Method of and apparatus for synthesizing hard material

Abstract: Disclosed herein is a method of and an apparatus for vapor-phase synthesizing a hard material by supplying raw material gas into a reaction tube (6) and introducing microwaves (18) of a prescribed frequency into a region of the reaction tube (6) for causing reaction for synthesizing the hard material along a prescribed direction, thereby generating plasma. In the reaction tube (6), at least two plate electrodes (17a, 17b, 19a, 19b) are oppositely arranged in parallel vertically to electric fields of the microwaves (18), so that the plasma is excited between the plate electrodes (17a, 17b, 19a, 19b) for vapor-phase synthesizing the hard material. According to the method of and apparatus for synthesizing a hard material, the microwaves (18) of high electric power are introduced into the reaction tube (6) by a waveguide (5) without loss, so that strong electric fields can be homogeneously and stably distributed between the opposite plate electrodes.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of and an apparatus for synthesizing a hard material, such as diamond or cubic boron nitride, which is applied to a tool, a thermal conductor or a semiconductor material, and more particularly, it relates to a method of and an apparatus for homogeneously vapor-phase synthesizing a high-quality hard material at a high rate over a wide area. Description of the Background Art Hard materials such as diamond, cubic boron nitride and wurtzite boron nitride are widely applied to tools such as cutting tools, wear-resistant components, abrasive grains and the like due to high hardness. These materials are also applied to heat radiation substrates for semiconductor elements due to high thermal conductivity. Further, development has been made in order to apply these materials to optical elements and semiconductor materials for effectuating light transmissivity thereof. These materials, which are stable under high pressure as high pressure phase materials, were artificially synthesized only under superhigh pressure at the beginning, while various methods have been recently developed for synthesizing such hard materials, including diamond, in decompressed vapor phases. Among such methods of vapor-phase synthesizing hard materials, a microwave plasma CVD process of generating plasma by microwaves for decomposing gas and depositing a hard material on a substrate is an excellent method which can synthesize a hard material of high purity. Japanese Patent Laying-Open Gazettes Nos. 58-110494, 59-3098 and 59-63732, U.S. Patent No. 4,434,188 and the like disclose methods of synthesizing hard materials through the microwave plasma CVD process. Fig. 1 shows a conventional apparatus for vapor-phase synthesizing diamond through microwaves. This apparatus is now schematically described. Referring to Fig. 1, microwaves oscillated by a magnetron oscillator 1 are applied to a substrate 8, which is placed on a support 7 provided in a reaction tube 6, through an isolator 2, a power monitor 3, a tuner 4 and a waveguide 5. Raw material gas, which is prepared by mixing methane gas, hydrogen etc. at a prescribed ratio, for example, is introduced from a gas inlet port 9 and exhausted from an exhaust port 10 through a pump (not shown). The discharge of the raw material gas is so adjusted as to maintain the reaction tube 6 at prescribed pressure. Plasma 11 is generated by the energy of the microwaves, to form diamond on the substrate 8. A cooling water supply tube 13 supplies an applicator 12 with cooling water, which in turn is discharged from a cooling water discharge tube 14, in order to prevent excessive heating of the reaction tube 6. The waveguide 5 implements optimum conditions of compatibility by movement of a plunger 15 or adjustment of the tuner 4 in response to the wavelength of the introduced microwaves. Referring to Fig. 1, the raw material gas is introduced into and exhausted from the apparatus along arrows A and B respectively. Japanese Patent Laying-Open Gazette No. 61-174378 or the like also reports a method of synthesizing cubic boron nitride through such a microwave plasma CVD apparatus. In general, electromagnetic waves in a frequency domain of 300 MHz to 300 GHz are called microwaves. The electromagnetic waves of this frequency domain are easily absorbed by molecules, and have wavelengths of 1 m to 1 mm, which are shorter as compared with the size of a reaction tube. Thus, it is possible to efficiently generate plasma in gas under a wide range of pressure. Further, the microwaves of this frequency domain can efficiently supply electric power by a waveguide having a practical size of not more than scores of centimeters, with smaller problems of loss and leakage