Thermal CVD apparatus for diamond synthesis

Abstract

We aim to improve diamond quality by minimizing metallic contamination, and to increase production efficiency by accelerating the diamond synthesis rate and enabling the creation of larger diamond surfaces. [Solution] A substrate stage 6 is provided in a vacuum chamber 1 to hold the substrate 5, and a blowout port 10 is positioned to blow out a raw material gas toward the composite surface of the substrate 5. The raw material gas is thermally decomposed and the thermal decomposition products of the raw material gas are bonded to the composite surface of the substrate 5 to synthesize diamond. An induction heating means 20 is provided to inductively heat the blowout port 10 to thermally decompose the raw material gas by the blowout port 10. The blowout port 10 is made up of a shower head 11 having a blowout wall 13 in which a large number of small holes 12 for blowing out gas are formed, and the induction coil 22 of the induction heating means 20 is positioned facing the outer wall surface of the shower head 11, excluding the blowout wall 13, at a required distance apart.

Description

Technical Field

[0001] The present invention relates to a hot CVD apparatus for synthesizing diamond, and particularly to a hot CVD apparatus for diamond synthesis that thermally decomposes a raw material gas at a high temperature to synthesize diamond. Background Art

[0002] In general, as methods for synthesizing diamond, for example, microwave plasma CVD and hot CVD are known. In the plasma CVD method, due to the influence of microwave wavelength, the size of the base material on which diamond is synthesized is limited to about 4 inches, and there is a restriction on the synthesis range. On the other hand, the hot CVD method has few restrictions on the generation range of a synthetic product, can achieve a large area and process three-dimensional objects, and is advantageous from the viewpoint of productivity, so there is an actual need to carry out production using this hot CVD method.

[0003] Conventionally, as this type of hot CVD apparatus for diamond synthesis, for example, the one described in Japanese Unexamined Patent Publication No. Hei 3-103396 (Patent Document 1) is known. As shown in FIG. 10(a), this hot CVD apparatus synthesizes diamond as a synthetic product by a so-called HF-CVD method. A base material stage 101 for holding a base material 100 is provided in a vacuum chamber (not shown), a blow-out port portion 102 formed of a tubular nozzle that blows out raw material gas toward the synthesis surface of the base material 100 is arranged, a resistance-heated filament 103 is interposed between the blow-out port portion 102 and the base material stage 101, the raw material gas is thermally decomposed by the filament 103, and the thermal decomposition product of the raw material gas is bonded to the synthesis surface of the base material 100 to synthesize diamond. As the filament 103, a material made of a high-melting-point metal such as tungsten or tantalum is used, for example. The temperature of the filament 103 is set to 1500° C. or higher, preferably 1800° C. or higher, and the surface temperature of the base material is set to 700° C. to 1200° C.

[0004] Alternatively, as shown in Figure 10(b), the tip 104 of the nozzle outlet 102, which is a tubular nozzle, is resist-heated, and the raw material gas is thermally decomposed by this tip 104. The temperature of the tip 104 of this nozzle outlet 102 is set to the same temperature as the filament described above. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 3-103396 [Overview of the project] [Problems that the invention aims to solve]

[0006] Incidentally, in this conventional thermal CVD apparatus for diamond synthesis, the former apparatus shown in Figure 10(a) uses high-melting-point metals such as tungsten or tantalum as the filament 103. However, since the filament 103 is a thin metal wire, in a synthesis process like diamond synthesis, the carbonized filament 103 is very brittle, and metal contamination from the filament 103 makes it difficult to produce high-quality diamonds. In addition, its relatively short lifespan makes long-term synthesis difficult. Furthermore, because the filament 103 has a short lifespan, it needs to be replaced after each process, which makes the work complicated and reduces production efficiency. Moreover, a mixed film of carbon and filament metal adheres to the filament holder, and after repeated synthesis, this can cause the filament 103 to get stuck and sag, resulting in stability problems. In addition, the synthesis speed is generally slower compared to plasma CVD.

