Electrostatic chuck member having an alumina-titania spray coated layer and a method of producing the same

What is claimed is:

1. An electrostatic chuck member comprising:

a metal substrate;

an undercoat of a metallic spray-coated layer formed on the metal substrate; and

an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer formed by spraying an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic material comprising 2-30 wt % of TiO.sub.2 on the metallic spray-coated layer such that at least a portion of the TiO.sub.2 is changed into at least one TiO.sub.2 n-.sub.1, where n=1 to 9, compound.

2. The electrostatic chuck member according to claim 1, wherein a thickness of the metallic spray-coated layer is 30-150 .mu.m and a thickness of the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer comprising the at least one Ti.sub.n O.sub.2n-1 compound is 50-500 .mu.m.

3. The electrostatic chuck member according to claim 1, wherein the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer comprising the at least one Ti.sub.n O.sub.2n-1 compound has a porosity of 0.4-3.0% and a surface roughness Ra of 0.1-2.0 .mu.m.

4. The electrostatic chuck member according to claim 1, further comprising a seal-treated layer of one of an organic silicon compound and an inorganic silicon compound on a surface of the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer comprising the at least one Ti.sub.n O.sub.2n-1 compound, and wherein the Al.sub.2 O.sub.3 TiO.sub.2 based spray-coated layer comprising the at least one Ti.sub.n O.sub.2n-1 compound has a volume resistivity of 1.times.10.sup.9 -1.times.10.sup.11 .omega..multidot.cm.

5. The electrostatic chuck member according to claim 1, wherein the metallic spray-coated layer is a layer made from at least one starting substance selected from the group consisting of Ni, Al, Cr, Co, Mo, and alloys thereof.

6. The electrostatic chuck member according to claim 1, wherein the at least one Ti.sub.n O.sub.2n-1 compound included in the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer is at least one compound selected from the group consisting of Ti.sub.3 O.sub.5, Ti.sub.2 O.sub.3, TiO, Ti.sub.4 O.sub.7, Ti.sub.5 O.sub.9, Ti.sub.6 O.sub..sub.11, Ti.sub.8 O.sub.15, Ti.sub.7 O.sub.13, and Ti.sub.9 O.sub.17.

7. A method of producing an electrostatic chuck member, which comprises:

subjecting a metal substrate to a blast treatment;

forming an undercoat of a metallic spray-coated layer on a surface of the substrates and applying an Al.sub.2 O.sub.3 TiO.sub.2 based ceramic spraying material comprising 2-30 wt % of TiO.sub.2 on the metallic spray-coated layer in Ar gas or air atmosphere adjusted to a pressure of 30-750 hPa through a plasma spraying process using hydrogen gas to form a top spray-coated layer in which one of a part and a whole of TiO.sub.2 in the spraying material is changed into at least one Ti.sub.n O.sub.2n-1 crystalline compound.

8. A method of producing an electrostatic chuck member, which comprises:

subjecting a metal substrate to a blast treatment;

forming an undercoat of a metallic spray-coated layer on a surface of the substrate;

applying an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spraying material comprising 2-30 wt % of TiO.sub.2 on the metallic spray-coated layer in Ar gas or air atmosphere adjusted to a pressure of 30-750 hPa through a plasma spraying process using hydrogen gas to form a top spray-coated layer in which one of a part and a whole of TiO.sub.2 in the spraying material is changed into at least one Ti.sub.n O.sub.2n-1 crystalline compound; and

thereafter finish-grinding the top spray-coated layer to a surface roughness Ra of 0.1-2.0 .mu.m.

9. A method of producing an electrostatic chuck member, which comprises:

subjecting a metal substrate to a blast treatment;

forming an undercoat of a metallic spray-coated layer on a surface of the substrate;

applying an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spraying material comprising 2-30 wt % of TiO.sub.2 on the metallic spray-coated layer in Ar gas or air atmosphere adjusted to a pressure of 30-750 hPa through a plasma spraying process using hydrogen gas to form a top spray-coated layer in which one of a part and a whole of TiO.sub.2 in the spraying material is changed into at least one Ti.sub.n O.sub.2n-1 crystalline compound;

thereafter finish-grinding the top spray-coated layer to a surface roughness Ra of 0.1-2.0 .mu.m; and

then subjecting the finish ground surface to a seal treatment with a silicon compound.

10. The method according to claim 9, wherein the seal treatment is carried out by applying an organic or inorganic silicon compound to the surface of the top spray-coated layer and then heating at 120-350.degree. C. for 1-5 hours.

