Coating processes – Coating by vapor – gas – or smoke – Carbon or carbide coating
Reexamination Certificate
1997-08-18
2001-06-19
Hendrickson, Stuart L. (Department: 1754)
Coating processes
Coating by vapor, gas, or smoke
Carbon or carbide coating
C423S446000
Reexamination Certificate
active
06248400
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for vapor phase synthesis of diamond. More particularly, it relates to a method and apparatus for producing a diaphragm of diamond which is used for a speaker or the like.
2. Description of the Related Art
Since diamond has an extremely high Young's modulus and a small specific gravity, its propagation speed of an acoustic wave is 18,000 m/s and this value is by far higher than those of other materials. Accordingly, diamond is an extremely excellent material for a diaphragm of a speaker, a headphone, a microphone, and so forth. Furthermore, a diamond film produced by vapor phase synthesis has a suitable internal loss irrespective of an extremely high propagation speed of the acoustic wave and in this sense, it is an ultimate material for a diaphragm.
Conventionally, the diamond diaphragm has been produced by the steps of depositing a diamond film on the surface of a substrate of Si, etc., shaped into the shape of a diaphragm (dome shape) by a hot filament assisted chemical vapor deposition (hereinafter referred to as HFCVD) and then removing the substrate by etching (for example, “Radio Technology”, April, 1991, published by Radio Gijutsusha).
FIG. 1
is a schematic view of a method of producing a diamond diaphram according to the conventional HFCVD. In drawing, reference numeral
1
denotes a diamond film;
2
is a Si substrate;
3
is a filament;
4
is a gas; and
5
is an exhaust system.
However, this method involves the following problems.
(1) The film deposition rate according to the HFCVD is as low as about 1 &mgr;m/hr, and an extended period of time is necessary to form the film.
(2) The arrangement of the filament and a support structure are extremely complicated because a diamond film having a uniform thickness must be deposited on a curved plane.
(3) The filament must be frequently changed due to its elongation and deterioration (carbonization), and the film deposition conditions are not stable, so that film quality is not constant.
(4) A step of removing the substrate by etching is necessary after the deposition of the diamond film, and this etching, too, requires a long time.
(5) The same number of dome-like Si substrates as the number of the diaphragms to be produced must be prepared.
(6) Gas efficiency for the vapor phase synthesis of diamond is as low as only several percents.
For the reasons described above, the conventional diamond diaphragm is very expensive, and producibility is also very low.
It is therefore an object of the present invention to provide a method and apparatus for vapor phase synthesis of diamond capable of solving the problems described above and economically producing the diamond diaphragm with a high producibility.
SUMMARY OF THE INVENTION
To accomplish the object described above, the present invention provides a method of producing a diamond shaped film which comprises the steps of preparing a substrate having a predetermined shape, depositing a diamond film onto the substrate by a DC plasma jet CVD process, and after the deposition of the film, cooling said substrate so as to automatically peel the diamond film from the substrate due to the difference of thermal expansion coefficients between diamond and the substrate during the cooling step.
When the diamond film to serve as the diaphragm of the speaker, etc., is produced according to the present invention by the DC plasma jet CVD process (refer to Japanese Unexamined Patent Publication (Kokai) No. 64-33096), the diamond film can be produced by far more economically in a shorter time with a reduced number of production steps but with a higher producibility than the conventional production method.
The DC plasma jet CVD process is a high speed synthesis method of a diamond film or powder in which a high temperature thermal plasma generated by D.C. arc discharge is discharged as a plasma jet from a torch nozzle, and this plasma jet is applied onto the substrate, by which a film deposition rate of some hundreds of microns per hour is attained. This method of depositing a diamond film makes it possible;
(1) to reduce the film deposition time to only one several tenth to one several hundredth of the HFCVD;
(2) to eliminate the necessity for the complicated works such as adjustment and replacement of the filament that have been required in the HFCVD, and to continuously produce large quantities of products, because this method jets the plasma jet from the plasma torch to the substrate; and
(3) to shape the substrate into a concaved or convexed shape, so that a sound emission plane can be smoothed in the former case and a voice coil bobbin can be integrally synthesized in the latter.
When a material having low adhesion with diamond is used for at least the surface of the substrate, the diamond film automatically peels from the substrate due to the difference of thermal expansion coefficients between diamond and the substrate material when the substrate temperature is lowered after completion of the synthesis of diamond. Therefore, the peeling step can be eliminated, and the diamond film can be continuously formed by one substrate.
The material for at least the surface of the substrate is preferably such a material which has low bonding strength with diamond but has great thermal expansion coefficient and thermal conductivity. More specifically, those metals which are likely to form carbides, such as silicon, molybdenum, tungsten, etc., are not preferred because they have higher adhesion to diamond than other materials. Those metals which are likely to form a solid solution with carbon, such as iron, nickel, cobalt, etc., are not preferred, either, because they cannot easily form high quality diamond. Suitable examples are those metals which do not easily form compounds and solid solutions with carbon, such as copper, gold and platinum, and ceramic materials. A composite material such as superhard WC-C and a TiC-clad stainless steel are effective materials for the substrate. When the thickness of diamond film is great, the diamond film can peel from the substrate without cracks even when a metal which is likely to form a carbide, such as molybdenum, is used.
According to the present invention, there is provided a method for vapor phase synthesizing diamond which includes the steps of synthesizing diamond by a chemical vapor phase synthesis using a carbon compound as a starting gas, and recycling the gas after being used for the synthesis of diamond as a part of the starting gas.
In the synthesis of diamond by the DC plasma jet CVD process, argon and hydrogen are generally used as the plasma gas, and a few percents, on the basis of the total amount, of hydrocarbon gas such as methane gas is added as a carbon source. Since the efficiency of conversion from the hydrocarbon gas to diamond is only several percents, efficiency in utilization of the gas is extremely low. Accordingly, if the used gas is circulated for re-use, the hydrocarbon gas needs be supplied only in the amount corresponding to the amount of the carbon content consumed for the synthesis of diamond. Accordingly, the quantity of the used gas can be drastically reduced. The composition of hydrocarbons in the gas exhausted from the torch is different from the composition first supplied to the torch, but the influences due to the difference of the kind of hydrocarbons are small in the DC plasma jet CVD process.
Therefore, the gas consumption can be drastically reduced and the cost of the diamond synthesis can also be drastically reduced, by recycling and reusing the gas used for the vapor synthesis of diamond. This method can be effectively applied not only to the production of a diaphragm of the speaker, or the like, but also to plasma CVD of diamond, in general. However, this effect is particularly great in the DC plasma jet CVD because it uses greater quantity of gas than the method as shown in FIG.
1
.
In the vapor phase synthesis of diamond using a DC plasma torch, an inert gas plus hydrogen are typically supplied to the
Akashi Shinobu
Itani Tsukasa
Kurihara Kazuaki
Sasaki Ken-ichi
Armstrong Westerman Hattori McLeland & Naughton LLP
Fujitsu Limited
Hendrickson Stuart L.
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