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
1998-09-14
2001-03-13
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...
C257S655000
Reexamination Certificate
active
06201260
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thin film transistor and a method of manufacturing the same and, more particularly, to a polysilicon thin film transistor formed on an insulating substrate such as a glass substrate, and a method of manufacturing the same.
In a thin film transistor having a polysilicon channel region, the field-effect mobilities of electrons and holes are larger than in a thin film transistor having an amorphous silicon channel region. The polysilicon thin film transistor therefore has a high current drive capability and is being used in liquid crystal display apparatuses and the like.
Regardless of its excellent characteristics, however, the polysilicon thin film transistor suffers the following problem.
More specifically, in the polysilicon thin film transistor, its characteristics and particularly its threshold voltage are greatly influenced by defects, impurities, and the like in the channel region. As a result, the threshold voltage may greatly shift from a value necessary for the operation of the transistor circuit. For example, in an n-channel transistor, the threshold voltage shifting to the negative side with respect to 0V makes the transistor have normally ON characteristics, failing normal switching.
To solve this problem, the threshold voltage is controlled by implanting, e.g., boron ions in the channel region using an ion implanter or an ion doping apparatus. According to this method, however, ion implantation or ion doping must be performed in addition to formation of the channel region. Accordingly, the number of steps of manufacturing a liquid crystal display device or the like increases, and the throughput as the productivity per unit time decreases. In addition, this method requires another apparatus, and thus the manufacturing cost increases.
The amount of boron ions implanted in the channel region to control the threshold voltage is very small. However, when an ion implanter or ion doping apparatus is used, the boron dose cannot be accurately controlled, and the thin film transistor cannot be manufactured with high reproducibility and high stability.
For example, boron ions are implanted in the channel region using an ion implanter or an ion doping apparatus in manufacturing a thin film transistor having a structure in which a silicon nitride film, a silicon oxide film, and a polysilicon thin film forming the channel region are sequentially stacked on a glass substrate. In this case, the boron concentration distribution shown in
FIG. 1
is generally obtained.
FIG. 1
is a graph showing the boron concentration distribution obtained when a thin film transistor is formed by a conventional method. In
FIG. 1
, the abscissa represents the boron dose, and the ordinate represents the distance from the glass substrate.
As is often the case with the conventional method, the boron concentration in the silicon oxide film gradually decreases from the polysilicon thin film toward the silicon nitride film as shown in FIG.
1
. It is noted that a high boron concentration is obtained in not only the polysilicon thin film but also the interface region between the silicon oxide film and the silicon nitride film. This is because the surface of the silicon nitride thin film is contaminated before forming the silicon oxide film.
The threshold voltage does not greatly vary between thin film transistors as far as the amount of boron ions implanted in the silicon oxide thin film and the silicon nitride thin film is very small or constant.
However, when an ion implanter or ion doping apparatus is used, it is difficult to prevent a non-negligible amount of boron ions from being implanted in the silicon oxide thin film and the silicon nitride thin film. The amount of boron ions implanted in the silicon oxide film and the silicon nitride film greatly depends on implantation conditions such as the boron ion acceleration voltage, and these implantation conditions cannot be kept unchanged. For this reason, in the conventional method, the threshold voltage greatly varies between thin film transistors.
BRIEF SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems, and has as its object to provide a thin film transistor which can be manufactured without increasing the number of manufacturing steps and varying the threshold voltage, and a method of manufacturing the same.
To achieve the above object, according to the present invention, there is provided a thin film transistor comprising an insulating substrate, a silicon nitride film formed on the insulating substrate, a silicon oxide film formed on the silicon nitride film, a polysilicon thin film formed on the silicon oxide film, the polysilicon thin film having a channel region containing boron as an impurity at a concentration of 5×10
16
to 1.5×10
18
atoms/cm
3
, and source and drain regions facing each other via the channel region, an insulating film formed on the polysilicon thin film, and a gate electrode formed on the insulating film, wherein the boron concentration decreases from the channel region toward the silicon nitride film in a region of the silicon oxide film between the channel region and the silicon nitride film, and the region of the silicon oxide film between the channel region and the silicon nitride film is made up of a first region in contact with the channel region, which has a boron concentration of not less than 1×10
16
atoms/cm
3
, and a second region between the first region and the silicon nitride film, which has a boron concentration of less than 1×10
16
atoms/cm
3
, the first region having a thickness of not more than 200 Å.
According to the present invention, there is provided a method of manufacturing a thin film transistor, comprising the steps of sequentially stacking a silicon nitride film and a silicon oxide film on an insulating substrate, stacking a non-singlecrystal silicon thin film on the silicon oxide film, depositing boron on an upper surface of at least one of the silicon oxide film and the non-singlecrystal silicon thin film, and irradiating light onto the non-singlecrystal silicon thin film to fuse and crystallize the non-singlecrystal silicon thin film, and to diffuse the deposited boron into the fused non-singlecrystal silicon thin film, thereby forming a boron-doped polysilicon thin film.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
REFERENCES:
patent: 4-315441 (1992-11-01), None
Kamagami Shinichi
Nakazono Takuji
Suzuki Mitsuaki
Kabushiki Kaisha Toshiba
Pillsbury & Winthrop LLP
Prenty Mark V.
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