Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor
Reexamination Certificate
2001-05-30
2002-12-03
Lee, Eddie (Department: 2815)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Amorphous semiconductor
C438S487000, C148SDIG009
Reexamination Certificate
active
06489222
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having a circuit constituted of a thin film transistor (hereafter referred to as a TFT). For example, the present invention relates to an electro-optical device, typically an EL display device or a light-emitting device, and to the structure of electronic equipment in which an electro-optical device is included as a part. Further, the present invention relates to a method of manufacturing the above device. Note that, throughout this specification, the category “semiconductor device” indicates general devices which can function by utilizing semiconductor characteristics, and the above electro-optical devices and electronic equipment are within the semiconductor device category.
2. Description of the Related Art
Research into techniques of increasing crystallinity when implementing laser annealing on an amorphous semiconductor film formed on an insulating substrate such as glass, thereby crystallizing the amorphous semiconductor film, has been widely performed in recent years. Silicon is often used as the amorphous semiconductor film.
Glass substrates are blessed with low cost and good workability compared with the synthetic quartz substrates often used conventionally, and possess an advantage of being easily manufactured into a large surface area substrate. This is the reason that the above research is being carried out. Further, the preferable use of a laser for crystallization is due to the melting point of glass substrates. Lasers are capable of imparting high energy only to the amorphous semiconductor films, without raising the temperature of the substrate very much.
Crystalline semiconductor films are formed from many crystal grains, and therefore they are also referred to as polycrystalline semiconductor films. A crystalline semiconductor film formed by implementing laser annealing has a high mobility, and TFTs are formed using the crystalline semiconductor film, for example TFTs for a pixel portion and a driver circuit portion formed on one glass substrate, are enthusiastically utilized in such devices as a monolithic type liquid crystal electro-optical device.
Further, a high output pulse laser beam such as an excimer laser beam widely used due to the fact that a method for performing laser annealing, in which the laser beam is formed into a square spot shape of several centimeters, or into a linear shape having a length of 10 cm or more by an optical system, on a surface to be irradiated, and then scanned (or the laser beam irradiation apparatus can be moved relative to the surface to be irradiated), has high productivity and superior workability.
In particular, the productivity is high if a linear shape beam is used, because differing from a case of using a spot shape laser beam in which it is necessary to scan forward and backward, and left and right, laser irradiation can be performed over the entire surface to be irradiated by scanning only in a direction perpendicular to the longitudinal direction of the linear shape beam. Scanning in a direction perpendicular to the longitudinal direction is performed because that is the scanning direction having the maximum efficiency. The use of a linear shape beam, formed by an appropriate optical system from a pulse emission excimer laser beam, in the laser annealing method at present due to superior productivity is becoming the main production technique for liquid crystal display devices which use TFTs. This technique makes possible a monolithic type liquid crystal display device in which TFTs forming a pixel portion (pixel TFTs), and driver circuit TFTs formed in the periphery of the pixel portion, are all formed on one glass substrate.
However, a crystalline semiconductor film manufactured by the laser annealing method is formed by a plurality of crystal grains, and the position and size of the crystal grains is random. The TFTs formed on the glass substrate are separated by element, and therefore formed by separating the crystalline semiconductor film into island shape patterns. The size and the position of the grains cannot be set in this case. Compared to within a crystal grain, there are an almost limitless number of re-crystallization centers and capture centers in the boundaries of the crystal grains (grain boundaries) which are the cause of amorphous structure and crystal defects. If a carrier is trapped in a capture center, then the potential of the grain boundary increases and this becomes a barrier with respect to the carrier, and it is known that the electric current transporting characteristics therefore drop. The crystallinity of the semiconductor film of a channel forming region has a great influence on the TFT properties, but it is nearly impossible to form the channel forming region by a single crystal semiconductor film, eliminating the effect of the grain boundaries.
In order to solve this type of problem, position is controlled for laser annealing, and various tests have been performed for forming large size crystal grains. A solidification process of the semiconductor film after the laser beam is irradiated to the semiconductor film is explained here first.
A certain amount of time is necessary until crystal nuclei develop within a semiconductor film which has been completely melted by laser beam irradiation, a large number of crystal nuclei are generated uniformly (or non-uniformly) in the completely melted region, and the solidification process of the completely melted semiconductor film is completed by crystal growth. The position and the size of the crystal grains obtained in this case becomes random.
Further, for cases in which the semiconductor film is not completely melted by the laser beam irradiation and solid semiconductor regions remain partially, crystal growth begins from the solid semiconductor regions immediately after laser beam irradiation. As stated above, a certain amount of time is necessary until crystal nuclei develop in the completely melted region. Thus, during the period until crystal nuclei develop in the completely melted region, the solid-liquid interface (indicating the interface between the solid semiconductor region and the completely melted region), which is the crystal growth leading edge, moves in a direction parallel to the film surface of the semiconductor film (hereafter referred to as a lateral direction), and crystal grains grow to a length several tens of times longer than the film thickness. A very large number of crystal nuclei develop uniformly (or non-uniformly) in the completely melted region with this type of growth, which is completed by crystal growth. This type of phenomenon is hereafter referred to as “super lateral growth.”
A laser beam energy region for also achieving super lateral growth in amorphous semiconductor films and in polycrystalline semiconductor films exists. However, this energy region is extremely narrow, and positions at which large size crystal grains are obtained cannot be controlled. In addition, microcrystalline regions, in which a very large number of crystal nuclei develop, and amorphous regions exist in regions outside the large size crystal grains.
As explained above, the position of grain growth and the growth direction can be controlled provided that the lateral direction temperature gradient is controlled by a laser beam energy region in which the semiconductor film is completely melted (making heat flow arise in a lateral direction). Several tests have been performed in order to realize this method.
For example, Ishihara, R., and Burtsev, A., (AM-LCD '98, pp. 153-156, 1998) reported on a laser annealing method in which they formed a high melting point metallic film between a substrate and a base silicon oxide film, and formed an amorphous silicon film above the high melting point metallic film, and then irradiated an excimer laser beam from both the top surface side of the substrate (defined in this specification as the face upon which the film is formed) and from the bottom surface sid
Brock II Paul E
Fish & Richardson P.C.
Lee Eddie
Semiconductor Energy Laboratory Co,. Ltd.
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