Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2001-01-30
2004-01-06
Loke, Steven (Department: 2811)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S150000, C438S166000, C438S486000, C257S347000
Reexamination Certificate
active
06673659
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted by thin-film transistors (hereinafter referred to as TFTs) and a method of producing the same. More specifically, the invention relates to an electro-optical device as represented by an active matrix-type liquid crystal display device having, for example, a pixel portion and a driver circuit formed on the same substrate, and to an electronic device mounting the above electro-optical device as a part.
In this specification, the semiconductor device stands for devices that work utilizing the semiconductor characteristics, in general. Electro-optical devices as represented by the active matrix-type liquid crystal display devices formed by using thin-film transistors, electronic devices mounting such electro-optical device as a part, and semiconductor circuits, are all semiconductor devices.
2. Prior Arts
There has been developed a thin-film transistor (TFT) by using a semiconductor thin film formed on an insulating substrate such as a glass. As a material of the semiconductor thin film, there can be used an amorphous silicon film or a crystalline silicon film formed by crystallizing the amorphous silicon film by a laser annealing method or a heat annealing method. Among them, the TFT using the polycrystalline silicon film as an active layer features a high current drivability owing to its high degree of electric field mobility, can be finely machined, and makes it possible to increase the numerical aperture in the pixel portion.
Such a TFT has been widely applied to electronic devices such as ICs and electro-optical devices, and is used, for example, as a pixel switching element or driver circuit in the active matrix-type display device. As the TFT substrate, there is typically used a glass substrate such as of barium borosilicate glass or alumino-borosilicate glass. The glass substrate has a heat resistance inferior to that of a quartz substrate, but offers such an advantage that the substrate of a large area can be easily and cheaply produced to cheaply realize a display device of a large screen.
The glass substrate such as barium borosilicate glass or alumino-borosilicate glass contains trace amounts of impurities such as alkali metal elements like sodium (Na), and the electric characteristics of the TFT vary as the impurity ions infiltrate into the active layer from the glass substrate. To prevent this variation in the electric characteristics, a base film (blocking layer) comprising a silicon oxide film, a silicon nitride film or a silicon oxynitride film has been formed on the surface of the glass substrate on where TFTs are formed.
It has been known that the TFT characteristics can be effectively stabilized if there is used, as a base film, a laminated film comprising a silicon nitride film on the side of the substrate and a silicon oxide film on the side of the active layer.
In the case of a top-gate TFT, the TFT characteristics are greatly affected by the quality of the base film since the channel-forming region is in contact with the base film.
When a voltage is applied to the gate electrode of TFT, an electric field is formed in the active layer and impurity ions in the glass substrate are pulled toward the side of the active layer. Depending upon the quality of the base film, therefore, the impurity ions infiltrate into the active layer or into the gate-insulating layer through the base film, and the electric characteristics of the TFTs undergo a change accompanying thereto to deteriorate the reliability.
A silicon nitride film as the base film exhibits a merit in that it exhibits a large effect for blocking impurity ions but has a defect in that it exhibits many large trap levels to affect the TFT characteristics. Besides, the silicon nitride film produces a large internal stress and causes a distortion on the interface relative to the active layer, resulting in the deterioration in the TFT characteristics such as negative shift of Vth (threshold voltage) and an increase in the S-value (subthreshold constant). Further, the silicon oxide film as the base film exhibits merits such as a wider band gap than that of the silicon nitride film, a high insulating property and a low trap level accompanied, however, by such defects as easy absorption of water and small effect for blocking impurity ions. The silicon oxynitride film exhibits properties that differ depending upon the nitrogen concentration and the oxygen concentration in the film. To enhance the effect for blocking impurity ions, therefore, the film must be formed dense by increasing the nitrogen concentration in the film. However, an increase in the nitrogen concentration in the silicon oxynitride film develops the same defect as that of the silicon nitride film.
Further, when a film containing large amounts of fixed electric charge is brought, as a base film, into contact with the active layer, a trap level is formed on the interface between the base film and the active layer causing the TFT characteristics to be deteriorated. In general, the fixed electric charge increases with an increase in the nitrogen concentration in the film.
When the silicon nitride film and the silicon oxide film are laminated one upon the other, special film-forming chambers are required since they are constituted by different elements. In this case, the temperature of the film-forming chambers drops since the substrate is cooled as it is being conveyed. Accordingly, preheating is required so as not to drop the temperature of the substrate. The preheating time accounts for an increase in the treating time.
SUMMARY OF THE INVENTION
The present invention is concerned with a technique for solving the above-mentioned problems, and provides a base film which helps stabilize TFT characteristics, improves reliability, and can be excellently produced, and a method of forming the same.
In order to solve the above-mentioned problems according to the present invention, a silicon oxynitride film is formed by a plasma CVD method by using SiH
4
, N
2
O and H
2
, and this film is used as a base film for a TFT. The properties of the silicon oxynitride film that is formed is controlled by, chiefly, varying the flow rates of N
2
O and H
2
. The hydrogen concentration and the nitrogen concentration are increased in the film upon increasing the flow rate of H
2
. Upon increasing the flow rate of N
2
O, further, the hydrogen concentration and the nitrogen concentration decrease in the film, and the oxygen concentration increases. On the other hand, the silicon concentration does not almost change even if only a ratio of N
2
and N
2
O gas flows is changed. This makes it possible to form a silicon oxynitride film on the side of the substrate, the silicon oxynitride film having a composition exhibiting properties which are merits of the silicon nitride film, and to form a silicon oxynitride film on the side of the active layer, the silicon oxynitride film having a composition exhibiting properties which are merits of the silicon oxide film, while continuously changing the compositions thereof, thereby to form a base film of good quality picking up merits of both the silicon nitride film and the silicon oxide film. The silicon oxynitride films exhibiting the above-mentioned properties are formed by the same film-forming method by simply changing the gas flow rate ratios, and can be formed in the same film-forming chamber contributing to enhancing the productivity.
Concretely speaking, there are formed a silicon oxynitride film formed over SiH
4
, N
2
O and H
2
flow rate ratios of Xh=0.5 to 5 (Xh=H
2
/(SiH
4
+N
2
O)), Xg=0.94 to 0.97 (Xg=N
2
O/(SiH
4
+N
2
O)), and a silicon oxynitride film formed over flow rate ratios of Xh=0 (Xh=H
2
/(SiH
4
+N
2
O)), Xg=0.97 to 0.99 (Xg=N
2
O/(SiH
4
+N
2
O)). these silicon oxynitride films being separately used.
In forming the silicon oxynitride film by the plasma CVD method, H
2
is added to a mixture gas of SiH
Ishimaru Noriko
Iwai Michinori
Miwa Masahiko
Sakama Mitsunori
Loke Steven
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
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