JPH0797641B2 - Thin film transistor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor, and more particularly to a thin film transistor in which instability of thin film transistor characteristics due to an interface between a gate insulating film and a semiconductor film is improved.

2. Description of the Related Art A thin film transistor is a so-called field effect transistor in which the electric conductivity of a semiconductor between a source electrode and a drain electrode is controlled by a voltage applied to a third electrode (gate electrode) provided through an insulating film in contact with the semiconductor. Known as. Conventional thin film transistors have a purpose of forming a switching array over a large area, or a driving circuit or a switching array of an image sensor, a liquid crystal display, an EL display device, etc., because they are inexpensive and can be manufactured at low cost. Research continues. The most important point of such a thin film transistor is that it operates stably over a long period of time without fluctuations in device characteristics.

It is considered that the cause of the change in characteristics of the thin film transistor over time is a charge trap capable of trapping electrons in the semiconductor film, the interface between the semiconductor film and the gate insulating film, or the gate insulating film. Of these, the number of charge traps existing in the gate insulating film is larger than that of other charge traps, and since the conductivity in the insulating film is low, a long relaxation time is usually required. It is considered to be the main cause of long-term aging. If there are many charge traps in the insulating film or the leakage current of the insulating film is large, the electrons moving in the channel formed at the interface between the semiconductor film and the insulating film are drawn into the insulating film and are trapped in the charge trap. It is captured and the effective gate voltage changes, and the drain current fluctuates. From the above points, in order to realize an element having stable transistor characteristics, it is desirable to use an insulating film having a small number of charge traps and a small leak current as the gate insulating film.

Conventionally, as the gate insulating film of the thin film transistor,
Al ₂ O formed by electron beam evaporation or sputtering
Thin films of ₃ , Ta ₂ O ₅ , SiO ₂ , Si ₃ N _4, etc. have been used.

Problems to be Solved by the Invention Al ₂ O ₃ and Ta ₂ O ₅ or composite insulating films thereof have a high relative dielectric constant, but generally have poor adhesion to other materials and are physically damaged during the manufacturing process. It had the drawback of being easily peeled off and having a poor yield. Further, for these reasons, there is a problem that many charge traps are generated at the interface and the change over time is large. Further, an insulating film containing silicon as a main component such as SiO ₂ or Si ₃ N ₄ has a defect that the relative dielectric constant is generally small and the mutual conductance of the thin film transistor cannot be increased. In addition, in an insulating film obtained by an electron beam evaporation method, a vapor deposition substance is heated to a high temperature in a high vacuum, so thermal dissociation occurs during vapor deposition, and a composition ratio is often deviated from a stoichiometric composition. In the case of oxides, oxygen defects are formed, which not only serve as charge traps but also cause an increase in leak current. For the above reasons, the conventional thin film transistor has a large change with time, a small mutual conductance, and a poor reproducibility.

Therefore, an object of the present invention is to solve the above problems and to provide a thin film transistor which has stable characteristics for a long period of time and can be manufactured with good reproducibility.

Means for Solving the Problems In a thin film transistor, the gate insulating film comprises a first gate insulating film made of a composite insulator sputtered thin film containing at least aluminum and tantalum as main components,
Between the gate insulating film and the semiconductor film, and a second gate insulating film composed of a composite insulator sputtered thin film containing at least aluminum and silicon as main components.

Effect According to the present invention, the insulating thin film containing aluminum and silicon as the main components formed by the sputtering method is used as the gate insulating film in contact with the semiconductor film, and these films have a stoichiometric composition. Since the deviation is small and therefore the charge trap due to defects is small, and the composite insulator sputtered thin film containing aluminum and tantalum as the main components, which is the first gate insulating film, has a part of the same composition, it has excellent adhesion. Since defects are less likely to be generated in the vicinity of the interface, electrons flowing in the semiconductor channel are less likely to be trapped. Further, the composite insulator sputtered thin film containing aluminum and tantalum as the main components, which is the second gate insulating film, has excellent adhesion to the semiconductor film. Further, since the growth of defects such as pinholes is cut off at the interface in the middle because of the multi-layer structure, the leak current becomes very small. Further, similarly to the composite insulator sputtered thin film containing aluminum and tantalum as the main components, the composite insulator sputtered thin film containing aluminum and silicon as the main components has a relative dielectric constant lower than that of the insulating film containing only silicon as the main component. Since it is large, the capacitance becomes large as a whole, and the thin film transistor has a large mutual conductance.

With the operation described above, in the thin film transistor of the present invention, 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 has a small leak current. Will be small.

