Three-dimensional ceramic heating system

Abstract

A three-dimensional ceramic heating device comprising: a support base body made of an electrically insulating ceramic, wherein the support base body is in the shape of a cylinder or a container with a bottom; a thin-film heating element made of a conductive ceramic applied to the support base body and cut to form a heating element pattern comprising folded-back sections, the multiple folded-back sections being arranged parallel to each other on a side face of the support base body in the shape of a cylinder or a container with a bottom; an insulating layer made of an electrically insulating ceramic applied over the heating element;and several power supply terminals arranged at one end of the support base body for connecting the heating element to a power source, wherein the heating element pattern forms current paths over which a current flows between the several power supply terminals; wherein each of the folded-back sections is divided into N electrical current paths with splitting bands extending in directions of an electrical current flow, where N is two or more, and an electrical power density Xn in an nth electrical current path satisfies an inequality 1 (n is 1 to N): 0.7 ≤ Xn ∑ n = 1 NXn N ≤ 1.3;

Description

[Area of ​​technology] The present invention relates to a three-dimensional ceramic heating device used for heating a crucible that holds a raw or starting material in a vacuum evaporation device used for manufacturing a semiconductor, a solar cell, an organic EL display, etc. [Background technology] The vacuum evaporation process is known as one of the procedures for forming a thin film, and this vacuum evaporation process consists of the steps of placing a raw material(s) for deposition in a crucible, heating the crucible to a temperature above the evaporation point of the deposition material in the vacuum evaporation device in order to generate the vapor of the deposition material, and depositing the substance onto a substrate in order to form a thin film.Methods used to heat the crucible include resistance heating, electron beam heating, high-frequency induction heating, or the like; however, resistance heating is the most well-known of these, in which a heating element consisting of a tantalum wire or tungsten wire is held outside the crucible by an electrical insulating device in such a way that the heating element is wound around the outer circumference of the crucible (IP publication 1). However, in the case of the resistance heating method described in IP Publication 1, the heating element lacks a sheath, leaving the conductor exposed during use, thus creating a risk of electrical discharge and short circuit. Furthermore, when heated to high temperatures, the element is prone to wear due to embrittlement and degradation, potentially leading to wire breakage. On the other hand, IP publication 2 teaches a ceramic heating element according to which a support base body made of pyrolytic boron nitride (PBN) by a chemical vapor deposition process is coated with an electrically conductive thin film of pyrolytic graphite (PG) by means of a chemical vapor deposition process, and subsequently, by machining this PG layer, a heating element with the desired shape is obtained, and this heating element is attached or coated with a pyrolytic boron nitride insulating layer by means of a chemical vapor deposition process, and thereby the ceramic heating device (PG / PBN heating device) is produced. In the ceramic heating device of IP publication 2, its heating element is attached or coated with an insulating layer, so that there is no risk of electrical discharge or short circuit, and likewise, because the support base body, the heating element and the insulating layer are each manufactured by a chemical coating process, the ceramic heating device has a relatively high purity and releases hardly any impurities. However, this ceramic heating device is a flat-plate type heating device used for uniform heating of a base plate to produce a semiconductor wafer or thin film, so it is not possible to use this ceramic heating device as is for the application of heating a raw material holding crucible in a vacuum evaporation device.Similarly, if this is used to heat a raw material holding crucible to a temperature of 1000 degrees Celsius or higher, the heating element design of the ceramic heating element is such that an electric current passing through a folded section of the heating element design tends to converge towards the inner corner, so that the area around the inner circumference of the folded section is locally heated to a higher temperature than its outer circumference, and consequently there is a problem that the heating element cannot be expected to operate stably and permanently during long-term use. IP Publication 3 describes a ceramic heating device in which the folded-back or reversed section (reversed section) of the heating element design or pattern is divided into a number of lanes running along the direction of the electric current flow to solve the problem of uneven electric current flow that occurs in conventional flat-plate heating devices. However, this improved ceramic heating device is designed to heat a flat plate body uniformly and is therefore not suitable for heating a three-dimensional crucible containing raw materials to an elevated temperature. Likewise, IP Publication 3 does not disclose whether the improved ceramic heating device can withstand long-term use where the crucible containing raw materials is heated to a temperature of 1000 degrees Celsius or higher. [State of the art documents] [IP publications] [IP Publication 1] Japanese published patent application JP 2007 / 262478A [IP Publication 2] Japanese published patent application JP 2001 / 006854A [IP Publication 3] Japanese published patent application JP 2001 / 342071A Furthermore, JP 2001 / 176 646 A discloses a method for measuring layer thickness to detect spectrally emitted light from a workpiece during or after layer deposition, by dividing the waveform of the spectral emission light from the object by the spectral emission waveform and by a predetermined spectral emission waveform. JP 2001 / 342 071 A discloses a ceramic heater consisting of a substrate material of electrically insulating ceramic, a thin, heat-generating layer formed in a heating pattern with folds on the substrate material, and a connector for connecting the heat-generating part to a power source, wherein the heating pattern is subdivided into several channels along the current flow direction, at least at the folds. [Disclosure of the invention] [Problems to be solved by the invention] It should