METHOD FOR SYNTHESIS OF DIAMOND

Abstract: The following are disclosed: A method for synthesis of diamond which is characterized by contacting a gas obtained by excitation of carbon monoxide and hydrogen in such a ratio as carbon monoxide being at least 1 mol% per total of carbon monoxide and hydrogen with a substrate in the presence of a reducing metal. A method for synthesis of diamond which is characterized by contacting with a substrate a gas obtained by excitation of carbon dioxide and hydrogen mixed at such a ratio of carbon dioxide being 0.1-20 mol% per hydrogen. A method for synthesis of diamond by depositing diamond on the surface of a substrate by introducing onto the surface of the substrate a plasma obtained from hydrogen and carbon source gas by irradiation of microwave in a plasma generator which is characterized in that progress of microwave oscillated from one microwave oscillator is divided and thus divided respective microwaves and led to a plurality of plasma generators. A diamond synthesis apparatus which is characterized by comprising a microwave oscillator for oscillation of microwave, a branched waveguide for dividing the microwave oscillated from said microwave oscillator and a plurality of plasma generators which are connected with said branched waveguide and have a substrate for deposition of diamond, respectively.

Description: BACKGROUND OF THE INVENTION This invention relates to a method and an apparatus for synthesis of diamond and more particularly it relates to a method for the synthesis of diamond usable widely for, for example, various protective films, optical materials, electronic materials and chemical industry materials by rapid and efficient deposition of diamond. PRIOR ART AND PROBLEMS THEREOF Recently, the technique for synthesis of diamond has been remarkably developed and there have been known CVD methods such as the plasma CVD method which comprises decomposing a hydrocarbon by plasma to form hard carbon on a substrate, and the chemical transport method which comprises forming a hard carbon film on a substrate by a disproportionation reaction as well as ionization vapor deposition methods which use hot-cathode PIG guns, cold-cathode PIG guns or sputtering guns. The conventionally proposed methods use hydrocarbon or a mixed gas of hydrocarbon and hydrogen as raw material [cf. Japanese Patent Application Kokai (Laid-open) Nos.91100/83, 110494/83, 135117/83, 63732/84 and 103099/85] and these methods suffer from the problem of low deposition rate of diamond. In order to improve the deposition rate, it has been proposed to use a raw material gas comprising a mixed gas of hydrocarbon and hydrogen which further contains carbon monoxide [cf. Japanese Patent Application Kokai (Laid-open) No.191097/85]. However, this method has a new problem that carbon monoxide must be additionally contained in the conventionally used raw material gas and thus the preparation of the raw material gas becomes complicated and the operation therefor is troublesome. This method has further the significant problem that when the amount of carbon monoxide added to hydrocarbon is more than 10 vol%, the content of oxygen in the artificial diamond deposited on the surface of the substrate becomes too high and the crystal structure of diamond is damaged. US-A 4 434 188 discloses a method for the synthesis of diamond wherein a mixture of hydrocarbon and hydrogen gas after passing through a microwave non-electrode discharge is contacted with a substrate heated to a temperature of from 300 to 1300°C to decompose hydrocarbon in its energetically activated state for the diamond deposition. Among various synthetic techniques referred to hereabove, especially the microwave plasma CVD method is promising because it is superior in reproducibility of the activated state and besides it includes no consumption parts. However, since in the conventional microwave plasma CVD method, the microwave oscillated from one microwave oscillator is introduced into one plasma generator [Japanese Patent Application Kokai (Laid-open) No.91100/84], many apparatuses each of which consists of a pair of microwave oscillator and plasma generator are required in order to produce diamond thin films simultaneously and in a large quantity. Therefore, this CVD method is low in production efficiency and industrially disadvantageous. Therefore, before the filing of the Japanese patent applications from which the present application claims convention priority, a method and an apparatus for industrial synthesis of diamond which is simple in operation, high in deposition rate and superior in production efficiency has been demanded. BRIEF EXPLANATION OF THE DRAWINGS Fig.1(A) is a diagrammatical view of an apparatus for synthesis of diamond according to this invention and Fig.1(B) illustrates one example of a plasma generator which constitutes said apparatus. SUMMARY OF THE INVENTION The object (1) of this invention is to provide a method for the synthesis of diamond free from disorder of crystal structure which is high in deposition rate, very simple in operation for preparation of raw material gas and superior in production efficiency by any methods of high radio frequency plasma CVD method, microwave plasma CVD method, hot filament CVD method, chemical