Substrate support device

A substrate support device formed of a metal and having a high withstand voltage and a high thermal resistance is provided. A substrate support device according to the present invention includes a plate section formed of a metal; a shaft section connected to the plate section and formed of a metal; a heating element provided in the plate section; and an insulating film formed on a first surface of the plate section, the first surface opposite to the shaft section, by ceramic thermal spraying. The substrate support device may further include an insulating film formed on a second surface of the plate section which intersects the first surface of the plate section approximately perpendicularly.

FIELD

The present invention relates to a substrate support device usable for producing a semiconductor device, and specifically to a metal substrate support device having a built-in heating element.

BACKGROUND

In production of a semiconductor device, specifically in processing steps of chemical vapor deposition (CVD), surface reforming and the like, a substrate support device is provided in a semiconductor production apparatus. In the case where the substrate support device is used in the state of being heated, a substrate support device having a built-in heating element is provided in the semiconductor production apparatus. Such a substrate support device has a structure in which a plate formed of a metal or ceramic material is supported by a shaft. There are cases where a plasma electrode or a heating element is provided in the plate and is connected to a control device provided outside the semiconductor production apparatus via a line provided in the shaft.

For a plate of such a substrate support device, bulk ceramics such as aluminum nitride (AlN), aluminum oxide (Al2O3) and the like are often used. However, in order to provide a plasma electrode or a heating element in the substrate support device, the inside of the plate needs to have a complicated structure. It is difficult to process bulk ceramics into such a complicated structure. By contrast, a metal plate allows a complicated structure to be formed therein easily, and also at lower cost than bulk ceramics. In the case where a metal plate is used, a surface of the metal plate on which a substrate is to be mounted needs to be covered with an insulating material, and contamination of the substrate with metal caused by contact needs to be reduced. For example, Japanese Laid-Open Patent Publication No. 2007-184289 describes a metal plate which is given an alumite treatment (an anodizing).

However, when a metal plate is anodized, the anodized film has a thickness of about 50 to 75 μm and a withstand voltage of about 0.8 to 1 kV. It is difficult to realize a higher withstand voltage. For producing a precision semiconductor device of, for example, the 25 nm process or the like, the metal plate needs to be covered with a material having a high thermal resistance in order to prevent the metal from being contaminated.

The present invention for solving the above-described problems has an object of providing a substrate support device formed of a metal and having a high withstand voltage and a high thermal resistance.

SUMMARY

An embodiment of the present invention provides a substrate support device comprising a plate section formed of a metal; a shaft section connected to the plate section and formed of a metal; a heating element provided in the plate section; and an insulating film formed on a first surface of the plate section, the first surface opposite to the shaft section, by ceramic thermal spraying.

The substrate support device may further comprise an insulating film formed on a second surface of the plate section which intersects the first surface of the plate section approximately perpendicularly.

The substrate support device may include a chamfer plane or a curvature at an edge between the first surface and the second surface of the plate section, and/or may include a chamfer plane or a curvature curving toward a third surface of the plate section from the second surface of the plate section, the third surface being connected to the shaft section.

The substrate support device may include a 0.5 mm or more chamfer plane or a curvature radius at an edge between the first surface and the second surface of the plate section, and/or may include a 0.5 mm or more chamfer plane or a curvature radius curving toward a third surface of the plate section from the second surface of the plate section, the third surface being connected to the shaft section.

In the substrate support device, from the second surface of the plate section may have a portion having a curvature radius, having a value nearly equal to a thickness of the plate section, which is curved outward toward a third surface of the plate section, the third surface being connected to the shaft section.

In the substrate support device, the third surface may have an insulting film formed thereon.

In the substrate support device, the first surface of the plate section may have a recessed portion; and an edge of the recessed portion of the first surface may have 0.5 mm or more chamfer plane or a curvature radius.

The insulation film may be formed by thermal spraying the plate section at a work temperature being a residual stress in which cracks are unproduced at an actual usage temperature of the substrate support device.