Embodiments Embodiments of the present invention will be described below 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 having a film thickness of about 100 nm on an insulating substrate 1 such as glass
a first gate electrode 2 made of l and further having a film thickness of about 300 nm on the insulating substrate 1 including the gate electrode 2 and made of a composite insulating film of Al and Ta formed by a high frequency magnetron sputtering method. Gate insulating film 3 and further on it
A second gate insulating film 4 having a film thickness of about 30 nm, which is also formed by a high frequency magnetron sputtering method and is made of a composite insulating film of Al and Si, and having a film thickness of about 50 nm on the second gate insulating film 4. A semiconductor film 5 made of CdSe formed by a resistance heating method, and a source electrode 6 and a drain electrode 7 made of Al and having a film thickness of about 100 nm with a predetermined interval of several to several tens of microns on the semiconductor film 5. Has been done.

FIG. 2 shows the relative permittivity when the molecular ratio of Al ₂ O ₃ and SiO ₂ in the second gate insulating film is changed. In this example, the condition that the relative permittivity is 5 was selected. It is.

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. 1 was changed, and as shown in Table 1, the thin film transistor (A),
(B), (C), and (D) were prototyped. Further, in the thin film transistor (D), the second gate insulating film was SiO ₂ . Here, (A) is the thin film transistor of the present invention shown above. The channel length of each sample was 10 μm, and the channel width was 50 μm.

FIG. 3 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 relative dielectric constant of the composite insulating film of Al and Si, which is the second gate insulating film, and therefore is almost the same as the thin film transistor (B). It is shown that the electric characteristics of (3) are obtained and the characteristics of the composite insulator sputtered thin film of Al and Ta having a large relative dielectric constant are not impaired. On the other hand, (C) and (D)
Compared with the composite insulating film of Al and Ta, the thin film transistor of
To obtain a large drain current, the relative dielectric constant of the composite insulating film of Al and Si is smaller, or the relative permittivity of SiO ₂ is smaller than that of the composite insulating film of Al and Si. ,
It can be seen that a large gate voltage is required.

FIG. 4 shows a leak current when the source electrode 6 and the drain electrode 7 are used as common electrodes in FIG. 1 and a voltage is applied between them and the gate electrode 2. It can be seen that the thin film transistor (A) of the present invention has a sufficiently smaller leak current than the thin film transistor (B) or (C).
It is considered that this is because the multilayer structure has a high probability that defects such as binholes generated during sputtering are cut off at the interface between the first gate insulating film and the second gate insulating film. When the leak current is small, the number of electrons injected into the charge traps in the insulating film also decreases, so that the change over time in thin film transistor characteristics can be reduced. Also,
Since the leak current is smaller than that of the thin film transistor of (D), it can be seen that a more honeydew film is formed by using the composite insulating film of Al and Si than the single film of SiO ₂ .

FIG. 5 shows changes over time in the drain current for the thin film transistors of (A), (B), (C), and (D). In the thin film transistor (A) of the present invention, the change with time is very small. This means that in the vicinity of the interface between the semiconductor film and the gate insulating film, the adhesion is very good, so that the number of charge traps, which is the main cause of the change over time, is small and the number of charge traps is very small. Further, it is considered that the number of electrons injected into the charge trap itself is small because the leak current is small as described above.

As the gate insulating film shown in FIG. 1, an insulating film manufactured by a resistance heating method or an electron beam method cannot obtain the above characteristics, and it can be understood that the gate trapping film is reduced by the sputtering method.

If the second gate insulating film contains nitrogen in addition to Al and Si, the nitrogen has a function of capturing mobile impurity ions,
Instability of characteristics due to movement of impurity ions can be improved.

The thickness of the second gate insulating film has a small relative dielectric constant,
In order to obtain a large transconductance, it is desirable that it is 50 nm or less. Also, stable film characteristics could not be obtained at 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 great also in the case of CdS, CdTe, or a solid solution thereof.

EFFECTS OF THE INVENTION As is clear from the above description, 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 leak current is small, the injection of electrons into the charge trap itself is Since it hardly occurs, the electrical characteristics and stability of the thin film transistor can be greatly improved. Further, since the relative dielectric constant of the gate insulating film is increased as a whole, the mutual conductance is large and it can be widely used for driving various display devices.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of the thin film transistor of the present invention, FIG. 2 is a view showing the relationship between the composition and relative permittivity of a composite insulating film of Al and Si, and FIG. 3 is an electrical characteristic of the thin film transistor. FIG. 4 is a diagram showing a leak current of a gate insulating film, and FIG. 5 is a diagram showing a change with time of thin film transistor characteristics. 1 ... Insulating substrate, 2 ... Gate electrode, 3 ... First gate insulating film, 4 ... Second gate insulating film, 5 ... Semiconductor film, 6 ... Source electrode, 7 ... Drain electrode .

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoichi Harada 1006 Kadoma, Kadoma City, Osaka Prefecture, Matsushita Electric Industrial Co., Ltd. (72) Kuni Ogawa, 1006 Kadoma, Kadoma City, Osaka Prefecture 72) Inventor Noboru Yuue 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Kenji Kumabe 2-3-8 Shimomeguro, Meguro-ku, Tokyo Matsushita Electric Transmission Co., Ltd. (56) JP-A-59-182572 (JP, A)

Claims (4)

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