METHOD OF MANUFACTURING PLASMA-RESISTANT COATING FILM

Abstract

Disclosed herein is a method of manufacturing a plasma-resistant coating film. The method includes (1) forming a lower coating layer through a thermal spray process, on a base member, from a first rare earth metal compound powder including 90 to 99.9 wt % of first rare earth metal compound particles and 0.1 to 10 wt % of silica (SiO.sub.2) particles, (2) processing the surface of the lower coating layer formed in step (1) to have an average surface roughness of 1 to 6 μm, and (3) forming an upper coating layer through a suspension plasma spray process, on the lower coating layer which is surface-treated in step (2), from second rare earth metal compound particles, to obtain a structurally dense and chemically stable plasma-resistant coating film with improved plasma resistance.

Claims

1. A method of manufacturing a plasma-resistant coating film, the method comprising: (1) forming a lower coating layer on a base member to be coated from a first rare earth metal compound powder comprising 90 to 99.9 wt % of first rare earth metal compound particles and 0.1 to 10 wt % of silica (SiO.sub.2) particles through a thermal spray process; (2) processing a surface of the lower coating layer formed in step (1) to have an average surface roughness of 1 to 6 μm; and (3) forming an upper coating layer on the lower coating layer that is surface-treated in step (2), through a suspension plasma spray process from a second rare earth metal compound powder.

2. The method of claim 1, wherein the first rare earth metal compound powder comprises 95 to 99.9 wt % of rare earth metal compound particles and 0.1 to 5 wt % of silica (SiO.sub.2) particles.

3. The method of claim 1, wherein the first rare earth metal compound powder has a particle size of 10 to 60 μm, and the lower coating layer has a thickness of 50 to 500 μm.

4. The method of claim 1, wherein the second rare earth metal compound has a particle size of 0.1 to 10 μm, and the upper coating layer has a thickness of 50 to 150 μm.

5. The method of claim 1, wherein the lower coating layer has a porosity of less than 2 vol % and the upper coating layer has a porosity of less than 1 vol %.

6. The method of claim 1, wherein each of the first rare earth metal compound and the second rare earth metal compound is selected from the group consisting of yttria (Y.sub.2O.sub.3), yttrium fluoride (YF), and yttrium oxyfluoride (YOF).

7. The method as set forth in claim 1, wherein the first rare earth metal compound is yttria (Y.sub.2O.sub.3).

8. The method of claim 1, wherein the thermal spray process in step (1) is atmospheric plasma spray.

9. The method of claim 1, wherein the surface process in step (2) is performed by polishing using a diamond pad.

10. A plasma-resistant member manufactured by the method of claim 1.

11. A plasma-resistant coating film comprising: a lower coating layer formed on a base member to be coated and made from a first rare earth metal compound powder comprising 90 to 99.9 wt % of first rare earth metal compound particles and 0.1 to 10 wt % of silica (SiO.sub.2) particles through a thermal spray process, the lower coating layer having an adhesive strength of 20 MPa or more with respect to the base member; and an upper coating layer formed on the lower coating layer and made from a second rare earth metal compound particles through a suspension plasma spray process, wherein the plasma-resistant coating film has a porosity of less than 1 vol %.

12. The film of claim 11, wherein each of the first rare earth metal compound and the second rare earth metal compound is selected from the group consisting of yttria (Y.sub.2O.sub.3), yttrium fluoride (YF), and yttrium oxyfluoride (YOF).

13. The film of claim 11, wherein the first rare earth metal compound is yttria (Y.sub.2O.sub.3).

14. The film of claim 11, wherein the lower coating layer has a porosity of less than 2 vol % and the upper coating layer has a porosity of less than 1 vol %.