According to the present invention, a metal substrate support device having a high withstand voltage and a high thermal resistance can be provided.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a substrate support device according to an embodiment of the present invention will be described with reference to the drawings. The following embodiment is directed to an example of substrate support device according to the present invention, and the substrate support device according to the present invention is not limited to the following embodiment.

With a thickness of an anodized film, it is difficult to provide a metal plate with a sufficiently high withstand voltage. Therefore, the present inventors made researches to find a method for forming a ceramic film on a metal plate, which can realize a higher withstand voltage. One method for forming an insulating ceramic film on a metal plate is ceramic thermal spraying. With this method, a high withstand voltage can be realized but the resultant ceramic layer is cracked under a high temperature condition and as a result, the plate is corroded or arc discharge occurs between the plate and the substrate. For this reason, it is difficult to improve the yield of semiconductor devices. Accordingly, it is difficult to realize a high withstand voltage and a high thermal resistance at the same time by merely performing ceramic thermal spraying with a low-fusing.

As a result of conducting active studies on these problems, the present inventors have found that by forming a thinnest possible insulating film in the range in which a desired withstand voltage can be realized, a substrate support device which is not cracked even under a high temperature condition can be provided.

FIG. 1is a perspective view of a substrate support device100according to an embodiment of the present invention.FIG. 2is a cross-sectional view ofFIG. 1taken along line A-A′ inFIG. 1. The substrate support device100in this embodiment includes a plate section110, a shaft section150, and a heating element120provided in the plate section110. A top surface of the plate section110has a recessed portion113for supporting a substrate. The shaft section150is connected to a central part of a rear surface, of the plate section110, opposite to the top surface having the recessed portion113. The shaft section150has a hollow structure170. In the hollow structure170of the shaft section150, a line160connected to the heating element120and also to a control device (not shown) outside the substrate support device100is provided.

The substrate support device100includes an insulating film119formed on the surface113of the plate section110for supporting the substrate and also on a side surface of the plate section110. The top surface and the side surface of the plate section110intersect each other approximately perpendicularly, and a convex curvature is included on an edge of the top surface and side surface of the plate section110. In addition, the side surface of the plate section110has a portion having a curvature which is curved outward toward a connection position on the rear surface of the plate section110, at which the plate section110is connected to the shaft section150. In the plate section110, an edge between the recessed portion113and a peripheral non-recessed portion of the top surface is rounded off so as to curve outward with a curvature.

The plate section110and the shaft section150in this embodiment are formed of a metal. A metal to be used can be selected from materials known as being usable for producing substrate support devices. Usable metals include, for example, aluminum, stainless steel, copper, nickel, titanium and the like. The plate section110includes two components110aand110b. A groove is formed in either the component110aor110b, and the heating element120is located in the groove. The components110aand110bare joined together by soldering or welding.

By forming the plate section110and the shaft section150of a metal, a coolant flow path190shown inFIG. 2may be formed in the shaft section150. The coolant flow path190is a mechanism for allowing a gas such as air or the like or a liquid such as oil, an aqueous solution of ethylene glycol or the like to circulate to assist the shaft section150in adjusting the temperature of a heater. Such a complicated structure cannot be easily formed when the shaft section is formed of a ceramic material, but can be formed in the shaft section150by soldering or welding since the shaft section150is formed of a metal. This mechanism provides higher-level temperature adjustment means. In addition, the above-mentioned rounding can be conducted easily when the plate section110is formed of a metal.

According to the present invention, the insulating film119is formed by ceramic thermal spraying. When ceramic thermal spraying is used, it is possible to form the insulating film119to be thicker than an anodized film. However, when the insulating film119is made thicker by ceramic thermal spraying, the insulating film119is cracked under a high temperature condition. The insulating film119according to the present invention is formed to be as thin as possible in the range in which a desired withstand voltage can be realized. Accordingly, the thickness of the insulating film119is set to any value in accordance with a required withstand voltage. The plate section110, which is rounded as described above, can suppress a stress from being concentrated on the edges of the insulating film119so that the insulating film119is not easily cracked even under a high temperature condition.

