an alumina sintered body in which, of the total constituent components in 100% by mass, the content of Na (calculated as Na2O) is 200 ppm to 500 ppm, the content of Si (calculated as SiO2) is 200 ppm or more, the content of Ca (calculated as CaO) is 200 ppm or more, and the content of Al (calculated as Al2O3) is 99.4% by mass or more, wherein the dielectric loss tangent value at 8.5 GHz is 0.5 times or less of the value of the Na (calculated as Na2O) content, the dielectric loss tangent value at 1 MHz is 0.3 times or less of the value of the Na (calculated as Na2O) content, and the dielectric loss tangent value at 12 MHz is 0.6 times or less of the value of the Na (calculated as Na2O) content. In other words, an alumina sintered body in which the dielectric loss tangent value is determined based on the amount of Na contained is shown. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 8-143358 [Patent Document 2] Japanese Patent Application Publication No. 5-217946 [Patent Document 3] Patent No. 6352686 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] As mentioned above, the presence of Na oxide increases the dielectric loss tangent, and attempts have been made to reduce the Na oxide content. However, this Na oxide is present as an impurity in the alumina raw material, which is the main component. Therefore, it is necessary to manufacture alumina sintered bodies using high-purity alumina raw material with a reduced Na oxide content. However, there is a problem in that manufacturing high-purity alumina raw material with a reduced Na oxide content increases manufacturing costs, and consequently, the manufacturing cost of alumina sintered bodies with low dielectric loss increases. Furthermore, even when using high-purity alumina raw material, it is not possible to completely remove Na, and trace amounts of Na remain, resulting in a dielectric loss tangent (tanδ) of 1 × 10⁻¹⁰. -4 It was difficult to obtain an alumina sintered body with a molecular weight of less than [amount missing].

[0007] This invention was made to solve the above problems and aims to provide an alumina sintered body with a small dielectric loss tangent (tanδ) and a method for manufacturing the same. [Means for solving the problem]

[0008] To achieve the above objective, the present invention provides an alumina sintered body containing 99.50% by mass or more and 99.95% by mass or less of Al2O3, as well as Na and Si, characterized in that the concentration ratio of Na to Si in the surface layer A of any cross-section and in the central part B of the cross-section in the depth direction from the surface layer A is smaller than the concentration ratio of Na to Si in the central part B.

[0009] Thus, the concentration ratio of Na to Si in the surface layer A (the concentration ratio of Na to Si is expressed as the Na / Si ratio, and the concentration ratio of Na to Si in the surface layer A is A Na / Si The concentration ratio of Na to Si at the central part B (the concentration ratio of Na to Si is expressed as the Na / Si ratio, and the concentration ratio of Na to Si at the central part B is expressed as B Na / SiIt is formed in a smaller size than the central part B. Therefore, the formation of β-alumina (Na2O-11Al2O3), which is a major cause of dielectric loss in the surface layer A, is suppressed. In other words, the formation of β-alumina (Na2O-11Al2O3) in the surface layer A is suppressed compared to the central part B, resulting in an alumina sintered body with low dielectric loss overall.

[0010] Furthermore, since the alumina sintered body according to the present invention contains Na and Si as impurities, it does not require the use of high-purity alumina raw materials with reduced Na oxide content, and can be manufactured at low cost.

[0011] Here, it is desirable that the surface layer A is a region within a depth of 10 mm from the surface, and the central part B of the cross-section is a region within a depth of 10 mm centered on the center of the cross-section. Also, the dielectric loss tangent (value of tanδ) at a frequency of 4 GHz is 10 -4 The following is preferable:

[0012] It is desirable that the aspect ratio of the Al2O3 crystal grains in the surface layer A is smaller than the aspect ratio of the Al2O3 crystal grains in the central part B. β-alumina (Na2O-11Al2O3) is formed from Al2O3 and the impurity Na, and this β-alumina (Na2O-11Al2O3) produces crystals with a large aspect ratio. As described above, because the aspect ratio of the Al2O3 crystal grains in the surface layer A is small, the formation of β-alumina with a large aspect ratio is suppressed, and the aspect ratio of the β-alumina in the surface layer A becomes smaller than the aspect ratio of the β-alumina in the central part.

