High temperature operating element

Abstract: A high temperature operating element includes a porous high temperature operating element film with low film density formed into a predetermined configuration on one surface of an insulating member with good heat conductivity, a resistive film with a high fusion point and good heat conductivity whose film density is higher than that of the high temperature operating element film, formed into a predetermined configuration on the other surface of the insulating member with good heat conductivity, a lead wire connected to the resistive film, an insulating protective film formed on the insulating member so as to cover the resistive film.

Description: FIELD OF THE INVENTION The present invention relates to a structure of a high temperature operating element which is heated by a heater at a high temperature to operate and, more particularly, to a structure of a high temperature operating element such as a laminated type electron emitting element which effectively emits electrons at a high temperature of approximately 1000°C using thermoelectron emission, such as an electron gun for a cathode ray tube, a hot cathode X-ray tube, an electron microscope or the Braun tube. In addition, the present invention relates to a method for manufacturing a heater for heating up to a high temperature of approximately 1000°C such as a compact heater for heating the high temperature operating element or a heater for the electron gun. BACKGROUND OF THE INVENTION Heretofore, a high temperature operating element has been manufactured using so-called thick film circuit forming technique such as screen printing as disclosed in Japanese Patent Publication Gazzete No. 55-24646. Figure 7 is a sectional view showing the thus manufactured conventional high temperature operating element. First, a raw material for forming a ceramic substrate 10 is prepared and a heat generating layer 11 having a predetermined configuration is formed on a sheet by printing technique such as extrusion through a roll or casting method. Then, an insulator 12 is formed on the substrate 10 with the heat generating layer 11 formed thereon and then a cathode lead layer 13, a base metal layer 14 and a cathode material layer 15 are formed on this insulator 12 by the same printing method, so that the high temperature operating element is formed. The heat generating layer 11 is formed on the substrate 10 by screen printing a paste in which baking assistant is applied to a heater material and the operating element is formed on the substrate 10 by screen printing a paste in which the baking assistant is applied to the desired material. After the screen printing, they are baked at a high temperature (1000°C - 2000°C) and then the high temperature operating element is formed. In this method, high temperature treatment is performed when it is manufactured. Therefore, if the heater is used below this processing temperature, a change of resistance with time is little, so that it is stable at a high temperature for a long time as a heater. However, pattern precision obtained by the screen printing is low and it is difficult to control (reduce) the thickness of the heat generating layer 11, therefore the power consumption is large and the resistance scatters in a plurality of heaters. Therefore, as a method for forming a pattern with high reliability, a PVD method (Physical Vapor Deposition) and a CVD method (Chemical Vapor Deposition) have been developed. Figure 8 shows a method for manufacturing the conventional high temperature operating element by a thin film forming method. First, a resistive (heat generating) film 20 and a high temperature element film 40 are uniformly formed on one surface and the other surface of the plain ceramic substrate 10, respectively. Then, predetermined heater pattern and an element pattern are formed by etching and then a lead wire 50 is connected to the heater side thereof, whereby the high temperature operating element is produced. Figure 10 shows a structure of an electron emitting apparatus produced by the thin film forming method as a example of the conventional high temperature operating element. First, a resistive (heat generating) film 20 for a heater and a film for a base metal 18 (reduction member) are uniformly formed on one surface and the other surface of a plain ceramic substrate 10, respectively. Then, a desired heater pattern and a pattern for a cathode are formed by etching and an electron emitting member 19 is applied to the base metal film and then a lead wire 50 is connected to the heater side, whereby an electron emitting apparatus is produced. A description is given of a method for manufacturing a conventional plain thin heater used in such high temperature operating element. Figure 11(a) to (d) are process diagrams showing a method for manufacturing the plain thin heater by the conventional thin film forming method. For example, a resistive (heat generating) film 30 for heater is uniformly formed on a plain ceramic substrate 10 of such as Al₂O₃ [figure 11 (b)], then a desired heater pattern is formed by etching [figure 11(c)] and then, a lead wire 50 is connected thereto [figure 11(d)]. As a result, the plain thin heater is provided. In the conventional high temperature operating element produced by the above method, the resistance changes while it is used as a plain thin heater with a voltage applied to the lead wire 50. This is because the resistive (heat generating) film 20 is thin. Figure 9 shows a change of a resistance value of the heater with time. In figure 9, the ordinate designates a resistance value and the abcissa designates time. As shown in the figure 9, resistance falls at an early stage because the thin film is recrystallized and a crystal grain in the film becomes large. For example, when the resistive (heat generating) film 