JPH04125971A - Thin film transistor - Google Patents

Abstract

PURPOSE:To prevent the generation of a leakage current by a method wherein a semiconductor layer processed into an insular form is formed into a structure, in which the sidewalls of the semiconductor layer are processed into the form of an overhang and are not brought into contact directly with source and drain electrodes. CONSTITUTION:A semiconductor layer 3 is formed into a structure, in which sidewalls 5 of the layer 3 are processed into the form of an overhang and are not brought into contact to source and drain electrodes 6 and 7. Thereby, the generation of a leakage current due to holes, which flow when a negative voltage is applied to a gate electrode, can be prevented and an increase in an OFF current in a transistor can be suppressed.

Description

[Detailed description of the invention] [Industrial application field] INDUSTRIAL APPLICATION This invention is utilized for a thin film transistor.

The present invention is particularly applicable to active matrix type and inverted stagger type thin film transistors using amorphous silicon used in liquid crystal displays and the like.

〔overview〕

The present invention reduces leakage current in an inverted staggered thin film transistor by making the sidewalls of the island-shaped semiconductor layer have an overhanging shape so that they do not come into direct contact with the source and drain electrodes. It is something that

[Conventional technology]

Conventionally, as shown in FIG. 3, this type of thin film transistor has a gate electrode 2 made of metal such as aluminum or tantalum, a gate insulating film 4 made of amorphous silicon nitride, etc. on an insulating substrate made of, for example, a glass substrate 1, a gate insulating film 4 made of amorphous silicon nitride, etc., and a hydrogenated amorphous A semiconductor layer 3 made of silicon processed into an island shape (hereinafter referred to as islanding), an ohmic contact layer 8 made of amorphous silicon doped with phosphorus (P), a source electrode 6 and a drain electrode 7 made of aluminum, chromium, etc. It was composed of

[Problem to be solved by the invention]

In the structure of this conventional thin film transistor, as shown in FIG. 4, the side wall 5 of the semiconductor layer 3 is processed into a vertical or tapered shape, and the drain electrode 7 or the source electrode 6 is in contact with this side wall 5. , a negative voltage ■ to the gate electrode 2. A leakage current caused by the holes 9 generated when the voltage is applied flows through the contact surfaces between the side wall 5 and the source electrode 6 and drain electrode 7, and as shown in FIG.
This has the disadvantage of increasing the off-state current of the transistor.

An object of the present invention is to provide a thin film transistor in which leakage current is prevented by eliminating the above-mentioned drawbacks.

[Means to solve the problem]

The present invention provides a thin film transistor including a gate electrode, a gate insulating film, a semiconductor layer processed into an island shape, a source electrode, and a drain electrode, which are formed by sequentially stacking them on an insulating substrate, wherein the semiconductor layer is formed on the sidewalls of the thin film transistor. is characterized in that it has an overhanging shape and does not come into contact with the source electrode and the drain electrode.

Further, the present invention may include auxiliary layers respectively provided on the source electrode and the drain electrode.

[Effect]

The sidewalls of the islanded semiconductor layer are processed into an overhang shape so as to eliminate contact between the sidewalls and the source and drain electrodes.

Therefore, since there is no current path from the source electrode to the drain electrode through the sidewall of the semiconductor layer, leakage current can be reduced.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic vertical sectional view showing a first embodiment of the present invention.

In the present embodiment, a gate electrode 2, a gate insulating film 4, a gate insulating film 4,
In a thin film transistor including an islanded semiconductor layer 3, an ohmic contact layer 8, a source electrode 6, and a drain electrode 7, the present invention is characterized in that the sidewall 5 of the semiconductor layer 3 has an overhang shape, The structure is such that the source electrode 6'J6 and the drain electrode 7 do not come into contact with each other.

Next, the main points of the manufacturing method of this embodiment will be explained.

A film of 1,000 metal chromium is formed by sputtering on a low-alkaline glass substrate 1 having a thickness of about 1 mm, and this is patterned by photolithography and wet etching to form a gate electrode 2.

Next, a silicon nitride film (
5000 people), hydrogenated amorphous silicon film (150
0 people) and phosphorus-doped amorphous silicon film 30
After 0 layers are continuously grown, about 200 layers of chromium are formed by sputtering, and this is patterned by photolithography and dry etching to form a chromium silicide layer 11.

