JPS5821868A - Manufacture of thin polycrystalline silicon film transistor - Google Patents

Abstract

PURPOSE:To obtain a thin polycrystalline Si film transistor having preferable characteristics by forming source and drain electrodes, plasma oxidizing the surfaces, and removing Si oxide with a solution containing hydrofluoric acid formed by the plastic oxidation. CONSTITUTION:After source and drain electrodes 4 and 5 are formed, the surfaces are plasma oxidized. Then, the surfaces are etched with a solution containing hydrofluoric acid. According to this method, the surface of a polycrystalline Si semiconductor film 2 can be oxidized at a low temperature. Thus, it can prevent the invasion of undesired impurity, and since the oxide of the Si produced at that time can be readily removed with the solution containing the hydrofluoric acid, the surface including aluminum which is subjected to solid solution on the polycrystalline Si surface layer can be cleaned. The source and drain electrode patterns are simultaneously oxidized, but the produce aluminum oxide is hardly dissolved in the hydrofluoric acid. Accordingly, only the surface of the Si film can be selectively etched. As a consequence, the Si surface can be cleaned without affecting adverse influence to the other, thereby obtaining preferable characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a thin film transistor using polycrystalline silicon.

A thin film transistor is an active element formed by depositing a semiconductor thin film on an insulating substrate by vapor deposition or the like, and is usually a field effect type, and is similar in structure and operation to a MOS-FET. However, while MOS-FETs are usually formed using a single crystal substrate, thin film transistors are constructed from a semiconductor thin film formed on an insulating substrate, and therefore have the advantage of being able to fabricate large-area transistor arrays. are doing. Therefore, it is extremely suitable as a switching element for preventing crosstalk in, for example, a liquid crystal matrix display. In other words, in recent years, liquid crystal matrix displays have been developed as devices for pocket TVs and computer terminals, and there is a demand for even higher definition images, but in order to prevent crosstalk due to the increase in the number of pixels. An effective method is to attach a switching element to each pixel. In this case, it is advantageous to use thin film transistors because they can be fabricated on one substrate of the display panel. Compounds such as CdS and CdSe, amorphous silicon, and the like are also used as semiconductors constituting the thin film, but polycrystalline silicon is the best in terms of stability of properties and non-polluting properties.

FIGS. 1 and 2 are cross-sectional views showing an example of this type of thin film transistor commonly used. In the figure, 1 is an insulating substrate such as glass, 2 is a semiconductor film, 3 is an insulating film, 4 and 5 are source and drain electrodes, and 6 is a gate electrode.

However, in a thin film transistor having the above structure, when the semiconductor is polycrystalline silicon, if the film thickness is too small, the crystallinity is insufficient and good operating characteristics cannot be obtained. The above is preferably 5000 AJJ or more. However, if the thickness of the semiconductor film is increased in this way, in the case of the structure shown in Figures 1 and 2, even if voltage is applied to the gate, carriers will not be sufficiently excited in the semiconductor near the source and drain electrodes, making it difficult to operate. There is a drawback. Therefore, when polycrystalline silicon is used as the semiconductor film, the third and fourth layers are advantageous for carrier excitation.
The structure shown in the figure is considered desirable.

In addition, in FIGS. 3 and 4, the same parts as in FIGS. 1 and 2 are given the same reference numerals. However, in the structure shown in FIG. 3, the polycrystalline silicon semiconductor film 2 is formed after forming the source and drain electrodes 4 and 5, but in order to form the polycrystalline silicon film, the substrate temperature must be set at 500°C. However, there is a drawback that the electrode material may diffuse into the silicon or react with the silicon at the time of forming the silicon film, so that it cannot be used in practice. In the end, if polycrystalline silicon is used as the semiconductor film, the structure shown in FIG. 4 has to be adopted.

In the case of the structure shown in Figure 4, it is possible to form the source and drain electrodes by mask evaporation, but there are drawbacks such as insufficient precision of the electrode pattern and leakage between the source and drain. be. On the other hand, photoetching can easily form a predetermined electrode pattern and produce desirable results. 1. As an electrode material, it does not easily react with polycrystalline silicon, and has good 'tri' properties.
Considering conditions such as the ability to make physical contact, it can be almost limited to Ad. In the end, it is desirable that the source and drain 'th< of the polycrystalline silicon thin film transistor be formed with an Al pattern formed by photo-etching.

However, when such a polycrystalline silicon thin film transistor was manufactured, it was found that the following problems occurred. That is, when depositing Al on a polycrystalline silicon film,
When the substrate temperature is raised to improve adhesion, A/ tends to diffuse into the polycrystalline silicon and the surface layer becomes P-type. This is extremely sensitive to the substrate temperature, so the operating characteristics remain constant. Moreover, it is difficult to obtain good operating characteristics because the polycrystalline silicon surface is not clean after the source/drain electrodes are formed by photoetching the A1 layer.

These can be cleaned by etching the surface with a suitable solution, but when attempting to clean the silicon surface, the problem arises that Al also dissolves.

The present invention has been conceived in view of these points, and its purpose is to provide a manufacturing method for obtaining polycrystalline silicon thin film transistors with good characteristics. The present invention will be explained in detail below.

First, in FIG. 4, after forming source and drain electrodes 4.5, their surfaces are plasma oxidized.

Next, the surface is etched with a solution containing 7-acid.

According to this method, since the surface of the polycrystalline silicon semiconductor film 2 can be oxidized at low temperature, it is possible to prevent the intrusion of undesirable impurities, and furthermore, the silicon oxide produced at that time can be easily removed with a solution containing hydrofluoric acid. Since Al is removed as a solid solution in the polycrystalline silicon surface layer, the surface is cleaned. Further, the source and drain electrode patterns are also oxidized at the same time, but since the generated Al oxide is extremely difficult to dissolve in hydrofluoric acid, only the surface of the polycrystalline silicon film can be selectively etched.

As described above, according to the manufacturing method of the present invention, it is possible to clean the polycrystalline silicon surface without adversely affecting others, so that it is possible to manufacture transistors with good reproducibility and good characteristics. It has the excellent effect of being able to

[Brief explanation of drawings]

1 to 3 are cross-sectional views of conventional polycrystalline silicon thin film transistors, and FIG. 4 is a cross-sectional view of a polycrystalline silicon thin film transistor in which the manufacturing method of the present invention is implemented. 1... Insulator substrate, 2... Semiconductor film, 3...
- Insulating film, 4... fist source electrode, 5... drain electrode, 6... fist/gate electrode. 7-

Claims (1)

[Claims] A source is formed by forming an Ag layer on a polycrystalline silicon film formed on an insulating substrate and photoetching it. A method of manufacturing a polycrystalline silicon thin film transistor with a drain electrode formed therein includes plasma oxidizing the surface after forming the source and drain electrodes, and removing silicon oxide formed by the plasma oxidation with a solution containing hydrofluoric acid. A method for manufacturing a characteristic polycrystalline silicon thin film transistor.