JPH05304294A - Thin film transistor - Google Patents

Abstract

PURPOSE:To prevent peeling of a first conductive layer at the time when a transparent conductive layer or the first conductive layer is heated, by providing a barrier layer between the transparent conductive layer and the first conductive layer consisting of two layers of a lower part of niobium and an upper part of alpha-tantalum. CONSTITUTION:A transparent conductive layer 2 is formed on a substrate 1, and a barrier layer 3 is formed on the transparent conductive layer 2. The barrier layer is composed of primarily oxide niobium which is slow in dispersion speed of oxygen and does not react with oxygen, or has almost the same volume even after reaction, if reaction happens. A first conductive layer 4 is formed on the barrier layer 3. The first conductive layer consists of a conductive layer lower part 4a composed of niobium, and a conductive layer upper part 4b composed of tantalum of alpha phase. As a result, it is possible to suppress the volume expansion, which is caused by oxidization, of the conductive layer lower part 4a, and to prevent peeling off of the conductive layer lower part 4a, even if the transparent conductive layer 2 or the first conductive layer 4 is heated, at the time when an active matrix-type liquid crystal display device is formed.

Description

Detailed Description of the Invention

[0001]

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 which can be preferably used in an active matrix type liquid crystal display device and the like.

[0002]

2. Description of the Related Art FIG. 2 shows a cross-sectional shape of a thin film transistor used in a conventional active matrix type liquid crystal display device or the like. Hatching is omitted to avoid making the drawing complicated. In FIG. 2, 21 is a substrate made of a glass substrate or the like, 22 is a first conductive layer to be a gate electrode, 23 is an insulating layer to be a gate insulating film, 24 is a first semiconductor layer to be a channel layer, 25
Is a second semiconductor layer serving as an ohmic contact layer, 26
Is a second conductive layer serving as a source / drain electrode, and a stopper layer 27 for dividing the source / drain electrode 26 and the second semiconductor layer 25 by etching is provided on the first semiconductor layer 24. The stopper layer 27 for this etching is formed of, for example, a silicon nitride film.

This thin film transistor has a gate electrode 22.
To the pixel electrode connected to the source electrode 26 via the first semiconductor layer 24 and the source electrode 26, while electrically connecting the source / drain electrode 26 to the pixel electrode ( An image signal or the like is supplied to (not shown) or the like.

The above-mentioned first conductive layer 22 is generally formed of tantalum (Ta) or the like because of its corrosion resistance and ease of forming an anodic oxide film. That is, in this type of transistor, even after the first conductive layer 22 is patterned, there are steps using various etching solutions until the transistor is completed. In such a process, it is common to use tantalum, which can easily form a tantalum oxide (TaO _x ) film having excellent corrosion resistance, in order to prevent the first conductive layer 22 from being corroded. When the first conductive layer 22 is formed using tantalum, the substrate 2 is formed by the sputtering method.
Tantalum was deposited on the entire surface of No. 1 and patterned by etching away other portions so that only predetermined portions remained.

However, in this conventional thin film transistor, since the first conductive layer 22 is formed by directly sputtering on the glass substrate 21 by using bulk tantalum as a target, the tantalum layer is a tetragonal β. -It becomes tantalum. β-tantalum has a specific resistance of 180 to 20
It is as large as 0 μΩ · cm. On the other hand, body-centered cubic α-tantalum having a small specific resistance is also known, but a manufacturing method for stably manufacturing α-tantalum has not been established yet.
When a material having a large specific resistance is used for the gate electrode 22 of a transistor used in an active matrix liquid crystal display device or the like, a signal delay occurs as the distance from a drive circuit (not shown) increases. Since the width of 22 must be widened, there is a problem that it is difficult to achieve high definition.

Therefore, the inventor of the present invention has filed Japanese Patent Application No. 3-3076.
In No. 16, a body-centered cubic niobium (Nb) layer is formed before the tantalum layer is formed, and the tantalum layer is continuously formed using this niobium layer as a base layer.
It has been disclosed that the tantalum layer can be stably formed.

