A synthesis process of diamond, synthesis apparatus and synthetic diamond

Abstract: An object of the invention is to provide a high quality synthetic diamond having a high bonding strength between a substrate surface and synthetic diamond or between synthetic diamond films. The object can be attained by a process for the synthesis of diamond comprising pre-heating mixed gases of a hydrocarbon and hydrogen by a heating body heated and feeding the heated mixed gases to a substrate surface heated to deposit diamond thereon, characterized by allowing the distance between the heating body and substrate surface to be apart from each other as far as possible in the carburizing step of the heating body and allowing the distance between the heating body and substrate surface to be nearer in the step of synthesizing diamond than in the former case.

Description: This invention relates to a process for the gaseous phase synthesis of diamond and synthesis apparatus for this process. As a method for the production of diamond by gaseous phase synthesis, a number of methods are known such as microwave plasma CVD methods, high frequency plasma CVD methods, DC plasma jet methods, etc. On a commercial scale for mass production, thermal filament CVD methods have been employed because of the low installation costs. The thermal filament CVD method is a method for the synthesis of a diamond film, comprising subjecting mixed gases of a hydrocarbon and hydrogen to thermal cracking by thermoelectron emission from a heating body heated at 1000 °C or above, followed by deposition on to a substrate surface heated at 500 to 1300°C In a filament CVD method of the prior art, diamond has been synthesized by the following steps: (1) positioning a heating body at a fixed distance from a substrate (2) introducing mixed gases of a hydrocarbon and hydrogen and gradually heating the heating body and (3) when the temperature of the heating body reaches a temperature suitable for the synthesis of diamond, maintaining the heating body at a constant temperature and synthesizing diamond on to the substrate. A problem associated with the filament CVD method is metallic contamination of the synthetic diamond when using, in general, a high melting point metal such as W, Ta, Re, etc. as the heating body. Since the heating body is covered with carbon atoms as a raw material at a certain temperature, the surface of the heating element is protected by a so-called carburization treatment and metallic contamination of the synthetic diamond is decreased. Nevertheless, metallic contamination still occurs during the step of heating the heating body until the surface of the heating body is completely covered due to the carburization treatment. Consequently, the substrate surface or previously synthesized diamond surface may become contaminated with a metallic coat and stripping of the diamond film during or after the synthesis can occur due to low bonding strength of the metallic coating. Furthermore, the presence of the metallic coat reduces the heat conductivity of the synthetic diamond. US-5071708 relates to a composite diamond grain and a method for the production thereof. During the carburizing of the filament the substrate is kept outside of the reactor. It is an object of the present invention to provide a process for the gaseous phase synthesis of diamond whereby a substrate surface is prevented from adhering to a metal. Accordingly, the present invention provides a process for the synthesis of diamond comprising the steps of: (a) providing a substrate in a reactor; (b) heating a mixed gas of a hydrocarbon and hydrogen by a heating body; and (c) feeding the heated mixed gas to a heated surface of the substrate such that diamond is deposited thereon during a diamond deposition step, characterized in that the heating body is carburized during a carburization step prior to the diamond deposition step, wherein the distance between the heating body and substrate surface in the reactor is greater during the carburization step than during the diamond deposition step. The heating body and substrate surface are preferably as far apart from each other as possible in the carburizing step. The present invention further provides an apparatus for the synthesis of diamond comprising a heating body, substrate, substrate-supporting base, raw material gas feeding nozzle, reactor vertically moving shaft having an ascending and descending mechanism of the substrate-supporting base, shielding plate having a mechanism moving in parallel with the substrate and high speed gas flow nozzle. The accompanying drawings are to illustrate the principle and merits of the present invention in detail. Fig. 1 is a schematic view of one embodiment of the apparatus for the synthesis of diamond used in the present invention. Fig. 2 is a schematic view of another embodiment of the apparatus for the synthesis of diamond used in Comparative Examples. In the carburizing treatment of the heating body, an atmosphere is required such that metal from the heating element of the heating body does not adhere to the surface of the substrate. To this end, the following means are effective: (1) In the step of raising the temperature of the heating body, the heating body and substrate surface are allowed to be