[0013] The method for manufacturing an alumina sintered body according to the present invention, which is made to achieve the above object, is the method for manufacturing an alumina sintered body described above, comprising the steps of adding Si content to alumina powder as a raw material using SiO2 powder, mixing a binder and a solvent, and granulating with a spray granulator; forming a molded body by molding the obtained granulated powder; degreasing the molded body; firing the degreased body at 1300 to 1800 °C in a reducing atmosphere or a vacuum atmosphere, and in the firing step, by volatilizing Na content from the degreased body, the concentration ratio of Na to Si in the surface layer portion A at an arbitrary cross section is formed to be smaller than the concentration ratio of Na to Si in the central portion B of the cross section in the depth direction from the surface layer portion A. By such a manufacturing method, an alumina sintered body with a small dielectric loss can be obtained.

[0014] Here, it is desirable that the surface layer portion A is a region within a range of 10 mm in the depth direction from the surface, and the central portion B of the cross section is a region within a range of 10 mm in the depth direction centered on the center of the cross section.

Advantages of the Invention

[0015] According to the present invention, an alumina sintered body with a small dielectric loss and a method for manufacturing the same can be obtained.

Brief Description of the Drawings

[0016] <o000095> [Figure 1] Figure 1 is a conceptual diagram showing the distribution state of the Na / Si ratio of the alumina sintered body according to the present invention. [Figure 2] Figure 2 is a schematic diagram for explaining the aspect ratio. [Figure 3] Figure 3 is a flowchart showing an example of the method for manufacturing an alumina sintered body according to the present invention.

Embodiments for Carrying Out the Invention

[0017] The embodiments of the alumina sintered body and its manufacturing method according to the present invention will be described in detail below. However, the present invention is not limited by these embodiments.

[0018] The alumina sintered body according to the present invention is an alumina sintered body containing 99.50% by mass or more and 99.95% by mass or less of Al2O3, and also containing Na and Si. The Na contained in the alumina sintered body is an oxide of Na contained as an impurity in the alumina raw material. Similarly, the Si contained in the alumina sintered body is an oxide of Si contained as an impurity in the alumina raw material, or an oxide of Si that is added.

[0019] As already mentioned, it is preferable for alumina sintered bodies to be high-purity alumina sintered bodies that contain as little Na and Si as possible as impurities, in order to reduce the dielectric loss tangent (tanδ). However, in order to produce high-purity alumina sintered bodies, it is necessary to use high-purity alumina raw materials with reduced Na oxide content, but this is undesirable because it increases manufacturing costs. In other words, when the Al2O3 content exceeds 99.95% by mass, the manufacturing costs increase, which is undesirable. Therefore, alumina sintered bodies containing at least 99.5% by mass of Al2O3 and containing Na and Si as impurities are used.

[0020] Since the aforementioned sodium affects the formation of β-alumina, reducing the sodium content is necessary to suppress the formation of β-alumina. However, as mentioned above, even when using high-purity alumina raw materials, it is not possible to completely eliminate the sodium content.

[0021] On the other hand, if Si is present, Na is attracted to Si. 、β The formation of an alumina structure is suppressed. More specifically, since Si is tetravalent and Al is trivalent, the presence of Si in Al2O3 disrupts the charge balance, and monovalent Na is attracted to Si mainly by Coulomb forces, thereby suppressing the formation of β-alumina. In other words, when Si is insufficient, monovalent Na is difficult to be attracted to Si mainly by Coulomb force, and Na forms a network with Al (Na binds to Al). As a result, β-alumina is likely to be formed. Therefore, the alumina sintered body according to the present invention needs to contain 99.50% by mass or more and 99.95% by mass or less of Al2O3, and contain Na and Si in a specific content relationship.

[0022] Next, the relationship between the contents of Na and Si will be described. The Na content of the alumina sintered body that causes an increase in the dielectric loss tangent is 5 ppm or more and 50 ppm or less. Incidentally, general alumina raw materials contain 50 ppm or more and 500 ppm or less of Na. Thus, an alumina sintered body containing 5 ppm or more and 50 ppm or less of Na can be manufactured by using general alumina raw materials without extremely reducing the content of the oxide of Na, which is an impurity (without costly high purification), so that the alumina sintered body can be manufactured at a low cost.

[0023] Also, as described above, Si attracts the contained Na 、β and suppresses the formation of the alumina structure. Therefore, the Si content of the alumina sintered body is in a specific content relationship with Na. Here, the relationship between the contents of Na and Si is represented by the concentration ratio of Na to Si (Na / Si ratio). And in an alumina sintered body with a small dielectric loss, such as an alumina sintered body with a dielectric loss less than 10 -4 the concentration ratio of Na to Si (Na / Si ratio) is 0.3 or less.

