YTTRIUM-BASED SPRAYED COATING AND MAKING METHOD

Abstract

An yttrium-base sprayed coating is obtained by thermally spraying yttrium oxide, yttrium fluoride or yttrium oxyfluoride onto a substrate to form a coating of 10-500 m thick, and chemically cleaning the coating with a cleaning liquid of organic acid, inorganic acid or a mixture thereof until the population of particles with a size of up to 300 nm becomes no more than 5 particles/mm.sup.2 of the coating surface. The yttrium-base sprayed coating exhibits high corrosion resistance even in a halogen gas plasma atmosphere and prevents yttrium-base particles from spalling off during etching treatment.

Claims

1. A method for preparing a yttrium-base sprayed coating, comprising the steps of: chemically cleaning a surface of a yttrium-base sprayed coating having a thickness of 10 to 500 m that is formed on a substrate readily dissolved in acid by thermally spaying a spray material comprising particles of one or two or more selected from the group consisting of yttrium oxide, yttrium fluoride and yttrium oxyfluoride with a cleaning liquid composed of an organic acid aqueous solution so as to be a number of particles with a size of up to 300 nm which exist on the surface of the coating of not more than 5 particles per square millimeters, wherein the organic acid aqueous solution is an aqueous solution of at least one selected from the group consisting of a monofunctional carboxylic acid, a difunctional carboxylic acid, a trifunctional carboxylic acid and a hydroxy acid, or an aqueous solution of a mixture of at least two thereof.

2. The method of claim 1 wherein the monofunctional carboxylic acid is formic acid or acetic acid, the difunctional carboxylic acid is maleic acid or tartaric acid or phthalic acid, the trifunctional carboxylic acid is citric acid, and the hydroxy acid is lactic acid.

3. The method of claim 1 wherein the substrate readily dissolved in acid is selected from the group consisting of stainless steel, and aluminum, nickel, chromium, zinc and an alloy thereof, and metal silicon.

4. The method of claim 2 wherein the substrate readily dissolved in acid is selected from the group consisting of stainless steel, and aluminum, nickel, chromium, zinc and an alloy thereof, and metal silicon.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0019] FIGS. 1, 2, 3 and 4 are SEM images of the surface of yttrium-base sprayed coatings in Examples 1, 2, 3 and 4, respectively.

[0020] FIGS. 5 and 6 are SEM images of the surface of yttrium-base sprayed coatings in Comparative Examples 1 and 2, respectively.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0021] The yttrium-base sprayed coating of the invention is formed by thermally spraying one or more compounds selected from among yttrium oxide, yttrium fluoride, and yttrium oxyfluoride.

[0022] Thermal spraying to a substrate is desirably atmospheric plasma spraying or vacuum plasma spraying. The plasma gas used herein may be nitrogen/hydrogen, argon/hydrogen, argon/helium, argon/nitrogen, argon alone, or nitrogen gas alone, but not limited thereto. Examples of the substrate subject to thermal spraying include, but are not limited to, substrates of stainless steel, aluminum, nickel, chromium, zinc, and alloys thereof, metal silicon, aluminum oxide, aluminum nitride, silicon nitride, silicon carbide, and quartz glass when parts or components of the semiconductor fabrication equipment are contemplated. The conditions under which yttrium oxide, yttrium fluoride or yttrium oxyfluoride is thermally sprayed are not particularly limited. The thermal spraying conditions may be determined as appropriate depending on the identity of substrate, the particle size and composition of spray material, and a particular application of the resulting sprayed component.

[0023] For example, when an yttrium oxide coating is formed on a metal aluminum substrate, it may be deposited by argon/hydrogen atmospheric plasma spraying using yttrium oxide powder having an average particle size D50 of about 20 m and a gas mixture of 40 L/min of argon and 5 L/min of hydrogen. The thermal spraying conditions including a spray distance, current value and voltage value may be determined as appropriate depending on a particular application of the sprayed component. Likewise, the feed rates of argon and hydrogen gases may be suitably adjusted.

[0024] The sprayed coating, i.e., yttrium-base sprayed coating should have a thickness of 10 to 500 m. A coating of less than 10 m thick may be less corrosion resistant or allow the substrate surface to be partly exposed in the cleaning step to be described below. A coating of more than 500 m thick may simply add to the cost because no further improvement in corrosion resistance is expectable. The thickness of the coating is preferably 80 to 400 m, more preferably 100 to 400 m, and even more preferably 100 to 300 m.

[0025] According to the invention, the surface of the yttrium-base sprayed coating is then cleaned with a preselected cleaning liquid to remove yttrium-base particles anchored thereto until the population (or number) of yttrium-base particles with a size of up to 300 nm becomes no more than 5 particles/square millimeters (mm.sup.2) of the coating surface. It is, of course, most preferred that the population of yttrium-base particles with a size of up to 300 nm on the coating surface be 0. As long as the population is no more than 5 particles/mm.sup.2, dusting to such an extent as to invite a substantial loss of production yield does not occur during etching treatment in the semiconductor device fabrication process. As used herein, the size of yttrium-base particles refers to the maximum diameter of individual particles measured by microscopy under a scanning electron microscope (SEM) or the like. As seen from the images of FIGS. 5 and 6, no or only a few particles with a size in excess of 300 nm are present on the sprayed coating surface. Removal of particles with a size of up to 300 nm means removal of substantially all inhibitory particles.

