Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-07-07
2003-02-18
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S139000, C349S171000, C349S187000, C349S201000
Reexamination Certificate
active
06522382
ABSTRACT:
FIELD OF INVENTION
This invention relates to a liquid crystal device.
DESCRIPTION OF PRIOR ART
Liquid crystal devices typically comprise a thin layer of a liquid crystal material sandwiched between substrates acting as cell walls. Optically transparent electrode structures on the walls allow an electric field to be applied across the layer, causing a re-alignment of the liquid crystal molecules.
Many different modes of liquid crystal devices are known in the art. Examples of known device types are the twisted nematic device, the cholesteric phase change device, the dynamic scattering device, the supertwisted nematic device and the surface stabilised ferroelectric device. It is well known in all of these device modes to provide a surface on the interior walls of the device which will control the alignment of the liquid crystal fluid in close proximity to the surface. The surface or surfaces may provide only one stable alignment of the liquid crystal within the cell, in which case the device is termed “monostable”. If two or more zero-field alignments are afforded, the device is termed “multistable”. In general where only two stable alignments, whose optical properties are significantly different, are obtained, devices are termed “bistable”.
The class of bistable nematic devices can be sub-divided into “azimuthal”, “zenithal” and “twisted” types. In “azimuthal” types, the switching between the stable states requires a general rotation of the liquid crystal bulk director in the plane of the substrates, whereas in “zenithal” types this movement is in a plane normal to the substrates surface. For “twisted” types, the movement between states is characterised by a significant change in the nematic director twist angle through the device about an axis normal to the substrates.
The liquid crystal may be realigned by use of electric fields applied between the electrodes on the cell walls. For liquid crystals with positive dielectric anisotropy, the result is alignment parallel to the field, whereas for liquid crystals with negative dielectric anisotropy, the result is alignment normal to the field.
For bistable and multistable devices, a key issue is switching between two different stable states—this should not occur uncontrollably, but should be reliable, repeatable, and should require little power. Switching is accomplished by realigning liquid crystals through the electric fields between electrodes. In standard cells, this realignment is a bulk effect, and it requires higher electric fields than is desirable to work efficiently.
Other approaches have been tried. Ohta et al, in “Development of Super-TFT LCDs with In Plane Switching Display Mode”, IDRC 95, p707, proposes use of two different electrodes interdigitated on a single substrate. International Patent Publication No. WO 81/01618 teaches use of different electrodes on a single substrate to achieve more than one field geometry. Such approaches have had problems in effective application. The electrode density is important for in-plane switching to be effective, but it is limited by the tendency for shorting between opposed conductors to occur, because such shorting results in the failure of the pixel concerned.
A further modification beneficial in aligning liquid crystals at a cell surface is to introduce topographic features of a microscopic scale. This approach has been followed for monostable alignment (discussed in Cheng, J., Boyd, G. D., Applied Physics Letters, Vol. 35, p1326, 1970) and for bistable alignment (see European Patent Publication No. 0744041). Use of a grating structure is discussed in “The Alignment of Liquid Crystals by Grooved Surfaces”, D. W. Berreman, Mol. Liq. Cryst. 23, pp 215-231, 1973. Use of such topographic features allows control of the ordering of the liquid crystal at the surface. More recently (as summarised in Konovalov et al, Asia Display 98, p379), the dielectric effect of larger scale surface features on the field geometry has been used to allow consistent alignment of Vertically Aligned Nematic (VAN) modes.
A key problem still remains in that in conventional arrangements, the effect of an applied electric field is realised mainly in the bulk, and a large bulk effect is required to change the alignment of the director near or at the surface. Consequently high voltages are required to generate such high fields across the cell, and control of switching could be usefully improved.
SUMMARY OF INVENTION
Accordingly, the invention provides a liquid crystal display cell, comprising: a liquid crystal layer having a thickness dimension and a bulk region bounded by first and second limits of extent of the liquid crystal layer in the thickness dimension; first and second electrode structures disposed for orienting liquid crystals in the liquid crystal layer; wherein said first and second electrode structures are displaced with respect to each other in said thickness dimension, and wherein at least one of the first and second electrode structures lies within the bulk region.
This has a number of advantages. As the vertical gap between electrodes is small, low drive voltages are needed to give a significant field between them (moreover, in preferred constructions a barrier layer can readily be used to protect the electrodes from each other). Moreover, this field is located in the region in which it can be particularly effective—adjacent to the surface. Electrodes can be patterned at high resolution without risk of shorting (because of the vertical displacement). Considerable control can be made of the topology of electric fields resulting, which can have important significance as the relationship of electric field direction to liquid crystal director is of central importance to effective switching. This relationship provides the torques experienced by the liquid crystal, and as these torques are provided near the surface (rather than in the bulk of the liquid crystal), they are faster acting and more effective. Yet a further advantage is that designs can be made which result in little or no electric field acting on the bulk, which can be beneficial as electric fields can affect liquid crystal optical properties.
Preferably, the first electrode structure lies adjacent to the bulk region and the second electrode structure lies within the bulk region: a particularly practical arrangement is for the first electrode structure to be formed on a first substrate and lie between the first substrate and the bulk region, with the second electrode formed on a raised structure formed on the first substrate and extending into the bulk region. This raised structure may have the same role as existing microscopic topographic structures do, of aligning the liquid crystal. The raised structure can thus be an array of ridges, but a new and useful alternative is to provide the raised structure as a layer perforated by an array of holes extending to the first electrode structure.
There may also be a third electrode structure on the opposite cell surface, and even a fourth electrode structure—the third and fourth electrode structures may have essentially the same types of features of first and second electrode structures. A useful approach, if there are first, second, third and fourth electrode structures, is for orthogonal liquid crystal alignments (by raised structures for liquid crystal alignment or otherwise) to be favoured at the two surfaces.
The present invention is particularly useful for nematic liquid crystals, both in cells that have a monostable or bistable (either zenithal or azimuthal) alignment without electric field.
REFERENCES:
patent: 5905557 (1999-05-01), Yaniv
patent: 6147666 (2000-11-01), Yaniv
patent: 0744041 (1997-10-01), None
patent: 99/18474 (1999-04-01), None
Bryan-Brown et al., Grating Aligned Bistable Nematic Devices, IDW '97, Defence Evaluation and Research Agency, pp. 261-264.*
Mike Towler, Future LC Technologies, Sharp Laboratories of Europe, Ltd., 1997, pp. 1-10.*
“Development of Super-TFT-LCDs with In-Plane Switching Display Mode” M. Ohta, M. Oh-e, K. Kondo. Electron Tube & Devices Div.,
Chowdhury Tarifur R.
Hewlett--Packard Company
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