JPH0575125A - Thin-film transistor - Google Patents

Abstract

PURPOSE:To allow carriers in an active layer to travel at a different energy level, thereby to improve field-effect mobility, by using a multilayered film as the active layer in which undoped microcrystalline silicon films are formed alternately with silicon nitride layers having a larger bandgap than the silicon film. CONSTITUTION:A silicon oxide film 3 is deposited to cover a gate electrode 2 formed on a transparent substrate 1, and then a silicon nitride film 4 is deposited. Undoped microcrystalline silicon film 5a for active layers, and silicon nitride layers 5b having a larger bandgap than the silicon film 5a are alternately deposited by photo CVD to form a multilayered film of about ten layers. A silicon nitride film 6a and a silicon oxide film 6b are deposited in sequence on the multilayered film to provide a protective film. After patterned, the structure is shaped to become wider downward so that a tapered active layer can be formed. Finally, an ohmic contact 8 and source and drain electrodes 9 are formed to complete a thin-film transistor.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to thin film transistors.

[0002]

2. Description of the Related Art A display device using electroluminescence, light emitting diode, plasma, fluorescent display, liquid crystal or the like can have a thin display portion, and is therefore a terminal for measuring instruments, office equipment, computers and the like. There is an increasing demand for use as a display device or a special display device. Among these, in recent years, an active matrix type liquid crystal display device using a thin film transistor (TFT) as a switching element has been receiving attention. In such a liquid crystal display device, generally, when the screen becomes a large screen with a diagonal of 10 inches or more, a TFT-LCD is produced using a glass substrate to suppress an increase in production cost. Therefore, as the active layer of the TFT, amorphous silicon (a-
Si) film is used.

However, the TFT-LCD using an a-Si film as an active layer has a problem that it is not suitable for a large screen and high definition. This is because the TFT-LCD requires a high-speed operation in order to have a large screen and high definition. That is, T using an a-Si film as an active layer
The FT has a drawback that its field effect mobility is smaller than that of a TFT using a single crystal film or a polycrystalline silicon film formed at high temperature as an active layer.

A method of forming a TFT at a low temperature using a microcrystalline silicon film as an active layer instead of the a-Si film has been proposed (J. Kanicki et a.
l. : Mat. Res. Soc. Symp. Proc.
Vol. 149, p. 173), since its field effect mobility is similar to that of a TFT using a-Si as an active layer, even if such a TFT is used, a TFT-LCD
It did not lead to a larger screen or higher definition.

[0005]

As described above, the conventional T using the a-Si film or the fine crystalline silicon film as the active layer is used.
The FT has a problem that the field effect mobility is small and it is difficult to increase the screen size and the definition of the TFT-LCD.
The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a TFT having a large field effect mobility.

[0006]

The essence of the present invention is to quantize the electron (hole) state of the active layer so that the wave nature of the electron, which normally does not appear, is strongly observed.

In other words, in order to achieve the above-mentioned object, the TFT of the present invention comprises thin films made of microcrystalline or polycrystalline silicon formed on a substrate and thin films having a band gap larger than that of the thin films. An active layer formed by stacking,
A source / drain electrode joined to the active layer and a gate electrode disposed above or below the active layer via a gate insulating film are provided.

Further, in the above-mentioned TFT manufacturing method, an active layer formed on a substrate, a source / drain electrode joined to the active layer, and a gate insulating film above or below the active layer. In a method of manufacturing a thin film transistor having a gate electrode arranged via a substrate, a thin film made of microcrystalline or polycrystalline silicon and a thin film having a bandgap larger than that of the thin film are alternately formed on a substrate by using a photo-CVD method. It is desirable to include a step of forming a multilayer film by depositing the multilayer film and a step of patterning the multilayer film to form the active layer.

[0009]

In the present invention, a multilayer film in which thin films made of microcrystalline or polycrystalline silicon and thin films having a band gap larger than that of the thin film are alternately laminated is used as the active layer. As a result, when the carrier in the active layer is an electron, the electron travels in a new energy level formed at an energy level higher than the conduction band of microcrystalline or polycrystalline silicon, so that the electron is trapped at the grain boundary. , The probability of scattering is reduced, and the field effect mobility is improved. Similarly, when the carrier of the active layer is a hole, the hole travels to a new energy level formed at an energy level lower than the valence band of microcrystalline or polycrystalline silicon, so that the field effect mobility is increased. Is improved.

[0010]

Embodiments will be described below with reference to the drawings. 1 and 2 are cross-sectional views of manufacturing steps of a TFT according to an embodiment of the present invention.

First, as shown in FIG. 1A, a gate electrode 2 made of a metal such as a Ta film or a MoTa alloy film having a thickness of about 200 nm is formed on a transparent insulating substrate 1 made of glass or the like. ..

Next, as shown in FIG. 1B, plasma CV
Using method D, the thickness of the first gate insulating film is about 300 n.
m of silicon oxide film 3 is deposited on the substrate 1 so as to cover the gate electrode 2, and then a silicon nitride film 4 having a thickness of about 50 nm to be a second gate insulating film is formed by the same method. Deposit on the silicon oxide film 3.

