Heating element

There is disclosed a heating element 10 comprising:

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heating element at least including a heating portion in which a heater pattern is formed in a plate portion of a heat-resistant base member, and a power-supply-terminal portion in which a power-supply terminal is formed in the heat-resistant base member.

2. Description of the Related Art

A heater in which a line or foil of metal having a high melting point such as molybdenum or tungsten is wrapped around or bonded to a heat-resistant base member made of sintered ceramic such as alumina, aluminum nitride or zirconia has been used for heating semiconductor wafers in manufacturing steps of semiconductor devices.

However, such a heater has drawbacks of being prone to deform or vaporize because the heating element is made of metal, being short-life, and being complicated to assemble (see the pyrolytic graphite/pyrolytic boron nitride heater from Union Carbide Services provided in “Vacuum” No. 12, (33), p. 53). Furthermore, use of sintered ceramic for the heat-resistant base member causes a problem that the binder in the sintered ceramic becomes impurities.

Then, to prevent such deformation or scattering of impurities due to a heat cycle, a ceramic heater is developed. The ceramic heater has a heat-resistant base member of pyrolytic boron nitride (PBN) having high mechanical strength and enabling high-efficiency heating, and a conductive layer of pyrolytic graphite on the heat-resistant base member (for example, see the pyrolytic graphite/pyrolytic boron nitride heater from Union Carbide Services provided in “Vacuum” No. 12, (33), p. 53; published Japanese translations of PCT international publication for patent applications No. H08-500932; Japanese Patent Laid-open (Kokai) No. H05-129210; and Japanese Patent Laid-open (Kokai) No. H06-61335).

An example of a heating element of such a heater is shown inFIG. 4. A heating element20has at least a heating portion20ain which a heater pattern3ais formed on a plate-shaped heat-resistant base member21, and a power-supply-terminal portion20cin which power-supply terminals3care formed at the rim of the surface of the heat-resistant base member21on which the heater pattern is formed. A protection layer4covers the heater pattern3a. To the power-supply terminal3ca power-supply member or a power terminal5is connected.

However, pyrolytic graphite used for the heating body is prone to undergo corrosion due to oxidation. Pyrolytic graphite has also reactivity with high-temperature gases used in the heating process. For example, hydrogen gas changes pyrolytic graphite into methane gas. Therefore, there is a problem that remaining oxygen or high-temperature gases in the process environment corrodes pyrolytic graphite in the power-supply-terminal portion exposed for power supply, and the power-supply-terminal portion is short-life.

To solve the problem, an attempt to locate the power-supply-terminal portion away from the heating portion is made. For example, the following solution is suggested: a power-supply terminal is connected to a power-terminal member via a power-supply member having a heater pattern which produces heat by energization. Insulating ceramic such as PBN is used for a protection layer covering the heater pattern, thereby preventing overheating of the power-supply-terminal portion to increase longevity of the power-supply terminal (see Japanese Patent Laid-open (Kokai) No. H11-354260).

Furthermore, the following method is suggested: assembling the power-supply-terminal portion made of carbon with an assembly part and forming a protection layer (see published Japanese translations of PCT international publication for patent applications No. H08-500932; International Publication WO2004/068541).

However, such a heating element has protrusions on the heating surface. It is necessary to provide a space between the heating surface and an object to be heated, which hampers compact design of the heating element. In addition, a protection layer in the vicinity of a connected part assembled from plural components is apt to produce cracks through usage. A conductive layer begins to corrode from the cracks, which causes a problem to shorten the life of the heating element. Furthermore, when the heating element is used in an environment corroding boride such as using halide etching gas, there is a drawback that an outermost layer of boride lacks resistance to corrosion, and corrosion of the outermost layer shortens the life of the heating element.

Use of pyrolytic boron nitride for a material of a heat-resistant base member as mentioned above gives high mechanical strength and enables high-efficiency heating. However, pyrolytic boron nitride has high anisotropy and is apt to warp. Pyrolytic boron nitride is also expensive. Therefore, use of sintered boron nitride for the heat-resistant base member is also suggested (see Japanese Patent Laid-open (Kokai) No. H04-358074).

However, when sintered boron nitride is used for the heat-resistant base member, the base member lacks mechanical strength, and it is necessary to thicken the base member. In addition, an amount of heat escaping from the side of the base member is huge. Therefore, it is impossible to rise temperature of a test sample sufficiently, especially high temperature of more than 700° C.

In addition, an advanced ceramic heater with electrostatic chuck on a heater for holding a semiconductor wafer to be heated is suggested currently (see Japanese Patent Laid-open (Kokai) No. H05-129210; Japanese Patent Laid-open (Kokai) No. H06-61335; Japanese Patent Laid-open (Kokai) No. H04-358074; Japanese Patent Laid-open (Kokai) No. H05-109876). However, the heater does not have sufficient heat resistance.

SUMMARY OF THE INVENTION

The present invention has been accomplished to solve the above-mentioned problems, and a main object of the present invention is to provide a heating element resistant to corrosion of a power-supply terminal, with a long-life protection layer, with high durability, compact in size, and capable of being produced at a low cost.

To achieve the above object, the present invention provides a heating element comprising: at least

a single-piece heat-resistant base member with a plate portion in which a heater pattern is formed, a rod portion extending from one surface of the plate portion and in which a current-carrying part is formed, and an end portion at an end of the rod portion on the opposite side of the plate portion and in which a power-supply terminal is formed;

a dielectric layer with insulating property on the surface of the heat-resistant base member;

a conductive layer with conductive property on the dielectric layer, in which the conductive layer is formed as the heater pattern in the plate portion, as the current-carrying part in the rod portion, and as the power-supply terminal in the end portion; and

a protection layer with insulating property which covers the surface of the heater pattern and the current-carrying part, in which the dielectric layer, the conductive layer and the protection layer are formed to unite with the heat-resistant base member.

In such a heating element, a heating portion in which the heater pattern is formed in the plate portion and a power-supply-terminal portion in which the power-supply terminal is formed in the end portion are separated by the rod portion in which the current-carrying part is formed. Therefore, the power-supply terminal exposed at the power-supply-terminal portion resists corrosion due to high-temperature gases, and has a long life.

In addition, the heat-resistant base member is single-piece, and not assembled from plural components. Therefore, the heating element is compact in size, and capable of being produced at a low cost. The layers formed on the heat-resistant base member resists producing cracks through usage, and have long life.

Furthermore, as mentioned above, the conductive layer is formed as the heater pattern, the current-carrying part, and the power-supply terminal. The protection layer covers the surface of the heater pattern and the current-carrying part. The dielectric layer, the conductive layer and the protection layer are formed to unite with the heat-resistant base member. Therefore, the heating element is compact in size, and capable of being produced at a low cost. The protection layer resists producing cracks through usage, and has long life.

In the above case, it is preferable that the heat-resistant base member is made of graphite.

When the heat-resistant base member is made of graphite, graphite is low-cost, and easy to process to produce even a complicated shape. Therefore, the cost of manufacturing the heating element can be reduced further. Moreover, such a heating element has high heat-resistance.

In addition, it is preferable that the dielectric layer is made of any one of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride and CVD aluminum nitride, or combination of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride or CVD aluminum nitride.

When the dielectric layer is made of any one of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride and CVD aluminum nitride, or combination of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride or CVD aluminum nitride, the dielectric layer has high insulating property, and does not release impurities under high temperature conditions. Therefore, a heating element with the dielectric layer can also be used for a heating process requiring high purity.

Furthermore, it is preferable that the conductive layer is made of pyrolytic carbon or glassy carbon.

The conductive layer made of pyrolytic carbon or glassy carbon can be heated up to high temperature, and is easily worked. Therefore, the heater pattern is formed as a meandering pattern with different pattern widths, whereby the heater pattern can easily have arbitrary temperature gradient or heating distribution with reference to the thermal environments to heat uniformly.

