Method of aligning liquid crystal molecules in an active matrix

Details Method of aligning liquid crystal molecules in an active matrix Method of aligning liquid crystal molecules in an active matrix

1990-12-04

1993-03-30

Howell, Janice A.

Optical: systems and elements

Holographic system or element

Using a hologram as an optical element

359 75, G02F 11337

Patent

active

051989174

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a method of aligning liquid crystal molecules in an active matrix liquid crystal display element.
The liquid crystal display element is widely used as a flat panel display having a low electric consumption. In particular, a liquid crystal display of an active matrix type, in which a switching element such as a thin film transistor (TFT) or a thin film diode is provided in each pixel for driving the molecules in the pixel, has a large capacity and high quality, so that it is used for a television set and an information terminal equipment. In manufacturing process of the liquid crystal display element, it is very important to align the molecules for controlling the orientation of the liquid crystal molecules. However, in the active matrix display, since the switching elements are projected on a substrate, the surface of the substrate is uneven. Therefore, it is difficult to obtain preferable alignment.
FIGS. 2 and 3 show a pixel of a conventional active matrix liquid crystal display element having a TFT switch. FIG. 2 is a sectional view and FIG. 3 is a plan view of a substrate having a switching element. The display element comprises a substrate 1 having a switching element (hereinafter called an active substrate), a substrate 2 without a switching element (hereinafter called an opposite substrate), a liquid crystal layer 3 sealed between the substrates 1 and 2. On the active substrate 1, a TFT type switching element 10 for driving liquid crystal molecules 6, a data electrode 11, a scanning electrode 12, and a display electrode 13 are provided. On the opposite substrate 2, an opposite electrode 14 is provided. Further, on both substrates, alignment films 7 and 8 made of polyimide are formed respectively, for controlling initial alignment of the molecules.
FIGS. 4 and 5 show another conventional liquid crystal display element having a diode ring (DR) as a switching element. FIG. 4 is a sectional view and FIG. 5 is a plan view of an active substrate. The liquid crystal layer 3 is sealed between the active substrate 1 and the opposite substrate 2. On the active substrate 1, a DR type switching element 15 for driving liquid crystal molecules 6, a scanning electrode 16, and the display electrode 13 are provided. On the opposite substrate 2, a data electrode 17 is provided. Further, on both substrates, alignment films 7 and 8 are formed respectively, for controlling initial alignment of the molecules. The data electrode 17 is provided opposite to the display electrode 13 and disposed in the direction to cross the scanning electrode 16.
As aligning treatment applied on the alignment films 7 and 8 of the active matrix display element, there are some methods such as a rubbing process and a vacuum deposition method. The rubbing process is usually used. In the rubbing process, the surface of the alignment film is rubbed with a brush in one direction so that the molecules are lined up in the rubbing direction.
A twist angle between the substrates and a preferential viewing direction of the liquid crystal layer 3 are determined by the alignment direction. The preferential viewing direction is a visual direction from which an image having the highest contrast can be observed. For example, in order to form a preferential viewing field in front of the liquid crystal display of 90.degree. TN (twisted nematic) mode, it is necessary to align in the direction at an angle of 45 degrees with the edge of the substrate as shown in FIG. 6. In FIG. 6, an arrow 31 represents a rubbing direction for an upper substrate and an arrow 32 is a rubbing direction for a lower substrate.
In the active matrix display element, the uneven surface of the active substrate caused by projections of the switching elements formed thereon makes it difficult to align the molecules. As shown in FIGS. 2 and 4, the switching elements and the electrodes are considerably thick, for example about 0.8 to 2.5.mu. in thickness. Since shadow portions of the switching elements and the electrodes are not rubbed, the

REFERENCES:
patent: 4796979 (1989-01-01), Tsuboyama
patent: 4836653 (1989-06-01), Yoshino et al.
Buczek, "A Thin Film Process to Improve Off Axis Viewing of Liquid Crystal Displays," Mol. Cryst. Liq. Cryst., vol. 47, No. 3-4, (1978), pp. 145-154.

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