JPH06135797A - Method and device for synthesizing diamond - Google Patents

Abstract

PURPOSE:To prevent the melting of a cathode even in the case of coming into contact with a carbon source gas in the DC plasma method, therefore to recycle a raw gas to reduce cost, and prevent the infiltration of the melted cathode into diamond as impurities to enhance the diamond quality. CONSTITUTION:In the DC plasma method (or a DC plasma device 1), a cathode 2 to be used is formed with one kind of metal selected from group IVa or Va metals or the carbides obtained by carbonizing the metals having >=3000 deg.C m.p. The m.p. of the metal is raised when carbonized. Consequently, the cathode 2 formed from the metals or metal carbides is not melted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diamond synthesizing method and a synthesizing apparatus, and more particularly to a diamond synthesizing method and a synthesizing apparatus in which a cathode does not melt even when contacted with a carbon source gas.

[0002]

2. Description of the Related Art Since a diamond synthesis method utilizing thermal plasma (Japanese Patent Laid-Open No. 62-158195) was proposed, many techniques such as a radio frequency (RF) plasma method and a direct current (DC) plasma method have been developed. There is. The thermal plasma method has the advantage that the synthesis rate is extremely fast, but on the other hand, most of the source gas is discharged unreacted,
There is a problem that a large amount of gas is used, the cost is extremely high, and the gas is not practical. Therefore, by paying attention to the fact that most of the exhausted gas is unreacted gas and its composition is almost the same as that of the raw material gas, the exhaust gas is circulated and reused to reduce the cost. Attempts have been made to improve the practicality of the above (Japanese Patent Laid-Open No. 1-164).
795).

[0003]

However, in the DC plasma method, if the exhaust gas is circulated and reused, the hydrocarbon gas comes into contact with the cathode and reacts with it to melt the cathode. There's a problem. The melting of the cathode causes not only the problem that the life of the cathode is shortened but also the problem that the synthesis becomes unstable and that the melted cathode is mixed as an impurity in the diamond.

As a technique for eliminating the melting of the cathode, a hydrocarbon such as methane is not allowed to flow between the discharge electrodes, and a discharge is caused by argon that does not react with the electrodes and hydrogen that hardly reacts with the electrodes. A technique for protecting the electrode and stabilizing the plasma by mixing a hydrocarbon such as methane as a raw material is known (Japanese Patent Laid-Open No. 16479/1989).
No. 5, JP-A-1-179789). However, if such a technique is applied to the above-mentioned technique for reusing the raw material gas, hydrocarbons such as methane have to be separated from the gas discharged after the reaction, which is too costly and the emission There is a problem that there is no point in reusing gas. Further, regarding the material of the cathode, there has not yet been developed a material that solves the above-mentioned cathode melting problem. For example, W (tungsten) mixed with a small amount of W (tungsten) or Th (thorium) is often used as the cathode in the DC plasma method.
This is because tungsten is a material that easily discharges and has heat resistance, but the tungsten cathode reacts with the hydrocarbon gas and its melting point is lowered. In addition, JP-A-63
No. 139095 discloses a tungsten-based electrode (W-Th, W-Ba, W-Sr, etc.) as an arc discharge electrode, and JP-A-64-33096 discloses a rare earth element as an arc discharge electrode. Although a tungsten electrode containing an oxide is disclosed, these electrodes also cannot react with the hydrocarbon gas to lower the melting point and maintain the initial heat resistance.

The applicant of the present application has found that the raw material gas can be reused by selecting carbon monoxide and hydrogen as the raw material gas (Japanese Patent Application No. Hei 2-13981).
No. 1). However, this method has a problem that the raw material gas is limited.

The present invention has been made in view of the above circumstances, and the cathode is not melted even when it comes into contact with a carbon source gas. Therefore, the cost and quality of diamond synthesis can be reduced. An object of the present invention is to provide a diamond synthesizing method and a diamond synthesizing device.