in a supply path of a coaxial cable etc., dissimilarly to waves of a lower frequency domain. However, a method of and an apparatus for synthesizing a hard material through the aforementioned conventional microwave plasma CVD apparatus have the following problems: While microwaves of 2.45 GHz in frequency are widely industrially employed, standing waves are caused in a cycle of a half-wave length thereof if the microwaves are trapped in a reaction tube. Thus, plasma is heterogeneously generated in response to strength distribution of the standing waves. Therefore, it is impossible to homogeneously generate plasma over a wide area for synthesizing a hard material. The microwaves have such a property that the same can stably generate plasma under higher gas pressure and improve the growth rate of the hard material as the frequency thereof is increased. When the frequency of the microwaves is thus increased, however, the wavelength is shortened to reduce areas of large amplitude portions of the standing waves. In order to improve such growth heterogeneity in vapor synthesis of a hard material, there have been reported a method of applying a dc magnetic field for controlling the streamline of plasma and a method of reducing gas pressure in a reaction tube in Japanese Patent Laying-Open Gazette No. 63-107899. In any of these methods, however, mean free paths of active molecule species and electrons contained in the plasma must be kept long. Thus, gas pressure in the reaction tube must be reduced to not more than 1.33 kPa (10 Torr), and the hard material is grown at an extremely slow rate of less than 1µm/h. Thus, the conventional method of synthesizing a hard material such as diamond through the plasma CVD process has been significantly restricted in growth area and growth rate, and it has been difficult to mass-synthesize the hard material. EP-A-0 279 895 discloses an apparatus for generating a plasma in a tube using microwaves and a magnetic field. The substrate is placed below a chamber defined by conductive plates thereby allowing to move the substrate through the tubes, the magnetic field being essential for the operation of the disclosed apparatus. SUMMARY OF THE INVENTION An object of the present invention is to provide a method of and an apparatus for vapor-phase synthesizing a hard material such as diamond on the surface of a wide-area substrate at a high growth rate by enabling generation of high-concentration plasma over a wide range. The inventive method of synthesizing a hard material is adapted to vapor-phase synthesize a hard material by supplying raw material gas into a reaction tube and introducing microwaves of a prescribed frequency into a region of the reaction tube for causing reaction for synthesizing the hard material along a prescribed direction. The feature of this synthesizing method resides in that at least two plate electrodes are oppositely arranged in parallel vertically to the electric fields of the microwaves and plasma is excited between the plate electrodes for vapor-phase synthesizing a hard material. According to this method of synthesizing a hard material, it is possible to introduce microwaves of high electric power into the reaction tube by a waveguide at small loss, as well as to homogeneously and stably distribute strong electric fields between the opposite plate electrodes. The plasma generated by the electric fields is stable under high reaction pressure, and homogeneous over a wide area. Although the inventive method of synthesizing a hard material is similar to a synthesizing method employing a plane-parallel plate electrode type plasma CVD apparatus using a high frequency which is lower than the frequency of microwaves (refer to U.S. Patent No. 4,414,085, for example), it is difficult to stably generate strong plasma under relatively high pressure of at least 100 Torr in a frequency domain lower than that of the microwaves, and this method is improper for synthesis of a hard material. When the frequency is lower than that of the microwaves, further, significant problems are caused such as loss in an inlet path for a coaxial cable etc. and power reflection caused by impedance unconformity at an inlet port for a reaction tube. The directions of electric fields and magnetic fields of microwaves which are guided by a waveguide depend on the configuration of the waveguide. The counter electrodes must necessarily be arranged vertically to the direction of the electric fields of the microwaves, so that strong electric fields can be generated between the counter electrodes. Further, the microwaves must be introduced so that the same progress in parallel with the electrodes. The inventive apparatus for synthesizing a hard material comprises a reaction tube, means for supplying raw material gas into the reaction tube, and means for generating plasma by introducing microwaves in a prescribed direction in the vicinity of a base material provided in the reaction tube for depositing a hard material. According to the inventive apparatus for synthesizing a hard material, the reaction tube is provided therein with at least two plate electrodes which are arranged in parallel vertically to electric fields of the microwaves through a region provided with a base material substrate for depositing the hard material. It is possible to provide such counter electrodes in a large number exceeding two. Such a large number of counter electrodes themselves may serve as base materials for growing the hard material, or a base material may be arranged between each adjacent pair of these electrodes, so that the hard material can be grown over a wider area. Thus, the yield of the hard material can be increased. Alternatively, fine particles can be arranged between the counter electrodes as base materials, thereby easily synthesizing abrasive grains of the hard material. The space between the opposite plate electrodes is preferably at least 5 mm, in order to facilitate flow of reaction gas. More preferably, the space is not more than a distance of about the wavelength of the microwaves. At least the reaction tube is preferably prepared from a material such as quartz, alumina, beryllia or boron nitride, which has small microwave transmission loss. The plate electrodes are preferably prepared from a heat-resistant metal material having a high microwave reflection factor, such as SUS, Mo or W, for example. If the plate electrodes also serve as the base materials for growing the hard material, the base materials are prepared from a conductive material such as a metal or semiconductor material. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view typically showing a conventional vapor synthesizing apparatus employed for vapor-phase synthesizing diamond; Fig. 2 is a sectional view typically showing an apparatus for synthesizing a hard material employed in Example 1 of the present invention; Fig. 3 is a sectional view typically showing the structure of an apparatus for synthesizing a hard material employed in Example 3 of the present invention; Fig. 4 is a sectional view typically showing the structure of an apparatus for synthesizing a hard material employed in Example 4 of the present invention; and Fig. 5 is a sectional view taken along the line D - D in Fig. 4. DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the present invention are now described with reference to Figs. 2 to 5. Example 1 According to the present invention, a hard material is vapor-phase synthesized by a basic apparatus shown in Fig. 2. Referring to Fig. 2, microwaves 18 are first applied through a waveguide 5 to a space defined between two base materials 17a and 17b which are oppositely placed on a support 16 provided in a reaction tube 5. While no detail of microwave generation means is shown in Fig. 2, the microwaves 18 are oscillated by a magnetron oscillator 1 and introduced into the reaction tube 6 through an isolator 2, a power monitor 3, a tuner 4 and the waveguide 5, similarly to the conventional apparatus shown in Fig. 1. The two base materials 17a and 17b, which are made of silicon, are arranged vertically to the direction (shown by arrows C) of electric fields of the microwaves 18, to serve as counter electrodes. Plasma 11 is generated by the microwaves 18 which are introduced between the base materials 17a and 17b, whereby raw material gas is vapor-phase synthesized to simultaneously form hard material films on the base materials 17a and 17b. A cooling water supply tube 13 supplies an applicator 12 with cooling water, which in turn is discharged from a cooling water discharge tube 14, in order to prevent arrows A and B respectively. Such an apparatus was employed in Example 1 for synthesizing diamond under the following concrete conditions: The reaction tube 6 was 60 mm in inner diameter and the base materials 17a and 17b were prepared from silicon discs of 50 mm in diameter and 3 mm in thickness, at a spacing of 25 mm. reaction gas, containing H₂ and CH₄ in the ratio 10:2, was supplied into the reaction tube 6, and the discharge thereof was so adjusted that the pressure in the reaction tube 6 was 60 Torr. Input power of the microwaves was 600 W. As the result, diamond films were formed on both of the base materials 17a and 17b over ranges of 40 mm in diameter at a high growth rate of 5 µm/h. Such an effective result was obtained since the plasma 11 was concentrated between the base materials 17a and 17b. Although the base materials 17a and 17b were prepared from silicon in Example 1, a material for such base