Coating processes – Direct application of electrical – magnetic – wave – or... – Plasma
Reexamination Certificate
1999-02-16
2002-12-03
Diamond, Alan (Department: 1753)
Coating processes
Direct application of electrical, magnetic, wave, or...
Plasma
C427S588000, C427S585000, C427S595000, C427S573000, C427S578000, C438S096000, C438S097000, C438S484000, C438S482000, C438S488000, C438S485000, C438S490000, C136S258000
Reexamination Certificate
active
06488995
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of forming a microcrystalline silicon film, a method of fabricating a photovoltaic device using this method, and a photovoltaic device fabricated thereby, and particularly to a method of forming a microcrystalline silicon film using a plasma CVD method, a method of fabricating a photovoltaic device using this method, and a photovoltaic device fabricated thereby.
2. Related Background Art
Since the valence electron control of a thin amorphous silicon film became feasible in the latter half of 1970s, headway has come to be made in research and development for applying the thin amorphous silicon film to photovoltaic devices typified by a solar cell. As a process for fabricating a photovoltaic device having a thin amorphous silicon film or the like, there has heretofore been widely known a plasma CVD method utilizing radio frequency (RF) typified by 13.56 MHz.
According to the photovoltaic device having the thin amorphous silicon film or the like fabricated by the RF plasma CVD method, a comparatively good photoelectric conversion efficiency is achieved with a less material compared with bulk-monocrystalline or polycrystalline silicon. However, there is room for improvement in process speed. More specifically, the thickness of the amorphous silicon layer used in an active layer of the photovoltaic device is required to be about several thousands Å. In order to obtain a good-quality amorphous silicon layer suitable for use in the photovoltaic device, however, it must be deposited at an extremely low rate. It has thus been difficult to reduce process cost.
By the way, in the plasma CVD method using 13.56 MHz, it has been confirmed that the quality of a thin film formed tends to be more rapidly deteriorated as the deposition rate of the film is increased, and so it is difficult to enhance throughput upon mass production.
A plasma CVD method using a microwave (MW) typified by 2.45 GHz has also been known as a method capable of forming a comparatively good-quality thin film even when the deposition rate of the thin film is comparatively high. For example, “a-Si Solar Cell according to Microwave plasma CVD Method”, Kazufumi Azuma, Takeshi Watanabe, and Toshikazu Shimada, Preprints in 50th Science Lecture Meeting of Applied Physics Society, pp. 566 is mentioned as an example where an i- type layer is formed by the microwave plasma CVD method.
It has also been known that the use of the frequency of about 100 MHz within the VHF band is effective from the viewpoints of the provision of a high-quality thin amorphous silicon film and the high-speed formation of the film. For example, U.S. Pat. No. 4,933,203 describes the fact that “a good-quality amorphous silicon film is obtained at a ratio, f/d of frequency, f (MHz) to an interelectrode distance, d (cm) of from 30 to 100 in a frequency range of from 25 to 150 MHz”.
In this publication, the relationship between the frequency and the interelectrode distance is defined with respect to the production process of an amorphous silicon film. However, it describes neither the production process of a microcrystalline silicon film nor the deposition pressure and raw gas residence time in a frequency range higher than 150 MHz.
By the way, the thin film photovoltaic device using the thin amorphous silicon film generally has a pin junction structure, and the photoelectric conversion thereof is mainly conducted in an i-type layer. Many attempts to microcrystallize a p-type layer and/or an n- type layer in order to improve junction characteristics have been made to date. For example, Japanese Patent Application Laid-Open No.
57-187971
discloses a method of improving output current and output voltage by forming an i-type layer with amorphous silicon and forming at least a layer of a p-type layer and an n-type layer, which is situated on the side struck by light, with microcrystalline silicon having an average grain size of at most 100 Å.
However, at the present time, in any forming process, a phenomenon (the so-called Staebler-Wronski effect) that the defect density of an i-type layer is increased upon exposure to light to cause the reduction of photoelectric conversion efficiency has become a great problem in practical use in pin-type solar cells using amorphous silicon for the i-type layer.
In recent years, it has been attempted to use i-type microcrystalline silicon for a photoelectric conversion layer of an amorphous silicon type thin film photovoltaic device. For example, in 25th IEEE PV Specialists Conference, Washington, May 13-17, 1996, a group of Shah et al. in Neuchatel University has reported a pin-type microcrystalline silicon solar cell having a photoelectric conversion efficiency of 7.7% without being attended with deterioration by light, which was fabricated by using microcrystalline silicon for all the layers of a p-type layer, an i-type layer and an n-type layer.
A process for forming the microcrystalline silicon layers adopted by this group is basically the same as the constitution of the conventional RF plasma CVD method and does not use a high-temperature process above 500° C. required for the formation of a thin crystalline silicon film such as a thin polycrystalline silicon film. It is also characterized in that the frequency of 110 MHz within the VHF band is adopted as plasma forming frequency.
As described above, the pin-type solar cell using the i-type microcrystalline silicon film formed by using the frequency within the VHF band has a great advantage in that the solar cell is not attended with deterioration by light though it may be fabricated by a low-temperature process.
According to the above-described report from the group of Shah et al. in Neuchatel University, the deposition rate of the i-type layer of microcrystalline silicon was 1.2 Å/s, and the thickness thereof was 3.6 &mgr;m. It is found by a simple calculation that the time required to form the i-type layer of microcrystalline silicon is as long as at least 8 hours. Therefore, its throughput is very small though it has a comparatively high conversion efficiency and is not attended with deterioration by light. As a result, it is difficult for such a process to reduce its production cost.
In order to realize the mass production of the pin- type solar cell using microcrystalline silicon for an i- type layer, it is essential to enhance the deposition rate of the i-type layer of microcrystalline silicon by leaps and bounds. However, pin-type solar cells using the microcrystalline silicon for the i-type layer and having a comparatively good photoelectric conversion efficiency have been comparatively lately fabricated, and so the technique for forming the i-type microcrystalline silicon layer at a high speed has been scarcely known under the existing circumstances.
For example, when a high-temperature process above 500° C. is used, it is expected that energy for crystallization can be obtained as thermal energy from a substrate, and so the formation of a film at a high speed can be conducted with comparative ease. However, the use of the high-temperature process incurs a possibility that the deterioration of cell characteristics may occur due to mutual diffusion at a cell interface, and involves a problem that process cost is increased.
SUMMARY OF THE INVENTION
The present invention has been completed in view of the foregoing circumstances, and it is an object of the present invention to provide a method of forming a microcrystalline silicon film having excellent semiconductor characteristics.
Another object of the present invention is to provide a method of forming a microcrystalline silicon film, by which a microcrystalline silicon film having good characteristics can be formed even when the formation is conducted at a low temperature and a high deposition rate.
A further object of the present invention is to provide a method of forming a microcrystalline silicon film, by which a microcrystalline silicon film suitable for use in an i-type layer of a pin-type
Nishimoto Tomonori
Sano Masafumi
Canon Kabushiki Kaisha
Diamond Alan
Fitzpatrick ,Cella, Harper & Scinto
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