11. An electrostatic chuck member comprising:

a metal substrate;

an undercoat of a metallic spray-coated layer formed on the metal substrate; and

an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer comprising at least one Ti.sub.n O.sub.2n-1 where n=1 to 9, compound formed on the metallic spray-coated layer.

12. The electrostatic chuck member according to claim 11, wherein a thickness of the metallic spray-coated layer is 30-150 .mu.m and a thickness of the Al.sub.2 O.sub.3 TiO.sub.2 based spray-coated layer comprising the at least one Ti.sub.n O.sub.2n-1 compound is 50-500 .mu.m.

13. The electrostatic chuck member according to claim 11, wherein the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer comprising the at least one Ti.sub.n O.sub.2n-1 compound has a porosity of 0.4-3.0% and a surface roughness Ra of 0.1-2.0 .mu.m.

14. The electrostatic chuck member according to claim 11, further comprising a seal-treated layer of one of an organic silicon compound and an inorganic silicon compound on a surface of the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer comprising the at least one Ti.sub.n O.sub.2n-1 compound, and wherein the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer comprising the at least one Ti.sub.n O.sub.2n-1 compound has a volume resistivity of 1.times.10.sup.9 -1.times.10.sup.11 .omega..multidot.cm.

15. The electrostatic chuck member according to claim 11, wherein the metallic spray-coated layer is a layer made from at least one starting material selected from the group consisting of Ni, Al, Cr, Co, Mo, and alloys thereof.

16. A method of producing an electrostatic chuck member, comprising:

subjecting a metal substrate to a blast treatment;

spray-coating a metallic undercoat on a surface of the metal substrate; and

spray-coating Al.sub.2 O.sub.3 .multidot.TiO.sub.2 on to the metallic undercoat to form a Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based layer in which at least a portion of the TiO.sub.2 is changed into Ti.sub.n O.sub.2n-1, where n=1-9.

17. The method according to claim 16, wherein the spray-coating of the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 comprises plasma spraying hydrogen gas and Al.sub.2 O.sub.3 .multidot.TiO.sub.2 in one of Ar gas and air atmosphere at a pressure of 30-750 hPa.

18. The method according to claim 16, further comprising finish-grinding the Al.sub.2 O.sub.3 TiO.sub.2 based layer to a surface roughness Ra of 0.1-2.0 .mu.m.

19. The method according to claim 18, further comprising subjecting the finish ground surface to a seal treatment with a silicon compound.

20. The method according to claim 19, wherein the seal treatment comprises applying one of an organic silicon compound and an inorganic silicon compound to the finish ground surface and then heating at 120-350.degree. C. for 1-5 hours.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrostatic chuck members and a method of producing the same. The electrostatic chuck members are used when a conductive member, semi-conductive member, insulative member or the like is held at adsorption state by static electricity and particularly incorporated into a dry etching device, an ion implantation device, a CVD device or a PVD device used in the production process of large size integrations or liquid crystals display.

2. Description of Related Art

Recently, treatments such as dry etching, ion implantation, CVD, PVD and the like constituting a part of a production process for the large size integrations or liquid crystals display, e.g. a production device of semiconductors changes from a wet process into a dry process from viewpoints of automation and anti-pollution. A greater part of the treatment through the dry process is usually carried out under vacuum.

In the dry treatment, it becomes recently important to improve a positioning accuracy in the formation of pattern on silicon wafer, glass plate or the like, for example, used as a substrate from viewpoints of high integration of circuits and fine work.

In order to satisfy such a demand, vacuum chuck or mechanical chuck has hitherto been adopted in the transportation, adsorption and fixation of the substrate. However, since the vacuum chuck is used under vacuum, the pressure difference is small and the adsorption effect is less. Even if the substrate can be adsorbed, an adsorbing portion becomes local and strain is caused in the substrate. Furthermore, the gas cooling can not be carried out with the temperature rising in the treatment of the wafer, so that the vacuum chuck can not be applied to the recent production process of high-performance semiconductor devices. On the other hand, the mechanical chuck becomes complicated in the structure and takes a long time in the maintenance and inspection thereof.

In order to avoid the above drawbacks of the conventional technique, electrostatic chuck utilizing static electricity is recently developed and widely adopted. However, this technique is indicated to have the following problems.

