Plasma chemical vapor deposition apparatus

Details Plasma chemical vapor deposition apparatus Plasma chemical vapor deposition apparatus

1999-01-19

2002-04-02

Mills, Gregory (Department: 1763)

Coating apparatus

Gas or vapor deposition

With treating means

C118S7230MP, C315S248000, C315S111010, C156S089210, C346S106000, C219S121430

Reexamination Certificate

active

06363881

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a plasma CVD (Chemical Vapor Deposition) apparatus for preparation of a thin film used in various electronic devices such as an amorphous silicon solar cell, a microcrystalline solar cell, a thin film polycrystalline solar cell, a thin film semiconductor device, an optical sensor, and a semiconductor protective film.
Various plasma CVD apparatuses are used for preparation of an amorphous silicon (hereinafter referred to as “a-Si”) thin film, a microcrystalline thin film, a polycrystalline thin film, or a silicon nitride (hereinafter referred to as “SiNx”) thin film. The conventional plasma CVD apparatus can be classified typically into a type in which is used a ladder type electrode for discharge generation and another type in which are used plate electrodes arranged in parallel. The ladder type electrode includes, for example, a ladder antenna electrode and a ladder inductance electrode.
Japanese Patent Disclosure (Kokai) No. 4-236781 discloses a plasma CVD apparatus using a ladder type electrode of various shapes.
FIG. 10
shows a typical example of the plasma CVD apparatus disclosed in JP '781 quoted above. As shown in the drawing, a ladder type electrode
2
for discharge generation and a heater
3
for heating a substrate are arranged in parallel within a reaction vessel
1
. A high frequency power having a frequency of, for example, 13.56 MHz is supplied from a high frequency power source
4
to the ladder type electrode
2
for discharge generation through an impedance matching device
5
. As shown in
FIG. 11
, the ladder type electrode
2
for discharge generation is connected at one end to the high frequency power source
4
via the impedance matching device
5
and is also connected at the other end to a ground lead
7
and, thus, to the ground. Also, the reaction vessel
1
is connected to the ground.
The high frequency power supplied to the ladder type electrode
2
for discharge generation serves to generate a glow discharge plasma in a free space between the substrate heater
3
, which is also connected to the ground together with the reaction vessel
1
, and the ladder type electrode
2
for discharge generation. After generation of the glow discharge plasma, the high frequency power flows through the discharge space into the wall of the reaction vessel
1
and into the ground through the ground lead
7
connected to the ladder type electrode
2
. A coaxial cable is used as the ground lead
7
.
A mixed gas consisting of, for example, monosilane and hydrogen is supplied from a bomb (not shown) into the reaction vessel
1
through a reactant gas introducing pipe
8
. The reactant gas introduced into the reaction vessel
1
is decomposed by a glow discharge plasma generated by the ladder electrode
2
for discharge generation so as to be deposited on a substrate
9
disposed on the heater
3
and heated to a predetermined temperature. On the other hand, the gas within the reaction vessel
1
is exhausted by a vacuum pump
11
through an exhaust pipe
10
.
In preparing a thin film by using the apparatus described above, the inner space of the reaction vessel
1
is exhausted first by operating the vacuum pump
11
, followed by introducing a mixed gas consisting of, for example, monosilane and hydrogen into the reaction vessel
1
through the reactant gas introducing pipe
8
. In this step, the inner pressure of the reaction vessel
1
is maintained at 0.05 to 0.5 Torr. Under this condition, a high frequency power is supplied from the high frequency power source
4
to the ladder type electrode
2
for discharge generation so as to generate a glow discharge plasma. Therefore, the reactant gas is decomposed by the glow discharge plasma generated in the free space between the ladder type electrode
2
and the substrate heater
3
so as to generate Si-containing radicals such as SiH
3
, and SiH
2
. These radicals are attached to a surface of the substrate
9
so as to form an a-Si thin film.
FIG. 12
shows another type of the conventional plasma CVD apparatus in which are used plate electrodes arranged in parallel. As shown in the drawing, the apparatus comprises a reaction vessel
21
. A high frequency electrode
22
and a substrate heater
23
are arranged in parallel within the reaction vessel
21
. A high frequency having a frequency of, for example, 13.56 MHz is supplied from a high frequency power source
24
to the high frequency electrode
22
through an impedance matching device
25
. The substrate heater
23
is connected to the reaction vessel
21
. Also, the reaction vessel
21
is connected to the ground. It follows that the substrate heater
23
is indirectly connected to the ground to constitute a ground electrode, with the result that a glow discharge plasma is generated in the free space between the high frequency electrode
22
and the substrate heater
23
.
A mixed gas consisting of, for example, monosilane and hydrogen is supplied from a bomb (not shown) into the reaction vessel
21
through a reactant gas introducing pipe
26
. On the other hand, the gas within the reaction vessel
21
is exhausted by a vacuum pump
28
through an exhaust pipe
27
. A substrate
29
is disposed on the substrate heater
23
so as to be heated to a predetermined temperature.
For forming a thin film by using the apparatus shown in
FIG. 12
, the inner space of the reaction vessel
21
is exhausted first by operating the vacuum pump
28
, followed by introducing a mixed gas consisting of, for example, monosilane and hydrogen into the reaction vessel
21
through the reactant gas introducing pipe
26
. In this step, the inner pressure of the reaction vessel
21
is maintained at 0.05 to 0.5 Torr. If a high frequency power is supplied from the high frequency power source
24
to the high frequency electrode
22
, a glow discharge plasma is generated within the reaction vessel.
The monosilane gas contained in the mixed gas supplied through the reactant gas introducing pipe
26
into the reaction vessel
21
is decomposed by the glow discharge plasma generated in the free space between the high frequency electrode
22
and the substrate heater
23
so as to generate Si-containing radicals such as SiH
3
and SiH
2
. These Si-containing radicals are attached to a surface of the substrate
29
so as to form an a-Si thin film.
However, any of the prior arts using a ladder type electrode and plate electrodes arranged in parallel gives rise to problems as described below.
(1) In the apparatus shown in
FIG. 11
, a reactant gas, e.g., SiH
4
, is decomposed by an electric field generated in the vicinity of the ladder type electrode
2
into Si, SiH, SiH
2
, SiH
3
, H, H
2
, etc. so as to form an a-Si film on the surface of the substrate
9
. However, if the frequency of the high frequency power is increased from the present level of 13.56 MHz to 30 to 150 MHz in an attempt to increase the rate of forming the a-Si film, the electric field in the vicinity of the ladder type electrode fails to be distributed uniformly, leading to a markedly poor uniformity in the thickness of the formed a-Si film.
FIG. 13
is a graph showing the relationship between the plasma power source frequency and the film thickness distribution in respect of a substrate having an area of 30 cm×30 cm. It should be noted that the size of the substrate which permits ensuring a uniformity in the film thickness distribution, i.e., deviation of ±10% from an average film thickness, is 5 cm×5 cm to 20 cm×20 cm.
The reason why it is difficult to increase the frequency of the high frequency power source
4
in the apparatus using a ladder type electrode is as follows. Specifically, non-uniformity of impedance derived from the construction of the ladder type electrode is inherent in the apparatus shown in
FIG. 10
, with the result that a strong plasma light emission is localized, as shown in FIG.
14
. For example, a strong plasma is generated in a peripheral portion alone of the ladder type electrode, and is not generated in a

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