be noted that ceramic heating elements made of PG and PBN with a three-dimensional structure, such as a cylindrical one, were used in addition to those with a flat plate structure. For example, in the case of manufacturing a three-dimensional PG / PBN ceramic heating element with a cylindrical contour, the heating element pattern is produced by methods such as: using a machining tool such as a machining centering device together with a flat-end or round-end end mill to cut the deposited conductive film layer into a desired pattern; or applying a mask with a desired cutout pattern to the conductive film layer and applying sandblasting to remove unnecessary sections of the conductive film layer. However, a three-dimensional PG / PBN ceramic heating device manufactured in such a manner has the problem that its service life tends to be short when used to heat a raw material holding crucible in a vacuum evaporation device, with the heating temperature being as high as a thousand and a few hundred degrees Celsius, and therefore this problem is explained in detail using the drawings. First, the problem arising from the heating element pattern is explained. Fig. 6(a) is a top view of a cylindrical PG / PBN ceramic heating element seen from above, and Fig. 6(b) is a side view of the cylindrical PG / PBN ceramic heating element seen from one side. Furthermore, Fig. 7 is an unfolded view of the cylindrical PG / PBN ceramic heating element, showing the pattern or structure of the ceramic heating element. The shape of this ceramic heating element consists of grooves 2a and the heating element 2b, which define current channels. The grooves 2a are created by subjecting the pyrolytic graphite (PG) conductive film layer, deposited over a pyrolytic boron nitride (PBN) support base, to machining by a device such as an end mill or sandblasting, followed by the removal of unnecessary sections of the conductive film layer to form the grooves 2a; the PG conductive film layer remaining on the support base becomes the heating element 2b. One end of the heating element 2b thus created is equipped with a power supply terminal 1, and the electric current originating from this power supply terminal 1 flows along the heating element (passage) 2b, rotating on the folded-back sections.Reversing sections 3 of the heating element pattern and reaching another power supply connection 1, which is provided at one end of the heating element 2b on the opposite side of the cylinder body. Fig. 8 is an enlarged view of a folded-back section of the heating element pattern. In Fig. 8, the dashed lines, which are curved in a semi-elliptical shape, indicate the paths taken by the electricity in the heating element 2b. As shown in Fig. 8, the current in the folded-back section 3 of the heating element pattern tends to concentrate towards the inner corner 4, resulting in the phenomenon of local heating in the vicinity of the inner corner 4 of the folded-back section, or inverted section 3. This is caused by a property of electric current that it takes a more selective path when it encounters lower resistance.The closer a conductor is to the inner corner 4 of the folded-back section 4, the shorter the length of the conductor becomes, thus reducing its resistance, and therefore the electric current concentrates in the vicinity of the inner corner 4 of the folded-back section 4; therefore, the problematic local heating occurs at as many locations as there are folded-back sections 3 of the heating element 3a. Next, a problem encountered in the manufacture of a cylindrical ceramic heating element will be explained. In the case of manufacturing a cylindrical ceramic heating device, if the diameter irregularity of the cylindrical outer surface of the support base body is significant, when the PG conductive thin film on the outer surface of the support base body is milled to produce the heating element pattern, the milling cutter edge would either penetrate too deeply into the support base body, forming an excessively deep groove, or too shallowly into the support base body, forming an excessively shallow groove. In areas where the end mill has penetrated too deeply, the groove becomes excessively deep, and therefore the PBN insulating layer formed over the heating element pattern is more easily detached. Consequently, as the temperature of the heating element increases, detachment from the excessively deep grooves occurs more rapidly. This type of manufacturing problem is also encountered when the heating element pattern is formed by machining with a ball-end milling cutter. In the case of using a ball-end milling cutter, because the edge has a spherical shape, if the edge penetrates too deeply into the support base body, the groove widens, reducing the width of the adjacent heating element to such an extent that a localized heating area is created. To solve this type of problem, a technique called profile machining is known, in which the movement of the cutting tool is controlled in response to variations in the diameter of the support base body, thus keeping the depth to which the end milling cutter edge penetrates the support base body constant; however, profile machining makes the machining tool excessively expensive. Similarly, if the irregularity of the diameter of the cylindrical outer surface of the support base body is large, when a heating element pattern is produced in such a way that a cutting template in a desired heating element pattern is applied to the surface of the PG conductive thin film, and sand is blasted over the mask to engrave the heating element pattern, it is difficult to apply the template or mask without offset, so that the resulting width of the heating element may be larger than some parts and smaller than others, or the width of the groove may be larger than some parts and smaller than others, with the consequence that the problematic local heating may occur in those parts of the heating element that have smaller widths.In addition to this, the probability of short-circuiting in the parts where the groove width is smaller is higher, and because in this situation the adjacent heating element bodies are closer together, the proportion by which the heating element bodies occupy the area is higher than normal, and at the same time the proportion by which the grooves occupy the area is smaller than normal, so that the problematic local heating is more likely to occur. Next, the problems associated with such localized heating and the selection of materials for the heating element will be explained. In a process where a film of an inorganic material or metal is formed using a vacuum evaporation process, heating to several thousand degrees