Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2000-05-10
2002-01-08
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S096000, C438S097000, C438S488000, C438S935000, C438S763000
Reexamination Certificate
active
06337224
ABSTRACT:
TECHNICAL FIELD
The present invention relates to methods of manufacturing thin film photoelectric converters and plasma CVD apparatuses used for the methods, and more particularly, to reduction in the cost of silicon-based (silicon or silicon alloy) photoelectric converters and improvements in the performance thereof. Note that in this specification, the terms, “polycrystalline”, “microcrystalline” and “crystalline” refer not only to complete crystalline states, but also to partially amorphous states.
Background Art
Typical thin film photoelectric converters include amorphous silicon-based solar cells. Since amorphous photoelectric conversion materials are formed by plasma CVD, normally at low temperatures around 200° C., the materials can be formed on inexpensive substrates such as glass, stainless steel, and organic films and this is why they are believed to be promising as materials for manufacturing low cost photoelectric converters. Furthermore, since in amorphous silicon, the absorption coefficient is large in the visible light region, short-circuit current of 15 mA/cm
2
or higher has been achieved in a solar cell using an amorphous photoelectric conversion layer having a film thickness of 500 nm or less.
The amorphous silicon-based materials, however, suffer from so-called Stebler-Wronskey effect, i.e. deterioration in the photoelectric conversion characteristic by long-term light irradiation, and the effective sensitivity wavelength region ranges up to about 800 nm. Therefore, in a photoelectric converter using an amorphous silicon-based material, the reliability and performance improvement is limited, and essential advantages of the material that it allows for flexibility in selecting a substrate or applicability in low cost process are not fully taken advantage of.
Meanwhile, in recent years, much energy has been devoted to development of photoelectric converters using a thin film including a crystalline silicon layer such as polycrystalline silicon and microcrystalline silicon. The development is an attempt to compatibly achieve reduction in the cost of a photoelectric converter and improvement in the performance by forming a high quality, crystalline silicon thin film on an inexpensive substrate in a low temperature process and its application to various photoelectric converters such as photo sensor in addition to solar cells is expected.
There are methods of forming such crystalline silicon thin films including directly depositing a film on a substrate by CVD or sputtering and depositing an amorphous film on a substrate in a similar process, followed by thermal anneal or laser anneal for crystallization. Any of these methods requests that the process must be carried out at a temperature of 550° C. or less in order to use an inexpensive substrate as described above.
Among such processes, it is expected that the method of directly depositing a crystalline silicon thin film by plasma CVD can most readily achieve reduction in the process temperature and increase in the area of the film, and that a high quality film can be relatively easily obtained. If a polycrystalline silicon thin film is obtained by this method, a high quality, crystalline silicon thin film is formed on a substrate by some process, and then, using the film as a seed layer or a crystallization control layer, a film may be formed thereon such that a high quality, polycrystalline silicon thin film can be formed at a relatively low temperature.
Meanwhile, there is a well known method of forming a film, using a silane-based material gas diluted 10 times or more with hydrogen, by setting the pressure in a plasma reaction chamber in the range from 10 mTorr to 1 Torr to obtain a microcrystalline silicon thin film. In this method, a silicon thin film can be readily formed into microcrystal at a temperature around 200° C. For example, a photoelectric converter including a photoelectric conversion unit formed by a pin junction of microcrystalline silicon is disclosed in Appl. Phys. Lett., Vol. 65, 1994, p. 860. The photoelectric conversion unit includes a p-type semiconductor layer, an i-type semiconductor layer which is a photoelectric conversion layer and an n-type semiconductor layer, simply deposited sequentially by plasma CVD, and all the semiconductor layers are of microcrystalline silicon. However, the deposition speed is less than 10 nm/min in the thickness-wise direction by conventional methods and under conventional conditions and too low for obtaining a high quality, crystalline silicon film and a high performance, silicon-based thin film photoelectric converter, and the speed is not more than the case of forming amorphous silicon films.
Meanwhile, an example of a silicon film formed at a relatively high pressure of 5 Torr by low temperature plasma CVD is disclosed by Japanese Patent Laying-Open No. 4-137725. In this example, however, the silicon thin film is directly deposited on a substrate of glass or the like and is presented simply as an example in comparison to the invention disclosed by Japanese Patent Laying-Open No. 4-137725, and the quality of the film is too low to be applied to photoelectric converters.
In general, if the pressure condition is raised in plasma CVD, a considerable amount of powdery product or dust is generated in the plasma reaction chamber, in which case such dust is likely to fly and come into the deposited film and generate pin holes in the film. In order to reduce such deterioration in the quality of the film, the reaction chamber must be frequently cleaned inside. Particularly when a film is formed at a low temperature of 550° C. or less and the pressure in the reaction chamber is raised, such disadvantages can be noticed. In the manufacture of a photoelectric converter such as a solar cell, a thin film having a large area must be deposited, which could lower the yield and increase the labor and cost for maintaining the film forming device.
Therefore, when a thin film photoelectric converter is manufactured by plasma CVD, conventional methods have employed a pressure condition of 1 Torr or less as described above.
A polycrystalline photoelectric converter including a crystalline silicon-based thin film photoelectric conversion layer as described above suffers from the following disadvantage. More specifically, when used as a photoelectric conversion layer for a solar cell, the film thickness must be at least several &mgr;m to several tens Ktm so that the film is allowed to absorb enough sunlight in view of the absorption coefficient of crystalline silicon, whether it is polycrystalline silicon or partly amorphous microcrystalline silicon. This thickness is larger than that of an amorphous silicon photoelectric conversion layer by about one or two digits.
Meanwhile, by conventional methods, the forming speed of a film is not more than that of an amorphous silicon film, as low as about 10 nm/min for example if various condition parameters including temperature, pressure in the reaction chamber, high frequency power and gas flow ratio are considered for obtaining a high quality, crystalline silicon-based thin film at a low temperature by plasma CVD. Stated differently, the crystalline silicon thin film photoelectric conversion layer requires time for forming several to several ten times as long as that of an amorphous silicon photoelectric conversion layer, which impedes the throughput in the manufacturing process of photoelectric converters from increasing and the cost from being reduced.
Conventional apparatuses to produce solar cells include those of the inline type according to which a plurality of film deposition chambers are linearly coupled as shown the block diagram in FIG.
6
and those of the multi-chamber type according to which a plurality of chambers are arranged around an intermediate chamber as shown in the block diagram in FIG.
7
. Note that for an amorphous silicon solar cell, there is also known a simple method called “single chamber method”, according to which all the semiconductor layers are formed in the same chamber. However, in order to preven
Okamoto Yoshifumi
Yamamoto Kenji
Yoshimi Masashi
Kaneka Corporation
Niebling John F.
Simkovic Viktor
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