20 is W (tungsten) and it is used at 1000°C, it is recrystallized because 1000°C is the recrystallization temperature of W. In addition, resistance is increased with time because impurities enter the film from an atmosphere or the film is oxidized. Therefore, it is not stable as a heater and reliability over a long period of time is not guaranteed. Since the oxide series substrate such as Al₂O₃ is easily available in a monocrystalline state and a surface thereof capable of receiving mirror grinding, pattern precision thereof is better than that of a sintered substrate such as SiC and AlN when a thin film is formed. However, in the heater using the oxide series substrate such as Al₂O₃ as shown in figure 11, a part of the substrate below the resistance wiring end is selectively damaged by a thermochemical or electrochemical action caused by oxygen during its use, as shown in an attached reference photograph 3 showing a sectional view of an end of the conventional plain thin heater of the high temperature operating element after its use and this damage causes the heater to reduce its life. In addition, in the high temperature operating element such as an electron emitting apparatus provided by the thus described method, the film peels off the substrate 10 while a voltage is applied to the lead wire to heat the heater and the cathode is heated through the ceramic substrate 10 to emit electrons. More specifically, the ceramic substrate 10 peels off the resistive film 20 or the ceramic substrate 10 peels off the base metal film 18 in the structure shown in figure 10. The reason for this is that an adhering force between the film and the substrate is originally weak, a change is generated in a balance of an internal stress due to a heat history during its use and thermal expansion coefficients of the film and the substrate are different. Therefore, heat capacity changes due to the peeling off, a resistance value as a heater fluctuates, a wire brakes in the heater and an amount of electron emission by the cathode changes with the change of the heat capacity. Furthermore, the base metal (reduction member) film 18 does not well adhere to the cathode material and an electron emitting characteristic are deteriorated, therefore the heater and the cathode are unstable and long-term reliability thereof is reduced. Therefore, performance thereof is not sufficient. As described above, the conventional high temperature operating element is formed alternatively by providing a porous film with low film density on both surfaces by thick film circuit forming technique or providing a fine film with high film density and with less adherence by the thin film forming method, therefore performance of the heater and the element are not effectively implemented. SUMMARY OF THE INVENTION The present invention was made in order to solve the conventional problem and it is intended to provide a high temperature operating element with long-term high reliability having a thin film high temperature heater with high reliability in which resistance changes are minimal and the film is not likely to peel off the substrate during its use. The present invention provides a high temperature operating element comprising: a substrate of electrically insulative thermally conductive material; a porous high temperature operating element film of low film density, disposed on a surface of said substrate; and a thin-film resistor of high fusion point, thermally conductive, material having a coefficient of thermal expansion close to that of said substrate and low vapour pressure at high temperature, which resistor is of a higher film density than said element film and is disposed on an opposite surface of said substrate; which element is characterised by: an electrically insulative protective film covering said resistor and an area of said opposite surface of said substrate at the periphery of said resistor, which protective film is of high temperature stable material, is adherent to the resistor to exclude the atmosphere and thus prevent oxidation of said resistor, and is effective to inhibit peeling of the resistor from said substrate. The preferred materials used, for such an element, for said substrate, resistor and protective film are as claimed in claim 2. Furthermore, said protective film may also be disposed between said resistor and said opposite surface of said substrate, and be of a non-oxide material. The preferred materials used, for such an element, for said substrate, resistor and protective film are as claimed in claim 4. Furthermore, said element film may be of a porous reducing agent material covered by a pore-penetrative electron emissive material. A method of producing a high temperature operating element, comprises the following steps: forming a porous high temperature operating element film of low film density on a surface or a substrate of electrically insulative thermally conductive material; applying a thin-film deposition technique to produce a patterned thin-film resistor on the opposite surface of said substrate, which resistor is of high fusion point, thermally conductive material having a coefficient of thermal expansion close to that of said substrate and a low vapour pressure at high temperature; and heating said resistor to recrystallise its constituent material; which method is characterised by: covering said resistor, and an area of said opposite surface of said substrate at the periphery of said resistor, with an electrically insulative protective film the material of which is high temperature resistant, adherent to the resistor to exclude the atmosphere therefrom thus to prevent oxidation of said resistor, which protective film also is effective to inhibit peeling of the resistor from said substrate and by using the thin-film deposition technique a high precision thin film pattern can be