JPS5914673A - Manufacture of thin film transistor - Google Patents

Abstract

PURPOSE:To obtain a stable transistor by forming an Si layer on an insulator substrate, forming source and drain electrodes at the prescribed interval on the layer, directly plasma oxidizing the surface layer of the Si layer to form an insulator layer, plasma anodically oxidizing the electrode surface layer as an insulator layer, and superposing it on the vicinity of both ends of the electrode to form a gate electrode. CONSTITUTION:An amorphous or polycrystalline Si layer 4 is accumulated by a low voltage plasma discharge decomposition method on an insulator substrate 1, and source and drain electrodes 5, 6 made of aluminum are formed at the prescribed interval on the layer. Then, the surface of the layer 4 is oxidized by a plasma anodic oxidizing method to convert it into an SiO2 layer 7, the surfaces of the electrodes 5, 6 are directly anodically oxidized to convert them into Al2O3 layers 8, 9. Thereafter, a gate electrode 2 is formed while superposing the end on the electrodes 5, 6 on the layer 7 between the electrodes 5 and 6. In this manner, H2 is not mixed in the insulator layer, and even if the insulator layer is reduced in thickness, leakage current is not increased.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a thin film transistor having a thin silicon semiconductor layer, and more particularly to a method for manufacturing a thin transistor having a coplanar electrode structure.

Conventionally, the electrode structures of thin film transistors are known as a staggered electrode structure and a coplanar electrode structure.

The coplanar electrode structure is superior to the staggered electrode structure in that the thickness of the semiconductor layer does not affect the electrical characteristics of the thin film transistor.

FIG. 1 is a cross-sectional view of an example of a thin film transistor with a conventional staggered electrode structure.

This transistor is manufactured as follows.

A gate electrode 2 is formed on an insulating substrate 1, and a silicon oxide film is formed as an insulating layer 3 on the entire surface of the substrate including the gate electrode 2 by a low pressure plasma decomposition method. Next ζ9, an amorphous silicon semiconductor layer 4 is formed by a low pressure plasma decomposition method. A source electrode 5 and a drain electrode 6 are formed at a predetermined interval on this silicon semiconductor layer 4 to form a thin film transistor. As an example of dimensions of this transistor, the channel length is 10 μm, the channel width is 100 μm, the amorphous silicon film thickness is 500 nm, and the silicon oxide film thickness is 200 nm.

The thin film transistor manufactured in this way has low voltage resistance, with a gate voltage of 10μ and a drain-source voltage of 1
Even under the on-state voltage condition of 0V, dielectric breakdown occurs at a rate of several percent. Elements that did not cause dielectric breakdown had a source-drain resistance of 10'Ω or less in the on state, a gate voltage of Ov,
1010Ω in off state with source-drain voltage 10V
Although a satisfactory value can be obtained for the switching of the liquid crystal element as described above, there is a problem that the electrical characteristics have large drift and hysteresis. Also, when the gate voltage is IOV, the leakage current from the gate electrode to the source electrode -bs 1
Many are as high as 0'A. The drift and hysteresis in the electrical characteristics are thought to be due to the presence of many interface states at the interface between the silicon oxide film and the amorphous silicon film, since a silicon oxide film produced by low-pressure plasma decomposition was used as the insulator film. . Furthermore, the large leakage current and low insulation resistance are thought to be due to the presence of a large amount of hydrogen in the insulator film because the insulator layer was formed by a low-pressure plasma discharge decomposition method.

FIG. 2 is a cross-sectional view of another example of a conventional staggered structure thin film transistor.

This transistor is also manufactured in the same manner as the transistor shown in FIG. That is, the upper source electrode 5 of the insulator substrate 1. A drain electrode 6 is formed, a silicon semiconductor layer 4 is formed on the entire surface, an insulator layer 3 is provided thereon, and a gate electrode 2 is provided thereon. The manufacturing conditions are similar to the example shown in FIG.

FIG. 3 is a cross-sectional view of an example of a conventional thin film transistor having a coplanar electrode structure.

A gate electrode 2 is provided on an insulating substrate 1, an insulating layer 3 is formed on the entire surface, and a source electrode 5 and a drain electrode 6 are formed thereon at a predetermined interval. Then, a silicon semiconductor layer 4 is formed on the entire surface to produce this transistor.

The manufacturing conditions are shown in Figure 1.

This is similar to the staggered electrode structure transistor shown in FIG.

FIG. 4 is a sectional view of another example of a conventional thin film transistor having a coplanar electrode structure.

This transistor has a source electrode 5. The process is the same as the example shown in FIG. 2, except that the steps for forming the drain electrode 6 and the silicon semiconductor layer 4 are reversed from the example shown in FIG. The manufacturing conditions were also the same as in the three examples above.

In all of the above four examples, the plasma discharge decomposition method is used to form the insulator layer 3. This is a method of forming a silicon oxide film on a substrate by low-pressure plasma discharge using a mixed gas containing silane, oxygen, etc.

The silicon oxide film produced by this low-pressure plasma discharge decomposition method is
Since it can be formed at a lower temperature than a silicon oxide film formed by thermal decomposition, it has the advantage that an inexpensive substrate with low heat resistance, such as glass, can be used as an insulating substrate. However, the silicon oxide film formed by this low-pressure plasma discharge decomposition method is different from the silicon oxide film formed by the sputtering method, vapor deposition method, gate electrode oxidation method, etc. that was used previously. Although the number of levels at the interface with the silicon oxide film has decreased, there are still many units compared to a silicon oxide film formed by thermal oxidation. Also, silane.

Since a mixed gas containing oxygen is decomposed by plasma discharge, there is a problem in that hydrogen easily enters the silicon oxide film and the characteristics of the thin film transistor change greatly due to subtle changes in the formation conditions. on the other hand.