transport method, ionization vapor deposition method and ion beam vapor deposition method. Another object (2) of this invention is to provide a method for the production of industrially useful diamond according to which the above problems are solved and mass production of diamond thin film can be accomplished by microwave plasma CVD method and an apparatus suitable for carrying out this method. This invention for attaining the above object (1) is a method for the synthesis of diamond which comprises contacting a substrate with a gas obtained by excitation of carbon monoxide and hydrogen, the content of carbon monoxide being at least 1 mol% per total of carbon monoxide and hydrogen. This invention for attaining the above objects (1) and (2) also is a method for the synthesis of diamond which comprises contacting a substrate with a plasma obtained from hydrogen gas and carbon source gas by irradiation with microwave in a plasma generator to deposit diamond on the surface of the substrate wherein progress of a microwave oscillated from one microwave oscillator is branched and thus branched microwaves are respectively led to a plurality of such plasma generators. This invention for attaining the above objects (1) and (2) further is an apparatus for the synthesis of diamond according to the above method which comprises a microwave oscillator for oscillation of microwave, a branching waveguide which in use branches the microwave oscillated from said microwave oscillator into a plurality of directions, and a plurality of plasma generators which are connected with said branching waveguide through an applicator such that a mixed gas of hydrogen gas and carbon source gas in use of said waveguide is excited to produce plasma which reaches the substrate contained in each of said plasma generators. DESCRIPTION OF THE INVENTION (A) Raw material gases Raw materials for synthesis of diamond in this invention are a carbon source gas and hydrogen gas. In more detail, raw materials in this invention are (1) a mixed gas containing carbon monoxide and hydrogen at such a ratio that the content of carbon monoxide being at least 1 mol%, or (2) hydrogen gas and carbon source gas when the apparatus for synthesis of diamond according to this invention is used. The carbon monoxide has no special limitation and there may be used, for example, producer gas or water gas which is obtained by reaction of coal or coke with air or water vapor with heating and which is sufficiently purified. As the carbon source gas, there may be effectively used, for example, paraffinic hydrocarbons such as methane, ethane, propane and butane; olefinic hydrocarbons such as ethylene, propylene and butylene; hydrocarbons of acetylene series such as acetylene and allylene; olefinic hydrocarbons such as butadiene; alicyclic hydrocarbons such as cyclopropane, cyclobutane, cyclopentane and cyclohexane; aromatic hydrocarbons such as cyclobutadiene, benzene, toluene, xylene and naphthalene; ketones such as acetone, diethyl ketone and benzophenone; alcohols such as methanol and ethanol; amines such as trimethylamine and triethylamine; carbon monoxide; and furthermore, although not single substances, the following may also be used: petroleums such as gasoline, kerosine, turpentine oil, camphor oil, oil obtained by dry distillation of roots of pine, heavy oil, gear oil and cylinder oil. These carbon compounds may also be used as mixtures thereof. Among these carbon source gases, preferred are oxygen-containing compounds such as ketones eg. acetone and benzophenone, alcohols eg. methanol and ethanol, carbon monoxide and nitrogen-containing compounds such as amines, eg., triethylamine and especially preferred are carbon monoxide and further preferred are carbon monoxide. The hydrogen usable has no special limitation and there may be used, for example, those which are obtained by gasification of petroleums, conversion of natural gas and water gas, electrolysis of water, reaction of iron and water vapor and complete gasification of coal and are sufficiently purified. When carbon monoxide and hydrogen are used as raw material gases, the content of carbon monoxide is at least 1 mol%, preferably at least 3 mol%, more preferably at least 5 mol% per total of carbon monoxide and hydrogen. When the content of carbon monoxide is less than 1 mol%, diamond tends not to be produced or, even if produced, the deposition rate tends to be extremely low. When the carbon source gas and the hydrogen gas are used as raw material gases, the amount of the carbon source gas used varies depending on kinds of the carbon source gas and cannot be generally determined. When carbon monoxide is used as the carbon source gas, the amounts thereof are as mentioned above. In the method of this invention, an inert gas can be used as carrier for the raw material gases. Typical examples of the inert gas are argon gas, neon gas, helium gas, xenon gas and nitrogen gas. These may be used alone or in combination of two or more. (B) Substrate The substrate with which the gas obtained by excitation of said raw material gases is allowed to contact is not critical and there may be used any of, for example, metals such as silicon, aluminum, titanium, tungsten, molybdenum, cobalt and chromium, oxides, nitrides and carbides of these