Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within nematic phase
Reexamination Certificate
1999-10-19
2003-01-28
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within nematic phase
C349S129000
Reexamination Certificate
active
06512569
ABSTRACT:
The present invention relates to a liquid crystal display device, and in particular to a surface mode LCD such as a pi-cell device or a splay-bend device (SBD). The application also relates to a method of manufacture of these devices. The invention also relates to a substrate having one region of high pre-tilt and another region of lower pre-tilt, and to a method of manufacture of such a substrate.
The term “surface mode LCD” as used herein means a LCD in which the optical change caused by varying the electric field across the liquid crystal layer occurs primarily in the surface layers of the liquid crystal. Examples of surface mode LCDs are the pi-cell and the splay-bend device, although other types of surface mode LCDs are known. Surface mode LCDs are disclosed in “Sov. J. QE”, 1973, Vol 3, p78-79.
The pi-cell (otherwise known as an “optically compensated birefringent device” or OCB) is described in “Mol. Cryst. Liq. Cryst.”, 1984, Vol 113, p329-339, and in U.S. Pat. No. 4,635,051. The structure of a pi-cell is schematically illustrated in FIG.
1
. The device comprises transparent substrates
1
,
1
′ on which are disposed alignment layers
2
,
2
′. A layer of nematic liquid crystal
3
is disposed between the substrates
1
,
1
′.
The alignment layers
2
,
2
′ create parallel alignment of the liquid crystal molecules in the liquid crystal layer
3
at its boundaries with the alignment layers
2
,
2
′. This can be achieved by using parallel-rubbed polyimide alignment layers.
Addressing electrodes (not shown) are provided on the substrates
1
,
1
′, so that an electric field can be applied to selected areas of the liquid crystal layer. The liquid crystal layer
3
is placed between linear polarisers
4
,
4
′, whose transmission axes are crossed with one another and are at 45° to the optic axis of the liquid crystal layer.
A retarder
5
, with its optic axis parallel to the optic axis of the liquid crystal layer, may optionally be provided to compensate for the retardation of the liquid crystal layer. The retarder lowers the required range for the operating voltage by allowing zero retardation of the LCD to be achieved at a finite voltage across the liquid crystal layer.
The principle of operation of the pi-cell device is illustrated in FIGS.
2
(
a
) to
2
(
c
).
When no electric field is applied across the liquid crystal layer, the liquid crystal is in an H-state (homogenous state), in which the liquid crystal molecules in the centre of the liquid crystal layer are substantially parallel to the substrates. This is shown in FIG.
2
(
a
). The short lines in the figures represent the director of the liquid crystal molecules.
When an electric field greater than a threshold value is applied across the liquid crystal layer, the liquid crystal molecules adopt a V-state (or a bend state). In this state, the liquid crystal molecules in the centre of the liquid crystal layer are substantially perpendicular to the substrates. FIG.
2
(
b
) shows a first V-state which occurs at a low applied voltage across the liquid crystal layer, and FIG.
2
(
c
) shows a second V-state which occurs when a higher voltage is applied across the liquid crystal layer. The pi-cell is operated by switching the liquid crystal layer between the first, low voltage V state and the second, higher voltage V-state.
If the electric field across the liquid crystal layer should be reduced below the threshold value, the liquid crystal layer will relax to the H-state of FIG.
2
(
a
); in order to re-commence operation of the device, it is necessary to put the liquid crystal layer back into the V-state. This generally requires a large applied voltage, owing to the low pre-tilt of the liquid crystal molecules. The pre-tilt is usually below 45° and typically between 2 and 10° so as to provide sufficient optical modulation and fast switching between the two V-states (for instance of the order of a millisecond or less).
One problem with known OCB devices is the difficulty of nucleating and stabilising the V-state, which is topologically distinct from the H-state. One prior art technique is described in UK Patent Application 9521043.1. In this prior technique, the V-state is nucleated under the application of a high voltage, and is stabilised by the polymerisation of a network whilst a high voltage is applied. This prior art technique is, however, unsuitable for use in active matrix devices, since it is difficult to apply voltages having the required magnitude in a TFT panel. A further disadvantage is that the in-situ polymerisation can lead to ionic contamination of the liquid crystal layer, and result in image sticking.
The SBD device, which is also a surface mode device, is described in UK Patent Application No. 9712378.0. The structure of a SBD device is generally similar to that of a pi-cell, except that the alignment layers in a SBD device have a high pre-tilt whereas the alignment layers in a pi cell have a low pre-tilt. An SBD device uses a liquid crystal material with a negative di-electric anisotropy, whereas a pi-cell uses a liquid crystal material having a positive di-electric anisotropy.
The principle of operation of a SBD is illustrated in FIGS.
3
(
a
) to
3
(
c
). When no voltage is applied across the liquid crystal layer, a V-state is stable as shown in FIG.
3
(
a
). When an electric field greater than a threshold value is applied across the liquid crystal layer, an H-state becomes stable. FIG.
3
(
b
) shows a first H-state which occurs at a low applied voltage across the liquid crystal layer, and FIG.
3
(
c
) shows a second H-state which occurs when a higher voltage is applied across the liquid crystal layer. In operation, the device is switched between the low voltage H-state of FIG.
3
(
b
) and the high voltage H-state of FIG.
3
(
c
). If the electric field across the liquid layer is reduced below the threshold value, the liquid crystal will relax into the V-state, and it will be necessary to put the liquid crystal back into the H-state before operation can be re-commenced.
The high pre-tilt alignment layers required for a SBD can be produced, for example, by the photo-polymerisation of a mixture of reactive mesogens, as described in UK Patent Application No. 9704623.9.
SID 97 Digest, page 739, discloses a method of promoting nucleation of the V-state in a pi-cell. Voltages of the order of 20 V are applied across the liquid crystal layer to switch the liquid crystal from the H-state to the V-state. However, it is difficult to provide voltages of this magnitude in a TFT (thin film transistor) substrate.
Japanese published Patent Application JP-A-9 90432 (Toshiba) discloses the provision of nucleation sites within a pi-cell panel. The nucleation sites are provided by including spacer balls or pillars within the pi-cell panel, and cooling the liquid crystal material from an isotropic phase to a nematic phase while an electric field is applied across the panel. This results in some of the spacer balls/pillars acting as nucleation sites for growth of the V-state into the H-state. This prior art has a number of disadvantages. Firstly, it requires additional process steps during fabrication of the panel, since it is necessary to align the liquid crystal molecules under the influence of an applied electric field. These additional process steps complicate the fabrication of the panel. Secondly, some spacer balls/pillars can nucleate the H-state into the V-state thus destabilising the operating state of the panel.
Japanese published Patent Application JP-A-9 218411 discloses an LCD having a bend alignment state which is stabilised, in the absence of an applied field, by the presence of spacers in the form of spherical particles. The spacers have a surface energy such that liquid crystal molecules adjacent the alignment layers are mainly aligned parallel to the alignment layers. However, in order for this technique to work, a field has to be applied during the initial alignment of the device. Also, the particles cannot be positioned so as to be outside the pixe
Acosta Elizabeth J.
Tillin Martin D.
Towler Michael J.
Walton Harry G.
Dudek James
Renner , Otto, Boisselle & Sklar, LLP
Sharp Kabushiki Kaisha
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