JPH1050998A - Fabrication of thin film transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for fabricating a semiconductor device in which a thin film transistor having a high on-current and a low off-current can be formed easily. SOLUTION: The method for fabricating a thin film transistor on an insulating substrate 1 comprises a step for forming a source electrode 3 and a drain electrode 4 of transparent conductive film on the insulating substrate 1, a step for forming heavily doped layers 6, 7 covering the source electrode 3 and the drain electrode 4 while isolating from each other, a step for exposing the heavily doped layers 6, 7 to hydrogen plasma generated through hydrogen discharge, a step for depositing a semiconductor layer 9 covering the heavily doped layers 6, 7 after exposure to hydrogen discharge, and a step for depositing a gate insulation film 10 on the semiconductor layer 9 and then forming a gate electrode thereon.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a thin film transistor used for a matrix display device, a contact type image sensor, or the like.

[0002]

2. Description of the Related Art In a thin film transistor (TFT), it is important that a current in a conductive state of a transistor, that is, an on current, is high and a current in a non-conductive state of a transistor, that is, an off current is low.

Conventionally, as a material of a semiconductor layer forming a channel region of a thin film transistor, amorphous silicon (a-
Si), polycrystalline silicon, microcrystalline silicon, and the like have been variously studied. In order to increase the on-state current of the transistor, it is necessary to improve the mobility of electrons or holes in the channel region of the thin film transistor. At present, as such a method, a method of forming a stacked silicon layer such as two layers or three layers having different crystallinity into a semiconductor layer has been energetically studied.

Alternatively, various methods have been studied for improving the film quality of a semiconductor layer constituting such a channel region by performing a hydrogen plasma treatment.

On the other hand, in order to reduce the off current,
Various methods have been studied for improving the quality of a PN junction formed between a channel region and a drain or reducing the electric field applied to the junction between the channel and the drain.

As a means for obtaining a high on-current and a low off-current by a simple method, a method of improving ohmic contact between a source electrode and a drain electrode and a semiconductor layer is disclosed in Japanese Patent Application Laid-Open No. 61-232673. I have. Such an ohmic contact allows a drain current to smoothly flow through the channel region when a positive voltage is applied to the gate electrode in the case of, for example, an N-channel thin film transistor, and applies a zero or negative voltage to the gate electrode. At the time of turning off, the drain current is provided to keep sufficiently low.

The technique disclosed in the above publication will be described below with reference to FIG. FIG. 5 is a cross-sectional view of a thin-film transistor having improved ohmic contact as described above.

[0008] As shown in FIG.
A source electrode 22 and a drain electrode 23 made of a transparent conductive layer such as TO are formed. Then, high-concentration impurity added layers 24 and 25 are formed on the surface portions of the source electrode 22 and the drain electrode 23 so as to cover the source electrode 22 and the drain electrode 23, respectively. The high-concentration impurity-added layers 24 and 25 are a-Si films to which phosphorus or arsenic is added as an impurity.

A semiconductor layer 26, which is an a-Si film, is formed so as to cover the high-concentration impurity added layers 24 and 25. Further, a gate insulating film 27 is formed on the surface of the semiconductor layer 26, and a gate electrode 28 is formed on the gate insulating film 27 with a transparent conductive layer such as ITO.

[0010]

In such a conventional thin film transistor, the drain current in the off state of the thin film transistor is greatly reduced. For example, when the thin film transistor is of an N-channel type, almost no drain current flows when the gate voltage becomes negative. This is an effect of the high-concentration impurity added layers formed on the source electrode and the drain electrode.

However, in such a structure, the drain current in the ON state is greatly reduced. this is,
This is because a natural oxide film having a thickness of about 3 nm is easily formed on the surface of the high-concentration impurity-added layer, and the ohmic property with the semiconductor layer 26 may be lost.

This phenomenon will be described with reference to FIG. FIG. 6 shows the gate voltage dependence of the drain current of the thin film transistor. Here, the case where a high-concentration impurity added layer is provided on the surface of the source electrode and the drain electrode and the case where it is not provided are shown.

As shown in FIG. 6B, when there is no high-concentration impurity added layer, the drain current increases when the gate voltage increases to the negative side in the off state. On the other hand, when there is a high-concentration impurity added layer as shown in FIG. 6A, the gate voltage is almost constant on the negative side, and such an increase in drain current is not observed.

However, as shown in FIG. 6A, it can be seen that the drain current does not increase even when the gate voltage becomes positive and the transistor is turned on. As can be seen from comparison with FIG. 6B, in the ON state, the drain current decreases by two to three digits. Then, the variation of the drain current in the ON state becomes large.

An object of the present invention is to provide a method for manufacturing a thin film transistor having a high on-current and a low off-current.

