JP2621619B2 - Method for manufacturing thin film transistor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in characteristics of a thin film transistor used as a switching element of, for example, an active matrix liquid crystal display device, and particularly to a reduction in off current.

[Prior Art] FIGS. 5A to 5E are cross-sectional views showing a conventional method for manufacturing a thin film transistor in the order of steps. In the figure,
(1) is an insulating substrate using an insulating material such as glass,
(2) is a gate electrode formed on a substrate (1) using a metal such as Cr, (3) is a gate insulating film made of Si nitride or the like formed to cover the gate electrode (2), (4) Is a semiconductor film using a semiconductor such as a non-doped Si film formed so as to be in contact with the upper part of the gate insulating film (3); (5) is a Si nitride film covering the semiconductor film (4) and the channel portion of the thin film transistor; The protective film (6) is formed by connecting the semiconductor film (4) through a contact hole formed by patterning the protective film (5) on the protective film (5), and the film is formed. A contact film formed by doping impurities such as P into a semiconductor film such as Si by removing an upper portion of an active region which is a part of the active region by etching or the like; (7) is a contact film formed from a metal formed on the contact film (6); Source / drain electrodes, (8) This is a second protective film formed of silicon nitride or the like for protecting the entire thin film transistor.

First, a metal film such as Cr is formed on the insulating substrate (1),
A gate electrode (2) pattern is formed by photolithography (FIG. 5a). Next, a gate insulating film (3), a semiconductor film (4), and a protective film (5) are successively formed and patterned by photolithography to form Si islands (FIG. 5b). Next, patterning is performed by photolithography to open a contact hole for connecting the contact film (6) and the semiconductor film (4) (FIG. 5c). Thereafter, a contact film (6) and a source / drain electrode (7) are formed, and a pattern is formed by photolithography (FIG. 5d). Finally, a second protective film (8) is formed (FIG. 5e).

Next, the operation of the thin film transistor will be described. A voltage of, for example, about 10 to 20 V is applied between the source and drain electrodes (7), and the voltage applied to the gate electrode (2) is changed, for example, between −5 V and 20 V, to flow between the source and drain electrodes. The switching operation is performed by controlling the current.
For example, in the case of an n-channel thin film transistor, when the gate electrode is made negative, the switching state of the transistor is turned off, and when the gate voltage is made positive, the switching state is turned on.

[Problem to be Solved by the Invention] In order to obtain a good switching operation, for example, n
In the case of a channel thin film transistor, the drain current (hereinafter referred to as on-current) flowing between the source and drain electrodes when the gate electrode is applied positively is large, and the drain current flowing between the source and drain when the gate electrode is negatively applied. It is important that the current (hereinafter referred to as off-state current) be small. However, when a conventional thin film transistor is used as a switching element of a matrix addressing type liquid crystal display or the like, the off current is not sufficiently low, and the use margin is not always large.

The present invention has been made to solve the above problems, and inactivates a back channel of a thin film transistor. As a result, for example, an off current that can be sufficiently applied as a switching element such as a liquid crystal display is sufficiently reduced. It is intended to manufacture a thin film transistor.

[Means for Solving the Problems] According to a method of manufacturing a thin film transistor of the present invention, a film surface of a semiconductor film on a semiconductor / protective film interface side is formed in a plasma formed by introducing an H ₂ gas before a protective film of the thin film transistor is formed. Exposure increases the interface state at the interface between the semiconductor film and the protective film.

[Operation] In the present invention, the semiconductor film is exposed to the plasma to raise the interface state at the interface between the semiconductor film and the protective film. Therefore, the back channel of the thin film transistor is inactivated, and the back channel is turned off when the thin film transistor is turned off. Of the thin film transistor, and as a result, the off current of the thin film transistor can be reduced.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

A method for manufacturing a thin film transistor according to an embodiment of the present invention will be described with reference to the cross-sectional views shown in the order of steps of FIGS.

First, a metal film such as Cr is formed on the insulating substrate (1),
A gate electrode pattern (2) is formed by photolithography (FIG. 1a). Next, after forming a gate insulating film (3) and a semiconductor film (4) (FIG. 1b), the surface of the semiconductor film (4) is removed.
The H ₂ gas is introduced and exposed to the formed plasma (FIG. 1c). Next, a protective film (5) is formed and patterned by photolithography to form Si islands (FIG. 1d).
Thereafter, patterning is performed by photolithography to open a contact hole for connecting the contact film (6) and the semiconductor film (4) (FIG. 1e). Next, a contact film (6) and a source / drain electrode (7) are formed, and then a pattern is formed by photolithography (FIG. 1f). Finally, a second protective film (8) is formed (FIG. 1g).

 Next, the operation of the thin film transistor will be described.

A voltage of, for example, about 10 to 20 V is applied between the source and drain electrodes (7), and the voltage applied to the gate electrode (2) is changed, for example, between −5 V and 20 V, to flow between the source and drain electrodes. The switching operation is performed by controlling the current. For example, for an n-channel thin film transistor,
When the gate electrode is made negative, the switching state of the transistor is turned off, and when the gate voltage is made positive, the switching state is turned on.