materials 17a and 17b, also serving as counter electrodes, may alternatively be prepared from a carbon material, a conductive metal material, or a semiconductor material other than Si, such as SiC or Ge, for example. Such a material may be applied onto the surface of an insulating material. Carbon source gas for serving as the reaction gas may be prepared from hydrocarbon other than CH₄, alcohol, ketone or the like. Alternatively, solid carbon may be decomposed in a vapor phase to be supplied as a raw material. Example 2 An apparatus employed in Example 2 and concrete conditions were substantially common with those of Example 1. Example 2 was different from Example 1 in a point that reaction gas containing H₂ and CH₄ in the ratio 100:2 was supplied and Ar was added to the same while its discharge was so adjusted as to increase the pressure in a reaction tube 6 to 100 Torr. As the result, it was possible to synthesize diamond films at a growth rate of 10 µm/h. A similar effect can be obtained by adding inactive gas such as He, Ne, Kr, Xe or Rn in place of Ar for similarly stabilizing the plasma. Example 3 As shown in Fig. 3, microwaves were supplied to two portions of one reaction tube 6 with input power of 600 W, to vapor-phase synthesize diamond films on two pairs of base materials 17a and 17b under conditions identical to those of Example 1. As the result, it was possible to synthesize diamond films on the base materials 17a and 17b over ranges of 30 mm in diameter at a rate of 5 µm/h. Example 4 Example 4 was carried out through an apparatus shown in Fig. 4. Referring to Fig. 4, a reaction tube 6 and microwave generation means are common to those of Example 1. In Example 4, no base materials were employed as counter electrodes but a quartz jig 20 was formed by a cylindrical member having counter electrodes 19a and 19b on both end surfaces thereof. Fig. 5 is a sectional view taken along the line D - D in Fig. 4. As shown in Fig. 5, the quartz jig 20 has small partition plates 21, so that diamond abrasive grains 22 provided in its interior as base materials can be stirred by rotating the quartz jig 20 about a shaft 23, which is coaxial with a reaction tube 6, along an arrow E. Such an apparatus was employed for vapor-phase synthesizing diamond under conditions identical to those of Example 2. As the result, the diamond abrasive grains of 100 µm in diameter, which were provided in the quartz jig 20, were grown into particles of 200 µm in diameter within one hour. Example 5 Similarly to Example 1, the apparatus shown in Fig. 2 was employed and three silicon discs of 50 mm in diameter and 3 mm in thickness were arranged as base materials vertically to the direction C of electric fields of microwaves 18 under the same conditions of reaction gas and microwaves as Example 1. Diamond was synthesized under pressure of 20 Torr so that the overall base materials were covered with plasma. As the result, diamond films were formed over the entire surfaces of the base materials of 50 mm in diameter at a growth rate of 0.2 µm/h. When the pressure was increased to 40 Torr, the mean growth rate of diamond was reduced to 0.05 µm/h. When the pressure was further increased to 50 Torr, diamond was merely partially grown on the base materials. According to Example 5, it was possible to homogeneously form diamond films on all of three or more base materials under pressure of not more than 40 Torr, thereby increasing the amount of generation of diamond. While carbon source gas and hydrogen were mixed to prepare raw material gas and inactive gas was further added thereto in the aforementioned Examples, it has been experimentally confirmed that such a well-known fact that the quality of diamond is improved with increase of transmissivity of the synthesized diamond etc. by employing or adding a material containing oxygen as or to the raw material gas is also effective in the present invention. The material containing oxygen can be prepared from alcohol, ketone, ether, O₂, H₂O, CO, CO₂, NO₂, NO, O₃ or the like, for example. Example 6 A synthesizing apparatus similar to that shown in Fig. 2 was employed and two Si base materials were arranged also similarly to Example 1, while reaction gas containing H₂, Ar, NH₃ and B₂H in the ratios 100:40:10:3 was supplied and the pressure was maintained at 25 Torr. Microwaves of 800 W were introduced into the reaction gas and reaction was caused for 20 hours, whereby films of a hard material were grown on both base materials over ranges of 25 mm in diameter. The central portions of the hard material films were at the maximum thickness of 7 µm. The crystal structure of this hard material was examined by X-ray diffraction, whereby it was found that the material was made of cubic boron nitride.