On the other hand, when forming an active matrix type liquid crystal display device or the like, a transparent conductive layer for forming a pixel electrode may exist immediately below the gate electrode. This transparent conductive layer is formed of tin oxide (SnO ₂ ) or indium tin oxide (ITO). When a niobium layer is formed on the transparent conductive layer and heat of 400 ° C. or higher is applied,
A reaction occurred at the interface between the transparent conductive layer and the niobium layer, which caused a problem that the niobium layer and the tantalum layer were separated. This is because the oxygen atoms in the transparent conductive layer diffused into the niobium layer and the oxygen atoms were heated to react with niobium (Nb) to become niobium oxide (NbO _x ), and the volume expanded rapidly. It is thought that this is due to a matter.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art, and is characterized in that a transparent conductive film serving as a base layer of a gate electrode is formed on a substrate. Layer, a first conductive layer to be a gate electrode, an insulating layer to be a gate insulating film, and a first semiconductor layer to be a channel layer are sequentially laminated and provided, and an ohmic contact layer is provided on the first semiconductor layer. In a thin film transistor in which a second semiconductor layer formed of and a second conductive layer serving as a source / drain electrode are provided separately, the first conductive layer includes a lower conductive layer made of niobium and an upper conductive layer made of α-tantalum. And a barrier layer is provided between the first conductive layer and the transparent conductive layer, and the barrier layer is preferably niobium oxide, niobium nitride, molybdenum silicide, or Pa Consisting of any of the indium.

[0009]

When the barrier layer is provided between the transparent conductive layer and the first conductive layer as described above, the first conductive layer peels off even when the transparent conductive layer or the first conductive layer is heated. There is no such thing. That is, the barrier layer is formed of a material that has a slow oxygen diffusion rate and that does not react with oxygen or whose volume does not change significantly even when it reacts, and therefore suppresses the volume expansion due to the oxidation of the niobium layer and removes the niobium layer. Can be prevented. Therefore, even in a thin film transistor in which a transparent conductive layer is formed as an underlayer of a gate electrode, α- with a low specific resistance is used for the gate electrode.
Tantalum can be used.

[0010]

The present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 is a sectional view showing an embodiment of the thin film transistor according to the present invention, in which 1 is a substrate, 2 is a transparent conductive layer, and 3 is a transparent conductive layer.
Is a barrier layer, 4 is a first conductive layer, 5 is an insulating layer, 6 is a first semiconductor layer, 7 is a second semiconductor layer, and 8 is a second conductive layer.

The substrate 1 is made of a glass substrate such as # 7059 substrate.

A transparent conductive layer 2 is formed on the substrate 1. The transparent conductive layer 2 is made of tin oxide (SnO ₂ ) or indium tin oxide (ITO) and is formed to have a thickness of about 1000Å by a sputtering method or the like.
The transparent conductive layer 2 is originally provided to form a pixel electrode of an active matrix type liquid crystal display device, but in order to simplify the manufacturing process, the lower layer portion of the gate electrode is left without being removed. To do.

A barrier layer 3 is formed on the transparent conductive layer 2. The conditions required for this barrier layer 3 are:
The oxygen diffusion coefficient is small, and it does not react with oxygen, or the volume does not change much even if it reacts. Materials that satisfy such conditions include niobium oxide, niobium nitride and other niobium compounds, molybdenum silicide, and palladium (Pd). When the barrier layer 3 is made of niobium oxide or niobium nitride,
In the argon (Ar) gas, oxygen gas (O ₂ ) is mixed at 2% or less and nitrogen gas (N ₂ ) is mixed at 25% or less, and bulk niobium or the like is used as a target material. Formed by reactive sputtering. When forming the barrier layer 3 with molybdenum silicide, the niobium layer and the tantalum layer are formed by continuous film formation after sputtering using a molybdenum silicide target material. Such a barrier layer 3 has a specific resistance of 160 to 2
Since it is not so small as about 00 μΩcm, for example, 30
It is desirable to form as thin as possible so that the thickness is 0 Å or less.