[0024] Moreover, the alumina sintered body according to the present invention is characterized in that the concentration ratio of Na to Si (Na / Si ratio) is different between the surface layer portion A in an arbitrary cross section and the central portion B of the cross section in the depth direction from the surface layer portion A. Specifically, compared with the concentration ratio of Na to Si (Na / Si ratio B Na / Si ) at the central portion B, in the firing process of the alumina sintered body, the Na in the surface layer portion A volatilizes, so that the Na / Si ratio becomes smaller, and the concentration ratio of Na to Si (Na / Si ratio A Na / SiThe ratio of Na to Si becomes smaller. That is, as shown in Figure 1, the concentration ratio of Na to Si (Na / Si ratio) is highest in the central part B of the alumina sintered body 1, and the concentration ratio of Na to Si is lowest in the surface part A.

[0025] Compared to the concentration ratio of Na and Si in the central part B, the concentration ratio of Na and Si in the surface part A (Na / Si ratio A) Na / Si A small value indicates that Na is volatilizing efficiently. Conversely, if the concentration ratio at the surface is high, it is possible that even a small amount of Na is forming a network with Al, potentially creating a β-alumina structure and worsening dielectric loss.

[0026] For example, if the dielectric loss is 10 -4 The concentration ratio of Na to Si in an alumina sintered body less than 0 is such that, as mentioned above, the concentration ratio of Na to Si (Na / Si ratio) is 0.3 or less, and the concentration ratio of Na to Si (Na / Si ratio B) in the central part B is 0.3 or less. Na / Si Compared to the surface layer A, the concentration ratio of Na to Si (Na / Si ratio A Na / Si It is necessary that ) be small.

[0027] Furthermore, the surface layer A refers to the region within a depth t1 from the surface, and the center of the cross-section refers to the region within a depth t2 centered on the center of the cross-section. Specifically, t1 is 10 mm and t2 is 10 mm. Also, Figure 1 is a conceptual representation and does not accurately show the Na / Si ratio of the alumina sintered body.

[0028] Furthermore, during the firing process of the alumina sintered body, whether Na volatilizes and whether the reduction of Na is efficient can be determined by evaluating the magnitude of the Na / Si ratio between the surface and the core, or the aspect ratio between the surface and the core, as explained below.

[0029] Next, we will explain the aspect ratio between the surface and the core. As shown in Figure 2, β-alumina (Na2O-11Al2O3)1 produces crystals with a large aspect ratio. In Figure 2, the symbol 2 indicates an Al2O3 crystal grain. Therefore, in the surface layer of the sintered body where the formation of β-alumina (Na2O-11Al2O3) is suppressed, the aspect ratio of the Al2O3 crystal grains is smaller than that of the Al2O3 crystal grains in the core.

[0030] Here, in the case of β-alumina (Na2O-11Al2O3), particles with a large aspect ratio undergo anisotropic crystal growth during sintering, progressing while generating defects. Therefore, when growing anisotropically, defects tend to occur along specific crystal axes, and it is presumed that these unique defects worsen dielectric loss. Furthermore, there are differences in the aspect ratio of Al2O3 crystal grains in the surface and the core. A small aspect ratio in the surface is presumed to be a structure resulting from efficient volatilization of Na. Conversely, when the aspect ratio of Al2O3 crystal grains in the surface is large (or equivalent), it is presumed that a β-alumina structure is likely to form in the surface, and that Na is not volatilizing. Therefore, by evaluating the aspect ratio of Al2O3 crystal grains in the surface and the core, it is possible to evaluate the degree of β-alumina structure formation and the dielectric loss tangent.

[0031] Thus, the alumina sintered body of this embodiment is an alumina sintered body with a low dielectric loss tangent, even while containing a predetermined amount of Na as an impurity. Furthermore, because the content of Al, calculated as Al2O3, is 99.5% or more of the total constituent components (100% by mass), the alumina sintered body has high corrosion resistance to highly reactive halogen-based corrosive gases and their plasmas.

[0032] Incidentally, the Na content and Al content (calculated as Al2O3) in 100% by mass of all constituent components can be determined by crushing a portion of the alumina sintered body, dissolving the resulting powder in a solution such as hydrochloric acid, and then measuring the Na and Al content using an ICP (Inductively Coupled Plasma) emission spectrometer (for example, Shimadzu Corporation's ICPS-8100).