as far apart from each other as possible so as to prevent metal from the heating body adhering to the substrate surface. (2) In the step of raising the temperature of the heating body, a shielding substance is provided between the heating body and the substrate surface in order to protect the substrate surface from contamination of metal from the heating body. (3) In the step of raising the temperature of the heating body, a high speed gas is allowed to flow over the substrate surface or between the heating body and the substrate surface to form a so-called curtain of a gas flow. (4) The surface of the heating body may be protected by a carburization treatment wherein the temperature of the heating body is maintained at a temperature as low as possible. During the step of synthesizing diamond, different processing conditions are required. In the step of raising the temperature of the heating body during the step of synthesizing diamond after the carburization treatment, little metallic contamination takes place because the surface of the heating body has already been carburized. If the step of synthesizing diamond is successively carried out to the step of the carburization treatment, the steps can be shortened. Accordingly, it is preferable to synthesize diamond by carrying out the carburization treatment using at least one means from the above described means (1) to (4), releasing the above described means by an exterior operation, setting up diamond synthesis conditions and then synthesizing diamond by means of a synthesis apparatus of diamond. In the above described means (1), the carburization treatment of the heating body is generally carried out by allowing the distance between the heating body and substrate surface to be apart by at least 50 mm, preferably at least 200 mm and the synthesis of diamond is generally carried out by allowing the distance between the heating body and substrate to be apart by at most 50 mm, preferably 4 to 20 mm. In the above described means (2), it is preferable to use a shielding plate consisting of at least one member selected from metals excellent in high temperature strength such as Mo, W, etc. or alloys thereof as the shielding substance between the heating body and substrate surface. In the above described means (3), there are used gases such as H 2 , N 2 , Ar, etc. as the high speed gas to be allowed to flow over the substrate surface or between the heating body and substrate surface, the flow rate being at least 5 cm/sec, preferably at least 20 cm/sec. In the above described means (4), the temperature of the heating body in the step of the carburization treatment is lower than 2000 °C. preferably 1500 to 2000 °C. In the present invention, in general, it is preferable to use a tungsten filament as the heating body and an Si substrate as the substrate, but any materials can be chosen which have commonly been used in the methods of this kind. Generally, in the step of synthesizing diamond, the temperature of the heating body is adjusted to at least 2000°C, preferably 2100 to 2400 °C, and the temperature of the substrate surface is adjusted to normal temperature to 1000 °C. In the present invention, to the raw material gas can be added at least one of oxygen-containing gases such as CO 2 , CO, O 2 , H 2 O, NO, NO 2 , etc. to decrease hydrogen gas in the synthesized diamond. Furthermore, an inert gas such as H 2 , Ar, He, etc. can be added thereto to control the stress in the synthesized diamond by the CVD method. According to the diamond synthesis of the present invention, the following merits or advantages can be given: Stripping of a diamond film due to stress can be decreased since the step of the carburization treatment is carried out by at least one of the above described means (1) to (4) and the synthesized diamond film meets with less metallic contamination of an interface between the substrate surface and diamond film. In synthetic diamond films obtained by repeating at least two times the step of the carburization treatment by the use of at least one of the above described means (1) to (4) and the step of synthesizing diamond, metallic contamination is also decreased in the interface between the diamond films to such a great extent that the interface of the diamond films is not stripped, the heat conductivity is not lowered and the high heat conductivity diamond intrinsically has can be given. Metallic contamination of the synthesized diamond film somewhat occurs and even in the case of the above described means, in particular, the interface is subject to metallic contamination to a much greater extent than other parts than the interface, thus forming a thin metallic film sometimes. In the synthetic diamond obtained by the repeated synthesis as such, if a film formed of other materials than diamond component has a thin thickness to such an extent that the whole surface of the diamond film is not completely covered, a newly synthesized diamond film thereon takes over the diamond crystal grains below it and can be freed from lowering of the heat conductivity. When the concentration of other components than diamond component in the interfacial part of the repeated synthesis is very small and this zone occupies a small proportion to the whole of the synthesized diamond