[0007] On the other hand, in the latter apparatus for resistance heating the nozzle tip 104 shown in Figure 10(b), since no filament is used, metal contamination of the diamond is reduced. However, because the heating electrode, which has external contact that allows heat to escape easily, is localized, uneven heating of the tip 104 occurs, and the thermal decomposition efficiency of the gas is not necessarily good, resulting in a problem of inferior production efficiency.

[0008] This invention has been made in view of the above problems, and aims to provide a thermal CVD apparatus for diamond synthesis that improves quality by minimizing metal contamination of diamonds, and improves production efficiency by accelerating the diamond synthesis rate and enabling the production of large-area diamonds. [Means for solving the problem]

[0009] To achieve this objective, the present invention provides a thermal CVD apparatus for synthesizing diamond, comprising a substrate stage for holding a substrate in a vacuum chamber, a nozzle for blowing a raw material gas toward the synthesis surface of the substrate, and a thermal CVD apparatus for synthesizing diamond by thermally decomposing the raw material gas and bonding the thermal decomposition products of the raw material gas to the synthesis surface of the substrate, The system is configured to include an induction heating means that inductively heats the outlet portion to cause thermal decomposition of the raw material gas by the outlet portion.

[0010] Here, the substrates include not only flat substrates but also three-dimensional objects such as tools like drills and taps. Diamond is deposited (synthesized) onto the surface of these substrates. In this diamond synthesis, methane (CH4) and hydrogen (H2) are mainly used as raw material gases.

[0011] In this process, when diamonds are synthesized, a raw material gas is supplied, which is thermally decomposed at the outlet and blown out towards the synthesis surface of the substrate. The thermal decomposition products of the raw material gas then combine with this surface to synthesize the diamond. In this case, since the outlet is induction heated, it becomes a relatively large solid, unlike a brittle, thin metal wire like a filament. Therefore, localized overheating is less likely to occur, metal contamination can be prevented as much as possible, and the quality of the diamond can be improved accordingly.

[0012] Furthermore, since it is no longer necessary to replace the filament for each process, work efficiency is improved, and maintenance of the filament holder is also unnecessary, thus significantly increasing production efficiency. In addition, compared to the filament type, the gas outlet is heated directly, which increases the surface area of ​​the heating element and improves the thermal decomposition efficiency of the gas. As a result, the diamond synthesis rate increases significantly, further improving production efficiency.

[0013] Furthermore, because there is no filament, the distance between the outlet and the substrate can be reduced, allowing the thermal decomposition products of the raw gas to reach the substrate before they become inactive, thereby increasing the amount of thermal decomposition products that reach the substrate. As a result, the diamond synthesis rate can be significantly increased, improving production efficiency in this respect as well. Moreover, because induction heating is used, the entire outlet can be heated more evenly compared to resistance heating, improving the thermal decomposition efficiency of the gas, and thus improving production efficiency in this respect as well.

[0014] Furthermore, the induction heating means is effective in heating the outlet portion to a heating temperature of 1500°C to 3000°C. In this configuration, the substrate temperature holding means is provided to heat or cool the substrate stage to maintain the substrate temperature at a required temperature. This substrate temperature holding means maintains the substrate temperature at, for example, 700°C to 1200°C. This ensures that the thermal decomposition products of the raw material gas are reliably bonded to the substrate surface, enabling the synthesis of diamond.

[0015] In this case, it is effective to form the outlet portion using a conductive material with a melting point higher than the heating temperature of the outlet portion as the main material. More preferably, the outlet portion is formed primarily of a conductive material with a melting point higher than the heating temperature of the outlet portion and a low vapor pressure.

[0016] Here, "low vapor pressure" refers to a vapor pressure of 10 at temperatures above 1500°C. -4 This refers to a vapor pressure of Pa or less. Conductive materials classified as having this low vapor pressure are used.

[0017] More specifically, the outlet portion is formed from one of the following materials: carbon, a material obtained by coating carbon with a substance having a different high melting point and low vapor pressure than the carbon, metal carbide, a material obtained by coating metal carbide with a substance having a different high melting point and low vapor pressure than the metal carbide, a conductive metal having a high melting point and low vapor pressure, or a conductive metal having a high melting point and low vapor pressure coated with a substance having a different high melting point and low vapor pressure than the conductive metal.