achieved. Furthermore, this protective film is preferably of a non-oxide material and may be applied to said opposite surface of said substrate as well as to said resistor and said area. Furthermore, following application of said non-oxide material, said heating is preferably performed by baking. The surface of the thin film resistive film is covered with a protective film of non-oxide series insulating material in accordance with the present invention, oxidation of the resistive material and a change of resistance are prevented while it is used and it is not likely to be deteriorated by the atmosphere, therefore temperature distribution on the surface is uniform regardless of the pattern configuration and reliability is improved. In addition, provided the insulating substrate surface opposite to the thin film resistive film is formed of a protective film of non-oxide series insulating material, damage of the substrate due to a chemical action between the substrate and the thin film resistive film can be prevented and a heater function is prevented from being reduced. Furthermore, provided the thin film resistive film is baked, the resistive film is recrystallized before it is used as a heater and resistance thereof is prevented from changing while it is used. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional view showing a high temperature operating element according to a first embodiment of the present invention; Figure 2 is a sectional view showing a high temperature operating element according to a second embodiment of the present invention; Figure 3 is a sectional view showing an electron emitting apparatus showing an example of the high temperature operating element of the present invention; Figure 4 is a sectional view showing a thin high temperature heater of the high temperature operating element according to a third embodiment of the present invention; Figure 5 is a sectional view showing a thin high temperature heater of the high temperature operating element according to a fourth embodiment of the present invention; Figure 6 is a sectional view showing a thin high temperature heater of the high temperature operating element according to a fifth embodiment of the present invention; Figure 7 is a sectional view showing a conventional high temperature operating element formed by a thick film forming method; Figure 8 (a) to (d) are sectional views showing a method for manufacturing the conventional high temperature operating element formed by the thin film forming method; Figure 9 is a graph showing a change of a resistance value of a heater of the conventional high temperature operating element with time; Figure 10 is a sectional view showing a conventional electron emitting apparatus formed by the thin film forming method; Figure 11(a) to (d) are sectional views showing a conventional method for manufacturing a plain thin heater by the thin film forming method; Reference photograph 1 shows a surface of the porous W-sintered substrate used in the embodiment; Reference photograph 2 shows a surface of the fine W-sputtered film; and Reference photograph 3 shows a sectional view of an end of the conventional plain thin heater of the high temperature operating element after its use. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the drawings. Figure 1 is a sectional view showing a high temperature operating element in accordance with an embodiment of the present invention. In figure 1, reference numeral 1 designates a ceramic substrate (insulating substrate), reference numeral 2 designates a fine resistive film with high film density for a heater, reference numeral 3 designates a vitreous protective coating layer (insulating protective film), reference numeral 4 designates a porous film with low film density for a high temperature operating element film and reference numeral 5 is a lead wire. It is desirable that the following requirements are met for respective materials. That is, for the substrate 1, it is desirable that its material has good heat conductivity, its coefficient of thermal expansion is close to that of the resistive film 2, it is a good insulator, it is not likely to cause dielectric breakdown at a high temperature and it is planar. Therefore, AlN, Al₂O₃ and the like are suitable for that in view of availability. For the resistive film 2, it is desirable that its material has a low vapour pressure and a stable electric characteristic in a high temperature region. Therefore, Mo, W, Pt, Ta, TiN, TiC, TiCN and the like are considered suitable. For the protective coating layer 3, it is desirable that its material is diffused in little at a high temperature and has a softening point or a fusion point higher than a working temperature. Therefore, a vitreous material which is stable at a high softening point and a high fusion point such as SiO₂, Al₂O₃ and the like is considered suitable. For example, in case of SiO₂, its softening point is 1710°C (rock crystal) and fusion point is 1470°C (crystal) and in case of Al₂O₃, its fusion point is 2030°C. Alternatively, a material such as CaO and Y₂O₃ may be used. For the lead wire 5, it is desirable that its material has the same characteristic and the same diffusion coefficient as those of the resistive film 2 and it is most desirable that its material is the same as that of the resistive film 2. For the element film 4, it is desirable that it is porous, so that it highly adheres to a protective layer, a layer to which an electron emitting assistant is applied, an insulating layer and the like which are provided to improve its performance. If they are constructed as shown in figure 1 with the above-described materials, it is possible to improve performance of the high temperature operating element which is operated when a voltage is applied to the lead wire 5, a heater, that is, the resistive film 2 is