Next, the hydrogenated amorphous silicon film and the phosphorus-doped amorphous silicon film are patterned by photolithography and dry etching to form an island semiconductor layer 3 and a gate insulating film 4. At this time, by selecting the dry etching conditions, the side wall 5 of the semiconductor layer 3 can be etched.
can be processed into an overhang shape.

In this embodiment, carbon tetrafluoride and oxygen were selected as the etching gas, and the ratio was 4=1, and the pressure was 50P.
Isotropic etching with a selectivity ratio of 5=1 between amorphous silicon and resist was performed by increasing the etching depth to about a. Since the chromium silicide layer 11 is not etched,
A large undercut was placed underneath, creating an overhanging shape.

Next, a 2,500-layer metal chromium film is again formed by sputtering, and this is patterned by photolithography and dry etching to form source electrode 6 and drain electrode 7. The ohmic contact layer 8 is formed by removing the phosphorus-doped amorphous silicon film exposed between the source electrode 6 and the drain electrode 7 by dry etching.

Finally, the entire structure is annealed for about 2 hours in a nitrogen atmosphere at 250° C. to complete the thin film transistor.

In this embodiment, the ratio of the thickness of the islanded semiconductor layer 3 to the thickness of the source electrode 6 and drain electrode 7 is 1:
16, but in order to prevent wire breakage at step portions, 1:
It is desirable to set it to 15 or more.

FIG. 5 shows the characteristics of the drain-source current Ins with respect to the gate voltage V for the thin film transistor of this embodiment and the conventional thin film transistor of FIG. Gate voltage■. In the conventional example, the off-state current when is negative is 1
Although 0-'' to 10-9 A also flows, it can be seen that in the present embodiment, as a result of the elimination of leakage current through the side wall of the semiconductor layer, the current is about 10-'' A, which is almost no flow.

FIG. 2 is a schematic vertical sectional view showing a second embodiment of the present invention.

In the second embodiment, an ITO film with a thickness of 1000 A, which is a feature of the present invention, is formed by sputtering on the source electrode 6 and drain electrode 7 of the first embodiment shown in FIG. 1 by photolithography. A source and drain electrode auxiliary film 10 is provided which is patterned and formed using a wet etching method.

In this second embodiment, the source electrode 6 and the drain electrode 7
The auxiliary film 10 covers the step coverage portion of the island-shaped semiconductor layer 3, which has the advantage that disconnections and the like are less likely to occur.

〔Effect of the invention〕

As explained above, in the present invention, by processing the sidewall of the islanded semiconductor layer of an inverted staggered thin film transistor into an overring shape, this part is made into a structure that does not come into contact with the source and drain electrodes. This has the effect of preventing leakage current caused by holes that flow when a negative voltage is applied to the transistor, and suppressing an increase in the off-state current of the transistor.

[Brief explanation of the drawing]

FIG. 1 is a schematic vertical sectional view showing a first embodiment of the present invention. FIG. 2 is a schematic vertical sectional view showing a second embodiment of the present invention. FIG. 3 is a schematic longitudinal sectional view showing a conventional example. FIG. 4 is a diagram illustrating the occurrence of leakage current in a conventional example. FIG. 5 is a current-voltage characteristic curve diagram of the thin film transistor of the embodiment and the conventional example. DESCRIPTION OF SYMBOLS 1...Glass substrate, 2...Gate electrode, 3...Semiconductor layer, 4...Gate insulating film, 5...Side wall, 6...
- Source electrode, 7... Drain electrode, 8... Ohmic contact layer, 9... Hole, 10... Auxiliary film,
11...Chromium silicide layer.

Claims (1)

[Claims] 1. A gate electrode formed by sequentially stacking layers on an insulating substrate;
In a thin film transistor including a gate insulating film, a semiconductor layer processed into an island shape, a source electrode, and a drain electrode, the semiconductor layer has a structure in which a side wall thereof has an overhang shape and does not contact the source electrode and the drain electrode. A thin film transistor characterized by: 2. The thin film transistor according to claim 1, further comprising auxiliary films provided on the source electrode and the drain electrode, respectively.