Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-06-07
2004-04-27
Chowdhury, T. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S122000, C349S169000
Reexamination Certificate
active
06727968
ABSTRACT:
This invention relates to liquid crystal device alignment.
Liquid crystal devices typically comprise a thin layer of a liquid crystal material contained between cell walls or substrates. 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.
Many different modes of liquid crystal devices are known in the art, for example the twisted nematic device the cholesteric phase change device, the dynamic scattering device, the supertwisted nematic device and the surface stabilised ferroelectric device modes. 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. For many applications of liquid crystal devices, such a treatment is considered necessary in order to impose a particular configuration on the alignment of the liquid crystal fluid throughout the device and/or to provide an optical appearance in the device which is free of apparent optical defect. The particular significance of this factor for different classes of liquid crystal device is described in greater detail below.
The terms azimuth or azimuthal is used hen in to define the molecular (or director n) alignment angle movement or energy in the plane of the substrate surface. The terms zenith or zenithal is used herein to define the molecular alignment angle movement or energy in a plane normal to the substrate surface.
In respect of use of nematic and long pitch cholesteric materials for devices known as twisted nematic liquid crystal devices, the relevance of alignment and the problems associated therewith are as follows.
In order to provide displays with a large number of addressable elements it is common to make the electrodes as a series of row electrodes on one wall and a series of column electrodes on the other cell wall. These form typically an x,y matrix of addressable elements or pixels and for twisted nematic types of device are commonly addressed using rms addressing methods.
Twisted nematic (TN) and phase change devices are switched to an ON state by application of a suitable voltage and allowed to switch to an OFF state when the applied voltage falls below a lower voltage level, i.e. these devices are monostable. For a twisted nematic type of device (90° or 270° twist as in U.S. Pat. No. 4,596,446) the number of elements that can be rms addressed is limited by the steepness of a device transmission verses voltage curve (as described by Alt and Pleschko in IEEE Trans ED vol ED 21, (1974) P.146-155). One way of improving the number of pixels is to incorporate thin film transistors adjacent to each pixel; such displays are termed active matrix displays.
An advantage of nematic types of devices is the relatively low voltage requirements. They are also mechanically stable and have a wide temperature operating range. This allows for the construction of small and portable battery powered displays. An alternative twisted nematic device is one which is switched from a non-twisted state at zero volts to a twisted state at a higher voltage, as described in GB 9607854.8, which will be referred to in this patent as a VCT device.
One problem with the twisted nematic device is that the contrast ratio of a normally white display remains at a low value until the voltage is increased to a value considerably higher than the threshold voltage. This is due to the nematic material close to the cell walls which does not fully reorient in the applied field due to the strong zenithal anchoring imposed by the surface alignment layer. This lack of surface reorientation also leads to higher voltage operation in the VCT device.
In respect of use of nematic and long pitch cholesteric materials for devices known as bistable nematic liquid crystal devices, the relevance of alignment and the problems associated therewith are as follows.
As described above, twisted nematic and phase change type of liquid crystal devices are switched to an ON state by application of a suitable voltage, and allowed to switch to an OFF state when the applied voltage falls below a lower voltage level, i.e. these devices are monostable. An advantage of nematic type of devices is that they have relatively low voltage requirements. They are also mechanically stable and have wide temperature operating ranges. This allows for the construction of small and portable battery powered displays. A disadvantage of such devices is that their monostable switching characteristic limits the number of lines that can be multiplex addressed.
Another way of addressing large displays is to use a bistable liquid crystal device. Ferroelectric liquid crystal displays can be made into bistable devices with the use of smectic liquid crystal materials and suitable cell wall surface alignment treatment. Such a device is a surface stabilised ferroelectric liquid crystal device (SSFELCDs) as described by:- L J Yu, H Lee, C S Bak and M M Labes, Phys Rev Lett 36, 7, 388 (1976); R B Meyer, Mol Cryst Liq Cryst. 40, 33 (1977); N A Clark and S T Lagerwall, Appl Phys Lett, 36, 11, 899 (1980). One disadvantage of ferroelectric devices is the relatively large voltage needed to switch the material. This high voltage makes small portable, battery powered displays expensive. Also these displays suffer from other problems such as lack of shock resistance, limited temperature range and also electrically induced defects such as needles.
If a bistable switching characteristic can be achieved using nematics then a display can be made which has the merits of both the above mentioned technologies but without their problems.
It has already been shown by Durand et al that a nematic can be switched between two alignment states via the use of chiral ions or flexoelectric coupling: A Charbi, R Barberi, G Durand and P Martinot-Largarde, Patent Application No WO 91/11747, (1991) “Bistable electrochirally controlled liquid crystal optical device”, G Durand, R Barberi, M Giocondo, P Martinot-Largarde, Patent Application No WO 92/00546 (1991) “Nematic liquid crystal display with surface bistability controlled by a flexoelectric effect”.
U.S. Pat. No. 4,333,708 describes a multistable liquid crystal device in which cell walls are profiled to provide an array of singular points. Such substrate configurations provide multistable configurations of the director alignments because disclination must be moved to switch between stable configurations. Switching is achieved by application of electric fields
Patent Application No:. WO97/14990, (PCT-96/02463, GB95 21106.6) describes a bistable nematic device having a grating surface treatment to at least one cell wall that permits nematic liquid crystal molecules to adopt either of two pretilt angles in the same azimuthal plane. The cell can be electrically switched between these two states to allow information display which can persist after the removal of power.
Another bistable nematic device is described in GB.2,286,467-A. This uses accurately formed bigratings on at least one cell wall. The bigrating permits liquid crystal molecules to adopt two different angular aligned directions when suitable electrical signals are applied to cell electrodes e.g. dc coupling to flexoelectric polarisation as described in Patent Application No. WO.92/00546. Since in the two splayed states the director is quite close to being in the plane of the layer, the coupling between director and flexoelectric component can be small, which may hinder switching in some circumstances.
The bistable nematic device of GB2286467-A also has a further problem which is not present in ferroelectric devices, that is, the need to switch the surface layer of molecules in order to eliminate image sticking effects. Surface layer switching usually requires high voltages which leads to both high power consumption and the need for customised driver circuitry.
In respect of devices using smectic materials, the relevance of alignment and the problems associated therewith are
Bryan-Brown Guy Peter
Hui Victor Chaklam
Jones John Clifford
Sage Ian Charles
Wood Emma Louise
Chowdhury T.
Di Grazio Jeanne
Hewlett--Packard Development Company, L.P.
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