heated and then the element film 4 is heated from the rear. If electron emitting assistant is applied to the element film 4, high current density can be obtained from the element film 4, because resistance thereof changes in little even after it is used for a long time, therefore it is stable as a heater. In addition, the element film will not peel off if it is covered with a vitreous protective film and the electron emitting assistant can be well contained therein because it is porous. As a result, high current density can be obtained from the element film 4. In this embodiment, in view of the above condition, a description is given of a method for manufacturing the high temperature operating element in which W is applied to the substrate, W/AlN is used as the ceramic substrate provided with both W and the substrate baked at the same time, W is formed by sputtering as the resistive film 2 and a vitreous "glaze" containing SiO₂ as a main component is applied as the protective coating layer 3. In addition, reference photograph 1 shows a surface of the porous W-sintered substrate used in the embodiment and reference photograph 2 shows a surface of the fine W-sputtered film (scale factor is the same as that in the reference photograph 1). In the photograph 1, reference numeral 0067 designates a film number, 15.0KV designates an accelerating voltage of a scanning electron microscope, X2, 000 designates a scale factor and a length of "10µm" on a line corresponds to real 10 µm. The side of W of the ceramic substrate (W/AlN substrate) in which W and the AlN substrate are baked at the same time is patterned into a predetermined configuration by etching and the side of AlN of the W/AlN substrate is mechanically polished to provide a mirror finish. A mask of a desired heater pattern is set on the substrate 1 and the W resistive film 2 with a predetermined thickness (a few µm - 10µm) is formed by sputtering. Then, the lead wire 5 is connected to a desired place by a method such as resistance welding. Then, the vitreous "glaze" is sprayed so as to cover the heater resistive film 2 and dried to form the coating layer 3. Then, it is baked for 5 - 10 minutes in vacuum, hydrogen or argon atmosphere to fuse on the W resistive film. A processing temperature at this time depends on a composition of the "glaze" and it is approximately 800 - 1400°C. This "glaze" is solution containing the so-called vitreous material of oxide. For example, compositions of three kinds of glaze A, B and C are shown in a table 1 and these are available in the market as glass type ceramic coating materials. The composition of frit shown in the table 1 is shown in a table 2 and it is the so-called vitreous material of oxide. This vitreous material dissolves a metal oxide generated in little in the resistive film 2 while it is used as a heater and serves as a seal coat which buries a gap between metals, so that there is provided good adherence to the resistive film 2. In addition, since it is a vitreous material, it has high electric insulating property and it will satisfactorily function as a high temperature heater. When a protective layer, a layer to which electron emitting assistant is applied, an insulating layer and the like are provided on the element film surface in next process, they are likely to adhere to it because it is a sintered porous substrate made from W particles. Distortion could be generated between the substrate 1, the resistive film 2 and the protective film 3 due to a difference in coefficients of thermal expansion at high temperature, but the vitreous material can flexibly bury the gap as described above and it acts to reduce the distortion. Therefore, even if it covers the whole surface, there is no problem in regard to the distortion. In addition, if the lead wire 5 which has also been covered with the same vitreous material is used, the effect is further improved. As shown in figure 2, the process can be performed over a large area of the ceramic substrate. In addition, although a method for applying the "glaze" is described in the above embodiment, the protective coating layer 3 can be also formed by a PVD or CVD method in which a vitreous target is prepared and then a film is formed thereon by sputtering. As for the composition of the vitreous material, it is not necessarily the composite composition shown in the table 2 and it may be a single composition such as SiO₂ and Al₂O₃. For example, when the substrate is made of Al₂O₃ and the protective layer 3 of Al₂O₃ is coated, it is not necessary to consider an influence of an impurity and diffusion. In the above embodiment, although a description was given of an example in which the high temperature operating element film is formed into a predetermined configuration on the simultaneously sintered W/AlN substrate by etching W, a W/AlN substrate having a screen printed pattern for elements may be used for that. In addition, the high temperature operating element film may be formed by etching a film formed by another method such as thermal spraying and cladding into a predetermined configuration so long as a porous surface is formed. Although a method for forming the W resistive film 2 by sputtering was described in the above embodiment, it is needless to say that the PVD method such as electron beam deposition, a laser PVD method and ion plating or the CVD method using WF₆, W(CO)₆ and WCl₆ gas may be used. In addition, the same may be said in a case where a film made of, for example Mo and the like instead of W is formed. Although wet process was not used in the above embodiment because the insulating substrate was made of AlN which reacts with water or alkali, the wet process may be used if the substrate is made of Al₂O₃ and the like. Then, as an example of the high temperature operating element an electron emitting apparatus is described. Figure 3 is a sectional view showing an electron emitting apparatus embodying the present invention. In figure 3, reference numeral 7 designates an electron emitting member and reference numeral 6 designates a reduction member (base metal) comprising porous material with low film density which reduces the electron emitting member 7. A part of the electron emitting member 7 penetrates into the pores of the porous material. Reference numeral 2 designates a fine resistive film with high film density as a heater for heating a cathode comprising the electron emitting member 7 and the reduction member 6. The film is formed into a predetermined configuration using a material having good electrical conductivity and high fusion point. Reference numeral 1 designates an insulating member made of electrical insulating material with good heat conductivity, which is interposed between the reduction member 6 and the resistive film 2 to insulate both electrically and. effectively conduct heat generated from the resistive film 2 to the reduction member 6. Reference numeral 3 is a protective film formed so as to cover the resistive film 2 for protecting the resistive film 2 from an outside atmosphere. Similar to the high temperature operating element shown in figure 1, the following properties are required for respective materials. For the insulating member 1 corresponding to the substrate of the electron emitting apparatus, it is desirable that. its material has good heat conductivity, its coefficient, of thermal expansion is close to that of the resistive film 2 and the reduction member 6, it is a good insulator, it is not likely to cause dielectric breakdown at high temperature and it is plain. Therefore, AlN, Al₂O₃ and the like are suitable for that in view of availability. For the resistive film 2 for a heater, it is desirable that its material has a low vapor pressure and a stable electric characteristic in a high temperature region. Therefore, Mo, W, Pt, Ta, TiN, TiC, TiCN and the like are considered suitable. Especially, TiN, TiC and TiCN of ceramic series are suitable because their recrystallized temperature is high and they are stable at high temperature. For the protective coating layer 3, it is desirable that its material is diffused in little at high temperature and has a softening point or a fusion point higher than a working temperature and it is a good insulator. Therefore, a vitreous material such as SiO₂, Al₂O₃ and the like which are stable at a high softening point and a high fusion point or ceramics such as AlN and BN are considered stable. For example, in case of SiO₂, a softening point is 1710°C (rock crystal) and a fusion point is 1470°C (crystal) and in case of Al₂O₃, a fusion point is 2030°C. Alternatively, a material such as CaO and Y₂O₃ may be used. For the reduction member 6, it is desirable that its material has a low vapour pressure and a stable electric characteristic in a high temperature region, it can highly reduce the electron emitting member 7 and it is porous so as to highly adhere to the electron emitting member 7. In this embodiment, in view of the above-described condition, a description is given of a method for manufacturing the electron emitting apparatus, in which a monocrystalline sapphire substrate (Al₂O₃) is used as the insulating member 1, powder W is sintered on the sapphire substrate as the reduction member 6, TiN is formed by sputtering as the resistive film 2, AlN is formed by sputtering as the insulating protective film 3 and the electron emitting member 7 (Ba, Sr, Ca) CO₃ is applied to the reduction member 6 of W. In addition, the surface of the porous W film used as the reduction member 6 in the embodiment is the same as that shown in the reference photograph 1. First, a sapphire substrate 1 having one surface finished by mirror grinding is prepared and a desired pattern for a cathode is screen printed on the other surface using W paste containing organic solvent or baking assistant. Then, it is baked at a high temperature (1000 - 1800°C). The pattern on the side of the cathode is relatively simple, so that precision can be reliable even if it is not finished by mirror grinding. Then, a mask of a desired heater pattern is set on the surface finished by the mirror grinding to form a TiN film 2 with a desired thickness (a few µm - 10µm) by sputtering. Then, the AlN film 3 is formed on the surface having the heater pattern by sputtering to cover the resistive film 2 for the heater. On the other hand, the electron emitting member 7 such as (Ba, Sr, Ca)CO₃ is applied to the surface of the reduction member W 6. As a result, the electron emitting apparatus is provided. Then, operation thereof is described. A constant voltage is applied to the resistive film 2 and then the resistive film 2 is heated at a predetermined temperature. The reduction member 6 and the electron emitting member 7 are heated through the insulating member 1 and then a voltage is applied between the grid and the cathode to emit electrons from the electron emitting member 7. Since the resistive film 2 serving as a heater is recrystallized at high temperature and is stable at high temperature during the long term use of the electron emitting apparatus, there is a little resistance change and then it is stable as a heater. In addition, since it is covered with the protective film 3, it is not damaged by an outer atmosphere such as a residual gas which could cause corrosion and the like. Furthermore, it can be prevented from peeling off the substrate. Since the reduction member 6 is sintered and formed on the surface on which mirror grinding has not been performed, it is highly adherent and it is stable at a high temperature. In addition, since it is porous, the electron emitting member 7 partially enters it, so that it can well adhere to the electron emitting member 7 and a