JPH05315615A - Thin film transistor - Google Patents

Abstract

(57) [Abstract] [Purpose] A gate electrode is formed of a material having a low electric resistance and to which anodization is applicable, and an oxide film having a high relative dielectric constant formed by anodization is used as an insulating layer. A thin film transistor is provided. [Structure] An aluminum layer 12 and a tantalum layer 14 to be a gate electrode are laminated on a substrate 10. A photoresist 16 is formed thereon and patterned, and tantalum is over-etched in a predetermined region by a plasma etching method using CF ₄ . At this time, since aluminum is not etched by the plasma etching method of CF ₄ , the side portion of the tantalum layer is etched,
The aluminum layer 12 and the tantalum layer 14 have a stepwise cross section. On the other hand, by performing anodic oxidation, most of the tantalum layer is anodized to form the tantalum oxide film 20.

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 (TFT) used in a liquid crystal display device or the like.

[0002]

2. Description of the Related Art FIG. 5 is a sectional view of a conventional bottom gate type thin film transistor used in a liquid crystal display device. This thin film transistor is configured by sequentially forming a gate electrode 32, an insulating film 34, a semiconductor layer 36, and an ohmic contact layer 38 on a substrate 44, and further forming a source electrode 40 and a drain electrode 42 on the gate electrode 32. .. Such a thin film transistor is provided for each pixel in the liquid crystal display device, the gate electrode 32 and the source electrode 40 are connected to a large number of electrodes formed in the vertical and horizontal directions, and the drain electrode 42 is provided for each pixel. It is connected to the pixel electrode.

In FIG. 5, chromium is a common material used for the gate electrode 32. The reason for this is that chromium has a strong adhesion to the glass substrate, chromium has a high melting point, so there is no problem even after a process including heat treatment after that, and chromium has amorphous silicon or silicon layered on top of it. This is because it has good compatibility with the nitride film (SiN _x ).

However, chromium has the drawback of high electrical resistance. That is, the resistivity of aluminum is about 3 μΩ · cm, while that of chromium is about 55 μΩ · c.
m. When such a material having a high electric resistance is used as the material of the gate electrode, the CR constant of the circuit including the gate electrode and the thin film transistor becomes large, which causes problems such as propagation delay. This is a big obstacle especially when the display is upsized to 10 inches or more and when a high-definition and high-performance display is manufactured.

For this reason, in recent years, aluminum having a low specific resistance is often used as the gate electrode (see, for example, Nikkei Microdevices separate volume "Flat Panel Display '91", page 88). Aluminum can also be applied with the anodizing method, which is also a feature not found in chromium. Since the anodic oxidation is a wet process, even if there is a foreign substance, the solution can enter underneath it to form an oxide film, so that the effect of preventing an interlayer short circuit is high. When aluminum is anodized, an aluminum oxide film (alumina Al ₂ O ₃ ) having excellent insulating properties is formed on the surface as an oxide. Then, a silicon nitride film is further formed thereon as an insulating film.
If only the silicon nitride film directly formed on the gate electrode is used as the insulating film without providing the aluminum oxide film layer, there is a concern that the gate electrode and the carrier layer may be short-circuited due to pinholes generated in the silicon nitride film layer. is there.
However, when the insulating film has a two-layer structure of an aluminum oxide film and a silicon nitride film, the risk of such a short circuit is reduced and the insulating effect is further improved.

[0006]

However, when aluminum is used as the gate electrode, when etching aluminum, it is difficult to perform the taper etching as compared with chrome, and the edge portion may stand up, resulting in a large step difference. There is a problem that defects such as step breakage are likely to occur when another layer is formed on top of this. Further, the thin film transistor has a characteristic that a higher current can flow when the dielectric constant of the oxide film is higher when the voltage applied to the gate is constant, but the relative dielectric constant of the aluminum oxide film formed by anodizing aluminum is Is also relatively small and therefore has a low current drive capability.

On the other hand, it is possible to use tantalum (Ta) as the gate electrode as another material capable of anodic oxidation. A tantalum oxide film (Ta ₂ O ₅ ) formed when anodizing using tantalum is known as a high dielectric material and has a very high relative dielectric constant (about 25). This means that the current drive capability of the thin film transistor can be increased. However, the electrical resistance of tantalum is higher than that of chromium, so that it is not suitable to use as a gate electrode as it is.

The present invention has been made in view of the above circumstances, in which the gate electrode is formed of a material having a low electric resistance and to which the anodizing method can be applied, and the relative dielectric constant formed by the anodizing is improved. An object of the present invention is to provide a thin film transistor using a high oxide film as an insulating layer, and a method for manufacturing such a thin film transistor.

