JPH07106589A - Thin film field effect transistor - Google Patents

Abstract

PURPOSE:To cut off the path of a leakage current on the surface of a protective insulating film, by forming an oxide film or a nitride film covering the surface of a source electrode or a drain electrode. CONSTITUTION:A gate electrode 2 is formed on a substrate 1 by using a photo etching method. A gate insulating film 3, a semiconductor layer 4 forming a channel, and an etching stopper 5 are continuously formed. An ohmic contact layer 6 is formed by etching an insulating film as the etching stopper 5. A picture element transparent electrode 7 and a source.drain electrode 8 are formed and patterned in desired shapes. An anode oxide film 10 is formed on the surface of the source-drain electrode 8. A protective insulating film 9 is formed by a plasma CVD method, and the insulating film on the picture element electrode 7 is etched and eliminated. Thereby the surface of the source- drain electrode 8 is not exposed, and the generation of a leakage current can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film field effect transistor used in a liquid crystal display device or the like.

[0002]

2. Description of the Related Art Nonlinear elements such as thin film transistors are used as switching elements in each pixel of an active matrix type liquid crystal display. Conventionally, this type of element has an element having a cross section as shown in FIG. A gate electrode 51 formed by a sputtering method or the like on a transparent insulating substrate 51 such as glass, a gate insulating film 53 such as SiOx or SiNx formed on the gate electrode 51 by plasma CVD, and a gate insulating film 53. I as a semiconductor layer formed by plasma CVD or the like
-A-Si: Si formed by plasma CVD or the like as an etching stopper on the H film 54 and the semiconductor layer 54.
An Ox or SiNx film 55, an n + a-Si: H film 56 as an ohmic contact layer, and an Al / Mo layer 58 formed by a sputtering method or the like as a source / drain electrode.
Are formed from etc. Finally, for the purpose of wiring protection and element stabilization, an insulating film 5 such as SiOx or SiNx is formed.
9 is formed by a method such as plasma CVD. 57
Is a pixel electrode connected to the source electrode.

In such a thin film transistor in which the source / drain electrodes are the uppermost layer, there has been a problem that a leak current occurs in the off region as compared with the conventional case.
When a leak current is generated in the off region, the holding operation of the transistor cannot be performed well, and as a result, the display quality of the liquid crystal display is deteriorated.

One of the causes of such leakage current is defective coverage of the protective insulating film covering the surface of the transistor. The source / drain electrodes that must be covered have a film thickness of about 400 nm to 500 nm, and poor coverage occurs at the stepped portions of the source / drain electrodes when the protective insulating film is formed. When the source / drain electrodes are sufficiently covered, the inside of the insulating film must be conducted in order for current to flow between the source / drain. Such a leak current will not occur unless a protective insulating film having a very poor insulating property is formed. However, if the coverage is not good on both sides of the source / drain electrode and the electrode surfaces A and B are exposed, the current between the source and drain is conducted to the surface of the protective insulating film. Conduction inside the bulk is unlikely to occur, but conduction at the surface easily occurs. Therefore, if the coverage is poor on both sides of the source / drain electrode, it causes a leak current that is conducted on the surface of the protective insulating film.

As a method of preventing the generation of the leakage current due to such poor coverage, a method of reducing the film thickness of the source / drain electrodes or increasing the film thickness of the protective insulating film can be considered. However, reducing the film thickness of the source / drain electrodes leads to an increase in wiring resistance, the transmitted signal is attenuated, the desired voltage is not applied to the liquid crystal layer, and the display quality of the display is deteriorated. . Increasing the thickness of the protective insulating film is effective in surely improving the coverage. However, at the same time, it causes unfavorable results in terms of device characteristics such as increase in threshold voltage and decrease in mobility. As described above, the conventional methods cannot effectively prevent the leak current without changing the film thickness of the protective insulating film or the source / drain electrodes.

[0006]

As described above, the protective film is formed on the uppermost layer of the thin film transistor. But,
Since the source / drain electrodes have a film thickness of about 400 nm to 500 nm, defective coverage may occur at the step portion. If the coverage on both sides of the source electrode and the drain electrode is not good and the electrodes are exposed, it may cause a leak current that is conducted on the surface of the protective insulating film.

The present invention has been made to solve the above-mentioned problems, and a thin film field effect transistor in which the leakage current path on the surface of the protective insulating film is cut off to stabilize the characteristics in the off region of the thin film transistor. For the purpose of providing.

[0008]

According to the present invention, a source / drain electrode is formed on an insulating substrate, and a source / drain electrode is arranged on a semiconductor layer in which a channel is formed, and an insulating film is formed between the source / drain electrodes. Another object of the present invention is to provide a thin film field effect transistor comprising an oxide film or a nitride film of the source electrode or the drain electrode, which covers the surface of the source electrode or the drain electrode.

In particular, the oxide film is preferably an anodized film, and after the source / drain electrodes are formed,
It is preferable to cover the surface of the source / drain electrodes with an insulating film formed by forming a film of anodizable metal and then anodizing this.

