METHOD AND APPARATUS FOR SYNTHESIZING DIAMOND

Abstract: A method and apparatus for synthesizing diamond by applying low-pressurized vapor phase capable of precipi­ tating diamond on the surface of substrate (4) by activ­ ating gas composed of carbons including solid carbon, hydrocarbon, and carbon tetrachloride respectively being mixed with hydrogen. More particularly, the invention relates to an apparatus and method of synthesizing dia­ mond by first disposing grid (3) between thermoelectron­ radiating material (2) heated with activated material gas having a minimum of 1,600°C and substrate to be coated, where the thermoelectron-radiating material (2) and grid (3) are respectively used for the negative and positive electrodes, followed by provision of potentials by causing the positive and negative electrodes to res­ pectively be connected to the grid (3) and the substrate (4) by arranging another power-supply source. The method of synthesizing diamond related to the invention implemented by the processes mentioned above securely generates extremely fine crystal diamond par­ ticles each having a maximum of 2 µm of diameter and provides extremely consummate diamond film having a minimum of 10¹⁰ Ω·cm of electric resistance, which is substantially the guide for determining the residual amount of carbon such as amorphous carbon and graphite.

Description: BACKGROUND OF THE INVENTION < Field of the Invention > The present invention relates to an improved vapor­ phase diamond synthesizing method, more particularly, to a method of synthesizing extremely consummate crystalline diamond which is almost free from carbon ingredient having structure other than diamond. < Description of the Prior Art > Conventionally, diamond is synthesized under presence of high temperature and high pressure with quite expensive processing cost. Recently, a variety of methods have been developed for synthesizing diamond under low pressure with­ out applying ultra-high pressure. Typical examples are cited below. (1) Japanese Patent Laid-Open No. 58-135117 (1983), Japanese Patent Application No. 61-8458 (1986), and Japanese Patent Application No. 61-8459 (1986) have respectively pro­ posed plasma-applied high-frequency plasma CVD methods. Japanese Patent Laid-Open No. 59-3098 (1984), Japanese Patent Laid-Open No. 58-110494 (1983), and U.S. Patent No. 4,434,188, have respectively proposed microwave-applied plasma CVD methods. (2) Some prior arts have proposed ionized vaporization method and ion-beam vaporization method by applying ionized particles. (3) Japanese Patent Laid-Open No. 58-91100 (1983) has proposed thermoelectron radiating material CVD method by applying neutral ion. (4) Japanese Patent Laid-Open No. 60-221395 (1985) (BP-161829) has proposed a CVD method by applying electron shower. Of these, methods cited in (1) and (3) respectively allow the synthesis of crystalline diamond film. Those methods cited in (2) respectively generate non-crystalline diamond-like carbon film. The method cited in (4) generates better effect than others mentioned above by allowing the diamond film to grow itself at a faster rate, i.e., at 3 through 5 µm per hour rate. Furthermore, this method proved to have thickened the density of the diamond nucleus generated during the initial period of growing diamond film. However, this occurs only when the volume ratio between CH₄ and H₂, i.e., CH₄/H₂ rises. When this condition is present, ac­ cording to Raman spectrum evaluation, peak of 1,550 cm⁻¹ belonging to amorphous carbon is too high, thus causing a problem of lowering the perfectness of diamond to easily occur. This is apparent from the description of J. Appl. Phys. 55 (1986) 640. To securely generate bulk material, the diamond film growth rate is not yet sufficient. Since crystalline diamond film is extremely rigid, those prior arts cited earlier have respectively proposed application of crystalline diamond film to the surface coating of wear-resistant parts, the surface of cutting tools to be coated, the heat-sink member of semiconductor device by effectively applying high thermal conductivity, and in addition, due to presence of considerable band gap, these prior arts positively study application of crystalline diamond film for making up semiconductor devices by means of doping impurities. By applying microwave plasma CVD method, any of those prior arts cited earlier can generate diamond almost being free from amorphous carbon and graphite having structure other than diamond as being identified by Raman spectrum method, while featuring extremely high electric resistance and consummation degree. However, when applying any method generating non-polar discharge of microwaves inside of the reactive tube which vertically penetrates a conventional waveguide tube, due to short wavelength of microwaves, if a reaction tube has a diameter longer than the wavelength of microwaves is made available, since this system cannot per­ fectly seal microwaves and causes microwaves to leak out of the reaction tube, the diameter of the reaction tube is constrained by the wavelength of microwaves, thus making it difficult for the reaction tube to provide wide coating zone and resulting in the difficulty to implement mass produc­ tion. More particularly, Japanese industries normally use 915 MHz and 2,350 MHz of industrial frequencies. These frequencies constrain the diameter of the reaction tube within a maximum of 120 mm and 50 mm, respectively. Conse­ quently, these diameters are not sufficient for generating a large-dimension coating zone durable for implementing mass production on the industrial basis. On the other hand, when applying RF plasma CVD method, in order to generate extreme­ ly consummate diamond, the system needs to generate extreme­ ly intensified plasma. Only the induction-coupling type RF power-supply source is actually made available for exciting plasma. It should be noted that capacity-coupling type RF power-supply source cannot synthesize crystalline diamond