To operate thin film transistors with low gate voltage,
However, it is necessary to reduce the thickness of the insulator layer 3.

If the insulator layer 3 is thinned, the silicon oxide film produced by low-pressure plasma decomposition contains hydrogen, so electrical problems such as low voltage resistance of the silicon oxide film and large leakage current from the gate electrode to the drain electrode occur. This had the disadvantage of causing the problem □ which deteriorates the characteristics.

The purpose of the present invention is to provide a method for manufacturing a thin film transistor with a coplanar electrode structure that eliminates the above drawbacks, has low leakage current, high voltage resistance, and has an insulator layer with high durability and excellent electrical characteristics. It is about providing.

The method for manufacturing a thin film transistor of the present invention includes a step of forming a silicon semiconductor layer on an insulating substrate, a step of forming a source electrode and a drain electrode at a predetermined interval on the silicon semiconductor layer, and a step of forming a silicon semiconductor layer on the silicon semiconductor layer. Direct plasma oxidation of the surface layer of the semiconductor layer to form an oxide insulator layer, and at the same time direct anodic plasma oxidation of the surface layer of the source electrode and the drain electrode to form the oxide insulator layer; The method includes the step of forming a gate electrode on the insulating layer so that the vicinity of both ends thereof overlap the source electrode and the drain electrode.

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 5 is a sectional view of a thin film transistor for explaining one embodiment of the present invention.

Although barium borosilicate glass is used for the insulator substrate 1, it is not limited thereto, and it goes without saying that substrates made of other insulators can also be used. A silicon semiconductor layer 4 on top of the insulator substrate 1 is formed.

An amorphous silicon semiconductor layer can be obtained using a low-pressure plasma discharge decomposition method. An example of the formation conditions is silane flow rate 20 cc/n1in, hydrogen flow rate f3 Q cc/m
In, the pressure was 0.3 torr, the high frequency power was 50 W, and the substrate temperature was 300°C. The silicon semiconductor layer 4 is not limited to being amorphous, and may of course be polycrystalline.

Next, a source electrode 5. is placed above the silicon semiconductor layer 4 at a predetermined interval. A drain electrode 6 is formed.

The electrode material uses a metal that can be anodized. There are many metals that can be anodized, such as Kl, 8i, Ta, and Nb, but in this example, At will be used.

Note that when high-temperature heat treatment is performed in a later step, it is necessary to select a metal that has a high melting point and can be anodized.

Next, a silicon semiconductor layer 4 is formed using plasma anodic oxidation.
At the same time, the surfaces of the source electrode 5 and drain electrode 6 are directly anodized to form aluminum oxide insulator layers 8 and 9. An example of plasma anodization is:
Oxygen flow rate 200cc/mln, pressure 0.5torr,
High frequency power 100W.

The substrate temperature is 300°C. Aluminum oxide 8. from above silicon oxide layer 7 between source electrode 5 and drain electrode 6. A gate electrode 2 is formed so as to cover the gate electrode 9.

As a result, a thin film transistor with a coplanar electrode structure is obtained.

The thin film transistor thus manufactured has a gate electrode 2, a source electrode 5. The leakage current between the drain electrode 6 and the drain electrode 6 was small, and was less than 1O-12A when the gate voltage was 10V in all devices. In addition, the voltage resistance was high, and no insulation failure occurred even when a gate voltage of 20 cm and a source-drain voltage of 20 V were applied. Gate voltage 10'■, source-drain voltage 10 for each element
With V on.

The resistance is 106Ω or less, and the gate voltage Ov, source voltage
In the off-state with a drain-to-drain voltage of 10 V, the resistance is 1010 Ω or more, which is sufficient for switching the liquid crystal element.

Ivy. In addition, no drift or hysteresis was observed in the electrical characteristics.This is because the semiconductor layer is directly plasma oxidized and at the same time the source and drain electrodes are directly plasma anodized. Despite the fact that there are few levels at This is thought to be due to the fact that the edges of the insulator were completely covered with an insulator layer.

As explained in detail above, according to the present invention, since the insulating layer is formed by plasma oxidation of the silicon semiconductor layer and the source and drain electrodes, no hydrogen is mixed into the insulating layer. Even if the insulator layer between the gate electrode and the semiconductor layer is made thinner to lower the operating gate voltage, the insulator layer between the gate electrode and the source and drain electrodes is made thicker to keep leakage current low. In addition, a thin film transistor with high stability and reliability without electrical characteristic drift or hysteresis can be obtained, so the effect is very large.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of an example of a thin film transistor with a conventional staggered electrode structure, Figure 2 is a cross-sectional view of another example of a thin film transistor with a conventional staggered electrode structure, and Figure 3 is a cross-sectional view of an example of a thin film transistor with a conventional coplanar electrode structure. 4 is a sectional view of another example of a conventional thin film transistor having a coplanar electrode structure, and FIG. 5 is a sectional view of a thin film transistor for explaining an embodiment of the present invention.

Claims (1)

[Claims] a step of forming a silicon semiconductor layer on an insulating substrate; a step of forming a source electrode, a drain electrode, and a drain electrode at predetermined intervals on the silicon semiconductor layer; Direct plasma oxidation of the surface layer to create an oxide insulator layer! At the same time, the surface layer of the source electrode and the drain electrode is directly anodic plasma oxidized to form an oxide insulating layer, and the source electrode, the drain electrode and the pot are formed on the insulating layer and the source electrode and the drain electrode. 1. A method of manufacturing a thin film transistor, comprising the step of forming a gate electrode so that the vicinity of both ends thereof overlap.