Diamond synthesizing apparatus

Abstract: A diamond synthesizing apparatus comprises a reaction tube for internally making reaction for vapor-phase synthesizing diamond, and plasma generation means for generating prescribed microwave plasma in this reaction tube. The reaction tube is coupled with a gas reservoir, which defines a circulation system with the reaction tube. The circulation system comprises means for evacuating its interior. This circulation system further comprises means for circulating raw material gas, which is a compound containing carbon, in a sealed state, and raw material gas supply means for intermittently or continuously supplying the raw material gas into the circulation system. According to this diamond synthesizing apparatus, it is possible to suppress pressure fluctuation and composition fluctuation following reaction and gas addition to the minimum and to vapor-phase synthesize diamond in extremely restricted ranges of gas composition and pressure. Further, consumption of the raw material gas is extremely reduced and gas concentration in the reaction tube is extremely slowly and smoothly changed in gas supply, whereby it is possible to suppress influence exerted by the changes in gas concentration to the film quality.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a diamond synthesizing apparatus, and more particularly, it relates to a diamond synthesizing apparatus which provides a technique for vapor-phase synthesizing high-quality diamond in microwave plasma at a low cost. Description of the Background Art A technique for vapor-phase synthesizing diamond was started by Derjaguin (USSR Inv. Certif. No. 339134, 1958) in 1956 and by Eversole (USP Nos. 3030187 and 3030188) in 1958, and has been vigorously developed since 1982, when study was made by Matsumoto et al. (Japanese Journal of Applied Physics, Vol. 21, 1982, L183 to L185). A microwave plasma CVD process of synthesizing diamond in plasma excited by microwaves was invented by Kamo et al. (Journal of Crystal-Growth, 62, 1983, pp. 642 - 644) in 1983. According to this process, it is possible to obtain a film containing no impurity due to electrodeless discharge. Thus, the cost for such a film has been reduced by increase in formation area and growth rate. Such a conventional method of vapor-phase synthesizing diamond in plasma has been basically carried out through the so-called open type apparatus which is adapted to circulate no material gas, as shown in Fig. 1. Referring to Fig. 1, a raw material gas supply system 1 supplies raw material gas, which is prepared by mixing about 1 to 2 % of carbon source gas with about 99 % of medium gas, into a reaction tube 3 through a raw material gas supply valve 2. The carbon source gas is prepared from hydrocarbon such as methane, ethane, propane, acetylene or benzene, alcohol such as methanol, ethanol or propanol, organic acid such as acetic acid, ketone such as acetone, ester such as methyl acetate, carbon monoxide, carbon dioxide, or the like. The medium gas is prepared from hydrogen or oxygen. The carbon source gas and the medium gas are reacted in the plasma for synthesizing diamond, while merely 1 to 2 % of the carbon source gas employed for such reaction is converted into diamond in practice. The raw material gas, including unreacted carbon source gas, once subjected to the reaction is entirely exhausted by an exhaust system 5 through an exhaust valve 4. Further, the pressure in a vacuum vessel is regulated by controlling the raw material gas supply system 1 and the exhaust system 5. Plasma generation means 6 generates the plasma in the reaction tube 3. This plasma generation means 6 is mainly formed by a microwave oscillator 7 and a waveguide 8. In general, the microwave oscillator 7 oscillates microwaves of 2.45 GHz in frequency for synthesizing diamond, while a tuner 10 and a plunger 11 adjust the waveguide 8, which is monitored by a power monitor 9, in order to conform the same to the frequency of the microwaves. It has been confirmed possible to obtain film type or thin-plate type diamond of high quality, performance of which is equivalent to that of natural diamond or bulk type diamond synthesized by a superhigh pressure synthesizing method, in accordance with diamond vapor-phase sinthesis, particularly the microwave plasma CVD process, by restricting synthesis conditions. In more concrete terms, it has been possible to obtain diamond having Vickers hardness of 10000, which is equivalent to that of a single crystal of diamond. Further, it has also been possible to attain thermal conductivity of 16 W/cm·K, which is by no means inferior to the value 22 W/cm·K of a single crystal of type IIa diamond and the value 11 W/cm·K of type Ia diamond. Further, a heat sink or a rear portion is used in the form of a plate-type film or a film provided on a base material. Thus, it is possible to considerably save time for working according to the