Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-04-19
2003-04-29
Cuneo, Kamand (Department: 2829)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
Reexamination Certificate
active
06555875
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor film having a crystal structure, formed on a substrate having an insulating surface, and a method of manufacturing a semiconductor device using the semiconductor film as an active layer. In particular, the present invention relates to a method of manufacturing a thin film transistor having an active layer formed of a crystalline semiconductor. In this specification, a semiconductor device generally refers to those capable of functioning by utilizing semiconductor characteristics, and includes an electro-optical device such as an active matrix type liquid crystal display device formed by using a thin film transistor, and electronic equipment provided with such an electro-optical device as a component.
2. Description of the Related Art
A technique has been developed, in which an amorphous semiconductor film is formed on a light-transparent substrate having an insulating surface, and a crystalline semiconductor film obtained by crystallizing the amorphous semiconductor film by laser annealing, thermal annealing, or the like is used as an active layer for a thin film transistor (hereinafter, referred to as a “TFT”). As the light-transparent substrate having an insulating surface, in most cases, a glass substrate made of barium borosilicate glass, aluminoborosilicate glass, or the like is used. Although such a glass substrate has poor heat resistance, compared with a quartz substrate, it is inexpensive. Furthermore, a glass substrate has the advantage of enabling a substrate with a large area to be easily produced.
Laser annealing is known as a crystallization technique that gives high energy only to an amorphous semiconductor film to crystallize it without substantially increasing the temperature of a glass substrate. In particular, an excimer laser that emits light with a short wavelength (400 nm or less) is a representative laser that has been used from the beginning of the development of laser annealing. In recent years, a technique using a YAG laser that is a solid-state laser has also been developed. According to laser annealing using these lasers, a laser beam is formed by an optical system so as to have a spot shape or a linear shape on a surface to be irradiated, and the surface to be irradiated on the substrate is scanned by the resultant laser light (i.e., an irradiation position of laser light is moved relative to the surface to be irradiated). For example, according to excimer laser annealing using linear laser light, the entire surface to be irradiated can be subjected to laser annealing by scanning only in a direction orthogonal to a longitudinal direction, and such laser annealing is excellent in productivity. Therefore, excimer laser annealing is becoming the mainstream in production of a liquid crystal display device using TFTs. This technique realizes a monolithic liquid crystal display device in which TFTs forming a pixel portion (pixel TFTs) and TFTs for a driver circuit provided on the periphery the pixel portion are formed on one glass substrate.
However, a crystalline semiconductor film formed by subjecting an amorphous semiconductor film to laser annealing includes a collection of a plurality of crystal grains, and the position and size of the crystal grains are random. TFTs are formed on a glass substrate by patterning a crystalline semiconductor layer in an island shape for device separation. In this case, the position and size of crystal grains cannot be specified. It is known that an interface of crystal grains (grain boundary) involves factors that cause current transporting characteristics of carriers to be degraded, due to the influence of a recombination center or a trapping center caused by an amorphous structure, a crystal defect, and the like, and the influence of a potential level at a grain boundary. However, it is almost impossible to form a channel formation region, crystal properties of which have a serious effect on the TFT characteristics, using single crystal grains while avoiding the influence of a crystal boundary. Therefore, a TFT using a crystalline silicon film as an active layer has not been obtained, which has characteristics equivalent to those of a MOS transistor formed on a single crystal silicon substrate.
In order to solve such problems, an attempt to grow a large crystal grain has been made. For example, in ┌“High-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Method”, K. Shimizu, O. Sugiura, and M. Matumura, IEEE Transactions on Electron Devices vol. 40, No. 1, pp 112-117, 1993┘, there is a report on a laser annealing method in which a film of three-layer structure of Si/SiO
2
/Si is formed on a substrate, and an excimer laser beam is irradiated from both sides of a film side and a substrate side. This report discloses that according to this method, the size of a crystal grain can be enlarged by irradiation of a laser beam at predetermined energy intensity.
The above-mentioned method of Ishihara et al. is characterized in that heat characteristics of an under material of an amorphous silicon film are locally changed and the flow of heat to the substrate is controlled, so that a temperature gradient is caused. However, for that purpose, the three-layer structure of high melting point metal layer/silicon oxide layer/semiconductor film is formed on the glass substrate. Although it is possible to form a top gate type TFT by using the semiconductor film as an active layer in view of structure, since a parasitic capacitance is generated by the silicon oxide film provided between the semiconductor film and the high melting point metal layer, power consumption is increased and it becomes difficult to realize high speed operation of the TFT.
On the other hand, when the high melting point metal layer is made a gate electrode, it is conceivable that the method can be effectively applied to a bottom gate type or reverse stagger type TFT. However, in the foregoing three-layer structure, even if the thickness of the semiconductor film is omitted, with respect to the thickness of the high melting point metal layer and the silicon oxide layer, since the thickness suitable for a crystallizing step is not necessarily coincident with the thickness suitable for the characteristics as a TFT element, it is impossible to simultaneously satisfy both the optimum design in the crystallizing step and the optimum design in the element structure.
Besides, when the opaque high melting point metal layer is formed on the entire surface of the glass substrate, it is impossible to fabricate a transmission type liquid crystal display device. Although the high melting point metal layer is useful in that its thermal conductivity is high, since a chromium (Cr) film or titanium (Ti) film used as the high melting point metal material layer has high internal stress, there is a high possibility that a problem as to adhesiveness to the glass substrate occurs. Further, the influence of the internal stress is also exerted on the semiconductor film formed as the upper layer, and there is a high possibility that the stress functions as force to impart distortion to the formed crystalline semiconductor film.
On the other hand, in order to control a threshold voltage (hereinafter referred to as Vth) as an important characteristic parameter of a TFT within a predetermined range, in addition to valence electron control of the channel formation region, it is necessary to reduce the charged defect density of an under film and a gate insulating film formed of an insulating film to be in close contact with the active layer, or to consider the balance of the internal stress. To such requests, a material containing silicon as its constituent element, such as a silicon oxide film or a silicon nitride oxide film, has been suitable. Thus, there is a fear that the balance is lost by providing the high melting point metal layer to cause the temperature gradient.
SUMMARY OF THE INVENTION
The present inve
Kasahara Kenji
Kawasaki Ritsuko
Ohtani Hisashi
Costellia Jeffrey L.
Sarkar Asok Kumar
Semiconductor Energy Laboratory Co,. Ltd.
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