Alignment of ferroelectric liquid crystal devices

Details Alignment of ferroelectric liquid crystal devices Alignment of ferroelectric liquid crystal devices

1998-08-04

2001-10-23

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S123000, C349S175000

Reexamination Certificate

active

06307610

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the alignment of ferro electric liquid crystal display (FLCD) devices.
2. Discussion of Prior Art
Such devices are formed by a thin layer of a chiral smectic liquid crystal material contained between two cell walls. The walls carry electrode structures, e.g. row and column electrodes forming an x,y matrix of addressable pixels. Electrical voltages are applied to the row and column electrodes in sequence so that all pixels are addressed in sequence. Prior to assembly, one or both of the inner faces of the cell wall are treated to an alignment treatment that provides both alignment direction and an amount of surface tilt to contacting liquid crystal material. Typical chiral tilted smectic materials have the following phases with decreasing temperature:—isotropic-cholesteric-smectic A-S*, where the asterisk denotes chirality and the smectic phase is tilted smectic such as smectic C, smectic I, etc.
One known type of FLCD is a surface stabilised (SSFLCD) device which is a bistable device. Switching between its two stable states is by application of a dc pulse of appropriate sign, amplitude, and time duration. Smectic liquid crystal molecules can be envisaged as rotating around the surface of a cone as they switch; the cone angle varies with material. Some devices show a minimum in their voltage vs. time switching characteristics (so called &tgr;v minimum). Such a device and a typical addressing scheme is described in GB-2,232,802.
It is known that FLCD can show a patchy appearance due to the smectic liquid crystal materials forming into a mixture of two different micro layer arrangements known as the C
1
and C
2
states (ref. J. Kanbe et al Ferroelectrics (1991) vol 114, pp 3). These micro layers are chevron shape and the angle between cell wall and micro layer tilt angle can be indicated by &dgr;. Both C
1
and C
2
can form as the material cools down from an isotropic state during manufacture; boundaries between these two states may be indicated by the known zigzag defect. Also, the two different states may form in a completed device, due to shock, e.g. mechanical damage when dropped or by application of too high an electric field etc.
Devices with a single state are required; preferably alignment in the C
2
state is obtained since this allows a faster switching at lower voltages. It has been a problem for several years to provide a clear, defect free device. Existing solutions include devices where only the C
1
state occurs as in U.S. Pat. No. 5,543,943 Hanyu et al. It is also known to use a high surface tilt &xgr; GB-2,210469, U.S. Pat. No. 4,977,264, to provide a device free of zigzag defects. GB-A-2,274,519 describes selection of a C
2
state by a surface pretilt &xgr; equal to or greater than 10°, a chevron angle &xgr; in the range 2° to 15°, a one half chiral smectic cone angle &thgr;
c,
such that &thgr;
c
−&dgr;≧&xgr;.
Good and consistent alignment is necessary for good devices. There are several ways of providing alignment. For example the walls may be coated with a polymer then unidirectionally rubbed, silicon oxide may be evaporated on to the cell (the so called oblique evaporation technique), and grating surfaces (formed by embossing or by photolithographic techniques). Such alignment treatment both give an alignment direction and a surface tilt to liquid crystal molecules in contact with the treated surface; this may be termed a surface alignment induced pretilt and given the symbol &xgr;; conventionally the pretilt is measured in a nematic phase. It is known to use long pitch in a cholesteric phase above the smectic phase e.g. GB-2,209,610, U.S. Pat. No. 5,061,047, GB-2,210,468B to improve alignment in the chiral smectic phase.
SUMMARY OF THE INVENTION
According to this invention the above problem is alleviated by a combination of material characteristics, surface alignment induced pretilt and alignment anchoring energy. Additionally, a voltage may be applied to the cell whilst the material is held at a reasonably constant temperature at or below a phase transition to a chiral smectic phase and above a device operating temperature range.
According to this invention a liquid crystal device comprises;
a liquid crystal cell including a layer of ferroelectric smectic liquid crystal material contained between two walls bearing electrodes and surface treated to give both an alignment and a surface tilt to liquid crystal molecules;
the surface pretilt &xgr; is equal to or greater than &thgr;−|&dgr;| over a device operating temperature range, a ratio of azimuthal to zenithal anchoring energies &bgr;/&agr; of at least 0.05 and a positive energy difference &Dgr;W
s
between the liquid crystal director at the surface in the C
1
and C
2
states the arrangement of angles and energies being arranged to cooperate so that the liquid crystal material preferentially adopts the C
2
state at device operating temperatures.
This may be achieved by careful control of the surface alignment properties by suitable election of material, rubbing strength, and baking temperatures. For example &bgr; is high on surfaces that have been strongly rubbed (pressure and time), whereas &agr; may be varied by choice of polymers (and possibly surfactants) to give the required surface free energy. The ration &bgr;/&agr; may be higher than 0.05, e.g. 0.1 or more.
The smectic layers form the C2 chevron geometry in which the component of the layer normal at each surface which is parallel to the surface normal has opposite sign to the component of the liquid crystal molecular director n at the pretilt and in the preferred alignment direction.
This is achieved by ensuring that the pretilt &xgr; is close to &thgr;−|&dgr;| (where &thgr; is the smectic C cone angle and &dgr; is the smectic layer tilt angle) at some temperature (T
2
) in the smectic C phase above an operating range (e.g. −10 to 60°, preferably 0 to 40° C.) and uniformly throughout the device. Preferrably &xgr;≧&thgr;−|&dgr;| within about 5° of smectic phase transition T
1
.
Having obtained the desired C2 state it is maintained by ensuring &xgr;≧&thgr;−|&dgr;|, preferably &xgr;>&thgr;−|&dgr;|, across the temperature range for operation.
Preferably &Dgr;W
s
and the ratio &bgr;/&agr; are as large as possible. For a given value of &bgr;/&agr;, the pretilt &xgr; is optimised to give &Dgr;W
s
positive and as large as possible across a temperature range of operation. Preferably the material changes from a C
1
state to a C
2
as close to a smectic A to a chiral smectic phase transition (T
1
) as possible, e.g. within 10° C. or less.
The device may further comprise means for applying voltages to row and column electrodes in the cell whereby the complete device may be addressed.
The device may further include means for heating the cell, allowing the cell to cool a predetermined temperature, and means for applying an ac voltage across the liquid crystal layer for a predetermined time. After the heating, the cell may then be cooled to ambient at a controlled rate. Such treatment may be performed to remove damage to alignment within the cell, e.g. caused by impact or other mishap subsequent to device manufacture.
According to this invention a method of making a ferroelectric liquid crystal device comprises the steps of:
providing a liquid crystal cell having a layer of a chiral smectic liquid crystal material contained between two cell walls bearing electrodes and surface treated to provide both an alignment direction and a surface pretilt to liquid crystal molecules;
providing a surface treatment giving a pretilt &xgr; and a ratio of azimuthal to zenithal anchoring energies &bgr;/&agr; which gives a positive energy difference &Dgr;W
s
between the liquid crystal director at the surface in the C
1
and C
2
states whereby the liquid crystal material preferentially adopts the C
2
state at device operating temperatures;
heating the liquid crystal mater

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