JPH0691255B2 - Thin film transistor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insulated gate thin film transistor (hereinafter referred to as "TFT"), and particularly to a TFT having a semiconductor layer of amorphous silicon having excellent characteristics.

[0002]

2. Description of the Related Art The structure of a conventional general TFT and its forming method will be described with reference to FIG. A TFT is manufactured by sequentially depositing a gate electrode 2, a gate insulating film 3, and a semiconductor layer 4 on an insulating substrate 1 and forming a source electrode 5 and a drain electrode 6 on the semiconductor layer 4. A glass plate, a ceramic plate, a quartz plate or the like is generally used as the insulating substrate 1. The gate electrode 2 is made of Cr, Al, Ni, Au.
And the like, and the gate insulating film 3 is made of SiO, SiO ₂ , A
l ₂ O ₃ , Ta ₂ O ₅ , Y ₂ O ₃ , Si ₃ N ₄ , M ₉ F _2, etc. oxides, nitrides or fluorides, the semiconductor layer 4 is CdS, CdS
e, TePbS, amorphous silicon, or the like. As the source electrode 5 and the drain electrode 6, Al, A
A metal such as u, Ni, Cr, In or the like that can make ohmic contact with the semiconductor layer 4 is used.

When the TFT having the above structure is used as a switching element for, for example, a multiplex drive of a liquid crystal display device, the off resistance (R _OFF ) of the TFT is sufficiently high and the blocking property is good, and the on resistance (R _ON ) Is sufficiently low, the on / off ratio (R _OFF / R _ON ) is high, and the switching speed is high, and it is required to be stable for long-term operation.

In order to realize a TFT satisfying such characteristics, the gate insulating film 3 of the TFT has (1) good insulation (no pinhole), high reliability and high breakdown voltage. ) It is necessary that the mobile ion density is low, (3) the interface state density with the semiconductor is small, (4) the electric field effect on the semiconductor is large, and the like. ) And (4) are contradictory requirements, and it is difficult to satisfy these requirements at the same time. For example,
When forming a thin film of SiO ₂ , Si ₃ N ₄ or the like by a sputtering method, a CVD method or the like, it is extremely difficult to form a pinhole-free thin film with a thickness of 2000 to 3000 Å or less. However, according to the anodic oxidation method using an appropriate material, it is possible to obtain a dense insulating film having a thickness of several hundred liters and no pinhole, and a high breakdown voltage. The electric field effect on the semiconductor surface is proportional to the dielectric constant of the gate insulating film and inversely proportional to the thickness if the voltage applied to the gate is constant, so using an anodized film with a high dielectric constant keeps good insulation. The thickness of the gate insulating film can be reduced,
An extremely large electric field effect is expected.

On the other hand, when amorphous silicon is used alone as the semiconductor layer 4, there is no adverse effect due to deviation from the stoichiometric composition, which is a problem in compound semiconductors such as CdSe, and the energy gap is large and the intrinsic carrier is large. It is possible to obtain excellent advantages as a semiconductor layer for a TFT, such as a small number of TFTs. Therefore, it is considered that by using amorphous silicon as the semiconductor layer 4 and combining the anodic oxide film as the gate insulating film 3, a TFT having excellent characteristics can be manufactured.

[0006]

However, an amorphous silicon film is generally formed by using glow discharge. However, a thin film such as glow discharge is directly formed on a gate insulating film made of an anodic oxide film. When the amorphous silicon layer is deposited through a shocking process, the anodic oxide film is damaged and deteriorated, resulting in a problem that the insulating property is significantly deteriorated. Therefore, the function as the gate insulating film 3 of the TFT cannot be fulfilled.

When the anodic oxide film is used as the gate insulating film 3 of the TFT, the step of forming the semiconductor layer 4 must be after the step of forming the gate insulating film 3, and the silicon is in a good amorphous state. It is said that a film forming technique such as glow discharge is most desirable for the deposition. Therefore, it is a very important requirement for manufacturing a good TFT to avoid the deterioration of the insulating property of the gate insulating film.

[0008]

In view of the above problems, the present invention is a thin film transistor including a gate electrode having an anodized film formed on the surface thereof, and an amorphous silicon semiconductor layer laminated on the gate electrode. In, the anodic oxide film was made of Ta ₂ O ₅ and the gate electrode was made of Ta, and an insulating layer was inserted between the Ta ₂ O ₅ anodic oxide film and the amorphous silicon semiconductor layer to form a double gate insulating film. It is characterized by that.

[0009]

With the above structure, even during glow discharge, the insulation film of the anodic oxide film is prevented from being deteriorated by the inserted insulation film, and the amorphous silicon layer can be formed as a semiconductor layer without deterioration of insulation. Therefore, the obtained TFT has a gate insulating film having a good field effect characteristic and a switching characteristic with high reliability.
Becomes

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view of the structure of a TFT showing an embodiment of the present invention.

After depositing a Ta film on the glass substrate 10,
This is immersed in an ammonium tartrate aqueous solution and subjected to chemical conversion treatment. A Ta ₂ O ₅ film of about 1000 Å was formed by a constant voltage formation of 65 V, and as a result, the gate electrode 20 made of Ta and Ta were formed.
A first insulating film 30 made of a thin anodic oxide film on the surface is formed.