[0018] In this configuration, in addition to carbon, the conductive metal is effectively selected from tungsten (W), rhenium (Re), osmium (Os), molybdenum (Mo), tantalum (Ta), niobium (Nb), iridium (Ir), hafnium (Hf), zirconium (Zr), platinum (Pt), and alloys thereof, and the metal carbide is effectively selected from tungsten carbide (WC), titanium carbide (TiC), zirconium carbide (ZrC), hafnium carbide (HfC), niobium carbide (NbC), and tantalum carbide (TaC). Materials equivalent to these can be used for the coating. The material does not necessarily have to be conductive.

[0019] Furthermore, in the present invention, the outlet portion may be configured as a shower head having an outlet wall formed with numerous small holes for blowing out gas, the induction heating means may be configured to include an induction coil connected to a high-frequency power supply, and the induction coil may be arranged facing the outer wall surface of the shower head, excluding the outlet wall, at a required distance apart.

[0020] As a result, since the outlet section is constructed as a showerhead, the surface area of ​​the outlet wall can be made larger compared to a tubular nozzle. Furthermore, since the induction coil is placed on the outer wall surface excluding the outlet wall, the distance between the outlet wall and the substrate can be reduced, thereby increasing the diamond synthesis rate and improving production efficiency. In addition, because there are many small holes in the outlet wall, the contact with the raw material gas is good, which improves the thermal decomposition efficiency of the gas, and thus also improves production efficiency.

[0021] In this configuration, it is effective to have a connecting pipe, one end of which is connected to a gas supply pipe that supplies raw material gas and the other end of which is connected to the shower head, with at least the other end facing into the chamber, and to form the connecting pipe and the shower head as a single unit by integrally forming them from the same material. Since the connection pipe facing the inside of the chamber can also contribute to heating, the thermal decomposition efficiency of the raw material gas can be improved accordingly. Furthermore, the connection pipe can also be induction-heated, in which case the thermal decomposition efficiency of the raw material gas can be further improved. Furthermore, since it is configured as a head unit, it is easy to attach to and detach from the chamber, has good mountability, and is also easy to handle. [Effects of the Invention]

[0022] As described above, according to the present invention, it is possible to improve quality by minimizing metal contamination of diamond, and also to improve production efficiency by increasing the synthesis rate of diamond and enabling diamond to have a larger area. [Brief Description of the Drawings]

[0023] [Figure 1] It is a diagram showing a thermal CVD apparatus for diamond synthesis according to an embodiment of the present invention. [Figure 2] In the thermal CVD apparatus for diamond synthesis according to the embodiment of the present invention, the shower head serving as an outlet and an induction coil are shown, where (a) is a perspective view, (b) is a front view, and (c) is a bottom view. [Figure 3] It is a cross-sectional view showing the shower head serving as an outlet and an induction coil in the thermal CVD apparatus for diamond synthesis according to the embodiment of the present invention. [Figure 4] In the thermal CVD apparatus for diamond synthesis according to the embodiment of the present invention, a modified example of the shower head serving as an outlet and an induction coil is shown, where (a) is a perspective view, (b) is a front view, and (c) is a bottom view. [Figure 5] In the thermal CVD apparatus for diamond synthesis according to the embodiment of the present invention, another embodiment of the shower head serving as an outlet and an induction coil is shown, where (a) is a perspective view, (b) is a front view, and (c) is a bottom view. [Figure 6]In the thermal CVD apparatus for diamond synthesis according to an embodiment of the present invention, another configuration of the shower head and induction coil as the outlet section is shown (A) and (B), where (Aa) and (Ba) are perspective views, (Ab) and (Bb) are front views, and (Ac) and (Bc) are bottom views. [Figure 7] In a thermal CVD apparatus for diamond synthesis according to an embodiment of the present invention, yet another embodiment (A) and (B) of the shower head and induction coil as the outlet section are shown, where (Aa) and (Ba) are perspective views, (Ab) and (Bb) are front views, and (Ac) and (Bc) are bottom views. [Figure 8] Another embodiment of the present invention relates to a thermal CVD apparatus for diamond synthesis, showing its outlet and induction coil, where (a) is a perspective view, (b) is a front view, and (c) is a bottom view. [Figure 9] This is a photograph illustrating the heating state of a shower head by an induction coil, relating to an experimental example of the present invention. [Figure 10] The image shows a conventional thermal CVD apparatus for diamond synthesis. (a) is a diagram of the main components of an apparatus that thermally decomposes the raw material gas using a filament, and (b) is a diagram of the main components of an apparatus that thermally decomposes the gas by resistance heating at the tip of a nozzle. [Modes for carrying out the invention]

