Thin high temperature heater and method for manufacturing the same

Abstract: A thin high temperature heater includes an adhesive layer (2) comprising Ti and formed on an insulating material (1) and a resistor layer (3) comprising a Ti compound and formed on the insulating material via the adhesive layer.

Description: FIELD OF THE INVENTION The present invention relates to a small heater for heating to high temperature and a manufacturing method therefor and, more particularly, to a thin heater for heating to high temperature such as a heater for an electron gun, a hot cathode X-ray tube, a Braun tube and so on which is used at approximately 1000 °C, and a manufacturing method therefor. BACKGROUND OF THE INVENTION A plane type heater has been conventionally manufactured using technique of forming a thick film circuit such as screen printing as disclosed in, for example Japanese Patent Publication Gazette No. 55-24646. Figure 3 is a sectional view showing a cathode of an electron tube using the conventional heater. Figure 3, discloses a ceramic substrate 10, a heating element layer 11, an insulating layer 12, a cathode lead layer 13, a base metal layer 14 and a cathode material layer 15. A manufacturing method therefor will be described in detail hereinafter. First, a raw material for forming the ceramic substrate 10 is prepared and then the heating element layer 11 having a desired pattern is formed on a sheet by printing technique such as extrusion, by which a material is extruded between rollers or by casting method. The heating element layer 11 is formed on the ceramic substrate 10 by screen printing a paste in which baking assistant is added to a heater agent. Thereafter, it is baked at high temperature (1000 ∼ 2000 °C) and then the plane type heater is provided. In this method, it is expected that the heater is stable at a high temperature for a long time and that the change of resistance of the heater with lapse of time is small even though the temperature is high in the manufacturing steps, when the heater is used at this temperature or less. However, pattern precision obtained by the screen printing is low and it is also difficult to control (thin) the thickness of the heating element 11, with the result that power consumption is large and variation of resistance among a plurality of heaters is large. Therefore, a method for forming a film by PVD (Physical Vapor Deposition) or CVD (Chemical Vapor Deposition) has been developed as a method for forming pattern with high precision. Figure 4 are sectional views showing manufacturing steps for forming a conventional plane thin type heater by a thin film forming method. A resistor film 3 for a heater usually formed of metal such as tungsten is uniformly formed on a plane ceramic substrate as insulating material and then desired pattern is formed by etching and then a lead wire (not shown) is connected thereto and finally the plane type thin heater is provided. However, when the plane type thin heater is manufactured by the above method, adhesion between the resistor film 3 for the heater and the insulating material 1 is small. Therefore, in order to increase the adhesion between the resistor film 3 and the insulating material 1, an adhesive layer is used. Usually, Ti film with a thickness of several tens to several hundreds nm is formed as the adhesive layer and then the resistor layer is formed thereon to provide the thin high temperature heater. However, while the heater is used with a voltage applied thereto, that is, high temperature load of 1000°C is applied, Ti is degraded by high temperature, causing the heater to break down. The reason for breakdown is considered that the transformation point of phase α to phase β (referred to as a transformation point hereinafter) in the phase diagram of Ti is 882°C and this transformation point is repetitively passed through while the heater is used. In addition, resistance of the thin film resistor 3 changes in use. Figure 5 shows a change of resistance value of the thin film resistor with lapse of time. The reason why the resistance is reduced in an early stage is that recrystallization occurs in the thin film and then the size of a crystal grain in the thin film is increased. For example, when the resistor (heating element) comprises W (tungsten) without an adhesive layer and then is used at 1000 °C recrystallization occurs, since the temperature of 1000 °C corresponds to recrystallization temperature of W. The reason why the resistance is increased with lapse of time is that an impurity is mixed into the film or the film is oxidized by the ambience in use. In addition, since the insulating substrate of an oxide group such as Al₂O₃ is available in its monocrystalline state and a surface thereof can be mirror-finished, pattern precision thereof is better than sintered substrate such as SiC, AlN. Al₂O₃ has therefore been conventionally used as the insulating substrate (insulating material) of the conventional thin film high temperature heater manufactured by the thin film forming method. However, in the conventional heater using Al₂O₃, when the heating element is directly formed on the insulating material, the insulating material reacts on the heating element by thermochemical or electrochemical action caused by oxygen when it is used. Therefore, a substance which is likely to be highly sublimated is formed. As a result, an edge part in which the Al₂O₃ substrate and the heating element such as W are both in contact with an ambience in the vicinity of resistance wiring end (heating element end) is selectively damaged. Thus, the conventional thin high temperature heater formed by the thin film forming method is unstable as a heater and also unreliable as far as a long term use is concerned. US-A-4 296 309 discloses a thermal head for use in thermal print recording. The thermal head comprises an electrically insulative substrate on which is carried a thin film resistive heater composed of metal boride. An adhesive layer of titanium may cover part of the resistive heater. A conductive layer overlies the adhesive layer so as to supply electric power to the resistive heater. SUMMARY OF THE INVENTION The present invention was made in order to solve the above problems. The present invention provides a thin high temperature