In addition, it is preferable that the protection layer is made of any one of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride and CVD aluminum nitride, or combination of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride or CVD aluminum nitride.

When the protection layer is made of any one of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride and CVD aluminum nitride, or combination of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride or CVD aluminum nitride, the protection layer has high insulating property, and does not exfoliate or release impurities under high temperature conditions. Therefore, a heating element with the protection layer can also be used for a heating process requiring high purity.

Furthermore, it is preferable that the protection layer comprises at least 2 or more layers, and an outermost layer is made of any one of aluminum, yttrium, silicon, and a compound of aluminum, yttrium or silicon.

When the protection layer comprises at least 2 or more layers, and an outermost layer is made of any one of aluminum, yttrium, silicon, and a compound of aluminum, yttrium or silicon, the heating element with the protection layer can be used with stability under a corrosive environment using halide etching gas, oxygen, etc. In this case, it is particularly preferable that the outermost layer of the protection layer is made of aluminum nitride and so on as mentioned above. Such an outermost layer is more effective under the corrosive environment.

In addition, it is preferable that the rod portion is 10 to 200 mm long.

When the rod portion is 10 to 200 mm long, the distance between the terminal portion and the heating portion is long enough to keep the terminal portion to be sufficiently a low temperature. Thus corrosion of the terminal portion can be reduced more effectively.

Furthermore, it is possible that the heater pattern is formed in the surface of the plate portion from which the rod portion extends, and an electrostatic-chuck pattern for holding an object to be heated is formed in the opposite surface of the plate portion.

When the heater pattern is formed in the surface of the plate portion from which the rod portion extends, and an electrostatic-chuck pattern for holding an object to be heated is formed in the opposite surface of the plate portion, the object can be heated with being retained. Thus the object can be heated with efficiency and the object can be positioned with high precision. Therefore, when it is required to position the object to be heated with high precision in ion implantation, plasma etching, sputtering and so on, desired heating process can be conducted more accurately with such a heating element.

As described above, the present invention provides a heating element in which the heating portion and the power-supply-terminal portion are separated by the rod portion. Thus the power-supply terminal exposed at the power-supply-terminal portion resists corrosion due to high-temperature gases in a process environment. Because the heat-resistant base member is single-piece, the heating element is compact in size, and capable of being produced at a low cost. Further, the formed layers resist producing cracks.

As described above, the conductive layer is formed as the heater pattern, the current-carrying part, and the power-supply terminal. The protection layer covers the surface of the heater pattern and the current-carrying part. The dielectric layer, the conductive layer and the protection layer are formed to unite with the heat-resistant base member. Thus the heating element is compact in size, and capable of being produced at a low cost. Further, the protection layer resists producing cracks through usage. Therefore, the heating element of the present invention has a very long life.

In particular, when the heat-resistant base member is made of graphite, the cost of manufacturing the heating element can be reduced further because the material to produce the base member is low-cost, and easy to process to produce even a complicated shape. Such a heating element has high heat-resistance.

DESCRIPTION OF THE INVENTION AND A PREFERRED EMBODIMENT

As shown inFIG. 4, a heating element20of a conventional ceramic heater has a heating layer3aof pyrolytic graphite on a heat-resistant base member21of pyrolytic boron nitride. Pyrolytic graphite is prone to undergo corrosion due to oxidation. Pyrolytic graphite has reactivity with high-temperature gases used for a heating process. Therefore, there is a problem that remaining oxygen or high-temperature gases in the process environment corrodes pyrolytic graphite of a power-supply terminal3cexposed for power supply, and conventional ceramic heaters are short-life.

To solve the problem, several attempts to locate the power-supply-terminal portion away from the heating portion are made. However, there are following problems: such a heating element has protrusions on the heating surface, which hampers compact design of the heating element. In addition, a protection layer in the vicinity of a connected part of the heating element assembled from plural components is apt to produce cracks through usage, which shortens the life of the heating element.

Then, the present inventors have investigated thoroughly. They have found that the following heating element has high durability because deterioration of the power-supply terminal and the protection layer is reduced; the heating element is compact in size; and capable of being produced at a low cost.