[0024] The following describes in detail an embodiment of the present invention of a thermal CVD apparatus for diamond synthesis based on the attached drawings. This thermal CVD apparatus synthesizes diamond by thermally decomposing a raw material gas. The raw material gas mainly used is methane (CH4) and hydrogen (H2).

[0025] As shown in Figures 1 to 3, the thermal CVD apparatus S for diamond synthesis according to this embodiment comprises a chamber 1 that is evacuated, a gas supply unit 3 that supplies raw material gas into the chamber 1, and a vacuum exhaust unit 4 that evacuates and exhausts the chamber 1. Reference numeral 2 denotes the gas supply pipe of the gas supply unit 3 through which the raw material gas flows.

[0026] A substrate stage 6 for holding the substrate 5 is provided inside the chamber 1. This substrate stage 6 is rotatably supported on a base 7 and is rotated during operation by a well-known driving means (not shown). In addition, a substrate temperature holding means is provided on the substrate stage 6 side to heat or cool the substrate stage 6 to maintain the temperature of the substrate at a required temperature.

[0027] Furthermore, a blowout section 10 for blowing raw material gas toward the composite surface of the substrate 5 is located inside the chamber 1. This blowout section 10 is composed of a shower head 11. The shower head 11 is cylindrical in shape and has a blowout wall 13 with numerous small holes 12 formed therein for blowing out gas, and a hollow section 14 inside which receives the raw material gas supplied from the gas supply pipe 2 and blows it out through the small holes 12 in the blowout wall 13. The hollow section 14 is formed in a conical shape that widens toward the blowout wall 13.

[0028] The shower head 11 is coaxially and integrally provided with a connecting pipe 15 that guides the raw material gas from the gas supply pipe 2 into the hollow section 14. The connecting pipe 15 is formed to have a smaller diameter than the shower head 11. The shower head 11 and the connecting pipe 15 constitute the head unit 16. At least one end of the connecting pipe 15 faces into the chamber 1, and one end of the connecting pipe 15 is fixed to the upper wall of the chamber 1, positioning the shower head 11 in the required position such that its discharge wall 13 faces the composite surface of the substrate 5 on the substrate stage 6.

[0029] The head unit 16, consisting of a shower head 11 (outlet portion 10) and a connecting pipe 15, is formed primarily of a conductive material with a melting point higher than the heating temperature of the shower head 11 and a low vapor pressure. Here, "low vapor pressure" refers to a vapor pressure of 10 at a temperature of 1500°C or higher. -4 This refers to a vapor pressure of Pa or less. Conductive materials classified as having this low vapor pressure are used.

[0030] More specifically, the head unit 16 is integrally formed from one of the following materials: carbon, a material obtained by coating carbon with a different high-melting-point, low-vapor-pressure substance, metal carbide, a material obtained by coating metal carbide with a different high-melting-point, low-vapor-pressure substance, a conductive metal with a high-melting-point, low-vapor-pressure, or a conductive metal obtained by coating a conductive metal with a different high-melting-point, low-vapor-pressure, substance.