Celsius is required. For example, such a heating temperature is necessary when copper is used as the vapor source, and if a PG / PBN ceramic heating element is used in this application, the ceramic heating element will be heated to a temperature of approximately 1300 degrees Celsius. The temperature will be further increased in the additionally heated areas, such as the folded sections of the heating element pattern and the cylindrical outer surface of the support base.PBN, which is one of the materials used to manufacture the ceramic heating element, is relatively stable; however, it starts an infinitesimal self-degradation when the temperature reaches 1300 degrees Celsius or so, and the higher the temperature gets, the faster this self-degradation continues, so that if the aforementioned locally heated areas exist in the ceramic heating element, the insulating layer will wear out faster than those locally heated areas. Similarly, if oxygen or moisture exists in a vacuum chamber, the oxygen and moisture would cause oxidation and wear of the PBN at high temperatures, and the higher the temperature, the faster the oxidation of the PBN progresses, so that the oxidation and wear of the insulating layer occur more progressively in the locally additionally heated areas. If such self-degradation and oxidative wear of the PBN occur continuously for a long period of time, the insulating layer at the inner corner of the folded-back section of the heating element pattern would be completely consumed to expose the heating element, followed by degradation and consumption of the heating element, and subsequently the heating element would break apart. If, separately, the locally additionally heated parts caused by the concentration of electric current towards the inner corner of the folded-back section of the heating element pattern and the locally additionally heated parts caused by the irregularity of the diameter of the support base body exist, the detachment of the insulating layer of the PBN and the consumption of the PBN itself on the locally additionally heated parts are triggered and caused during the heating process, increasing the temperature to several thousand and several hundred degrees Celsius, so that the service life of the ceramic heating element tends to be unsatisfactory. Therefore, the objective of the present invention is to solve the problems described above associated with a three-dimensional ceramic heating device, and thus to provide a three-dimensional ceramic heating device for heating a raw material holding crucible in a vacuum evaporation device which has a long service life. The inventors in question have conducted intensive research to achieve this goal and have found that when the heating element pattern (structure) described in the aforementioned IP publication 3 is applied to a three-dimensional ceramic heating element for heating a raw material holding crucible, it is possible to essentially remove the locally additional heated part in the folded-back section of the heating element pattern, which adversely affects the service life of the heating element;and that, if the electrical power density in the corresponding divided electrical current bands is limited within a range of plus / minus 30% of the average value of the electrical power densities of all bands, then no excessively high temperature heating occurs in a particular divided electrical current band, so that it becomes rare for temperature irregularities to occur among the divided electrical current bands, which leads to a significant extension of the service life of the ceramic heating element, wherein the present invention is made. [Means of solving the problems] In other words, the present invention provides a three-dimensional ceramic heating device comprising: a support base body made of an electrically insulating ceramic, wherein the support base body has the shape of a cylinder or a container with a bottom; a thin-film heating element made of a conductive ceramic applied to the support base body and cut to form a heating element pattern comprising folded-back sections, the multiple folded-back sections being arranged parallel to one another on a side face of the support base body in the shape of a cylinder or a container with a bottom; an insulating layer made of an electrically insulating ceramic applied over the heating element;and several power supply terminals arranged at one end of the support base body for connecting the heating element to a power source, wherein the heating element pattern forms current paths over which a current flows between the several power supply terminals; wherein each of the folded-back sections is divided into N electrical current paths with splitting bands extending in directions of an electrical current flow, where N is two or more, and an electrical power density Xn in an nth electrical current path satisfies an inequality 1 (n is 1 to N):; Likewise, the heating element pattern of the present invention is preferably designed such that at least one split electric current band extends continuously from a folded-back section to a section folded back immediately downstream, without transitioning into another split electric current band; and likewise, each split electric current band preferably extends through an even number of folded-back sections. Furthermore, the heating element pattern of the present invention is characterized in that the electrical current passage is divided into two or more bands, wherein the division extends continuously from a folded-back section immediately downstream of one electrical power supply connection to a folded-back section immediately upstream of the other electrical power supply connection along the electrical current passage; preferably, the two or more divided bands extend beyond the final folded-back sections as close as possible to the corresponding electrical power supply connections. The three-dimensional ceramic heating element of the present invention is preferably designed in the form of a cylinder; and in the case of a cylindrical ceramic heating element, it is preferably the case that the irregularity of the diameter, which is measured between the outermost surfaces of the cylindrical heating element, lies within a range of plus / minus 0.025 mm. The support base body of the present invention consists of pyrolytic boron nitride, and the heating element preferably consists of either pyrolytic graphite or a pyrolytic graphite containing boron and / or boron carbide, and the insulating layer preferably consists of pyrolytic boron nitride or a pyrolytic boron nitride containing carbon. Furthermore, it is preferably the case that the ceramic heating