15. The film of claim 11, wherein the lower coating layer has a thickness of 50 to 500 μm and the upper coating layer has a thickness of 50 to 150 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 is a schematic diagram for explaining a shadow effect that occurs during thermal spray coating;

[0042] FIG. 2 is a schematic diagram for explaining the structure of a plasma-resistant coating film including a first rare earth metal compound coating layer and a second rare earth metal compound coating layer of the present invention;

[0043] FIG. 3(A). FIG. 3(B), FIG. 3(C), FIG. 3(D) show scanning electron microscope (SEM) images, in which FIG. 3(A) and FIG. 3(B) show coating films manufactured according to Comparative Example 1 and Comparative Example 4, respectively, FIG. 3(C) shows a lower coating film of a plasma-resistant coating film of the present invention, and FIG. 3(D) shows a side surface of a plasma-resistant coating film manufactured according to Example 1; and

[0044] FIG. 4(A), FIG. 4(B), FIG. 4(C), FIG. 4(D) show SEM images in which FIG. 4(A) and FIG. 4(B) show coating films manufactured according to Comparative Example 1 and Comparative Example 4, respectively, FIG. 4(C) shows a lower coating film of a plasma-resistant coating film according to the present invention, and FIG. 4(D) shows a surface of a plasma-resistant coating film manufactured according to Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by experts skilled in the art to which the present invention pertains. In general, the nomenclature used herein is well known and commonly used in the art.

[0046] Throughout the present specification, when a part “includes” a certain component, this means that it may further include other components rather than excluding other components, unless specifically stated to the contrary.

[0047] In one aspect of the present invention, there is provided a method of manufacturing a plasma-resistant coating film, the method including:

[0048] (1) forming a lower coating layer on a base member to be coated from a first rare earth metal compound powder including 90 to 99.9 wt % of first rare earth metal compound particles and 0.1 to 10 wt % of silica (SiO.sub.2) particles through a thermal spray process; and

[0049] (2) processing the surface of the lower coating layer formed in step (1) to have an average surface roughness of 1 to 6 μm; and

[0050] (3) forming an upper coating layer on the lower coating layer which is surface-treated in step (2) from a second rare earth metal compound particles through a suspension plasma spray process.

[0051] More specifically, as illustrated in FIG. 2, a method of manufacturing a plasma-resistant coating film according to the present invention forms a lower coating layer (B) on a base member to be coated (A) from a first rare earth metal compound powder including 0.1 to 10 wt % of silica (SiO.sub.2) particles through a thermal spray process. After processing the surface of the lower coating layer (B) to have an average surface roughness of 1 to 6 μm, an upper coating layer (C) from a second rare earth metal compound is formed on the lower coating layer (B) which is surface-treated through a suspension plasma spray (SPS) process having a high coating density to obtain a high dense plasma-resistant coating film having excellent adhesive strength between layers, plasma-resistant and breakdown voltage.

[0052] A method of manufacturing a plasma-resistant coating film according to the present invention, first, forms a lower coating layer on a base member to be coated from a first rare earth metal compound powder including 90 to 99.9 wt % of first rare earth metal compound particles and 0.1 to 10 wt % of silica (SiO.sub.2) particles through a thermal spray process [Step (1)].

[0053] The base member to be coated may be a plasma device part such as electrostatic chuck applied inside the plasma device, a heater, a chamber liner, a shower head, a boat for CVD, a focus ring, a wall liner, etc., the material of the base member to be coated may be metal such as iron, magnesium, aluminum or ally thereof; ceramic such as SiO.sub.2, MgO, CaCO3, alumina, etc.; and polymer such as polyethylene terephthalate, polyethylene naphthalate, polypropylene adipate, polyisocyanate, etc., but is not limited thereto.

[0054] In addition, by sanding the surface of the base member to be coated to provide a certain surface roughness, the adhesive characteristics of the lower coating layer including the base member to be coated and the first rare earth metal compound to be formed thereafter can be improved.

[0055] In the present embodiment, the base member to be coated may be sanded to have a surface roughness having an average center roughness value of about 1 to 6 μm. When the surface roughness of the base member to be coated is less than 1 μm, the adhesive characteristics of the lower coating layer including the first rare earth metal compound and the silica compound formed thereafter and the base member to be coated are lowered, there may be a problem that the lower coating layer may be easily peeled off from the base member to be coated by an external impact. On the other hand, when the surface roughness of the base member to be coated exceeds 6 μm due to the sanding, it may affect the surface roughness of the lower coating layer formed thereafter and thus, there may be a problem that the upper coating layer including the second rare earth metal compound formed on the lower coating layer may not be formed in a uniform thickness.

[0056] Meanwhile, as illustrated in Table 1 below, when a coating film is manufactured using a rare earth metal compound powder including a small amount of silica component as a thermal spray material in addition to the rare earth metal compound particles, there is an effect of improving the adhesive strength of the coating layer.