The present invention provides a heating element comprising: at least

a single-piece heat-resistant base member with a plate portion in which a heater pattern is formed, a rod portion extending from one surface of the plate portion and in which a current-carrying part is formed, and an end portion at an end of the rod portion on the opposite side of the plate portion and in which a power-supply terminal is formed;

a dielectric layer with insulating property on the surface of the heat-resistant base member;

a conductive layer with conductive property on the dielectric layer, in which the conductive layer is formed as the heater pattern in the plate portion, as the current-carrying part in the rod portion, and as the power-supply terminal in the end portion; and

a protection layer with insulating property which covers the surface of the heater pattern and the current-carrying part, in which the dielectric layer, the conductive layer and the protection layer are formed to unite with the heat-resistant base member.

Hereinafter, an embodiment according to the present invention will be further described in detail referring to the appended drawings. However, the present invention is not limited thereto.

FIG. 1andFIG. 2are schematic views of embodiments of heating elements according to the present invention.

The present invention provides a heating element10. The heating element10includes a single-piece heat-resistant base member1with a plate portion1ain which a heater pattern3ais formed, a rod portion1bextending from one surface of the plate portion1aand in which a current-carrying part3bis formed, and an end portion1cat an end of the rod portion1bon the opposite side of the plate portion1aand in which a power-supply terminal3cis formed;

a dielectric layer2with insulating property on the surface of the heat-resistant base member1;

a conductive layer3with conductive property on the dielectric layer2, in which the conductive layer3is formed as the heater pattern3ain the plate portion1a, as the current-carrying part3bin the rod portion1b, and as the power-supply terminal3cin the end portion1c; and

a protection layer4with insulating property which covers the surface of the heater pattern3aand the current-carrying part3b, in which the dielectric layer2, the conductive layer3and the protection layer4are formed to unite with the heat-resistant base member1.

In such a heating element10, the heating portion10ain which the heater pattern3ais formed in the plate portion la and the power-supply-terminal portion10cin which the power-supply terminal3cis formed in the end portion1care separated by the conductive portion10bin which the current-carrying part3bis formed in the rod portion1b. Thus the power-supply-terminal portion10cremains at low temperature, and the exposed power-supply terminal3cresists corrosion and deterioration due to high-temperature gases in a process environment.

In addition, the heat-resistant base member1is single-piece, and not assembled from plural components. Therefore, the base member is compact in size, and capable of being produced at a low cost. Furthermore, the whole member is made of an identical material, the layers2,3and4formed on the heat-resistant base member1resist producing cracks through usage.

It is preferable that the heat-resistant base member1is made of graphite. Then, the material to produce the base member is low-cost, and easy to process to produce even a complicated shape. Therefore, the manufacturing cost of the base member can be reduced further. And such a base member has high heat-resistance. Besides, other materials with heat resistance such as sintered boron nitride can also be used.

The plate portion1asatisfies the present invention as long as the dielectric layer2, the heater pattern3aand the protection layer4are formed in the plate portion1ato provide the heating portion10a. The plate portion1ais not necessarily a circular plate as shown inFIG. 1andFIG. 2. The plate portion la can be a polygonal plate.

The rod portion1bsatisfies the present invention as long as the rod portion1bextends from one surface of the plate portion1aand in which the dielectric layer2, the current-carrying part3band the protection layer4are formed to provide the conductive portion10bas shown inFIG. 1C. The rod portion1bis not necessarily a cylinder as shown inFIG. 1andFIG. 2. The rod portion1bcan be a prism. In addition, the rod portion1bcan be one portion as shown inFIG. 1, two portions as shown inFIG. 2, or more portions. The heating element inFIG. 2has the heater pattern3ain both surfaces of the plate portion1a. The heater pattern3ais energized via two pieces of the rod portion1band heated.

When the rod portion1bis 10 to 200 mm long, the distance between the power-supply-terminal portion10cand the heating portion10ais long enough to reduce corrosion of the terminal portion more effectively.