high current density can be stably obtained. In addition, it is possible to uniformly mass-produce the electron emitting apparatus by collectively forming heaters and cathodes on the insulating member 1 over a large area and then dividing it into minimum chips. The reason why the resistive film 2 is formed of a simple substance of TiC, TiN and TiCN or their mixture is that its recrystallized temperature is high and electrically stable at a high temperature. Although it may be formed of a general heater material such as W or Mo like the reduction member 6, this material removes oxygen (deoxidize) from the substrate of Al₂O₃ to form an oxide having a high vapour pressure and then scatters while used at a high temperature of approximately 1000°C. More specifically, the heater is etched away and its configuration changes. Therefore, circumstances in which it can be stably used as a heater, for example a material of the substrate 1, an atmosphere and a temperature are limited. However, W or Mo can be used below approximately 800°C. Although a description was given of a method for forming TiC, TiN and TiCN serving as a heater material by sputtering, it is needless to say that it can be formed by a PVD method such as ion plating, electron beam deposition and laser PVD method. Since it is used at a high temperature, a thermal CVD method using TiCl₄, CH₄, NH₃ and the like is considered best in view of thermal equilibrium. In addition, as a method for forming a film, a plasma CVD method using the same gas is also considered. Although the monocrystalline Al₂O₃ substrate was used as the insulating member 1 in the above embodiment, an AlN sintered substrate and a substrate on which an AlN film is further-formed thereon may be used when the condition of a leak current or an insulation breakdown voltage is not strict. Although a description was given of a method for forming the reduction member 6 by screen printing using W paste, it may be formed by thermal spraying, cladding or the like and then a pattern for a cathode may be formed by etching depending on pattern precision. Next, a description is given of a structure of a thin high temperature heater part composing a part of the high temperature operating element of the present invention. Figure 4 is the sectional view showing the thin high temperature heater in accordance with an embodiment of the present invention. In figure 4, reference numeral 1 designates an insulating substrate comprising a plain ceramic plate 100 and a protective film 8 of non-oxide series insulating material, reference numeral 2 designates a thin film resistive film for a heater and reference numeral 5 is a lead wire. It is desirable that the following requirements are met for respective material films as in the above embodiment. For example, for the insulating substrate, it is desirable that its material has good heat conductivity, its coefficient of thermal expansion is close to that of the resistive film, it is a good insulator, it is not likely to cause breakdown at high temperature and it is plain so that it is not likely to be damaged by an atmosphere. In order to provide an insulating substrate satisfying the above requirement, the protective film of non-oxide series insulating material such as AlN and BN which has good heat conductivity and heat expansion coefficient close to that of the resistive film and is not likely to be damaged by the atmosphere is provided on the ceramic insulating material of oxide series such as Al₂O₃ and BeO which is easily available in a monocrystalline state and whose surface can be finished by mirror grinding in such a manner that it opposed to the thin film resistive film. However, this is not limited and the single insulating material of non-oxide series satisfying the above-described condition can be also used in the same manner. In addition, as shown in figure 4, the protective film of non-oxide series insulating material is not necessarily provided on the whole surface of the insulating material of oxide series and the protective film of non-oxide series insulating material may be provided only on the surface opposite to the thin film resistive film. Although the conventional thick resistive film formed by the screen printing is several tens µm, the thin resistive film in accordance with the present invention has a thickness of 10µm or less, so that its vapor pressure is low and its electrical characteristic is stable in a high temperature region. Therefore, Mo, W, Pt, Ta, TiN, TiC, TiCN and the like are considered suitable. For the protective film of non-oxide series insulating material covering the thin resistive film, it is desirable that its material is diffused in little at a high temperature, it has a softening point or a fusion point higher than the working temperature and it is not likely to be damaged by the atmosphere. Therefore, AlN, BN and the like are considered suitable as in the above. For the lead wire, it is desirable that its material has the same characteristic and diffusion coefficient as those of the resistive film and it is most desirable that it is the same as that of the resistive film. Hereinafter, in view of the above condition, a description is given of a method for manufacturing the thin high temperature heater in which Al₂O₃ (which is called alumina or sapphire) is used as a ceramic substrate, W is formed by sputtering as the thin film resistive film and AlN is used as a protective film of non-oxide series insulating material. An AlN film having a desired thickness (several µm - 100µm) is uniformly formed on a plain Al₂O₃ substrate and then a W film having a desired thickness (several µm - 10µ m) is uniformly formed by sputtering. Then, it is formed into a desired pattern configuration by wet or dry etching. For example, if it is formed by the wet etching, etching is performed in the