Claims: A method of vapor-phase synthesizing a hard material, comprising the steps of: supplying raw material gas into a reaction tube (6); and introducing microwaves (18) of a prescribed frequency into a region of said reaction tube (6) for causing reaction for synthesizing a hard material along a prescribed direction thereby generating plasma (11); at least two mutually insulated plate counter electrodes (17a, 17b; 19a, 19b) being oppositely arranged in parallel vertically to electric fields of said microwaves so that strong electric fields are generated between the counter electrodes for vapor-phase synthesizing said hard material by exciting said plasma (11) between said plate counter electrodes (17a, 17b; 19a, 19b) and wherein the microwaves are introduced so that the same progress in parallel with the counter electrodes. A method of synthesizing a hard material in accordance with claim 1, wherein two opposite silicon plates are arranged as base materials (17a, 17b), so that said base materials (17a, 17b) also serve as said plate counter electrodes. A method of synthesizing a hard material in accordance with claim 1, wherein hydrogen is added to carbon source gas for preparing raw material gas, while inactive gas is further added thereto for synthesizing diamond as said hard material. A method of synthesizing a hard material in accordance with claim 1, wherein a material containing oxygen is employed as said raw material gas. A method of synthesizing a hard material in accordance with claim 1, wherein a material containing NH₃ and B₂H₆ is employed as said raw material gas, for synthesizing cubic boron nitride. An apparatus for synthesizing a hard material, comprising: a reaction tube (6); means for supplying raw material gas into said reaction tube (6); and means for introducing microwaves (18) along a prescribed direction in the vicinity of base materials (17a, 17b, 22) provided in said reaction tube (6) for depositing a hard material, thereby generating plasma (11), at least two mutually insulated plate counter electrodes (17a, 17b; 19a, 19b) being oppositely arranged in parallel vertically to electric fields of said microwaves so that in use strong electric fields are generated between the counter electrodes through a region of said reaction tube (6) being provided with said base materials (17a, 17b, 22) for depositing said hard material and that in use the introduced microwaves progress in parallel with the counter electrodes. An apparatus for synthesizing a hard material in accordance with claim 6, wherein said base materials (17a, 17b) are formed by two opposite flat planes which are arranged in parallel on a support (16) vertically to the direction of said electric fields of said microwaves, said flat plates containing a conductive material and also serving as said plate counter electrodes. An apparatus for synthesizing a hard material in accordance with claim 6, wherein at least the material for said reaction tube (6) is prepared from any one of quartz, alumina, beryllia and boron nitride. An apparatus for synthesizing a hard material in accordance with claim 7, wherein said base materials (17a, 17b) are formed of silicon plates. An apparatus for synthesizing a hard material in accordance with claim 7, wherein said base materials (17a, 17b) are formed of any one of a semiconductor material other than silicon, a carbon material and a conductive metal material. An apparatus for synthesizing a hard material in accordance with claim 7, wherein said base materials (17a, 17b) are prepared by coating the surfaces of plates of an insulating material with any one of a carbon material, a conductive metal material and a semiconductor material. An apparatus for synthesizing a hard material in accordance with claim 6, further comprising a hollow cylindrical member (20) having said opposite plate electrodes (19a, 19b) on both ends thereof and means for rotating said cylindrical member (20) about its central axis. An apparatus for synthesizing a hard material in accordance with claim 12, wherein said cylindrical member (20) is provided in its inner periphery with partition plates (21) which are arranged at regular intervals along its normal direction. An apparatus for synthesizing a hard material in accordance with claim 6, wherein said plate electrodes (17a, 17b) are made of any one of stainless steel, molybdenum and tungsten. An apparatus for synthesizing a hard material in accordance with claim 6, further including exhaust means for decompressing said reaction tube (6). An apparatus for synthesizing a hard material in accordance with claim 12, wherein said base materials are prepared from fine particles.

Inventors: FUJIMORI NAOJI C O ITAMI WORKS, IMAI TAKAHIRO C O ITAMI WORKS, OTA NOBUHIRO C O ITAMI WORKS, SHIOMI HIROMU C O ITAMI WORKS, FUJIMORI, NAOJI, C/O ITAMI WORKS, IMAI, TAKAHIRO, C/O ITAMI WORKS, OTA, NOBUHIRO, C/O ITAMI WORKS, SHIOMI, HIROMU, C/O ITAMI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: C23C16
IPC codes: C23C16