Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1998-06-10
2001-10-16
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S164000, C438S165000, C257S066000
Reexamination Certificate
active
06303415
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating TFTs.
2. Description of Related Art
Thin-film transistors (TFTs) using an amorphous silicon film as an active layer have been well known. TFTs using this amorphous silicon film as an active layer are employed in active matrix liquid crystal displays (AMLCDs).
However, TFTs using an amorphous silicon film have low characteristics and so the P-channel type cannot be put into practical use. Therefore, the actual situation is that the use of the TFTs using an amorphous silicon film is limited to active matrix circuits.
An active matrix liquid crystal display with which peripheral driver circuits are integrated to reduce the fabrication cost and to miniaturize the liquid crystal panel is also known. In this configuration, even the peripheral driver circuits are composed of TFTs. Therefore, this kind of display must meet some requirements, i.e., high-speed operation and realization of P-channel TFTs.
TFTs using a crystalline silicon film are known as a configuration satisfying these requirements. One main method of obtaining a crystalline silicon film is heat treatment for an amorphous silicon film.
Another main method is to irradiate the amorphous silicon film with laser light.
The former method results in a clear polycrystalline state. However, the electrical characteristics tend to be nonuniform because of clear crystal grain boundaries. This problem is deeply concerned with the fact that the positions and the state of the crystal grain boundaries cannot be controlled.
The latter method can produce a crystalline silicon film of uniform quality. However, the production yield is low. In addition, the crystallinity is not sufficiently high.
In an attempt to solve these problems, we have proposed a technique for obtaining a crystalline silicon film having desired film quality and electrical characteristics. In particular, nickel is introduced into an amorphous silicon film, and then a heat treatment is performed.
However, doping is utilized in forming source and drain regions. The resulting damage must be healed. Nickel element remaining in the active layer adversely affects the characteristics of the completed device. These problems must be alleviated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of fabricating TFTs in which the foregoing problems have been mitigated.
One embodiment of the present invention lies in a device comprising: a gate electrode comprising a heat-resistant material; and an active layer comprising a semiconductor film comprising silicon to which a catalytic element is intentionally added to promote crystallization of silicon. The source and drain regions of the active layer are more heavily doped with the metallic element than other regions.
Tantalum or a material mainly comprising tantalum can be used as the heat-resistant material described above. Nickel can be the most favorable element as the catalytic element for promoting crystallization of silicon.
In the configuration described above, the concentration of the catalytic element in the source and drain regions is higher than in other regions by two or more orders of magnitude. This can lower the catalytic element concentration in the channel region. In consequence, the characteristics and reliability of the finished TFTs can be enhanced.
More particularly, the concentration of the catalytic element in the channel region is reduced less than 5×10
16
atoms/cm
3
. Consequently, the characteristics and the reliability of the TFTs can be improved.
One or more elements selected from the group consisting of Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au can be used as the aforementioned catalytic element, though the merits are less conspicuous than where nickel is used alone. Furthermore, Ni may be added to one or more elements selected from this group, and the resulting compound may be used together with other catalytic element.
Another structure of the invention lies in a method of fabricating a semiconductor device having a gate electrode comprising a heat-resistant material and an active layer comprising a semiconductor film comprising silicon to which a catalytic element for promoting crystallization of silicon is added. This method starts with forming an active layer comprising a crystalline semiconductor film comprising silicon by using of the catalytic element described above. Phosphorus is introduced into regions to be source and drain. A heat treatment is performed at 550 to 700° C. to getter the catalytic element in the active layer to the phosphorus-doped regions.
Other objects and features of the invention will appear in the course of the description thereof, which follows.
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Bowers Charles
Nixon & Peabody LLP
Robinson Eric J.
Schillinger Laura M
Semiconductor Energy Laboratory Co,. Ltd.
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