except for amorphous diamond. In other words, in order to generate a large-dimension coating zone by applying RF induction-coupling type power-supply source, the system should be provided with an extremely large-capacity RF power-supply source which in turn involves much difficulty in the industrial application. On the other hand, since the prior art cited in (4) Japanese Patent Laid-Open No. 60-221395 (1985) proposing the electron-shower-applied CVD method uses electron shower, the amount of power to be supplied between filaments and the substrate is limited, and thus, it cannot generate as much the film growth rate as that is normally achieved by the diamond synthesis method related to the invention. Further­ more, since it is essential for this prior art to apply shower by means of electrons, while the electron shower does not reach the substrate itself under presence of high atmo­ spheric pressure, the system related to this prior art can­ not actually generate diamond film by applying 50 through 500 Torr of extremely high pressure proposed by the pre­ ferred embodiment of the present invention. In principle, due to diversified configurations and application of a plurality of thermoelectron radiating mate­ rials, and in addition, as a result of proper arrangement of these materials and the substrate, the thermoelectron radi-­ ating material CVD method can process a large number of diamond and implement coating of a large area as well. However, this prior art still has problems to solve, in which more amount of carbon other than diamond is present in the synthesized diamond than the amount of carbon present in the diamond synthesized by the microwave plasma CVD method, thus eventually degrading the degree of the consummation of diamond itself, while also causing electric resistance to lower. SUMMARY OF THE INVENTION In the light of those problems mentioned above, after judging that the thermoelectron radiating material CVD was quite advantageous for implementing mass production of dia­ mond film, inventors followed up efforts for establishing a method of improving the quality of diamond film. Inventors considered that plasma advantageously func­ tioned for generating excellent quality of diamond film by applying the microwave plasma CVD method, in which raw-­ material gas significantly decomposed and activated itself, while this method could apply a variety of concentrations and kinds of radicals and ions having relationship with reaction. As a result, inventors considered that these conditions were advantageous for synthesizing extremely consummate diamond film, and inventors eventually detected useful application of plasma in order to securely promote the activation degree of raw-material gas based on the thermoelectron radiating material CVD method. More particularly, the invention relates to a method of synthesizing diamond, which is substantially the method of synthesizing diamond by applying low-pressurized vapor phase for causing diamond to precipitate itself on the surface of substrate heated to 600°C through 1,200°C by activating raw-material gas which is composed of the mixture of hydro­ gen with solid carbon, or hydrocarbon, or hydrocarbon con­ taining O and N in the bonded radical, or carbon tetra­ chloride, wherein the method is comprised of the sequential steps of; activating the above raw-material gas by applying thermoelectron radiating material heated to a minimum of 1,600°C, providing potential by connecting positive elec­ trode to grid disposed between thermoelectron radiating material which is provided with negative electrode and the substrate to be treated with coating process by applying DC power-supply source; extracting thermoelectrons for gen­ erating plasma; providing potential by connecting the posi­ tive and negative electrodes to the grid and the substrate by applying another DC power-supply source; generating plas­ ma by holding the potential of the substrate negative in order that the effect of the activation generated by thermo­ electron radiating material and the generated DC plasma can jointly be applied; and maintaining the pressure inside of the reactor generating the DC plasma to be a minimum of 10 Torr and a maximum of 400 Torr, respectively. By applying negative potential to the substrate, inven­ tors arranged the system to simultaneously generate DC bias effect which is conventionally made available for ion-­ plating process. Based on knowledges of the inventors, in order to securely generate extremely consummate diamond, extremely pressurized gas ranging from 10 Torr to several hundred Torr was compulsorily needed for generating the designed plasma. It should be noted that extremely consum­ mate diamond film denotes the one having a maximum of 2 microns of mean particle size, while the diamond film is mainly composed of surface (111) having a minimum of 10¹⁰ Ω·cm of electric resistance. Although the diamond film may also be provided with surfaces (100) or (110) as well, due to convenience of tools, diamond surface should desirably be provided with surface (111). It should also be noted that electric resistance normally indicates a certain amount of residual carbon. In other words, the higher the electric resistance, the less is the amount of residual carbon such as amorphous carbon and graphite. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from the fol­ lowing detailed description and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the invention and wherein: FIG. 1 is the front view of a preferred embodiment of the apparatus for implementing the method of synthesizing diamond related to the invention, which is comprised of the following constituents denoted by respective reference nu­ merals shown below; 1---Raw-material gas inlet 2---W-filament of thermoelectron radiating material 3---Grid 4---Substrate 5---Substrate holding base 6---AC power-supply source for heating thermoelectron radiating material 7---DC power-supply source generating plasma between thermoelectron radiating material and grid 8---DC power-supply source providing the substrate with negative potential 9---Vacuum exhaust outlet 10---Insulation seal 11---External furnace 12---Transparent quartz reaction tube and FIG. 2 is the constitution of the substrate holding base improved from the one used for the preferred embodiment of the invention, which is provided with cooling device. Arrowed directions respectively denote the flow path of refrigerant. The cooling device properly adjusts tempera­ ture of the substrate by allowing fluid like water and/or gas to flow through it. It is also possible for the im­ proved substrate holding base to more precisely control temperature by internally providing a temperature-measuring instrument in part of the substrate or the substrate holding base. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is the simplified block diagram of a preferred embodiment of the apparatus for implementing the method of synthesizing diamond related to the invention. First, CH₄ and H₂ mixed gas are led into quartz-made reaction tube 12 via raw-material gas inlet 1, and then, the mixed gas is thermally activated by thermoelectron radiating mate­ rial 2 heated to more than 1,600°C. At the same time, the apparatus causes negative electrode and positive electrode to respectively be connected to thermoelectron radiating material 2 and grid 3 by applying DC power-supply source 7 so that plasma can be generated between thermoelectron radi­ ating material which has extracted thermoelectrons from thermoelectron radiating material 2 and the grid 3. The substrate 4 is then held in the negative potential so that the positive ion can positively be drawn onto the surface of the substrate 4 by allowing the negative electrode to be connected to the substrate 4 on the holding base 5. While executing these processes, the system uses extremely pres­ surized gas ranging from a minimum of 10 Torr to a maximum of 400 Torr. The apparatus shown in FIG. 1 is provided with AC power-supply source 6 for heating thermoelectron radiat­ ing material, vacuum exhaust outlet 9, insulation seal 10, and external furnace 11, respectively. The invention provides four features which are sum­ marized below. (1) By jointly applying DC plasma together with pyro­ lysis and activation of raw-material gas generated by extremely-heated thermoelectron radiating material, degree of the activation of raw-material gas is promoted and causes kinds of ion to be generated that otherwise cannot be generated merely by means of pyrolysis. (2) Negative potential is applied to the substrate to allow positive ion to be drawn onto the substrate surface positively. (3) By disposing grid between thermoelectron radiating material and substrate, the apparatus generates plasma be­ tween thermoelectron radiating material and the grid and also between the grid and the substrate, respectively. (4) The apparatus related to the invention generates plasma by applying extremely high pressure ranging from a minimum of 10 Torr to a maximum of 400 Torr. Normally, plasma cannot stably be generated or maintained merely by applying DC power-supply source under presence of such a high pressure mentioned above. In order to stably generate and maintain plasma by applying such an extremely high pressure ranging from a minimum of 10 Torr to a maximum of 400 Torr, it is essential for the system to extract thermoelectrons from thermoelectron radiating material. When feeding negative potential to the substrate, if the negative electrode and the positive electrode were respectively connected to the substrate and thermoelectron radiating material by applying DC power-supply source, thermoelectrons cannot be extracted from thermoelectron radiating material but conversely thermoelectrons will be set back, and as a result, no plas­ ma can be generated under presence of such a high pressure mentioned above. Now, the apparatus related to the invention disposes a grid between thermoelectron radiating material and the sub­ strate so that plasma can be generated between these members by allowing thermoelectrons to be extracted from thermo­ electron radiating material after connecting the negative electrode to thermoelectron radiating material and the posi­ tive electrode to the grid by applying DC power-supply source. Furthermore, the apparatus causes the already-­ generated positive ion to be drawn onto the substrate and generates plasma by respectively connecting the positive electrode and the negative electrode to the grid and the substrate by applying another DC power-supply source. It is considered that the preferred embodiment of the diamond-synthesizing method and apparatus related to the invention can securely generate such an extremely consummate diamond based on the reasons explained below. The invention allows generation of DC plasma in addi­ tion to the pyrolysis and the activation of raw-material gas done by thermoelectron radiating material in order that the degree of activation can securely be promoted. This in turn intensifies energy of the activated gas and causes the gas volume remaining in the activated state to increase itself, where the activated gas includes hydrogen radical capable of selectively eliminating carbon (other than diamond) precip­ itating itself together with diamond and/or methyl radical having SP³ mixed track needed for generating diamond. In addition, since the life of the activated gas is durable, graphite and/or amorphous carbon having structure other than diamond can be prevented from precipitating itself, thus eventually allowing such extremely consummate diamond film to be generated securely. Furthermore, inventors detected in surprise that the bonded strength between diamond film and the substrate has significantly been promoted by feeding negative potential with DC power-supply source. Although the reason for this effect is not yet clear, it is probable that the positive ion of the raw-material gas ionized by plasma generated between