element of the present invention is used in a vacuum evaporation device in which a crucible containing a single source or starting material for vacuum evaporation, selected from Ag, Al, Au, Cr, Cu, Ga, Ge, In and Si, is heated to a temperature of 1000 degrees Celsius or higher, thereby melting or subliming the source or starting material. [Effects of the invention] According to the present invention, it is possible to attenuate the local additional heating at the folded-back sections of the heating element pattern, and also to prevent each divided electrical current band from being heated to an excessively high temperature than other bands, thus preventing the irregularity of the temperature to which each divided electrical current band is heated, making it possible to provide a three-dimensional ceramic heating device made of PG / PBN that has a long service life when operating for heating the raw material holding crucible in a vacuum evaporation device. [Brief description of the drawings] Fig. 1 is an unfolded view of a heating element pattern of a ceramic heating device according to an embodiment of the present invention. Fig. 2 is an enlarged view of a folded-back section of a heating element pattern of a ceramic heating device of the present invention, showing electrical current bands. Fig. 3 is an unfolded view of a heating element pattern of a ceramic heating device according to a further embodiment of the present invention. Fig. 4 is an unfolded view of a heating element pattern of a ceramic heating device according to yet another embodiment of the present invention, which is described below in Example 2. Fig. 5 is a schematic drawing to illustrate the electrical power density in the corresponding divided electrical current bands.Figure 6 is a top view and a side view of a three-dimensional ceramic heating device according to a non-inventive comparative example. Figure 7 is an unfolded view of a heating element pattern of the ceramic heating device according to non-inventive comparative example 3. Figure 8 is an enlarged view of a folded-back section of the heating element pattern of the ceramic heating device of the comparative example, showing the electrical current band. Figure 9 shows photographs taken of the external appearance of corresponding folded-back sections of the heating element pattern of the ceramic heating elements according to Example 1, Comparative Example 1, and Comparative Example 2, in order to show the heating environment of these. [Examples embodying the invention] An embodiment of the present invention is explained, but the invention is not limited to this embodiment. The ceramic heating device of the present invention is a three-dimensional PG / PBN ceramic heating device in the shape of a cylinder, a container with a bottom, a boat or a half-tube, and the most suitable shape is selected based on the shape of the raw material holding crucible to be heated. The ceramic heating device of the present invention comprises a support base body consisting of an electrically insulating ceramic and an electrically conductive ceramic formed on the support base body, wherein this conductive ceramic forms a thin-film heating element in the form of a heating element pattern with folded-back sections. Likewise, an insulating layer consisting of an electrically insulating ceramic is located on this heating element, and electrical power supply connections are provided for connecting the heating element to the power source. Fig. 1 is an unfolded view of a heating element pattern of a ceramic heating device of the present invention, in which each of the folded-back sections 5 of the heating element pattern between one power supply terminal 1 and the other power supply terminal 1 is divided into two paths, an inner electrical current path 6 and an outer electrical current path 7. How the electric current flows in this heating element pattern is explained with reference to Fig. 2. Fig. 2 is an enlarged view of a folded-back section of the heating element divided into two paths extending along the direction of the current flow path, and the curved dashed lines indicate paths that the electric current can take in the corresponding paths 6, 7.Now the electric current flowing in the heating element is split into a branch current 8, which runs in the inner electric current band 6, and a branch current 9, which runs in the outer electric current band 7, so that the tendency for the electric current to concentrate at the innermost part of the folded-back section, which causes local additional heating, is reduced. However, in this heating element pattern, the length of the inner electric current band 6 is shorter than that of the outer electric current band 7, so that the resistance of the inner electric current band 6 is smaller than that of the outer electric current band 7. Consequently, in the heating element pattern of Fig. 1, although the concentration of the electric current at the innermost part of the folded-back section is reduced, the amount of electric current that collects in the inner electric current band 6 is greater; therefore, there is still room for improvement to reduce the concentration of the electric current. Therefore, if the heating element pattern is divided into tracks, with one or more dividing bands extending continuously through an even number of successive folded-back sections such that each band does not coincide with another track, it is possible to further reduce local additional heating. Fig. 3 shows an improved example, where the heating element pattern is divided into two tracks 10 and 11 that extend continuously through two successive folded-back sections along the electric current flow path without converging. If the number of successive folded-back sections through which the split electric current paths extend is an even number, the length of the split electric current paths becomes identical, so that virtually there is no difference in resistance among the split electric current paths, and therefore the amount of electricity flowing in each path becomes approximately identical, with the consequence that preventing local additional heating at the inner corner of each folded-back section of the heating element pattern is achieved more effectively. In contrast, the electrical current band 12 and the electrical current band 13 extend through an even number of the folded-back sections along the electrical current path, resulting in a resistance difference between the electrical current band 12 and the electrical current band 13. In such a situation, it is preferably the case that the split electrical current bands extend close to an electrical power supply connection, as