Photochemical vapor deposition apparatus

In a photochemical vapor deposition apparatus, a reaction space in which a substrate is to be placed and a discharge space adjacent to the reaction space, in which electric plasma discharge is generated for radiating ultraviolet rays which cause photochemical decomposition reaction of a photoreactive gas, are surrounded by the same vessel, and discharging electrodes are provided in the discharge space so as to be opposite to each other in a first level and a second level, which are different in level in the direction in which the spaces align. The discharging electrode arranged in the first level, which is closer to the reaction space, has such a configuration or arrangement that an ultraviolet ray-passing opening is formed. According to the apparatus, a vapor-deposited film can be formed with high efficiency, because a large quantity of ultraviolet rays can be applied to the substrate without any damage of the vapor-deposited film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a photochemical vapor deposition apparatus. 
2. Description of the Prior Art 
Recently, there are studied methods for forming a vapor-deposited film of 
amorphous silicon for use in the photosensitive drum of a duplicating 
machine or a solar cell. On the other hand, a vapor depositing method is 
further utilized in the formation of diverse insulating films or 
protective films, and a variety of vapor depositing methods have been 
proposed in answer to various uses. Among these methods, a photochemical 
vapor depositing method utilizing a photochemical reaction is being now 
particularly watched because of having such advantages that the 
film-deposition rate is remarkably high and a uniform film can be formed 
on a portion of large area of a substrate, too. 
A conventional chemical vapor depositing method utilizing a photochemical 
reaction comprises placing a substrate in an air-tight vessel made of 
material through which ultraviolet rays can be fully transmitted, feeding 
a photoreactive gas to flow through the vessel and applying ultraviolet 
rays radiated from an ultraviolet discharge lamp outside of the vessel 
through the wall thereof onto the substrate so that a photochemical 
reaction is caused to decompose the photoreactive gas and the resulting 
reaction product is vapor-deposited onto the substrate. In spite of having 
the above-mentioned remarkable advantages, this conventional photochemical 
deposition method that may be called "outer discharge type", has however 
been found to have such a defect that the reaction product is also 
vapor-deposited on the inner wall of the vessel, with impeding the 
transmission of ultraviolet rays seriously. 
Thus, a photochemical vapor deposition apparatus, that may be called "inner 
discharge type", has been studied and developed. In the apparatus of this 
type, a reaction space and a discharge space are air-tightly surrounded by 
the same one vessel. The reaction space forms a passage for a 
photoreactive gas and in this reaction space a substrate is to be placed. 
In the discharge space, electric plasma discharge is generated and 
ultraviolet rays radiated from the plasma are applied onto the substrate 
to cause a photochemical reaction of decomposition of the photoreactive 
gas. Between the plasma and the substrate, there is no partition member 
that impedes passing of the ultraviolet rays. 
In the photochemical vapor deposition apparatus of an inner discharge type, 
a substrate is horizontally placed on the bottom of the vessel and 
discharging electrodes are arranged in such a state that they are opposite 
to each other in a horizontal direction with the discharge space 
therebetween, whereby plasma discharge is generated in the horizontal 
direction. Since the plasma of electric discharge generated between the 
electrodes diffuses or expands mainly in the direction perpendicular to 
the discharging direction, however, there is a fear that the diffused or 
expanded plasma will damage the vapor-deposited film on the substrate 
placed below. In order to prevent such damaging of the film, it is 
required that the substrate be placed at a position more remote from the 
plasma than the mean free path of ions or electrons in the plasma. On the 
other hand, in order to increase the intensity of ultraviolet rays applied 
onto the substrate to improve the efficiency of the vapor deposition, the 
substrate should be kept near an ultraviolet ray source as much as 
possible. After all, in the photochemical vapor deposition apparatus of an 
inner discharge type, vapor depositing with sufficiently high 
film-deposition rate is not carried out, because the applying efficiency 
of ultraviolet rays is lowered due to the placement of the substrate at a 
position remote enough from the ultraviolet ray source, namely the plasma 
of the discharge. 
SUMMARY OF THE INVENTION 
With the foregoing in view, an object of this invention is to provide a 
photochemical vapor deposition apparatus of an inner discharge type in 
which electric plasma discharge is generated without diffusing or 
expanding of the plasma in the direction of a substrate by a simple 
construction, thereby permitting a substrate to be placed at a position 
close enough to an ultraviolet ray source of plasma that a 
vapor-deposition can be carried out with high efficiency. 
In accordance with this invention, there is provided a photochemical vapor 
deposition apparatus wherein a reaction space, in which a substrate for 
vapor deposition is to be placed, and a discharge space adjacent to the 
reaction space, in which electric plasma discharge is generated for 
radiating ultraviolet rays which cause a photochemical decomposition 
reaction of a photoreactive gas, are surrounded by the same vessel, and 
discharging electrodes are arranged in the discharge space in a first 
level and a second level, which are different in level in the direction in 
which the reaction space and the discharge space align so that they are 
opposite to each other, wherein the discharging electrode arranged in the 
first level which is closer to the reaction space, has such a 
configuration or arrangement that an opening for passing ultraviolet rays 
is formed. 