When the substrate is adsorbed and held by such an electrostatic chuck, charge retains between the substrate and the electrostatic chuck (through the action of adsorption force) even after the applied voltage is stopped, so that the detaching of the substrate can not be carried out unless the charge is completely removed.

For this end, it has been attempted to improve the insulating dielectric material used in the electrostatic chuck. For example, there are proposed 1 JP-A-6-8089 . . . a sintered body of a mixture of aluminum nitride powder and titanium nitride powder or a spray-coated layer thereof is used as a high insulative material; 2 JP-A-6-302677 . . . titanium oxide is applied onto the high insulative material and aluminum is applied thereto to contact with Si+SiC plate; 3 JP-Y-6-36583 . . . use of high insulative substance (aluminum oxide); 4 JP-A-4-304942, JP-A-5-235152, JP-A-6-8089 . . . use of aluminum oxide, aluminum nitride, zinc oxide, quartz, boron nitride, sialon and the like; 5 JP-A-62-94953, JP-A-2-206147, JP-A-3-147843, JP-A-3-204924 and the like . . . volume resistivity is lowered to improve static electricity by adding TiO.sub.2 (titania) having a high dielectric constant to the high insulative material in case of further requiring a higher static electricity.

However, the conventional Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based (alumina-titania) spray-coated layer has the following problems to be solved.

(1) When Al.sub.2 O.sub.3 mixed with TiO.sub.2 is used as a spray-coated layer having an electrostatic adsorption performance, the volume resistivity is small and a slight current flows, so that it can be expected to improve the static electricity through Johonson-Larbec effect. However, TiO.sub.2 (titania) is a semiconductive substance, so that the moving rate of electrical charge is slow and the responsibility (arrival time of saturated adsorption, adsorption disappearing time ) when the application of voltage is stopped is poor. This responsibility becomes more remarkable under low-temperature environment.

In order to render the value of volume resistivity into, for example, a practical state of 1.times.10.sup.9 .OMEGA..multidot.cm, it is necessary to add 25% by weight of titania. In the production process of semiconductors, the addition of a great amount of titania means the incorporation of impurity, which brings about the degradation of quality and results in the contamination of working environment.

Furthermore, when the temperature of the semiconductor wafer to be adsorbed is higher than room temperature, there is a high possibility that a large leak current is passed to break wafer circuit because the volume resistivity is too low.

(2) The Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer is formed by a spraying process. In the resulting layer, however, the volume resistivity and adsorption force are largely scattered and also the productivity is low to bring about the rise of the cost.

(3) The Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer is porous, so that the high surface finishing can not be conducted and the remaining adhesion of foreign matter is frequently caused. Furthermore, the adhesion property to the substrate is low, so that the coated layer is peeled off from the substrate under service condition, particularly thermal change.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electrostatic chuck member having a large volume resistivity, a small scattering thereof and a stable quality.

It is another object of the invention to provide an electrostatic chuck member having a strong adsorption force and an excellent responsibility (release property) in the stop of voltage application.

It is the other object of the invention to establish a technique of advantageously producing the electrostatic chuck member having the above properties with a high productivity.

The inventors have made various studies with respect to the electrostatic chuck member having the aforementioned problems, particularly the member having an Al.sub.2 O.sub.3 TiO.sub.2 based spray-coated layer formed on a substrate and have the following knowledge.

1 The inventors have confirmed from experiments that the problem in the conventional Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer results mainly from TiO.sub.2 (titania). Further, it has been found that this cause can be overcome when TiO.sub.2 is changed into a crystal form of Ti.sub.n O.sub.2n-1 (n=1-9). 2 It has been found that the following methods are effective as a technique of surely providing the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer containing Ti.sub.n O.sub.2n-1 (n=1-9).

a. Method of spraying an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 material under an atmosphere having a low oxygen partial pressure to isolate oxygen from TiO.sub.2 to change into Ti.sub.n O.sub.2n-1 (n=1-9).

In this way, TiO.sub.2 is changed into Ti.sub.n O.sub.2n-1 (n=1-9), whereby the responsibility is improved and the scattering of the volume resistivity is small and also the duality and productivity are improved.

b. The spray-coated layer containing Ti.sub.n O.sub.2n-1 (n=1-9) is obtained by spraying in an atmosphere containing substantially no oxygen or an air atmosphere capable of controlling to a pressure lower than atmospheric pressure with a plasma containing hydrogen as a thermal source. In this point, when the spraying is carried out under a pressure lower than the atmospheric pressure, spraying particles flying in the thermal source has a strong impact force to the substrate because the resistance through gas is small, and hence the dense coated layer having a good adhesion force is formed. Moreover, the dense spray-coated layer is possible to be subjected to a high surface finishing and also has an effect of decreasing the scattering of volume resistivity.

c. When hydrogen gas having a strong reducing action is used in plasma as a spraying heat source, the change of TiO.sub.2 into Ti.sub.n O.sub.2n-1 (n=1-9) rapidly proceeds and the function and mechanism of the above items a and b can more effectively be promoted.