[0033] Furthermore, the alumina sintered body of this embodiment contains Si as a component other than Al and Na, as described above. Preferably, the Si content is 20 ppm or more of 100% by mass of the total constituent components. This Na and Si exist as a crystalline phase or amorphous phase made of oxides in the grain boundary phase between the alumina crystals, which are the main crystals.

[0034] The concentration ratio of Na and Si in the surface layer A and the central part B (Na / Si ratio A Na / Si ) and (Na / Si ratio B Na / Si The ratio A / B is calculated as follows: First, a portion of the alumina sintered body is etched using a processing device such as an ion thinning device, and the measurement surface is observed by TEM at a magnification of 10,000 to 100,000 times under the condition of an acceleration voltage of 200 kV. EDS measurement (energy dispersive X-ray spectrometry) of the grain boundary phase is performed, and the content of each component obtained is determined. Let the content of Na be A and the content of Si be B, and the ratio A / B be calculated. It is preferable to perform EDS measurements at multiple locations, for example, three locations, in one grain boundary phase and calculate the ratio A / B using the average value, in which case it is rounded to the third decimal place.

[0035] If the grain boundary phase does not contain Al, a portion of the alumina sintered body may be pulverized, the resulting powder dissolved in a solution such as hydrochloric acid, and then the Na and Si content may be measured using an ICP emission spectrometer. The ratio (A / B) may then be calculated using these values.

[0036] Furthermore, the dielectric loss tangent of the alumina sintered body in this embodiment can be measured by, for example, preparing a disc-shaped sample with a diameter of 50 mm and a thickness of 1 mm, and measuring the dielectric loss tangent value at a frequency of 4 GHz using a capacitance meter (HP-4278A), an impedance analyzer (HP-4291A), and a cavity resonator (network analyzer 8722ES), respectively.

[0037] Next, the method for producing an alumina sintered body according to the present invention will be described. First, as a raw material, alumina powder is prepared with an Al2O3 purity of 99.5% or more and 99.95% by mass or less, and an average particle size of 1 to 2 μm (Step S1 in Figure 3). It is preferable that this alumina powder be of high purity, but it may contain 200 ppm or less of Na as an impurity. In this embodiment, the content of Na is merely specified as an impurity, and Na is not actively added to the alumina powder.

[0038] Si is added to the alumina powder used as the raw material, using SiO2 powder or similar materials. Besides SiO2 powder, colloidal silica and tetraethoxysilane can also be used. The amount of Si added is 40-300 ppm. In this way, silicon dioxide (SiO2) is added to prepare the raw material powder (Step S2 in Figure 3).

[0039] Next, the binder, such as PVA (polyvinyl alcohol), and the solvent are mixed (step S3 in Figure 3). At this time, the mixture was mixed at room temperature using a mixer at 200 rpm.

[0040] Next, the granulated powder is filled into a mold, and a molded body is produced by cold isohydrostatic pressing (CIP) at a pressure in the range of 80 to 200 MPa (Step S4 in Figure 3). This molded body is then degreased to form a degreased body. This degreasing is performed by firing at 700°C to 1300°C in an atmospheric environment (Step S5 in Figure 3).

[0041] Furthermore, this degreased body and the molded body are fired in a reducing atmosphere or a vacuum atmosphere at a temperature of 1300 to 1800°C to obtain an alumina sintered body (step S6 in Figure 3). The reducing atmosphere is preferably a hydrogen atmosphere or a halogen gas atmosphere. By firing in a reducing atmosphere or a vacuum atmosphere at 1300 to 1800°C in this way, Na volatilizes. As a result, an alumina sintered body is obtained in which the Na / Si ratio of the surface layer A in any cross-section and the Na / Si ratio of the central part B of the cross-section in the depth direction from the surface layer A is smaller than the Na / Si ratio of the central part B.

[0042] Thus, the Na / Si ratio decreases due to the volatilization of Na, which suppresses the formation of β-alumina (Na2O-11Al2O3), a major cause of dielectric loss, and the dielectric loss is reduced by 10. -4 An alumina sintered body with a ratio of less than 1 is obtained. β-alumina (Na2O-11Al2O3) produces crystals with a large aspect ratio. Therefore, the aspect ratio of the Al2O3 crystal grains in the surface layer of the sintered body, where β-alumina formation is suppressed, becomes smaller than the aspect ratio of the Al2O3 crystal grains in the center. The aspect ratio is determined by observing the structure of the alumina sintered body with a scanning electron microscope and selecting the particle with the largest aspect ratio in the observed image.

