Liquid crystal alignment
a liquid crystal and alignment technology, applied in the direction of non-linear optics, instruments, optics, etc., can solve the problems of static and mechanical damage of active matrix elements, unsatisfactory rubbing process, and the effect of increasing the average lc tilt angle away from the normal
Inactive Publication Date: 2004-12-30
KITSON STEPHEN CHRISTOPHER +2
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Benefits of technology
[0022] Because the features are arranged in a random or pseudorandom array instead of a regular lattice, diffraction colours which result from the use of regular grating structures are reduced and may be substantially eliminated. Such an array can act as a diffuser, which may remove the need for an external diffuser in some displays. Of course, if a diffraction colour is desired in the display, the array may be made less random, and the posts may be spaced at intervals which produce the desired interference effect. Thus, the structure may be separately optimised to give the required alignment and also to mitigate or enhance the optical effect that results from a textured surface.
[0023] Using a random or pseudorandom array also mitigates optical and LC alignment effects that arise as a result of variations of phasing between regular arrays on two surfaces, for example Moire effects.
[0028] Shorter and wider posts tend to induce a planar alignment, whilst taller and thinner posts tend to induce a homeotropic alignment. Posts of intermediate height and width can induce tilted alignments and may give rise to bistable alignments in which the director may adopt either of two tilt angles in substantially the same azimuthal direction. By providing posts of suitable dimensions and spacing, a wide range of alignment directions, planar, tilted and homeotropic, can easily be achieved, and the invention may therefore be used in any desired LC display mode.
[0031] If the posts are perpendicular to the cell walls, the LC may be homeotropically aligned at substantially 90.degree. to the plane of the cell walls. However, for some applications it is desirable to achieve a homeotropic alignment which is tilted by a few degrees. This may readily be achieved by using posts which are inclined from the perpendicular. As the posts are inclined more, the average LC tilt angle away from the normal will increase. The invention therefore provides a simple way of inducing LC homeotropic alignment with any preferred tilt angle.
[0032] When exposing a photoresist, a desired post tilt angle can readily be achieved by exposing the photoresist through a suitable mask with a light source at an angle related to the desired angle by Snell's law as is known to allow for the refractive index of the photoresist material.
[0040] For many displays, it is desirable to have a uniform alignment throughout the field of view. For such displays, the posts may all be of substantially the same shape, size, orientation and tilt angle. However, where variation in alignment is desired these factors, or any of them, may be varied to produced desired effects. For example, the posts may have different orientations in different regions where different alignment directions are desired. A TN cell with quartered sub-pixels is an example of a display mode which uses such different orientations, in that case to improve the viewing angle. Alternatively, if the heights of the posts are varied, the strengths of interactions with the LC will vary, and may provide a greyscale. Similarly, variation of the shape of the posts will vary the strength of interaction with the LC.
Problems solved by technology
Whilst having a desirable effect on the optical characteristics of the device, the rubbing process is not ideal as this requires many process steps, and high tolerance control of the rubbing parameters is needed to give uniform display substrates.
Moreover, rubbing may cause static and mechanical damage of active matrix elements which sit under the alignment layer.
Rubbing also produces dust, which is detrimental to display manufacture.
This avoids some of the problems with rubbing, but the coatings are sensitive to LC materials, and typically produce only low pre-tilt angles.
This also effects a desired optical response; however the process is complicated by the addition of vacuum deposition and a lithography process.
Moreover, control of process parameters for SiO evaporation is critical to give uniformity, which is typically difficult to achieve over large areas.
An asymmetric bigrating structure can cause tilt in one or both angular directions.
LC homeotropic alignment is also a difficult process to control, typically using a chemical treatment of the surface, such as lecithin or a chrome complex.
These chemical treatments may not be stable over time, and may not adhere very uniformly to the surface to be treated.
These polyimides need high temperature curing which may not be desirable for low glass transition plastic substrates.
This requires vacuum processes which are subject to the problems discussed above in relation to planar alignment.
However, PTFE gives only weak control of alignment angle and may be difficult to process.
Method used
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[0050] The liquid crystal cell shown schematically in FIG. 2 comprises a first cell wall 2 and a second cell wall 4 which enclose a layer of nematic LC material of negative dielectric anisotropy. The molecules of the LC are represented as ellipses, with the long axis indicating the local director. The inner surface of each cell wall is provided with a transparent electrode pattern, for example row electrodes 12 on the first cell wall 2 and column electrodes 14 on the second cell wall 4, in a known manner. The LC alignment is bistable.
[0051] The inner surface of the first cell wall 2 is textured with an array of square posts 10, and the inner surface of the second cell wall 4 is flat. The posts are in a pseudorandom array, as will be described below with reference to FIG. 4. The posts 10 are approximately 1 .mu.m high and the cell gap is typically 3 .mu.m. The flat surface is treated to give homeotropic alignment. The posts are not homeotropically treated.
[0052] Such an array of squa...
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Abstract
A liquid crystal device has a surface alignment structure comprising a random or pseudorandom two dimensional array of alignment features (10) which are shaped and / or orientated to produce a desired alignment. Depending on the geometry and spacing of the features (10), the liquid crystal may be induced to adopt a planar, tilted, or homeotropic alignment.
Description
FIELD OF THE INVENTION[0001] The present invention relates to alignment of liquid crystals in liquid crystal devices.BACKGROUND OF THE INVENTION[0002] Liquid crystal (LC) materials are rod-like or lath-like molecules which have different optical properties along their long and short axes. The molecules exhibit some long range order so that locally they tend to adopt similar orientations to their neighbours. The local orientation of the long axes of the molecules is referred to as the "director". There are three types of LC materials: nematic, cholesteric (chiral nematic), and smectic. For a liquid crystal to be used in a display device, it must typically be made to align in a defined manner in the "off" state and in a different defined manner in the "on" state, so that the display has different optical properties in each state. Two principal alignments are homeotropic (where the director is substantially perpendicular to the plane of the cell walls) and planar (where the director is...
Claims
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IPC IPC(8): G02F1/1337G02F1/139
CPCG02F1/133753G02F1/1391G02F1/133788G02F1/1337
InventorKITSON, STEPHEN CHRISTOPHERRUDIN, JOHN CHRISTOPHERNEWTON, CHRISTOPHER JAMES P.
OwnerKITSON STEPHEN CHRISTOPHER