The invention is based on the above knowledge and the main part and construction thereof are shown below.

(1) An electrostatic chuck member comprises a metal substrate, an undercoat of a metallic spray-coated layer formed thereon and an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer containing a Ti.sub.n O.sub.2n-1 (n=1-9) type compound formed thereon.

(2) In the electrostatic chuck member, a thickness of the metallic spray-coated layer is 30-150 .mu.m and a thickness of the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer containing the Ti.sub.n O.sub.2n-1 (n=1-9) type compound is 50-500 .mu.m.

(3) In the electrostatic chuck member, the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer containing the Ti.sub.n O.sub.2n-1 (n=1-9) type compound has a porosity of 0.4-3.0% and a surface roughness Ra of 0.1-2.0 .mu.m.

(4) The Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer containing the Ti.sub.n O.sub.2n-1 (n=1-9) type compound is provided at its surface with a seal-treated layer of an organic or inorganic silicon compound and has a volume resistivity of 1.times.10.sup.9 -1.times.10.sup.11 .OMEGA..multidot.cm.

(5) The metallic spray-coated layer is a layer made from at least one starting substance selected from Ni, Al, Cr, Co, Mo and alloys containing at least one of these metals.

(6) The Ti.sub.n O.sub.2n-1 (n=1-9) type compound included in the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer is one or more compounds selected from the group consisting of Ti.sub.3 O.sub.5, Ti.sub.2 O.sub.3, TiO, Ti.sub.4 O.sub.7, Ti.sub.5 O.sub.9, Ti.sub.6 O.sub.11, Ti.sub.8 O.sub.15, Ti.sub.7 O.sub.13 and Ti.sub.9 O.sub.17.

The above electrostatic chuck member can be produced by adopting the following methods.

(7) A method of producing an electrostatic chuck member comprises subjecting a metal substrate to a blast treatment, forming an undercoat of a metallic spray-coated layer on a surface of the substrate, and applying an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spraying material containing 2-30 wt % of TiO.sub.2 thereonto in Ar gas or air atmosphere adjusted to a pressure of 30-750 hPa through a plasma spraying process containing a hydrogen gas to form a top spray-coated layer in which a part or whole of TiO.sub.2 in the spraying material is changed into a crystalline compound represented by Ti.sub.n O.sub.2n-1 (n=1-9).

(8) A method of producing an electrostatic chuck member comprises subjecting a metal substrate to a blast treatment, forming an undercoat of a metallic spray-coated layer on a surface of the substrate, applying an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spraying material containing 2-30 wt % of TiO.sub.2 thereonto in Ar gas or air atmosphere adjusted to a pressure of 30-750 hPa through a plasma spraying process containing a hydrogen gas to form a top spray-coated layer in which a part or whole of TiO.sub.2 in the spraying material is changed into a crystalline compound represented by Ti.sub.n O.sub.2n-1 (n=1-9) and thereafter finish-grinding the top spray-coated layer to a surface roughness Ra of 0.1-2.0 .mu.m.

(9) A method of producing an electrostatic chuck member comprises subjecting a metal substrate to a blast treatment, forming an undercoat of a metallic spray-coated layer on a surface of the substrate, applying an Al.sub.2 O.sub.3 .multidot.Ti-O.sub.2 based ceramic spraying material containing 2-30 wt % of TiO.sub.2 thereonto in Ar gas or air atmosphere adjusted to a pressure of 30-750 hPa through a plasma spraying process containing a hydrogen gas to form a top spray-coated layer in which a part or whole of TiO.sub.2 in the spraying material is changed into a crystalline compound represented by Ti.sub.n O.sub.2n-1 (n=1-9) and thereafter finish-grinding the top spray-coated layer to a surface roughness Ra of 0.1-2.0 .mu.m, and then subjecting the finish ground surface to a seal treatment with a silicon compound.