shown in Fig. 3, so that the effect of the resistance difference that occurs between the inner band and the outer band in the folded-back section is relatively attenuated. Furthermore, for the same reason as in the case of Fig. 3, because the electrical current passage of the heating element pattern extends continuously from a folded-back section immediately downstream of one electrical power supply terminal to a folded-back section immediately upstream of the other electrical power supply terminal, it is advantageous if the electrical current passage is divided into two or more paths that extend continuously from the folded-back section adjacent to one electrical power supply terminal to the folded-back section adjacent to the other electrical power supply terminal along the electrical current passage, thereby further preventing local additional heating. Fig. 4 shows an improved example in which a division of the electric current passage into an electric current band 16 and an electric current band 17 extends continuously from a folded-back section 14 immediately downstream of one electric power supply terminal to a folded-back section 15 immediately upstream of the other electric power supply terminal 1. As is the case in this improved example, where the division of the electric current path extends continuously, even if the number of folded-back sections through which the division extends is not an even number, the difference in resistance between the divided electric current paths is small due to the excessive length of the divided paths, so that the prevention of local additional heating is effectively achieved. As shown in Fig. 4, the electric current passage is further divided into two paths along its length between an electrical power supply terminal and the immediately downstream folded section 14, and between the other electrical power supply terminal and the immediately upstream folded section 15, so that the difference in resistance between the electric current path 16 and the electric current path 17 is reduced. It is preferable that the electric current path be divided in such a way that the widths of the resulting electric current paths are identical; however, this is not essential. Since it is desirable to control the temperature distribution of the ceramic heating element with precision, it may be appropriate to design the widths of the paths differently if necessary, or it may be appropriate to vary the width of an individual path from place to place.Next, the current density in the corresponding electrical current paths into which the current flow is divided will be explained with reference to Fig. 5. Fig. 5 is a schematic drawing showing a situation in which the electrical current flow is divided into N rows of paths. The temperature of the ceramic heating element is determined by the electrical power density (energy consumption per unit area), so the higher the electrical power density, the higher the temperature in that area; the lower the electrical power density, the lower the temperature in that area. For example, in the case of a [missing information] in Fig.In the heating element pattern shown in Figure 5, the following conditions are met with respect to the nth electric current path, where Pn, Rn, Sn, and Xn are the electrical energy, resistance, area, and power density in the nth electric current path, respectively: where In: the current flowing in the nth electric current path; Ln: the total length of the nth electric current path (measured along the dashed line); Wn: the width of the nth electric current path; V: the voltage measured between terminals A and A' of the split electric current path; T: the thickness of the heating element; ρ: the resistance of the heating element. According to the above relationships, the electrical power density Xn in the n-th divided electrical current path is obtained by the following equation. Furthermore, when a ceramic heating element is used to heat a raw material holding crucible in a vacuum evaporation device, the temperature of the ceramic heating element is increased to approximately 1300 degrees Celsius. Therefore, if a temperature irregularity exists between the electrical circuits, the service life of the ceramic heating element is negatively affected. Consequently, according to the present invention, measures are taken to maintain the current density in each electrical circuit within a specific range in order to prevent the occurrence of a temperature irregularity between the divided electrical circuits. In particular, by dividing a section of a heating element into N rows of electrical current paths (N is 2 or greater) and limiting the electrical power density in each divided electrical current path to a range of plus / minus 30% of the mean electrical power densities of the first to Nth paths, it is possible to minimize the temperature irregularity between the paths as well as the excessively high temperature heating in each specific electrical current path, and it is possible to attenuate the local additional heating in the folded-back sections contained in the section of the heating element, with the result that the service life of the ceramic heating element is significantly extended. Therefore, the present invention is characterized in that at least one section of an electric current path is divided into N rows of paths (N is 2 or greater), and that the electrical power density Xn in the nth path (n is 1 to N) satisfies the following inequality. The effect of this invention becomes clear from the results of Example 1 and Comparative Example 1, which are described below. If the ratio of the electrical power density in one of the paths to the average value calculated from inequality 1 above is 1.3 or greater, temperature irregularity among the electrical paths occurs more readily, and consequently, the service life of the ceramic heating element is greatly affected. Similarly, as shown in Fig. 5, if the first or the nth electrical path contains an innermost part of a folded-back section of the heating element pattern, as indicated by dashed circles, abnormal overheating is caused by a compression of the electrical current at the innermost part of the folded-back section, which can cause the insulating layer in the innermost part to disintegrate, resulting in damage to the heating element.On the other hand, if the ratio in each track is 0.7 or smaller, the temperature irregularity among the divided electrical current paths is equally favored, and the service life of the ceramic heating element is also greatly reduced. Furthermore, in a case where the support base body of the present invention, made of an insulating ceramic, has a