The photochemical vapor deposition apparatus of an inner discharge type 
according to this invention can achieve the formation of a vapor-deposited 
film with extremely high efficiency, because the quantity of ultraviolet 
rays applied onto the substrate is increased. 
The principle and construction of this invention will be clearly understood 
from the following detailed description and appended claims, taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
The present invention will be concretely described with reference to 
embodiments shown in the accompanying drawings. 
FIG. 1 shows the first embodiment of the photochemical vapor deposition 
apparatus of an inner discharge type according to this invention. In this 
embodiment, a gas supply port 11 through which a photoreactive gas and a 
gas for electric discharge are supplied is provided in the lower part of a 
cylindrical air-tight vessel 1, a gas exhaust port 12 through which these 
gases are to be exhausted is provided in the upper part thereof, and a 
substrate support 13 with a heater contained therein is provided at the 
top part of the vessel 1. A substrate 4 to be subjected to vapor 
deposition, which is put in and out of the vessel 1 through an opening 
(not shown) with a shutter therefor, is supported on the lower surface of 
the substrate support 13, and the space just under the substrate support 
13 constitutes a reaction space 5. A discharge space 3 in which plasma 
discharge will be generated is formed adjacent to and under the reaction 
space 5, and no partition such as quartz glass or the like is provided 
between the discharge space 3 and the reaction space 5. An anode 6 is 
arranged in the first level L.sub.1 at which lies a border between the 
reaction space 5 and the discharge space 3. As shown in FIG. 2, the anode 
6 is an electrode consisting of a wire-netting of circular form, made of 
tungsten for example, having meshes N which are openings through which 
ultraviolet rays pass. The meshes N of the wire-netting of the anode 6 are 
relatively rough so as not to obstruct substantially the passage of 
ultraviolet rays. A cathode 7 is arranged in the second level L.sub.2 of 
the bottom part of the vessel 1. As the cathode 7, there may be used an 
electrode comprising of one or a plurality of elements which have been 
generally used up to now for such purposes. In this embodiment, the 
cathode 7 consists of a wire-netting, made of tungsten for example, like 
the anode 6, and it has denser meshes than the anode 6. A heater 8 is 
provided in contact with the lower surface of the cathode 7, and by this 
heater 8, the heating of the cathode 7 required for emitting electrons is 
conducted. In order to improve the efficiency of electron emission of the 
cathode 7, a paste of an electron-emitting substance such as an oxide of 
alkaline-earth metal may be applied on the cathode 7 in such a manner that 
the meshes thereof are clogged. Thus, plasma discharge is vertically 
generated between the anode 6 and the cathode 7, and ultraviolet rays 
radiated from the plasma are passed through the meshes N of the anode 6 so 
as to be applied to the substrate 4 supported above. 
A concrete example of the vapor deposition using the above-mentioned 
apparatus will be described. A photochemically reactive gas which was a 
mixed gas consisting of argon of 5 mmHg as a carrier gas, mercury vapor of 
3.times.10.sup.-3 mmHg as a photosensitizer and silane gas of 1 mmHg as a 
photoreactive gas for vapor deposition was fed through the gas supply port 
11 to flow through the reaction space 5, and a mixed gas for discharging 
which comprised argon of 8 mmHg and mercury vapor of 2.times.10.sup.-3 
mmHg was supplied into the discharge space 3 with the photoreactive gas. A 
substrate 4 consisting of an alumina plate was supported on the substrate 
support 13 and was heated at about 150.degree. C. by the heater. Under 
such a condition, an electric power with a voltage of 60 V and an amperage 
of 30 A was applied between the anode 6 and the cathode 7 to generate 
electric plasma discharge in the discharge space 3, and ultraviolet rays 
radiated from the plasma of the electric discharge in the gas of argon and 
mercury were applied to the substrate 4 through the meshes of the anode 6 
so that the silane gas was photochemically decomposed and the resultant 
product was deposited to form an amorphous silicon film on the substrate 
4. 
In the first embodiment, it is important that in the discharge space 3, the 
anode 6 and the cathode 7 are arranged respectively, in the first level 
L.sub.1 and the second level L.sub.2 different in level from each other in 
the direction in that the reaction space 5 and the discharge space 3 
align, namely in the up-and-down direction in FIG. 1, whereby the plasma 
discharge is generated in vertical direction. Although the plasma diffuses 
or expands in the direction perpendicular to the direction of discharging, 
namely, it diffuses or expands in horizontal direction in FIG. 1 in this 
case, it can scarcely diffuse over the anode 6 upward into the reaction 
space 5 where the substrate 4 is placed. Therefore, when a substrate 4 is 
positioned at a position closer to the discharge space 3, a 
vapor-deposited film is not damaged by ions or electrons produced in the 
plasma. 