thermoelectron radiating material and grid has pref­ erentially been drawn onto the surface of the substrate. Furthermore, the diamond synthesis method related to the invention proved to have generated significantly fine composition of the diamond film which was formed on the surface of the substrate. It is probable that both the impact of ion against the surface of the substrate and the plasma generated on the surface of the substrate advanta­ geously affected the degree of activation of the activated ingredients, concentration, and the life of these activated ingredients as well, thus eventually promoting the density of the diamond nucleus being generated. It is obvious that execution of these processes under presence of high pressure advantageously promoted the density of the generated diamond nucleus. The invention securely provides an extremely consummate diamond film which is free from such carbon matter having shape other than diamond and features a maximum of 1 micron of fine constitution of the diamond film even after being exposed to a minimum of 1,000°C of high temperature by generating extremely intensified bonding strength against the substrate. The method of synthesizing diamond related to the in­ vention allows the use of carbon ingredients, for example, such as solid carbon, or hydrocarbon, or hydrocarbon con taining O and N in the bonded radical, or carbon tetra­ chloride. It is possible for the diamond synthesis method related to the invention to effectively use raw-material gas composed of the mixture of at least more than one kind of those carbon ingredients and hydrogen. When solid carbon is made available for carbon source, it is desirable for the system to use carbon for making up thermoelectron radiating material. Of the raw-material gas present in the reactor, there is a specific proportional number of carbon atoms against the number of hydrogen atoms, i.e., by a minimum of 0.01% and a maximum of 20%. If the proportional rate of carbon atoms present in the raw-material gas is less than 0.01%, diamond film will grow itself at an extremely slow rate, which is not durable enough for actual use. Conversely, if the proportional rate of carbon atoms in the raw-material gas exceeds 20%, compared to diamond, a greater amount of carbon having structure other than diamond will precipitate itself, thus eventually inhibiting formation of diamond film. In addition to those gas ingredients delivered to the reactor, Ar, Kr, Xe, Rn, and N₂, which are respectively independent of the reaction may also be used in mixture with those gas ingredients mentioned above. Each of these can effectively be used for cooling down temperature of the substrate owing to low thermal conductivity, and yet, each properly adjusts the strength of plasma as another phase of advantage. It is essential for the thermoelectron radiating material and the grid material to provide low-vapor pressure under presence of high temperature and have high melting point as well. In particular, it is essential for thermoelectron radi­ ating material to be superb in the capacity of releasing thermoelectrons. Consequently, it is desirable that both the thermoelectron radiating material and the grid be made from those materials having high melting point like tungsten (W), tantalum(Ta), or molybdenum(Mo), or LaB₆, or gra­ phite, for example. Thermoelectron radiating material should be used only after being heated to a minimum of 1,600°C of ultra-high temperature. If temperature goes down from 1,600°C, graphite and amorphous carbon will dominantly precipitate themselves. Unless the surface of the substrate bears a minimum of 600°C and a maximum of 1,200°C, diamond does not precipitate at all. Reactor should internally be provided with a minimum of 10 Torr and a maximum of 400 Torr of pressure, preferably the internal pressure should a minimum of 50 Torr and a maximum of 300 Torr. If the pressure inside of the reactor exceeds 400 Torr, it is difficult for the apparatus related to the invention to stably maintain plasma between the grid and the substrate. Conversely, if the pressure inside of the reactor is lower than 10 Torr, evaporation speed will significantly lower, thus generating unwanted results. The invention features the capability of stably gen­ erating and maintaining plasma under presence of high pressure, which cannot actually be realized by any con­ ventional system that merely uses DC power-supply source. This is because the expansion of gas pressure effectively promotes the density of the generated diamond nucleus. The intensity of plasma generated between thermoelec­ tron radiating material and the grid is dependent on the gas atmosphere, configuration of thermoelectron radiating mate­ rial and grid, temperature of thermoelectron radiating mate­ rial, arrangement of thermoelectron radiating material and grid, and the power output from DC power-supply source. Since the plasma radiation is observed in the position close to the grid, it is desirable for the apparatus to properly control the plasma-generating condition by applying DC pow­ er-supply source which is delivered per unit area of the grid. When embodying the invention, it is desirable for the apparatus to feed a minimum of 10 W/cm² of DC power between the thermoelectron radiating material and the grid. If the apparatus receives less than 10 W/cm² of DC power, the appa­ ratus cannot stably generate and maintain plasma between the grid and the substrate. Furthermore, the intensity of plas­ ma generated between the grid and the substrate is dependent on the intensity of plasma generated between thermoelectron radiating material and grid, gas atmosphere, configuration of the grid, substrate, and the substrate holding base, arrangement of the grid and the substrate, and the power output from DC power-supply source. Like the above case, since plasma is generated in the position close to the grid and the surface of the substrate facing the grid, it is desirable for the apparatus to properly