vapor-phase synthesizing method, which is adapted to already define a film or a plate having a thickness of several to hundreds of millimeters in film formation. On the other hand, a step of cutting diamond in the form of a plate by laser cutting or the like is required when single-crystalline diamond is employed. In the existing circumstances, however, sufficient results cannot be obtained as to the vapor-phase synthesized diamond, due to problems in cost. To this end, reduction in cost is studied in every aspect, in relation to extension of the formation area and increase of the growth rate. In order to extend the formation area in the microwave plasma CVD process, study has been made as to the method of generating plasma in various ways, in order to extend the formation area. In general, the formation area is extended by the following two means: The first means employs an apparatus using a large-diameter microwave plasma process, as shown in Fig. 2. According to this means, introduced microwaves are enlarged by a tapered waveguide 21 so that plasma 23 is generated in a bell jar 22 by the enlarged microwaves, thereby vapor-phase synthesizing a diamond film on a wafer 26 provided on a holder 25 which is rotated by a rotation mechanism 24. According to this means, the wafer 26 is prepared from a silicon substrate of 4 inches in diameter, and the growth rate is about 0.2 µm/hr. The second means employs an apparatus using an ECR plasma process, as shown in Fig. 3. According to this means, electromagnetic coils 33 applies magnetic field to microwaves which are enlarged from a waveguide 31 of a small sectional area having a rectangular section into a trapezoidal cylindrical waveguide 32. Thus, ECR plasma 36 is generated in the vicinity of the surface of a substrate 35 which is heated by a heater 34, thereby vapor-phase synthesizing a diamond film on the surface of the substrate 35. When this means is employed, the substrate 35 is prepared from a silicon substrate of 2 inches in diameter, with a diamond film growth rate of about 0.1 µm/hr. When the aforementioned open type vapor-phase synthesizing apparatus is employed, however, the film quality is extremely deteriorated in the means for extending the film forming area shown in Figs. 2 and 3. Although attempts have been made to increase the pressure, concentration of fuel gas and the like in order to increase the growth rate, in particular, no success has been attained due to instability of the plasma and deposition of a non-diamond component such as graphite. While an attempt has also been made to rotate the substrate in order to homogenize the plasma, portions of relatively good quality overlap with defective portions, to reduce the film quality as a whole. Further, the film quality tends to be lowered as the same is separated from the central portion of the plasma. These techniques are still in the stages of development in the existing circumstances. According to these means, the growth rate is generally limited to about 1 µm/hr. under pressure of 40 Torr. Area increase and high-speed film formation are unlimited, and values of at least 4 inches in diameter and about 10 µm/hr. are regarded preferable. The conventional open type apparatus cannot possibly attain such values. Other problems of the conventional open type vapor synthesizing apparatus for vapor-phase synthesizing diamond are as follows: Merely a few percent of the supplied carbon source gas is subjected to reaction, and the remaining part thereof is exhausted from the system in an unreacted state. Further, the medium gas, which is required to occupy approximately 99 % of the overall raw material gas, is finally exhausted in an unreacted and unchanged state. In other words, this apparatus requires the raw material gas in an amount 5000 times that of the carbon source gas which is converted into diamond in practice. Thus, the material expense takes the greater part of the cost for diamond which is vapor-phase synthesized at a relatively small equipment cost. Namely, it is necessary to effectively use the raw material gas, in order to reduce the material expense. On the other hand, it has been confirmed that fluctuation of the pressure and that of raw material composition exert bad influence on synthesis of a high quality film, while valves of the supply and exhaust systems must be driven in complicated operation for regulating the pressure and the raw material composition, and it is difficult to stabilize conditions for long-time operation. Thus, the valves must be finely controlled by an operator, leading to a high cost. If such pressure regulation is automated, the equipment cost is extremely increased to raise up the overall cost. Namely, it is also necessary to simplify valve operation and pressure regulation, in order to implement automated or unmanned operation. In order to solve these problems, a chemical reactor may be sealed to circulate raw material gas