JPS6390857A - Thin film transistor - Google Patents

Abstract

PURPOSE:To obtain a thin-film transistor whose characteristics are stable for a long period of time and which can be manufactured with good reproducibility, by producing a gate insulation film as a multilayered film consisting of an insulating film provided by a sputtered composite oxide thin film principally composed of aluminium and tantalum and of another insulating film provided by a sputtered thin film of insulator principally composed of silicon. CONSTITUTION:A gate insulation film is a multilayered film consisting of a first gate insulation film 3 provided by a sputtered composite oxide thin film principally composed of aluminium and tantalum, and a second gate insulating film 4 disposed between said first gate insulating film 3 and a semiconductor layer 5 and principally composed of silicon. For example, on an insulating substrate 1 of glass or the like, there are provided a gate electrode 2 of aluminium, a first gate insulating film 3 of Al and Ta composite oxide film formed by the high-frequency magnetron sputtering process, a second gate insulating film 4 of SiO2 also formed by the high-frequency magnetron sputtering process, a semiconductor layer 5 of CdSe, and source and drain electrodes 6 and 7 of aluminium.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a thin film transistor, and more particularly to a thin film transistor in which instability in thin film transistor characteristics caused by an interface between a gate insulating film and a semiconductor layer is improved.

Conventional technology A thin film transistor is a so-called field effect transistor in which the electrical conductivity of a semiconductor between source and drain electrodes is controlled by a voltage applied to a third electrode (gate electrode) provided through an insulating layer in contact with the semiconductor. known as. Conventional thin film transistors have been researched for the purpose of switching arrays for image sensors, liquid crystals, EL display devices, etc. because they are easy to form switching arrays over a large area, and because the materials are cheap, they can be made at low cost. It is being The most important point in such thin film transistors is that they operate stably over a long period of time without fluctuations in device characteristics.

The cause of changes in thin film transistor characteristics over time is thought to be due to charge traps that can trap electrons in the semiconductor thin film, at the interface between the semiconductor thin film and the insulator layer, or in the insulator layer. Of these, the number of charge traps that exist in the insulator layer is larger than other charge traps, and because the conductivity in the insulator layer is low, it usually requires a long relaxation time, which affects the characteristics of thin film transistors. It is believed that this is the main cause of long-term changes over time. If there are many charge traps inside the insulator layer or if the leakage current in the insulator layer is large, electrons moving in channels formed at the interface between the semiconductor layer and the insulator layer will be drawn into the insulator layer. , they are captured by a charge trap, and the effective gate voltage changes, the drain voltage changes, and the drain current fluctuates. From the above points, in order to realize an element with stable transistor characteristics 2,
It is desirable to use a thin film with fewer charge traps and less leakage current as the insulator layer.

Conventionally, the insulator layer of the above-mentioned thin film transistor is made of AIOTa formed by electron beam evaporation method or sputtering method.
Thin 23' 251 films such as OSiO2 and Si3N4 were used.

Problems to be Solved by the Invention Although Al2O3, T a 205, or their composite insulating films have a high dielectric constant, they generally have poor adhesion with other materials and may physically peel off during the manufacturing process. The drawback was that it was easy to use, and the yield was poor. Furthermore, for these reasons, many charge traps are generated at the interface, resulting in a large change over time.

In addition, insulating films whose main component is silicon, such as S z O2 and Si3N4, generally have a small dielectric constant and the mutual conductance of thin film transistors. The drawback was that it could not be made larger.

In addition, in insulating films obtained by electron beam evaporation, the evaporation material is heated to high temperatures in a high vacuum, so thermal dissociation occurs during evaporation and the composition ratio often deviates from the stoichiometric composition. In the case of oxides, oxygen defects are created,
This not only becomes a charge trap but also causes an increase in leakage current. For the above-mentioned reasons, conventional thin film transistors exhibit large changes over time, low mutual conductance, and poor reproducibility.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide a thin film transistor that has stable characteristics over a long period of time and can be manufactured with good reproducibility.

Means for Solving the Problems In order to achieve the above object, the present invention provides a thin film transistor in which the gate insulating film includes a first gate insulating film made of a composite oxide sputtered thin film containing at least aluminum and tantalum as main components; The first gate insulating film and the second gate insulating film are made of an insulating sputtered thin film mainly composed of silicon, and the semiconductor layer is a multilayer film.

According to the present invention, an insulating thin film mainly composed of silicon formed by sputtering is used as the gate insulating film in contact with the semiconductor layer, and these films are free from deviations from the stoichiometric composition. Therefore, there are fewer charge traps due to defects, and it has excellent adhesion to the thin films that are in contact with both sides of the semiconductor thin film and the first gate insulating film, a composite oxide sputtered thin film whose main components are aluminum and tantalum. Since defects are less likely to be generated near the interface,
Electrons flowing through the semiconductor channel are less likely to be trapped. Furthermore, due to the multilayer structure, the growth of defects such as pinholes is cut off at intermediate interfaces, so leakage current becomes extremely small. Furthermore, since the composite oxide sputtered thin film mainly composed of aluminum and tantalum has a high dielectric constant, the capacitance as a whole becomes large, resulting in a thin film transistor with high mutual conductance.

Due to the above-described effects, the thin film transistor of the present invention has
Since the structure of the gate insulating film has few charge traps at the interface with the semiconductor layer, has a high relative dielectric constant as a whole, and reduces leakage current, it has a small change over time.

Embodiments Hereinafter, embodiments of the present invention will be described based on the accompanying drawings.

FIG. 1 is a sectional view showing an embodiment of a thin film transistor of the present invention.

A gate electrode 2 made of M having a film thickness of about 100 nm is formed on an insulating substrate 1 such as glass, and a film thickness of about 300 nm is formed on the insulating substrate 1 including the gate electrode 2, using high-frequency magnetron sputtering. A formed by the law
A first gate insulating film 3 made of a composite oxide film of l and Ta
, and 5102 having a film thickness of about 30 nm thereon, which was also formed by high frequency magnetron sputtering.
A second gate insulating film 4 consisting of
A semiconductor layer 5 made of CdSe having a film thickness of about
It is composed of a source electrode 6 and a drain electrode 7 made of M and having a film thickness of about 0.00 nm.

In order to investigate the effect of the thin film transistor of the present invention, the thickness of the first gate insulating film 3 and the second gate insulating film 4 shown in FIG.
, (ff), (2) was prototyped. Here, CI) is the thin film transistor of the present invention shown above.

The channel length of each sample was 1 μm, and the channel width was 50 μm.

Table 1, Figure 2 shows the drain current when the gate voltage is changed. As is clear from the figure, in the thin film transistor (I) of the present invention, since the second gate insulating film S102 is sufficiently thin, electrical characteristics almost equivalent to those of the thin film transistor (ff) can be obtained, and the dielectric constant is large. (εr~15)
This shows that the features of the composite oxide sputtered thin film of M and Ta are not impaired. On the other hand, it can be seen that in the thin film transistor of @), a large gate voltage is required in order to obtain a large drain current because the dielectric constant of 5102 is low (εr~3.5).

FIG. 3 shows the leakage current when the source electrode 6 and the drain electrode in FIG. 1 are used as common electrodes and the full voltage is applied between them and the gate electrode 2.

In the thin film transistor (I) of the present invention, (u)t
It can be seen that the leakage current is sufficiently smaller than that of the thin film transistor II). This is considered to be because the multilayer structure increases the probability that defects such as pinholes generated during sputtering will be cut off at the interface between the first gate insulating film and the second gate insulating film. When the leakage current is small, the number of electrons injected into charge traps in the insulating film is also reduced, so that changes over time in thin film transistor characteristics can be reduced.

FIG. 4 shows the change in drain current over time for the thin film transistors (1), (If), and (7).

The thin film transistor (I) of the present invention shows very little change over time. This means that near the interface between the semiconductor layer and the gate insulating film, the adhesion is very good, so there are few defects and the number of charge traps, which is the main cause of aging, is very small. Further, as mentioned above, it is thought that this is because the number of electrons injected into the charge trap is reduced because the leakage current is small.

It can be seen that the above-mentioned characteristics cannot be obtained with S102, which is produced as the second gate insulating film in FIG. 1 by a resistance heating method or an electron beam method, and charge traps are reduced by forming a sputtered thin film.

The thickness of the second gate insulating film has a small dielectric constant, so in order to obtain a large mutual conductance 2, the thickness of the second gate insulating film is 50 nm.
The following is desirable.

Further, although the effect when SiO2 is used as the second gate insulating film is as described above, the same characteristics were obtained when Si3N4 is used as the second gate insulating film.

In the example, the case where CdSe was used as the semiconductor layer was described, but it was found that the effect of the present invention is also effective in the case of solid solutions of CdS and CdTe (L/'1).

Effects of the Invention As is clear from the above explanation, in the thin film transistor of the present invention, there are few charge traps at the interface between the gate insulating film and the semiconductor layer, and since the leakage current is small, injection of electrons into the charge traps itself occurs. This makes it possible to greatly improve the electrical characteristics and stability of thin film transistors, making them widely applicable to driving various display devices.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of the thin film transistor of the present invention, FIG. 2 is a diagram showing the electrical characteristics of the thin film transistor,
FIG. 3 is a diagram showing leakage current of the gate insulating film, and FIG. 4 is a diagram showing changes in thin film transistor characteristics over time. DESCRIPTION OF SYMBOLS 1... Insulating substrate, 2... Gate electrode, 3... First gate insulating film (sputtered composite oxide thin film of AI and Ta), 4... ...Second gate insulating film (Si insulator sputtered thin film), 5...
- Semiconductor layer, 6... Source electrode, 7...
・Drain electrode. Name of agent: Patent attorney Toshio Nakao and one other person, i
:z −−''Xze tilt Figure 3 (1) −,! 9 Jin/August 642 Blade mouth 4θ:A(v) Fig. 4

Claims (5)

[Claims] (1) A substrate comprising at least a drain electrode, a gate electrode, a source electrode, a semiconductor layer, and a gate insulating film on an insulating substrate, the gate insulating film being a composite oxide sputtered thin film containing at least aluminum and tantalum as main components. a second gate insulating film formed of an insulating sputtered thin film containing at least silicon as a main component, the second gate insulating film being between the first gate insulating film and the first gate insulating film and the first gate insulating film and the semiconductor layer; A thin film transistor characterized by being made of a multilayer film. (2) The thin film transistor according to claim 1, wherein the thickness of the second gate insulating film is 50 nm or less. (3) The thin film transistor according to claim 1, wherein the second gate insulating film is made of silicon oxide. (4) The thin film transistor according to claim 1, wherein the second gate insulating film is made of silicon nitride. (5) Claim 1, characterized in that the semiconductor layer is CdS, CdSe, CdTe, or a solid solution thereof.
The thin film transistor described in Section 1.