[0016]

For this purpose, a method of manufacturing a thin film transistor according to the present invention is directed to a thin film transistor formed on an insulating substrate, wherein a source electrode and a drain electrode formed of a transparent conductive film are formed on the insulating substrate. Forming a high-concentration impurity-added layer that covers and separates the source electrode and the drain electrode, respectively, and exposing the high-concentration impurity-added layer to hydrogen plasma generated by hydrogen discharge. Depositing a semiconductor layer covering the high-concentration impurity-added layer after the exposure to the hydrogen plasma, and forming a gate electrode on the gate insulating film while forming a gate insulating film on the semiconductor layer. including.

The semiconductor layer is formed in the same device that performs the hydrogen discharge.

Here, the high-concentration impurity-added layer is a silicon semiconductor layer, and the semiconductor layer is an amorphous silicon semiconductor layer, a polycrystalline silicon semiconductor layer, or a microcrystalline silicon semiconductor layer.

[0019]

FIG. 1 shows an embodiment of the present invention.
This will be described with reference to FIG. 1 to 3 are sectional views in the order of the manufacturing process of the thin film transistor of the present invention.

As shown in FIG. 1A, a transparent conductive film 2 is formed on a glass substrate 1. This transparent insulating film 2 is an ITO film having a thickness of 150 nm.

As shown in FIG. 1B, the transparent insulating film 2 is processed by a photolithography technique and a dry etching technique, and a source electrode 3 is formed on a predetermined region of the glass substrate 1.
And a drain electrode 4 are formed.

Next, as shown in FIG. 1C, an n ⁺ semiconductor film 5 is formed. The n ⁺ semiconductor film 5 is a 30 nm-thick amorphous silicon film containing a phosphorus impurity, and the concentration of the phosphorus impurity is set to about 10 ²⁰ atoms / cm ³ .

This amorphous silicon film is made of silane (SiH
₄ ) Deposition is performed by a plasma CVD apparatus using a mixed gas of gas, phosphine (PH ₃ ) gas and hydrogen gas. Here, in forming the amorphous silicon film, the pressure of the mixed gas is 100 Pa, the substrate temperature at the time of film formation is 250 ° C., and 13.
The RF power density at 56 MHz is set so as to satisfy the condition of 0.05 W / cm ² .

Next, as shown in FIG. 1D, the n ⁺ semiconductor film 5 is patterned by a photolithography technique and a dry etching technique. Then, high concentration impurity added layers 6 and 7 are formed on source electrode 3 and drain electrode 4.

Next, as shown in FIG. 2A, the high-concentration impurity added layers 6 and 7 are subjected to a hydrogen plasma treatment.
Then, the surfaces of high concentration impurity added layers 6 and 7 are irradiated with hydrogen ions 8 in the hydrogen plasma. By this hydrogen ion irradiation, the natural oxide film formed on the surface of the high-concentration impurity added layer is reduced and removed.

This hydrogen plasma treatment is performed by the above plasma C
This is achieved by a hydrogen discharge in the chamber of the VD device. The electrode in the chamber used for hydrogen discharge is a parallel plate type.
The frequency of the high-frequency power supply of F is 13.56 MHz. The temperature of the electrode substrate on which the silicon substrate is placed at the time of hydrogen discharge is 250 ° C., and this electrode substrate is connected to the anode side (earth side) of the above-mentioned parallel plate. That is, the configuration of the anode coupling is adopted. Further, the radio frequency (RF) power density is fixed at 0.05 watts (W), and the pressure of the hydrogen gas is fixed at 50 Pa.

In such a hydrogen discharge, a similar effect can be obtained by mixing helium or argon gas with hydrogen gas. Further, this hydrogen discharge may be performed in a configuration of a cathode coupling in which the electrode substrate is connected to the cathode side.

Next, as shown in FIG. 2B, a semiconductor layer 9 is formed so as to cover the source electrode 3, the drain electrode 4, and the high-concentration impurity added layers 6 and 7. Here, the semiconductor layer 9 is an amorphous silicon film having a thickness of 100 nm deposited by the above-mentioned plasma CVD apparatus.

This amorphous silicon film uses a mixed gas of silane and hydrogen as a reaction gas, the pressure of the reaction gas is 120 Pa, the substrate temperature during film formation is 250 ° C., and 13.56.
It is deposited under the condition of an RF power density of 0.04 W / cm ^{2 in} MHz. Here, the flow ratio between the silane gas and the hydrogen gas is set to about 1: 3.

Then, the film thickness is 300 n by the plasma CVD method.
An amorphous silicon nitride film of about m is deposited, and a gate insulating film 10 is formed.

Here, a mixed gas of silane gas, ammonia (NH ₃ ) gas and nitrogen gas is used as a reaction gas for the amorphous silicon nitride film serving as a gate insulating film. Substrate temperature of 300
RF power density of 0.08 W / c at 13.56 MHz
m ² and deposited.

Next, as shown in FIG. 2C, the gate insulating film 10 and the semiconductor layer 9 are patterned by photolithography and dry etching to form islands.

