Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Patent
1996-08-06
1998-05-19
Sikes, William L.
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
349172, 349201, 349133, G02F 11337, G02F 1141, G02F 113
Patent
active
057542643
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to ferroelectric liquid crystal device alignment.
2. Discussion of Prior Art
Liquid crystal display devices are well known. They typically comprise a liquid crystal cell formed by a thin layer of a liquid crystal material held between two glass walls. These walls carry transparent electrodes which apply an electric field across the liquid crystal layer to cause a reorientation of the molecules of liquid crystal material. The liquid crystal molecules in many displays adopt one of two states of molecular arrangement. Information is displayed by areas of liquid crystal material in one state contrasting with areas in the other state. One known display is formed as a matrix of pixels or display elements produced at the intersections between column electrodes on one wall and row electrodes on the other wall. The display is often addressed in a multiplex manner by applying voltages to successive row and column electrodes.
Liquid crystal materials are of three basic types, nematic, cholesteric, and smectic each having a distinctive molecular arrangement.
The present invention concerns chiral smectic liquid crystal materials particularly ferroelectric smectic liquid crystal materials. Devices using this material include the surface stabilised ferroelectric liquid crystal (SSFLC) device. These devices can show bistability, ie the liquid crystal molecules, more correctly the molecular director, adopt one of two alignment states on switching by positive and negative voltage pulses and remain in the switched state after removal of the voltage. This behaviour depends, in part, upon the surface alignment properties. The switched states may be stabilised by the presence of an ac bias which may be provided by the data (column) voltages in a multiplexed addressed device. Another type of ferroelectric liquid crystal device (FELCD) is the electro-clinic device. SSFLC are described for example in N. A. Clark & S. T. Lagerwall, App Phys Letters 36(11) 1980 pp 899-901, and U.S. Pat. No. 4,367,924. Yet another type is an antiferroelectric device (Ref: A. Fukuda et al. J later. Chem. (1994) 4, 7, 997-1016).
Alignment of ferroelectric liquid crystals is often carried out using rubbed polymer alignments, however this method has several limitations. These include dirt, electrical damage due to discharge between in-plane electrodes and uniformity of pretilt and anchoring. Another known alignment is that of oblique evaporation of eg SiO. This is difficult to employ for large area displays.
It can be shown that if the alignment used introduces a splay in a higher temperature cholesteric phase, then a predominantly (zig-zag) defect free alignment can be achieved in the ferroelectric phase. This may be due to the balance between surface pretilt and surface (zenithal and azimuthal) anchoring energies being able to bias the presence of either the C1 or C2 chevron at the operating temperature. In order to be able to optimise this balance for a given liquid crystal, ideally it is necessary to be able to change surface pretilt and anchoring energies controllably and independently. This is difficult with rubbed polymers as all these properties are manifestations of the same physical interaction.
For some liquid crystal materials the alignment may be very sensitive to this balance and so good uniformity of pretilt and anchoring energies is required over the surface, this is not always achievable with rubbed alignment techniques.
Furthermore in some FLC devices it is crucial to align the preferred directions (defined in the cholesteric phase) accurately with respect to each other, be these parallel or offset at some required angle. This is very difficult with conventional alignments since rubbing leaves no easily observable direction on the surface.
SUMMARY OF THE INVENTION
The above problems are solved according to this invention by the use of gratings on the inner surface of liquid crystal cell walls.
According to this invention a liquid crystal device comprises a
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Lee et al., "Control of the LC Alignment Using a Stamped Morphology Method and its Application to LCDs", SID 93 Digest, pp. 957-960.
Nakamura et al., "Alignment of Nematic Liquid Crystals on Ruled Grating Surfaces", Journal of Applied Physics, vol. 52, No. 1, Jan., 1981, pp. 210-218.
Molecular Crystals and Liquid Crystals, vol. 23, No. e/4, 1973, GB pp. 215-231, Berreman D W `Alignment of liquid crystals by grooved surfaces`.
Bryan-Brown Guy Peter
Jones John Clifford
McDonnell Damien Gerard
Towler Michael John
Sikes William L.
The Secretary of State for Defence in Her Britannic Majesty's Go
Ton Toan
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