Furthermore, since the anode 6 arranged in the first level L.sub.1 that is 
closer to the discharge space 3 consists of a wire-netting with relatively 
rough meshes, the radiated ultraviolet rays are not obstructed 
substantially by the anode 6 and therefore can pass upward through the 
anode 6 to the substrate 4 with a smaller loss thereof. The larger the 
thickness of plasma becomes, the more the quantity of ultraviolet rays 
radiated is increased. In the first embodiment described above, the 
distance between the anode 6 and the cathode 7 is regarded as the 
thickness of the plasma, taken from the reaction space 5 where the 
substrate 4 is placed, because the electric plasma discharge is generated 
in the vertical direction. In this embodiment, accordingly, it is easy to 
make the thickness of plasma large so that the quantity of ultraviolet 
rays radiated toward the substrate 4 increases more than that in the 
lateral direction, and after all, a large quantity of ultraviolet rays are 
permitted to be applied onto the substrate 4, as compared with a 
conventional apparatus in which electric discharge is generated in the 
horizontal direction. Since it is possible to arrange the substrate 4 at a 
position closer to the discharge space 3 and it is further possible to 
increase the quantity of ultraviolet rays to be applied onto the substrate 
4, as mentioned above, the quantity of ultraviolet rays effectively 
applied to the substrate 4 can be widely increased, and as a result, the 
deposition rate of the vapor-deposited film can be remarkably increased. 
In the apparatus of the embodiment illustrated in FIG. 1, the 
above-mentioned effects can be realized even if the apparatus is set in 
any position in which another portion thereof is positioned at the top. 
For example, there may be allowable the following states, namely a state 
wherein the apparatus of FIG. 1 is turned by an angle of 90.degree. so 
that the reaction space and the discharge space adjoin each other in the 
horizontal direction, or a state wherein the apparatus of FIG. 1 is turned 
by an angle of 180.degree. so that the reaction space is positioned at the 
lower part of the vessel and the discharge space is adjacent to and above 
the reaction space. In the case of the state wherein the substrate is 
placed opposite and below the cathode with the anode interposed, however, 
there arises the problem that, when the cathode has an electron-emitting 
substance applied thereon, the electron-emitting substance may possibly 
peel off and deposit on the vapor-deposited film to contaminate the same. 
In order to solve this problem, it is effective to use a cathode 
consisting of a wire-netting of fine meshes and having electron-emitting 
substance thereon in such a state that the fine meshes are clogged 
therewith. Since it is desired that the cathode has an excellent 
electron-emitting ability, an electrode which is made up of a high-melting 
point metal containing an electron-emitting substance such as thoria or 
the like is used as the cathode, or an electron-emitting substance is 
applied to the cathode as in the above-mentioned embodiment. In the latter 
case, as can be seen from the foregoing, it is required that the cathode 
be arranged so as not to be positioned directly above the substrate, and 
otherwise that the electron-emitting substance is applied to the cathode 
so that the peeling-off thereof does not happen. 
The second embodiment of the photochemical vapor deposition apparatus of 
inner discharge type according to this invention is illustrated in FIG. 3 
and FIG. 4. In this embodiment, the construction of the vessel 1, the gas 
exhaust port 12 and the substrate support 13 and the relative positions of 
the discharge space 3 and the reaction space 5 are essentially similar to 
those of the embodiment shown in FIG. 1, except for the parts elsewhere 
specified. In this embodiment, a plurality of cathode elements 17 are 
arranged on the side wall of the vessel 1 in the first level L.sub.1, at 
which lies a border between the discharge space 3 and the reaction space 
5, in such a state that, each cathode element is isolated from other 
cathode elements and arranged along the inner circumference of the vessel 
1, and a relatively large hemispherical anode 16 is arranged on the bottom 
of the vessel 1 in the second level L.sub.2 so as to be positioned at the 
center of the vessel 1. Each of the cathode elements 17 consists of a 
double coil made by coiling a tungsten wire densely in a coil and further 
coiling the thus-coiled tungsten wire roughly in a coil, to which double 
coil a paste of electron-emitting substance such as an oxide of an 
alkaline-earth metal, for example, is applied to improve its electron 
emitting ability. A plurality of gas supply ports 11 are provided in the 
lower part of the vessel 1. 
When electric voltage is applied between the anode 16 and the cathode 
elements 17, plasma discharge is generated in the form of a cone having a 
vertical axis in accordance with the arrangement of the anode 16 and the 
cathode elements 17 and ultraviolet rays radiated from the plasma are 
applied to the substrate 4 supported above the space around which the 
cathode elements 17 are arranged. 