The insulating film119in this embodiment can be formed of any known material which realizes a desired withstand voltage and can be ceramic-thermal-sprayed. For example, as the material of the insulating film119, an oxide of at least one of alkaline earth metals, rare-earth metals, Al, Ta and Si can be selected. Specifically, aluminum oxide (Al2O3), magnesium oxide (MgO), yttrium oxide (Y2O3) and the like are usable. According to the present invention, a combination of a metal and an insulating material which have a small difference in the coefficient of thermal expansion is used. When the difference in the coefficient of thermal expansion is large between the plate section110and the insulating film119, the insulating film119is easily cracked under a high temperature condition. An example of combination of a metal and an insulating material which have a small difference in the coefficient of thermal expansion is a combination of aluminum (Al) and aluminum oxide. When the plate section110is formed of aluminum and the insulating film119is formed of aluminum oxide, the insulating film119is not cracked easily. In general, ceramics have properties weak against a tensile stress. It is considered that when the plate section110is formed of aluminum, which is thermally expandable, the plate section110is expanded along the insulating plate119under a high temperature condition and thus the insulating plate119is not cracked easily.

In this embodiment, it is preferable that the ceramic insulating film119formed by ceramic thermal spraying has a stoichiometric composition or a composition close thereto. When being formed of a stoichiometric composition or a composition close thereto, the insulating film119is not cracked easily even under a high temperature condition. When the amount of oxygen in the ceramic film is significantly lower than that of the stoichiometric composition, the insulating film119is easily cracked and thus the substrate support device cannot exhibit a sufficiently high withstand voltage. By contrast, when the amount of oxygen in the ceramic film is significantly higher than that of the stoichiometric composition, the adhesiveness of the insulating film119to the plate section110is lowered, which is not preferable.

For ceramic thermal spraying conducted to form the insulating film119in this embodiment, oxygen gas or oxygen-containing gas is used as plasma working gas. By using oxygen gas or oxygen-containing gas as plasma working gas, the composition of the insulating film119formed by ceramic thermal spraying can be made closer to the stoichiometric composition than the composition of a film formed by conventional thermal spraying. Thus, a high electrical insulating property and a high corrosion resistance can be realized at the same time.

For forming the insulating film119in this embodiment, a thermal-sprayed film having a larger thickness than the thickness at which a desirable withstand voltage can be realized is formed by ceramic thermal spraying. Then, the thermal-sprayed film is processed with surface polishing to obtain the desired thickness. In this embodiment, it is preferable that the ceramic thermal spraying is performed in at least two directions, i.e., a direction toward the top surface of the plate section110and a direction toward the side surface thereof. In order to provide the surface of the plate section110with the insulating property without fail to prevent the metal from being contaminated, the insulating film119is formed on two surfaces, i.e., the top surface and the side surface. As described above, in this embodiment, the side surface of the plate section110has a portion having a curvature which is curved outward toward the connection position with the shaft section150on the rear surface. Therefore, on the side surface of the plate section110, the insulating film119is formed to be gradually thinner toward the connection position with the shaft section150on the rear surface, and thus the insulating film119is not easily delaminated from the plate section110.

In this embodiment, for the purpose of reducing stress concentration, it is desirable that the curvature radius (R1) be 0.5 mm or more at the edge between the top surface and the side surface of the plate section110. In addition, for the same reason, it is desirable that the edge of the recessed portion113of the plate section110have a curvature radius (R3) of 0.5 mm or more. However, it is desirable that the portion of the side surface which is curved outward toward the connection position with the shaft section150has a convex curvature (1/R value) and the curvature radius (R2) is large. In this embodiment, it is preferable that this curvature is nearly equal to the thickness of the plate section110. Owing to this, the insulating film110can be formed to be gradually thinner from the side surface of the plate section110toward the connection position with the shaft section150on the rear surface, by thermal spraying conducted from a position to the side of the plate section110.