[0007]

In order to achieve the above object, the method for synthesizing diamond of the present invention is to convert a source gas into thermal plasma by direct-current arc discharge between an anode and a cathode, and collide it with a substrate as a plasma jet. In a method for synthesizing diamond, which comprises rapidly cooling and vapor-depositing diamond on a substrate, at least the discharge surface of the cathode has a melting point of 3000 ° C. or higher when a metal is carbonized to form a metal carbide or a melting point of 3000 ° C. or higher. The gas phase synthesis of diamond is carried out using the cathode formed of metal carbide, and if necessary, the gas discharged after the reaction should be recovered, circulated and recycled as the raw material gas for use again. I am doing it. Further, the diamond synthesizing apparatus of the present invention has an anode and a cathode, and a direct current arc discharge is generated between these electrodes to turn the raw material gas into thermal plasma, thereby synthesizing diamond on the substrate in a vapor phase. In, at least the discharge surface of the cathode has a melting point of 3 when the metal is carbonized to form a metal carbide.
A metal formed at a temperature of 000 ° C. or higher or a metal carbide having a melting point of 3000 ° C. or higher is preferably used.
As a metal selected from a group Va metal or a group Va metal, or a metal selected from a group IVa metal or a group Va metal selected from the above metal carbides, if necessary. The gas exhausted after the reaction is recovered and circulated.

The present invention will be described in detail below. First, the diamond synthesizing apparatus of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an example of the diamond synthesizing apparatus of the present invention.

In the drawing, a direct current (DC) plasma device main body 1 is composed of a cathode (cathode) 2, an anode (anode) 3, a vacuum reaction container 4 and the like. Here, at least the discharge surface of the cathode 2 is formed of a metal having a melting point of 3000 ° C. or higher when a metal is carbonized into a metal carbide or a metal carbide having a melting point of 3000 ° C. or higher.

"A metal having a melting point of 3000 ° C. or higher when the metal is carbonized to form a metal carbide" means that the melting point of the metal itself is 3000 ° C. or lower, but when the metal is carbonized, the melting point rises and the melting point increases. A metal having a melting point of 3000 ° C. or higher, or a metal having both the melting point of the metal itself and the melting point of a metal carbide being 3000 ° C. or higher.
Examples of such a metal include Ti (titanium) (melting point 1680 ° C.), Zr (zirconium) (melting point 1860).
IV) such as Hf (hafnium) (melting point 2230 ° C)
Group a metal, Nb (niobium) (melting point 2520 ° C.), T
Va group metals such as a (tantalum) (melting point 3000 ° C.) can be used.

Examples of the “metal carbide having a melting point of 3000 ° C. or higher” include carbides of the above group IVa and Va metals. Specifically, TiC (melting point 3250 ° C.),
ZrC (melting point 3180 ° C), HfC (melting point 3900)
℃), NbC (melting point 3500 ℃), Ta ₂ C (melting point 34
00 ° C.), TaC (melting point 3880 ° C.) and the like.

Since these metal carbides are used as electrodes, they need to have electrical conductivity. In addition, W (tungsten) which is a VIa group metal (melting point 33
80 ° C.) has a melting point of 3000 ° C. or higher as a metal, but as the carbonization progresses, the melting point decreases and metal carbide (W ₂
C; melting point 2750 ° C., WC; melting point 2600 ° C.) is 3000 ° C. or less. Therefore, when this is used as a cathode, the cathode is melted, the molten tungsten is scattered on the substrate, the diamond becomes impure, and the discharge becomes unstable, so that the synthesis cannot be continued, which is unsuitable. Further, Mo (molybdenum) (melting point 2630 ° C.), which is also a VIa group metal, has a melting point of 3 for both metal and metal carbide (MoC; melting point 2690 ° C.).
The temperature is 0000 ° C or lower, which is unsuitable as a cathode.

At least the discharge surface of the cathode 2 should be formed of the above metal or metal carbide. This is because only the discharge surface of the cathode 2 is in contact with the high temperature plasma. Therefore, if the above metal or metal carbide is adhered to water-cooled copper to form a cathode, the inside of the cathode 2 will not have a high temperature. Internal heat resistance is 2000 ~
This is because 2500 ° C. is sufficient. Of course, the entire cathode 2 may be formed of the above metal or metal carbide, or only the portion of the discharge surface of the cathode 2 that comes into contact with plasma may be formed of the above metal or metal carbide.

The anode 3 is formed of a material having a good cooling performance (eg Cu). The shapes and the like of the cathode 2 and the anode 3 are not particularly limited. Further, cooling water (not shown) may be caused to flow through the electrodes. In addition, it is preferable that a slight amount of pure Ar gas is caused to flow around the cathode to form a sheath, because it serves to suppress the deterioration of the cathode.