Next, as shown in FIG.
1 is deposited by sputtering. Here, this chromium film 1
1 is set to about 150 nm.

Next, as shown in FIG.
1 is patterned to form a gate electrode 12. Finally, a passivation film 13 made of a silicon nitride film is formed.

As described above, the source electrode 3 and the drain electrode 4 partially covered with the high-concentration impurity-added layers 6 and 7 are provided on the glass substrate 1. 6, the drain electrode 4 and the high-concentration impurity-added layer 7 are covered with the semiconductor layer 9 and the gate insulating film 10, and a thin film transistor in which the gate electrode 12 is formed on the gate insulating film 10 is completed.

Next, the effect of the present invention will be described with reference to FIG. FIG. 4 shows the gate voltage dependence of the drain current of the thin film transistor formed by the method of the present invention. Here, the thin film transistor is an N-channel type.

As shown in FIG. 4, in this case, the gate voltage is almost constant on the negative side, and there is no increase in drain current.

On the other hand, as shown in FIG. 4, when the gate voltage is on the positive side, the drain current is greatly increased, which is two to three orders of magnitude higher than that of the conventional thin film transistor.
Then, the variation of the drain current becomes very small.

As described above, the thin film transistor formed by the manufacturing method of the present invention has a high on-current and a low off-current.

In the above embodiment of the present invention, the case where the semiconductor layer is formed of an amorphous silicon film of a silicon semiconductor has been described. The present invention is not limited to such a semiconductor film, and may be a microcrystalline semiconductor film such as silicon, silicon / germanium, or a polycrystalline semiconductor film. In this case, the mobility of the charge is improved, and the on-current is further increased.

The high-concentration impurity-added layers 6 and 7 may be composed of a polycrystalline silicon film or a microcrystalline silicon film.
Further, it should be noted that the gate electrode 12 may be made of an ITO film.

In the above embodiment, the case where the thin film transistor is formed on a glass substrate has been described. However, the present invention is not limited to this, and also mentions that the present invention can be effectively applied to a thin film transistor on an insulating film formed over an insulator such as a plastic or a semiconductor substrate such as a silicon substrate. Keep it.

[0043]

As described above, according to the present invention, a high-concentration impurity-added layer which is in ohmic contact with the surfaces of a source electrode and a drain electrode formed on a glass substrate is formed. Immediately before the semiconductor layer covering the high-concentration impurity-added layer is formed, the surface of the high-concentration impurity-addition layer is subjected to hydrogen plasma treatment by hydrogen discharge.

Therefore, the off current of the thin film transistor is greatly reduced, and the on current of the thin film transistor is increased. In addition, the on-current value stably becomes a large value.

Thus, a thin film transistor having a high on-current and a low off-current can be formed stably and easily.

According to the present invention, it is very easy to improve the performance of a matrix display element, a contact type image sensor, and the like.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a TFT according to an embodiment of the present invention in the order of manufacturing steps.

FIG. 2 is a cross-sectional view illustrating a TFT according to an embodiment of the present invention in the order of manufacturing steps.

FIGS. 3A to 3C are cross-sectional views illustrating a TFT according to an embodiment of the present invention in the order of manufacturing steps.

FIG. 4 is a diagram showing the gate voltage dependence of the drain current for explaining the effect of the present invention.

FIG. 5 is a sectional view of a TFT for explaining a conventional technique.

FIG. 6 is a diagram showing the gate voltage dependence of the drain current of a conventional TFT.

[Explanation of symbols]

 1,21 glass substrate 2 transparent conductive film 3,22 source electrode 4,23 drain electrode 5 n + semiconductor layer 6,7,24,25 high concentration impurity added layer 8 hydrogen ion 9,26 semiconductor layer 10,27 gate insulating film 11 Chromium film 12, 28 Gate electrode 13 Passivation film

Claims (3)

[Claims] In a thin film transistor formed on an insulating substrate, a step of forming a source electrode and a drain electrode made of a transparent conductive film on the insulating substrate, and covering the source electrode and the drain electrode, respectively. Forming a high-concentration impurity-added layer separated from each other, exposing the high-concentration impurity-added layer to hydrogen plasma generated by hydrogen discharge, and covering the high-concentration impurity-added layer after exposure to the hydrogen plasma. A method of manufacturing a thin film transistor, comprising: depositing a semiconductor layer to be formed; and forming a gate insulating film on the semiconductor layer and forming a gate electrode on the gate insulating film. 2. The method according to claim 1, wherein the semiconductor layer is formed in the same device that performs the hydrogen discharge. 3. The high-concentration impurity-added layer is a silicon semiconductor layer, and the semiconductor layer is an amorphous silicon semiconductor layer;
3. The method according to claim 1, wherein the method is a polycrystalline silicon semiconductor layer or a microcrystalline silicon semiconductor layer.