[0043] As described above, even when using alumina raw materials containing sodium oxide as an impurity, it is possible to manufacture alumina sintered bodies with low dielectric loss. In particular, when the dielectric loss tangent (tanδ) is 1 × 10⁻¹⁰ -4 An alumina sintered body less than 1 can be obtained.

[0044] Furthermore, alumina sintered bodies exhibit excellent heat resistance, chemical resistance, and plasma resistance, and also have a small dielectric loss tangent (tanδ) in the high-frequency range. As a result, they are used in semiconductor manufacturing equipment components such as inner wall materials (chambers), microwave introduction windows, shower heads, focus rings, and shield rings, as well as in liquid crystal panel manufacturing equipment components such as stages, mirrors, mask holders, mask stages, chucks, and reticles. [Examples]

[0045] The alumina sintered body according to the present invention and the method for producing the same will be further described based on examples.

[0046] (Experiment 1) In Example 1, alumina powder with a purity of 99.5% by mass and an average particle size of 1-2 μm was used as the raw material. To this alumina powder, silicon dioxide (SiO2) was added in the amounts shown in Table 1. The Na content in the raw material powder was 60 ppm. Furthermore, 2% by mass of polyvinyl alcohol was added as a binder to 100% by mass of the raw material powder containing the above additives to produce granulated powder.

[0047] Next, the granulated powder was filled into a mold, and a molded body was produced by cold isostatic pressing (CIP) at a pressure in the range of 100 MPa. After that, a degreasing treatment was performed, and then the body was fired at 1800°C in a hydrogen atmosphere to obtain an alumina sintered body (alumina ceramics).

[0048] Then, a portion of the alumina sintered body was etched using processing equipment such as an ion thinning device, and the measurement surface was observed at a magnification of 50,000 times using a TEM (Transmission Electron Microscope) under the condition of an acceleration voltage of 200 kV. EDS measurements of the grain boundary phase were performed, and the content of each component obtained was determined. The Na content was denoted as X, the total Si content as Y, and the ratio X / Y was calculated.

[0049] Here, the measurement surface etched using the processing device was defined as the surface layer A in the arbitrary cross-section and the center B of the cross-section in the depth direction from the surface layer A. Then, the ratio X / Y in the surface layer A was defined as A Na / Si Let the ratio X / Y at the central point B be B Na / Si Furthermore, the dielectric loss tangent (tanδ) in the high-frequency region (4 GHz) of the surface layer A and the central part B was measured in accordance with JIS R1641 (Method for measuring microwave dielectric properties of fine ceramic substrates). The results are shown in Table 1.

[0050] [Table 1]

[0051] As can be seen from Examples 1 to 4 in Table 1 above, the Na content decreases on the surface of the sintered body, and the Na / Si ratio (A) of the surface decreases. Na / Si ) is the Na / Si ratio in the central part (B Na / Si It is smaller than ). Furthermore, it was confirmed that dielectric loss decreases as the Na / Si ratio decreases.

[0052] In Comparative Examples 1 and 2 of Table 1 above, the amount of Si added was increased too much in an attempt to raise the Si concentration in the center, exceeding the appropriate amount of 200 ppm, resulting in a tendency for the dielectric loss tangent to increase due to excess Si. In other words, the reason why the Na / Si ratio is lowered throughout the entire sintered body is that, due to the effects of Si segregation and non-uniformity during the manufacturing process, the Si concentration is higher in the center than in the surface.

[0053] In this way, a low Na / Si ratio can be achieved by volatilizing Na during the firing process. Furthermore, by reducing the Na / Si ratio, the dielectric loss value can also be reduced by 10. -4 This achieves a value of less than [value missing]. Furthermore, by adding Si in the range of 130 ppm to 200 ppm while keeping the Na / Si ratio low, it is possible to further reduce dielectric loss. The Na / Si ratio can be adjusted by the amount of Na and Si added.

[0054] (Experiment 2) In Experiment 2, the aspect ratios of the Al2O3 crystal grains of Example 1 and Comparative Example 3 from Experiment 1 were measured using a scanning electron microscope. Also, similar to Experiment 1, the dielectric loss tangent (tanδ) in the high-frequency region (4 GHz) of the surface layer A and the central layer B was measured according to JIS R1641 (Method for measuring microwave dielectric properties of fine ceramic substrates). The results are shown in Table 2.

[0055] As Comparative Example 3, the aspect ratio of Al2O3 crystal grains was changed by adjusting the firing time and the cooling rate after firing compared to Example 1. Specifically, the firing temperature was set to 1400°C and the cooling rate to 300°C / h compared to Example 1. All other conditions were the same as in Example 1. The results are shown in Table 2.