The DC voltage (arc voltage) and current (arc current) values applied between the electrodes are not theoretically limited, but in a practical range, the voltage is 30 to 200.
V, current range 30-2000A, especially voltage 70-12
It is preferable that the voltage is 0 V and the current is 50 to 500 A.

The gas used in the present invention includes a carbon source gas as a raw material gas and hydrogen (H) as a plasma gas.
₂ ) or an inert gas such as argon (Ar) is used. Here, the carbon source gas means diamond (C)
A gas of a molecule containing a carbon atom (C) which is a constituent element of. Further, the plasma gas refers to a gas used to generate plasma by arc discharge.

As the carbon source gas, various hydrocarbons,
It is also possible to use a gas such as a halogen-containing compound, an oxygen-containing compound or a nitrogen-containing compound, or a gas such as graphite in which carbon is gasified.

Examples of the hydrocarbon include paraffinic hydrocarbons such as methane, ethane, propane and butane; olefinic hydrocarbons such as ethylene, propylene and butylene; acetylene hydrocarbons such as acetylene and allylene.
Diolefin hydrocarbons such as butadiene; cyclopropane, cyclobutane, cyclopentane, cyclohexane,
Examples thereof include alicyclic hydrocarbons such as cyclobutadiene; aromatic hydrocarbons such as benzene, toluene, xylene and naphthalene.

Examples of the halogen-containing compound include halogenated methane, halogenated ethane, halogenated hydrocarbon such as benzene, carbon tetrachloride and the like.

Examples of the oxygen-containing compound include alcohols such as methanol, ethanol, propanol and butanol; methyl ether, ethyl ether, ethyl methyl ether, methyl propyl ether, ethyl propyl ether, phenol ether, acetal, cyclic ether ( Dioxane, ethylene oxide, etc.) ethers; acetone, pinacholine, methyl oxide, aromatic ketones (acetophenone, benzophenone, etc.), diketones, cyclic ketones, and other ketones; formaldehyde, acetaldehyde, butyraldehyde, benzaldehyde, and other aldehydes; formic acid Organic acids such as acetic acid, propionic acid, succinic acid, butyric acid, oxalic acid, tartaric acid, stearic acid; Acid esters such as methyl acetate and ethyl acetate; Ethylene glycol, diethyl Dihydric alcohols such as glycol; carbon monoxide, such as carbon dioxide oxygen-containing inorganic gas, and the like.

Examples of the nitrogen-containing compound include amines such as trimethylamine and triethylamine.

Among these carbon source gases, paraffinic hydrocarbons such as methane, ethane and propane, which have a high gas or vapor pressure at room temperature, ketones such as acetone and benzophenone, alcohols such as methanol and ethanol, and monoxide. Oxygen-containing inorganic gases such as carbon and carbon dioxide are preferable.

In the diamond synthesizing apparatus of the present invention, a substrate holder 6 on which a substrate 7 is placed is provided in a vacuum reaction vessel 4. The inside of the vacuum reaction container 4 is maintained at a desired ultimate vacuum pressure by the pump 8. The gas discharged by the pump 8 is collected in the buffer tank 9 as needed and is supplied again between the electrodes (plasma torch).

Next, the operation of the above apparatus, that is, the method for synthesizing diamond of the present invention will be described. First, in order to start the reaction, a source gas composed of a carbon source gas and plasma gas is supplied between the electrodes (between the cathode 2 and the anode 3), and thermal plasma is generated by DC arc discharge in the electrode (plasma torch), and the plasma jet 5 Gush out as. The plasma jet 5 collides with the substrate 7 placed on the substrate holder 6 and is rapidly cooled to deposit diamond on the substrate. In this case, film-like or granular diamond can be synthesized by appropriately selecting the conditions such as the composition of the source gas and the surface condition of the substrate. For example, granular diamond can be synthesized by vapor-depositing diamond on the surface of nucleation particles such as diamond particles. The carbon source gas may be introduced downstream of the discharge electrodes instead of flowing between the discharge electrodes.

Since the diamond synthesizing apparatus and synthesizing method of the present invention use the cathode made of the above-mentioned specific material, the cathode is less likely to melt than the conventional synthesizing apparatus and synthesizing method using the cathode. Further, the cathode is not melted and the plasma becomes unstable, and the material forming the cathode is not mixed into the diamond. Therefore, high-quality diamond can be synthesized stably for a long time. In particular, in the device and method for synthesizing diamond by collecting and recycling exhaust gas described later,
In the conventional W-based cathode, carbon source gas such as hydrocarbon reacts with the cathode, and the cathode is easily melted, so that it is difficult to put it into practical use, and the present invention can be put to practical use for the first time. Is remarkable.