heater with high reliability in which adhesion between a resistor film for a heater and an insulating material is high and a resistance change while it is used is small. Other advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. According to a first aspect of the present invention, there is provided a thin high temperature heater comprising: an adhesive layer composed of Ti carried on an insulating material; the heater being characterised in further comprising: a layer of a Ti compound carried on the adhesive layer; wherein the Ti compound exhibits a recrystallisation temperature in excess of 1000°C, and is electrically stable at temperatures in excess of 1000°C. According to a second aspect of the present invention there is provided a method for manufacturing a thin high temperature heater comprising the steps of: forming on an insulating material an adhesive layer composed of Ti, said method being characterised in comprising the further steps of: forming on the adhesive layer a layer of Ti compound, said Ti compound exhibiting a recrystallisation temperature in excess of 1000°C, and electrical stability at temperatures in excess of 1000°C; both steps taking place at a temperature either: below the transformation point of α to β of Ti; or at the temperature of the transformation point of α to β of Ti or more. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional view showing a thin high temperature heater in accordance with an embodiment of the present invention; Figure 2 are views showing general etching process which is a part of manufacturing process for the thin high temperature heater shown in figure 1; Figure 3 is a sectional view showing a cathode of an electron tube using a conventional thin type high temperature heater; Figure 4 are views showing manufacturing steps for the conventional thin high temperature heater; Figure 5 is a graph showing a change of resistance value in the conventional thin film with lapse of time; and Figure 6 is a sectional view showing a thin high temperature heater in accordance with another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described in detail with reference to the drawings. Figure 1 is a sectional view showing a thin high temperature heater in accordance with an embodiment of the present invention. In figure 1, reference numeral 1 designates an insulating material, reference numeral 2 designates a Ti layer serving as an adhesive layer to the insulating material 1, and reference numeral 3 designates a resistor layer comprising a Ti compound of a simple substance of TiC, TiN or TiCN or a mixture of them and formed on the adhesive layer 2. It is desirable that the insulating material satisfies the following requirements : it should have a good heat conductivity and thermal expansion coefficient close to that of the resistor layer 3, it should serve as a good insulator, it should not break down by high temperature and it should be plane. Therefore, in view of availability, it may be AlN, Al₂O₃ or the like. The choice of the material of the adhesive layer 2, is not particularly limited as long as it increases adhesion between the resistor layer 3 and the insulating material 1. For example, metal such as Ti, V, Cr, Y, La, Zr, Nb or Hf which has a thickness of 10nm or less, may be applicable. Particularly Ti having the above thickness is most preferable among them, because it is highly adhesive between the resistor layer 3 and the insulating material 1. When the thickness of Ti exceeds 10nm, degradation by high temperature is generated after repetitive use above the transformation point of Ti, which could cause a heater to break down. The reason why the resistor layer 3 is formed of a Ti compound of a simple substance of TiC, TiN or TiCN or Ti compound or a mixture of them is that it has high recrystallization temperature and it has high electrical stability at high temperature. A general heater material such as W or Mo may be used. However, such material takes oxygen (deoxidizes) from the substrate 1 and forms oxide having high vapor pressure and then scatters when it is formed on a substrate I made from for example Al₂O₃ and used at high temperature of approximately 1000°C. More specifically, the resistor layer is etched away so that its configuration deforms. Therefore, circumstance in which it is stable while used as a heater, such as the material of the substrate 1, ambience or temperature is limited. Two methods for manufacturing a thin high temperature heater using a monocrystalline sapphire substrate (Al₂O₃) as the insulating material 1 will be described hereinafter. A first method will be described whereby a Ti film having a desired thickness (several microns to 10 microns) is uniformly formed on the Al₂O₃ substrate at 200∼300 °C by sputtering. Then, it is etched away so as to have desired pattern configuration by wet or dry method. In case of the wet method, the etching is performed through general steps as shown in figure 2. A sample having the pattern is arranged in a vacuum chamber for ionitriding. Ionitriding is then performed below temperature of transformation point of Ti, for example 400∼500°C and thereby N is diffused from the Ti surface. As a result, TiN is formed. Nitriding is performed to reach or include a layer which contributes adhesion between Ti and Al₂O₃ at an interface with the substrate (10nm or less), a depth which is approximately several microns to 10 microns. If the ionitriding is performed using DC power supply, Al₂O₃ of the insulating material is not damaged and electrically conductive Ti is only nitrided. Although the method for forming the Ti film by sputtering is described in the above embodiment, it is needless to say that it may be formed by a PVD method such as an electron beam deposition method, a laser PVD method or an ion plating method. Although the pattern of Ti is formed and then ionitriding is performed in the above embodiment, the same effect can be obtained if ionitriding is performed after the Ti film is formed and the film is etched away by thermal nitric acid or the like. Although ionitriding is performed using N₂ gas in the above embodiment, a simple substance of TiC, TiN or TiCN or a mixture thereof may be formed by carbonization using mixture gas of CH₄ and N₂ gas. As described above, according