[0026] The cleaning liquid is an aqueous solution of organic acid, aqueous solution of inorganic acid or aqueous solution of mixed organic and inorganic acids. The organic acid is not particularly limited as long as it is water-soluble. Suitable organic acids include, but are not limited to, monofunctional carboxylic acids such as formic acid and acetic acid, difunctional carboxylic acids such as maleic acid, tartaric acid and phthalic acid, trifunctional carboxylic acids such as citric acid, hydroxy acids such as lactic acid, and sulfonic acids such as methanesulfonic acid. Inter alia, tartaric acid and citric acid are preferred because they are edible, nontoxic and easy to handle. The inorganic acid is not particularly limited as long as it is water-soluble. Suitable inorganic acids include nitric acid, sulfuric acid, carbonic acid, hydrofluoric acid, and acidic ammonium fluoride.

[0027] The cleaning technique is not particularly limited. Preferably, a part or component in the form of a substrate having the yttrium-base sprayed coating formed on its surface is wholly immersed in the cleaning liquid because this technique is effective and efficient. For those substrates of metallic aluminum and silicon which are readily dissolved in acid, the area of the substrate that should avoid corrosion with acid is desirably masked with resin tape or sheet when a strong acid is used for cleaning. Cleaning without masking is possible when a weak organic acid is used for cleaning, for example, a carboxylic acid or hydroxy acid such as phthalic acid, tartaric acid or citric acid. For those substrates of quart glass or Al.sub.2O.sub.3 ceramics which are acid resistant, cleaning without masking is possible even with a strong acid solution such as nitric acid. In some cases, a buffer solution based on a combination of acid and salt may be used as the cleaning liquid.

[0028] The yttrium-base sprayed coating is chemically cleaned with the cleaning liquid to dissolve a thin layer from the coating surface for removing particles with a size of up to 300 nm which become a source of dusting. The dissolution depth is preferably at least 0.01 m from the original coating surface. Although the upper limit of dissolution depth is not critical, the dissolution depth is preferably up to 20 m. More preferably the dissolution depth is 1 to 20 m from the coating surface. A dissolution depth of less than 0.01 m may be insufficient to remove particles with a size of up to 300 nm and fail to reach a population of no more than 5 particles/mm.sup.2. A dissolution depth in excess of 20 m may simply make the coating thinner without further improvements in particle removal.

[0029] After cleaning, the coating is rinsed with ultrapure water to thoroughly remove the acid and dried in vacuum or under atmospheric pressure.

[0030] When a secondary electron image (magnification 10,000 or more) of the dry coating surface is observed under SEM, yttrium-base particles having a size of up to 300 nm on the coating surface are detectable. According to the invention, yttrium-base particles are removed from the coating surface by the cleaning step until the population of particles reaches no more than 5 particles/mm 2 of the surface.

EXAMPLE

[0031] Examples are given below by way of illustration and not by way of limitation.

Examples 1 to 4 and Comparative Examples 1 and 2

Preparation of Sprayed Coating

[0032] An yttrium-base sprayed coating was obtained by thermally spraying the coating material shown in Table 1 onto a surface of a substrate of the material shown in Table 1, immersing the coated substrate in a cleaning liquid, which was an aqueous solution of the cleaning agent shown in Table 1, to clean the coating surface, thoroughly rinsing with ultrapure water, and vacuum drying. The surface of the yttrium-base coating thus obtained was observed under SEM, and yttrium-base particles having a size of up to 300 nm on the surface were inspected and counted. The results are shown in Table 1 and SEM images are shown in FIGS. 1 to 6. Notably, the yttrium-base sprayed coating was formed by atmospheric plasma spraying using a gas mixture of 40 L/min of argon and 8 L/min of hydrogen.

TABLE-US-00001 TABLE 1 Example Comparative Example 1 2 3 4 1 2 Spray Material Y.sub.2O.sub.3 Y.sub.2O.sub.3 + YF.sub.3 YF.sub.3 YOF Y.sub.2O.sub.3 YF.sub.3 Coating thickness, 200 300 100 200 200 200 m Substrate material Al Al.sub.2O.sub.3 Si Al Al Al Cleaning Cleaning agent tartaric citric hydrofluoric lactic no no conditions acid acid acid + acid cleaning cleaning acidic ammonium fluoride Concentration, 2 1 0.05 + 0.1 2 mol/L Temperature, 30 50 30 50 C. Time, hr 4 12 0.5 12 Dissolution 2 20 2 10 depth, m Particle population 0 0 0 0 numerous numerous on surface (particles/mm.sup.2) SEM image FIG. 1 FIG. 2 FIG. 3 FIG. 4 FIG. 5 FIG. 6

[0033] As is evident from Table 1 and FIGS. 1 to 6, the yttrium-base sprayed coatings in Examples 1 to 4 bear no particles on their surface whereas numerous particles are on the yttrium-base sprayed coatings in Comparative Examples 1 and 2 which omit cleaning with an aqueous solution of acid or cleaning agent. It is readily presumed that these particles cause dust generation during etching treatment. When parts or components having yttrium-base sprayed coatings of Examples 1 to 4 deposited thereon are used, dusting as a result of yttrium-base particles spalling off during etching treatment in a semiconductor device fabrication process is substantially prevented. This will eventually improve the fabrication yield of semiconductor devices.

[0034] Japanese Patent Application No. 2015-151568 is incorporated herein by reference.

[0035] Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.