The end portion1csatisfies the present invention as long as the end portion1cis at an end of the rod portion1bon the opposite side of the plate portion1aand in which the dielectric layer2and the power-supply terminal3care formed to provide the power-supply-terminal portion10c. In the end portion, the protection layer4is not formed on the power-supply terminal3c. Thus the power-supply terminal3cis connected electrically to the power terminal5to feed direct current or alternating current.

In addition, when the rod portion1bis one portion as shown inFIG. 1, the end portion1cis preferably enlarged to form a plate as shown inFIG. 1to provide structural stability.

The dielectric layer2satisfies the present invention as long as the dielectric layer2is made of a material with insulating property and heat resistance. When the dielectric layer is made of any one of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride and CVD aluminum nitride, or combination of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride or CVD aluminum nitride, the dielectric layer has high insulating property, and does not release impurities under high temperature conditions. Therefore, a heating element with such a dielectric layer can also be used for a heating process requiring high purity.

The conductive layer3is formed as the heater pattern3ain the plate portion1a, as the current-carrying part3bin the rod portion1b, and as the power-supply terminal3cin the end portion1c. And the protection layer4covers the surface of the heater pattern3aand the current-carrying part3b. The dielectric layer2, the conductive layer3and the protection layer4are formed to unite with the heat-resistant base member1. Therefore, the heating element is compact in size, and capable of being produced at a low cost. Furthermore, the conductive layer3is not assembled from plural components and resists exfoliating. The protection layer4does not produce cracks in the vicinity of a connected part of components through usage, and has a long life.

It is preferable that the conductive layer3is made of pyrolytic carbon or glassy carbon. Such a conductive layer3can be heated up to high temperature, and is easily worked. Therefore, the heater pattern is formed as a meandering pattern with different pattern widths, whereby the heater pattern can have arbitrary temperature gradient or heating distribution with reference to the thermal environments to heat uniformly. In particular, use of pyrolytic graphite is more preferable to further decrease the cost of producing heating elements. However, other materials which heats by the passage of electric current and has high heat resistance can also be used. The shape of the heater pattern is not limited to the meandering pattern (a zigzag pattern) inFIG. 1. For example, a concentric scroll pattern can also be used.

The heater pattern3ais formed between the dielectric layer2and the protection layer4in the plate portion1a. The heater pattern3aheats by the passage of electric current and heats a target object sufficiently. As shown inFIG. 1andFIG. 2, a pair of current-introducing portion connected to the current-carrying part3bcan be used. Besides, use of two or more pairs of current-introducing portion provides a heater with 2 or more zones which can be controlled independently.

The heater pattern3ais preferably formed in a surface of the plate portion1awhich is opposite with another surface from which the rod portion1bextends as shown inFIG. 1BandFIG. 2B. However, the heater pattern3acan be formed in a surface of the plate portion1afrom which the rod portion1bextends as shown inFIG. 3B. The heater pattern3acan also be formed in both surfaces of the plate portion1a.

The power-supply terminal3cis formed on the dielectric layer2in the end portion1c. The power-supply terminal3cis connected to the power terminal5, and the protection layer4with insulating property is not formed on the power-supply terminal3c. The power terminal5is connected to the power-supply terminal3cto feed direct current or alternating current to the conductive layer3.

The current-carrying part3bis formed between the dielectric layer2and the protection layer4in the rod portion lb. The current-carrying part3bexists between the heater pattern3aand the power-supply terminal3c, and connects the power-supply terminal3cto the heater pattern3a. The current-carrying part3b, the dielectric layer2and the protection layer4are formed to unite with the rod portion lb. In the case of the heater pattern3abeing formed in a first surface of the plate portion1awhich is opposite with a second surface from which the rod portion1bextends as shown inFIG. 1andFIG. 2, the current-carrying part3bis formed in the side and the second surface of the plate portion la to connect with the heater pattern3a.

