JP3043870B2 - Liquid crystal display - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix type liquid crystal display device used for an image display section of OA equipment, a home television, and the like. In particular, a drive circuit section is formed of a thin film transistor such as polycrystalline silicon on a substrate. The present invention relates to an integrally formed liquid crystal display device.

[0002]

2. Description of the Related Art An active matrix type liquid crystal display device using a polycrystalline silicon thin film transistor is used because a large screen is easily obtained and a conventional semiconductor manufacturing technique can be applied to the manufacturing. For this reason, it is desired to increase the speed and quality of the liquid crystal display device of this system.

In a conventional active matrix type liquid crystal display device using a polycrystalline silicon thin film transistor,
The driving circuit portion and the pixel portion are formed over the same substrate surface, and the thin film transistors of the driving circuit portion and the pixel portion are simultaneously formed by the same process. FIG. 4 shows an example of the current-voltage characteristics of such a polycrystalline silicon thin film transistor.
Figure 4 is the channel length and channel width (W) for the ratio (W / L) of 10/10 NMOS of (L), a drain voltage (V _D) as a parameter, the drain current (I _D)
A gate voltage (V _G) experiment to measure the dependence results.

[0004] Polycrystalline silicon having higher mobility by one or two orders of magnitude than amorphous silicon can obtain a high ON current, but at the time of gate reverse bias, a leak current which increases exponentially with respect to a gate voltage is observed. This leakage current observed in polycrystalline silicon is a tunnel current flowing through a defect level in the crystal in a high electric field portion near the drain, and is observed even when the mobility is high and the crystallinity is good.

In this liquid crystal display device, a transistor in a drive circuit section requires a sufficiently large current drive capability to operate the circuit at a required operation speed, while a transistor serving as an analog switch in a pixel section is turned on when turned on. A sufficiently large current driving capability for charging the capacitance of the pixel portion and a sufficiently small OFF current for holding the pixel potential when OFF are required.

Although the current driving capability required for the transistors in the driving circuit portion and the pixel portion differs depending on the driving method and dimensions, the former is usually one to two orders of magnitude larger than the latter. Therefore, for example, in a liquid crystal display device using an amorphous silicon thin film transistor having low mobility by one or two digits,
Only an analog switch of a pixel portion is formed by a thin film transistor, and an external IC is used for a drive circuit portion. As a result, it is required that the OFF current of the transistor in the pixel portion be sufficiently smaller than the current driving capability.

As a method of reducing the OFF current of the transistor in the pixel portion, as can be easily inferred from FIG. 4, the transistor may be used in a region where the gate voltage is small. However, in order to drive the liquid crystal cell, the thin film transistor in the pixel portion needs to be able to operate in a region where the gate voltage is at least about ± 6 V. Therefore, the gate voltage is effectively reduced by increasing the thickness of the gate oxide film, and the OFF current is reduced in a necessary power supply voltage region.

[0008]

However, if the gate oxide film is thickened and the gate voltage is effectively lowered, the current driving capability at the time of ON is sacrificed, and the driving circuit has sufficient characteristics corresponding to the transistor characteristics. There was a problem that it could not be put out.

When the drive circuit section and the pixel section are formed on the same substrate surface, it is difficult to increase the thickness of one gate oxide film in the same manufacturing process. Furthermore, there is a problem that if both are to be manufactured in completely separate manufacturing steps, the number of steps will be significantly increased and the yield will be reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and does not cause a large increase in the number of processes and a decrease in yield, and does not reduce the current driving capability at the time of the ON state. It is an object of the present invention to provide a liquid crystal display device having a sufficiently high-speed drive circuit suitable for transistor characteristics by reducing the current.

[0011]

A liquid crystal display device according to the present invention comprises a plurality of pixel portions formed in a matrix on the same surface of a substrate, and a drive circuit portion formed around the plurality of pixel portions. Wherein the gate capacitance per unit area of one polycrystalline silicon thin film transistor forming the switching element of the pixel portion is changed per unit area of one polycrystalline silicon thin film transistor forming the driving circuit portion. The gate capacitance is set to be 0.5 times or less as compared with the gate capacitance.

In the liquid crystal display device of the present invention, there are various configurations for changing the gate capacitance of the thin film transistor, and any configuration may be used. For example, when a gate oxide film is used as the gate insulating film, the gate capacitance can be changed by changing the thickness of the oxide film. Further, the gate capacitance can be changed by using an insulating film having a different dielectric constant such as a nitride film or a multilayer film.