and supply carbon source gas only for an amount consumed for synthesizing diamond, thereby effectively using the raw material. Further, it is conceivable that no pressure regulation is required in such a sealed system since the reaction space is sealed, and hence valve operation can be simplified. However, in such reaction that carbon source gas is decomposed in plasma to generate diamond, the following reaction (1) generally progresses if the carbon source gas is prepared from hydrocarbon, for example: C x H y → xC + y/2H₂ Due to such reaction, the pressure in the sealed reaction vessel is increased by generated hydrogen gas to exert influence on the plasma state and deteriorate the film quality, except for the case of y = 2. Further, the carbon source gas itself is bulked as expressed in the following formula (2), to cause similar problems: nC x H y → C nx H ny-α + α/2H₂ Thus, it is difficult to stably control the raw material gas concentration or suppress pressure increase by merely sealing the reaction tube 3 of the conventional open type apparatus for vapor-phase sinthesis. SUMMARY OF THE INVENTION An object of the present invention is to provide a diamond synthesizing apparatus which can stably vapor-phase synthesize diamond with smaller consumption of raw material gas at a low cost by stably controlling the pressure of the raw material gas and a plasma state in a sealed circulation system. The inventive diamond synthesizing apparatus comprises a reaction tube for internally making reaction for vapor-phase synthesizing diamond, and plasma generation means for generating prescribed microwave plasma in the reaction tube. The reaction tube is coupled with a gas reservoir which defines a circulation system with the reaction tube. This circulation system comprises means for evacuating its interior. The circulation system further comprises means for circulating raw material gas, which is a compound containing carbon, in a sealed state and raw material gas supply means for intermittently or continuously supplying the raw material gas into the circulation system. According to this diamond synthesizing apparatus, the raw material gas is circulated in the circulation system in a sealed state so that only required raw material gas is intermittently or continuously supplied following reduction of the raw material for vapor-phase synthesizing diamond in the reaction tube, whereby it is possible to suppress pressure fluctuation and composition fluctuation following reaction and gas addition to the minimum and to vapor-phase synthesize diamond with extremely limited ranges of gas composition and pressure. Further, consumption of the raw material gas is so extremely reduced that the amount of the raw material gas can be reduced to not more than 2 % as compared with the conventional open type apparatus. In addition, the circulation system has the gas reservoir and hence the gas concentration in the reaction tube is extremely slowly and smoothly changed in gas supply. Thus, even if the gas is intermittently supplied by a simple carbon source gas supply system which is prepared by combining an electromagnetic valve and a timer, for example, no influence is exerted on the film quality. In another aspect of the present invention, the diamond synthesizing apparatus comprises a plurality of reaction tubes which are arranged in parallel in a circulation system defining a single sealed system, and each of the plurality of reaction tubes is provided with independent plasma generation means for generating prescribed microwave plasma. Due to such structure, it is possible to enlarge capacity for vapor-phase reaction without significantly increasing the equipment cost by increasing only numbers of the reaction tubes and the plasma generation means while maintaining the raw material gas supply system, the exhaust system and the circulation system single. 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 typically illustrates the structure of a conventional open type diamond synthesizing apparatus; Fig. 2 is a sectional view showing a conventional diamond synthesizing apparatus using a large-diameter microwave plasma process; Fig. 3 is a sectional view showing a conventional diamond synthesizing apparatus utilizing an ECR plasma process; Fig. 4 typically illustrates the structure of a diamond synthesizing apparatus according to a first embodiment of the present invention; and Fig. 5 typically illustrates the structure of a diamond synthesizing apparatus according to a second embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention is now described with reference to Fig. 4. Referring to Fig. 4, a raw material gas supply valve 2 of a raw material gas supply system is closed and an exhaust valve 4 of an exhaust system 5 is opened first, to evacuate a reaction tube 3 and a gas reservoir 13 defining a circulation system 1. Then the exhaust valve 4 is closed and the raw material gas supply valve 