[0009]

The invention according to claim 1 for achieving the above object comprises a gate electrode, an insulating film, and
In a thin film transistor having a semiconductor film layer, the gate electrode is made of aluminum and tantalum, and a tantalum oxide film is formed on the gate electrode as a part of the insulating film.

According to a second aspect of the invention for achieving the above object, in a thin film transistor having a gate electrode, an insulating film, and a carrier layer, the gate electrode is made of aluminum and niobium, and the gate electrode A niobium oxide film is formed as a part of the insulating film on.

In order to achieve the above object, the invention according to claim 3 forms an aluminum layer, forms a tantalum layer on the aluminum layer, and overetches the tantalum layer by a plasma etching method. ,
The gate electrode and the insulating film are formed by oxidizing the surfaces of the aluminum layer and the tantalum layer by an anodic oxidation method, and forming an insulating film on the anodized oxide film. ..

In order to achieve the above object, the invention of claim 4 forms an aluminum layer, forms a niobium layer on the aluminum layer, and overetches the niobium layer by a plasma etching method. The surface of the aluminum layer and the niobium layer is oxidized by an anodic oxidation method, and an insulating film is formed on the anodized oxide film to form a gate electrode and an insulating film. is there.

[0013]

According to the present invention as set forth in claim 1 or 2, by using aluminum for the gate electrode, the electric resistance of the gate electrode is greatly reduced as compared with the case where chromium is used. Further, by forming an oxide film of tantalum or niobium as a part of the insulating film, the current driving capability of the thin film transistor is improved due to the high relative dielectric constant of the tantalum oxide film or niobium oxide film formed when these are anodized. However, the on-current can be increased.

According to the third and fourth aspects of the present invention, the tantalum layer or the niobium layer formed on the aluminum layer is etched by the plasma etching method according to the above-mentioned structure, whereby the tantalum layer or the niobium layer and the aluminum layer are formed. And can have a stepwise cross section, which can effectively prevent disconnection at the edge portion of the gate electrode. Aluminum, tantalum, and niobium are all materials that can be anodized, and an oxide film having a high relative dielectric constant can be formed on the surface by the anodization method. As a result, it becomes possible to manufacture a thin film transistor having a large current drive capability.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. 1 to 4 are cross-sectional views in each manufacturing process of a thin film transistor which is an embodiment of the present invention. In FIG. 1, first, aluminum (A
The layer 12 of 1) is formed, and the tantalum (Ta) layer 14 is laminated thereon. These two layers serve as the gate electrode of the thin film transistor of this embodiment. Therefore, since the electric resistance of aluminum is small, the CR constant can be reduced when this thin film transistor is used in a liquid crystal display device,
Therefore, the liquid crystal display device can have a large screen and high definition without causing problems such as propagation delay.

Further, a photoresist 16 is formed on this and patterned, and a predetermined portion is etched. At this time, tantalum is etched by a plasma etching method using CF ₄ , and aluminum is wet-etched with a solution containing phosphoric acid. Then, the tantalum is overetched again by the plasma etching method of CF ₄ . At this time, since aluminum is not etched by the CF ₄ plasma etching method, the side portions of the tantalum layer 14 are etched, as shown in FIG. 2, so that the aluminum layer 12 and the tantalum layer 14 have a stepwise cross section. That is, the shape is close to a tapered cross section. Therefore, the gate electrode of this embodiment has an advantage that there are few steps and it is difficult for a step break to occur in another layer formed thereon. Moreover, since such a tapered cross section can be obtained by over-etching, a special mask for forming a step shape when patterning is not required, and only one mask is required for patterning as in the conventional case. There are advantages.

Next, the stepwise aluminum layer 12 and tantalum layer 14 as shown in FIG. 2 are anodized. Some materials such as chromium cannot be anodized depending on the material of the gate electrode, but both aluminum and tantalum used here as the gate electrode are materials that can be anodized. Therefore, various materials described later can be obtained by performing anodization. Benefits are obtained. When anodic oxidation is performed, an aluminum oxide film (Al ₂ O ₃ ) 18 is formed on the surface of the side portion where aluminum is exposed as shown in FIG.

On the other hand, most of the tantalum layer is anodized to form a tantalum oxide film (Ta ₂ O ₅ ) 20.