(10) The seal treatment is carried out by applying an organic or inorganic silicon compound to the surface of the top spray-coated layer and then heating at 120-250.degree. C. for 1-5 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relation between atmosphere pressure in plasma spraying using an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spraying material and porosity of the resulting spray-coated layer; and

FIG. 2 is a diagrammatic outline of an apparatus for measuring a volume resistivity of an electrostatic chuck member provided with an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer formed through plasma spraying process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An essential feature of the invention lies in that the component of the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer formed on the substrate includes a crystalline compound represented by Ti.sub.n O.sub.2n-1 (n=1-9).

The electrostatic chuck member according to the invention will be described in the order of the production steps together with the method of producing the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spray-coated layer and its function and mechanism.

(1) Formation of undercoat on metal substrate

In the electrostatic chuck member according to the invention, Al, Mo, W, C or the like is used as a substrate and then a surface of the metal substrate is uniformly roughened and cleaned by blowing Al.sub.2 O.sub.3 particles (#60) thereonto.

Then, an undercoat of metallic spray-coated layer having a thickness of 30-150 .mu.m is formed by spraying a metal of Ni, Al, Cr, Co, Mo or an alloy thereof as a spraying material through an arc spraying process or a plasma spraying process.

The action of the metallic spray-coated layer is based on the consideration of not only the adhesion force to the substrate but also the adhesion property to an Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spray-coated layer subsequently formed as a topcoat. When the thickness of this layer is less than 30 .mu.m, the function as the undercoat is low, while when it exceeds 150 .mu.m, the remarkable effect is not obtained and the formation of the layer undesirably takes a long time.

(2) Formation of topcoat

After the formation of the metallic spray-coated layer as the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spray-coated layer is formed thereonto as a topcoat. The ceramic spray-coated layer will be described in detail below.

The spray-coated layer obtained by plasma spraying the commercially available Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based spraying material has strong peaks of Al.sub.2 O.sub.3 and TiO.sub.2 detected by an X-ray diffractometry, from which it is clear that the components of the spraying material are rendered into coated components as they are. However, the coated layer consisting of such crystalline components has problems that the response rate is slow and the leak current is large as mentioned above.

According to the invention, therefore, the same commercially available Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spraying material is used and sprayed in an Ar gas atmosphere containing substantially no air (oxygen) or in an atmosphere containing a slight amount of air by using a hydrogen gas having a strong reducing action as a plasma working gas. In this case, it has been found that a part of TiO.sub.2 releases oxygen to change into a crystalline compound represented by a general formula of Ti.sub.n O.sub.2n-1 (n=1-9).

The reason why TiO.sub.2 releases oxygen to produce Ti.sub.n O.sub.2n-1 (n=1-9) compound when the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spraying material is plasma-sprayed using hydrogen gas is considered due to the fact that Ar, He, H.sub.2 or the like is isolated into ion and electron in the plasma as a spraying heat source to form a region being electrically neutral as a whole of the plasma but having locally a high electron density, while TiO.sub.2 spraying particles pass through this region to release oxygen and change into the form of the Ti.sub.n O.sub.2n-1 (n=1-9) compound.

This phenomenon becomes more conspicuous when the plasma spraying is carried out under a condition that the spraying atmosphere contains hydrogen but does not contain oxygen.

According to the inventors' experiments, Ti.sub.3 O.sub.5, Ti.sub.2 O.sub.3, TiO, Ti.sub.4 O.sub.7, Ti.sub.5 O.sub.9, Ti.sub.6 O.sub.11, Ti.sub.8 O.sub.15, Ti.sub.7 O.sub.13, Ti.sub.9 O.sub.17 and Ti.sub.10 O.sub.19 are found as the Ti.sub.n O.sub.2n-1 (n=1-9) compound. Among them, Ti.sub.3 O.sub.5 and Ti.sub.2 O.sub.3 are effective.

In the formation of the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spray-coated layer containing Ti.sub.n O.sub.2n-1 (n=1-9) as the topcoat according to the invention, when the layer is formed in an atmosphere containing no oxygen under a pressure lower than atmospheric pressure, resistance of gas to the spraying particles flying in the heat source reduces, so that the impact energy of the spraying particles to the substrate becomes large and the deposition density of the particles becomes large accompanied therewith and hence the porosity of the coated layer becomes considerably small. It can be said that this method is suitable.

For example, FIG. 1 shows a relation between porosity of plasma spray-coated layer obtained by using commercially available 85 wt % Al.sub.2 O.sub.3 -15 wt % TiO.sub.2 spraying material and pressure of spraying atmosphere. As seen from this result, the porosity of the coated layer becomes smaller under a lower pressure.