Next, as shown in FIG. 1C, an AND microcrystalline silicon film 5a having a thickness of about 2.5 nm and a silicon nitride film 5b having a thickness of about 5 nm to be an active layer are formed by using a photo-CVD method.
Are alternately laminated on the silicon nitride film 4 to form a multilayer film of about 10 layers. Then, photo CVD method or plasma C
Using the VD method, a silicon nitride film 6a having a thickness of about 50 nm to be a first protective film and a thickness of about 100 n to be a second protective film.
m silicon oxide film 6b is sequentially deposited on the multilayer film.

The formation of the AND-microcrystalline silicon film 5a and the silicon nitride film 5b, which will be the active layers, is performed by plasma C
Alternatively, the VD method may be used. However, the photo-CVD method is more preferable from the viewpoint of reducing the damage of the film.

Next, as shown in FIG. 2A, the silicon nitride film 6a and the silicon oxide film 6b are patterned to form protective films 6a and 6b, and thereafter, isotropic properties such as chemical dry etching (CDE) are performed. By using etching, a multilayer film of the AND-microcrystalline silicon film 5a and the silicon nitride film 5b is processed so that the width becomes larger toward the substrate 1 side,
A taper-shaped active layer 7 is formed.

Next, as shown in FIG. 2 (b), for example, a plasma CVD method is used to form n ⁺ with a thickness of about 50 nm to be an ohmic contact layer. The microcrystalline silicon film 8 is deposited on the entire surface. Then, using a sputtering method, a Mo film with a thickness of about 50 nm and an Al film with a thickness of about 500 nm are n ⁺ A laminated metal film 9 to be a source / drain electrode is formed by sequentially depositing on the microcrystalline silicon film 8. Finally, n ⁺ The microcrystalline silicon film 8 and the laminated metal film 9 are patterned to form an ohmic contact layer 8 and a source / drain electrode 9 to complete a TFT.

When the field effect mobility of the TFT obtained by the above method was examined, it was about 5 times larger than that of the conventional TFT using an a-Si film as an active layer. The reason why such a result is obtained is considered as follows.

In the conventional TFT, a large number of traps exist at the grain boundaries in the active layer, and the band structure is single crystal Si.
Since the film is more complicated than that of the film, carriers are easily trapped or scattered at the grain boundaries, and the field effect mobility is reduced.

On the other hand, in the TFT of this embodiment, since the laminated film of the thin AND-microcrystalline silicon film 5a and the silicon nitride film 5b is used for the active layer 7, the microcrystalline silicon film 5 is formed.
A new energy level is formed at an energy level higher than the conduction band of a. As a result, the electron travels to a new energy level, and the probability of being trapped or scattered at the grain boundary is reduced. That is, the electron behaves as a wave, and as a result, the electron passes through a barrier such as a grain boundary and is not affected by it, so that the field effect mobility increases.

Further, in this embodiment, the source of the active layer 7
Since the end face shape on the drain electrode side is tapered, even if the active layer 7 has the same thickness and channel width as those of the conventional one, the contact area between the active layer 7 and the ohmic contact layer 8, that is, Since the effective contact area between the active layer 7 and the source / drain electrode 9 becomes large, there is an advantage that the ohmic contact characteristics are improved. Next, a TFT according to another embodiment of the present invention will be described. TF of this embodiment
The difference of T from that of the previous embodiment is that a p-type amorphous SiC film was used instead of the silicon nitride film to form the multilayer film.

Even in the TFT having such a structure, the band gap of the p-type amorphous SiC film is larger than that of the microcrystalline silicon film, so that the same effect as that of the previous embodiment can be obtained.

Further, in this embodiment, since the entire active layer apparently becomes a p-type semiconductor layer, an pn junction is formed between the ohmic contact layer and the active layer, which has the advantage of reducing the off current. There is.

The present invention is not limited to the above embodiment. For example, although the microcrystalline silicon film is used for the active layer in the above embodiment, a polycrystal silicon film may be used for forming the active layer instead. In addition, in the above embodiment, the case of the inverted stagger type TFT has been described, but the present invention is a stagger type or coplanar type TFT.
Can also be applied to. Besides, various modifications can be made without departing from the scope of the present invention.

[0024]

As described above in detail, according to the present invention, a laminated film of a thin film made of microcrystalline or polycrystalline silicon and a thin film having a band gap larger than that of the thin film is used as an active layer. As a result, the electrons in the active layer travel to a new energy level formed at an energy level higher than the conduction band of microcrystalline or polycrystalline silicon. Therefore, the probability of being trapped or scattered at the grain boundaries is reduced, and a TFT having a high field effect mobility can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of a manufacturing process of a TFT according to an embodiment of the present invention.

FIG. 2 is a sectional view of a manufacturing process of a TFT according to an embodiment of the present invention.

[Explanation of symbols]

1 ... Transparent insulating substrate, 2 ... Gate electrode, 3 ... Silicon oxide film (first gate insulating film), 4 ... Silicon nitride film (second gate insulating film), 5a ... AND -P microcrystalline silicon film, 5b ... Silicon nitride film, 6a ... Silicon nitride film (first protective film), 6b ... Silicon oxide film (second protective film), 7 ... Active layer, 8 ... N ⁺ Microcrystalline silicon film (ohmic contact layer), 9 ... Laminated metal film (source / drain electrode).

Claims (1)

[Claims] 1. An active layer formed by alternately laminating thin films made of microcrystalline or polycrystalline silicon formed on a substrate and thin films having a bandgap larger than that of the thin film, and a soft layer bonded to the active layer. -A thin film transistor having a source / drain electrode and a gate electrode disposed above or below the active layer via a gate insulating film.