The protection layer4with insulating property and heat resistance satisfies the present invention. The protection layer is preferably made of any one of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride and CVD aluminum nitride, or combination of boron nitride, pyrolytic boron nitride, silicon nitride, CVD silicon nitride, aluminum nitride or CVD aluminum nitride. Such a protection layer has high insulating property, and does not release impurities under high temperature conditions. Therefore, a heating element with such a protection layer can also be used for a heating process requiring high purity. Incidentally, pyrolytic boron nitride is resistant to hydrogen. However, fluoride corrodes pyrolytic boron nitride, and use of a protection layer made of pyrolytic boron nitride under fluoride environments is not recommended.

Furthermore, it is preferable that the protection layer4comprises at least 2 or more layers as shown inFIG. 5B, and an outermost layer4pis made of any one of aluminum, yttrium, silicon, and a compound of aluminum, yttrium or silicon. A heating element with such a protection layer can be used with stability under a corrosive environment using halide etching gas, oxygen, etc. For example, the outermost layer can be made of metal aluminum or metal yttrium. Or, as to a compound of aluminum, yttrium or silicon, for example, alumina, aluminum fluoride, yttria, yttrium fluoride, silicon oxide can be used. Combination of two or more of metal aluminum, metal yttrium, alumina, aluminum fluoride, yttria, yttrium fluoride, silicon oxide can also be used. In such a case, the outermost layer is made of aluminum and so on. As for an under layer of the outermost layer to protect the conductive layer, an oxide or a conductive material is not preferable, and metal cannot be used for causing a short. Therefore, boron nitride, pyrolytic boron nitride and so on as mentioned above can preferably used for the under layer.

Furthermore, an electrostatic-chuck pattern6can be formed as shown inFIG. 3. The electrostatic-chuck pattern6is an electrode pattern providing static electricity to hold an object to be heated. In particular, the following heating element is preferable: the heater pattern3ais formed on a surface of the plate portion1afrom which the rod portion1bextends as shown inFIG. 3B; the electrostatic-chuck pattern6for holding an object to be heated is formed on another surface of the plate portion1aas shown inFIG. 3A. In such a heating element, the object can be heated with being retained securely. Thus the object can be heated and positioned with high precision. Therefore, when it is required to position the object to be heated with high precision in ion implantation, plasma etching, sputtering and so on, desired heating process can be conducted more accurately with the heating element. When the chuck pattern is formed, a protection layer of the chuck pattern preferably has insulating property and contains nitride as mentioned above.

The heating element10of the present invention as explained above heats by being connected electrically with the power terminal5. The heating portion10aand the power-supply-terminal portion10care separated by the conductive portion10bin which the current-carrying part3bis formed in the rod portion lb. Therefore, the power-supply terminal3cexposed at the power-supply-terminal portion10cresists corrosion due to high-temperature gases in a process environment, and the heating element has a long life.

In addition, the conductive layer3is formed as the heater pattern3a, the current-carrying part3b, and the power-supply terminal3c. And the protection layer4covers the surface of the heater pattern3aand the current-carrying part3b. The dielectric layer2, the conductive layer3and the protection layer4are formed to unite with the heat-resistant base member1. Therefore, the heating element is compact in size, and capable of being produced at a low cost. Furthermore, the protection layer4resists producing cracks through usage, and has a long life.

EXAMPLE

Hereinafter, the present invention will be explained further in detail with reference to Examples and Comparative Example, however, the present invention is not limited thereto.

A single-piece heat-resistant base member1as shown inFIG. 1was prepared. The base member1was made of carbon and included a plate portion1a, a rod portion1b, and an end portion1c. The plate portion1ais 10 mm thick with an outside diameter of 250 mm. The rod portion1bextends from one surface of the plate portion1a. The rod portion1bis 100 mm long with a diameter of 30 mm. The end portion1cwas at an end of the rod portion1bon the opposite side of the plate portion1a. The end portion1cwas a small circular plate which is 10 mm thick with a diameter of 60 mm. Four holes with a diameter of 6 mm to which a power terminal5can be connected were formed in the plate of the end portion1c.

