Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-06-01
2002-07-30
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S129000
Reexamination Certificate
active
06426786
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a liquid crystal display (LCD) device.
More particularly, the present invention relates to a homeotropic-alignment or a tilted homeotropic-alignment LCD device of either a single-domain-type, a two-domain-type, or a four-domain type for use in a flat panel display, a projection display, and a method of producing such homeotropic-alignment or tilted homeotropic-alignment liquid crystal display devices, in which improvements in the display quality and mass production capability of the homeotropic-alignment or the tilted homeotropic-alignment liquid crystal display devices are achieved.
2. Description of the Related Art
In homeotropic-alignment liquid crystal display devices, a nematic liquid crystal having a negative dielectric anisotropy is used. Liquid crystal molecules, nearly homeotropically aligned with respect to substrate surfaces, are caused to tilt by a drive voltage, whereupon a display operation begins.
With this construction, if the molecule tilt direction is not uniform in the activated state, a remarkable non-uniformity in brightness is observed. To avoid this phenomenon, a small pre-tilt angle is typically provided when a liquid crystal homeotropic-alignment is achieved. However, as the pre-tilt angle increases, the contrast ratio and the threshold level decrease with an increase in pre-tilt angle. This is problematic in that the liquid crystal molecule is a birefringent molecule and depends on its orientation.
The pre-tilt angle may be provided by using an angle-deposited film (e.g., a film formed by an angle vapor deposition process also referred to as an “oblique evaporation process”). The angle vapor deposition process is a process used for aligning liquid crystal molecules, wherein the vapor of an oxide, such as SiO, is deposited on a substrate surface from an oblique direction. However, the pre-tilt angle should be larger than about 0.50°. Otherwise, if the drive voltage undergoes a sudden change, then the liquid crystal molecules may tilt in diametrically opposite directions. This phenomenon is observed as a dynamic alignment defect. The pre-tilt angle cannot exceed a certain value. Otherwise, the birefringence due to the pre-tilt of the liquid crystal molecules will cause a light leakage in a dark state and reduce the contrast ratio.
To solve this problem, a two-step angle vapor deposition process, such as disclosed in Japanese Patent Publication No. 55-13338, has been proposed as a process which is suitable for controlling the molecule tilt direction of the liquid crystal.
However, the above-mentioned conventional angle vapor deposition process has a problem that the pre-tilt angle varies greatly with a slight change in film thickness or incident angle.
As is also reported in
Proceedings of the SID
, Vol. 31/4, page 321 (1990), when the angle vapor deposition process is used in the production of liquid crystal light valves, a great change in the contrast ratio is observed with a slight change in the pre-tilt angle. This phenomenon will bring about the occurrence of non-uniform liquid crystal alignment and an inadequate reproducibility problem.
According to another conventional alignment method as disclosed in Japanese Patent Laid-open Publication No. 51-129251, an SiO
2
film deposited on a substrate is etched off by an Argon ion beam directed obliquely onto the SiO
2
film, so as to produce a surface profile change which is used for liquid crystal alignment. Due to a large etch-off amount, this alignment method is time-consuming. Further, a simple ion beam etching effected in an oblique direction may cause an alignment non-uniformity problem in the activated state. This problem is particularly significant for a high resolution spatial light modulator (SLM) constructed to display dynamic images.
According to another conventional alignment method, it involves two glass substrates having patterned transparent electrodes being set obliquely at a first angle with respect to an evaporation source. Then, a first SiO
2
film is deposited on each of the glass substrates while irradiating the glass substrates with an ion beam emitted from an ion gun.
Subsequently, the glass substrates are turned in their plane over an angle of 90° and set obliquely at a second angle with respect to the evaporation source. Thereafter, a second SiO
2
film is deposited on the respective first SiO
2
films while irradiating the first SiO
2
film with the ion beam. The first and second SiO
2
films thus deposited on the glass substrates jointly form a homeotropic-alignment undercoat film. A homeotropic alignment agent is applied to the undercoat film, and thereafter the glass substrates are assembled together via spacers and filled with a liquid crystal.
However, as evident from the description above, the processing steps to form tilted homeotropic alignment of liquid crystals are very complicated, involving ion-beam assisted two-step oblique evaporations of SiO
2
with an orientation change of substrates between two oblique evaporations and the application of the homeotropic alignment film on top of the SiO
2
films. The manufacturing cost to produce such homeotropic alignment layers is high, and the throughput is low because of the complicated process steps.
Another conventional method to form stable tilted homeotropic alignment in liquid crystals involves depositing a layer of silica upon the substrate by in-line magnetron sputtering as the substrate is moved past the sputtering target. Thereafter, the silica-coated substrate is coated with a long-chain alcohol to induce the liquid crystal director assuming a field-off state tilted about 1° to 3° from the perpendicular direction toward an azimuthal direction parallel to the direction of movement of the substrate during sputtering.
A similar conventional alignment method describes a deformation of aligned phase (DAP) mode liquid crystal display panel of a homeotropic structure type. The alignment films are formed so that a homeotropic orientation of the liquid crystal is achieved by a combination of oblique vacuum deposition and a perpendicular orientating processing agent.
However, both conventional approaches require two layers of films, thereby requiring additional processing steps.
That is, a first layer of dielectric film either by sputtering or oblique evaporation and a second layer of homeotropic aligning agent, are required. The second layer of homeotropic aligning film is usually made of organic material, such as a long-chain alcohol which will be photo-degradable by ultraviolet, violet, or blue light impinging thereon, either from a back-light or a projection light source, to change the pre-tilt angle of the adjacent liquid crystal molecules. As a result, the display image will be defective or the display will have a decreasing contrast ratio as a function of illumination dosage.
SUMMARY OF THE INVENTION
In view of the foregoing and other problems of the conventional methods and structures, an object of the present invention is to provide a photo-stable homeotropic-alignment or tilted homeotropic-alignment liquid crystal display unit which is free from the occurrence of alignment non-uniformity both in the inactivated state and in the activated state, and which has a high contrast ratio.
Another object of the present invention is to provide a method of making the photo-stable homeotropic or tilted homeotropic-alignment liquid crystal display unit which uses an inorganic material, such as oblique evaporated SiO
2
, as compared to the conventional techniques of using an organic homeotropic alignment agent, such as long chain alcohols.
Another object of the present invention is to provide a simple and inexpensive method to generate homeotropic or tilted homeotropic liquid crystal alignment.
Another object of the present invention is to provide a method, compatible to semiconductor integrated circuit (IC) processing, of generating homeotropic- or tilted homeotropic-alignment.
According to a first aspect of the present invention,
Lu Minhua
Yang Kei-Hsiung
International Business Machines - Corporation
McGinn & Gibb PLLC
Schechter Andrew
Ton Toan
Zarick, Esq. Gail H.
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