[0031] More specifically, in addition to carbon, conductive metals can be selected from tungsten (W), rhenium (Re), osmium (Os), molybdenum (Mo), tantalum (Ta), niobium (Nb), iridium (Ir), hafnium (Hf), zirconium (Zr), platinum (Pt), and any alloys thereof. Furthermore, as metal carbides, one can be selected from tungsten carbide (WC), titanium carbide (TiC), zirconium carbide (ZrC), hafnium carbide (HfC), niobium carbide (NbC), and tantalum carbide (TaC). Materials equivalent to these can be used for coating. For example, a carbon base coated with tantalum carbide is effective. The material used for coating does not necessarily have to be conductive.

[0032] Furthermore, the diamond synthesis thermal CVD apparatus S according to this embodiment includes an induction heating means 20 that induction heats the shower head 11 (outlet portion 10) to thermally decompose the raw material gas using the shower head 11. The induction heating means 20 is configured to include an induction coil 22 connected to a high-frequency power supply 21. The induction coil 22 is positioned opposite the outer wall surface of the shower head 11, excluding the outlet wall 13, at a required distance apart. In this embodiment, the induction coil 22 is formed in a spiral and cylindrical shape, with extended portions 23 at both ends supported by an insulator on the upper wall of the chamber 1, and is positioned coaxially with the shower head 11, facing and surrounding the side wall surface of the shower head 11.

[0033] In this embodiment, the induction heating means 20 heats the shower head 11 to a heating temperature of 1500°C to 3000°C using a high-frequency power supply 21 (high frequency of 10 kHz or higher). In this embodiment, the substrate temperature holding means, for example, holds the substrate 5 at a temperature of 700°C to 1200°C.

[0034] Chamber 1 is also equipped with a first radiation thermometer 30 for measuring the temperature of the shower head 11 and a second radiation thermometer 31 for measuring the temperature of the substrate 5.

[0035] Therefore, when diamond is synthesized using the thermal CVD apparatus S for diamond synthesis according to this embodiment, the process is as follows: When raw material gas is supplied from the gas supply unit 3 to the head unit 16, the raw material gas is thermally decomposed by the shower head 11 (outlet 10) and blown out from the shower head 11 toward the synthesis surface of the substrate 5. The thermal decomposition products of the raw material gas then combine with this synthesis surface to synthesize diamond.

[0036] In this case, since the showerhead 11 is induction heated, it becomes a relatively large solid, unlike a brittle, thin metal wire like a filament. Therefore, localized overheating is less likely to occur, metal contamination can be prevented as much as possible, and thus the quality of the diamond can be improved. In addition, since there is no need to replace the filament for each process, work efficiency is improved, and maintenance of the filament holder is also unnecessary, so production efficiency can be greatly improved. Furthermore, compared to the filament method, since the showerhead 11 is heated directly, the surface area of ​​the heating element is increased, which improves the thermal decomposition efficiency of the gas. As a result, the diamond synthesis rate is greatly increased, which also improves production efficiency.

[0037] In particular, in this embodiment, the outlet portion 10 is composed of a shower head 11 having an outlet wall 13 with numerous small holes 12 for blowing out gas, and the induction coil 22 of the induction heating means 20 is positioned opposite the outer wall surface (side wall surface in this embodiment) excluding the outlet wall 13 at a required distance. Compared to a tubular nozzle, the area of ​​the outlet wall 13 can be made larger, and since the induction coil 22 is positioned on the outer wall surface excluding the outlet wall 13, the distance between the outlet wall 13 and the substrate 5 can be reduced. As a result, the thermal decomposition products of the raw material gas can reach the substrate 5 before they become inactive, thereby increasing the amount of thermal decomposition products that reach the substrate. Therefore, the diamond synthesis rate can be significantly increased, and in this respect as well, production efficiency can be improved.

[0038] Furthermore, because it uses induction heating, the entire showerhead 11 can be heated more evenly compared to resistance heating. Also, because there are many small holes 12 in the outlet wall 13, the raw material gas has good contact with it, which improves the thermal decomposition efficiency of the gas, and thus improves production efficiency.