The heat-resistant base member1was placed in a thermal CVD reactor. A reaction gas mixed in the volume proportions of 4 parts ammonia to 1 part boron trichloride was fed to the surface of the base member1. Reaction was carried out under conditions of 1900° C. and 1 Torr to form a dielectric layer2of pyrolytic boron nitride with 0.3 mm thick on the surface of the base member1.

Next, methane gas was pyrolyzed under conditions of 1800° C. and 1 Torr to form a conductive layer3made of pyrolytic graphite with 0.1 mm thick. The conductive layer3was worked so that a heater pattern3awas formed in the heating surface in the plate portion1a, a current-carrying part3bis formed in the side and the back surface of the plate portion1aand in the rod portion1b, and a power-supply terminal3cwas formed in the end portion1c. Two power-supply terminals3cwere formed, and other two holes were left unused.

The base member1was again placed in the thermal CVD reactor with masking the power-supply terminals3c. A reaction gas mixed in the volume proportions of 4 parts ammonia to 1 part boron trichloride was fed. Reaction was carried out under conditions of 1900° C. and 1 Torr to form a 0.1 mm thick protection layer4of pyrolytic boron nitride with insulating property on a surface of the heater pattern3aand the current-carrying part3b.

Thus produced heating element10inFIG. 1was electrically connected to heat in vacuum of 1×10−4Pa. The heating portion10awas heated up to 800° C. with 1.5 kW of electric power. At this time, the power-supply-terminal portion10cwas 150° C. The power-supply-terminal portion10cthus remained much lower temperature than the heating portion10a.

Then, hydrogen gas was fed to the vacuum to be 1×10−2Pa, and the heating element10heated as long as 200 hours. During 200 hours the terminal portion and the heater portion of the heating element10remained unchanged, and heating was conducted properly.

Furthermore, another heating element10inFIG. 5Awas produced: an outermost layer4pof aluminum nitride 10 m thick was formed by reactive sputtering on a surface of a heating element10inFIG. 1produced as with above.

Thus produced heating element10was electrically connected to heat in vacuum of 1×10−4Pa. The heating portion10awas heated up to 500° C. with 1.0 kW of electric power. At this time, the power-supply-terminal portion10cwas 150° C. The power-supply-terminal portion10cthus remained much lower temperature than the heating portion10a.

Then, CF4gas was fed to the vacuum to be 1×10−2Pa, and the heating element10heated as long as 200 hours. An amount of corrosion was 5 μm or less and heating was conducted unchangingly.

A single-piece heat-resistant base member1as shown inFIG. 2was prepared. The base member1was made of carbon and included a plate portion1a, a pair of rod portion1b, and an end portion1c. The plate portion1ais 10 mm thick with an outside diameter of 250 mm. The pair of rod portion1bextends from two peripheral ends across a surface of the plate portion1arespectively. The rod portion1bis 50 mm long with a diameter of 20 mm. The end portion1cwas an end of the rod portion1bon the opposite side of the plate portion1a. At the end portion1c, a hole 10 mm deep with a female M10 screw thread was formed to establish electric connections with a screw.

The heat-resistant base member1was placed in a thermal CVD reactor. A reaction gas mixed in the volume proportions of 4 parts ammonia to 1 part boron trichloride was fed to the surface of the base member1. Reaction was carried out under conditions of 1900° C. and 1 Torr to form a dielectric layer2of pyrolytic boron nitride with 0.3 mm thick on the surface of the base member1.

Next, methane gas was pyrolized under conditions of 1800° C. and 3 Torr to form a conductive layer3of pyrolytic graphite with 0.1 mm thick. The conductive layer3was worked so that a heater pattern3awas formed in the heating surface in the plate portion1a, a current-carrying part3bis formed in the rod portion1b, and a power-supply terminal3cwas formed in the end portion1c.

The heat-resistant base member1was again placed in the thermal CVD reactor with masking the power-supply terminals3c. A reaction gas mixed in the volume proportions of 4 parts ammonia to 1 part boron trichloride was fed. Reaction was carried out under conditions of 1900° C. and 1 Torr to form a 0.1 mm thick protection layer4of pyrolytic boron nitride with insulating property on a surface of the heater pattern3aand the current-carrying part3b.