film, lowering of the heat conductivity to a large extent can be saved. In this case, it is preferable that a zone formed of other materials than diamond is not present or if any, there is only a zone which is not uniform nor thick to such an extent that it covers completely the whole surface or which does not have a structure of separating crystal grains of the whole surface. In the synthetic diamond as described above, the concentration of other components than diamond component, formed of at least one member selected from the group consisting of Group 4A, 5A, 6A and 7A metals of Periodic Table or carbides thereof, is at most 1/100 (atomic ratio) in the whole diamond film. Preferred embodiments of the present invention are summarized below: (1) A process for the synthesis of diamond, as claimed in Claim 1, which comprises allowing the distance between the heating body and substrate surface to be apart from each other by at least 50 mm in the carburizing step of the heating body and allowing the distance between the heating body and substrate surface to be nearer by at most 50 mm in the step of synthesizing diamond than in the former case. (2) The process for the synthesis of diamond, as claimed in Claim 3, wherein the shielding substance is a shielding plate consisting of a metal excellent in high temperature strength, such as Mo, W, etc. or an alloy thereof. (3) The process for the synthesis of diamond, as claimed in Claim 4, wherein the high speed gas flow has a flow rate of at least 5 cm/sec, preferably 20 cm/sec. (4) The process for the synthesis of diamond, as claimed in Claim 4, wherein the high speed gas flow consists of a gas selected from the group consisting of H 2 , N 2 and Ar. (5) The process for the synthesis of diamond, as claimed in Claim 1, wherein the temperature of the heating body in the step of the carburization treatment is lower than 2000°C, preferably 1500 to lower than 2000°C and the temperature of the heating body in the step of synthesizing diamond is at least 2000°C, preferably 2100 to 2400 °C. (6) The process for the synthesis of diamond, wherein a combination of two or more processes as claimed in Claims 1 to 4 is carried out as an indispensable element. (7) The process for the synthesis of diamond, wherein a combination of two or more processes as claimed in the above described (1) to (6) is carried out as an indispensable element. Examples The present invention will now be illustrated by Examples wherein diamond was synthesized by the use of a synthesis apparatus whereby the above described processes are rendered feasible and of the above described processes, and by Comparative Examples wherein diamond was synthesized by the prior art process. Fig. 1 is a schematic view of a synthetic apparatus whereby all the processes (1) to (7) described above are rendered feasible, comprising a heating body 1, substrate 2, substrate-supporting base 3, raw material gas feeding nozzle 4, reactor 5, vertically moving shaft 6 having an ascending and descending mechanism of the substrate-supporting base, Mo shielding plate 7 having a left and right moving mechanism and high speed gas flow nozzle 8. Example 1 In an apparatus for the synthesis of diamond shown in Fig. 1, a synthetic experiment was carried out in which a distance between the heating body and substrate surface was changed in the carburization treatment step of the heating body and diamond synthesis step. The heating body 1 was made up of three tungsten filaments (length: 250 mm, diameter: 0.5 mm) stretched at an interval of 6 mm and an Si substrate of 10 mm square was used as the substrate 2. In the carburization treatment step of the heating body, the distance between the heating body and substrate surface was adjusted to 100 mm and in the diamond synthesis step, the distance between the heating body and substrate surface was adjusted to 6 mm. In the carburization treatment step of the heating body, 'the distance between the heating body and substrate surface was first adjusted to 100 mm, H 2 at 1000 cc/min and CH 4 at 20 cc/min were then introduced from the raw material gas feeding nozzle 4, while maintaining the inside of the reactor 5 at 13332 Pa (100 Torr), electric current was applied to the heating body 1 to heat the heating body up to 1900 °C and maintained for 1 hour, during which the temperature of the substrate surface was maintained at lower than 150 °C. The heating body was carburized and after 1 hour, the surface of the heating body was covered with carbon and tungsten carbide. After the carburization treatment step, the substrate-supporting base 3 was moved upward by the vertically moving shaft 6 to adjust the the distance between the heating body and substrate surface to 6 mm and the temperature of the heating body was subsequently raised to 2200 °C, followed by maintaining for 40 hours. During the diamond synthesis step, the temperature of the substrate surface was maintained at 750 to 800 °C. The carburization treatment step of the heating body and diamond synthesis step, as described above, were repeated three times under the same conditions to obtain a synthetic diamond film of 180 µm in thickness on the substrate. The synthesized diamond film was not stripped, and interfaces between the substrate and diamond