Discharge plasma generating method, discharge plasma...

Details Discharge plasma generating method, discharge plasma... Discharge plasma generating method, discharge plasma...

2001-01-10

2002-09-24

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Discharge device load with fluent material supply to the...

Plasma generating

C315S111710, C313S231310

Reexamination Certificate

active

06456010

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-069044, filed Mar. 13, 2000; and No. 2000-085281, filed Mar. 24, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a discharge plasma generating method, discharge plasma generating apparatus, semiconductor device fabrication method, and semiconductor device fabrication apparatus used in the deposition of thin films of semiconductors such as amorphous silicon, nanocrystalline silicon, polycrystalline silicon, and silicon nitride for use in various electronic devices such as an amorphous Si-based solar cell, a thin-film polycrystalline Si-based solar cell, an LCD (liquid crystal display), a thin-film transistor of a flat panel display, a photosensitive body of a copying machine, and an information recording device, or used in the etching of thin films of semiconductor elements.
The surface processing technologies using a discharge plasma of a reactive gas are being more and more demanded to increase the area and processing speed in the fabrication of diverse electronic devices every year. To meet these needs, research and development are recently being extensively made, not only in the industrial world but also in learned societies, particularly for the use of plasma chemical vapor deposition (to be referred to as PCVD hereinafter) and a very-high-frequency (VHF) power supply having a frequency of 30 to 800 MHz.
Apparatuses and methods of generating a discharge plasma of a reactive gas are disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-325759 (to be referred to as reference
1
hereinafter) and L. Sansonnens et al., “A voltage uniformity study in large-area reactors for RF plasma deposition”, Plasma Source Sci. Technol. 6(1997), pp. 170-178 (to be referred to as reference
2
hereinafter).
Unfortunately, the conventional PCVD and plasma etching have the following problems.
(1) The first problem is an increased size of a substrate to be processed. Needs for large-sized solar cells and flat panel display LCDs are increasing. When an amorphous silicon film (to be referred to as an a-Si film hereinafter) is formed on a 50 cm×50 cm substrate by the conventional PCVD method, the film thickness distribution is, e.g., as shown in FIG.
6
A. Also, when an a-Si film is formed on a 100 cm×100 cm substrate by the conventional method, the film thickness distribution is, e.g., as shown in FIG.
6
B. As shown in
FIGS. 6A and 6B
, as the frequency of a high-frequency power supply increases, the film thickness distribution of an a-Si film increases, so the film thickness uniformity significantly lowers. In the field of LCDS, a film thickness distribution of ±5% is permitted. In the field of solar cells, a film thickness distribution of a maximum of ±20%, preferably ±10% is permitted. Accordingly, when such large-sized substrates are used in the conventional PCVD method, the only practical power-supply frequency is 13.56 MHz, and no VHFs in a frequency band exceeding this frequency are practical.
(2) The second problem is a method of increasing the surface processing speed (improving the productivity) and improving the film quality. To increase the speed of surface processing, the discharge plasma density must be increased. As a method of increasing the discharge plasma density, the use of a VHF in a frequency band exceeding a versatile power-supply frequency of 13.56 MHz is recommended. Also, to improve the film quality, ion damage to a film must be reduced. To this end, reducing the plasma potential related to the ion energy is effective. The use of a VHF which reduces the plasma potential is also recommended for this purpose.
As examples of needs for a high surface processing speed, low cost (a high film formation speed and a large area) and high quality (a low defect density and high crystallinity) are demanded in the formation of thin films for solar cells and thin-film transistors for flat panel displays. Mat. Res. Soc. Symp. Proc. Vol. 424, p. 9, 1997 (to be referred to as reference
3
hereinafter) disclosed a method of increasing the film formation speed and improving the quality of films by using a VHF power supply. A VHF is recently found to be suited to the high-speed, high-quality formation of particularly a thin nanocrystalline Si film which is attracting attention as a new thin film replacing an a-Si film.
When a VHF is used, however, a VHF progressive wave A
1
propagating in the forward direction on an electrode and a VHF reflected wave A
2
propagating in the opposite direction interfere with each other as shown in
FIG. 2A
, generating a standing wave A
3
as shown in FIG.
2
B. This standing wave A
3
promotes the spatial nonuniformity of the discharge plasma density, thereby increasing the film thickness distribution of a thin film formed on a substrate
804
. This is contrary to the need described in item (1) above.
Other causes which promote the film thickness nonuniformity resulting from this standing wave are as follows.
(a) The first cause is an increase and nonuniformity of the impedance of a propagation path resulting from the skin effect. When a VHF is supplied from a power supply
807
to an electrode
802
via feeding point (feeder distribution center)
809
,
809
a
, and
809
b
as shown in
FIGS. 1A and 1B
, the skin effect causes this VHF to propagate, along the surface portion of the electrode
802
, as the progressive wave A
1
in one direction and the progressive wave A
2
in the other direction on the electrode surface as shown in FIG.
2
A. This interference generates the standing wave as shown in FIG.
2
B. This skin effect appears more notably for a VHF than for a versatile frequency of 13.56 MHz as a high frequency. A surface depth &dgr; at which a current flows by this skin effect is given by
&dgr;=(3.14
×f
·&mgr;·&sgr;)
−0.5
where f is the frequency, &mgr; is the permeability, and a is the conductivity.
When the electrode
802
is copper, for example, its surface depth is about 19 &mgr;m for a frequency of 13.56 MHz as a high frequency, about 10 &mgr;m for a VHF of 50 to 60 MHz, and about 5.8 &mgr;m for a VHF of 150 MHz. Therefore, in PCVD or plasma etching using a VHF, the impedance of a power-supply propagation path from a power supply to a discharge portion of a discharge electrode increases. This increases the impedance and makes voltage distribution nonuniform. Accordingly, the spatial uniformity of the discharge plasma density cannot be held any longer.
(b) The second cause is mutual interference between a plasma and VHF. When a VHF becomes nonuniform as described above, the plasma distribution becomes nonuniform accordingly, thereby producing a load distribution (the impedance lowers as the plasma density rises) to an electrode. This influences the VHF distribution.
As described above, the discharge plasma density becomes nonuniform owing to the generation of a standing wave by a reflected wave from the electrode end, the influence which the presence of the electrode impedance has on the voltage distribution, and the mutual interference between a plasma and VHF. Consequently, the film thickness uniformity suffers in large-area PCVD using a VHF.
For example, when the dimensions of a parallel plate electrode exceed 30 cm×30 cm or the VHF frequency exceeds 30 MHz, the influence of the standing wave A
3
becomes conspicuous. This makes it difficult to achieve the minimum necessary film thickness uniformity of semiconductor devices.
FIG. 3
is a graph showing a voltage distribution and an ion saturation current distribution by plotting the position (cm) on a discharge electrode on the abscissa and a voltage vpp (V) and ion saturation current on the ordinate. This data is obtained by the conventional method which uses a VHF of 100 MHz. Referring to
FIG. 3
, a characteristic curve B
1
indicates the voltage distribution, and a characteristic curve

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