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
1998-02-23
2002-08-13
Prenty, Mark V. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S057000, C257S059000, C257S072000, C257S408000, C257S607000
Reexamination Certificate
active
06433363
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin-film transistor and its manufacturing method as well as to circuits and apparatuses constituted by using thin-film transistors.
2. Description of the Related Art
The thin-film transistor (hereinafter abbreviated as “TFT”) using a thin-film semiconductor is known. This is formed on a substrate by using a thin-film semiconductor, particularly a silicon semiconductor film.
The TFT is used in various kinds of integrated circuits, particularly the active matrix liquid crystal display device. In the active matrix liquid crystal display device, TFTs as switching elements are provided for respective pixel electrodes that are arranged in matrix form. There is known a version (called “peripheral driver circuits integrated type”) in which not only a matrix circuit but also peripheral driver circuits are constituted by using TFTs.
Examples of other uses of the TFT are various kinds of integrated circuits and multilayered integrated circuits (three-dimensional ICs).
As for a silicon film used in the TFT, it is simple and convenient to use an amorphous silicon film formed by a vapor-phase method such as plasma CVD. It can be said that this method has already been established almost completely.
However, the electrical characteristics of the TFT using an amorphous silicon film are far lower than those of the TFT using a single crystal semiconductor for common semiconductor integrated circuits. Therefore, at present, this type of TFT can be used for only limited purposes such as a switching element of an active matrix circuit.
The characteristics of a TFT using an amorphous silicon film can be improved by converting the amorphous silicon film into a crystalline silicon film. Silicon films having crystallinity other than a single crystal silicon film are called a polysilicon film, a microcrystal silicon film, etc.
Such a silicon film having crystallinity can be obtained by forming an amorphous silicon film and crystallizing it by heating (thermal annealing). This method is called a solid-phase growth method because conversion from an amorphous state to a crystal state is effected while the solid phase is maintained.
However, the silicon solid-phase growth has problems that the heating temperature and time need to be set at more than 600° C. and more than 10 hours, respectively, and that it is difficult to use an inexpensive glass substrate.
For example, the Corning 7059 glass, which is commonly used in the active matrix liquid crystal display device, has a glass strain point of 593° C. Therefore, in view of increasing the substrate area, it is problematic to subject this glass to thermal annealing of more than 600° C. for a long time.
Further, the fact that the heating time for crystallization is more than 10 hours is problematic in terms of productivity.
To solve the above problems, the inventors have developed a technique in which a certain kind of metal element such as nickel or palladium is deposited, by a very small amount, on the surface of an amorphous silicon film and then heating is performed. According to this technique, crystallization can be completed by performing heating at 550° C. for about 4 hours (refer to Japanese Unexamined Patent Publication No. 6-244103 (JP-A-6-244103).
Naturally a silicon film even higher in crystallinity can be obtained by annealing of 600° C. and 4 hours.
This technique can produce a large-area crystalline silicon film on an inexpensive glass substrate with high productivity.
To introduce a very small amount of metal element (for accelerating crystallization), various methods are available such as depositing a coating of a metal element or its compound by sputtering (refer to JP-A-6-244104), forming a coating of a metal element or its compound by such a means as spin coating (JP-A-7-130652), and forming a coating by decomposing a gas containing a metal element by thermal decomposition, plasma decomposition, or the like (JP-A-7-335548).
It is also possible to introduce a metal element selectively, i.e., into a particular portion, and then cause crystal growth to proceed from the portion where the metal element is introduced to the periphery (called “lateral growth method”). Having directivity in crystal structure, a crystalline silicon film produced by this method exhibits much superior characteristics when used properly in connection with the directivity.
Although the above-described methods of forming a crystalline silicon film by using a certain kind of metal element (for instance, nickel) are much superior, it is known that they cause the following problems when a TFT is formed by using such a crystalline silicon film.
First, the structure of a common TFT will be described.
FIG. 1
shows the structure of a typical n-channel TFT.
In the structure of
FIG. 1
, an active layer including a source region
102
, a low-concentration impurity region
103
, a channel region
104
, a low-concentration impurity region (LDD region)
105
, and a drain region
106
is provided on a glass substrate
101
. Reference numerals
111
,
112
, and
114
denote an interlayer insulating film, a source electrode, and a drain electrode, respectively.
The TFT structure as shown in
FIG. 1
is generally employed in which the low-concentration impurity region
105
that is less conductive than the drain region
106
is provided on the drain side, particularly in n-channel TFTs. The region
105
, which is called an LDD (lightly doped drain) region, such roles as reducing a strong electric field applied between the channel region and the drain region, reducing the leak current, and suppressing the hot carrier effect.
The following problems arise when a TFT having the above structure is formed by using a crystalline silicon film that has been crystallized by utilizing a particular metal element of the above kind.
A TFT formed by using a crystalline silicon film that has been obtained by utilizing a metal element exhibits much superior characteristics as a whole; for instance, it is even superior to a TFT formed by using a crystalline silicon film that has been obtained by irradiation with laser light.
However, the case of utilizing a metal element is associated with a problem that the characteristics vary to a large extent when a number of TFTs are produced. Further, TFTs having marked deteriorations in characteristics are found though the rate of occurrence is low. These deteriorations in characteristics increase variations in device characteristics as a group of TFTs.
The variations in device characteristics are a serious problem in making an integrated circuit. In general, in making an integrated circuit, it is important that the characteristics be uniform among devices used as well as the characteristics of each device be superior.
SUMMARY OF THE INVENTION
An object of the invention is to provide a technique for obtaining a TFT having small variations in device characteristics in forming it by using a crystalline silicon film that has been obtained by utilizing a metal element.
According to knowledge of the inventors, the variations in device characteristics are caused by the metal element that was used in the crystallization, which means that the problem of the variations in device characteristics can be eliminated by removing the metal element selectively from a crystalline silicon film obtained.
Studies of the inventors have revealed that where the nickel element is used, it can be removed (or its influences can be eliminated) by performing, on a crystalline silicon film obtained, a heat treatment at more than about 900° C. in an oxygen atmosphere containing chlorine at several percent.
By utilizing this technique, a TFT having superior characteristics can be obtained with vary small variations in characteristics. A patent application has already been filed for this technique (Japanese Patent Application No. 8-335152).
However, this technique still has a problem that an inexpensive glass substrate cannot be used because heating at more than 900° C. is needed to remove the nickel e
Ohnuma Hideto
Ohtani Hisashi
Teramoto Satoshi
Yamazaki Shunpei
Prenty Mark V.
Semiconductor Energy Laboratory Co,. Ltd.
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