In the present embodiment, it is possible to relax the stress concentration of the substrate support device100caused by heating and cooling during use by providing a convex curvature on an edge between an top surface and side surface of the plate section110and thereby prevent cracks from occurring on the insulation film119. In addition, it is possible to prevent the insulation film119from peeling from the plate section110by forming the insulation film119to be gradually thinner from the side surface of the plate section110toward to connection direction with the shaft section150on the rear surface.

The substrate support device100in this embodiment may include a buffer layer, for alleviating the difference in the withstand voltage, formed between the plate section110and the insulating film119. For the buffer layer in this embodiment, magnesium oxide (MgO), for example, is usable.

Where the coefficient of thermal expansion of the plate section110is αs, the coefficient of thermal expansion of the insulating film119is αf, the working temperature during the thermal spraying is T0, the room temperature is T1, the temperature of the heater in use is T2, and the Young's modulus of the insulating film119is E, the thermal stress σ generated in the insulating film119is represented by the following expressions in the case where the thickness of the plate section100is significantly larger than the thickness of the insulating film119.
During cold working: σ=(αs−α1)·E·(T0−T1)
During hot working: σ=(αs−α1)·E·(T0−T2)

During cold working, (T0−T1) has a positive value, and because αs>αf, σ has a positive value and the insulating film119receives a compressive stress. However, when the substrate support device100is used in the state of being heated at a temperature exceeding the temperature of the plate section110used during the thermal spraying, σ has a negative value and the insulating film119receives a tensile stress, which causes cracks.

Thus, in the present embodiment, the insulation film119may be formed by thermal spraying the plate section110at a work temperature which becomes a residual stress in which cracks are not produced at an actual usage temperature of the substrate support device100. The work temperature related to the present embodiment can be determined by considering the actual usage temperature of the substrate support device100, each thermal expansion coefficient of the plate section110material and insulation film119material, and the size (radius) of the plate section110and thickness of the insulation film119etc. In the present embodiment, the generation of the tensile stress in such a temperature range or the vicinity thereof can be suppressed by, for example, setting the working temperature during the thermal spraying to a range of 150° C. or higher and 250° C. or lower.

As described above, a substrate support device according to the present invention can be processed in a complicated manner because the plate section and the shaft section are formed of a metal, and is not easily cracked even under a high temperature condition and thus can realize a high withstand voltage because a thin insulating film is formed on the plate section by ceramic thermal spraying.

FIG. 3is a cross-sectional view of a substrate support device200according to an embodiment of the present invention taken along a line corresponding to line A-A′ inFIG. 1. The substrate support device200in this embodiment comprises a plate section210including three components210a,210band210cinstead of the plate section110. The heating element120is provided in a groove formed in the component210aor210b. The component210bor210chas a groove formed therein, which is used as a coolant flow path290. A top surface of the plate section210has a recessed portion213for supporting a substrate, and a shaft section150is connected to a central part of a rear surface, of the plate section210, opposite to the top surface having the recessed portion213. The shaft section150is substantially the same as that described in Embodiment 1 and therefore will not be described in detail.

The substrate support device200includes an insulating film219formed on the surface213of the plate section210for supporting the substrate, on a side surface of the plate section210, and on the rear surface of the plate section210which is connected to the shaft section150. The top surface and the side surface of the plate section210, and the side surface and the rear surface thereof, intersect each other approximately perpendicularly. An edge between the top surface and the side surface, and an edge between the side surface and the rear surface, are rounded off so as to be curve outward with a curvature. An edge between the recessed portion213and a peripheral non-recessed portion of the top surface of the plate section210is rounded off to be curved outward with a curvature.

In this embodiment, for the purpose of reducing stress concentration, it is desirable that the curvature radius (R1) be 0.5 mm or more at the edge between the top surface and the side surface of the plate section210. For the same reason, it is desirable that the edge of the recessed portion213of the plate section210has a curvature radius (R3) of 0.5 mm or more. In the present embodiment, for the same reason, it is desirable that the edge between the side surface and rear surface has a curvature radius (R2) of 0.5 mm or more. Therefore, in the present embodiment, R1and R2may be equal or different.