TABLE-US-00001 TABLE 1 Coating Adhesive Classification method Kind of coating film strength (MPa) 1 APS YF3 10.0 2 APS YOF 8.0 3 APS Y2O3 20.0 (+SiO.sub.2 0.1~10 wt. %)

[0057] Therefore, in manufacturing a plasma-resistant coating layer according to the present invention, in order to improve the adhesive strength of the lower coating layer, the first rare earth metal compound powder may include 90 to 99.9 wt % of the first rare earth metal compound particles and 0.1 to 10 wt % of silica (SiO.sub.2) particles, thereby further improving the adhesive characteristics of the lower coating layer, and more preferably, 95 to 99.9 wt % of the first rare earth metal compound particles and 0.1 to 5 wt % of silica (SiO.sub.2) particles.

[0058] In this case, the first rare earth metal compound may be selected from the group consisting of yttria (Y.sub.2O.sub.3), yttrium fluoride (YF), and yttrium oxyfluride (YOF), and specifically, yttria (Y.sub.2O.sub.3), is preferable.

[0059] In step (1), the lower coating layer is a coating layer formed on a base member to be coated from a first rare earth metal compound powder including a first rare earth metal compound and silica through a thermal spray process. It is preferable to manufacture it by using the first rare earth metal compound powder having a particle size of 10 to 60 μm, and more preferably, 20 to 40 μm. When the first rare earth metal compound powder is less than 10 μm, the granular powders may agglomerate with each other due to electrostatic attraction between the granular powders, making it practically difficult to transfer in the atmosphere, or due to low mass after transfer of the granular powder, it is highly possible that the transfer to the central frame of the thermal spray gun may not performed and deviate from the target position. When it exceeds 60 μm, the size of the droplet increases and the size of the defect is formed relatively large in the process of solidification of the droplet, resulting in a decrease in density, such that the surface roughness of the first rare earth metal compound coating layer cannot form a uniform thin film.

[0060] In addition, it is preferably that the first rare earth metal compound coating layer has a thickness of 50 to 500 μm, and more preferably, 100 to 200 μm. When the first rare earth metal compound coating layer has a thickness of less than 50 μm, the effect of improving the entire film formation speed is reduced, and when it exceeds 500 μm, the process time is increased, thereby reducing productivity.

[0061] In addition, it is preferable that the porosity of the lower coating layer formed by thermal spraying the first rare earth metal compound powder has a porosity of less than 2 vol %.

[0062] The thermal spray in step (1) may be applied without limitation as long as it is a thermal spray coating capable of forming a coating layer that satisfies the requirements such as strong bonding force and corrosion resistance between the base member to be coated and the lower coating layer, etc., and preferably in terms of high hardness and high electrical resistance of the coating layer, plasma thermal coating may be applied.

[0063] Specifically, the thermal spraying in step (1) may be performed by a plasma spray method, and the plasma spray method may be a form of atmospheric plasma spraying (APS) performed in the atmosphere, low pressure plasma spraying (LSP) performed spraying at a pressure lower than atmospheric pressure, and high pressure plasma spraying performed in a pressurized container at higher than atmospheric pressure.

[0064] According to such plasma spray, for example, a coating layer may be manufactured by generating plasma under condition of a voltage of 80.0V and a current of 600 A using, for example, argon gas 40NLPM and hydrogen gas 8NLPM.

[0065] In addition, thermal spray of the present invention may be performed by atmospheric pressure plasma spray. In this case, the plasma gas is not particularly limited, and may be appropriately selected, and for example, nitrogen/hydrogen, argon/hydrogen, argon/helium, argon/nitrogen, etc. may be used, and in the present invention, it is desirable that argon/hydrogen is sprayed.

[0066] In addition, as a specific example of plasma spray, in the case of argon/hydrogen plasma spray, atmospheric pressure plasma spray using a mixed gas of argon and hydrogen in an atmospheric atmosphere is mentioned. The thermal spray conditions such as a thermal spray distance, a current value, a voltage value, an argon gas supply amount, a hydrogen gas supply amount, etc. are set according to the use of the thermal spray member, etc. An amount of thermal spray material is filled in a powder supply device, and powder is supplied to the front end of the plasma spray gun by a carrier gas (argon) through a powder hose. By continuously supplying the powder in the plasma flame, the thermal spray material is melted and liquefied, and is liquid-framed with the force of the plasma jet. When liquid frame touches the substrate, the molten powder adheres, solidifies, and is deposited, thereby forming the first rare earth metal compound coating layer.