control the plasma by applying DC power-supply source delivered per unit area of the substrate surface to be coated. The apparatus related to the invention should desirably feed a minimum of 5 W/cm² and a maximum of 200 W/cm² of DC power between the thermoelectron radiating material and the grid. If less than 5 W/cm² of DC power is delivered, the apparatus will not be able to stably generate and maintain plasma between the grid and the substrate. Conversely, if more than 200 W/cm² of DC power is delivered, the plasma etching effect will be magnified, thus extremely slowing down the diamond precipitation speed. Temperature on the surface of the substrate is depen­ dent on the temperature of thermoelectron radiating materi­ al, arrangement and configuration of thermoelectron radiat­ ing material, grid, and the substrate, DC power being de­ livered between the thermoelectron radiating material and the grid, DC power being delivered between the grid and the substrate, gas atmosphere, and temperature of the external furnace, etc. The optimum effect of the invention is gen­ erated by properly selecting specific conditions from those mentioned above. It may be necessary for the diamond synthesizing appa­ ratus related to the invention to execute operations for cooling down temperature of the substrate holding base and the wall of the reactor in order to precisely constrain temperature on the surface of the substrate within an ade­ quate range. Control of temperature on the surface of sub­ strate is extremely important for the apparatus related to the invention in order to correctly control stress which remains on the diamond film. More particularly, there is the difference in the ther­ mal expansion coefficient between diamond and the substrate, and thus, since there is a difference between the diamond synthesizing temperature and the temperature actually being used, stress remains in the diamond film. If the remaining stress is too strong, diamond film will be stripped off from the substrate. The greater the thickness of the diamond film, the greater is the possibility of causing diamond film to be stripped off from the substrate. However, more satis­ factory diamond film will be generated if more than a spe­ cific amount of stress remains in the diamond film. The reason for this has not yet been clarified. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now more particularly to the accompanying drawings, preferred embodiments of the method and the appa­ ratus for synthesizing diamond related to the invention are described below. < The First Preferred Embodiment > Using substrate made from molybdenum(Mo) plate having 10 mm × 10 mm × 1 mm of dimension and the surface which is ground with #5000 diamond grinding particles while employing the diamond synthesizing apparatus shown in FIG. 1, inven­ tors experimentally carried out diamond coating operations. First, inventors decompressed pneumatic pressure. from the reactor to a maximum of 10⁻³ Torr. Then, inventors fed gas which was composed of a mixture of CH₄ and H₂ at the 1/100th rate into the reactor at 100 ml/min. of flow rate. While maintaining a specific pressure inside of the reactor and using tungsten(W)-made filament which substantially made up thermoelectron radiating material, inventors once heated the compound gas to more than 2,000°C in order that temperature on the surface of the substrate could be adjusted to 920°C. Inventors arranged the distance between tungsten filament and grid made from tungsten to be 5 mm and also the distance between tungsten filament and molybdenum-made substrate of the grid to be 4 mm, respectively. Next, using DC power-­ supply source, inventors fed power by connecting the fila­ ment to the negative electrode and the grid to the positive electrode. Then, using another DC power-supply source, inventors fed power by connecting the filament to the nega­ tive electrode and the grid to the positive electrode, and then, inventors adjusted power output from these two DC power-supply sources flowing through filaments in order that temperature on the surface of the substrate could precisely be stabilized at 920°C. Inventors set 4 hours of the dia­ mond coating time. Table 1 represents the coating condi­ tions and characteristic of the coated film. Depending on the coating conditions, inventors adjusted temperature on the surface of the substrate by cooling down temperature borne by the substrate holding base. For comparative evaluation, using the same molybdenum-­ made substrate and the 2.45 GHz microwave plasma CVD appara­ tus, inventors fed gas which was composed of a mixture of CH₄ and H₂ at the 1/100th rate into the reactor at 200 ml/min. of flow rate. As shown in Table 1, using 40 Torr of pressure, 250W of microwave power, and 910°C of temperature on the surface of the substrate, inventors carried out a coating process for 4 hours consecutively. Table 1 also represents characteristic of the coated film detected from the comparative evaluation. In order to generate ohmic contact for determining electric resistance of diamond film, using ion-plating apparatus, inventors prepared an electrode by coating titanium (Ti) and gold (Au) for making up the first and second layers on the diamond film. Using this electrode and another electrode made from molybdenum substrate plate, inventors measured V-I characteristic between these elec­ trodes for determining electric resistance. Experiments cited in Table 1 proved that satisfactory diamond film was uniformly generated except for experiments Nos. 1, 2, and 13 which were consequently deleted from the scope of the claims of the invention. Compared to No. 14 experiment which dispensed with plasma, other experiments significantly promoted electric resistance of the diamond film. Table 1 also represents that the experiments done for the first preferred embodiment of the invention satisfac­ torily generated superb diamond film featuring quite higher electric resistance than that of the diamond film