A concrete example of the vapor deposition using the above-mentioned 
apparatus will be described here. A photochemically reactive gas which was 
a mixed gas comprising argon of 5 mmHg as a carrier gas, mercury vapor of 
3.times.10.sup.-3 mmHg as a photosensitizer and silane gas of 1 mmHg as a 
photoreactive gas for vapor deposition was fed through the gas supply 
ports 11 to flow through the reaction space 5, and a mixed gas for 
discharging which comprises argon of 8 mmHg and mercury vapor of 
2.times.10.sup.-3 mmHg was supplied into the discharge space 3 with the 
photoreactive gas. A substrate 4 consisting of an alumina plate was 
supported and heated at about 150 .degree. C. just like as in the first 
embodiment. Under such a condition, an electric power with a voltage of 60 
V and an amperage of 20 A in total was applied between the anode 16 and 
the cathode elements 17 to generate electric plasma discharge in the 
discharge space 3, and ultraviolet rays radiated from the plasma of the 
electric discharge in the gas of argon and mercury were applied to the 
substrate 4 so that the silane gas was photochemically decomposed and the 
resultant product was deposited to form an amorphous silicon film on the 
substrate 4. 
In the second embodiment, it is important not only that the cathode 
elements 17 are arranged in the first level L.sub.1 and the anode 16 is 
arranged in the second level L.sub.2, but also that the plurality of the 
cathode elements 17 are arranged along the inner circumference of the 
vessel whereby the plasma discharge is generated in the form of a cone 
having a vertical axis. In a photochemical vapor deposition process 
utilizing ultraviolet rays, the more the quantity of applied ultraviolet 
rays is increased, the higher the deposition rate of a vapor-deposited 
film becomes, and the quantity of ultraviolet rays radiated is increased 
proportionally to the electric power applied to electrodes. Accordingly, 
it may be effective to increase the electric power applied to the 
electrodes in order to make the deposition rate of the vapor-deposited 
film higher. However, there arises the problem that a cathode may be 
damaged and degraded considerably when the current density in the cathode 
is excessive. That is to say, there is a limit to increasing the electric 
power to be applied. In the second embodiment, since the plurality of the 
cathode elements 17 are employed, the current density in each of the 
cathode elements 17 is not excessive and the cathode elements 17 are not 
damaged even when a large electric power in total is applied thereto so 
that a large quantity of ultraviolet rays are to be radiated. Accordingly, 
the deposition rate of the vapor-deposited film may be made higher by 
increasing the electric power to be applied without any problems. 
Since the direction in which the substrate 4 is supported is not a 
direction perpendicular to the discharging direction in which the plasma 
diffuses or expands, also in the second embodiment, the vapor-deposited 
film is not damaged by ions or electrons of the plasma, even when the 
substrate 4 is kept close to the discharge space 3 in which plasma 
discharge is generated. 
Moreover in the second embodiment, since the thickness of the plasma in the 
direction in which the reaction space 5 is positioned with respect to the 
discharge space 3 is nearly equal to the distance between the first level 
L.sub.1 and the second level L.sub.2, the plasma has a larger thickness in 
that direction and a larger quantity of ultraviolet rays are applied to 
the substrate 4, as compared with a conventional apparatus, and since the 
ultraviolet rays radiated from the plasma are applied to the substrate 4 
through the space surrounded by the cathode elements 17, they are not 
hindered or obstructed by the cathode elements 17. 
In the second embodiment, as mentioned above, there are the following 
advantages, namely, the ultraviolet rays radiated in the direction in 
which the thickness of the plasma is larger are to be utilized, the 
substrate is permitted to be closer to the discharge space and the 
ultraviolet rays directed to the substrate are not hindered, as in the 
embodiment shown in FIG. 1. In the second embodiment, furthermore, the 
deposition rate of a vapor-deposited film can be remarkably increased, 
because the quantity of ultraviolet rays radiated can be increased by 
applying a larger electric power to the discharging electrodes without 
damage thereof and therefore the quantity of the ultraviolet rays 
effectively applied to the substrate becomes large. In addition, the 
above-mentioned effects can also be realized in this embodiment, even 
though the apparatus is set in any state in which any portion thereof is 
positioned at the top. 
Now, a variation of the apparatus according to this invention is described. 
In the vessel 1, an additional gas supply port may be provided in the 
reaction space 5 so that the photoreactive gas fed through the additional 
gas supply port flows through the reaction space only. In such a case, 
only the gas for discharge is fed through the gas supply port 11 which is 
provided in the discharge space. 
Having now fully described the invention, it will be understood for those 
skilled in the art that various changes and modifications can be made 
thereto without departing from the spirit or scope of the invention as set 
forth herein.