to the first method, the Ti film is formed and then nitriding or carbonization is performed from a surface before or after patterning. A second method will now be described. A pattern mask is put on the Al₂O₃ substrate heated at desired temperature which is the transformation point of Ti or less, for example 200∼300°C and then a Ti film is formed by a normal Ar sputtering method. When an extremely thin film is formed N₂ gas is introduced into the sputtering atmosphere before the thickness of the Ti film reaches 10nm, and a TiN film having a desired thickness (several microns to 10 microns) is formed by reactive sputtering. Then, the simple substance of TiC, TiN or TiCN or a mixture thereof is formed by changing the introduced gas to gas comprising carbon, for example mixed gas of CH₄ and N₂. As described above, according to the second method, different films are continuously formed, for example Ti and then TiN during the process for forming the film. The thus manufactured thin type high temperature heater is not likely to be degraded by a change of volume because the adhesive layer and the resistor layer are both formed at low temperature which is below the transformation point of Ti and then transformation α to β of Ti does not occur while the heater is manufactured. In addition, even if extra Ti element for the adhesive layer diffuses into the resistor layer while the heater is used, the adhesive layer is stable without any bad influence exerted since the resistor layer comprises a Ti element. When the size of a heater is increased or it is mass-produced, the plurality of thin high temperature heaters in accordance with the second method may be formed on the substrate. In addition, a stable layer 4 can be formed of a material which has low vapor pressure and stable electrical characteristic at high temperature, such as W or Mo, on the insulator layer of the thin high temperature heater formed in accordance with the second method, as shown in figure 9, and then this can be used as the heating element. In this case, the resistor layer functions as an adhesive relaxation layer which prevents Ti element of the adhesive layer from diffusing into the heating element. Although the adhesive layer 2 and the resistor layer 3 (adhesive relaxation layer) are both formed at a temperature below the transformation point of Ti in the above embodiment, they may both be formed at the temperature of the transformation point of Ti or more. In this case, high temperature degradation caused by α to β transformation of Ti does not occur since the extra Ti element for the adhesive layer is diffused in the manufacturing process, even if it is used at temperature of the transformation point or more, for example 1000 °C. As described above, according to an aspect of the present invention, a thin high temperature heater comprises an adhesive layer comprising Ti formed on an insulating material and a resistor layer comprising a Ti compound formed on the insulating material through the adhesive layer. As a result, it is possible to provide a thin high temperature heater with high reliability in which adhesion between the resistor layer and the insulating material is high and resistance is not likely to change while it is used. According to another aspect of the present invention, an adhesive relaxation layer 3 (resistor layer) comprising a simple substance of TiC, TiN or TiCN or a mixture thereof and having a superior stability at high temperature than the adhesive layer is provided between the adhesive layer and the heating element comprising metal. Therefore, adhesion between the heating element and the insulating material is high and the Ti element of the adhesive layer does not diffuse into the heating element. Furthermore, the heating element and the insulating material are not damaged by the interaction therebetween. Furthermore, according to the present invention, when the adhesive layer comprising Ti and the resistor layer comprising the Ti compound are formed on the insulating substrate, those layers are both formed at temperature below the transformation point of α to β of Ti or both are formed at temperature of the transformation point or more. As a result, Ti is prevented from being degraded by high temperature while the heater is manufactured.

Claims: A thin high temperature heater comprising: an adhesive layer (2) composed of Ti carried on an insulating material (1); the heater being characterised in further comprising: a layer (3) of a Ti compound carried on the adhesive layer; wherein the Ti compound exhibits a recrystallisation temperature in excess of 1000°C, and is electrically stable at temperatures in excess of 1000°C. A thin high temperature heater according to claim 1, further comprising: a heating element (4) composed of metal carried on the layer of Ti compound. A thin high temperature heater according to Claim 1 in which said layer (3) of Ti compound constitutes a heating element. A thin high temperature heater according to any one of the preceding claims, wherein the thickness of the adhesive layer (2) is 10nm or less. A method for manufacturing a thin high temperature heater comprising the steps of: forming on an insulating material (1) an adhesive layer (2) composed of Ti, said method being characterised in comprising the further steps of: forming on the adhesive layer (2) a layer (3) of Ti compound, said Ti compound exhibiting a recrystallisation temperature in excess of 1000°C, and electrical stability at temperatures in excess of 1000°C; both steps taking place at a temperature either: below the transformation point of α to β of Ti; or at the temperature of the transformation point of α to β of Ti or more. A method for manufacturing a thin high temperature heater according to claim 5, comprising the further step of: forming a heating element (4) composed of metal on the layer of Ti compound. A method for manufacturing a thin high temperature heater according to either claim 5 or 6, wherein the thickness of said adhesive layer is 10nm or less.

Inventors: HOSHINOUCHI SUSUMU C O MITSUBI, KOBAYASHI MINORU C O MITSUBISH, KUSAKABE YOSHIHIKO C O MITSUBI, MORITA NORIKO C O MITSUBISHI D, HOSHINOUCHI, SUSUMU, C/O MITSUBISHI DENKI K.K., KOBAYASHI, MINORU, 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