[0067] Subsequently, step (2) is a step of processing the surface of the lower coating layer including the first rare earth metal compound and the silica component to have an average surface roughness of 1 to 6 μm.

[0068] In the method of manufacturing a plasma-resistant coating film according to the present invention, step (2) is a step of processing the surface of the lower coating layer formed in step (1) to have an average surface roughness of 1 to 6 μm, and after grinding to have a uniform thickness, the surface of the lower coating layer is roughly processed to have an average surface roughness of 1 to 6 μm.

[0069] In this case, the surface processing may be performed by polishing using a diamond pad but is not limited thereto. In addition to polishing using a diamond pad, it may be polished using chemical mechanical polishing (CMP) or other polishing procedures.

[0070] Through the surface processing, the surface of the lower coating layer formed in step (1) may be roughened to have an average surface roughness of 1 to 6 μm, thereby improving adhesive strength between the lower coating layer and the upper coating layer. When the surface of the lower coating layer has an average surface roughness of 6 μm or more, the surface roughness is excessively high and thus coating is not properly performed, thereby causing peeling of the upper coating layer.

[0071] Subsequently, step (3) is a step of forming an upper coating layer by depositing a second rare earth metal compound by suspension plasma spray to form a denser coating layer on the lower coating layer.

[0072] Meanwhile, in forming the double coating layer, an upper layer may be formed by aerosol deposition on a lower coating layer manufactured by atmospheric plasma spray as disclosed in the prior literature. In addition, as in the present invention, after forming the lower coating layer by atmospheric plasma spray, the upper coating layer may be formed by aerosol suspension plasma spray deposition.

[0073] In this case, as illustrated in Table below, as the coating layer manufactured by atmospheric plasma spray generates tensile stress, and the coating layer formed by aerosol deposition generates compressive stress due to mechanical collision, when the atmospheric plasma spray and the aerosol deposition are simultaneously applied, peeling and breaking of the coating layer may be induced due to the stress difference between the coating layers, whereas as the same tensile stress is generated in the coating layer manufactured by atmospheric plasma spray and the coating layer manufactured by suspension plasma spray, peeling between coating layers due to the stress difference does not occur.

TABLE-US-00002 TABLE 2 APS SPS Aerosol Deposition Coating stress −3.0 ± 6.0   −3.0 ± 6.0   −195.9 ± 49.9    (MPa) Base metal stress −26.9 ± 13.9    −30 ± 15.8  −63.9 ± 17.1   (MPa) Base metal stress-   −10~−43.8   −10~−55.8 99.2~199  coating stress (MPa) Stress Tensile Tensile Compressive stress stress stress

[0074] Therefore, in forming the plasma-resistant coating film of the present invention, in order to improve adhesive strength between the lower coating layer and the upper coating layer manufactured by atmospheric plasma spray, an upper coating layer including a second rare earth compound was formed using a suspension plasma spray method.

[0075] As an embodiment, a second rare earth metal compound suspension composition for forming an upper coating layer including a second rare earth metal compound will be described.

[0076] As an embodiment, the second rare earth metal compound powder is ball milled in the range of 100 to 140 revolution per minute (RPM) for 3 hours or more to manufacture solid contents and then distilled water is mixed. A slurry composition is prepared by adding a dispersant, etc. as an additive. In this case, the content of the second rare earth metal compound powder may be contained in an amount of 10 to 50 parts by weight based on 100 parts by weight of distilled water.

[0077] In this case, it is preferable for the second rare metal compound to use a powder having a particle size of 0.1 to 10 μm, and more preferably, 1 to 5 μm. When the second rare earth metal compounder powder is less than 0.1 μm, it is difficult to agglomerate and disperse the powder of the second rare earth metal compound powder in the solvent, and when it exceeds 10 μm, it is difficult to achieve the object of the present invention by increasing the surface roughness and porosity of the second rare earth metal compound coating layer.

[0078] According to such suspension plasma spray, for example, the plasma generation condition may be supplied at a flow rate of argon gas 340SCFH, nitrogen gas 100SCFH, and hydrogen gas 80SCFH to generate plasma under a condition of voltage 285.0V and current 380 A to form a coating player.

