JPH05267664A - Thin-film transistor and its manufacture - Google Patents

Abstract

PURPOSE:To provide a thin-film transistor of high dielectric strength and improved reproducibility, by forming a gate oxide film by thermally oxidizing single-crystal silicon. CONSTITUTION:A polysilicon film 2 for forming an active layer is formed on an insulating substrate 1. A single-crystal silicon film 3 for forming a gate oxide film is formed and thermally oxidized, and a silicon oxide film 3a for forming a gate oxide film is formed only on the surface of the polysilicon film 2 for forming an active layer. A polysilicon film 4 for forming a gate electrode is formed at the central part of the upper surface of the silicon oxide 3a. On the surface of the polysilicon film 4, a silicon oxide film 5a for protecting side etching is formed by thermally oxidizing single-crystal silicon. When both of the silicon films 5a, 3a are etched at the same time, the silicon oxide film 3a turning to a gate oxide film is left only between the polysilicon films 2 and 4. Single-crystal silicon which is excellent in quality and forms a gate oxide film can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor and its manufacturing method.

[0002]

2. Description of the Related Art In a thin film transistor using polysilicon as an active layer, the surface side of a polysilicon film for forming the active layer is thermally oxidized to form an active layer with a polysilicon film which is not thermally oxidized. There is one in which a gate oxide film made of silicon oxide is formed on the surface of the active layer. In this case, since the gate oxide film is formed by the high temperature thermal oxidation method, the film quality can be the best.

[0003]

However, in such a conventional thin film transistor, the surface of the unevenness is thermally oxidized because the unevenness due to the crystal particles is formed on the surface of the polysilicon film. There is a problem in that the gate oxide film has a weak portion against an electric field, which results in a low withstand voltage and poor reproducibility. It should be noted that if the film thickness of the gate oxide film is increased, the dielectric strength voltage can be increased, but in this case, the rate of formation of the thermal oxide film is exponentially slower with the passage of time, and thus the productivity is reduced. There was another problem. The object of the present invention is to increase the withstand voltage and also improve the reproducibility,
It is another object of the present invention to provide a thin film transistor and a method of manufacturing the thin film transistor that can prevent productivity from decreasing.

[0004]

The invention according to claim 1 is
In a thin film transistor using polysilicon as an active layer, a gate oxide film is formed by thermally oxidizing single crystal silicon. According to a fourth aspect of the present invention, a single crystal silicon film for forming a gate oxide film is formed on the surface of a polysilicon film for forming an active layer, and the single crystal silicon film is thermally oxidized to form a silicon oxide film. The gate oxide film is formed of the silicon oxide film.

[0005]

According to the present invention, since the gate oxide film is formed by thermally oxidizing single crystal silicon,
Even if the single crystal silicon film for forming the gate oxide film is formed to be relatively thin, good quality can be obtained, and thus the dielectric strength of the gate oxide film formed by thermally oxidizing the single crystal silicon film is increased. In addition, the reproducibility can be improved, and the productivity can be prevented from decreasing.

[0006]

1 to 6 show respective steps of manufacturing a thin film transistor to which an embodiment of the present invention is applied. Therefore, the structure of the thin film transistor will be described together with its manufacturing method with reference to these drawings in order.

First, as shown in FIG. 1, the upper surface of an insulating substrate 1 made of glass, ceramic, or a semiconductor substrate having an oxide film formed by thermal oxidation on the surface thereof is subjected to plasma CVD, low pressure CVD, sputtering, or the like. Polysilicon is deposited and patterned to form an island-shaped polysilicon film 2 for forming an active layer. Next, M on all surfaces
BE (Molecular Beam Epitaxy), MO (Metal Oxide) C
A single crystal silicon film 3 for forming a gate oxide film is formed by VD or the like. Next, the single crystal silicon film 3 for forming the gate oxide film is thermally oxidized, and then this thermally oxidized product is patterned. As shown in FIG. 2, the gate is formed only on the surface of the polysilicon film 2 for forming the active layer. A silicon oxide film 3a for forming an oxide film is formed.

Next, plasma CVD and reduced pressure CV are performed on the entire surface.
When polysilicon is deposited by D, sputtering or the like and patterned, a polysilicon film 4 for forming a gate electrode is formed in the central portion of the upper surface of the silicon oxide film 3a for forming a gate oxide film, as shown in FIG. It Next, the single crystal silicon film 5 for forming a silicon oxide film for preventing side etching is formed on the entire surface by MBE, MOCVD or the like, and the single crystal silicon film 3 for forming a gate oxide film (see FIG. 1).
The film thickness is the same as the film thickness of the reference). Next, the single crystal silicon film 5 for forming a silicon oxide film for preventing side etching is thermally oxidized, and then the thermally oxidized product is patterned to form a polysilicon film 4 for forming a gate electrode, as shown in FIG. A side-etching-preventing silicon oxide film 5a is formed only on the surface of the film with the same thickness as the gate oxide film-forming silicon oxide film 3a.