The plate section210and the shaft section150in this embodiment are formed of a metal. Usable metals are described in Embodiment 1 and will not be described in detail. By forming the plate section210and the shaft section150of a metal, the components210a,210band210ccan be joined together by soldering or welding, and also the plate section210and the shaft section150can be joined together by soldering or welding.

The coolant flow path290is a mechanism for assisting the adjustment of the temperature of a heater. As long as the temperature of the heater can be efficiently adjusted, the coolant flow path290may be located in the same manner as the heating element120, or spirally, in the plate section210. Such a complicated structure cannot be easily formed when the plate section is formed of a ceramic material, but can be formed in the plate section210since the plate section210is formed of a metal. This mechanism provides higher-level temperature adjustment means. In addition, the above-mentioned rounding can be conducted easily when the plate section210is formed of a metal.

According to the present invention, the insulating film219is formed by ceramic thermal spraying. The insulating film219according to the present invention is formed to be as thin as possible in the range in which a desired withstand voltage can be realized. Accordingly, the thickness of the insulating film219is set to any value in accordance with a required withstand voltage. The plate section210, which is rounded as described above, can suppress a stress from being concentrated on the edges of the insulating film219so that the insulating film219is not easily cracked even under a high temperature condition.

The insulating film219in this embodiment can be formed of any material which realizes a desired withstand voltage and can be ceramic-thermal-sprayed. Usable metals are described in Embodiment 1 and will not be described in detail. As described above, in this embodiment, it is preferable that the ceramic insulating film219formed by ceramic thermal spraying has a stoichiometric composition or a composition close thereto. When being formed of a stoichiometric composition or a composition close thereto, the insulating film219is not cracked easily even under a high temperature condition.

For ceramic thermal spraying conducted to form the insulating film219in this embodiment, oxygen gas or oxygen-containing gas is used as plasma working gas. By using oxygen gas or oxygen-containing gas is used as plasma working gas, the composition of the insulating film219formed by ceramic thermal spraying can be made closer to the stoichiometric composition than the composition of a film formed by conventional thermal spraying. Thus, a high electrical insulating property and a high corrosion resistance can be realized at the same time.

For forming the insulating film219in this embodiment, a thermal-sprayed film having a larger thickness than the thickness at which a desirable withstand voltage can be realized is formed by ceramic thermal spraying. Then, the thermal-sprayed film is processed with surface polishing to obtain the desired thickness. In this embodiment, it is preferable that the ceramic thermal spraying is performed in three directions, i.e., a direction toward the top surface of the plate section210, a direction toward the side surface thereof, and a direction toward the rear surface thereof. The side surface of the plate section210in this embodiment may be processed to have a portion having a curvature which is curved outward toward the connection position with the shaft section150on the rear surface as described above in Embodiment 1, instead of having the edge between the side surface and the rear surface. In this case, the formation of the insulating film219on the rear surface of the plate section210may be omitted to simplify the production process.

The substrate support device200in this embodiment may include a buffer layer, for alleviating the difference in the withstand voltage, formed between the plate section210and the insulating film219. For the buffer layer in this embodiment, magnesium oxide (MgO), for example, is usable. As explained in the first embodiment, in the present embodiment, the insulation film219may be formed by thermal spraying the plate section210at a work temperature which becomes a residual stress in which cracks are not produced at an actual usage temperature of the substrate support device200.

As described above, a substrate support device according to the present invention can be processed in a complicated manner because the plate section and the shaft section are formed of a metal, and is not easily cracked even under a high temperature condition and thus can realize a high withstand voltage because a thin insulating film is formed on the plate section by ceramic thermal spraying.