Since the topcoat of the spray-coated layer according to the invention is required to have a porosity of not more than 3%, the spraying atmosphere pressure satisfying the above condition is not more than 750 hPa as seen from the above figure.

Because, the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spray-coated layer containing Ti.sub.n O.sub.2n-1 (n=1-9) and having a porosity or not more than 3% develops properties suitable as a coating for the electrostatic chuck, i.e. the scattering of volume resistivity is low, the highly surface finishing is possible and the like. Particularly, the coated layer having a porosity of more than 3% has drawbacks that the scattering of volume resistivity is large to increase the reject ratio, the smoothly polished finish surface is not obtained and the like.

In the ceramic spray-coated layer according to the invention, the average surface roughness Ra is required to be within a range of 0.1-2.0 .mu.m. Preferably, it is within a range of 0.1-1.0 .mu.m. When the finish surface has Ra: less than 0.1 .mu.m, the number of polishing steps is uneconomically large and the residual adsorption force to the wafer is large. When the surface roughness Ra exceeds 2.0 .mu.m, the scattering of volume resistivity becomes large and the inaccuracy of fixing silicon wafer becomes undesirably large in the use as the electrostatic chuck.

The amount of TiO.sub.2 included in the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spray-coated layer according to the invention is 2-30 wt %, preferably 5-15 wt %. When the TiO.sub.2 amount is less than 2 wt %, the value of the volume resistivity of the spray-coated layer is too high, while when it exceeds 30 wt %, the -value of the volume resistivity is too low and a large leak current undesirably flows.

Moreover, the thickness of the topcoat is within a range of 50-500 .mu.m, preferably 100-300 .mu.m. When the thickness is less than 50 .mu.m, the function as the topcoat is not sufficiently developed and the dielectric strength is unsuitably low. When it exceeds 500 .mu.m, a long time is taken in the formation to degrade the productivity and also the peeling is apt to be caused by thermal shock.

(3) Sealing treatment or polished surface

If necessary, an organic silicon compound (commercially available organic silicone resin) or an inorganic silicon compound (commercially available silicon alkoxide compound) is applied to the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spray-coated layer containing Ti.sub.n O.sub.2n-1 (n=1-9) compound and having a given polished roughness according to the invention and then heated to 120-350.degree. C. for 1-5 hours.

This procedure is to prevent the retaining adhesion of foreign matter by filling the silicon compound in the fine porous portions of the spray-coated layer.

In general, the porosity of the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spray-coated layer containing Ti.sub.n O.sub.2n-1 (n=1-9) compound according to the invention is as very low as not more than 3%, so that the sealing treatment is not an essential step. However, the sealing treatment is said to be preferable because the sealed surface prevents the adhesion of foreign matter in the industrial use as the electrostatic chuck.

The following examples are given in illustration of the invention and are not intended as limitations thereof.

EXAMPLE 1

In this example, the influence of kind of atmosphere gas and thickness upon the state of forming Ti.sub.n O.sub.2n-1 (n=1-9) in the spray-coated layer is examined in% the plasma spraying of Al.sub.2 O.sub.3 TiO.sub.2 based spraying material.

(1) Substrate to be used:

Pure aluminum plate (size: width 50 mm.times.length 100 mm.times.thickness 8 mm)

(2) Undercoat of spray-coated layer:

it is formed at a thickness of 100 .mu.m by plasma spraying process of 90 wt %Ni-10 wt %.Al in air.

(3) Topcoat of spray-coated layer:

It is formed on the undercoat at a thickness of 300 .mu.m by using Al.sub.2 O.sub.3 .multidot.15 wt %TiO.sub.2 spraying material through plasma spraying process under various pressures and various kinds of atmosphere.

(4) Spraying atmosphere and pressure:

1 Ar gas: 30-1000 hPa

2 Air: 30-1000 hPa

(5) Plasma working gas:

A mixed gas of Ar and H.sub.2 is used.

(6) Evaluation method:

Each cut section of the spray-coated layers formed under the above conditions is observed by means of an optical microscope to measure a porosity, while a part of the coated layer is taken out and placed in an X-ray diffraction device to measure a change of TiO.sub.2 crystal system.