film and between the diamond films were present, but it was found by observation using an electron microscope that the thickness of metallic impurity film partially present in the interface was at most 1 µm and diamond crystal grains were substantially continuous even in the interface. It was made clear as a result of elementary analysis that this metallic impurity film was formed of tungsten, as the material of the heating body, but the tungsten concentration was very small and was assumed to be a concentration of at most 1/200 (atomic ratio) to the whole of diamond. Furthermore, the substrate was dissolved, the surface of the synthesized diamond was polished and the heat conductivity in the synthesis direction was then measured to obtain 10 W/cm · K. Example 2 In an' apparatus for the synthesis of diamond shown in Fig. 1, a synthetic experiment was carried out in which a distance between the heating body and substrate surface was changed in the carburization treatment step of the heating body and diamond synthesis step and moreover, a shielding plate of Mo was placed between the heating body and substrate surface in the carburization treatment step of the heating body, the shielding plate of Mo was moved and diamond was then synthesized. Synthesis of diamond was carried out under the same conditions as in Example 1 except placing, during the step of carburization treatment of the heating body, the shielding plate 7 of Mo between the heating body and substrate surface and below the heating body by 80 mm, as shown in Fig. 1, and after the carburization treatment of the heating body, moving right the shielding plate of Mo, withdrawing it from the space over the substrate and then adjusting the distance between the heating body and substrate surface to 6 mm. Stripping of the resulting synthetic diamond film was not found and the interfaces between the substrate and diamond film and between diamond films were found to such an extent that they were hardly observed by an electron microscope. The substrate was dissolved, the surface of the synthesized diamond was polished and the heat conductivity in the synthesis direction was then measured to obtain 13 W/cm · K. Example 3 In an apparatus for the synthesis of diamond shown in Fig. 1, a synthetic experiment was carried out in which a distance between the heating body and substrate surface was changed in the carburization treatment step of the heating body and diamond synthesis step and moreover, H 2 gas was allowed to flow at a rate of 5000 cc/min between the heating body and substrate surface, mainly toward the substrate surface, from the high speed gas feeding nozzle in the carburization treatment step of the heating body, the high speed gas flow was stopped and diamond was then synthesized. Synthesis of diamond was carried out under the same conditions as in Example l'except allowing H 2 gas to flow at a rate of 5000 cc/min and flow rate of 15 cm/sec toward the substrate surface from the high speed gas flow nozzle 8 shown in Fig. 1 during the step of carburization treatment of the heating body, and after the carburization treatment of the heating body, stopping the high speed gas flow and then adjusting the distance between the heating body and substrate surface to 6 mm. Stripping of the resulting synthetic diamond film was not found and the interfaces between the substrate and diamond film and between diamond films were found to such an extent that they were hardly observed by an electron microscope. The substrate was dissolved, the surface of the synthesized diamond was polished and the heat conductivity in the synthesis direction was then measured to obtain 12 W/cm · K. Example 4 In an apparatus for the synthesis of diamond shown in Fig. 1, a synthetic experiment was carried out in which a distance between the heating body and substrate surface was changed in the carburization treatment step of the heating body and diamond synthesis step, moreover, the temperature of the heating body was maintained relatively low in the carburization treatment step of the heating body and after the carburization treatment, the temperature of the heating body was raised to a temperature suitable for the synthesis and diamond was then synthesized. Synthesis of diamond was carried out under the same conditions as in Example 1 except adjusting the temperature of heating body to 1800 °C in the step of carburization treatment of the heating body. Stripping of the resulting synthetic diamond film was not found and the interfaces between the substrate and diamond film and between diamond films were found, but it was assumed by observation using electron microscope that the metallic impurity film partially present in the interface had a film thickness of at most 1 µm and crystal grains were substantially continuous even in the interface. Furthermore, the substrate was dissolved, the surface of the synthesized diamond film was polished and the heat conductivity in the synthesis direction was then measured to obtain 12 W/cm · K. Comparative Examples Fig. 2 is a schematic view of an apparatus for the synthesis of diamond, comprising a heating body 1, substrate 2, substrate-supporting base 3, raw material gas feeding nozzle 4 and reactor 5, the distance between the heating body and substrate