Third Embodiment

FIG. 4is a cross-sectional view of the substrate support device300according to the embodiment of the present invention taken along line A-A′ inFIG. 1. The substrate support device300related to the present embodiment is arranged with a plate section310having a chamfered edge instead of the plate section110. A groove is formed in either a component310aor310b, and a heating element120is located in the groove. A recessed portion313for supporting the substrate is formed on the top surface of the plate section310and a shaft section150is connected to a center section of the plate310on the opposite side to the recessed portion313. Because the shaft section150is the same as the shaft section explained in the first embodiment, and detailed explanation is omitted here. Furthermore, as explained in the second embodiment, the element310bor310cmay be formed in the groove and a cooling flow path may be arranged.

In the substrate support device300, an insulation film319is formed on a surface for supporting the substrate of the plate section310and a side surface of the plate section310. The top surface and the side surface of the plate section310intersect each other approximately perpendicularly, and an edge between the top surface and side surface of the plate section310is chamfered. In addition, the side surface of the plate section310has a portion having a curvature which is curved outward toward a connection position at which the plate section310is connected to the shaft section150. The edge of the recessed portion313of the plate section310is chamfered.

In this embodiment, for the purpose of reducing stress concentration, it is desirable that the edge between the top surface and the side surface of the plate section310be chamfered (C1) to 0.5 mm or more. For the same reason, it is desirable that the edge of the recessed portion313of the plate section310be chamfered (C3) to 0.5 mm or more. However, in the present embodiment, as explained in the first embodiment, it is desirable that the portion of the side surface which is curved outward toward the connection position with the shaft section150has a curvature (1/R value) and the curvature radius (R2) is large. In this embodiment, it is preferable that this curvature radius (R value) is nearly equal to the thickness of the plate section310. Owing to this, the insulating film319can be formed to be gradually thinner from the side surface of the plate section310toward the connection position with the shaft section150on the rear surface, by thermal spraying conducted from a position to the side of the plate section310.

In the present embodiment, it is possible to relax the stress concentration of the substrate support device300caused by heating and cooling during use by chamfering an edge between an top surface and side surface of the plate section310and thereby prevent cracks from occurring on the insulation film319. In addition, it is possible to prevent the insulation film319from peeling from the plate section310by forming the insulation film319to be gradually thinner from the side surface of the plate section310toward to connection direction with the shaft section150on the rear surface.

Furthermore, because the material of plate section310and the shaft section150and the manufacturing method are the same as in the first embodiment a detailed explanation is omitted here.

As described above, a substrate support device according to the present invention can be processed in a complicated manner because the plate section and the shaft section are formed of a metal, and is not easily cracked even under a high temperature condition and thus can realize a high withstand voltage because a thin insulating film is formed on the plate section by ceramic thermal spraying.

Fourth Embodiment

FIG. 5is a cross-sectional view of the substrate support device400according to the embodiment of the present invention taken along a line corresponding to line A-A′ inFIG. 1. The substrate support device400related to the present embodiment is arranged with a plate section410comprised of three elements410a,410band410cthe same as the plate section210. A groove is formed in either the element410aor410b, and a heating element120is located in the groove. In addition, the groove is formed in the element410bor410cand a cooling flow path490is arranged. A recessed portion413for supporting the substrate is formed on the top surface of the plate section410and a shaft section150is connected to a center section of the plate410on the opposite side to the recessed portion413. Because the shaft section150is the same as the shaft section explained in the first embodiment, and a detailed explanation is omitted here.

In the substrate support device400, an insulation film419is formed on a surface413for supporting the substrate of the plate section410, a side surface of the plate section410and a rear surface of the plate section410which is connected to the shaft section150. The top surface and the side surface of the plate section410and the side surface and rear surface of the plate section410intersect each other approximately perpendicularly, and an edge between the top surface and side surface of the plate section410and an edge between the side surface and rear surface of the plate section410is chamfered. In addition, the edge of the recessed portion413of the plate section410is chamfered.