[0010]

According to the present invention, of the source / drain electrodes,
Even if the electrode surface on the side where the insulating film is not formed has poor coverage and the electrode surface is exposed, the surface of the opposing electrode is covered with an oxide film or a nitride film, so a leak current flows into the electrode. The path of the leak current that cannot be prevented and is conducted on the surface of the protective insulating film is blocked. Further, if the surface of the electrode on the side where the coverage is deteriorated is covered with the insulating film, the electrode is not exposed and the generation of the leak current is prevented.

[0011]

EXAMPLE A case in which an anodic oxidation method is used as one of the methods for forming an insulating film according to the first example of the present invention will be described below. 1A to 1D are cross-sectional views in the manufacturing process order of the thin film transistor according to the first embodiment.

A transparent insulating substrate 1 such as cleaned glass
A low resistance metal film such as MoTa is formed thereon by using a sputtering method or the like, and the gate electrode 2 is formed by using a method such as photoetching (FIG. 1A).

Next, using a plasma CVD method or the like, SiO and SiN as the gate insulating film 3, ia-Si: H as the semiconductor layer 4 in which the channel is formed, and SiO and SiN as the etching stopper 5 are continuously formed. Then, a film is formed. Next, the insulating film as an etching stopper is etched into a desired shape (FIG. 1B).

Then, n + a-Si: H as the ohmic contact layer 6 is formed by plasma CVD. i
After -a-Si: H, n + a-Si: H, etc. are simultaneously patterned into a desired shape (FIG. 1C), ITO as a pixel transparent electrode 7 is formed into a film by a sputtering method or the like to obtain a desired shape. Patterning is performed (FIG. 1D).

Next, as source / drain electrodes 8, Al /
A film of Mo or the like is formed by a sputtering method or the like and patterned into a desired shape (FIG. 1E). Then, n + a-Si: H6 remaining between the source / drain electrodes is removed by etching (FIG. 2A).

Further, anodic oxidation, which is a feature of the present invention, is applied to the source / drain electrodes. The anodic oxide film 10 is formed on the surface of the source / drain electrodes using an ammonium borate solution, an ammonium tartrate solution, an oxalic acid solution, or the like (FIG. 2B). At this time, the surface of the portion such as the electrode pad where it is not convenient to form the insulating film is covered with a resist or the like. On the contrary, a method of removing an unnecessary portion by using a technique such as photoetching after forming the anodic oxide film is also possible. The anodic oxidation method is a self-aligned insulating film forming method, and is a very simple method. Further, since the treatment is performed in a solution, there is an advantage that it is difficult to form pinholes due to the influence of dust and the like, and a dense insulating film can be easily formed. In the case of this example, since n + a-Si: H is etched before anodizing,
Since no voltage is supplied to the electrode connected to the pixel electrode 7, only the electrode not connected to the pixel electrode is anodized.

Finally, the protective insulating film 9 is formed by a method such as plasma CVD, and the insulating film on the pixel transparent electrode 7 is removed by a method such as etching (FIG. 2C). In this embodiment, as shown in the cross-sectional view between the source and drain electrodes in the case of using this embodiment shown in FIG. 2D, when the method according to this embodiment is used, it is not connected to the pixel electrode. In the process of anodizing the electrode, the side surface of the electrode is also anodized. Further, since the side surface of n + a-Si: H is also anodized, the electrode surface is entirely covered with the insulating film. A liquid crystal display device was completed by making a substrate having such a field effect transistor and a counter substrate facing the substrate and having a counter electrode formed on the surface thereof face each other via a liquid crystal layer.

As described above, in the thin film transistor formed by this embodiment, the electrode not connected to the pixel transparent electrode is covered with the dense insulating film formed by the anodic oxidation method. Therefore, if a leak current tries to flow between the source / drain electrodes, that current must be conducted inside the insulating film, and if the surface of the source / drain electrode is exposed and leak current occurs due to surface conduction. In comparison, the generation of leak current is much suppressed. The thin film transistor of this example had a leakage current of 10 ^−12, and the thin film transistor similar to that of this example in which an anodized film was not formed was 10 ⁻⁹ for comparison. Therefore, the leakage current is reduced by three digits or more. You can see that it was done.

Next, a method for forming an insulating film by anodic oxidation on both sides of the source / drain electrodes will be described below as a second embodiment. In the following embodiments, the same parts as those in the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

The steps up to the step of patterning the source / drain electrodes into a desired shape (FIG. 1E) are the same as in the first embodiment. Next, anodic oxidation is performed. However, if anodic oxidation is performed as it is, n + a-Si: H remains on the opposite side surfaces of the source / drain electrodes, which causes a leak current. Therefore, in this embodiment, n + a-Si: H is etched to an appropriate film thickness before anodizing (FIG. 3A).

After that, by performing anodic oxidation, insulating films are formed on the surfaces on both sides of the source / drain electrodes (FIG. 3B). Next, the n + a-Si: H remaining between the source and the drain is etched (FIG. 3C).