following process. Then, it is baked at 800 - 1000°C in a hydrogen reducing atmosphere or an argon atmosphere until the resistance of the thin film resistive film becomes stable. Smoothness which is generally hard to obtain in a sintered material such as the AlN substrate can be implemented by covering the plain Al₂O₃ substrate with the AlN of the protective film of non-oxide series insulating material. As a result, profile irregularity of the resistive material formed thereon is improved and reliability of the heater is also improved. Then, the lead wire is connected to a desired place by a method such as resistance welding. Then, the AlN protective film of non-oxide series insulating material is formed by sputtering so as to cover the thin resistive film for the heater to obtain the thin high temperature heater in accordance with one embodiment of the present invention. Although the protective film of non-oxide series insulating material was formed only around the W film after the lead wire was connected in the thin high temperature heater in accordance with the above embodiment, it may be formed on the whole surface of the substrate including the connection part as shown in a sectional view of the thin high temperature heater in accordance with another embodiment in figure 5. In this case, distortion between the substrate, the thin resistive film and the protective film could be generated at high temperature because their thermal expansion coefficients are different. However, when AlN is used, distortion is prevented even if the whole surface is covered, because the thermal expansion coefficient of AlN is almost the same as that of W in comparison with Al₂O₃. As a result, AlN reduces the distortion generated between the substrate and the resistor. In addition, it is also possible to process both surfaces of the ceramic substrate over a large area as shown in a sectional view of a thin high temperature heater in accordance with a still another embodiment in figure 6. Although the AlN film was formed by sputtering in the embodiment, it is needless to say that it may be formed by the PVD method such as electron beam deposition, laser PVD method, ion plating method and ionized cluster beam deposition or the CVD method. Furthermore, although a description was given of the method for forming the W film by sputtering in the above embodiment, it may be formed by the PVD method such as the electron beam deposition, the laser PVD method and the ion plating or the CVD method using WF₆, W(CO)₆ and WCl₆ gas. In addition, the same is said when a film of Mo and the like, instead of W, are formed. Although the thin resistive film was baked immediately after it is formed on the insulating substrate in the above embodiment, the baking time is not limited to this and it is all right so long as the thin resistive film is baked at least one time before used. As described above, there is provided a high temperature operating element by forming a porous high temperature operating element film with low film density into a predetermined configuration on one surface of the insulating member, forming a resistive film having film density higher than that of the high temperature operating element film into a predetermined configuration on the other surface of the insulating member, connecting the resistive film to the lead wire, covering the resistive film and forming the insulating protective film on the insulating member. As a result, it is prevented from peeling off the substrate and there can be provided the high temperature operating element mounting the high temperature heater with high long-term reliability. Since the element film is porous, it well adheres to the layer provided on the element in accordance with the performance of the element, so that the performance of the element can be easily improved. Since the electron emitting apparatus comprises an insulating member with good heat conductivity, a fine resistive film with high film density, formed into a predetermined configuration on one surface of the insulating member using a material with good electrical conductivity and a high fusion point, an insulating protective film formed so as to cover the resistive film, a porous reduction member with film density lower than that of the resistive film, formed into a predetermined configuration on the other surface of the insulating member using a reduction material with good heat conductivity and an electron emitting member formed on the reduction member with one part entering the hole of the reduction member. As a result, the resistive film can be protected from the outer atmosphere by the protective film, the fine resistive film is prevented from peeling off the insulating member and a stable heater of the electron emitting apparatus can be provided. In addition, since the reduction member is formed of a porous material, it can well adhere to the electron emitting member provided on the reduction member. In addition. since a part of the electron emitting member enters the reduction member, the electron emission can be high]y effective. As a result, the electron emitting apparatus with a long life, high performance and high reliability can be provided. Furthermore, there is provided a thin high temperature heater composing the high temperature operating element in such a manner that the thin resistive film with a predetermined heater pattern is provided on the insulating material in which at least the surface opposite to the thin resistive film is formed of the protective film of non-oxide series insulating material, the surface of the thin resistive film is covered with the protective film of non-oxide series insulating material and then the thin resistive film is baked. As a result, the thin high temperature heater with high reliability in which resistance changes in little can be provided.

