Liquid crystal device alignment layer having an asymmetric profi

Details Liquid crystal device alignment layer having an asymmetric profi Liquid crystal device alignment layer having an asymmetric profi

1996-08-06

1998-03-03

Sikes, William L.

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

349124, 349201, G02F 11337, G02F 113

Patent

active

057241139

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to liquid crystal device alignment. Such devices typically comprise a thin layer of a liquid crystal material contained between cell walls. Optically transparent electrode structures on the walls allow an electric field to be applied across the layer causing a re-ordering of the liquid crystal molecules to an ON state. On removing the electric field the molecules relax back to their OFF state.
There are three known types of liquid crystal material, nematic, cholesteric, and smectic each having a different molecular ordering.
2. Description of Prior Art
The present invention particularly concerns devices using nematic or long pitch cholesteric materials and a surface alignment treatment to the cell walls. This surface alignment aligns liquid crystal molecules in contact with the wall in an alignment direction. By arranging these alignment directions orthogonal the liquid crystal is forced to adopt a twisted structure in its voltage OFF state. This device is known as a twisted nematic device. Addition of a small amount of cholesteric material to the nematic material imparts a preferred twist direction to ensure a uniform twist in the device. Also devices can be made with angles of twist greater than 90.degree.; eg the super twisted nematic device, or 270.degree. twisted nematic device described in U.S. Pat. No. 4,596,446. Another requirement for the alignment treatment is that it should also impart a surface tilt to liquid crystal molecules at the cell wall. Such a surface tilt is necessary in some devices to ensure a uniform display as described eg in GB-1,472,247, and 1,478,592.
One method of providing alignment is termed a rubbing process where a cell wall, with or without a polymer layer, is unidirectionally rubbed by a soft cloth. Liquid crystal molecules align along the rubbing direction, usually with a surface tilt of about 2.degree. or more depending upon the polymer layer.
Another alignment technique is known as oblique evaporation of eg SiO, which can produce surface tilts of zero, or high tilt eg 30.degree. depending upon evaporation direction angle. Such a technique is cumbersome for large scale manufacture; but a more important problem is that it is difficult to provide a uniform alignment direction and surface tilt over large areas of cell walls.
Short pitch cholesteric materials, forming thermochromic displays, have been aligned by grating structures embossed into plastic cell walls; this is described in GB-2,143,323 (McDonnell 1983). A previous use of gratings to obtain pretilted alignment has utilised a blazed grating crossed with a sinusoidal grating (E S Lee et al SID 93 Digest, 957). The liquid crystal director then runs along the sinusoidal grooves and hence runs over the blazed grooves which leads to pretilt. One method of producing blazed gratings using a photosensitive layer exposed by a two beam interference fringe, is described in Jp-A-60 060 624.
Other alignment techniques include the use of the Langmuir Bloggett technique (H Ikeno et al Japan J Appl Phys Vol 27, pp 495, 1988); application of magnetic field to the substrate (N Koshida and S Kikui, Appl Phys Lett vol 40, PP 541, 1982); or the use of polymers films having optical anisotropy induced by mechanical drawing (H Aoyama et al Mol Cryst Liq cryst vol 72 pp 127, 1981). Also twisted nematic structures have been made in which only one surface is rubbed (Y Toko et al Japan Display 92 491).
A variation on rubbed polymer alignment is described by M Schadt, K Schmitt, V Kozinkov and V Chigrinov, Jpn J Appl Phys 31, 2155 (1992). The material polyvinylcinnamate when illuminated with polarised light forms a cross linked structure with an anisotropic distribution. This anisotropy leads to a preferred azimuthal direction for liquid crystal alignment on the surface. The direction of the nematic director is perpendicular to the polarisation of the light used to cross link the surface, and with zero surface tilt (pretilt).
For most liquid crystal devices a molecu

REFERENCES:
patent: 5013141 (1991-05-01), Sakata
patent: 5299289 (1994-03-01), Omae et al.
patent: 5311339 (1994-05-01), Fertig et al.
patent: 5464669 (1995-11-01), Kang et al.
Japanese Journal of Applied Physics, Part 1, vol. 31, No. 7, Jul. 1992, JP pp. 2155-2164, Schadt et al `Surface-induced parallel alignment of liquid crystals by linearly polymerized photopolymers`.
Molecular Materials, UK, vo. 3, No. 2, 1993 pp. 161-168, Marusii et al, `Photosensitive orientants for liquid crystal alignment`.
Society for Information Display, International Symposium, Digest of Technical Papers, vo. XXIV, May 1993 pp. 957-960, Lee et al; `Control of the LC alignment using a stamped morphology method and its application to LCDs`.

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