Multi-domain surface mode device

Details Multi-domain surface mode device Multi-domain surface mode device

1999-05-27

2003-07-15

Parker, Kenneth (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S144000

Reexamination Certificate

active

06593987

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of visual displays and image processing devices such as flat-screen computer displays, televisions, 3D displays, projection devices, time-sequential color systems and head-mount displays.
DESCRIPTION OF RELATED ART
U.S. Pat. No. 4,56,758 discloses a uniformly aligned 180° twisted surface mode liquid crystal. The pi-cell and the chirally doped variant of the pi-cell described in U.S. Pat. No. 4,566,758 are so-called surface mode liquid crystal devices. Both are characterized by having the molecules in the central region of the liquid crystal remaining aligned almost perpendicularly to the cell surfaces during operation of the device, whilst molecules close to the surfaces undergo reorientation in response to varying electric fields. However these devices suffer a disadvantage in that the optical characteristics vary non-uniformly when the device is viewed from different angles. This variation in viewing characteristics with angle is undesirable when designing, for instance, a television screen to be viewed from a wide range of angles.
GB 2 318 880 A discloses a liquid crystal device with two aligned but only one pretilted surface. However embodiments include 90° twisted nematic rather than 180° twisted surface mode device or pi-cell.
EP 0 750 210 A1 discloses a multi-domain liquid crystal device, in which the liquid crystal is mixed with some amount of polymer but no chiral dopant and is preferably 90° twisted.
EP 0 768 560 A1 discloses a mult-domain liquid crystal device in which one surface has vertical alignment.
EP 0 632 311 A1 discloses a multi-domain liquid crystal device which allows an amount of chiral dopant giving d/p<0.75, but does not describe the operation and viewing performance of a 180° twisted surface mode device as in the present invention.
U.S. Pat. No. 5,710,609 describes a 180° twisted liquid crystal device which includes a liquid crystal of negative dielectric anisotropy.
U.S. Pat. No. 5,504,604 describes a multi-domain liquid crystal device which requires first and second liquid crystal layers with first and second pre-tilt angles.
U.S. Pat. No. 4,635,051 discloses the use of pi-cells as switchable optical retardation elements. The pi-cell is a surface switching device which utilizes a positive dielectric anisotropy liquid crystal confined between two surfaces which have been treated such as by rubbing in such a way as to induce unidirectional alignment of the liquid crystal with a small tilt (typically 2-5°) of the molecules away from the surface (‘pretilt’).
P. D. Berezin et. al., Sov.J.Quant.Electron Vol. 3(1) pp.78-79 (1973) discloses the principle of liquid crystal surface mode switching in the form of a voltage-tunable birefringent waveplate. A layer of nematic liquid crystal is arranged with its optic axis at 45° to the orthogonal polarizing directions of two polarizers. The device is operated between voltages such that significant molecular switching only occurs near the surfaces of the liquid crystal layer.
EP 0 616 240 A1 describes the use of a fixed uniform optical retardation film with optic axis crossed with respect to the rubbing direction of a pi-cell.
FIGS. 1A
to
1
D of the accompanying drawings illustrate diagrammatically the construction and operation of a typical pi-cell. The cell comprises a substrate
1
of transparent glass or plastic coated with a transparent electrode
2
, for instance of indium tin oxide (ITO). The electrode
2
is coated with a thin layer
3
of a polymer, such as polyimide, which is unidirectionally rubbed or buffed so as to define the alignment direction and pretilt angle for nematic liquid crystal molecules
4
. The opposite cell wall comprises a substrate
11
, an electrode
12
and an alignment layer
13
of the same type as the substrate
1
, the electrode
2
and the alignment layer
3
, respectively. The cell walls are spaced apart by spherical glass or plastic spacers (not shown) to define a cell thickness which is typically between 1 and 25 micrometers. The cell walls are aligned with their rubbing directions parallel and the cell is sealed so as to contain a layer of liquid crystal material of positive dielectric anisotropy.
The pi-cell is operated with its molecules in a “bend configuration”. Above a certain voltage between the electrodes
2
and
12
, typically of the order of 2 volts RMS, the liquid crystal director favors the adoption of the bend-configuration with the molecules in the central region
5
of the cell being aligned along the applied electric field. At higher fields, the bend-configuration distorts as more of the molecules align along the direction of the applied field.
FIG. 1A
illustrates a bend-configuration at a typical operating voltage whereas
FIG. 1B
illustrates the further distortion which occurs with increased operating voltage. Thus, in the bend-configuration, only the surface regions
6
distort when the applied voltage is varied whereas the central region
5
remains substantially static.
Because liquid crystal materials are optically anisotropic or “birefringent”, when the pi-cell is disposed between two polarizers with the surface alignment directions of the cell oriented at 45° to the absorption axes of the polarizers, a variation in optical transmission accompanies distortions in the liquid crystal director structure. Thus, a voltage-tunable optical shutter can be produced and can form the basis of a display device. Usually, when the pi-cell is operated as a transmissive device, a range of operating voltages is selected such that, at a lower operating voltage, the pi-cell exhibits approximately a half-wave optical retardation at a predetermined wavelength (normally approximately 550 nanometers). The device or picture element then appears bright. At an upper operating voltage, the pi-cell exhibits approximately zero optical retardation and so looks dark between the crossed polarizers. Switching between the upper and lower operating voltages produces optical switching between bright and dark states, with intermediate voltages producing intermediate grey levels.
It is known that, if a voltage is applied to a pi-cell in its bend configuration and the voltage is then reduced towards zero, below a certain level which is typically about 2 Volts, a 180° twisted director configuration appears briefly in the cell as illustrated in FIG.
1
D. This twisted structure is typically replaced shortly thereafter by the nucleation and growth of the splay configuration.
JP 9-90432 discloses twisted devices with twists of substantially 360°.
WO 97/12275 discloses another type of surface switching liquid crystal device using two parallel aligned surfaces. However, in this case, a relatively high pretilt, typically between 80° and 90°, is produced by the alignment layers and the liquid crystal material is of the negative dielectric anisotropy type.
The 180° twisted surface mode discussed herein is not to be confused with the 180° twisted so-called STN device disclosed, e.g., in UK Patent GB 2 123 163B. The STN is a device designed specifically to exhibit a very sudden change in optical transmission with voltage. This allows it to be used with so-called “multiplex drive schemes” and it does not use an active matrix of, e.g., thin film transistors. The surface mode devices described herein require an active matrix as discussed later in connection with
FIG. 12. A
disadvantage of the STN device is that its switching speed is relatively slow. In particular, switching between states typically takes more than 100 milliseconds whereas the switching times of typical surface mode devices are on the order of 10 milliseconds. Thus, STN devices are unsuitable for use as video rate displays or in other applications where switching speeds of 20 milliseconds or less are required.
SUMMARY OF THE INVENTION
The present invention relates to surface mode nematic liquid crystal devices with improved angular viewing characteristics. Conventional liquid crystal devices consist of a layer of liquid crystal confined between two glass or plas

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