[0079] In this case, the upper coating layer including the second rare earth metal compound may be formed by repeatedly laminating the second rare earth metal compound twice or more using the suspension plasma spray method.

[0080] In addition, it is preferable that the upper coating layer including the second rare metal compound has a thickness of 50 to 150 μm. When the second rare earth metal compound coating layer has a thickness of less than 50 μm, it is difficult to secure plasma resistance because the thickness of the chemically stable and dense second rare earth metal compound coating layer is not sufficient. When it exceeds 150 μm, peeling may occur due to residual stress of the coating layer and a phenomenon in which breakdown voltage characteristics deteriorate occurs.

[0081] As illustrated in Table 3 below, the coating layer formed by the suspension plasma spray of yttria (Y.sub.2O.sub.3) solution exhibits an effect of improving the breakdown voltage characteristics as the thickness of the coating layer increases in the range of 150 μm or less, whereas when the thickness of the coating layer exceeds 150 μm, the breakdown voltage characteristics rather deteriorate as the thickness of the coating layer increases.

TABLE-US-00003 TABLE 3 Coating Kind of coating Thickness of Breakdown classification method film coating film voltage (V) 1 SPS Y2O3 coating film 50 2,384 2 SPS Y2O3 coating film 100 2,453 3 SPS Y2O3 coating film 150 2,173 4 SPS Y2O3 coating film 200 1,987

[0082] In this case, it is preferable that the upper coating layer including the second rare earth metal compound has a low porosity and is dense in order to secure the mechanical strength and electrical characteristics of the plasma-resistant coating film.

[0083] Therefore, the upper coating layer including the second rare earth metal compound formed by the suspension plasma spray coating has a porosity of less than 1 vol %, and it is preferable to indicate a value lower than 2 vol %, which is the porosity of the lower coating layer including the first rare earth metal compound and the silica compound.

[0084] As an embodiment, it is more preferably that the upper coating layer including the second rare earth metal compound has a porosity less than 40% or less of the porosity of the lower coating layer including the first rare earth metal compound and the silica compound to form a chemically stable coating film with improved plasma resistance.

[0085] In addition, the second rare earth metal compound may be selected from the group consisting of yttria (Y.sub.2O.sub.3), yttrium fluoride (YF), and yttrium oxyfluoride, and preferably, yttria is desirable. More preferably, when the first rare earth metal compound and the second rare earth metal compound are the same compound, the adhesive strength between the first rare earth metal compound coating layer (lower coating layer) and the second rare earth metal compound coating layer (upper coating layer) is improved, thereby minimize peeling of the coating film, generation of particles during the manufacturing process and contamination of the wafer therefrom.

[0086] In another aspect of the present invention, there is provided a plasma-resistant member manufactured by the method of manufacturing a plasma-resistant coating film, the method including:

[0087] (1) forming a lower coating layer on a base member to be coated from a first rare earth metal compound powder including 90 to 99.9 wt % of first rare earth metal compound particles and 0.1 to 10 wt % of silica (SiO.sub.2) particles through a thermal spray process;

[0088] (2) processing the surface of the first rare earth metal compound coating layer formed in step (1) to have an average surface roughness of 1 to 6 μm; and

[0089] (3) forming an upper coating layer on the first rare earth metal compound coating layer which is surface-treated in step (2) from a second rare earth metal compound particles through a suspension plasma spray process.

[0090] In further another aspect of the present invention, there is provided a plasma-resistant coating film including:

[0091] a lower coating layer formed on a base member to be coated and made from a first rare earth metal powder including 90 to 99.9 wt % of first rare earth metal compound particles and 0.1 to 10 wt % of silica (SiO.sub.2) particles through a thermal spray process, the lower coating layer having an adhesive strength of 20 MPa or more with respect to the base member; and

[0092] an upper coating layer formed on the lower coating layer and made from second rare earth metal compound particles through a suspension plasma spray process,

[0093] in which the plasma-resistant coating film has a porosity of less than 1 vol %.

[0094] In this case, each of the first rare earth metal compound and the second rare earth metal compound may be selected from the group consisting of yttria (Y.sub.2O.sub.3), yttrium fluoride (YF), and yttrium oxyfluoride (YOF), and preferably, the first rare earth metal compound may be yttria (Y.sub.2O.sub.3).