metals, alloys of these metals, cermets such as Al₂O₃-Fe, TiC-Ni, TiC-Co and B₄C-Fe systems and various ceramics. (C) Excitation of raw material gas For obtaining gases by excitation of the raw material gas, known methods can be employed such as plasma CVD method, sputtering method, ionization vapor deposition method, ion beam vapor deposition method, hot filament method and chemical transport method. When plasma CVD method is used, said hydrogen produces atomic hydrogen after forming plasma by irradiation of high radio frequency or microwave and when CVD methods such as the chemical transport method and hot filament method are used, said hydrogen produces atomic hydrogen by heat or discharging. This atomic hydrogen has the action of removing carbon of graphite structure deposited simultaneously with the deposition of diamond. When said plasma CVD apparatus is used, diamond can be efficiently produced from any raw material gases of the above (1) and (2) by employing the method which comprises branching the progress of microwave oscillated from one microwave oscillator and leading the respective branched microwave to a plurality of plasma generators. This effective method for production of diamond can be realized by using an apparatus for the synthesis of diamond as described above. Next, the apparatus for the synthesis of diamond will be explained below. As shown in Fig.1(A), this apparatus for synthesis of diamond has one microwave oscillator 1, one branching waveguide 2 and two plasma generators 3. Said microwave oscillator 1 has an oscillating function at a microwave band (frequency 1000 MHz - 100 GHz) and examples thereof are conventional microwave tubes such as velocity modulation tube, klystron and magnetron. In this invention, especially ISM frequency band can be suitably used among microwaves. The branching waveguide 2 shown in Fig.1(A) is formed so that it uniformly branches the microwave oscillated from microwave oscillator 1 in two directions and leads respective microwaves branched in two directions to respective plasma generators 3. In this exmple, a two-direction branching waveguide is shown which uniformly branches the microwave oscillated from microwave oscillator 1 in two directions. When a microwave of one direction is to be branched to three or more directions, for example, the two-direction branching waveguides are combined and the microwave oscillated from the microwave oscillator is branched into two directions by the first two-direction branching waveguide and one of the branched microwaves is branched into two directions by the second two-direction branching waveguide. As a result, a microwave can be branched into three directions. In this case, the microwave is branched by two two-direction branching waveguides so as to obtain intensities of 2 : 1 : 1. However, it is also possible to branch a microwave into two or more, for example, by employing a T-type branching tube, E-cornered waveguide, three branching tube or combination thereof without using a plurality of the two-direction branching waveguide. Furthermore, not only a microwave can be branched in equal intensity by the combination of branching waveguides, but also it can be branched at a desired intensity ratio. In this invention, excitation of a mixed gas can be attained by leading the microwaves branched by branching waveguide 2 to a plurality of plasma generators 3 connected with said branching waveguide 2 and irradiating the microwaves to the mixed gas of hydrogen and carbon source gas or carbon monoxide gas with the microwaves to produce plasma. As shown in Fig.1(B), the plasma generator 3 has a gas feeding device 5 for feeding a mixed gas of hydrogen gas and carbon source gas, or carbon monoxide gas to reaction chamber 4 and an exhaustion device 6 for pumping the reaction gas after used from reaction chamber 4, this reaction chamber 4 containing a substrate 7 on which diamond is to be deposited. Substrate 7 is, if necessary, heated by a heating furnace and then is used. Substrate 7 is not critical as mentioned above. The feeding amount and pumping amount of the mixed gas can be adjusted by feeding amount adjusting valves 8 and pumping amount adjusting valve 9, respectively. The connection of plasma generator 3 and branching waveguide 2 is performed through an applicator per se known. In this invention, microwave oscillated from said one microwave oscillator 1 is branched so that the irradiation intensity thereof to the respective mixed gas of hydrogen gas and carbon source gas fed to the reaction chamber 4 of the plasma generator 3 is ordinarily at least 0.1 kw, preferably at least 0.2 kw. When the irradiation intensity per one plasma generator is less than 0.1 kw, sometimes, the deposition rate of diamond is low or no diamond is deposited. The following operation is carried out for producing diamond by the synthesis apparatus of this invention. That is, a mixed gas of said hydrogen gas and carbon source gas is fed to reaction chamber 4 of plasma generator 3 shown in Fig.1(B) by gas feeding device 5. Then, microwave oscillated from microwave oscillator 1 and branched by branching waveguide 2 is irradiated to the mixed gas fed to reaction chamber 4 in each plasma generator 3 to produce plasma, thereby to excite the mixed gas. The mixed gas thus