surface being adjusted to 6 mm. Comparative Example 1 In an apparatus for the synthesis of diamond shown in Fig. 2, a synthetic experiment was carried out under the following conditions: The heating body 1 was made up of three tungsten filaments (length: 250 mm, diameter: 0.5 mm) stretched at an interval of 6 mm and an Si substrate of 10 mm square was used as the substrate 2. In both of the carburization treatment step and synthesis step, the distance between the heating body and substrate surface was adjusted to a constant value, i.e. 6 mm. H 2 at 1000 cc/min and CH 4 at 20 cc/min were then introduced from the raw material gas feeding nozzle 4, while maintaining the inside of the reactor 5 at 13332 Pa (100 Torr), electric current was applied to the heating body 1 to heat the heating body up to 2100 °C and maintained for 1 hour. The temperature of the substrate surface was already 600 °C in this carburization treatment step. Subsequently, the temperature of the heating body was raised to 2200°C and maintained for 40 hours. In this diamond synthesis step, the temperature of the substrate surface was maintained at 750 to 800°C. The above described steps were repeated three times under the same conditions. After the second synthesis, the synthesized diamond film was somewhat stripped and when the third synthesis step was held for 15 hours, the synthesized diamond film was completely stripped. A clear deteriorated layer was present in the interface between the diamond films, which was assumed to be about 2 to 3 µm in thickness according to observation by an electron microscope. When the surface of the synthetic diamond film was polished and the heat conductivity in the synthesis direction was measured, it was in a large dispersion depending on the measurement position and only a heat conductivity of 5 W/cm · K was obtained. The dispersion of this measuremnt was assumed to be due to the thickness of the deteriorated layer and in particular, a part of the synthesized diamond film just below the heating body was extremely bad. Comparative Example 2 In an apparatus for the synthesis of diamond shown in Fig. 2, a synthetic experiment was carried out under the same conditions as in Example 1 except adjusting the distance between the heating body and substrate surface to 30 mm in the carburization treatment step of the heating body and adjusting the distance between the heating body and substrate surface to 6 mm in the diamond synthesis step. The temperature of the substrate surface was already 400 to 500 °C in this carburization treatment step. The synthesized diamond film was not stripped, but interfaces between the substrate and diamond film and between the diamond films were clearly present and according to observation by an electron microscope, a metallic impurity film present in the interface was assumed to be about 1 µm in thickness. The diamond film was clearly separated by this interface and as far as the observation concerned, substantial crystal grains were discontinuous. It was made clear as a result of elementary analysis that this metallic impurity film was formed of tungsten, as the material of the heating body. There was found no remarkable impurity metal in the synthesized diamond film other than the interface, but two or three layers of the metallic impurities of about 1 µm in thickness were present in the diamond film of 180 µm in thickness, which corresponded to 1/90 to 1/60 (atomic ratio) in simple terms of concentration. The substrate was dissolved, the surface of the synthesized diamond film was polished and the heat conductivity in the synthesis direction was then measured to obtain 7 W/cm · K. Comparative Example 3 A synthetic experiment was carried out under the same conditions as in Example 1 except adjusting the distance between the heating body and substrate surface to 100 mm in the carburization treatment step of the heating body and adjusting the distance between the heating body and substrate surface to 60 mm in the diamond synthesis step. However, the temperature of the substrate surface during the diamond synthesis step was at most 500 °C and in spite of the repeated experiments of three times, the film thickness did not reach 10 µm, by which the quality as diamond was not satisfied. Advantages of the Present Invention According to the present invention, metallic contamination from a heating body can largely be reduced to obtain synthetic diamond having a high bonding strength as well as high heat conductivity. Since such a diamond film that diamond crystal grains are not separated over the whole surface can be formed, while a deteriorated layer in the interface between synthetic diamond films by the repeated synthesis can be diminished or if any, the deteriorated layer is not uniform nor thick to such an extent that it covers completely the whole surface, according to the present invention, in particular, there can be obtained a high quality diamond with at least 100 µm thick. The synthetic diamond film obtained by such a synthesis process, in particular, such a high quality synthetic diamond film that the concentration of a deteriorated layer formed of other materials than diamond is at most 1/100 in the whole diamond film can be used as a radiation plate with a high heat conductivity.