In the liquid crystal display device of the present invention, the gate capacitance of the pixel portion is made smaller than the gate capacitance of the drive circuit portion. Preferably, one polycrystalline silicon thin film transistor constituting a switching element of the pixel portion The gate capacitance per unit area is 0.5 times or less the gate capacitance per unit area of one polycrystalline silicon thin film transistor constituting the drive circuit section. If it exceeds 0.5 times, it becomes difficult to achieve a sufficiently small OFF current when operated in a power supply voltage range necessary for driving the liquid crystal cell.

For convenience of the manufacturing process, it is preferable to use an oxide film in the liquid crystal display device of the present invention.

In the case where an oxide film is used, the object can be achieved by sufficiently increasing the thickness of the gate oxide film of the thin film transistor in the pixel portion and by sufficiently decreasing the thickness of the gate oxide film of the thin film transistor in the drive circuit portion.

There are various methods for changing the thickness of the gate oxide film, and any of them may be used. For example, there are the following methods.

In the case of an upper gate, coplanar type MOS transistor, a thin gate oxide film is first formed, and a drive circuit portion is formed on this gate oxide film by ordinary Po.
In addition to forming a ly gate, a transistor in a pixel portion further deposits an oxide film on the gate oxide film by a CVD method, a sputtering method, or the like, and forms a metal gate thereon to reduce the thickness of the gate oxide film. Can be changed.

Alternatively, a method may be used in which a thick gate oxide film is first formed simultaneously on the pixel portion and the drive circuit portion, and only the gate oxide film on the drive circuit portion is etched to a required thickness. An oxide film deposited by a CVD method, a sputtering method, or the like can also be used as an interlayer insulating film. But about 4000
When it is desired to reduce the thickness of the gate insulating film with respect to the interlayer insulating film requiring a thickness of about Å, the insulating film can be adjusted by selectively etching only the portion to be thinned.

Note that a glass or quartz substrate can be used for the liquid crystal display device of the present invention.

It is desirable to use a polycrystalline silicon thin film transistor as a circuit element formed on the same surface of the substrate. A polycrystalline silicon thin film transistor can be formed relatively easily on a transparent substrate such as glass, which is essential for a liquid crystal device, and requires a driving frequency (for example, 160 MHz in the case of HD-TV) required in a main scanning direction driving circuit.
This is because z) can also be achieved by performing split driving. Deposition of polycrystalline silicon thin film
It can be performed by a low pressure CVD method, a plasma CVD method, a sputtering method, or the like. At that time, instead of forming a polycrystalline silicon thin film from the beginning, an amorphous (amorphous) silicon thin film is formed, and solid phase growth is performed at about 600 ° C. to perform polycrystallization. A method of solid phase growth, furthermore, a silicon film after film formation or a thin film formed by the above method is further combined with hydrogen and dangling bonds of silicon by a method such as hydrogen plasma annealing so as to be electrically stabilized. Therefore, a semiconductor thin film having high mobility can be obtained by using a method of forming a polycrystalline silicon thin film having more excellent characteristics.

In the liquid crystal display device of the present invention, the transparent conductive film serving as the pixel electrode and the counter electrode is made of ITO (Indiu).
m Tin Oxide film, tin oxide (SnO ₂ ) film and the like can be used.

[0022]

By sufficiently reducing the thickness of the gate oxide film of the thin film transistor in the driving circuit portion, a sufficient current driving force for obtaining a required operation speed can be achieved. Further, by sufficiently increasing the thickness of the gate oxide film of the thin film transistor in the pixel portion, a sufficiently small OFF current can be achieved even when the liquid crystal cell is operated in a power supply voltage range necessary for driving the liquid crystal cell.

Further, by making the gate oxide film thickness of the thin film transistor in the pixel portion sufficiently large, the gate capacitance of the switching element in the pixel portion becomes small. Therefore, switch O
Variations in pixel potential (penetration voltage) caused by discharging the charge stored in the switch element during FF can also be suppressed.

[0024]

【Example】

 Embodiment 1 Embodiment 1 will be described with reference to FIGS.

FIG. 2 is an equivalent circuit diagram. The drive circuit is a CMOS circuit using polycrystalline silicon thin film transistors.
The vertical driving circuit comprises a shift register 12 and a gate line driving buffer 13, and the horizontal driving circuit comprises a shift register 1.
4. An analog switch 15 for selecting a signal line and a capacitor 16 for holding a signal potential. The pixel unit is a thin film transistor analog switch 17 for each pixel,
It has a liquid crystal cell 18 and a storage capacitor 19.

FIG. 1 is a partial sectional view of the liquid crystal display device of the present invention. On a quartz substrate 3, a thin film transistor 1 forming a driving circuit and a thin film transistor 2 forming an analog switch of a pixel portion are formed. The active layers 4a and 4b of the thin film transistor are simultaneously formed by the same process using a polycrystalline silicon thin film.