In the present invention, before forming the protective film of the thin film transistor, the film surface of the semiconductor layer on the semiconductor / protective film interface side is exposed to a plasma formed by introducing H ₂ gas to reduce the interface state at the semiconductor film / protective film interface. By increasing the thickness, the back channel of the thin film transistor is inactivated, the leakage current flowing through the back channel during the off operation of the thin film transistor is reduced, and as a result, the off current of the thin film transistor is reduced.

Silicon nitride film, semiconductor film (4) as gate insulating film (3)
Amorphous silicon, a protective film (5) as a silicon nitride film, and a contact film (6) as an adjacent doped impurity.
Using Si, the thin film transistor according to one embodiment of the present invention was exposed to H ₂ plasma under the conditions shown in the table. The temperature dependence of the off-state current in this example was measured under the measurement conditions of a drain voltage of 20 V, a gate voltage of −5 V, and a ratio of channel width to channel length of the transistor: W / L = 40/5. The result is
By the conventional method in the characteristic diagram of Fig together with comparative examples not exposed in H ₂ plasma. The vertical axis represents the off-current, and the horizontal axis represents the reciprocal of the absolute temperature. The characteristic number A represents the temperature dependence of the off-state current without hydrogen plasma irradiation of the comparative example, and B represents the hydrogen plasma irradiation of this embodiment. As can be seen from the figure, when the film surface of the semiconductor film (4) on the semiconductor / protective film interface side of the thin film transistor is exposed to H ₂ plasma, the off-state current is reduced, and the off-state current was 0.46 eV without hydrogen plasma irradiation. The activation energy of the current increased to 0.65 eV by plasma irradiation.

Further, a back gate electrode was formed on the protective film (5), and the capacitance-voltage characteristics between the back gate electrode and the source electrode (7) were measured. The results are shown in the characteristic diagram of FIG. The vertical axis indicates the capacitance (pF), the horizontal axis indicates the voltage (V), and the characteristic curve C indicates the capacity-voltage characteristics of the present example with hydrogen plasma irradiation, and the characteristic curve C indicates the comparative example without hydrogen plasma irradiation. If the film surface of the thin film transistor of the semiconductor-protective film interface side of the semiconductor film (4) of Comparative Example not exposed in H ₂ plasma, capacity -
Although the voltage characteristics increase when the back gate voltage is applied positively, in the case of this embodiment in which the film surface of the semiconductor film (4) on the semiconductor / protective film interface side of the thin film transistor is exposed to H ₂ plasma, Even if the gate voltage is applied positively, no change in the capacitance-voltage characteristics is observed. From this, believed interface state of the semiconductor-protective film interface by exposing in H ₂ plasma is increased inactivated.

From the above results, before the formation of the protective film, the film surface of the semiconductor film (4) on the semiconductor film / protective film interface side is exposed to the plasma formed by introducing H ₂ gas, whereby the semiconductor / protective film interface of the thin film transistor is formed. The interface level increases and becomes inactive. As a result, the energy band at the interface between the semiconductor and the protective film becomes as illustrated in FIG. 4 (b). It is considered that the flowing back channel current decreased, and as a result, the off current decreased. FIG. 4 (a) is an explanatory diagram showing an energy band at an interface between a semiconductor and a protective film of a conventional thin film transistor.

In the above embodiment, the semiconductor film (4) is amorphous.
The case where the Si film is used has been described, but the semiconductor film (4)
The same effect can be obtained even if a polycrystalline Si film is used.

In the above embodiment, the n-channel thin film transistor has been described. However, a similar effect can be obtained by using a p-channel thin film transistor.

[Effects of the Invention] As described above, according to the present invention, when manufacturing a thin film transistor, the film surface of the semiconductor film on the interface side of the semiconductor film and the protective film is formed by introducing H ₂ gas into the plasma before forming the protective film. Exposure to increase the interface state at the interface between the semiconductor film and the protective film compared to that before plasma irradiation, thereby decreasing the conductivity and mobility of the semiconductor film at the interface between the semiconductor film and the protective film, and inactivating the semiconductor film.
The off-current of the thin film transistor can be reduced.

[Brief description of the drawings]

1A to 1G are cross-sectional views showing a method of manufacturing a thin film transistor according to an embodiment of the present invention in the order of steps, and FIG. 2 compares the temperature dependence of off current of the thin film transistor according to an embodiment of the present invention. The characteristic diagram shown with the example, FIG.
FIG. 4 (a) is a characteristic diagram showing capacitance-voltage characteristics measured between a source electrode and a back gate electrode together with a comparative example. FIG. Explanatory drawing showing the figure, FIG.
5A to 5E are explanatory views of the comparative example, and FIGS. 5A to 5E are cross-sectional views showing a conventional method of manufacturing a thin film transistor in the order of steps. (1) is an insulating substrate, (2) is a gate electrode, (3) is a gate insulating film, (5) is a protective film, (4) is a semiconductor film,
(6) is a contact film, and (7) is a source / drain electrode. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

(57) [Claims] A thin film transistor having an insulating substrate, a gate electrode, a gate insulating film, a semiconductor film, a protective film formed on the semiconductor film, a contact film, a source electrode, and a drain electrode is manufactured. In the method, before forming the protective film, the film surface of the semiconductor film at the interface between the semiconductor film and the protective film is exposed to plasma formed by introducing H ₂ gas to raise the interface state at the interface between the semiconductor film and the protective film. A method for manufacturing a thin film transistor, characterized in that: