Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
2001-11-02
2004-07-20
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
C257S049000, C257S050000, C257S051000, C257S066000, C257S070000, C257S072000, C257S074000, C257S345000, C257S350000
Reexamination Certificate
active
06765229
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor device having TFTs (thin film transistors) provided on an insulating substrate of glass or the like, and a method for producing the semiconductor device.
2. Description of Related Art
TFTs have been conventionally formed on a glass substrate to form a semiconductor device such as an active matrix liquid crystal device or an image sensor. The TFTs are used, for example, to drive the pixels of the liquid crystal device.
The TFTs used in the above devices are generally formed of a silicon semiconductor layer in the form of a thin film. The silicon semiconductor of a thin-film type is classified into two types, an amorphous silicon semiconductor (a-Si) type and a crystalline silicon semiconductor type. The amorphous silicon semiconductor can be relatively easily produced at a low film-forming temperature by a vapor-phase deposition method. Therefore, this type is suitable for mass production, and it has been most generally used. However, this type of silicon semiconductor has inferior physical properties such as electrical conductivity, etc. to the crystalline silicon semiconductor. Therefore, in order to more improve a high-speed response characteristic of TFTs, a producing method for TFTs comprising crystalline silicon semiconductor has been strongly required to be established. As the silicon semiconductor having crystallinity have been known polycrystalline silicon, microcrystalline silicon, amorphous silicon containing crystal components, semi-amorphous silicon having an intermediate state between crystallinity and amorphousness, etc.
The following methods may be used to obtain thin film silicon semiconductors having the foregoing crystallinity:
(1) Crystallinity is established during the formation of the semiconductor film.
(2) An amorphous semiconductor film is formed in advance, and then a laser beam is irradiated to the film to crystalize the film.
(3) An amorphous semiconductor film is formed in advance, and then heated to crystalize the film.
However, in the method (1), it is technically difficult to form a film having excellent semiconductor physical properties on the whole surface of a substrate uniformly. In addition, the film formation must be performed at a temperature above 600° C. and thus an inexpensive glass substrate is unusable, so that a manufacturing cost is increased.
In the method (2), an excimer laser is most generally used at present as a laser beam source for irradiating a laser beam to an amorphous semiconductor film. In this case, the irradiation area of the laser beam is small, and thus this method has a disadvantage that a throughput is low. In addition, the stability of the laser beam is insufficient, so that the whole surface of a large-area substrate cannot be treated uniformly. That is, this method is not practically usable at present.
As compared to the methods (1) and (2), the method (3) has an advantage that it is more suitable to manufacture a large-area semiconductor film. However, this method requires a heating temperature above 600° C., and thus an inexpensive glass substrate is not usable. Therefore, this method must be developed to reduce the heating temperature. Particularly in case of present liquid crystal display devices, a large-area screen design is being promoted, and thus use of a large-size glass substrate is required. When a large-size glass substrate is used, contraction and distortion of a substrate occur in a heating process which is indispensable to produce semiconductors, and they cause a critical problem that the precision of a masking process is reduced. Particularly in a case of 7059 glass which is most generally used at present, it has a distortion point of 593° C., and it is greatly deformed in a conventional heat crystallization method. In addition to the heat problem as described above, a heating time required for crystallization is over several tens hours in a present process, and thus the heating time must be shortened.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method capable of solving the above problems, and specifically to provide a process for producing a silicon semiconductor thin film having crystallinity utilizing a method of heating an amorphous silicon thin film to crystallize the thin film, in which both of lowering of the temperature for crystallization and shortage of the heating time for crystallization can be performed. A silicon semiconductor having crystallinity which is manufactured using the process according to this invention has the same physical properties as or physical properties superior to that manufactured by a conventional technique, and it is usable in an active layer area of TFTs.
The inventors of this application have made the following experiments and consideration for a method of forming an amorphous silicon semiconductor film as described above by a CVD method or a sputtering method, and then heating the film to crystalize the film.
An amorphous silicon film is initially formed on a glass substrate, and then the film is crystallized by heating. The inventors investigated the mechanism of this crystallization. Through the experiments, it was observed that crystal growth of silicon starts at an interface between the glass substrate and the amorphous silicon and proceeds vertically to the substrate surface into a pillar shape in the case that the thickness of the film is larger than a certain thickness.
The above phenomenon is considered as progressing on the basis of a mechanism that crystalline nuclei serving as geneses for crystal growth (species serving as geneses for crystal growth) exist between the glass substrate and the amorphous silicon film, and crystal grow from the crystalline nuclei. These crystalline nuclei are considered as being impurity metal elements or crystal components (as is called as a crystallized glass, it is considered that crystal components of silicon oxide exist on the surface of the glass substrate) existing on the surface of the substrate in a very small amount.
Accordingly, it is expected that a crystallization temperature can be lowered by introducing crystal nuclei more positively. In order to confirm an effect of introducing crystal nuclei, the following experiment was tried. That is, a thin film of different metal in a very small amount was beforehand formed on a substrate, then an amorphous silicon thin film was formed on the different metal film, and then heat-crystallization was conducted on the amorphous silicon thin film. As a result, it was proved that the crystallization temperature was lowered when thin films of some different kinds of metal were beforehand formed on the substrate, and it was expected that crystal growth using foreigners as crystal nuclei had conducted. Accordingly, a more detailed mechanism for plural kinds of impurity metal which could lower the crystallization temperature was studied.
The crystallization mechanism can be considered to be classified into two stages which are an initial nucleus generation stage and a subsequent crystal growth stage from the nuclei. The speed of the initial nucleus generation can be detected by measuring a time elapsing until spotted fine crystals occur at a constant temperature. This time could be shortened in all cases where the thin films of the above kinds of impurity metal were formed on the substrate, and the effect of the introduction of the crystal nuclei on the lowering of the crystallization temperature can be proved. As an unexpected result, through an experiment for examining variation of growth of crystal grains with variation of the heating time after generation of crystal nuclei, it was observed that the speed of the crystal growth after the generation of the nuclei was also rapidly increased when a thin film of a certain kind of metal was formed on a substrate, an amorphous silicon thin film was formed on the metal thin film and then the amorphous silicon thin film was crystallized. A mechanism for this effect has not yet been elucidated at pres
Takayama Toru
Takemura Yasuhiko
Zhang Hongyong
Flynn Nathan J.
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Sefer Ahmed N.
Semiconductor Energy Laboratory Co,. Ltd.
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