generated by No. 15 microwave plasma CVD method. After executing Raman spectrum analysis, inventors observed the presence of amorphous carbon in the diamond film generated by No. 14 experiment. Conversely, inventors confirmed that no amorphous carbon was present in those diamond films generated from experiments which applied plas­ ma. Inventors confirmed from comparative evaluations of experiments Nos. 3, 4, 7, and 11 that particles of diamond film became finer relative to the intensified degree of plasma. Likewise, inventors also confirmed that crystalline particles of diamond became finer when plasma strength was intensified. It was extremely difficult for the microwave plasma CVD method to securely generate fine particle compo­ sition like the one mentioned above under presence of high substrate temperature. < The Second Preferred Embodiment > The second preferred embodiment of the method and appa­ ratus for synthesizing diamond related to the invention is described below. By implementing the second preferred embodiment of the invention, extremely consummate diamond surpassing that is achieved by the microwave plasma CVD method featuring a maximum of 1 micron of fine particle size and unsurpassed strength of bonding with the substrate is securely generated even under exposure to a minimum of 1,000°C of high temperature. In addition, it is possible for the second preferred embodiment of the invention to execute processing of a plurality of substrates and coating process over a wide area by applying a plurality of thermoelectron radiating materials. Inventors experimentally coated a molybdenum substrate having 200 mm × 150 mm × 10 mm of dimension ground with #600 diamond whetstone by applying 21 units of thermoelectron radiating materials which were also made available for im­ plementing the first preferred embodiment. In order to precisely adjust temperature of each thermoelectron radiat­ ing material, heating power sources were independently pro­ vided so that temperature of each thermoelectron radiating material could independently be controlled. However, in order to connect DC power-supply source, one-side of the electrodes were commonly connected to each other. According to the dimension of the substrate, inventors arranged the size of the grid disposed between the thermoelectron radiat­ ing material and the substrate to be 220 mm × 180 mm. First, inventors decompressed pneumatic pressure from the reactor to a maximum of 10⁻³ Torr. Then, inventors fed gas which was composed of a mixture of CH₄ and H₂ at the 1/100th rate into the reactor at 200 ml/min. of flow rate so that the pressure inside of the reactor could be held at 90 Torr. Inventors once heated tungsten filament made avail­ able for thermoelectron radiating material to a minimum of 2,000°C in order that temperature on the surface of the substrate could be adjusted in a range from 980°C to a maxi­ mum of 1,010°C. Next, using DC power-supply source, inventors fed 40W of power per unit area of grid by connecting tungsten fila­ ment and the grid to the negative and positive electrodes before eventually generating plasma between the filament and the grid. Next, using another DC power-supply source, in­ ventors fed 20W of power per unit area of the grid by con­ necting the grid and the substrate to the positive and nega­ tive electrodes before eventually allowing plasma to be generated on the surface of the substrate. Since the sup­ plied DC power could heat up temperature on the surface of the substrate, inventors decreased the amount of DC current flowing through the filament so that temperature of the filament could be lowered. Inventors also cooled the sub­ strate holding base with water so that temperature on the surface of the substrate could be adjusted to a range from 980°C to a maximum of 1,010°C. Now, as a result of coating process done for 4 hours consecutively, the second preferred embodiment of the inven­ tion successfully generated diamond film having 8 µm in average of thickness with a maximum of 10% of unevenness. Each crystalline particle had an extremely fine size ranging from 0.3 micron to a maximum of 0.8 micron. Evaluation of electric resistance resulted in quite satisfactory values ranging from 10¹¹ to a maximum of 10¹² Ω·cm. Consequently, inventors confirmed that the second pre­ ferred embodiment of the invention could extensively and uniformly coat the substrate with extremely consummate dia­ mond crystals featuring extremely fine constitution. < The Third Preferred Embodiment > For implementing the third preferred embodiment of the invention, inventors employed the substrate made from SPG 422 type ISO K-10 cemented carbide. Using the same appara­ tus as that was employed for the first preferred embodiment, inventors executed coating of substrate with diamond film. Inventors arranged the distance between the filament and the grid to be 5 mm and the distance between the grid and the upper surface of the insert to be 4 mm, respectively. First, inventors decompressed the pneumatic pressure from the reactor to a maximum of 10⁻³ Torr. Then, inventors fed gas which was composed of a mixture of CH₄ and H₂ at the 1/100th rate into the reactor at 100 m/min. of flow rate. While maintaining the pressure inside of the reactor at 90 Torr, inventors once heated tungsten filament to a maximum of 2,000°C in order that temperature on the surface of the substrate could be adjusted to 950°C. Next, using DC power-supply source, inventors connected the filament and the grid to the negative and positive elec­ trodes for generating potential before eventually generating plasma. In addition, using another DC power-supply source, inventors connected the grid and the substrate to the posi­ tive and negative electrodes for generating potential. Then, inventors adjusted amount of DC current flowing through the filament and the amount of current output of these two DC power-supply sources in order that temperature on the surface of the substrate could be held