Claims: A high temperature operating element comprising: a substrate (1) of electrically insulative thermally conductive material; a porous high temperature operating element film (4) of low film density, disposed on a surface of said substrate (1); and a thin-film resistor (2) of high fusion point, thermally conductive, material having a coefficient of thermal expansion close to that of said substrate (1) and low vapour pressure at high temperature, which resistor (2) is of a higher film density than said element film (4) and is disposed on an opposite surface of said substrate (1); which element is characterised by: an electrically insulative protective film (3) covering said resistor (2) and an area of said opposite surface of said substrate (1) at the periphery of said resistor (2), which protective film (3) is of high temperature stable material, is adherent to the resistor (2) to exclude the atmosphere and thus prevent oxidation of said resistor (2), and is effective to inhibit peeling of the resistor (2) from said substrate (1). An element as defined in claim 1 wherein: said substrate (1) is of AlN or Al₂O₃; said resistor (2) is of a material comprising Mo, W, Pt, Ta, TiN, TiC, or TiCN ; and said protective film (3) is of a vitreous oxide series material selected from SiO₂, Al₂O₃, CaO, Y₂O₃ or is one of the compositions A, B, or C listed in tables 1 and 2. An element as defined in claim 1 wherein said protective film (3) is also disposed between said resistor (2) and said opposite surface of said substrate, said protective film (3) being of a non-oxide material. An element as defined in claim 3 wherein: said substrate (1) is of Al₂O₃ or BeO; said resistor (2) is of a material comprising Mo, W, Pt, Ta, TiN, TiC or TiCN; and said protective film (3) is of AlN or BN. An element as defined in any preceding claim wherein said element film (4) is of porous reducing agent material (6) and is covered by a pore-penetrative covering (7) of electron emissive material. A method of producing a high temperature operating element, which method comprises the following steps: forming a porous high temperature operating element film (4) of low film density on a surface or a substrate (1) of electrically insulative thermally conductive material; applying a thin-film deposition technique to produce a patterned thin-film resistor (2) on the opposite surface of said substrate (1), which resistor is of high fusion point, thermally conductive material having a coefficient of thermal expansion close to that of said substrate and a low vapour pressure at high temperature; and heating said resistor (2) to recrystallise its constituent material; which method is characterised by: covering said resistor (2), and an area of said opposite surface of said substrate (1) at the periphery of said resistor (2), with an electrically insulative protective film (3) the material of which is high temperature resistant, adherent to the resistor (2) to exclude the atmosphere therefrom thus to prevent oxidation of said resistor (2), which protective film (3) also is effective to inhibit peeling of the resistor (2) from said substrate (1). A method as claimed in claim 6 wherein said protective film (3) is of non-oxide material and said step of applying is both preceded by and followed by a step of applying said non-oxide material, which material is applied to said opposite surface of said substrate (1), and to said resistor (2) and said area, respectively. A method as claimed in claim 7 wherein, following application of said non-oxide material to said resistor (2) and said area, baking is performed to effect said heating.

Inventors: HOSHINOUCHI SUSUMU C O MITSUBI, KUSAKABE YOSHIHIKO C O MITSUBI, MORITA NORIKO C O MITSUBISHI D, HOSHINOUCHI, SUSUMU, C/O MITSUBISHI DENKI K.K., KUSAKABE, YOSHIHIKO, C/O MITSUBISHI DENKI K.K., MORITA, NORIKO, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: H01J1
IPC codes: H01J9, H01J1