2 is opened to supply raw material gas until prescribed pressure is attained. Carbon source gas and medium gas are supplied in a prescribed ratio to prepare the raw material gas. The pressure in the circulation system 12 is regulated by controlling the raw material gas supply system 1 and the exhaust system 5. The gas reservoir 13 is coupled to a carbon source gas supply system 14, and supplied with the carbon source gas through a carbon source gas supply valve 15 at any time. When all of the raw material gas supply valve 2, the exhaust valve 4 and the carbon source gas supply valve 15 are closed, the circulation system 12 is cut off from the exterior to define a sealed system, while the raw material gas sealed in the circulation system 12 is circulated therein by a circulation pump 16. Plasma generation means 6 generates plasma in the reaction tube 3. The plasma generation means 6 is mainly formed by a microwave oscillator 7 and a waveguide 8, and the microwave oscillator 7 generates microwaves of 2.45 GHz in general. The waveguide 8 is adjusted by a tuner 10 and a plunger 11 while being monitored by a power monitor 9, to be conformed with the frequency of the microwaves. The apparatus according to this embodiment was employed for vapor-phase synthesizing diamond under the following concrete conditions: The apparatus was evacuated first, and then the raw material supply valve 2 was opened to supply 10 ml/min. of methane gas and 1000 ml/min. of hydrogen gas into the reaction tube 3 until the pressure in the circulation system 12 reached 40 Torr (5,3 kPa), and thereafter the raw material supply valve 4 was closed to seal the circulation system 12. The reaction tube 3 was prepared from a cylindrical silica tube of 40 mm in inner diameter. The microwave output was about 400 W. It is noted that the microwave output is not restricted to the general range of 300 to 500 W. Optimum film forming conditions, which are conceivably influenced by positional relation between the substrate and the plasma etc., are not univocally decided. As to relation between the configuration of the reaction tube and the microwave output, higher output is required as the thickness of the reaction tube is increased, while it is conceivable that the diameter of the reaction tube is limited to 40 to 60 mm in the apparatus of this embodiment, in consideration of leakage of the microwaves and the like. Then the plasma generation means 6 was driven to generate plasma in the reaction tube 3, thereby starting synthesis of diamond. Thereafter the carbon source gas supply valve 15 was opened every hour to intermittently introduce the methane gas. The amount of the intermittently introduced methane gas was decided on the basis of an experimentally obtained value of variation in methane gas concentration in the circulation system 12. As the result of such diamond synthesis, the pressure in the system was raised up to 41.5 Torr (5,3 kPa) after a lapse of hours from starting of vapor-phase sinthesis. The pressure in the circulation system 12 was thus changed from 40 to 41.5 Torr (5,3 to 5,5 kPa) since the methane gas serving as the raw material was decomposed and bulked with generation of the diamond film as hereinabove described, to cause pressure increase by generation of hydrogen. Thereafter the plasma was stopped and the sample was extracted after evacuation, to be subjected to SEM observation and Raman spectrum analysis. As the result, it was confirmed that diamond, which compared favorably with that synthesized by a conventional open type vapor-phase synthesizing apparatus with valve adjustment performed by an operator, was synthesized. Deterioration of film quality can be clearly observed through a break of a diamond crystal configuration in SEM observation. In general, diamond is in the form of a hexahedron or a hexa-octahedron, while high-quality diamond has flat planes as well as linear and clear contours. If the film quality is deteriorated, however, the planes and contours are rounded and slackened. The amount of the used raw material gas was calculated on trial, to find that diamond of about 10 mm square and 50 µm thick was synthesized with raw material gas of 1.4 % of that in the open type apparatus. A problem of pressure fluctuation in this embodiment is considered as follows: In a conventional method, the pressure is regulated by controlling the amount of supply of raw material gas and hydrogen gas by a mass flow controller and controlling the gas discharge by operation of a valve on a vacuum pump side. Thus, it can be said that composition fluctuation is hardly caused. On the other hand, pressure fluctuation is about ± 2.5 % in general, depending on performance of the valve and the vacuum pump. According to the method of this embodiment, on the other hand, pressure fluctuation and composition fluctuation are caused by generation of hydrogen gas caused by decomposition of the raw material gas, as