As described above, the oxide film obtained by anodic oxidation exhibits excellent characteristics as an insulating film. Therefore, there is an advantage that short-circuit defects can be reduced in a portion where wirings cross each other. Further, the tantalum anodic oxide film is characterized by having a very high relative dielectric constant. When a material having a high dielectric constant is used as an insulating film of a MOS type transistor, many carriers can be induced with a small voltage, so that the on-state current of the MOS type transistor can be increased and the current driving capability is improved. To do. Therefore, when this tantalum oxide film is formed,
There is also an advantage that the current driving capability of a thin film transistor, which is a type of MOS type transistor, can be improved.

Each layer formed thereafter and the forming method thereof are the same as in the case of a general thin film transistor. That is, first, as shown in FIG. 4, a silicon nitride film layer 22 is formed as an insulating film on the anodic oxide films 18, 18. That is, the insulating film has a two-layer structure of the aluminum oxide film 18 or the tantalum oxide film 20 and the silicon nitride film 22, and the reliability against a short circuit defect can be further improved. A semiconductor layer 24 serving as a carrier layer, an ohmic contact layer 26, source and drain metal electrodes 28, and a protective film 30 are formed on the insulating film 22.

In the above embodiment, the case where the gate electrode is composed of two layers of aluminum and tantalum has been described, but niobium (Nb) may be used instead of this tantalum. That is, niobium can be plasma-etched by CF ₄ like tantalum, and therefore niobium formed on aluminum can be over-etched by a plasma etching method, which results in steps similar to those in FIG. A cross section can be obtained. Further, niobium can be anodized, and the niobium oxide film obtained by anodization also has a high relative dielectric constant like the tantalum oxide film. Therefore, the current driving capability of the thin film transistor can be improved as in the case of the tantalum oxide film. Similar to the case of the tantalum oxide film, the reliability of the insulating property can be improved.

[0022]

As described above, according to the present invention as defined in claim 1 or claim 2, since aluminum having a low specific resistance is used for the gate electrode, the liquid crystal can be produced without causing problems such as propagation delay. A screen of a display device can be made large and high definition, and a tantalum oxide film or a niobium oxide film having a high relative dielectric constant obtained by oxidizing tantalum or niobium by an anodic oxidation method is used as a part of an insulating film. As a result, it is possible to provide a thin film transistor having a high current driving capability and a high reliability of insulation characteristics.

Further, according to the present invention as set forth in claim 3 or 4, by overetching a tantalum or niobium layer formed on aluminum, the cross section of the aluminum and tantalum or niobium layer is stepped. Since it can be formed into a shape, it is possible to effectively prevent step breakage of the layer formed thereon, and by using the anodization method, a thin film transistor having a high current driving capability and a highly reliable insulating characteristic can be manufactured. A method can be provided.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a state in which aluminum, tantalum, and a photoresist are formed on a substrate.

FIG. 2 is a cross-sectional view showing a state in which aluminum and tantalum have a stepwise cross section by overetching tantalum.

FIG. 3 is a cross-sectional view showing a state in which an anodized film of tantalum and aluminum is formed by performing an anodizing method.

4 is a cross-sectional view of a thin film transistor configured by forming each layer on the layer of FIG.

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

[Explanation of symbols]

 10 substrate 12 aluminum layer 14 tantalum layer 16 photoresist 18 aluminum oxide film 20 tantalum oxide film 22 insulating film 24 semiconductor layer 26 ohmic contact layer 28 metal electrode 30 protective film 32 gate electrode 34 insulating film 36 semiconductor layer 38 ohmic contact layer 40 source Electrode 42 Drain electrode 44 Substrate

Claims (4)

[Claims] 1. A thin film transistor having a gate electrode, an insulating film, and a semiconductor layer, wherein the gate electrode is made of aluminum and tantalum, and a tantalum oxide film is formed on the gate electrode as a part of the insulating film. A thin film transistor characterized by the above. 2. A thin film transistor having a gate electrode, an insulating film, and a semiconductor layer, wherein the gate electrode is made of aluminum and niobium, and a niobium oxide film is formed on the gate electrode as a part of the insulating film. A thin film transistor characterized by the above. 3. An aluminum layer is formed, a tantalum layer is formed on the aluminum layer, the tantalum layer is over-etched by a plasma etching method, and the surfaces of the aluminum layer and the tantalum layer are oxidized by an anodic oxidation method. A method for manufacturing a thin film transistor, characterized in that a gate electrode and an insulating film are formed by forming an insulating film on the anodized oxide film. 4. An aluminum layer is formed, a niobium layer is formed on the aluminum layer, the niobium layer is over-etched by a plasma etching method, and the surfaces of the aluminum layer and the niobium layer are oxidized by an anodic oxidation method. A method for manufacturing a thin film transistor, characterized in that a gate electrode and an insulating film are formed by forming an insulating film on the anodized oxide film.