cylindrical shape and the irregularity of the diameter of the cylindrical outer surface of the support base body is within a range of plus / minus 0.025 mm, when the heating element pattern is formed by applying a milling cutter to the conductive ceramic thin film layer located on the surface of the support base body, the edge of the milling cutter penetrates the support base body to an approximately constant depth, so that there is hardly any risk of the milling cutter penetrating too far to form an excessively deep groove, with the consequence that the insulating layer consisting of an electrically insulating ceramic, which is provided via the heating element pattern, is difficult to remove. Similarly, if the irregularity of the diameter of the cylindrical outer surface of the support base body is within the range of plus / minus 0.025 mm, if the heating element pattern is formed by applying a milling cutter to the conductive ceramic thin film layer located on the surface of the support base body, the edge of the milling cutter enters the support base body at an approximately constant depth, so that the width of the cut grooves becomes uniform, and therefore the width of the heating element also becomes uniform, with the result that hardly any additionally heated areas are created locally. Furthermore, if the irregularity of the diameter of the cylindrical outer surface of the support base body is within the range of plus / minus 0.025 mm, and if the heating element pattern is manufactured in such a way that a template in a desired heating element pattern is applied and sand is blasted onto the template to form the heating element pattern, it becomes easy to apply the template without offset, so that the resulting width of the grooves becomes quite uniform, and therefore the width of the heating element also becomes uniform, with the result that few locally additional heated areas are formed, and because the adjacent heating element parts are not formed too close to each other, the occurrence of a short circuit is also prevented. The support base body of the electrical insulating ceramic of the present invention is preferably produced by chemical vapor deposition of pyrolytic boron nitride. The support base body produced from this material can be used stably in a high-temperature heating process from ambient temperatures of 1500 degrees Celsius, and likewise in a heating process with a rapid temperature increase of 100 degrees Celsius per minute or more. Preferably, the thickness of the support base body is 0.5 to 2 mm, and more preferably 0.8 to 1.3 mm. If the thickness of the support base body is less than 0.5 mm, there is a high probability that the support base body will be damaged by a worker handling it, and if the thickness is greater than 2 mm, the time required for the formation of the support base body by the chemical vapor deposition process becomes so long that the costs are not justified.On the other hand, the thin-film heating element of an electrically conductive ceramic is preferably made of either pyrolytic graphite or pyrolytic graphite containing boron and / or boron carbide, wherein these (graphites) are produced by a chemical vapor deposition process. A heating element made of these materials can be used stably at high temperatures; and because they are produced by a chemical vapor deposition process, they exhibit higher purities and can form a layer of uniform thickness with greater precision than in the case where a heating element layer is formed over the ceramic heating element surface by screen printing a conductive paste, which is the commonly used method.The thickness of the heating element is not specified in the present invention and can be appropriately determined taking into account the combination of factors such as the desired heating temperature, the area to be heated, the capacity of the energy source, and the shape of the heating element pattern. Preferably, this thickness is 10–300 micrometers, and particularly 30–150 micrometers. The insulating layer of an electrically insulating ceramic is preferably made of pyrolytic boron nitride or pyrolytic boron nitride containing carbon, wherein these (nitrides) are produced by a chemical vapor deposition process. Such an insulating layer can function stably during a high-temperature heating process of approximately 1500 degrees Celsius, and likewise during a rapid heating and cooling process at a rate of 100 degrees per minute. The thickness of the insulating layer is not specified in the present invention, whereby 20–300 micrometers is acceptable and 50–200 micrometers would be preferable, since if the thickness of the insulating layer is less than 20 micrometers, there is a possibility of dielectric breakdown, and if it is greater than 300 micrometers, the layer could easily detach. The ceramic heating element of the present invention is designed as described above so that it can operate reliably for extended periods, even during high-temperature heating processes where temperatures can reach 1000 degrees Celsius or higher. Similarly, when used to heat a crucible in a vacuum evaporation device to melt or sublime a starting material for vapor deposition, such as Ag, Al, Au, Cr, Cu, Ga, Ge, In, and Si, it operates reliably for extended periods. [Examples] <Example 1 (not claimed)> In Example 1, a pyrolytic boron nitride circular plate, 140 mm in diameter and 1 mm thick, was prepared by reacting 4 slm (standard liters per minute) of ammonia with 2 slm of boron trichloride at a pressure of 10 Torr and a temperature of 1900 degrees Celsius. Next, a pyrolytic graphite layer, 50 micrometers thick, was formed on the circular plate by thermal cracking of methane at a pressure of 5 Torr and a temperature of 1750 degrees Celsius; this layer was machine-cut into a heating element pattern consisting of 8 mm wide passages of a heating element arranged in a meandering radial direction. As shown in Fig.As can be seen in Figure 9(a), a pair of folded sections formed in the heating element pattern is divided from position E, where the two folded sections are opposite each other, to a position 30 mm away from position E into two 4 mm wide electrical current paths 18 and 19, where path 18 is the inner path on the folded section and path 19 is the outer path there. A pyrolytic boron nitride insulating layer was deposited over this ceramic heating element by reacting 5 slm of ammonia with 2 slm of boron trichloride at a pressure of 10 Torr and a temperature of 1890 degrees Celsius, thus completing a ceramic heating element.With regard to this ceramic heating element of Example 1, the ratio of the electrical power density defined in the preceding inequality 