Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-06-29
2004-10-26
Kielin, Erik (Department: 2813)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S130000
Reexamination Certificate
active
06809788
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
The invention is based on patent application Nos. 2000-199023 Pat., 2000-236810 Pat. 2001-72054 Pat., and 2001-72911 Pat. filed in Japan, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display element, and in other words, a liquid crystal light modulation element and method of producing the same.
2. Description of the Background Art
The liquid crystal display element and, in other words, the liquid crystal light modulation element primarily includes a pair of substrates, between which a liquid crystal layer including liquid crystal material is held. For example, predetermined drive voltage is applied to the liquid crystal layer to control orientation of liquid crystal molecules in the liquid crystal layer so that external light incident on the liquid crystal light modulation element is modulated to perform intended display of images or the like.
The liquid crystal light modulation element using the cholesteric liquid crystal has been known as the above kind of liquid crystal light modulation element, and various studies have been made.
Examples of the cholesteric liquid crystal are, e.g., liquid crystal, which exhibits the cholesteric phase by itself, and chiral nematic liquid crystal obtained by adding a chiral agent to nematic liquid crystal.
The cholesteric liquid crystal has such a feature that the liquid crystal molecules form helical structures, and can exhibit three states, i.e., a planar state, focal conic state and a homeotropic state when it is held between a pair of substrates, and is subjected to an external stimulus such as an electric field, a magnetic field or a heat.
In the liquid crystal light modulation element (e.g., liquid crystal display element) using the cholesteric liquid crystal, these three states exhibit different light transparencies and reflectances. Therefore, the three states and the manner of applying the external stimulus can be appropriately selected to perform the display. For example, the display may be performed in the cholesteric-nematic phase transfer mode using the homeotropic state and the focal conic state, and may be performed in a bistable mode using the planar state and the focal conic state.
Among them, the display in the bistable mode has such a feature that the planar state and the focal conic state are stable even in the state where no external stimulus is applied, and thus has the bistability (memory property), which maintains the display state even when no external stimulus (e.g., voltage) is applied. For the above reason, the liquid crystal light modulation element using the cholesteric liquid crystal has been increasingly studied in recent years as the memorizable element (display element achieving the stable display state).
In particular, the liquid crystal light modulation element using the cholesteric liquid crystal, which exhibits the selective reflection property in the visible wavelength range when it is in the planar state, has the memorizable property, and can achieve a bright reflection state. In other words, it can perform bright display without using a polarizing plate or a color filter. Therefore, it is expected that the liquid crystal light modulation element described above can be used as a display element, which is very effective at reducing the power consumption, and can be used as a display element of, e.g., a mobile telephone requiring low power consumption.
The liquid crystal having the bistability can be stable in both the planar state (i.e., the state of the planar orientation), where the helical axis of the cholesteric liquid crystal is substantially perpendicular to the substrate surface, and the liquid crystal exhibits the selective reflection state, and the focal conic state (the state of the focal conic orientation), where the helical axis of the liquid crystal is substantially parallel to the substrate surface, and the liquid crystal is transparent to the visible light.
However, in the liquid crystal display element utilizing the selective reflection characteristics of the cholesteric liquid crystal, the reflection wavelength shifts toward the shorter side in accordance with the incident angle of the light and observation angle because it employs the reflection manner using the light interference.
This phenomenon becomes more remarkable as the helical axis of the cholesteric liquid crystal in the planar orientation is closer to the vertical direction to the substrate surface. In particular, a TN liquid crystal element and an STN liquid crystal element may use a pair of substrates having deposited and rubbed polyimide thin films thereon for holding a liquid crystal layer therebetween, in which case the helical axis of the cholesteric liquid crystal is perfectly or substantially perfectly perpendicular to the substrate surface, resulting in an extremely narrow view angle. If the above liquid crystal element is used as the display element, therefore, the viewability becomes extremely low.
The rubbing of the thin polyimide film increases the restricting force on a polyimide interface so that it becomes difficult to maintain the focal conic state. Consequently, the bistability, which is the distinctive feature of the cholesteric liquid crystal, may be lost.
For avoiding the above, it has been attempted to incline slightly the helical axis of the cholesteric liquid crystal with respect to the normal of the substrate. One of such attempts is called PSCT (Polymer Stabilized Cholesteric Texture), in which polymers are dispersed in the cholesteric liquid crystal so that the helical axes may be positioned in random directions owing to mutual operations between the polymers and the liquid crystal (U.S. Pat. No. 5,384,067). According to this method, however, mixing of the polymer in the liquid crystal material may lower the reliability of the element, and/or may require the increased drive voltage.
In another method, a polyimide film not subjected to the rubbing is deposited on substrate surface opposed to the liquid crystal so that the helical axis may be inclined. In this method, however, domains including different directions of the inclined helical axes (directions of the helical axes projected onto the substrate) are formed randomly so that scattering of the incident light is liable to occur due to the difference in refractive index between the domains, resulting in lowering of the purity of the display color in the selective reflection. In a multilayer liquid crystal display element employing a multilayer structure for multicolor display, the reflection light from the lower layer is liable to be affected by light scattering by an upper layer, which lowers both the contrast and color purity.
For improving the characteristics of the cholesteric liquid crystal element, in which the liquid crystal is held between the substrates provided with the polyimide films not subjected to the orientation processing, Japanese Laid-open Patent Publication No. 10-31205 (31205/1998) has disclosed the following manner. Different surface treatments are effected on the polyimide films formed on the substrates on the observation side and the non-observation (opposite) side, respectively. More specifically, the rubbing processing is effected on only the polyimide film on the non-observation side, and the liquid crystal domains on the observation side may be the non-orientation random domains (polydomain state). Thereby, the helical axes of the liquid crystal on the non-observation side may be substantially perfectly perpendicular to the substrate surface, and the liquid crystal domains on the non-observation side may be uniform (mono-domain state).
According to this manner, however, the rubbing is effected on the whole polyimide film area of the substrate on the non-observation side. Therefore, the liquid crystal domains form the monodomain state on the whole substrate so that the stability in the focal conic state is liable to lower, and the bistability, which is the
Hashimoto Kiyofumi
Miyai Mitsuyoshi
Okada Masakazu
Yamada Jun
Kielin Erik
Minolta Co. , Ltd.
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