(7) Test results:

The results are shown in Table 1. As seen from the results of Table 1, the porosity of the coated layer is within a range of 0.4-3.0% under conditions of 30-750 hPa in atmospheres of Ar and air and a part of TiO.sub.2 in 90 wt %Ni-10 wt %Al constituting the coated layer changes into Ti.sub.3 O.sub.5, Ti.sub.2 O.sub.3 and other Ti.sub.n O.sub.2n-1 type crystal system. Particularly, the peck of TiO.sub.2 disappears substantially completely in Ar atmosphere under 30-200 hPa (Test Nos. 1 and 2), and a great part thereof changes into Ti.sub.n O.sub.2n-1 (n=1-9) type having a less oxygen amount.

TABLE 1 __________________________________________________________________________ Spraying atmosphere Porosity of Results of X-ray diffraction Kind of Pressure coated layer another No. gas (hPa) (%) TiO.sub.2 Ti.sub.3 O.sub.5 Ti.sub.2 O.sub.3 Ti.sub.n O.sub.2n-1 Remarks __________________________________________________________________________ 1 Ar 30 0.4.about.1.5 not .largecircle. .circleincircle. .largecircle. Acceptable detected Example 2 200 0.8.about.1.7 not .circleincircle. .circleincircle. .largecircle. detected 3 600 1.5.about.2.1 .DELTA. .circleincircle. .largecircle. .largecircle. 4 750 2.4.about.3.0 .largecircle. .circleincircle. .largecircle. .largecircle. 5 900 3.8.about.5.8 .largecircle. .circleincircle. .largecircle. .largecircle. Comparative 6 1000 7.5.about.10.5 .largecircle. .circleincircle. .largecircle. .largecircle. Example 7 Air 30 0.6.about.1.8 .largecircle. .circleincircle. .largecircle. .largecircle. Acceptable 8 200 1.0.about.1.7 .largecircle. .circleincircle. .largecircle. .largecircle. Example 9 600 1.7.about.2.3 .largecircle. .circleincircle. .largecircle. .largecircle. 10 750 2.6.about.3.0 .largecircle. .circleincircle. .largecircle. .largecircle. 11 900 4.0.about.6.8 .circleincircle. -- -- .DELTA. Comparative 12 1000 8.5.about.13.0 .circleincircle. -- -- .DELTA. Example __________________________________________________________________________ (Note) (1) use of Ar/H.sub.2 mixed gas as a plasma gas (2) Ti.sub.n O.sub.2n-1 compound is a compound of n = 4- 9. (3) Symbol in column of Xray diffraction result as follows: .circleincircle.: strongest diffraction peak, .largecircle.: strong peak, .DELTA.: weak peak, --: hardly observed

EXAMPLE 2

In this example, a limit of finish polishing is measured by using the same coated layer as in Example 1, while a thermal shock test is made to measure an adhesion property of the coated layer and a mechanical resistance through thermal shock.

(1) Substrate to be used: same as in Example 1

(2) Undercoat of spray-coated layer: same as in Example 1

(3) Topcoat of spray-coated layer: same as in Example 1

(4) Spraying atmosphere and pressure:

Ar gas: 60, 200, 750, 900, 1000 hPa

(5) Plasma working gas: same as in Example 1

(6) Evaluation method:

The coated layer prepared by the above procedure is polished to render into a mirror surface as far as possible, which is heated in air at 300.degree. C. for 10 minutes and cooled in air to room temperature and this procedure is repeated 10 times to measure a change of appearance of the coated layer (average roughness Ra).

Moreover, this test is conducted with respect to the coated layer in which a silicon alkoxyde compound is applied three times and subjected to a driving treatment at 200.degree. C. for 30 minutes to measure the effect.

(7) Test results:

The results are shown in Table 2. As seen from Table 2, the coated layer formed by spraying under a low pressure (60-750 hPa) is small in the porosity and smooth in the finish polished surface. However, when the spraying pressure is 900 hPa or 1000 hPa, the smooth polished surface is not obtained. Because, it is considered that the coated layer formed under a low pressure (30-750 hPa) has a polished surface having Ra: 0.1-2.5 .mu.m owing to the low porosity, while in the coated layer having a high porosity (900-1000 hPa), the porous portion is exposed in form of pit and hence the surface roughness is necessarily large.

On the other hand, the thermal shock resistances of these coated layers develop relatively good properties under this test conditions independently of the presence or absence of the sealing agent. Only in the coated layers (Nos. 4 and 5) containing no sealing agent, the occurrence of slight cracking is observed after the repetition of the test 8 times.

