Coating processes – Coating by vapor – gas – or smoke – Carbon or carbide coating
Reexamination Certificate
2001-06-04
2002-10-01
Meeks, Timothy (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Carbon or carbide coating
C427S575000
Reexamination Certificate
active
06458415
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of forming a diamond film and a film-forming apparatus, and particularly, to a method of forming a diamond film and a film-forming apparatus utilizing microwave plasma.
2. Description of the Background Art
Various methods have been invented for forming diamond from vapor phase, such as a hot-filament CVD method, a microwave plasma assisted CVD method and so forth. The microwave plasma assisted CVD method is especially suitable, among others, for forming a high-purity polycrystalline diamond film and an epitaxial diamond film, whereby a high-quality diamond film can easily be obtained compared to the case with other methods. The other methods are associated with some problems that degrade the quality of the diamond film. For example, the hot-filament CVD method involves metal contamination from filament, and a plasma jet method involves metal contamination from an electrode. Moreover, in a combustion flame method, nitrogen in the air is mixed into diamond, degrading the quality of the diamond film. Thus, the microwave plasma assisted CVD method has been widely used as a method of obtaining a high-quality diamond film, and recently, developments have been propelled for obtaining a large area of high-quality diamond film.
The microwave plasma assisted CVD method has an advantage in that such a high-quality diamond film can easily be obtained, but suffers a drawback in that the resulting film is varied in its thickness and quality in a wide range of distribution, especially when compared to the case with the hot-filament CVD method. Thus, it is particularly difficult to obtain a large size of diamond film having a uniform thickness and quality by the microwave plasma assisted CVD method. Currently, there is not even a guideline for adjusting the variation as described above, and such a guideline is still being searched for. As a guideline in forming a diamond film by the microwave plasma method, it is known to use a temperature of a substrate measured using a radiation thermometer and a thermocouple within a reactor, and further spectrum analysis by plasma emission spectroscopy or the like. However, the substrate temperature measured by the radiation thermometer is essentially associated with the plasma emission, making it difficult to obtain an accurate temperature of the substrate. Furthermore, when the thermocouple is used for a temperature measurement, the temperature cannot directly be obtained unless the substrate is in direct contact with the thermocouple. Even if the direct contact was possible, such contact would cause disturbance, which affects formation of the diamond film. Whereas, when the plasma emission spectroscopy is used for diagnosing a plasma state, observation on the spot is possible without any contact, causing no disturbance to the plasma state. Thus, conventionally, the diagnosis using the plasma emission spectroscopy has been actively performed. The measurement using the plasma emission spectroscopy has been successful in certain ways, for instance, contamination by nitrogen, which significantly interferes with the formation of the diamond film, can be found instantly. However, the plasma emission spectroscopy has not yet reached the level where the quality and the deposition rate of the diamond film can be predicted.
SUMMARY OF THE INVENTION
The present invention is directed to provide a method for forming a diamond film from reaction gas excited by microwave, and particularly for forming a large size of a high-quality diamond film by controlling a manufacturing condition based on information on spectroscopic measurement of plasma emission, and to provide a film-forming apparatus for forming such a diamond film.
In the method of forming a diamond film according to the present invention, a gas mixture of hydrocarbon gas and hydrogen gas is introduced into a reactor where the gas mixture is excited by microwave which is also introduced into the reactor to generate plasma, in order to form a diamond film on a substrate. In the forming method, plasma emission is spectroscopically measured to control a formation condition of the diamond film such that the spectrum of a carbon molecule (C
2
: hereinafter referred to as “carbon molecule”) falls within a predetermined range of requirement.
The formation method of the diamond film from the microwave-excited plasma of this invention is based on a new idea in that the spectrum of the carbon molecule is strongly correlated with the quality of the diamond film and its distribution, and hence only an emission spectrum band of the carbon molecule is herein observed. Thus, a controlling method can be simplified, since the only requirement is to adjust the formation condition of the diamond film such that the spectrum of the carbon molecule is within the predetermined range, and therefore the formation condition of the diamond film can be precisely adjusted. As a result, the distribution of quality, i.e. spatial variation of quality, is suppressed, so that a large area of homogeneous and high-quality diamond film can be obtained. Any apparatus may be employed for forming the diamond film described above, in which reaction gas is excited by microwave to attain a plasma state and the diamond film is formed on a substrate by the plasma. A microwave plasma assisted CVD apparatus or another apparatus may be used.
In the method of forming a diamond film according to the present invention, the spectrum of a carbon molecule is a vibration spectrum of the carbon molecule, and a formation condition is controlled such that a vibration temperature obtained by such a spectrum falls within a predetermined range.
The inventors of the present invention spectroscopically measured the emission of microwave plasma, and found that the vibration temperature of a carbon molecule can be derived from the emission spectrum band of the carbon molecule, i.e. one of activated molecule species constituting the plasma. They also came to a new idea in that the vibration temperature is closely related to the deposition rate and quality of the diamond, and the distribution thereof. The vibration temperature of the carbon molecule can be derived using the procedure described below.
In plasma, electrons are much lighter than atomic nuclei, and hence moves much faster. This allows the movement of the electrons and that of the atomic nuclei to be precisely separated for further discussion. Such a way of discussing these movements independent of each other is a precise approximation method called Born-Oppenheimer approximation. When Born-Oppenheimer approximation is possible, the intensity I
ev′v″J′J″
of spectral lines contained in a band spectrum emitted due to the transition of a molecule between electron states can be represented by the equation (1) below.
I
ev′v″J′J″
=Cf
4
q
v′v″
S
J′J″
×exp
[[−(
hc/kT
ex
)
T
e
]+[−(
hc/kT
vib
)
G
(
v
′)]+[−(
hc/kT
rot
)
F
(
J
′)]] (1)
wherein e is a type of electron-term transition, v is the quantum number of vibration, J is the quantum number of rotation, and an addition of ′ indicates a high level whereas that of ″ indicates a low level. Moreover, C is a constant, f is the vibration number of the spectral lines, q
v′v″
is a Franck-Condon factor and S
J′J″
is a Honl-London factor. Furthermore, h is a Planck constant, c is the speed of light, and k is a Boltzmann constant. In addition, T
ex
, T
vib
and T
rot
indicate an excitation temperature, a vibration temperature and a rotation temperature, respectively, and T
e
, G(v′) and F(J′) indicate the term values in the electron state, in the vibration state and in the rotation state, respectively. Noting the transition between certain electron states, the equation (1) is separated by the term of the vibration temperature and that of the rotation tem
Imai Takahiro
Matsuura Takashi
Meguro Kiichi
Fasse W. F.
Fasse W. G.
Fuller Eric B
Meeks Timothy
Sumitomo Electric Industries Ltd.
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