Coating processes – Direct application of electrical – magnetic – wave – or... – Plasma
Reexamination Certificate
1999-03-03
2003-11-11
Chen, Bret (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Plasma
C427S578000
Reexamination Certificate
active
06645573
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates a process for forming a microcrystalline silicon series thin film (this film will be hereinafter referred to as “&mgr;c-silicon series thin film” or “&mgr;c-Si series thin film”) and an apparatus suitable for practicing said process. More particularly, the present invention relates to a process and an apparatus which enable one to form a highly reliable &mgr;c-Si series thin film having a large area and a high energy conversion efficiency which is usable in the production of semiconductor devices such as electrophotographic light receiving members (or electrophotographic photosensitive members), image input line sensors, image pickup devices, photovoltaic devices (including solar cells), and the like.
2. Related Background Art
Hitherto, solar cells comprising a photovoltaic element which converts sunlight into electric energy have been widely using as a small power source in daily appliances such as electronic calculators, wrist watches, and the like. Such a solar cell is expected to provide a practically usable power generation source which can replace the power generation source based on fossil fuels such as petroleum.
Incidentally, in a solar cell the photoelectromotive force of a pn junction is used in the functional portion. In general, the pn junction is constituted by a semiconductor material such as a semiconductor silicon material or a semiconductor germanium material. The semiconductor functions to absorb sunlight and generate photocarriers of electrons and holes, where the photocarriers drift due to an internal electric field of the pn junction, followed by being outputted to the outside.
Now, in view of the efficiency of converting light energy into electricity, it is preferred to use a single crystalline silicon material. However, crystalline silicon materials including a single crystalline silicon material have an indirect optical end, and therefore, they are small in light absorption. In this connection, in the case of a solar cell in which a single crystalline silicon is used (this solar cell will be hereinafter referred to as “single crystal solar cell”), it is necessary for the single crystal solar cell to have a thickness of at least 50 &mgr;m in order for the solar cell to sufficiently absorb incident sunlight. In this case, if the single crystalline silicon material is replaced by a polycrystalline silicon material in order to diminish the production cost of the solar cell, the problem of the above indirect optical end cannot be solved unless the thickness is increased. The polycrystalline silicon material has problems such as grain boundaries and others.
In view of attaining a large area and a reasonable production cost for a solar cell, a so-called thin film silicon solar cell which is represented by an amorphous silicon solar cell having a semiconductor layer comprising an amorphous silicon thin film produced by way of CVD (chemical vapor phase deposition) has been evaluated as being more advantageous. In fact, currently, amorphous silicon solar cells have been widely used as a small power source in daily appliances. However, in order for such a amorphous silicon solar cell to be used as an ordinary power generation source, the photoelectric conversion efficiency and must be improved the performance stabilized.
A solar cell in which a microcrystalline silicon (a &mgr;c-silicon) as a carrier generation layer has been proposed (see, A. Shah et al., 23th IEEE Photovoltaic Specialist Cont. (1993), p. 839).
The most popular film-forming method for depositing such &mgr;c-silicon series thin film or amorphous silicon thin film is a plasma CVD process. In the plasma CVD process, the formation of a &mgr;c-silicon series thin film or an amorphous silicon thin film is conducted, for instance, in the following manner., That is, a film-forming raw material gas such as silane (SiH
4
) or disilane (Si
2
H
6
) is introduced into a reaction chamber in which a substrate on which a film is to be deposited is arranged, if necessary, while being diluted by hydrogen gas (H
2
), a high frequency power with an oscillation frequency of 13.56 MHz in an RF band region is supplied in the reaction chamber to generate plasma whereby decomposing the film-forming raw material gas to produce reactive active species having, resulting in depositing a &mgr;c-silicon thin film or an amorphous silicon thin film on the substrate. In the case where the film formation is conducted by mixing a doping gas such as phosphine (PH
3
), diborane (B
2
H
6
) or boron fluoride (BF
3
) to the film-forming raw material gas, it is possible to form a doped &mgr;c-silicon thin film whose conductivity is controlled to n-type or p-type.
However, such &mgr;c-silicon thin film has a disadvantage. The photoelectric conversion efficiency of a solar cell in which such &mgr;c-silicon thin film is used is lower than that of a crystalline series solar cell. In addition, for the &mgr;c-silicon thin film, there is also a disadvantage in that the deposition rate thereof is low.
In general, the formation of a &mgr;c-silicon thin film is conducted by using RF glow discharge. However, the &mgr;c-silicon thin film thus formed has an indirect optical end as well as in the case of a crystalline silicon thin film, and therefore, its light absorption is small. In this connection, in the case of a &mgr;c-silicon solar cell in which a &mgr;c-silicon thin film is used, it is necessary for the &mgr;c-silicon solar cell to have a thickness of about 5 &mgr;m, and therefore, a lot of time is required to produce the &mgr;c-silicon solar cell.
Shah describes that the formation of a &mgr;c-silicon thin film is conducted using a high frequency power with an oscillation frequency of 70 MHz. The deposition rate in this case is about 1 Å/sec. which is smaller.
With respect to the formation of an amorphous silicon (a-Si) thin film by way of RF plasma CVD, there is a report in that for the high frequency discharge in the RF band region hitherto, discussion has been made by raising the oscillation frequency has been discussed. Particularly, in the Applied Physics-related joint lecture meetings of 1990 Autumn and 1991 Spring (28p-MF-14 and 28p-S-4), Oda et al. of Tokyo Institute of Technology have reported that amorphous silicon thin films were formed by conducting glow discharge using a high frequency power with an oscillation frequency of 144 MHz (which is of VHF (very high frequency) band region) and the amorphous silicon thin films were evaluated.
Additionally, U.S. Pat. No. 4,400,409 discloses a process of continuously preparing a photovoltaic element by using a continuous plasma CVD apparatus of a roll-to-roll system. This document describes that a plurality of glow discharge regions are separately arranged along the path of a sufficiently long flexible substrate having a desired width which is continuously transported to pass through each of said glow discharge regions, and while forming a desired semiconductor layer on the substrate in each glow discharge region, the substrate is continuously transported, whereby a photovoltaic element having a desired semiconductor junction can be continuously formed.
In the case of forming a &mgr;c-silicon series thin film by RF glow discharge using a high frequency power with an oscillation frequency of 13.56 MHz as in the foregoing prior art, the following problems need to be solved or improved.
(1) There are such disadvantages for semiconductor devices in which such &mgr;c-silicon thin film are used, because of the basic property of the thin film. That is, in the case of a thin film transistor, the carrier mobility is small. In the case of a photo sensor, its S/N ratio defined by a ratio between light conductivity and that dark conductivity. In the case of a solar cell, its photoconductivity (&sgr;p) is small.
(2) With respect to production yield, in the case of a large area semiconductor device in which such &mgr;c-silicon series thin film is used, a decrease in the yield is caused due to the distributions and
Higashikawa Makoto
Sano Masafumi
Canon Kabushiki Kaisha
Chen Bret
Fitzpatrick ,Cella, Harper & Scinto
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