From the above results, it has been confirmed that the smooth polishing is possible because the coated layer according to the invention is dense, and the resistance to thermal shock is good under the conditions in this example irrespectively of the sealing agent.

TABLE 2 __________________________________________________________________________ Properties Spraying coated layer atmosphere Surface Results of Pressure Porosity roughness Sealing thermal No. (hpa) (%) Ra (.mu.m) treatment shock test Remarks __________________________________________________________________________ 1 60 0.5.about.1.5 0.1.about.0.3 absence no cracking Acceptable 2 200 0.8.about.1.7 0.3.about.0.5 after 10 Example 3 750 2.4.about.3.0 0.8.about.2.5 times 4 900 3.8.about.5.8 3.2.about.3.5 Occurrence Comparative 5 1000 7.5.about.10.5 3.2.about.4.1 of slight Example cracking at 8 times 6 60 0.5.about.1.5 0.1.about.0.3 presence no cracking Acceptable 7 200 0.8.about.1.7 0.3.about.0.5 after 10 Example 8 750 2.4.about.3.0 0.8.about.2.5 times 9 900 3.8.about.5.8 3.2.about.3.5 Comparative 10 1000 7.5.about.10.5 3.2.about.4.1 Example __________________________________________________________________________ (Note) (1) Ar/H.sub.2 mixed gas is used as a plasma gas. (2) Crystal type of coated layer is the same as in Table 1. (3) Conditions of thermal shock test are 10 times repetition of 300.degree. C. .times. 15 minutes .revreaction. room temperature.

EXAMPLE 3

A volume resistivity of the Al.sub.2 O.sub.3 .multidot.TiO.sub.2 based ceramic spray-coated layer according to the invention is measured, and the scattering thereof is compared with that obtained by the conventional spraying process.

(1) Substrate to be used: same as in Example 1

(2) Undercoat of spray-coated layer: same as in example 1

(3) Topcoat of spray-coated layer:

It is formed at a thickness of 250 .mu.m or 500 .mu.m by using Al.sub.2 O.sub.5 -14 wt %TiO.sub.2 material through plasma spraying process under various pressures and various gas atmospheres.

(4) Spraying atmosphere and pressure:

1 Ar gas: 60, 750, 1000 hPa

2 Air: 60, 750, 1000 hPa

(5) Plasma working gas: use of Ar/H.sub.2 mixed gas

(6) Evaluation method:

A dotite (Ag paste) is applied onto the surface of the spray-coated layer as an electrode and a direct current of 500V is applied between the dotite and the aluminum substrate to measure a resistivity, from which a volume resistivity is calculated according to the following equation.

Volume resistivity .rho.=RA/d (.omega..multidot.cm)

A: electrode area (cm.sup.2) d: coated layer thickness (cm) R: resistivity (.omega.)

The measurement is 5 positions every one coated layer, and also the effect of sealing treatment with a silicon alkoxyde compound (200.degree. C..times.30 minutes drying after the application, 3 times repeat) is examined.

(7) Test results:

The results are shown in Table 3. As seen from the results of Table 3, the comparative spray-coated layers (Nos. 5, 6, 11, 12) formed under air or Ar spraying atmosphere of 1000 hPa are large in the scattering of volume resistivity and the effect of the sealing treatment is not clear.

In the coated layers according to the invention (Nos. 1-4, 7-10), the porosity is small and the property is dense and a part of TiO.sub.2 in the spraying material changes into Ti.sub.n O.sub.2n-1 (n=1-9), so that the scattering of the measured value is less and the value of volume resistivity required in the electrostatic chuck according to the invention is within a range of 1.times.10.sup.9 -10.sup.11 .omega..multidot.cm and it is confirmed that the quality control is very easy.

TABLE 3 __________________________________________________________________________ Spraying atmosphere Properties of spray-coated layer Kind Pres- thick- Volume of sure ness Porosity Ti.sub.n reality resistivity No. gas (hPa) (.mu.m) (%) O.sub.2n-1 treatment (.OMEGA. .multidot. cm.sup.2) Remark __________________________________________________________________________ 1 Ar 60 250 0.5.about.1.5 presence presence 1 .times. 10.sup.10 .about.10.sup.11 Acceptable 2 60 500 0.5.about.1.4 presence absence 1 .times. 10.sup.10 .about.10.sup.11 Example 3 750 250 2.4.about.3.0 presence presence 1 .times. 10.sup.9 .about.10.sup.11 4 750 500 2.2.about.2.8