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
1997-01-17
2004-06-01
Fahmy, Wael (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S066000, C257S074000, C257S075000, C257S353000
Reexamination Certificate
active
06744069
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a crystalline thin-film semiconductor and a manufacturing method thereof. The invention also relates to a semiconductor device using the above thin-film semiconductor and a manufacturing method thereof.
2. Description of Related Art
Techniques are known in which a crystalline silicon film is formed on a glass or quartz substrate and thin-film transistors (hereinafter referred to as TFTs) are formed by using the silicon film. Such TFTs are called high-temperature polysilicon TFTs or low-temperature polysilicon TFTs.
In the case of high-temperature polysilicon TFTs, a crystalline silicon film is formed by a technique including a heat treatment at a relatively high temperature of 800-900° C. It can be said that this technique is derived from an IC manufacturing process using a single crystal silicon wafer. Naturally, high-temperature polysilicon TFTs are formed on a quartz substrate, which withstand the above-mentioned high temperature.
On the other hand, low-temperature polysilicon TFTs are formed on a glass substrate, which is inexpensive but is apparently lower in heat resistance than a quartz substrate. To form a crystalline silicon film for low-temperature polysilicon TFTs, heating at lower than 600° C. which a glass substrate can withstand or laser annealing which causes almost no thermal damage on a glass substrate is performed.
The high-temperature polysilicon TFT is advantageous in that TFTs having uniform characteristics can be integrated on a substrate.
On the other hand, the low-temperature polysilicon TFT is advantageous in that a glass substrate can be used which is inexpensive and can easily be increased in size.
According to the current manufacturing techniques, there are no large differences in characteristics between the high-temperature polysilicon TFT and the low-temperature polysilicon TFT. That is, in both cases, the mobility is 50-100 cm
2
/Vs and the S value is 200-400 mV/dec. (V
D
=1 V).
However, these values are much worse than those of MOS transistors formed on a single crystal silicon wafer. In general, the S value of MOS transistors formed on a single crystal silicon wafer is 60-70 mV/dec.
At present, there are active matrix liquid crystal display devices in which an active matrix circuit and peripheral driver circuits are integrated on the same substrate by using TFTs. In this type of configuration, the source driver circuit of the peripheral driver circuits is required to operate at a frequency higher than a little more than 10 MHz. However, at present, a circuit using high-temperature polysilicon TFTs or low-temperature polysilicon TFTs can provide a margin of operation speed that is as small as several megahertz.
For this reason, at present, a liquid crystal display device is constituted by dividing its operation (called “divisional driving”). However, this method has several problems; for example, stripes appear on the screen due to, for instance, a slight deviation in the division timing.
SUMMARY OF THE INVENTION
It is now considered a configuration in which not only peripheral driver circuits (constituted of a shift register circuit and a buffer circuit) but also an oscillation circuit, a D/A converter, an A/D converter, and digital circuits for various kinds of image processing are integrated on the same substrate.
However, the above-mentioned oscillation circuit, D/A converter, A/D converter, and digital circuits for various kinds of image processing are required to operate even at higher frequencies than the peripheral driver circuits. Therefore, it is very difficult to constitute such circuits by using high-temperature polysilicon TFTs or low-temperature polysilicon TFTs as long as they are formed by the current manufacturing techniques.
On the other hand, integrated circuits of MOS transistors formed on a single crystal silicon wafer which circuits can operate at more than 100 MHz have already been put to practical use.
An object of the present invention is to provide a TFT which can constitute a circuit that is required to perform a high-speed operation (generally at more than tens of megahertz).
Another object of the invention is to provide a TFT whose characteristics are equivalent to those of a MOS transistor formed on a single crystal silicon wafer. It is also intended to provide a means for manufacturing such a TFT. It is further intended to provide a semiconductor device having a required function by using TFTs having so superior characteristics.
According to one aspect of the invention, there is provided a semiconductor device using a thin-film transistor that uses, as an active layer, a crystalline silicon film formed on a substrate having an insulating surface, wherein the crystalline silicon film has a crystal structure that is continuous in a predetermined direction, and grain boundaries extending in the predetermined direction; and the predetermined direction is at a predetermined angle with a direction connecting a source region and a drain region of the thin-film transistor.
FIGS. 6 and 7
show an example of a crystalline silicon film having the above-mentioned crystal structure.
FIGS. 6 and 7
are photographs of obtained by observing the surface of a 250-Å-thick crystalline silicon film with a transmission electron microscope (TEM).
FIG. 7
is an enlargement of part of the photograph of FIG.
6
.
The crystalline silicon film of
FIGS. 6 and 7
can be obtained by a manufacturing process of a first embodiment of the invention which will be described later.
FIGS. 6 and 7
show a crystal structure that continuously extends from the bottom left to the top right in these drawings, as well as many grain boundaries extending substantially parallel with the above direction.
As is apparent from the crystal structure shown in
FIG. 7
, this crystalline silicon film is a collection of many crystallizations (crystalline silicon grains) each having a crystal structure extending in the particular direction. The width of the crystallizations is 500-2,000 Å, or from about the thickness of the crystalline silicon film to 2,000 Å.
Many definite grain boundaries are arranged, at intervals, perpendicularly or substantially perpendicularly (in the direction from the bottom right to the top left in these drawings) to the direction in which the crystal structure has continuity; the crystal structure is discontinuous (continuity is lost) in the former direction.
The continuity of the lattice structure is substantially maintained in the direction in which the crystal structure has continuity. In this direction, the scattering and trapping of carriers during their movement occur at a much smaller possibility than in the other directions.
That is, it can be considered that a substantial single crystal state, in which carriers are not scattered or are hardly scattered by grain boundaries, is established in the direction in which the crystal structure has continuity.
The above-mentioned aspect of the invention defines the relationship between the direction in which the crystal structure has continuity and the direction connecting the source and drain regions of a thin-film transistor. To attain a high-speed operation, it is desired that the direction in which the crystal structure has continuity coincide or substantially coincide with the direction connecting the source and drain regions. This provides a configuration in which carriers can move most easily.
The characteristics of a thin-film transistor can be controlled by setting the angle between the above two directions at a proper value. For example, in the case of forming a number of thin-film transistor groups, the characteristics of a plurality of groups can be made different from each other by changing the angle between the two directions from one group to another.
A thin-film transistor in which the active layer is bent to assume an N-like or a square-bracket-like, or even an M-like shape, that is, the line connecting the source and drain regions is bent can be formed in the follow
Hamatani Toshiji
Hayakawa Masahiko
Koyama Jun
Ogata Yasushi
Ohtani Hisashi
Fahmy Wael
Ha Nathan W.
Robinson Intellectual Property Law Office P.C.
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