JPH0339722A - Image display device - Google Patents

Abstract

PURPOSE:To secure a large loading capacity and to maintain information despite of a miniaturized picture element by forming a thin film which has greater permittivity than a silicon oxide film and a gate, on a part of polysilicon of a thin film transistor constituted of the gate, the silicon oxide film and the polysilicon. CONSTITUTION:After a positive polysilicon film 12 is placed on a quartz substrate 11, the film is insularly formed by etching, and impurity ions are implented into a part becoming loading capacity. After the silicon oxide film 13 is formed as a gate oxide film, the silicon oxide film 13 for the only part of the loading capacity is removed. Then, a silicon nitride film 15 is formed, the film except for the part of the loading capacity is removed and a gate polysilicon film 14 is piled up. Under this constitution, the loading capacity size is ten times as large as a normally constituted picture element. Although the picture element capacity is declined accompanied by the miniaturization of the picture element, the great loading capacity is secured, and thereby, the maintained information is never leaked by a leak current.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to an image display device.

(Prior Art) In recent years, liquid crystal displays are already in practical use, as they are used in pocket-sized TVs. Especially from the point of view of image quality, each pixel is equipped with a thin film transistor (hereinafter referred to as
The active matrix type with a switch (abbreviated as TPT) is dominant, and research is being conducted to reduce the characteristics of the switch TPT, especially the leakage current.

Referring to FIG. 3, a pixel portion of a conventional liquid crystal display element will be explained. 3(a) is a plan view, and FIG. 3(b) is a sectional view taken along line BB' in FIG. 3(a). In the same figure, 31
is a quartz substrate, 32 is an active polysilicon film, 33 is a silicon oxide film, 34 is a gate polysilicon film, 35 is an interlayer insulating film, 36 is an aluminum wiring, 37 is a chrome plate, 38 is a contact hole, 39 is a transparent electrode.

As shown in FIG. 3, conventionally, a gate polysilicon film 34
A pixel structure has often been adopted in which an additional capacitance is formed using a silicon oxide film 33, which is a gate oxide film between the active polysilicon film 32 and the active polysilicon film 32.

(Problems to be Solved by the Invention) As mentioned above, in the conventional pixel display device, as the number of pixels increases and the pixel size decreases, the load capacity also decreases, and the information written in the liquid crystal is discharged by the TPT nok current. Therefore, it becomes difficult to retain information in the pixel portion. Furthermore, if the silicon oxide film is made thinner in order to increase the load capacity, there is a drawback that conduction between the active polysilicon film and the gate polysilicon film becomes poor.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the conventional drawbacks and provide an image display device with a large load capacity.

(Means for Solving the Problems) The image display device of the present invention has a thin film transistor including a gate, a silicon oxide film, and a polysilicon. A gate is formed with a thin film with a certain ratio.

(for production) With the above configuration, the size of the pixel load capacitance is as follows.

The dielectric constant of the thin film is ε, and the 1# electric constant of the silicon oxide film is ε,
If so, it becomes ε,/ε95 times compared with one area of the same film thickness. Therefore, even if the capacitance of the liquid crystal decreases with the miniaturization of one pixel, the held information is stored in the load capacitance, so that the held information will not be leaked due to leakage current of the TPT.

(Example) An example of the present invention will be described based on FIGS. 1 and 2.

FIG. 1(a) is a plan view of the image display device of the present invention,
Figure 1 (b) is a sectional view taken along line AA' in Figure 1 (a). FIG. 2 is a process flow forming FIG.

In FIGS. 1 and 2, 11 is a quartz substrate, 12 is an active polysilicon film, 13 is a silicon oxide film, 14 is a gate polysilicon film, 15 is a silicon nitride film, 16 is an interlayer insulating film, and 17 is an aluminum wiring. , 18 is a chromium electrode, 1
9 is a contact hole, 20 is a transparent electrode, and 21 is an impurity ion.

Next, the process up to the formation of FIG. 1 will be explained based on FIG. 2. Active polysilicon film 12 on quartz substrate 11
After depositing about 2000 Å, it is etched to form an island shape. Next, impurity ions 21 are implanted into the portion that will become the load capacitance (FIG. 2(a)).

Next, a silicon oxide film 13 with a thickness of 1300 mm is used as a gate oxide film.
After forming the silicon oxide film 13, only the portion corresponding to the load capacitance is removed (FIG. 2(b)). Next, LPCVD
About 500 people formed a silicon nitride film 15 using the method,
Remove the parts other than the load capacity (Fig. 2 (C)). Furthermore, after depositing a gate polysilicon film 14 (FIG. 2(d)), the structure shown in FIG. 1 is formed using a normal image display process.

If the structure is as shown in FIG. 1, the load capacitance will be about 10 times that of a pixel with a normal structure. Therefore, even if the pixel capacitance decreases as the pixel becomes smaller, a large load capacitance can be ensured, so that retained information will not be leaked due to leakage current of the TPT. Therefore, high image quality can be ensured.

Note that 5iON is used instead of the silicon nitride film.

Ta,○,,A#20. ．． Tie2. Sm2O3,5r
TiO,.

A similar effect can be obtained by using a thin film such as BaTa, O, etc., which has a dielectric constant greater than that of a silicon oxide film.

(Effects of the Invention) According to the present invention, a large load capacity can be ensured and information retention can be guaranteed even if one pixel becomes small, and its practical effects are great.

[Brief explanation of drawings]

FIG. 1(a) is a plan view of an image display device according to an embodiment of the present invention, FIG. 2(b) is a sectional view thereof, FIG. a) and (b) are a plan view and a cross-sectional view of a conventional image display device. 11... Quartz substrate, 12... Active polysilicon film,
13... Silicon oxide film, 14... Gate polysilicon film, 15... Silicon nitride film, 16... Interlayer insulation film, 17... Aluminum wiring, 18... Chrome electrode, 19... - Contact hole, 20...-transparent electrode. 21... Impurity ion.

Claims (1)

[Claims] In a thin film transistor composed of a gate, a silicon oxide film, and polysilicon, a thin film having a dielectric constant greater than that of the silicon oxide film and the gate are formed on at least a portion of the polysilicon. Characteristic image display device.