hereinabove described. The widths of such fluctuation are determined by the amount of generation of diamond and the capacity of a reservoir tank. If diamond of about 10 mm square and 50 µm thick is generated as hereinabove described and about the same amount of carbon is decomposed and deposited on a substrate support material or an inner wall of a reaction tube, the pressure is raised up from 40 Torr to 41.8 Torr (5,3 to 5,57 kPa) when the capacity of the reservoir tank is 100 lit. As to composition fluctuation, concentration of the methane gas is changed from 1 % to 0.96 % when only an amount of the consumed methane gas is added. Namely, the width of fluctuation is suppressed to 4.5 %. The pressure fluctuation must be suppressed within a range of ± 5 %. If the fluctuation is out of this range, the plasma state and the substrate temperature are changed out of optimum conditions, to deteriorate the film quality. The composition fluctuation leads to no deterioration of the film quality but causes reduction in film forming rate if concentration of the raw material gas such as methane gas is reduced. If concentration of the raw material gas is increased, on the other hand, it causes deterioration of the film quality even if the increase is small. In experience, the pressure fluctuation must be suppressed within a range of -10 % to 0 %. According to this embodiment, as hereinabove described, the circulation system 12 defined by the reaction tube 3 and the gas reservoir 13 is sealed and the raw material gas is circulated to vapor-phase synthesize diamond in the plasma, whereby consumption of the raw material gas can be greatly decreased to reduce the cost. Further, since the raw material gas is circulated in the sealed system which can easily maintain constant pressure and a small amount of methane gas is intermittently supplied, pressure fluctuation and composition fluctuation following reaction and gas addition can be suppressed to the minimum and it is possible to automatically and stably synthesize high-quality diamond, which can be synthesized in extremely restricted ranges of gas composition and pressure, without requiring manual operation. A second embodiment of the present invention is now described with reference to Fig. 5. The apparatus of this embodiment comprise material gas supply system 1, an exhaust system 5, a circulation system 12 and a carbon source gas supply system 14, commonly to the aforementioned first embodiment. This embodiment is different from the first embodiment in that a plurality of reaction tubes 3 and a plurality of plasma generation means 6 are arranged in parallel in the circulation system 12. The plasma generation means 6 are provided in one-to-one correspondence to the reaction tubes 3, similarly to the apparatus shown in Fig. 4. Referring to Fig. 5, each of the plasma generation means 6 is illustrated in a manner viewed from a right side of the plasma generation means 6 shown in Fig. 4. The apparatus of this embodiment was employed to synthesize diamond under concrete conditions similar to those for the aforementioned first embodiment. The amount of intermittent supply of methane gas in this embodiment was controlled/adjusted by a gas supply mass flow controller. As the result of such diamond synthesis, the pressure in the circulation system 12 reached 41.7 Torr (5,55 kPa) after a lapse of 50 hours from starting of synthesis. Thereafter plasma was stopped and the sample was extracted after evacuation and subjected to SEM observation and Raman spectrum analysis, to confirm that diamond, which compared favorably with that synthesized by a conventional open type apparatus with valves controlled by an operator, was synthesized. The amount of the used raw material was calculated on trial, to find that diamond was synthesized with raw material gas of 1.78 % of that in the open system apparatus. According to Raman spectrum analysis, observed are a sharp diamond peak appearing at 1333 cm⁻¹, a signal of amorphous carbon smoothly appearing at 1500 to 1700 cm⁻¹, and relatively broad peaks of graphite appearing at 1360 cm⁻¹ and 1600 cm⁻¹. It can be said that only a diamond peak appears in a high-quality film by Raman spectrum analysis. If a signal of amorphous or graphite appears, it means that the film quality is deteriorated. According to this embodiment, as hereinabove described, it is possible to increase only the numbers of the reaction tubes 3 and the plasma generation means 6 while maintaining the raw material gas supply system 1, the exhaust system 5 and the circulation system 12 single, thereby greatly reducing the equipment cost. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims: A diamond synthesizing apparatus comprising: a reaction tube (3) for internally making reaction for vapor-phase synthesizing diamond; plasma generation means (6) for generating prescribed microwave plasma in said reaction tube (3); a gas reservoir (13) coupled with said reaction tube (3) for defining a circulation system (12) with said reaction tube (3); means (5) for evacuating said circulation system (12); means (16) for circulating raw material gas, being a compound containing carbon, in a circulation system (12) which is cut off from the exterior to define a sealed system; raw material gas supply means (1) for intermittently or continuously supplying said reaction gas into said circulation system (12). A diamond synthesizing apparatus in accordance with claim 1, wherein said raw material gas supply means (1) is formed by a raw material gas supply system for mixing carbon source gas and medium gas and supplying the mixture into said circulation system (12), and a carbon source gas supply system (14) for supplying said carbon source gas into said gas reservoir (13). A diamond synthesizing apparatus in accordance with claim 1, wherein said plasma generation means (6) is formed by a waveguide (8) oppositely holding a position of said reaction tube (3) for generating said microwave plasma and a microwave oscillator (7) connected to one end of said waveguide (8). A diamond synthesizing apparatus in accordance with claim 3, wherein said plasma generation means (6) is further formed by a power monitor (9) for monitoring the output value of said microwaves, a tuner for adjusting the frequency of said microwaves, and a plunger (11) which is provided on an end portion of said waveguide (8) being opposite to said end connected with said microwave oscillator (7) for adjusting the line length of said waveguide (8). A diamond synthesizing apparatus in accordance with claim 1, wherein said means (16) for circulating said raw material gas in a sealed state is formed by a circulation pump (16) which is mounted on a pipeline coupling said reaction tube (3) with said gas reservoir (13). A diamond synthesizing apparatus in accordance with claim 1, wherein said carbon source gas contained in said raw material gas is prepared from any gas selected from hydrocarbon, alcohol, organic acid, carbon monoxide and carbon dioxide, and said medium gas is prepared from hydrogen gas. A diamond synthesizing apparatus comprising: a plurality of parallelly arranged reaction tubes (3) for internally making reaction for vapor-phase synthesizing diamond; a plurality of plasma generation means (6) provided in one-to-one correspondence to said plurality of reaction tubes (3) for generating prescribed microwave plasma in said reaction tubes (3); a gas reservoir (13) coupled with said reaction tubes (3) for defining a circulation system (12) with said reaction tubes (3); means (5) for evacuating said circulation system (12); means (16) for circulating raw material gas, being a compound containing carbon, in a circulation system (12) which is cut off from the exterior to define a sealed system; raw material gas supply means (1) for intermittently or continuously supplying said raw material gas into said circulation system (12). A diamond synthesizing apparatus in accordance with claim 7, wherein said raw material gas supply means (1) is formed by a raw material gas supply system (1) for mixing carbon source gas with medium gas and supplying the mixture into said circulation system (12) and a carbon source gas supply system (14) for supplying said carbon source gas to said gas reservoir (13). A diamond synthesizing apparatus in accordance with claim 7, wherein said plasma generation means (6) are formed by waveguides (8) oppositely holding positions of said reaction tubes (3) for generating said microwave plasma and microwave oscillators (7) connected to first ends of said waveguides (8). A diamond synthesizing apparatus in accordance with claim 9, wherein said plasma generation means (6) are further formed by power monitors (9) for monitoring the output value of said microwaves, tuners (10) for adjusting the frequency of said microwaves, and plungers (11) provided on end portions of said waveguides (8) being opposite to said first ends coupled with said microwave oscillators (7) for adjusting the line lengths of said waveguides (8). A diamond synthesizing apparatus in accordance with claim 7, wherein said means (16) for circulating said raw material gas in a sealed state is formed by a circulation pump (16) which is mounted on a pipeline coupling said reaction tubes (3) with said gas reservoir (13). A diamond synthesizing apparatus in accordance with claim 7, wherein said carbon source gas contained in said raw material gas is prepared from any gas selected from hydrocarbon, alcohol, organic acid, carbon monoxide and carbon dioxide, and said medium gas is prepared from hydrogen gas.

Inventors: FUJIMORI NAOJI C O ITAMI WORKS, OTA NOBUHIRO C O ITAMI WORKS O, FUJIMORI, NAOJI, OTA, NOBUHIRO, FUJIMORI, NAOJI, C/O ITAMI WORKS OF SUMITOMO ELEC., OTA, NOBUHIRO, C/O ITAMI WORKS OF SUMITOMO ELEC.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: C01B31
IPC codes: C23C16