in excited state reaches the surface of substrate 7 to deposit diamond on the surface of this substrate. Reaction gas after used is pumped by exhaustion device 6. (D) Reaction of synthesis of diamond In the method of this invention, reaction proceeds under the following conditions and diamond is deposited on the substrate. That is, the reaction temperature of the surface of said substrate varies depending on the excitation means of the raw material gas and cannot be indiscriminately determined. However, for example, when the plasma CVD method is used, it is usually 400-1,000°C, preferably 450-950°C. If this temperature is lower than 400°C, sometimes the deposition rate of diamond is lowered or carbon in excited state is not produced. On the other hand, if it is higher than 1,000°C, sometimes diamond deposited on the substrate is scraped by etching and an improvement of the deposition rate is not attained. The reaction pressure is usually 0.133 Pa - 0.133 MPa (10⁻³ - 10 3' torr), preferably 1.33 Pa - 106.6 kPa (1-800 torr). If the reaction pressure is lower than 0.133 Pa (10⁻³ torr), the deposition rate of diamond is lowered or diamond is not deposited. On the other hand, even if the reaction pressure is increased to higher than 0.133 MPa (10³ torr), the corresponding effect cannot be obtained. (E) Reducing metals In this invention, when carbon monoxide and hydrogen are used as raw material gas, a reducing metal may be used in the reaction system. The reducing metal is preferably at least one metal selected from the group consisting of Group VII metals and Group VIII metals of the periodic table. Specific examples thereof are manganese, iron, nickel, cobalt, platinum and palladium. These may be used alone or in combination of two or more. When two or more reducing metals are used, these metals may be in the form of a mixture or an alloy. A powder of the reducing metal may be allowed to be present previously in the reaction system or may be continuously fed to the reaction system together with the feeding of raw material gas. The reducing metal in this invention can also be used in the form of a molded body made by pressure molding said powder and moreover, in the form of a metal carried on a carrier. The inventors consider the mechanism of formation of diamond as follows. As shown in the above formula (I), hydrogen gas in the raw material gas is excited and becomes a hydrogen radical; as shown in the formula (I), this hydrogen radical reacts with carbon monoxide to produce HCO; and as shown in the formula (III), this HCO reacts with carbon monoxide to produce a CO addition product and this CO addition product is reduced with the reducing metal and oxygen atoms are eliminated to produce diamond. The diamond which can be obtained by the method of this invention can be suitably utilized for various protecting films such as surface protecting film of cutting tools, optical materials, electronic materials, chemical industry materials. According to this invention, diamond can be synthesized at high deposition rate using a mixed gas of carbon monoxide and hydrogen as raw material by any of high radio frequency plasma CVD method, microwave plasma CVD method, hot filament CVD method, chemical transport method, ionization vapor deposition method and ion beam vapor deposition method. Therefore, a method for synthesis of industrially useful diamond which has the following effects can be provided. (1) Complication in preparation of raw material gas or complexity in operation as in case of using hydrocarbons do not occur. (2) Excellent production efficiency. (3) No disorder in crystal structure of diamond. Especially when carbon monoxide and hydrogen are used as raw material gas, the deposition rate of diamond can be further increased. Furthermore, according to this invention, when a carbon source gas and hydrogen gas are used as raw material gas, the progress of microwave oscillated from one microwave oscillator is branched and thus branched microwaves are led to a plurality of plasma generators. Thus, a method for synthesis of diamond and an apparatus for synthesis of diamond having the following various effects can be provided. (1) A plurality of diamond thin films can be simultaneously produced and this is industrially advantageous. (2) Since the microwave is merely branched, the apparatus for synthesis can be simplified. (3) One microwave oscillator is used in common by a plurality of plasma generators and thus the production efficiency is superior. This invention will be further explained by the following examples and comparative examples. Example 1 Using a microwave source of 2.45 GHz in frequency, output was set at 500 W under the conditions of a substrate temperature of 900°C and a pressure of 6.66 kPa (50 torr). Then, into this reaction chamber were introduced carbon monoxide at a flow rate of 5 sccm and hydrogen at a flow rate of 95 sccm and synthesis of diamond by microwave plasma CVD method was effected for 1 hour to obtain a deposit of 10 µm thickness on a substrate controlled to the above mentioned temperature. A silicon wafer was used as the substrate. Raman spectroscopic analysis of the resulting deposit gave a peak due to diamond in the vicinity of 1333 cm⁻¹ of the Raman scattering spectrum. This shows that the deposit