In this embodiment, for the purpose of reducing stress concentration, it is desirable that the edge between the top surface and the side surface of the plate section410be chamfered (C1) to 0.5 mm or more. For the same reason, it is desirable that the edge of the recessed portion413of the plate section410be chamfered (C3) to 0.5 mm or more. In the present embodiment, for the same reason, it is desirable that the edge between the side surface and rear surface be chamfered (C2) to 0.5 mm or more. Therefore, in the present embodiment, C1and C2may be equal or different.

Furthermore, because the material of plate section410and the shaft section150and the manufacturing method are the same as in the second embodiment a detailed explanation is omitted here.

As described above, a substrate support device according to the present invention can be processed in a complicated manner because the plate section and the shaft section are formed of a metal, and is not easily cracked even under a high temperature condition and thus can realize a high withstand voltage because a thin insulating film is formed on the plate section by ceramic thermal spraying.

EXAMPLES

Hereinafter, an example of the substrate support device100according to the present invention described above will be shown and described in detail.

In this example, the plate section110was formed of aluminum, and the insulating film119was formed of aluminum oxide. For producing the insulating film119, ceramic thermal spraying was performed to form a thermal-sprayed film having a thickness of 250 μm to 300 μm, and then the thermal-sprayed film was polished to have a thickness of 100 μm to 150 μm.

Samples of the substrate support device100thus produced were heated at 120° C., 150° C., 200° C. and 250° C. for 5 hours in an oven to inspect whether the insulating film119of each sample was cracked or not. In any of the samples, no crack was generated. The investigation results are summarized in Table 1.

By contrast, in the case where the post-polishing thickness of the insulating film119was 300 μm or larger, cracks were generated. In a dielectric strength test in which a DC voltage (DC) of 2 kV was applied to the samples, the insulating film119was destroyed when the thickness thereof was less than 90 μm. From these results, it has been found that the thickness of the insulating film119is preferably 100 μm or larger and 200 μm or less.

When manufacturing a semiconductor device it is necessary to apply a thermal cycle and use a substrate support device. Therefore, when a thermal cycle is applied it is necessary to ensure that cracks do not occur in the insulation film119. In the present example, 1 cycle in which a normal temperature (40° C.) is raised to 250° C. and dropped from 250° C. to 40° C. was performed in 1 hour, and the insulation film119was evaluated by a thermal cycle experiment over a total of 60 cycles. For this evaluation, the insulation film119formed on the plate section110was used with a work temperature during thermal spraying of 80° C., 150° C. and 250° C. For producing the insulating film119, ceramic thermal spraying was performed to form a thermal-sprayed film having a thickness of 250 μm to 300 μm, and then the thermal-sprayed film was polished to have a thickness of 100 μm to 150 μm.

Table 2 shows the experiment results of the insulation film119formed with a work temperature of 80° C. during thermal spraying, table 3 shows the results at a work temperature of 150° C. and table 4 shows the results at a work temperature of 250° C.

TABLE 2R value ofwithstandsubstratevoltage(mm)Crack(2 kV DC)State0.5Occurred after∘Crack was occurred20 cyclesfrom R portion at anedge of the substrate1——N/A2——N/A3Occurred after∘Crack was occurred20 cyclesfrom R portion at anedge of the substrate

The substrate thermally sprayed at 80° C. showed cracks being produced beginning at the edge of the plate section110due to the effects of thermal stress under the cycle experiment of 250° C. However, while it was estimated that thermal stress would be produced in the plate section110thermally sprayed at 150° C. when raised at 250° C., no cracks were produced. The same as the plate section110thermally sprayed at 150° C., no cracks were produced in the plate section110thermally sprayed at 250° C. Because stress was more relaxed the larger the R value of the edge of the plate section110it is estimated that cracks would also not be produced in the case where R>0.5 mm. Consequently, if the plate section110having a 300 mm wafer substrate (about φ300 mm) is processed with R value of the plate section110being 0.5 mm or more and the work temperature of the plate section110during thermal spraying being 150° C. or more, no cracks are produced in 250° C. to 40° C.×60 cycle range.