1 was calculated; the ratio of the electrical power density of the inner track 18 was 1.30 and that of the outer track 19 was 0.70, whereby these satisfied inequality 1 of the present invention. Next, the ceramic heating element was placed in a vacuum chamber and a temperature sensor was attached for temperature measurement; the interior of the chamber was then evacuated to 1 Pa using a vacuum pump. The ceramic heating element was then electrified (electrically excited) and its temperature increased to 1000 degrees Celsius. Fig. 9(a) is a photograph taken of the folded sections of the ceramic heating element when the temperature had risen to 1000 degrees Celsius; the current flowing through the heating element splits at the folded sections into the inner current path 18 and the outer electric current path 19, and it is therefore assumed that no local heating occurs in the innermost region of the folded sections. < Comparison example 1 > In the non-inventive comparative example 1, a ceramic heating element was manufactured in the same manner as in example 1; as can be seen from Fig. 9(b), a pair of folded sections is divided from a position F, where the two folded sections are opposite each other, to a position 15 mm away from position F into an inner and outer electrical current path (18, 19). With respect to this ceramic heating element of comparative example 1, the ratio of the electrical power density was calculated as defined in the preceding equation 1; the ratio of the electrical power density of the inner path was 1.56 and that of the outer path was 0.44, which meant that they did not satisfy inequality 1 of the present invention. Next, this ceramic heating element was inserted into the vacuum chamber, and the temperature sensor was attached to the heating element for temperature measurement; then, the interior of the chamber was evacuated to 1 Pa using the vacuum pump. Subsequently, the ceramic heating element was electrified and heated to a temperature of 1000 degrees Celsius. Fig.Figure 9(b) is a photograph taken of the folded-back sections of the ceramic heating element when the temperature was increased to 1000 degrees Celsius; at the folded-back sections, the current flowing through the heating element splits into the inner electric current channel (18) and the outer electric current channel (19); however, the magnitude of the current flowing into the inner current channel (18) was much greater than that of the current flowing into the outer current channel (19), and therefore it is assumed that local heating occurred at the innermost region of the left folded-back section (as indicated by an arrow). < Comparison example 2 > In the non-inventive comparative example 2, a ceramic heating element was manufactured in the same manner as in example 1; however, as can be seen from Fig. 9(c), no splitting was performed on a pair of folded-back sections. This ceramic heating element was then placed in the vacuum chamber, and the temperature sensor was attached to the heating element for temperature measurement; subsequently, the interior of the chamber was evacuated to 1 Pa using the vacuum pump. The ceramic heating element was then electrified and heated to a temperature of 1000 degrees Celsius. Fig.Figure 9(c) is a photograph taken of the folded-back sections of the ceramic heating element when the temperature was increased to 1000 degrees Celsius; the current passing through the heating element concentrated towards the innermost parts of the folded-back sections and therefore it is assumed that local heating occurred in the innermost region of the left folded-back section (as indicated by an arrow). <Example 2> In Example 2, a pyrolytic boron nitride circular cylinder with an outer diameter of 85 mm, a height of 200 mm, and a thickness of 1.3 mm was prepared by reacting 4 slm of ammonia with 2 slm of boron trichloride at a pressure of 10 Torr and a temperature of 1900 degrees Celsius. Next, a pyrolytic graphite layer 40 micrometers thick was formed on the circular cylinder by thermal cracking of methane at a pressure of 5 Torr and a temperature of 1750 degrees Celsius; this layer was then machine-cut into a heating element pattern as shown in Fig. 4.In this heating element pattern, the passage between the two electrical power supply terminals 1, comprising the passage section between a folded-back section 14 and the nearest electrical power supply terminal, as well as the passage section between the folded-back section 15 and the nearest electrical power supply terminal, was divided into an electrical current path 16 and an electrical current path 17, which extend continuously parallel to the direction of current flow. The widths of the electrical current paths 16 and 17 were both 7.3 mm, and the total length of the electrical current path 16 was 141.9 cm and that of the electrical current path 17 was 145.2 cm. A pyrolytic boron nitride insulating layer was deposited on this ceramic heating element by reacting 5 slm of ammonia with 2 slm of boron trichloride at a pressure of 10 Torr and a temperature of 1900 degrees Celsius, thus completing a ceramic heating element. With respect to this ceramic heating element of Example 2, the ratio of the electrical power density, as defined in inequality 1 above, was calculated; the ratio of the electrical power density of the electrical current path 16 was 1.02 and that of path 17 was 0.98, thus satisfying inequality 1 of the present invention. Next, this ceramic heating element was placed in the vacuum chamber, and the temperature sensor was attached to the heating element for temperature measurement. The interior of the chamber was then evacuated to 1 Pa using the vacuum pump. The ceramic heating element was then electrified and heated to a temperature of approximately 1400 degrees Celsius. The temperature distribution of the heating element surface was measured using a thermograph (Neo Thermo TVS-700, a product name of Nippon Avionics Co., Ltd.) viewed through the inspection window of the vacuum chamber. It was found that the temperature of point A in Fig. 4, which is not located in a folded-back section of the heating element pattern, was 1406 degrees Celsius; conversely, the temperature of point B, which is located in the inner split current path of the folded-back section of the heating element pattern, was 1396 degrees Celsius. Based on this result, it was found that in the case of a cylindrical heating element pattern in which the folded-back sections are divided into two lanes, there is no substantial temperature difference between the inner lane in the folded-back section and that at a location outside the folded-back sections, thus confirming that the