Thus produced heating element10inFIG. 2was electrically connected to heat in vacuum of 1×10−4Pa. The heating portion10awas heated up to 800° C. with 1.5 kW of electric power. At this time, the power-supply-terminal portion10cwas 200° C. The power-supply-terminal portion10cthus remained much lower temperature than the heating portion10a.

Then, hydrogen gas was fed to the vacuum to be 1×10−2Pa, and the heating element10heated as long as 200 hours. During 200 hours the terminal portion of the heating element10remained unchanged, and heating was conducted properly.

Furthermore, another heating element10inFIG. 5Cwas produced: an outermost layer4pof yttria with 10 μm thick was formed by plasma spray coating on a surface of a heating element10inFIG. 2produced as with above.

Thus produced heating element10was electrically connected to heat in vacuum of 1×10−4Pa. The heating portion10awas heated up to 500° C. with 1.0 kW of electric power. At this time, the power-supply-terminal portion10cwas 150° C. The power-supply-terminal portion10cthus remained much lower temperature than the heating portion10a.

Then, CF4gas was fed to the vacuum to be 1×10−2Pa, and the heating element10heated as long as 200 hours. An amount of corrosion after a lapse of 200 hours was very small of 10 μm.

Comparative Example

A single-piece heat-resistant base member21inFIG. 4was prepared. The base member1was a plate made of carbon, 10 mm thick, with an outside diameter of 250 mm. Two holes were formed at peripheral ends across the surface of the plate to establish electric connections with a screw. The hole was 10 mm deep with a female M10 screw thread. The diameter of the hole was larger than a M10 screw by 0.4 mm.

The heat-resistant base member21was placed in a thermal CVD reactor. A reaction gas mixed in the volume proportions of 4 parts ammonia to 1 part boron trichloride was fed to the surface of the base member1. Reaction was carried out under conditions of 1900° C. and 1 Torr to form a dielectric layer2of pyrolytic boron nitride with 0.3 mm thick on the surface of the base member21.

Next, methane gas was pyrolyzed under conditions of 1800° C. and 3 Torr to form a conductive layer3of pyrolytic graphite with 0.1 mm thick. The conductive layer3was worked so that a heater pattern3awas formed on the heating surface of the base member, and power-supply terminals3cwere formed at peripheral ends across the heating surface.

The base member21was again placed in the thermal CVD reactor with masking the power-supply terminals3c. A reaction gas mixed in the volume proportions of 4 parts ammonia to 1 part boron trichloride was fed. Reaction was carried out under conditions of 1900° C. and 1 Torr to form a 0.1 mm thick protection layer4of pyrolytic boron nitride with insulating property on a surface of the heater pattern3a.

Thus produced heating element20inFIG. 4was electrically connected to heat in vacuum of 1×10−4Pa. The heating element20heated up to 800° C. with 1.5 kW of electric power. At this time, the power-supply-terminal portion was 480° C. and substantially did not resist being heated.

Then, hydrogen gas was fed to the vacuum to be 1×10−2Pa, and the heating element20heated. After a lapse of 75 hours, carbon of the power-supply terminal3cwas corroded and the heating element20was disconnected.

Furthermore, the heating element20heated up to 500° C. with 1.0 kW of electric power, and CF4gas was fed to the environment. After a lapse of 10 hours, an outermost layer of boron nitride was lost. And the conductive layer3such as the heater pattern3aproduces cracks, and the heating element20was disconnected.

The present invention is not limited to the above-described embodiments. The above-described embodiments are mere examples, and those having the substantially same structure as that described in the appended claims and providing the similar action and advantages are included in the scope of the present invention.

For example, the cases of forming the dielectric layer2and the protection layer4of pyrolytic boron nitride were described in the above embodiments. However, a case of using other materials such as silicon nitride or aluminum nitride to form those layers is also included in the scope of the present invention.