A first conductive layer 4 is formed on the barrier layer 3. The first conductive layer 4 is composed of a lower conductive layer 4a made of niobium and an upper conductive layer 4b made of α-tantalum. The lower conductive layer 4a made of niobium is formed so that the upper conductive layer 4b made of tantalum has an α phase. That is, niobium is body-centered cubic, and if a tantalum layer is continuously formed on the lower semiconductor layer 4a made of niobium, this tantalum can also be made body-centered cubic in the α phase. α-tantalum has a very small specific resistance of about 23 μΩcm and is suitable as a gate electrode of a transistor. When the barrier layer 3 is formed of a niobium compound such as niobium oxide or niobium and the lower conductive layer 4a of the first conductive layer 4 is formed of niobium, niobium is used as a target material and oxygen gas is introduced into the sputtering device. It is possible to continuously form the lower conductive layer 4a by introducing nitrogen gas or nitrogen gas to form the barrier layer 3 and then stopping introducing oxygen gas or nitrogen gas. In this case, the composition ratio of the barrier layer 3 may be changed continuously or stepwise.

The first conductive layer 4 is set to about 1 × 10 ⁻⁴ torr by introducing an inert gas such as argon gas or xenon gas into the sputtering apparatus, and bulk niobium or palladium is used as a target material. And then 5-10Å
A niobium or palladium layer 4a having a thickness of about 10Å is formed on the substrate 1 at a speed of 1 / sec, and then 1 to 10Å / se is used as a target material of bulk tantalum.
tantalum layer 4b having a thickness of 100 Å or more at a speed of c
Should be formed. In this case, the target material is sputtered with an ion beam energy of 900 to 1500 eV and an electron flow of about 35 mA. Further, when the thickness of the niobium or palladium layer 4a is 3 Å or more, the tantalum layer 4b rapidly increases the α phase, and when it is 10 Å or more, most tantalum becomes the α phase. Therefore, it is preferable that the lower niobium or palladium layer 4a has a thickness of 3Å or more. Further, it is optimal to continuously form the niobium or palladium layer 4a and the tantalum layer 4b with an ion beam sputtering device having a plurality of target holders. That is, once the niobium or palladium layer 4a is formed and the vacuum is broken, an oxide film is formed on the surface of the niobium or palladium layer, and the upper tantalum layer 4b cannot be stably made into the α phase. The same α-tantalum layer 4b can be formed not only by the ion beam sputtering method but also by the magnetron sputtering method.

The lower conductive layer 4a and the upper conductive layer 4
b is patterned into a predetermined shape by chemical dry etching or hydrofluoric nitric acid. Further, an oxide film 4c is formed on the outer surface of the first conductive layer 4 so that the first conductive layer 4 is not contaminated or eroded in the step of forming a transistor. Is desirable. That is, when niobium oxide or niobium nitride is used as the barrier layer 3, the oxide film 4c is formed by anodizing the barrier layer 3, the lower conductive layer 4a, and the upper conductive layer 4b. Such an oxide film 4c is formed by anodic oxidation in, for example, a 1% ammonium tartrate aqueous solution. In this case, at a voltage of 160V or less, 4 × 10 ^-4
Anodization may be performed at a current of about A / cm ^{2 and} a current density of about 2.5 nA / V. When molybdenum silicide is used as the barrier layer 3, a tantalum oxide film is formed in advance by sputtering, and then the barrier layer 3, the lower conductive layer 4a, and the upper conductive layer 4b are anodized to form an oxide film 4c. do it.

An insulating layer 5 serving as a gate insulating film is formed on the first conductive layer 4. The insulating layer 5 is formed of, for example, a silicon nitride film (SiN _x ), a silicon oxide film (SiO ₂ ), or a two-layer structure of these. The insulating layer 5 is formed to a thickness of about 2000 to 4000 Å by the plasma CVD method or the sputtering method.