exactly at 950°C. Inventors eventually carried out coating processes for 3 hours consecutively based on those conditions shown in Table 2. After completing the coating processes based on those conditions shown in Table 2, satisfactory diamond film was uniformly generated. For comparison with those inserts used for the experiments, inventors adjusted temperature on the surface of the substrate at 950°C by adjusting the amount of DC current flowing through filament. Substrate A was coated for 3 hours by conventional thermoelectron radiating CVD method and surface temperature, pressure, composition of raw material gas was same as mentioned above. Substrate B was not coated. Using these inserts, inventors carried out cutting tests based on conditions shown below. Objective material: Aluminum-25% silicon Cutting speed: 500 meters per minute Material feed rate: 0.1 mm/rev. Cutting depth: 0.5 mm Holder: FR 11R - 44A As a result of the above test, substrate B which was not yet treated with coating process generated 0.40 mm of Frank wear after the cutting process which lasted for 15 seconds, whereas substrate A which was complete with the coating process generated 0.16 mm of Frank wear after the cutting process which lasted for 20 minutes. Compared to these, after the cutting process which lasted for 30 minutes, those specimens made available for implementing the third preferred embodiment of the invention respectively generated less amount of Frank wear as shown below. Specimen No. 16 0.12 mm; Specimen No. 17 0.10 mm Specimen No. 18 0.07 mm; Specimen No. 19 0.10 mm Specimen No. 20 0.07 mm; Specimen No. 21 0.06 mm These results proved that the third preferred embodi­ ment of the invention securely generated significantly ad­ vantageous effect. The fourth preferred embodiment By additionally installing the cooling device shown in Fig. 2 to the apparatus shown in Fig. 1 and by applying those conditions shown in Table 3 as well as those methods explained in the preceding section related to the first preferred embodiment, inventors executed experimental diamond-film coating for two hours. The experiment success­ fully generated extremely consummate diamond film having the thickness shown in Table 3. Next, by applying the identical conditions and by ad­ justing the coating time, inventors processed cutting tools having the configuration No. SPG-422 in order that the thickness of diamond film could exactly become 6 µm. Final­ ly, inventors assessed the conditions of the processed cutting tools by executing cutting tests. Conditions em­ pleyed for executing the cutting tests are shown below. Objective material: Aluminum - 8% silicon Cutting speed: 1,800 meters per minute Material feed rate: 0.1 mm/rev. Cutting depth: 0.2 mm Duration of cutting tests: 2 hours Note that symbols , ○, Δ, and x shown in Table 3 respectively denote the grade order of worn-out amount between those tools which were coated with diamond film and those which are not coated with it. The least amount of wear is denoted by the double circle, followed by the single circle, the triangle, and x, respectively. Symbol x denotes the amount of wear which is equivalent to that is not coated with diamond film. Test No. 27 merely generated amorphous carbon. When executing the cutting tests in accordance with those conditions shown in Table 3, inventors properly ad­ justed temperature of substrates by applying the cooling device shown in Fig. 2. < Industrial Applicability > As is clear from the foregoing description, based on the thermoelectron radiating material CVD method, the pre­ ferred embodiments of the invention respectively activate raw-material compound gas by applying thermoelectron radiat­ ing material being heated to a minimum of 1,600°C, and then, using a DC power-supply source, potential is generated for the diamond synthesizing apparatus by connecting thermo­ electron radiating material to the negative electrode and the grid to the positive electrode, in which the grid is disposed between the thermoelectron radiating material and the substrate subject to coating with diamond film, thus allowing thermoelectrons to be extracted from this material so that plasma can eventually be generated. The apparatus related to the invention also uses another DC power-supply source for generating potential by connecting the grid and the substrate to the positive and negative electrodes for generating plasma while holding the substrate potential negative. The apparatus jointly uses activation generated by thermoelectron radiating material and the other activa­ tion caused by generation of DC plasma, and yet, by properly controlling pressure inside of the reactor to a minimum of 10 Torr and a maximum of 400 Torr for causing plasma to be generated, the apparatus securely generates extremely con­ summate diamond which is significantly better in quality than that which is normally generated by the thermoelectron radi­ ating material CVD method. Furthermore, since the apparatus related to the invention provides diamond with significantly satisfactory electric resistance which is incomparable to that which is normally generated by the microwave plasma CVD meth­ od, the diamond which is synthesized by the method and the apparatus related to the invention is usefully applicable to a wide variety of electronic materials requiring extremely satisfactory insulation effect such as heat-radiating print­ ed wiring boards and diamond semiconductors for example. Furthermore, since the method and the apparatus related to the invention securely provide fine-particle-size diamond film even under the presence of high temperature above 1000°C with extremely strong bonding effect against the substrate, the method and the apparatus for synthesizing diamond related to the invention securely and significantly improve the produc­ tivity by effectively being applied to those uses which effectively utilize mechanical characteristic of diamond such as cutting tools and wear-resistant parts for example.