[0039] Furthermore, in this embodiment, since a head unit 16 is used in which the connecting pipe 15 and the shower head 11 are integrally formed from the same material, the connecting pipe 15 facing the chamber 1 can also contribute to heating, thereby improving the thermal decomposition efficiency of the raw material gas. The connecting pipe 15 can also be induction heated, in which case the thermal decomposition efficiency of the raw material gas can be further improved. Moreover, the head unit 16 is easy to attach to and detach from the chamber 1, resulting in good installation and easy handling.

[0040] Figure 4 shows a modified head unit 16 and induction coil 22 in a thermal CVD apparatus S for diamond synthesis according to an embodiment of the present invention. In this modification, the diameter of the cylindrical shower head 11 is made larger than that of the connecting pipe 15, and the induction coil 22 of the induction heating means 20 is formed to face not only the side wall surface of the shower head 11 but also the top surface. The portion of the induction coil 22 facing the top surface is formed in a flattened spiral shape. This also produces the same effects and advantages as described above.

[0041] Figure 5 shows another configuration of the head unit 16 and induction coil 22 in a thermal CVD apparatus S for diamond synthesis according to an embodiment of the present invention. In this configuration, the outer shape of the shower head 11 is formed in a substantially conical shape, and the induction coil 22 of the induction heating means 20 is also formed in a substantially conical shape and arranged facing the side wall surface of the shower head 11. This configuration also produces the same effects and advantages as described above.

[0042] Figures 6(A) and 6(B) show yet another configuration of the head unit 16 and induction coil 22 in a thermal CVD apparatus S for diamond synthesis according to an embodiment of the present invention. The shower head 11 of the head unit 16 shown in Figures 6(A) and 6(B) is formed in a relatively flat disc shape with a top surface, and the induction coil 22 of the induction heating means 20 is formed in a flat spiral shape and is positioned opposite the top surface of the shower head 11. The induction coil 22 in Figure 6(B) is provided in pairs and is positioned in any area of ​​the shower head. For example, the induction coil 22 is positioned on the inner and outer circumferences and is individually connected to a pair of high-frequency power supplies 21. This makes it easy to adjust the temperature distribution. Other functions and effects are the same as described above.

[0043] Figures 7(A) and 7(B) show yet another embodiment of the head unit 16 and induction coil 22 in a thermal CVD apparatus S for diamond synthesis according to an embodiment of the present invention. The shower head 11 of the head unit 16 shown in Figures 7(A) and 7(B) is formed in the shape of a rectangular parallelepiped, and the induction coil 22 of the induction heating means 20 is formed to face each other on a pair of opposing sides of the shower head 11. In Figure 7(B), a pair of induction coils 22 are provided, and each induction coil 22 is formed in a flat, rectangular spiral shape to match its corresponding side, and is individually connected to a pair of high-frequency power supplies 21. The shape of the outlet portion 10 can be optimized for substrates that are angular, long, or arranged in a grid, and the temperature distribution can be easily adjusted. Other functions and effects are the same as described above.

[0044] Figure 8 shows a thermal CVD apparatus S for diamond synthesis according to another embodiment of the present invention, with its outlet 10 and induction coil 22. A connecting pipe 40 is also connected to this outlet 10, and these are integrally formed in a tubular shape of the same diameter to constitute a nozzle body 41. The induction coil 22 is formed in a spiral and cylindrical shape, is arranged coaxially with the outlet 10, and faces the side wall surface of the outlet 10. The other configurations are the same as described above. This also produces the same functions and effects as described above.

[0045] <Example of experiment> Next, an experimental example is shown. In this example, the head unit 16 and induction coil 22 were formed in the same shape as in Figures 1 to 3. The head unit 16 was made of tantalum, with a total length of 50 mm, the shower head 11 had a diameter of 8 mm and a length of 25 mm, and the connecting pipe 15 had a diameter of 7 mm and a length of 25 mm. The head unit 16 and induction coil 22 were installed in a chamber 1 with a capacity of 8300 CC, and the shower head 11 was induction heated by a high-frequency power supply 21 (frequency of approximately 70 kHz) to synthesize diamond. The heating temperature of the shower head 11 was set to 1900°C, and the temperature of the substrate 5 was set to 1000°C. Figure 9 shows the heating state of the shower head 11. By introducing hydrogen and methane and performing homoepitaxial growth using a diamond single crystal substrate as the base material, single-crystal diamond was efficiently synthesized.

[0046] In the above experimental example, the head unit 16 was formed from tantalum, but it is not necessarily limited to this, and as shown in the embodiments, it may be formed from other materials, and modifications may be made as appropriate. Furthermore, it goes without saying that the present invention is not limited to the synthesis of diamond, but can be applied to the synthesis of various substances. Those skilled in the art will find it easy to make many modifications to these exemplary embodiments without substantially departing from the novel teachings and effects of the present invention, and many of these modifications fall within the scope of the present invention. [Explanation of Symbols]

[0047] S. Thermal CVD apparatus for diamond synthesis 1 Chamber 2 Gas supply pipe 3. Gas Supply Department 4. Vacuum exhaust section 5 Base material 6. Substrate Stage 7 base 10. Air outlet section 11 Shower head 12 small hole 13. Air vent wall 14 Hollow part 15 connecting pipes 16 Head Units 20 Induction heating means 21 High frequency power supply 22 Induction Coil 30 1st radiation thermometer 31 Second radiation thermometer 40 connecting pipes 41 Nozzle body

Claims

1. A thermal CVD apparatus for synthesizing diamonds, comprising a substrate stage for holding a substrate in a vacuum chamber, and a nozzle for blowing a raw material gas toward the synthesis surface of the substrate, wherein the raw material gas is thermally decomposed and the thermal decomposition products of the raw material gas are bonded to the synthesis surface of the substrate to synthesize diamonds, A thermal CVD apparatus for diamond synthesis, characterized by comprising an induction heating means that inductively heats the outlet portion to cause the raw material gas to be thermally decomposed by the outlet portion.

2. The induction heating means is characterized by heating the outlet portion to a heating temperature of 1500°C to 3000°C, as described in claim 1.

3. The thermal CVD apparatus for diamond synthesis according to claim 2, characterized in that the outlet portion is formed mainly of a conductive material having a melting point temperature higher than the heating temperature of the outlet portion.

4. The thermal CVD apparatus for diamond synthesis according to claim 3, characterized in that the outlet portion is formed mainly of a conductive material having a melting point higher than the heating temperature of the outlet portion and a low vapor pressure.

5. The thermal CVD apparatus for diamond synthesis according to claim 4, characterized in that the outlet portion is formed of one selected from carbon, a material obtained by coating carbon with a substance having a different high melting point and low vapor pressure than said carbon, a metal carbide, a material obtained by coating a metal carbide with a substance having a different high melting point and low vapor pressure than said metal carbide, a conductive metal having a high melting point and low vapor pressure, and a conductive metal having a high melting point and low vapor pressure coated with a substance having a different high melting point and low vapor pressure than said conductive metal.

6. The conductive metal is one selected from tungsten (W), rhenium (Re), osmium (Os), molybdenum (Mo), tantalum (Ta), niobium (Nb), iridium (Ir), hafnium (Hf), zirconium (Zr), platinum (Pt), and any alloy thereof, and the metal carbide is one selected from tungsten carbide (WC), titanium carbide (TiC), zirconium carbide (ZrC), hafnium carbide (HfC), niobium carbide (NbC), and tantalum carbide (TaC), as described in claim 5, a thermal CVD apparatus for diamond synthesis.

7. The thermal CVD apparatus for diamond synthesis according to any one of claims 1 to 6, characterized in that the outlet portion is made of a shower head having an outlet wall formed with a number of small holes for blowing out gas, the induction heating means is made of an induction coil connected to a high-frequency power supply, and the induction coil is arranged facing the outer wall surface of the shower head, excluding the outlet wall, at a required distance apart.

8. The diamond synthesis thermal CVD apparatus according to claim 7, characterized in that it comprises a connecting pipe having one end connected to a gas supply pipe for supplying raw material gas and the other end connected to the shower head, with at least the other end facing into the chamber, and the connecting pipe and the shower head are integrally formed from the same material to constitute a head unit.

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