Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1997-10-14
2002-04-02
Pham, Long (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
Reexamination Certificate
active
06365933
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having an active layer of a semiconductor thin film formed on a substrate having an insulating surface, and particularly to a thin film transistor in which an active layer is made of a crystalline silicon film.
2. Description of the Related Art
In recent years, attention has been paid to a technique in which a thin film transistor (TFT) is constituted by using a semiconductor thin film (thickness of about hundreds to thousands Å) formed on a substrate having an insulating surface. The thin film transistor is widely applied to an electronic device such as an IC or an electro-optical device, and especially, its development as a switching element for an image display device has been hurried.
For example, in a liquid crystal display device, attempts have been made to apply TFTs to any electric circuit such as a pixel matrix circuit for respectively controlling pixel regions arranged in a matrix form, a drive circuit for controlling the pixel matrix circuit, and a logic circuit (processor circuit, memory circuit, etc.) for processing data signals from the outside.
In the present circumstances, although a TFT using an amorphous silicon film as an active layer is put into practical use, an electric circuit required to have further high speed operational performance, such as a drive circuit and a logic circuit, demands a TFT using a crystalline silicon film (polysilicon film).
As a method of forming a crystalline silicon film on a substrate, techniques disclosed in Japanese Patent Unexamined Publication No. Hei 6-232059 and No. Hei. 6-244103 by the present applicant are well known. The techniques disclosed in these-publications enable the formation of a crystalline silicon film having excellent crystallinity by using a metal element (especially nickel) for promoting crystallization of silicon and by a heat treatment at 500 to 600° C. for about four hours.
Japanese Patent Unexamined Publication No. Hei. 7-321339 discloses a technique for carrying out crystal growth substantially parallel to a substrate by utilizing the above techniques. The present inventors refer to the formed crystallized region as especially a side growth region (or lateral growth region).
However, even if a drive circuit is constituted by using such a TFT, the drive circuit is still far from the state of completely satisfying the required performance. In the present circumstances, especially it is impossible to constitute a high speed logic circuit requiring electric characteristics of extremely high performance to realize both high speed operation and high withstand voltage characteristics at the same time, by a conventional TFT.
As described above, in order to attain the higher performance of an electro-optical device and the like, it is necessary to realize a TFT having performance comparable with a MOSFET formed by using a single crystal silicon wafer.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide a thin film semiconductor device having extremely high performance as a breakthrough for realizing higher performance of an electro-optical device, and a method of manufacturing the same.
It is conceivable, as a reason why a high performance TFT as mentioned above has not been able to be obtained by a conventional method, that carriers (electrons or holes) are captured by crystal grain boundaries so that improvement of an field effect mobility as one of parameters showing TFT characteristics has been prevented.
For example, there are many unpaired bonds (dangling bonds) of silicon atoms and defect (capture) levels in the crystal grain boundaries. Accordingly, since carries moving in the inside of each crystal are easily trapped by the dangling bonds, defect levels or the like when they come close to or come into contact with the crystal grain boundaries, it is conceivable that the crystal grain boundaries have functioned as “malignant crystal grain boundaries” to block the movement of the carries.
In order to realize a semiconductor device of the present invention, it is indispensable to provide a technique to change the structure of such “malignant crystal grain boundaries” into “benign crystal grain boundaries” for carriers. That is, it is important to form crystal grain boundaries which have a low probability of capturing carriers, that is, a low possibility of blocking the movement of carriers.
Therefore, according to the present invention disclosed in the present specification, a method of manufacturing a semiconductor device including an active layer of a semiconductor thin film, comprises the steps of forming an amorphous silicon film on a substrate having an insulating surface, forming a mask insulating film selectively on the amorphous silicon film, making the amorphous silicon film selectively hold a metal element for promoting crystallization, transforming at least a part of the amorphous silicon film into a crystalline silicon film by a first heat treatment, removing the mask insulating film, forming an active layer made of only the crystalline silicon film by patterning, forming a gate insulating film on the active layer, carrying out a second heat treatment in an atmosphere containing a halogen element so that the metal element in the active layer is removed through gettering and a thermal oxidation film is formed in an interface between the active layer and the gate insulating film, and carrying out a third heat treatment in a nitrogen atmosphere to improve film qualities of the gate insulating film including the thermal oxidation film and the state of the interface, wherein the active layer is a crystalline structure body in which crystal grain boundaries are aligned substantially in one direction and which is constituted by an aggregation of a plurality of needle-shaped or column-shaped crystals substantially parallel with the substrate.
If a crystalline silicon film is formed in accordance with the above manufacturing method, a thin film having an appearance as shown in
FIG. 9
is obtained.
FIG. 9
is an enlarged micrograph of the thin film in the case where the present invention was practiced by using the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 7-321339 as means-for crystallizing an amorphous silicon film, and shows a lateral growth region
901
having a length of several tens to a hundred and several tens &mgr;m.
The lateral growth region
901
has a feature that since the needle-shaped or column-shaped crystals grow almost vertically to a region (designated by
902
) in which a metal element for promoting the crystallization has been added, and substantially parallel with each other, the directions of crystals are aligned. A portion designated by
903
is a macroscopic crystal grain boundary (differentiated from crystal grain boundaries between needle-shaped crystal and column-shaped crystals) formed by collision between needle-shaped crystal and column-shaped crystals extending from the opposing added regions
902
.
FIG. 10
is a TEM photograph in which a minute region of the inside of a crystalline grain is further enlarged with paying attention to the inside of the lateral growth region shown in FIG.
9
.
That is, although the crystalline silicon film of the present invention seems to be macroscopically composed of the large lateral growth region
901
as shown in
FIG. 9
, when the lateral growth region
901
is microscopically observed, the lateral growth region is such a crystalline structure body as to be constituted by a plurality of needle-shaped or column-shaped crystals
1001
as shown in FIG.
10
.
In
FIG. 10
, reference numeral
1002
denotes a crystal grain boundary showing a boundary between the needle-shaped or column-shaped crystals, and from the direction of extension of the crystal grain boundary
1002
, it is confirmed that the needle-shaped or column-shaped crystals
1001
grew substantially parallel to each other. Incidentally, the crystal grain boundary in the present specification indicat
Fukunaga Takeshi
Koyama Jun
Ohtani Hisashi
Yamazaki Shunpei
Coleman William David
Fish & Richardson P.C.
Pham Long
Semiconductor Energy Laboratory Co,. Ltd.
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