Claims: 1. A method of synthesizing diamond, which is substan­ tially a method of synthesizing diamond by applying low-pressurized vapor phase, wherein said method uses carbon source such as solid carbon, or hydrocarbon, or hydrocarbon containing O and N in the bonded radical, or carbon tetrachloride, where said method causes raw-­ material compound gas composed of a mixture of said carbon ingredient and hydrogen to be activated for eventually precipitating diamond on the surface of sub­ strate (4) being heated to a minimum of 600°C through a maximum of 1,200°C, wherein comprising steps and pro­ cesses of: causing thermoelectron radiating material (2) heated to a minimum of 1,600°C to activate said compound gas; generating potential by connecting said thermoelec­ tron radiating material (2) to negative electrode and grid (3) disposed between said thermoelectron radiating material (2) and substrate (4) subject to coating to positive electrode by applying a DC power-supply source; extracting thermoelectrons from said thermoelectron radiating material (2) for generating plasma; generating potential by connecting grid (3) to posi­ tive electrode and substrate (4) to negative electrode by applying another DC power-supply source (7) for gen­ erating plasma while maintaining potential of said sub­ strate (4) negative; jointly applying effect of activation generated by thermoelectron radiating material (2) and another activ­ ation caused by generation of DC plasma; and controlling pressure inside of plasma-generating reactor to be a minimum of 10 Torr and a maximum of 400 Torr, respectively. 2. The method of synthesizing diamond defined in Claim 1, wherein the proportional number of carbon atom against the number of hydrogen atom of raw-material compound gas in said reactor is a minimum of 0.01% and a maximum of 20%. 3. The method of synthesizing diamond defined in Claim 1, wherein said thermoelectron radiating material (2) is substantially comprised of any of those elements includ­ ing tungsten (W), tantalum (Ta), molybdenum (Mo), or LaB₆, or graphite. 4. The method of synthesizing diamond defined in Claim 1, wherein a minimum of 10 W/cm² of DC power is sup­ plied between said thermoelectron radiating material (2) and grid (3). 5. The method of synthesizing diamond defined in Claim 1, wherein 5 through 200 W/cm² of DC power is supplied between said grid (3) and substrate (4). 6. The method of synthesizing diamond defined in Claim 1, wherein temperature on the surface of said substrate (4) is adjustable as required. 7. An apparatus for synthesizing diamond, which is sub­ stantially an apparatus for synthesizing diamond by applying low-pressurized vapor phase, wherein said appa­ ratus uses carbon source such as solid carbon, or hydro­ carbon, or hydrocarbon containing O and N in the bonded radical, or carbon tetrachloride, where said apparatus causes raw-material compound gas composed of a mixture of said carbon ingredient and hydrogen to be activated before eventually precipitating diamond on the surface of said substrate (4), wherein comprising: a reactor which is internally provided with thermo­ electron radiating material (2), grid (3), and substrate holding base (5) being arranged in said order; and two units of DC power-supply sources (7, 8) which are respectively disposed between said thermoelectron radiating material (2) and grid (3) and also between said grid (3) and substrate (4). 8. The diamond synthesizing apparatus defined in Claim 7, further comprising compound-gas inlet part (1) and air-exhaust part (9) for vacuuming. 9. The diamond synthesizing apparatus defined in Claim 7, further comprising an external heating device (11). 10. The diamond synthesizing apparatus defined in Claim 7, further comprising a device for precisely controlling temperature of said substrate.

Inventors: IKEGAYA, AKIHIKO SUMITOMO ELECTRIC IND.LTD., MASAAKI, TOBIOKA SUMITOMO ELECTRIC IND.LTD.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1987
Main classification: C30B25
IPC codes: C30B25