Claims: A process for the synthesis of diamond comprising the steps of: (a) providing a substrate in a reactor; (b) heating a mixed gas of a hydrocarbon and hydrogen by a heating body; and (c) feeding the heated mixed gas to a heated surface of the substrate such that diamond is deposited thereon during a diamond deposition step, characterized in that the heating body is carburized during a carburization step prior to the diamond deposition step, wherein the distance between the heating body and substrate surface in the reactor is greater during the carburization step than during the diamond deposition step. A process for the synthesis of diamond as claimed in claim 1, wherein the heating body and substrate surface are positioned as far apart from each other as possible during the carburization step. A process for the synthesis of diamond as claimed in claim 1 or claim 2, wherein a shielding substance is provided between the heating body and substrate surface during the carburization step. A process for the synthesis of diamond as claimed in any one of the preceding claims, wherein a high speed gas flow is provided over the substrate surface or between the heating body and the substrate surface during the carburization step. A process for the synthesis of diamond as claimed in any one of the preceding claims, wherein the temperature of the heating body during the carburization step is less than approximately 2000°C and the temperature of the heating body during the diamond deposition step is at least approximately 2000°C. A process for the synthesis of diamond as claimed in claim 5, wherein the temperature of the heating body during the carburization step is from 1500°C to less than approximately 2000°C and the temperature of the heating body during the diamond deposition step is from 2100 to 2400°C. A process for the synthesis of diamond as claimed in any one of the preceding claims, wherein the distance between the heating body and substrate surface during the carburization step is at least 50 mm, and the distance between the heating body and substrate surface during the diamond deposition step is at most 50 mm. A process for the synthesis of diamond as claimed in any one of claims 3 to 7, wherein the shielding substance is a shielding plate consisting of Mo, W or an alloy thereof. A process for the synthesis of diamond as claimed in any one of claims 4 to 8, wherein the high speed gas flow consists of a gas selected from H 2 , N 2 and Ar. A process for the synthesis of diamond as claimed in any one of the preceding claims, wherein the said process is repeated at least two times on the same substrate. A process for the synthesis of diamond as claimed in any one of the preceding claims, further including the step of dissolving the substrate following the diamond deposition step. A process for the synthesis of diamond as claimed in any one of the preceding claims, wherein the synthesized diamond comprises no zone formed of other materials than diamond or, if any, a zone which is not uniform nor thick to such an extent that it covers completely the whole surface or which does not have a structure of separating crystal grains of the whole surface. A process for the synthesis of diamond as claimed in claim 12, wherein the concentration of components, other than diamond components, formed from at least one of Group 4A, 5A, 6A and 7A metals of the Periodic Table or carbides thereof, is at most 1/100 (atomic ratio) in the whole diamond film. An apparatus for the synthesis of diamond comprising a heating body, substrate, substrate-supporting base, raw material gas feeding nozzle, reactor vertically moving shaft having an ascending and descending mechanism of the substrate-supporting base, shielding plate having a mechanism moving in parallel with the substrate and high speed gas flow nozzle.

Inventors: FUJIMORI NAOJI, OJI MASATAKA, FUJIMORI, NAOJI, OJI, MASATAKA, FUJIMORI, NAOJI, C/O ITAMI WORKS OF, OJI, MASATAKA, C/O ITAMI WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1995
Main classification: C30B25
IPC codes: C30B29, C23C16, C30B25