Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
1999-02-19
2001-11-13
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S064000, C257S065000, C257S067000, C257S069000, C257S070000
Reexamination Certificate
active
06316789
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film transistor and a method for producing the same.
2. Description of the Related Art
Recently, much attention is paid on a liquid crystal device with a constitution comprising thin film transistors (TFTs). In a device of this type denoted as an active matrix type liquid crystal display device, a TFT is formed in each of several million pixels or more arranged in matrix and the charge stored in each pixel is controlled by the TFT. Since a liquid crystal display device of the active matrix type enables a fine display at a high speed, it is utilized in a display device of a hand-held word processor or a computer.
Generally, at present, in an active matrix type liquid crystal display device, the TFTs provided in each pixel is produced using an amorphous silicon film formed by plasma CVD, and a peripheral driver circuit for driving the TFTs provided to each pixel is constructed by an external IC. The peripheral driver circuit is constructed by an external IC because the operation speed of a TFT using amorphous silicon film is too slow to satisfactorily operate as a peripheral driver circuit. In addition, although the peripheral driver circuit is generally constructed by a CMOS circuit, there is a problem that a CMOS circuit cannot be fabricated, because the properties of a P-channel TFT based on an amorphous silicon film are too inferior as compared with those of an N-channel TFT.
A TFT based on an amorphous silicon film is utilized in a liquid crystal display device of an active matrix type because the heat resistance must be considered in case a glass substrate is used. In general, a liquid crystal display device requires a transparent substrate. Thus, a type of material of the substrate is limited. Generally, an inexpensive transparent material available as a large-area substrate is confined to glass. However, since distinct shrinking and warping appear on a glass substrate when it is heated at 600° C. or higher, a glass substrate can not be used practically in a process having heating treatment at 600° C. or higher. For example, the deformation (strain) temperature of a Corning 7059 glass substrate that is commonly used as a substrate for a liquid crystal display device of an active matrix type is 593° C. Thus, if heating treatment at a temperature higher than the deformation temperature is performed for the Corning 7059 glass substrate, a large warping or shrinking occurs on the substrate to make it practically unapplicable.
On the other hand, an amorphous silicon film can be formed easily over a large area at a low temperature (400° C. or lower) by plasma CVD.
As described above, in a conventional technique, when a glass substrate is used, a semiconductor portion of a TFT to be produced may be constructed by an amorphous silicon film.
An active matrix type liquid crystal display device using quartz substrate is also known. Since a device of this type allows a heating treatment at 800° C. or higher, or 900° C. or higher, a TFT using a crystalline silicon thin film can be produced. Since a TFT based on a crystalline silicon film can be operated at a speed far higher than that of a TFT based on an amorphous silicon film, a finer display can be realized at a higher speed. Also, by using a crystalline silicon film for the TFT, a peripheral driver circuit can be arranged on the same substrate (a quartz substrate) to realize a compact and light-weight liquid crystal display device.
However, a quartz substrate is very expensive, and it costs 10 times as high as the price of a glass substrate. Thus, a quartz substrate is economically unfeasible.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the above problems, and to provide a TFT having a crystalline silicon film which can be formed on a glass substrate. That is, an object of the present invention is to provide a technology for forming a crystalline silicon film on a glass substrate at 600° C. or lower, and thereby to produce a TFT using the resulting crystalline silicon film. Another object of the present invention is to provide a technology for producing a TFT which operates stably.
According to the present invention, there is provided a method for producing a TFT, which includes the steps of, forming a silicon film having an amorphous region and a crystalline region on a substrate having an insulating surface, and performing heating treatment, wherein the crystalline region contains a metal element which accelerates (promotes) the crystallization of silicon and the heating treatment allows the metal element to diffuse from the crystalline region into the amorphous region.
Also, according to the present invention, there is provided a method for producing a TFT, which includes the steps of, forming a silicon film having a crystalline region on a substrate having an insulating surface, and performing heating treatment, wherein the crystalline region includes a metal element for promoting crystallization of silicon and the metal element is diffused from the crystalline region to a region other than the crystalline region by the heating treatment.
In the above constitution, as a substrate having an insulating surface, there are a glass substrate, a quartz substrate, a glass substrate or a quartz substrate having an insulating film formed thereon, etc. The present invention is particularly useful in case a glass substrate is used as the substrate.
In forming a silicon film having an amorphous region and a crystalline region, more specifically, a metal element which accelerates the crystallization of silicon is selectively introduced into the amorphous silicon film, and a heating treatment at about 450 to 600° C. is performed. In this treatment, the region into which the metal element is introduced and a region in the periphery of the metal-introduced region can be selectively crystallized. The upper limit of the heating temperature depends on the heat resistance temperature, i.e., the deformation (strain) temperature, of the glass substrate. For example, by considering the heat resistance of the glass substrate and from the viewpoint of productivity, the heating temperature is preferably about 550° C. In case a quartz substrate or any heat-resistant material which resists to a heating treatment at 1,000° C. or higher, the heating temperature can be elevated in accordance with the heat resistance of the substrate material.
At least one selected from the group of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au can be used as the metal elements for accelerating the crystallization of silicon. Particularly effective among the metal elements above is nickel (Ni). The metal element can be introduced into the amorphous silicon film by forming a thin film of the metal element on the surface of the amorphous silicon film using a physical method such as sputtering, CVD, or evaporation, or by applying the amorphous silicon film to a solution containing the metal element. In the physical method, it is difficult to uniformly form an extremely thinner film on the amorphous silicon film. Thus, since the metal element can not be brought in uniform contact with the amorphous silicon film, its distribution becomes unbalanced easily during the crystal growth. On the other hand, the concentration of the metal element can be readily controlled by the method of using a solution. Further, the method using a solution maintains the metal element in uniform contact with the surface of the amorphous silicon film. Thus, the method using a solution is preferred as compared with the method for physically forming a metal film.
The metal element must be introduced into the amorphous silicon film at a concentration of 1×10
16
m
−3
or more at the crystallization by heating. However, it is not preferred to introduce the metal element into the amorphous silicon film at a concentration of 5×10
16
cm
−3
or higher, because a silicide forms inside the film.
As described above, by heating treatment to diffuse the metal element in
Miyanaga Akiharu
Ohtani Hisashi
Teramoto Satoshi
Yamazaki Shunpei
Costellia Jeffrey L.
Lee Eddie
Lee Eugene
Nixon & Peabody LLP
Semiconductor Energy Laboratory Co,. Ltd.
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