is diamond free from impurities. Comparative Example 1 Synthesis of diamond was carried out in the same manner as in Example 1 except that methane gas was used in place of carbon monoxide. As a result, only such deposit as having a thickness of 1.0 µm was obtained and the deposition rate was 1/10 of the deposition rate attained in Example 1. Example 2 Synthesis of diamond was effected in accordance with Example 1 by carrying out the reaction for 1 hour using WC-Co (Co content: 12%) as substrate and under the conditions of substrate temperature of 800°C, a pressure of 5.33 kPa (40 torr), flow rate of carbon monoxide of 30 sccm and flow rate of hydrogen of 70 sccm. A deposit of 6 µm thickness was obtained on the substrate. Raman spectroscopic analysis of the resulting deposit gave a peak due to diamond in the vicinity of 1333 cm⁻¹ of the Raman scattering spectrum. This shows that the deposit was a diamond free from impurities. Comparative Example 2 Synthesis of diamond was effected in the same manner as in Example 2 except that methane gas and carbon monoxide were used in place of carbon monoxide and flow rate of methane gas was 5 sccm and that of carbon monoxide was 3 sccm. As a result, only such deposit of 3 µm thickness was obtained and deposition rate was 1/2 of the rate in Example 2. Example 3 Synthesis of diamond was effected in the same manner as in Example 1 except that a silicon substrate was used and reaction was carried out for 1 hour under the conditions of a microwave output of 400 W, a substrate temperature of 800°C, a pressure of 5.33 kPa (40 torr), a flow rate of carbon monoxide of 80 sccm and that of hydrogen of 20 sccm. A deposit of 12 µm thickness was obtained on the substrate. Raman spectroscopic analysis of the deposit gave a peak due to diamond in the vicinity of 1333 cm⁻¹ of the Raman scattering spectrum which shows that the deposit was diamond free from impurities. Comparative Example 3 Synthesis of diamond was effected in the same manner as in Example 3 except that the flow rate of carbon monoxide was 0.5 sccm and the flow rate of hydrogen was 100 sccm. As a result, the whole surface of the substrate was not covered with a film of diamond even after reaction for 1 hour. Example 4 Methane gas and hydrogen gas were respectively introduced into a reaction tube containing a silicon wafer as a substrate. Then, output of microwave oscillator was set at 2.4 kw and said output was branched into equal two directions by a first two-direction branching waveguide and then said output was further branched into two direction by two second two-direction branching waveguides to branch into equal four outputs after all. 0.6 kw each of them was fed to each reaction tube to perform discharging. The amounts of methane gas and that of hydrogen gas were 0.5 sccm and 100 sccm, respectively. The deposition was carried out for 4 hours in the above state to obtain simultaneously thin films of 2 µm thickness on each of the four substrates. Raman spectroscopic analysis of the resulting thin films showed that all thin films were diamond free from impurities. Example 5 Example 4 was repeated except that acetone was used in place of methane gas to obtain simultaneously thin films of 10 µm thickness on the four substrates. Raman spectroscopic analysis of the thin films showed that all of the thin films were diamond free from impurities.

Claims: A method for the synthesis of diamond which comprises contacting a substrate with a gas obtained by excitation of carbon monoxide and hydrogen, the content of carbon monoxide being at least 1 mol% per total of carbon monoxide and hydrogen. The method according to claim 1, wherein the excitation is effected by a plasma CVD method. A method for the synthesis of diamond which comprises contacting a substrate with a plasma obtained from hydrogen gas and carbon source gas by irradiation with microwave in a plasma generator to deposit diamond on the surface of the substrate wherein progress of a microwave oscillated from one microwave oscillator is branched and thus branched microwaves are respectively led to a plurality of such plasma generators. The method according to any of claims 1 to 3, wherein the substrate is a material selected from the group consisting of metal. cermet and ceramics. The method according to claim 4, wherein the substrate is a material selected from the group consisting of silicon, WC and WC-Co. The method according to any of claims 1 to 5, wherein the reaction temperature of the surface of the substrate is 400 to 1000°C. An apparatus for the synthesis of diamond according to the method of claim 3, which comprises a microwave oscillator (1), a branching waveguide (2) which in use branches the microwave oscillated from said microwave oscillator (1) into a plurality of directions, and a plurality of plasma generators (3) which are connected with said branching waveguide (2) through an applicator such that a mixed gas of hydrogen gas and carbon source gas in use of said waveguide (2) is excited to produce plasma which reaches the substrate (7) contained in each of said plasma generators (3).

Inventors: HOSOYA, IKUO, ITO, TOSHIMICHI, NOSAKA, MASAAKI
Applicants: IDEMITSU PETROCHEMICAL CO. LTD.
Application year: 1988
Main classification: C23C16
IPC codes: C30B25, C23C16, C01B31