A first semiconductor layer 6 is formed on the insulating layer 5. The first semiconductor layer 6 is made of i-type amorphous silicon or the like and serves as a channel of a transistor. The first semiconductor layer 6 is formed to have a thickness of about 100 to 1000Å by a plasma CVD method using a carrier gas and silane gas (SiH ₄ ).

A second semiconductor layer 7 is formed on the first semiconductor layer 6. This second semiconductor layer 7 is n
It is composed of ⁺ type amorphous silicon and becomes the ohmic contact layer of the transistor. The second semiconductor layer 7 is a diborane gas (B) for supplying semiconductor impurities to a carrier gas and a silane gas (SiH ₄ ).
It is formed to a thickness of about 1000Å by a plasma CVD method mixed with ₂ H ₆ ).

On the second semiconductor layer 7, a second conductive layer 8 serving as an image signal wiring and source / drain electrodes is formed. The second conductive layer 8 is formed of tantalum, aluminum, chromium, titanium, or the like, and has a thickness of 1000 to 2000Å by a sputtering method or a vacuum deposition method.
Formed to a degree.

An etching stopper layer 9 made of a silicon nitride film or the like is formed in the central portion between the first semiconductor layer 6 and the second semiconductor layer 7. That is, when the source / drain of the transistor is divided by etching, the first semiconductor layer 6 is protected from being etched and disappeared, and the second semiconductor layer 7 is completely etched. Form. By providing the stopper layer 9 for such etching, it is possible to prevent a part of the second semiconductor layer 7 from remaining and increase the off-current of the transistor, and to prevent the first semiconductor layer 6 from being etched at all. can do. Note that the first semiconductor layer 6 and the second semiconductor layer 7 are formed by including, for example, a carbon element or a nitrogen element in the first semiconductor layer 6 and forming the second semiconductor layer from microcrystalline silicon or the like. The etching stopper layer 9 may be omitted by providing etching selectivity.

Although not shown, a passivation film made of a silicon nitride film or the like is formed on the transistor if necessary.

[0023]

As described above, according to the thin film transistor of the present invention, the first conductive layer serving as the gate electrode has a two-layer structure of the lower conductive layer made of niobium and the upper conductive layer made of α-tantalum. At the same time, since the barrier layer is provided between the first conductive layer and the transparent conductive layer which is the underlayer, the first conductive layer can be formed even when the transparent conductive layer or the first conductive layer is heated. It does not peel off. Therefore, even in a thin film transistor in which a transparent conductive layer is formed as a base layer of a gate electrode, α-tantalum having a low specific resistance can be used for the gate electrode.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a thin film transistor according to the present invention.

FIG. 2 is a diagram showing a conventional thin film transistor.

[Explanation of symbols]

1 ... Substrate, 2 ... Transparent conductive layer, 3 ... Barrier layer, 4 ... First conductive layer, 4a ... Lower conductive layer, 4
b ... upper conductive layer, 5 ... insulating layer, 6 ... first semiconductor layer, 7 ... second semiconductor layer, 8 ... second conductive layer.

Claims (2)

[Claims] 1. A transparent conductive layer to be a base layer of a gate electrode, a first conductive layer to be a gate electrode, an insulating layer to be a gate insulating film, and a first semiconductor layer to be a channel layer are sequentially laminated on a substrate. In the thin film transistor in which the second semiconductor layer to be the ohmic contact layer and the second conductive layer to be the source / drain electrodes are separately provided on the first semiconductor layer, the first conductive layer Is a two-layer structure of a lower conductive layer made of niobium and an upper conductive layer made of α-tantalum, and a barrier layer is provided between the first conductive layer and the transparent conductive layer. Thin film transistor. 2. The thin film transistor according to claim 1, wherein the barrier layer is made of any one of niobium oxide, niobium nitride, molybdenum silicide, and palladium.