Liquid crystal device alignment

Details Liquid crystal device alignment Liquid crystal device alignment

1996-10-03

1998-06-09

Sikes, William L.

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

349134, 349128, 349201, G02F 11337, G02F 141, G02F 113

Patent

active

057643253

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.
2. Discussion of Prior Art
There are three known types of liquid crystal material, nematic, cholesteric, and smectic each having a different molecular ordering.
The present invention 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.
In order to provide displays with a large number of addressable elements it is common to make the electrodes as a series of row electrode on one wall and a series of column electrodes on the other cell wall. These form eg an x.y matrix of addressable elements or pixels, and are commonly addressed using rms addressing methods. Due to material and device parameters there is an upper limit to the number of pixels that can be multiplex addressed. One way of improving the number of pixels is to incorporate thin film transistors adjacent each pixel; such displays are termed active matrix displays.
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. A disadvantage of the rubbing process is that it can generate dust which degrades the yield of the display and also causes electrostatic and mechanical damage to thin film transistors already formed on the cell wall.
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.
For these reasons it is highly desirable to develop non-rubbing alignment techniques. Such alignments have included 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); or by irradiation of polarised light (M Schadt et al Japan J Appl Phys vol 31 pp 2155, 1992). Also twisted nematic structures have been made in which only one surface is rubbed (Y Toko et al Japan Display 92 491).
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

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Journal of Applied Physics, vol.52, No.1, Jan. 1981, New York US pp. 210-218, Nakamura M et al Alignment of nematic liquid crystals on ruled grating surfaces.
Society for Information Display, International Symposium, Digest of Technical Papers, vo. XXIV, 1993 pp. 957-960, XPOOO470785, E.W. Lee et al `Control of the LC alignment using a stamped morphology method and its application to LCDs`.

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