Next, when both silicon oxide films 5a and 3a are simultaneously etched, as shown in FIG. 5, the gate is formed only between the polysilicon film 2 for forming the active layer and the polysilicon film 4 for forming the gate electrode. The silicon oxide film 3a for forming the oxide film remains, and the remaining silicon oxide film 3a forms the gate oxide film 3b. In this case, the presence of the silicon oxide film 5a for preventing side etching prevents side etching of the gate oxide film 3b. Next, as shown in FIG. 6, impurities are injected and diffused by the ion implantation device using the polysilicon film 4 for forming the gate electrode as a mask, and the active layer forming films on both sides of the polysilicon film 4 for forming the gate electrode are formed. The source / drain regions are formed in the polysilicon film 2 and the active layer 2a is formed by the polysilicon film 2 in which the source / drain regions are formed. At this time, the impurity is also implanted into the polysilicon film 4 for forming the gate electrode, and the resistance value thereof is reduced, so that the gate electrode 4a is formed by the polysilicon film 4 into which the impurity is implanted. Hereinafter, a thin film transistor is manufactured by forming an interlayer insulating film, contact holes, and source / drain electrodes by a known method.

In the thin film transistor thus manufactured, the single crystal silicon film 3 for forming the gate oxide film 3b can have a good quality even if it is formed relatively thin. 3, the gate oxide film 3b formed by thermal oxidation of 3 can be increased in withstand voltage,
In addition, reproducibility can be improved, and productivity can be prevented from lowering. In addition, the presence of the silicon oxide film 5a for preventing side etching can prevent side etching of the gate oxide film 3b, so that the breakdown voltage can be prevented from deteriorating.

In the above embodiment, the gate oxide film 3b is used.
The case where the invention is applied to the thin film transistor having the gate electrode 4a formed thereon has been described, but the invention is not limited to this, and the invention can be applied to, for example, a memory transistor. In this case, for example, in the state shown in FIG. 2, a single crystal silicon film for forming a tunnel oxide film, a silicon nitride film, and a polysilicon film for forming a gate electrode are formed on the single crystal silicon film 3 for forming a gate oxide film. After patterning these, a single crystal silicon film for forming a silicon oxide film for preventing side etching is formed on the entire surface, thermally oxidized, and then patterned, and then both single crystal silicon films are thermally oxidized. Impurities may be implanted after simultaneously etching both silicon oxide films. The memory transistor having such a structure can have high durability and reliability.

[0012]

As described above, according to the present invention, even if the single crystal silicon film for forming the gate oxide film is formed to be relatively thin, good film quality can be obtained. The withstand voltage of the gate oxide film formed by thermally oxidizing the silicon film can be increased, reproducibility can be improved, and productivity can be prevented from lowering.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a state in which a polysilicon film for forming an active layer and a single crystal silicon film for forming a gate oxide film are formed on an insulating substrate in manufacturing a thin film transistor according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a state in which a single crystal silicon film for forming a gate oxide film is thermally oxidized into a silicon oxide film and then patterned in manufacturing the same thin film transistor.

FIG. 3 is a cross-sectional view of a state in which a polysilicon film for forming a gate electrode and a single crystal silicon film for forming a silicon oxide film for preventing side etching are formed in manufacturing the same thin film transistor.

FIG. 4 is a cross-sectional view showing a state in which a single crystal silicon film for forming a silicon oxide film for preventing side etching is thermally oxidized into a silicon oxide film and then patterned in manufacturing the same thin film transistor.

FIG. 5 is a cross-sectional view showing a state in which both silicon oxide films are simultaneously etched in manufacturing the same thin film transistor.

FIG. 6 is a cross-sectional view of a state in which impurities are implanted in manufacturing the same thin film transistor.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Insulating substrate 2 Polysilicon film for forming active layer 2a Active layer 3 Single crystal silicon film for forming gate oxide film 3a Silicon oxide film for forming gate oxide film 3b Gate oxide film 4 Polysilicon film for forming gate electrode 4a Gate electrode 5 Single-crystal silicon film for forming silicon oxide film for preventing side etching 5a Silicon oxide film for preventing side etching

Claims (5)

[Claims] 1. A thin film transistor having polysilicon as an active layer, comprising a gate oxide film formed by thermally oxidizing single crystal silicon. 2. The thin film transistor according to claim 1, further comprising a gate electrode made of polysilicon. 3. The thin film transistor according to claim 2, wherein the gate oxide film is formed only in a region corresponding to the gate electrode. 4. A single crystal silicon film for forming a gate oxide film is formed on the surface of a polysilicon film for forming an active layer, and the single crystal silicon film is thermally oxidized to form a silicon oxide film. A method of manufacturing a thin film transistor, characterized in that a gate oxide film is formed of a silicon oxide film. 5. A polysilicon film for forming a gate electrode is formed at the center of the upper surface of the silicon oxide film formed by thermally oxidizing the single crystal silicon film for forming the gate oxide film, and a side surface is formed on these surfaces. A single crystal silicon film for forming a silicon oxide film for preventing etching is formed, the single crystal silicon film is thermally oxidized to form a silicon oxide film, and the both silicon oxide films are simultaneously etched to thereby remove the poly silicon film. 5. The method of manufacturing a thin film transistor according to claim 4, wherein the gate oxide film is formed only between silicon films.