[0095] In addition, the lower coating layer may have a porosity of less than 2 vol %, the upper coating layer may have a porosity of less than 1 vol %, the lower coating layer may have a thickness of 50 to 500 μm and the upper coating layer may have a thickness of 50 to 150 μm.

[0096] Hereinafter, the present invention will be described in more detail with reference to the examples. The following examples are intended to illustrate the present invention, but the present invention is not limited by the following examples.

Comparative Example 1 to 3

[0097] It was performed using an atmospheric plasma spray device (Oerlikon Metco, F4 MB), and using argon gas 40 NLPM and hydrogen gas 8 NLPM, plasma was generated under the condition of voltage 80.0V and current 600 A to form a coating layer with Y.sub.2O.sub.3, YF.sub.3 or YOF thermal spray coating powder to a thickness of 150 μm.

Comparative Example 4 to 6

[0098] It was performed using a suspension plasma thermal spray device (Progressive, 100HE), and using a flow rate of argon gas 340SCFH, nitrogen gas 100SCFH, and hydrogen gas 80SCFH, plasma was generated under the condition of voltage 285.0V and current 380 A to form a Y.sub.2O.sub.3, YF.sub.3 or YOF coating layer to a thickness of 100 μm.

TABLE-US-00004 TABLE 4 Coating Hardness Porosity Surface Adhesive Classification Material method (Hv) (%) roughness strength Comparative Y2O3 APS 415 4.5 4.5 ± 0.4 10.0 example 1 Comparative YF3 APS 272 1.7 5.1 ± 0.8 10.0 example 2 Comparative YOF APS 377 4.4 4.6 ± 0.5 8.0 example 3 Comparative Y2O3 SPS 524 0.6 2.0 ± 0.5 15.0 example 4 Comparative YF3 SPS 466 0.8 2.2 ± 0.4 13.0 example 5 Comparative YOF SPS 497 0.8 1.7 ± 0.2 10.0 example 6

Examples 1 to 3

[0099] 1-1: Forming Lower Coating Layer

[0100] It was performed using an atmospheric plasma spraying device (Oerlikon Metco, F4 MB), and using argon gas 40 NLPM and hydrogen gas 8 NLPM, plasma was generated under the condition of voltage 80.0V and current 600 A to form a coating layer with an average thickness of 200 μm. Then, surface polishing was performed so that the surface roughness of the coating layer was 1 to 3 μm and the thickness of the coating layer was 150 μm.

[0101] 1-2: Forming Upper Coating Layer

[0102] It was performed using a suspension plasma thermal spray device (Progressive, 100HE), and using a flow rate of argon gas 340SCFH, nitrogen gas 100SCFH, and hydrogen gas 80SCFH, plasma was generated under the condition of voltage 285.0V and current 380 A to form a Y.sub.2O.sub.3, YF.sub.3 or YOF coating layer to a thickness of 50 μm.

TABLE-US-00005 TABLE 5 Lower coating Upper coating layer layer Coating Coating Hardness Porosity Surface Adhesive classification Material method Material method (Hv) (%) roughness strength Example 1 Y2O3 + 1 APS Y2O3 SPS 542, 0.8 1.8 ± 0.2 20.0 wt % SiO.sub.2 531 Example 2 Y2O3 + 1 APS YF3 SPS 554, 0.9 1.9 ± 0.3 20.0 wt % SiO.sub.2 458 Example 3 Y2O3 + 1 APS YOF SPS 548, 0.9 1.8 ± 0.3 20.0 wt % SiO.sub.2 487

[0103] As illustrated in Table 4, it was confirmed that the plasma-resistant coating films according to Examples 1 to 3 not only have better adhesive strength than the coating films according to Comparative Examples 1 to 3, but also have excellent mechanical properties and form a dense thin film.

[0104] In addition, as illustrated in FIGS. 3(D) and 4(D) below, it was confirmed that the high-density upper coating layer in the plasma-resistant coating film manufactured by Example 1 forms a coating layer of a very dense structure compared to the lower coating layer.

DESCRIPTION OF LETTER IN DRAWINGS

[0105] A: base member to be coated [0106] B: first rare earth metal compound coating layer (lower coating layer) [0107] C: second rare earth metal compound coating layer (upper coating layer)