Next, the case where the exhaust gas is collected and recycled to synthesize diamond will be described. After the synthesis reaction, the gas discharged into the vacuum reaction container 4 is collected in the buffer tank 9 by the pump 8 as needed.
The inside of the vacuum reaction container 4 is evacuated by the pump 8 so as to have a predetermined ultimate vacuum pressure. As the pump 8, it is preferable to use an oil-free type pump such as a dry pump in order to prevent oil from entering the exhaust side.

The material, capacity, filling pressure, etc. of the buffer tank 9 are not particularly limited, but as an example, a 500 liter tank made of SUS304 with a filling pressure of 1-2 Kg / c.
m ² · G. A sensor 10 for measuring the gas composition in the tank is attached to the buffer tank 9 so that the pressure and composition in the buffer tank can be constantly monitored.

A carbon source gas supply pipe 13 and plasma gas supply pipes 14 and 15 are provided in the buffer tank 9. Further, the mass flow meter 1 is connected to each of the supply pipes 13 to 15.
6 to 18 are attached respectively. The other ends of the carbon source gas supply pipe 13 and the plasma gas supply pipes 14 and 15 are connected to gas cylinders (not shown), respectively.

The gas composition in the buffer tank is constantly replenished with the insufficient gas from the gas cylinder, or is replenished with the insufficient gas from the gas cylinder every several hours. This is because changes in gas consumption and gas composition per unit time are small, and slight changes in gas composition do not affect diamond synthesis.

The gas recovered in the buffer tank 9 and having its composition adjusted is supplied to the flow rate controller 11. As the flow rate controller 11, a flow rate controller 11 for synthesis gas that is calibrated with the composition of the carbon source gas and the plasma gas is used.

Although only the carbon atoms in the carbon source gas are consumed in the source gas accompanying the above diamond synthesis, graphite or diamond-like carbon (DLC:
By-products such as Diamond Like Carbon) are produced in an amount of about 1%, and the raw material is consumed by a leak that occurs during circulation. Therefore, in addition to carbon source gases such as CH ₄ and CO, plasma such as Ar and H ₂ is also generated. Gas also needs to be replenished.

As described above, in the present invention, when exhaust gas is collected and recycled to synthesize diamond,
Unlike the conventional case, carbon source gas such as hydrocarbon does not react with the cathode to melt the cathode, and the raw material gas can be recycled. Therefore, it is possible to put the diamond synthesis by the DC plasma method into practical use, which has been difficult to put into practical use in terms of cost. That is, the cost is 1/10 to 1/1
It can be greatly reduced to 00, and can be sufficiently put to practical use. In addition, since the raw material gas is recycled,
No exhaust gas is emitted and no environmental problems occur.

[0033]

The present invention will be described in more detail with reference to the following examples.

Example 1 The diamond synthesizer shown in FIG. 1 was prepared, and Ta (tantalum) having a size of 7 mmφ × 10 mm and a purity of 99.9% or more was used as the cathode. The plasma gas and its flow rate were Ar: 15 liter / min, H ₂ : 15 liter / min, and the carbon source gas and its flow rate were CH.
₄ : 0.5 liter / min, arc voltage 80V,
The operation of the device was started under the conditions of an arc current of 120 A and a pressure of 200 Torr. Diamond was synthesized by continuously operating for 120 minutes while collecting and recycling the exhaust gas. As a result of the above synthesis, 200 on the Mo substrate
mg of diamond (film) was obtained. During the synthesis, the plasma was stable and the voltage fluctuation was within 5%.
When the state of the cathode after the synthesis was examined, the cathode was scarcely damaged (melted). Also, under the same conditions as above, 20 minutes after the start of operation, the substrate was taken out of the apparatus, the surface state of the diamond film formed on the substrate was observed with a scanning electron microscope (SEM), and further elemental analysis of the surface was conducted. EPMA (Electron ProbeMic
ro Analysis; electron probe microanalysis method). As a result, no foreign matter was observed by SEM, and elements other than diamond were not detected by elemental analysis by EPMA.