[0056] [Table 2]

[0057] As a result, it was confirmed that Comparative Example 3 had a larger aspect ratio of Al2O3 crystal grains and an increased dielectric loss tangent compared to Example 1. Therefore, by adjusting the firing time and the cooling rate after firing, the tanδ(E-05) surface / center can be reduced, and the dielectric loss tangent can be decreased.

[0058] (Experiment 3) Based on the manufacturing conditions of Example 4, Example 5 was created by further fine-tuning the firing temperature and firing time so that the Si analysis value was between 130 ppm and 200 ppm in both the surface and the core. The amount of SiO2 added was 185 ppm.

[0059] As a result, in Example 5, A Na / Si 0.062, B Na / Si The values ​​were 0.095, Si analysis values ​​(ppm, surface / center) were 90 / 110, and tanδ(E-05) surface / center were 5.58 / 6.67. Thus, by setting the Si analysis values ​​to be between 130 ppm and 200 ppm for both the surface and center, the dielectric loss tangent can be reduced even further than in Example 4.

Claims

1. Al ２ O ３ An alumina sintered body containing 99.50% by mass or more and 99.95% by mass or less of, and also containing Na and Si, The area within a 10 mm depth from the surface is defined as the surface layer A, and the area within a 10 mm depth from the center of the cross-section is defined as the central part B. In any cross-section, the concentration of Si in the grain boundary phase in the surface layer A is 67 ppm or more and 120 ppm or less, the concentration of Si in the boundary phase in the central part B is 70 ppm or more and 130 ppm or less, the concentration of Na in the grain boundary phase in the surface layer A is 7.17 ppm or more and 9.72 ppm or less, and the concentration of Na in the boundary phase in the central part B is 13.02 ppm or more and 24.05 ppm or less. The concentration ratio of Na to Si in the grain boundary phase of the surface layer A is 0.119 or less. The concentration ratio of Na to Si in the grain boundary phase at the central part B is 0.214 or less. Furthermore, the concentration ratio of Na to Si in the surface layer A is smaller than the concentration ratio of Na to Si in the central part B. The dielectric loss tangent (value of tanδ) at a frequency of 4 GHz in the surface layer A is 6.53 × 10 －５ The above 7.09 x 10 －５ The following: The dielectric loss tangent (value of tanδ) at the central region B at a frequency of 4 GHz is 7.47 × 10⁻¹⁰. －５ The above 8.47 x 10 －５ An alumina sintered body characterized by the following:

2. Al ２ O ３ A method for producing an alumina sintered body according to claim 1, using alumina powder containing 99.50% by mass or more and 99.95% by mass or less of alumina, with Na as an impurity of 60 ppm or more and 200 ppm or less, wherein the method is as described in claim 1. Al ２ O ３ containing 99.50% to 99.95% by mass of Al ２ O ３ and 60 ppm to 200 ppm of Na as an impurity, to an alumina powder having an average particle diameter of 1 μm to 2 μm as a raw material, SiO ２ powder is added at 160 to 285 ppm to add Si component, and a step of mixing a binder and a solvent and granulating with a spray granulator, The process involves molding the obtained granulated powder to form a molded body, The process involves degreasing the molded body in an atmospheric environment at a temperature of 700°C to 1300°C, The process comprises a step of firing the degreased body, which has undergone the degreasing treatment, at 1300 to 1800°C under a reducing atmosphere or a vacuum atmosphere. In the firing process described above, by volatilizing the Na content from the degreased body, The concentration of Si in the grain boundary phase in the surface layer A of any cross-section is 67 ppm or more and 120 ppm or less, the concentration of Si in the grain boundary phase in the central part B of the cross-section in the depth direction from the surface layer A is 70 ppm or more and 130 ppm or less, and the concentration of Na in the grain boundary phase in the surface layer A is 7.17 ppm or more and 9.72 ppm or less, and the concentration of Na in the boundary phase in the central part B is 13.02 ppm or more and 24.05 ppm or less. The concentration ratio of Na to Si in the grain boundary phase of the surface layer A is 0.119 or less. The concentration ratio of Na to Si in the grain boundary phase at the central part B is 0.214 or less. A method for manufacturing an alumina sintered body, characterized in that the concentration ratio of Na to Si in the surface layer A is formed to be smaller than the concentration ratio of Na to Si in the central part B.

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