FIG. 3E shows an enlarged sectional view between the source and drain electrodes in this state. As a result of such a process, n + a is formed on the opposite side surfaces of the source / drain electrodes.
Although --Si: H remains, the possibility of causing a leak current is reduced because the film thickness is reduced by etching before anodization. As in Example 1,
The portion where it is not convenient to form an insulating film, such as on the electrode pad, is covered with a resist or the like, or is removed by a method such as photoetching after forming an anodic oxide film. Finally, by plasma CVD method,
An insulating film 9 such as SiO or SiN is formed, and the insulating film on the pixel transparent electrode is removed by a method such as etching (FIG. 3D).

This embodiment can also achieve the same effect of reducing the leak current as in the first embodiment. Next, the third aspect of the present invention
An example of the above will be described below.

N + remaining between the source and drain electrodes
The process up to the step of etching a-Si: H (FIG. 2A) is the same as in the first embodiment. After that, an anodizable metal film 40 such as Al is formed by a sputtering method.

Next, the metal film 40 on the portion where the insulating film is not needed, such as on the pixel transparent electrode, is finally removed by a method such as etching (FIG. 4A). Next left metal film 4
0 is anodized. At this time, anodic oxidation is performed until the metal film on the stopper is completely oxidized (see FIG. 4).
(B)). Further, as in the first embodiment, the surface of the electrode pad or the like is covered with a resist or the like, or the anodic oxide film is formed and then removed by a method such as photoetching. When the step of removing by using a method such as photoetching is selected, the step of removing the metal film on the pixel transparent electrode can be included in this step after the formation of the anodic oxide film. By performing such processing, the source
Since the drain electrode is completely covered with the dense insulating film, the source / drain electrodes are not exposed and the generation of the leak current conducted on the surface of the surface protective insulating film is prevented. This embodiment has the same advantages as those of the first embodiment, and has the following advantages. Normally, in the manufacturing process of a thin film transistor, an insulating film such as SiO or SiN is finally formed on the thin film transistor and the wiring for the purpose of stabilizing the element and protecting the wiring. Plasma CVD is used to form this protective insulating film.
Etc. are used. When the protective insulating film is formed by plasma CVD, heating is performed in a vacuum, and plasma may be damaged during the formation, which may deteriorate the characteristics of the already formed device. In addition, the process of forming the protective insulating film is film formation,
Etching is a very time-consuming process. On the other hand, according to the present embodiment, as shown in FIG. 4C, the thin film transistor and the wiring are already covered with the insulating film when the insulating film is formed by the anodic oxidation treatment. This insulating film is a very dense insulating film formed by anodic oxidation treatment, and plays a sufficient role as a protective film.

Therefore, by using this embodiment,
Not only the generation of leakage current is prevented, but also the process of forming the protective film can be omitted, the time required for forming the protective film can be shortened, and the risk of deterioration of device characteristics at the time of forming the protective film can be avoided. .

Further, a fourth embodiment of the present invention will be described. This embodiment differs from the first embodiment in that the source
That is, the anodic oxide film formed on the surface of the drain electrode was changed to a plasma oxide film. The surface of the source electrode or the drain electrode can also be modified into an oxide film by plasma oxidation.

First, before forming this plasma oxide film, the steps of forming the source / drain electrodes shown in FIG. 2A of the first embodiment are performed. After that, a protective film is formed on the surface of the non-oxidized portion of the ITO pixel electrode or the like, and then the substrate 1 is set in the chamber of the plasma oxidation device to oxidize the source / drain electrodes. This oxidation is performed in an atmosphere containing oxygen atoms, for example, oxygen. As a result, a molybdenum oxide film is formed on the surface of the source / drain electrodes.

After the oxidation, an insulating film covering the thin film transistor is formed as in the step of FIG. The same effects as those of the first embodiment can be obtained even with the above configuration.
Although the oxide film is formed in the above four embodiments, a nitride film or an oxide film including a nitride film in the oxide film may be formed in the same manner.

The source / drain electrodes are preferably made of a material on which a dense oxide film or nitride film is easily formed, such as Al. The present invention is not limited to the above embodiments, and various modifications can be carried out without departing from the spirit of the present invention.

[0031]

By using the present invention, the electrode surfaces on both sides of the source / drain electrodes are not exposed at the same time, and the generation of a leak current between the source / drain electrodes conducted on the surface of the protective insulating film is prevented. It

[Brief description of drawings]

FIG. 1 is a sectional view of a thin film transistor according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a thin film transistor according to a first embodiment of the present invention.

FIG. 3 is a sectional view of a thin film transistor according to a second embodiment of the present invention.

FIG. 4 is a sectional view of a thin film transistor according to a third embodiment of the present invention.

FIG. 5 is a cross-sectional view of a conventional thin film transistor.

[Explanation of symbols]

 1. Transparent insulating substrate 2. Gate electrode 3. Gate insulating film 4. Semiconductor layer 5. Etching stopper 6. Ohmic contact layer 7. Pixel transparent electrode 8. Source / drain electrodes 9. Protective insulating film 10. Anodized film

Claims (1)

[Claims] 1. A thin film field effect transistor in which a source / drain electrode formed on an insulating substrate is arranged on a semiconductor layer in which a channel is formed, and an insulating film is formed between the source / drain electrodes. A thin film field effect transistor comprising an oxide film or a nitride film of the source or drain electrode, which covers the surface of the source or drain electrode.