JPS63216378A - Thin film transistor - Google Patents

Abstract

PURPOSE:To have stable characteristics for a long period and manufacture a favorably reproducible gate insulating film, by causing its film to be composed of a multilayer film which is constructed by a composite insulating material spatter thin film comprising aluminum and tantalum at least as principal ingredients as well as the composite insulating material spatter thin film comprising tantalum and silicon at least as the principal ingredients. CONSTITUTION:Gate insulating films 3 and 4 in a thin film transistor which are constructed by drain, gate, and source electrodes 7, 2, and 6 respectively and a semiconductor film 5 as well as gate insulating films 3 and 4 at least that are mounted on an insulating substrate 1 are composed of a multilayer film which is constructed by the first gate insulating film 3 consisting of a composite insulating material spatter thin film comprising aluminum and tantalum at least as principal ingredients as well as the second gate film 4 consisting of the composite insulating material spatter thin film that is positioned between the above first gate insulating film 3 and the semiconductor film 5 and comprises tantalum and silicon at least as the principal ingredients. In this way, the gate insulating film has a little charge trap at an interface between its insulating film and the semiconductor film and has a high dielectric constant as a whole and then its leakage current is so limited that the thin film transistor having a little change with elapsed time is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a thin film transistor, and 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 film is improved.

Conventional technology Thin film transistors are so-called field effect type transistors in which the electric 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 film in contact with the semiconductor. Also known as a transistor. Conventionally, thin film transistors have been used for driving circuits and 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. Research continues. 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 film, at the interface between the semiconductor film and the gate insulating film, or in the gate insulating film. Of these, the number of charge traps that exist in the gate insulating film is larger than other charge traps, and because the conductivity in the insulating film is low, it usually requires a long relaxation time, which affects the characteristics of thin film transistors. It is thought to be the main cause of long-term changes over time. If there are many charge traps in the insulating film, or if the leakage current in the insulating film is large, electrons moving in channels formed at the interface between the semiconductor film and the insulating film will be drawn into the insulating film, causing charge traps. The effective gate voltage changes and the drain current fluctuates.From the above points, in order to realize a device with stable transistor characteristics, it is necessary to use an insulating film with fewer charge traps and less leakage current at the gate. It is desirable to use it as an insulating film.

Conventionally, the gate insulating film of the above thin film transistor is
A12 formed by electron beam evaporation method or sputtering method
Thin films such as 0G, Ta206.5i02.5iaNa, etc. were used.

Problems to be Solved by the Invention Al 203, Ta 205, or their composite insulating films have a high dielectric constant, but they generally have poor adhesion to other materials and are easily peeled off during the manufacturing process ( , had the disadvantage of poor yield. In addition, for these reasons, there was a problem that many charge traps occurred at the interface, and the change over time was large. In addition, 5i02 and 5iaN4
Insulating films whose main component is silicon, such as 4 m , generally have a small dielectric constant and have the disadvantage that the mutual conductance of a 4 m transistor cannot be increased. In addition, insulating films obtained by electron beam evaporation are heated in high vacuum to high temperatures, so thermal dissociation occurs during evaporation and the composition ratio often deviates from the stoichiometric composition ( , especially in the case of oxides, oxygen vacancies are formed, which not only act as charge traps but also cause an increase in leakage current.For the reasons mentioned above, conventional thin film transistors undergo large changes over time and The conductance was small and the reproducibility was poor.

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 is a first gate insulating film made of a composite insulating sputtered thin film containing at least aluminum and tantalum as main components. and a second sputtered thin film of a composite insulator which is located between the first gate insulating film and the semiconductor film and whose main components are at least tantalum and silicon.
It is characterized by being composed of a multilayer film including a gate insulating film and a gate insulating film.

According to the present invention, an insulating thin film mainly composed of tantalum and silicon formed by sputtering is used as the gate insulating film in contact with the semiconductor film, and these films have a stoichiometric composition. There is little misalignment (therefore, there are fewer charge traps due to defects), and because the composition is partially the same as the first gate insulating film, a composite insulator sputtered thin film whose main components are aluminum and tantalum, it is possible to achieve close contact. Because it has excellent properties and is difficult to generate defects near the interface, the electrons flowing in the semiconductor channel are not wrapped.
It becomes. Further, the second gate insulating film, a composite insulating sputtered thin film mainly composed of tantalum and silicon, has excellent adhesion to the semiconductor film. 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. In addition, similar to the composite insulator sputter thin film mainly composed of aluminum and tantalum, the composite insulator sputtered thin film mainly composed of tantalum and silicon also has a relative permittivity higher than that of an insulating film mainly composed only of silicon. Since it is large, the overall capacitance is large, resulting in a thin film transistor with large 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 film, 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 with reference to the accompanying drawings.

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

A gate electrode 2 made of AI having a thickness of about 1100 nm is provided on an insulating 11 substrate 1 made of glass or the like.

Further, on the insulating substrate 1 including the gate electrode 2, 30
A first gate insulating film 3 made of a composite insulating film of A1 and Ta having a film thickness of about 0 nm and formed by high frequency magnetron sputtering, and a 30 nm thick film formed on the first gate insulating film 3
A second gate insulating film 4 is provided which is made of a diagonal insulating film of Ta and Si, which is similarly formed by high-frequency magnetron sputtering and has a thickness of about 100 mL. 5 on top of this
A semiconductor film 5 made of CdSe having a film thickness of about 0 nm and formed by a resistance heating method, and further made of AI having a film thickness of about 1100 nm at predetermined intervals of several to several tens of microns thereon. Source electrode 6 and drain electrode 7
is provided.

The composite insulating film can be obtained, for example, by sputtering a target in which an AI plate having a large number of holes is stacked on a Ta plate.

Figure 2 shows Ta20s and 5i0 in the second gate insulating film.
The graph shows the dielectric constant when the molecular ratio of 2 is changed, and in this example, conditions were selected where the dielectric constant was 15.

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 in FIG.
, (B), '(C), and (D> were prototyped. In addition, the thin film transistor (D) has a second gate insulating film made of SiO2.
And so. Here, (A) is the thin film transistor of the present invention shown above.

The channel length of each tumble was 10 μm and the width of one channel was 50 μm.

FIG. 3 of Table 1 shows the drain current when the gate voltage is changed. As is clear from the figure, the thin film transistor (A) of the present invention has a second gate insulating film made of Ta and Si.
Since the dielectric constant of the composite insulating film with (B) is relatively large,
This shows that almost the same electrical characteristics as those of the thin film transistor were obtained, and the characteristics of the composite insulator sputtered thin film of AI and Ta, which have a large dielectric constant, are not impaired. On the other hand, in the thin film transistors (C) and (D), the dielectric constant of the composite insulating film of Ta and Si is smaller than that of the dielectric film of A1 and Ta, or the composite insulating film of Ta and Si It can be seen that since the dielectric constant of SiO2 is smaller than that of SiO2, a large gate voltage is required in order to obtain a large drain current.

FIG. 4 shows the leakage current when a voltage is applied between the source electrode 6 and the drain electrode 7 in FIG. 1 as common electrodes and the gate electrode 2. FIG. It can be seen that the leakage current in the thin film transistor (A) of the present invention is sufficiently smaller than that in the thin film transistor (B) or (C).

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. Also,(
Since the leakage current is smaller than that of the thin film transistor D), it can be seen that a composite insulating film of Ta and Si forms a more honeyed film than a single film of SiO2.

FIG. 5 shows changes in drain current over time for the thin film transistors (A), (B), (C), and (D). The thin film transistor (A) of the present invention shows very little change over time. This means that near the interface between the semiconductor film 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 described above, since the leakage current is small, the number of electrons injected into the charge trap is small.

It can be seen that the gate insulating film shown in FIG. 1 cannot have the above-mentioned characteristics with an insulating film produced by a resistance heating method or an electron beam method, and that charge traps are reduced by producing it by a sputtering method.

If the second gate insulating film contains nitrogen in addition to AI and Si, nitrogen has the effect of capturing movable impurity ions, and instability of characteristics due to movement of impurity ions can be improved.

The thickness of the second gate insulating film has a small dielectric constant, so
In order to obtain a large mutual conductance, the thickness is preferably 50 nm or less. Further, stable film characteristics could not be obtained at a thickness of 10 nm or less.

In this example, the case where CdSe was used as the semiconductor film was described, but it was found that the effect of the present invention is also great when using CdS, CdTe, or a solid solution thereof.

Effects of the Invention 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 film, and since the leakage current is small, injection of electrons into the charge traps itself is difficult to occur, which improves the electrical characteristics of the thin film transistor. Stability can be greatly improved. In addition, the relative dielectric constant of the gate insulating film is large as a whole, so the mutual conductance is large (it can be widely used for driving various display devices, etc.).

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing one embodiment of the thin film transistor of the present invention, Figure 2 is a graph showing the relationship between the composition and dielectric constant of a composite insulating film of Ta and Si, and Figure 3 is the electrical characteristics of the thin film transistor. FIG. 4 is a graph showing leakage current of the gate insulating film, and FIG. 5 is a graph showing changes in thin film transistor characteristics over time. l... Insulating substrate, 2... Gate electrode, 3... First gate insulating film (sputtered composite insulating film of At and Ta), 4... Second gate insulating film (Ta and Si composite insulator sputtered thin film), 5... semiconductor film, 6...
- Source electrode, 7... drain electrode. Name of agent: Patent attorney Toshio Nakao and one other person Fig. 1 Fig. 2 Molecular tail of TazO5 (γ,) Fig. 3 - Io o to
z. Figure 4

Claims (4)

[Claims] (1) at least a drain electrode provided on an insulating substrate;
a first gate insulating film composed of a gate electrode, a source electrode, a semiconductor film, and a gate insulating film, the gate insulating film being a composite insulating sputtered thin film containing at least aluminum and tantalum as main components; A thin film transistor comprising a multilayer film including a gate insulating film and a second gate insulating film between the semiconductor film and a second gate insulating film made of a composite insulating sputtered thin film containing at least tantalum and silicon as main components. (2) The thin film transistor according to claim 1, wherein the second gate insulating film is made of a composite insulating sputtered thin film containing at least tantalum, silicon, and nitrogen as main components. (3) The thickness of the second gate insulating film is 10 nm or more and 50 nm
2. The thin film transistor according to claim 1, wherein the thickness of the thin film transistor is less than or equal to m. (4) The thin film transistor according to claim 1, wherein the semiconductor film is CdS, CdSe, CdTe, or a solid solution thereof.