Example 2 In the diamond synthesizing apparatus shown in FIG. 1, as a cathode, one obtained by sintering HfC (hafnium carbide) powder in a pellet form and silver-brazing it on a copper holder for water cooling was used. Plasma gas and its flow rate are Ar: 1
5 liter / min, H ₂ : 10 liter / min, and the carbon source gas and its flow rate were C 0: liter / mi
The operation of the apparatus was started under the conditions of an arc voltage of 70 V, an arc current of 120 A, and a pressure of 200 Torr. Diamond was synthesized by continuously operating for 120 minutes while collecting and recycling the exhaust gas. As a result of the above synthesis, 250 mg of diamond (film) was formed on the Mo substrate.
was gotten. During the synthesis, the plasma was stable and the voltage fluctuation was within 5%. When the state of the cathode after the synthesis was examined, the cathode was scarcely damaged (melted).

Comparative Example 1 The diamond synthesizer shown in FIG. 1 was prepared, and W (tungsten) having a size of 7 mmφ × 10 mm and containing a small amount of Th (thorium) was used as the cathode. Plasma gas and its flow rate are Ar: 15 liters / mi
n, H ₂ : 15 liter / min, carbon source gas and its flow rate CH ₄ : 0.01 liter / min,
Arc voltage 80V, arc current 120A, pressure 200T
The operation of the device was started under the condition of orr. Diamonds were synthesized by continuous operation while collecting and recycling the exhaust gas. As a result, the carbon source gas (CH ₄ ) that causes the melting of the cathode in all the gases
The cathode melted, and a part of the melted cathode splattered on the substrate, although the amount of was very small. And, even for 5 minutes, the synthesis could not be stably performed. In addition, in the same manner as in Example 1, 10 from the start of operation
When SEM observation and EPMA elemental analysis were performed on the substrate after the minute, foreign matter was observed by SEM,
As a result of EPMA elemental analysis, it was confirmed that this foreign substance was a substance containing W (tungsten), that is, a flying substance from the cathode.

[0037]

As described above, according to the method and apparatus for synthesizing diamond of the present invention, the initial melting point of the cathode is high, or the melting point is high due to the carbonization at the initial stage of diamond synthesis, so that the cathode is deteriorated. Without this, diamond can be synthesized stably for a long period of time. Further, even when the discharged gas is collected and supplied to the plasma torch for recycling, the recycled carbon source gas does not melt the cathode, and thus stable recycling can be performed for a long period of time. The cost can be reduced. Further, no exhaust gas is emitted and no environmental problem occurs. Further, since the cathode does not melt and mix into diamond as an impurity, the quality of diamond can be improved.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an example of a diamond synthesizing apparatus of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Direct current (DC) plasma device 2 ... Cathode 3 ... Anode 4 ... Vacuum reaction container 7 ... Substrate 8 ... Pump

Claims (6)

[Claims] 1. A method for synthesizing diamond in which a raw material gas is converted into thermal plasma by direct-current arc discharge between an anode and a cathode, and a plasma jet is made to collide with a substrate to be rapidly cooled to vapor-deposit diamond on the substrate. The vapor phase synthesis of diamond is performed by using a cathode whose discharge surface is formed of a metal having a melting point of 3000 ° C. or higher when the metal is carbonized to form a metal carbide or a metal carbide having a melting point of 3000 ° C. or higher. The method of synthesizing diamonds. 2. The method for synthesizing diamond according to claim 1, wherein the gas discharged after the reaction is recovered, circulated, and recycled again as a raw material gas for use. 3. A diamond synthesizing apparatus having an anode and a cathode, in which direct-current arc discharge is generated between these electrodes to convert a raw material gas into thermal plasma to vapor-deposit diamond on a substrate. When the metal is carbonized to form a metal carbide on the discharge surface, the melting point is 300.
An apparatus for synthesizing diamond, which is formed of a metal having a temperature of 0 ° C. or higher or a metal carbide having a melting point of 3000 ° C. or higher. 4. The diamond synthesizing apparatus according to claim 3, wherein the metal is any one metal selected from Group IVa metals and Group Va metals. 5. The diamond synthesizing apparatus according to claim 3, wherein the metal carbide is obtained by carbonizing any one metal selected from the group IVa metals and the group Va metals. 6. The diamond synthesizing apparatus according to claim 3, 4 or 5, further comprising means for collecting and circulating the gas discharged after the reaction.