The gate insulating film 5a of the thin film transistor 1 is formed by thermally oxidizing the polycrystalline silicon thin film, and the gate insulating film 5b of the thin film transistor 2 is formed of the above thermal oxide film and an oxide film deposited thereon by the CVD method. Has a laminated structure. The thermal oxide film also serves as an insulating film (not shown) of the storage capacitor, and the oxide film deposited by the CVD method also serves as the interlayer insulating film 7.

The gate electrode 6a of the thin film transistor 1 is a polycrystalline silicon film having a low resistance by adding impurities.
The source and drain regions 8 in a are formed by ion implantation of active species by a self-alignment method using a gate electrode. On the other hand, the gate electrode 6b of the thin film transistor 2 is a metal wiring of Al, Mo-Ta, etc., and the source and drain regions 4b are simultaneously formed using a resist mask when forming the source and drain regions 4a. The source electrodes 9a and 9b and the drain electrodes 10a and 10b are formed of an Al thin film by providing a contact hole in an interlayer insulating film. The drain electrode 10b of the switching transistor 2 in the pixel portion is connected to the pixel electrode 11 made of a transparent conductive film.

When this liquid crystal display device was operated in the power supply voltage range necessary for driving the liquid crystal cell, a sufficiently small OFF current could be achieved.

Embodiment 2 FIG. 3 shows a partial cross-sectional view of Embodiment 2. This embodiment is an example in which the gate insulating film 5d of the transistor in the pixel portion and the gate insulating film 5c of the transistor in the driving circuit portion are both made of a thermal oxide film of an active layer. First, a thick thermal oxide film is formed. After that, the drive circuit portion is selectively etched to partially remove the oxide film to reduce the film thickness. The gate electrode 6d of the second embodiment is a polycrystalline silicon film having a low resistance by adding impurities as in the first embodiment.
The source and drain regions 8 are formed by ion implantation of active species by the self-alignment method using the gate electrode 6d.

When this liquid crystal display device was operated in the power supply voltage range necessary for driving the liquid crystal cell, a sufficiently small OFF current could be achieved.

[0032]

According to the liquid crystal display device of the present invention, the gate capacitance of the thin film transistor in the pixel portion formed on the same surface of the substrate is made smaller than the gate capacitance of the thin film transistor in the drive circuit portion. Thin-film transistors having different gate insulating film thicknesses can be obtained without greatly increasing the current. The current driving capability of the thin-film transistor in the drive circuit can be increased, and the OFF current of the thin-film transistor in the pixel can be kept sufficiently small. Thus, a high-quality liquid crystal display device can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a partial cross section of a liquid crystal display device according to a first embodiment.

FIG. 2 is a diagram illustrating an equivalent circuit of the liquid crystal display device according to the first embodiment.

FIG. 3 is a view showing a partial cross section of a liquid crystal display device according to a second embodiment.

FIG. 4 shows a thin film transistor (NMOS, W / L = 10/10 (μm), field effect mobility of carrier; μ = 60 (cm ² / Vs) used in a conventional liquid crystal display device,
Oxide layer thickness; tox = 450 (angstrom)
FIG. 4 is a characteristic diagram showing the gate voltage dependence of the drain current of FIG.

[Explanation of symbols]

1. Thin-film transistors constituting a drive circuit 2.
... Thin film transistors constituting analog switches in the pixel portion, 3... Quartz substrate, 4a, 4b... Thin film transistor active layers, 5a, 5b, 5c, 5d. ……… Gate electrode, 7…
... Interlayer insulating film, 8 source and drain regions, 9
a, 9b ... source electrode, 10a, 10b ... drain electrode, 11 ... pixel electrode, 12 ... shift register, 13 ... buffer, 14 ... shift register, 15 ... analog Switch, 16: Capacitance for holding signal potential, 17: Analog switch, 18:
Liquid crystal cell, 19: storage capacity.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-34463 (JP, A) JP-A-2-27320 (JP, A) JP-A-58-4180 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G02F 1/1362 G02F 1/136 H01L 29/78

Claims (1)

(57) [Claims] 1. A liquid crystal display device comprising: a plurality of pixel portions formed in a matrix on a same surface of a substrate; and a drive circuit portion formed around the plurality of pixel portions.
One polycrystalline silicon constituting a switch element of the pixel section
0 Compared gate capacitance per unit area of Con thin film transistor gate capacity per unit area of a single polycrystalline silicon thin film preparative <br/> transistor constituting the driving circuit portion.
A liquid crystal display device characterized in that the ratio is 5 times or less .