On the first insulating film 30, a Si ₃ N ₄ film having a thickness of 1000 Å is laminated as a second insulating film 31 by the CVD method or the sputtering method. Unlike the glow discharge, the CVD method and the sputtering method do not generate plasma or the like at the time of film formation, and are not manufacturing methods that give impact to a thin film, so that the anodic oxide film is not damaged. Second insulating film 3
In addition to Si ₃ N _4, 1 is SiO, SiO ₂ , Y ₂ O ₃ , Al ₂ O
₃ , MgF ₂ etc. were subjected to the operation and anodized Ta ₂ O ₅
It has a function of preventing deterioration of characteristics of the film, that is, the first insulating film. The first insulating film (Ta ₂ O ₅ ) 30 and the second insulating film (Si ₃ N ₄ ) 31 form a double gate insulating film.

An amorphous silicon layer as a semiconductor layer 40 is deposited thereon by 3000 Å by glow discharge, and then 3000 Å as a source electrode 50 and a drain electrode 60.
By vapor-depositing Al, the TFT of this embodiment is manufactured.

This TFT has a surface protection film 70 formed by CVD.
3000 Å of Si ₃ N ₄ is laminated by the method
The exposed part of is coated. This protective film 70 not only protects the amorphous silicon layer, but also depletes the surface of the semiconductor layer 40 on the back side, reduces the leak current in the off state, and greatly improves the characteristics of the TFT.

In the above embodiment, if the relative permittivity of Si ₃ N ₄ is 6.4 and the relative permittivity of Ta ₂ O ₅ is 26.0, the gate insulating film is formed of only Si ₃ N _4. In order to obtain the same electric field effect as this embodiment, it is necessary to layer Si ₃ N ₄ to a thickness of about 1250 Å, but this is necessary to obtain sufficient insulation characteristics for pinhole generation. Is almost impossible. However, as in the above embodiment, the gate insulating film is formed of Ta ₂ O ₅ film and S
When the double-layered composite film of i ₃ N ₄ film is used, the Ta ₂ O ₅ film does not generate pinholes or the like, and high insulation characteristics can be obtained. Other metals that can be anodized include Al and the like, but Ta is significantly superior from the viewpoint of eliminating pinholes. Also by depositing the Si ₃ N ₄ film on the Ta ₂ O ₅ film, will be the Si ₃ N ₄ film in glow discharge amorphous silicon layer protects the Ta ₂ O ₅ film Ta
_{The 2} O ₅ film is not damaged, and therefore, the Ta ₂ O ₅ film having a good insulating property can be maintained even after the semiconductor layer 4 is formed.
Although the gate electrode 20 is made of Ta in the above embodiment, the work function of Ta is larger in the n-channel operation TFT than in the case of Al or the like, so that the pinch-off voltage becomes positive, and the normal-off TFT. Is obtained, the resistance (OFF resistance) at a gate voltage of 0 V is increased, and the characteristics suitable for a liquid crystal matrix driving TFT are obtained.
In addition, the protective film 70 prevents the TFT from directly contacting the atmosphere, reduces the bending of the band on the surface (rear surface) of the amorphous silicon layer opposite to the gate, improves the stabilization of the characteristics, and at the same time, turns off. It has the function of keeping the resistance high. Further, when it is applied to one cell substrate for driving a liquid crystal display element, it prevents direct contact between the liquid crystal layer and the TFT and contributes to improvement of life characteristics of the TFT. In addition, the protective film 70 is important also when a metal layer is formed on the active region of the TFT for shielding light, and when the metal layer is provided on the protective film 70 to cover the active region of the TFT. There is no problem that the off resistance decreases due to leakage.

FIG. 3 shows the drain current-gate voltage characteristic (V _SD = + 10 V) in the above-mentioned TFT. The measured TFT has a source electrode 50 and a drain electrode 6
The channel length L corresponding to the interval between 0s is 40 μm, and the channel width W is 2000 μm. The source-drain voltage V _SD is 10V. Gate voltage is 0V
It can be seen that an on / off ratio (drain current ratio) of four digits is obtained in the range of + 5V and five digits or more in the range of 0V to + 10V.

[0017]

As described above in detail, according to the present invention, the gate electrode is made of Ta having a small electric resistance, and the semiconductor layer is made of amorphous silicon having a large energy gap, and the gate insulation is located between the gate electrode and the semiconductor layer. By forming the film with a double insulating film consisting of an anodic oxide film made of Ta ₂ O ₅ having high insulating properties and an insulating film laminated on this anodic oxide film, the electric field effect of the gate insulating film is high and the operating characteristics are good. The above TFT is configured, and its technical significance is great.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of a TFT showing an embodiment of the present invention.

FIG. 2 is a sectional view showing a basic structure of a conventional TFT.

FIG. 3 is an explanatory diagram showing drain current-gate voltage characteristics of the TFT shown in FIG.

[Explanation of symbols]

 10 glass substrate 20 gate electrode 30 first insulating film 31 second insulating film 40 semiconductor layer 50 source electrode 60 drain electrode 70 protective film

Claims (1)

[Claims] 1. A thin film transistor comprising a gate electrode having an anodized film formed on a surface thereof and an amorphous silicon semiconductor layer laminated on the gate electrode, wherein the anodized film is Ta ₂ O ₅ , The gate electrode is Ta
A thin film transistor, comprising an insulating layer inserted between the anodic oxide film and the amorphous silicon semiconductor layer to form a double gate insulating film with the insulating layer and the anodic oxide film.