occurrence of a local temperature increase can be prevented by this design of the heating element pattern. < Comparison example 3 > In the non-inventive comparative example 3, a pyrolytic boron nitride circular cylinder was produced in the same manner as in Example 2, and a pyrolytic graphite layer was formed on the circular cylinder. This layer was machine-cut into a heating element pattern shown in Fig. 7. In this heating element pattern, although two passages exist between the two electrical power supply terminals, each passage was not divided into lanes at the folded-back sections. As in Example 2, a pyrolytic boron nitride insulating layer was deposited over this ceramic heating element, thus completing a ceramic heating element. Next, this ceramic heating element was placed in the vacuum chamber, and the temperature sensor was attached to the heating element for temperature measurement. The interior of the chamber was then evacuated to 1 Pa using the vacuum pump. The heating element was then electrified and heated to a temperature of 1400 degrees Celsius. The temperature distribution of the heating element surface was measured using a thermograph (Neo Thermo TVS-700, a product name of Nippon Avionics Co., Ltd.) viewed through the inspection window of the vacuum chamber. It was found that the temperature of point C, located outside a folded section of the heating element pattern, was 1402 degrees Celsius, while the temperature of point D, located at the innermost position within a folded section of the heating element, was 1561 degrees Celsius. Based on this result, it was found that in the case of a cylindrical heating element pattern in which the folded-back sections are not divided into tracks, local heating at an abnormally high temperature occurred at an innermost point of a folded-back section. [Industrial applicability] The ceramic heating element of the present invention is not subject to malfunction due to local overheating, even in the most vulnerable inner area of ​​the folded-back sections, so that it can be used reliably for a long time and is therefore very useful industrially. [Explanation of reference symbols] 1 Power supply connection 2a Groove 2b Heating element 3, 5, 14, 15 Folded-back section or reversed section 4 Inner corner of a folded-back section 6, 11, 12, 16, 18 Inner electric current path 7, 10, 13, 17, 19 Outer electric current path 8, 9 Electric current I A folded-back section immediately downstream of a power supply connection II A folded-back section immediately upstream of the other power supply connection III The section between a power supply connection and its immediately downstream folded-back section and that between the other power supply connection and its immediately upstream folded-back sections are divided IV Direction of the electricity In flowing in the nth electric current path V Width Wn of the nth electric current path VI First electric current path VII Nth electric current path VIII Nth electric current path IX Although not recorded,There are further divided paths in these spaces. X The total length of the nth electric current path is measured along this dashed line. A three-dimensional ceramic heating device, such as a cylindrical ceramic heating device, is proposed in which the conductive ceramic heating element is divided into multiple tracks at least in the folded-back sections, so that the electric current tends to flow in a more uniform and therefore laminar manner, with the effect that local excessive heating is largely prevented.

Claims

A three-dimensional ceramic heating device comprising: a support base body made of an electrically insulating ceramic, wherein the support base body is in the shape of a cylinder or a container with a bottom; a thin-film heating element made of a conductive ceramic applied to the support base body and cut to form a heating element pattern comprising folded-back sections, the multiple folded-back sections being arranged parallel to each other on a side face of the support base body in the shape of a cylinder or a container with a bottom; an insulating layer made of an electrically insulating ceramic applied over the heating element;and several power supply terminals arranged at one end of the support base body for connecting the heating element to a power source, wherein the heating element pattern forms current paths over which a current flows between the several power supply terminals; wherein each of the folded-back sections is divided into N electrical current paths with splitting bands extending in directions of an electrical current flow, where N is two or more, and an electrical power density Xn in an nth electrical current path satisfies an inequality 1 (n is 1 to N): 0.7 ≤ X n ∑ n = 1 NX n N ≤ 1.3; Three-dimensional ceramic heating device according to claim 1, wherein the heating element pattern is configured such that at least one divided electric current path extends continuously from a folded-back section to an immediately downstream folded-back section without merging with another divided electric current path. Three-dimensional ceramic heating device according to claim 1 or 2, wherein the split electrical current path extends continuously between a folded-back section immediately downstream of one power supply connection and a folded-back section immediately upstream of the other power supply connection, without merging with any other split electrical current path. Three-dimensional heating ceramic device according to claim 2 or 3, wherein the divided electrical current paths extend continuously through an even number of folded-back sections without merging with any other divided electrical current path. Three-dimensional ceramic heating device according to one of claims 1 to 4, wherein the divided electrical current path extends continuously from a near vicinity of one power supply connection to a near vicinity of the other power supply connection without merging with another divided electrical current path. Three-dimensional ceramic heating device according to one of claims 1 to 5, wherein the support base body has a cylindrical shape, and any irregularity in the diameter of the cylindrical outer surface of the base support body is within a range of plus / minus 0.025 mm. Three-dimensional ceramic heating device according to one of claims 1 to 6, wherein the support base body consists of a pyrolytic boron nitride, and the heating element consists of either a pyrolytic graphite or a pyrolytic graphite containing boron and / or boron carbide, and the insulating layer consists of a pyrolytic boron nitride or a carbon-containing pyrolytic boron nitride.

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