Liquid crystal device and process for production thereof

Details Liquid crystal device and process for production thereof Liquid crystal device and process for production thereof

1997-03-06

2001-11-20

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S129000, C349S133000

Reexamination Certificate

active

06320639

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a liquid crystal device having a pixel region and an outside-pixel region providing different liquid crystal alignment states allowing improved performances and a process for production thereof, particularly such a liquid crystal device using a liquid crystal having bistability, such as a chiral smectic liquid crystal, and a process for production thereof.
A liquid crystal device of a type which controls transmission of light by utilizing the refractive index anisotropy of liquid crystal molecules in combination with a polarizing device has been proposed by Clark and Lagerwall (U.S. Pat. No. 4,367,924). The liquid crystal used in the liquid crystal device is generally a bistable liquid crystal, such as a chiral smectic liquid crystal which has chiral smectic C phase (Sm*C) or H phase (Sm*H) in specific temperature range and, under this state, shows a property of taking either one of a first optically stable state and a second optically stable state in response to an electric field applied thereto and maintaining such a state in the absence of an applied electric field, namely bistability, and also has a quick responsiveness to a change in the electric field. Accordingly, such a chiral smectic liquid crystal device (ferroelectric liquid crystal device) is expected to be widely utilized as a high-speed and memory-type display device.
Further, in recent years, a study on a bistable twisted-nematic (BTN)-type liquid crystal device using a liquid crystal in chiral nematic phase (N*) has been made.
Such a liquid crystal device is generally constituted by disposing, e.g., a chiral smectic liquid crystal between scanning electrodes and data electrodes constituting in combination an electrode matrix, and driven according to a multiplexing drive scheme of sequentially applying a scanning signal to the scanning electrodes and applying data signals to the data electrodes in synchronism with the scanning signal to change the orientation states of liquid crystal, i.e., turn on or off the liquid crystal, at pixels.
Such a liquid crystal device generally has a structure including a pair of substrates which have electrodes and optionally drive elements thereon and are provided with some aligning treatment, and a liquid crystal sandwiched between the substrates. As a result, such a liquid crystal device is provided with a plurality of display pixels (hereinafter simply referred to as “pixel(s)” functioning to effect a data display independently from each other and a separation region (hereinafter called “outside-pixel region”) separating adjacent pixels and allowing such an independent data display. The outside-pixel region may include a pixel-spacing region for electrically isolating the pixels from each other where no transparent electrodes or metal electrodes are present, and a region not effective for display where auxiliary metal electrodes, etc., are disposed, e.g., for preventing a delay in electrical signal transmission liable to occur accompanying a size enlargement of liquid crystal device. In the case where such auxiliary metal electrodes are not provided, the outside-pixel region and the pixel-spacing region are identical to each other.
Incidentally, when such a liquid crystal device is driven according such a multiplexing drive scheme, the liquid crystal in, e.g., an outside-pixel region
80
c
between pixels
80
a
and
80
b
as shown in
FIG. 22
assumes an ununiform mixture alignment state including white and black domains
81
a
and
81
b
due to influence of molecular alignments in the pixels
80
a
and
80
b
representing a white-displaying pixel and a black-displaying pixel, respectively. The presence of such mixture domains
81
a
and
81
b
in an outside-pixel region
80
c
is liable to result in a low-quality picture giving a rough appearance as a whole.
The above difficulty is presumably caused by a phenomenon that it is difficult for the liquid crystal at the outside-pixel region
80
c
to have an alignment state (principally, a pretilt) which is remarkably different from those at the pixels
80
a
and
80
b
, and the liquid crystal at the outside-pixel region
80
c
is caused to have a bistable alignment state similarly as at the pixels
80
a
and
80
b
, thus resulting in locally ununiform regions leading to a rough appearance of picture as a whole. This difficulty is also encountered in case where a bistable chiral nematic liquid crystal is subjected to multiplexing drive.
On the other hand, in order to suppress such a rough display appearance, it has been practiced to dispose a (light-)masking or shade layer at the outside-pixel region. If the masking layer is formed as a metal film, the formation and photolithographic steps for patterning thereof incur an increase in production cost. Further, it is well known that a liquid crystal device using a bistable liquid crystal is rather vulnerable to an impact, and the fixation of the substrates for alleviating the difficulty is also liable to incur a production cost increase.
Further, a chiral smectic liquid crystal device, such as a ferroelectric liquid crystal device requires a small cell gap (gap between the substrates) on the order of 1 &mgr;m—several &mgr;m and, when a liquid crystal device is enlarged in a planar size while keeping such a small cell gap, it becomes critically important to provide the device with a liquid crystal layer in a uniform thickness, i.e., a uniform cell gap, in order to ensure a uniform display over an entire effective optical modulation area (an entire display area in case of a display device).
Accordingly, in order to provide a liquid crystal device capable of a uniform display over the entire area, it has been generally practiced to disperse spacer beads
14
p
of a uniform diameter in the device as shown in FIG.
23
. Further, as the liquid crystal device is further enlarged in area, it becomes difficult to retain a uniform cell gap in a central region of the device by holding substrates llaa and llbb to each other only at their peripheries, so that it has been also practice to disperse a particulate adhesive resin
15
p
within the liquid crystal device so as to adhere to the substrates
11
aa
and
11
bb
, thereby preventing the cell gap from expanding in excess of the spacer diameter and retaining a uniform gap.
As shown in
FIG. 24
which is a sectional view taken along a line X-Y in
FIG. 23
, in such a conventional liquid crystal device, a stepwise unevenness appears at a boundary region (encircled by an oval OV in
FIG. 24
) between an auxiliary metal electrode
19
p
provided for alleviation of signal transmission delay along an edge of a transparent electrode
12
aa
or
12
bb
constituting a pixel P for data display and a pixel-spacing region for electrically separating adjacent pixels.
When an alignment film
13
aa
or
13
bb
covering such a stepwise unevenness on the substrate
11
aa
or
11
bb
is subjected to rubbing as a uniaxial aligning treatment generally used at present, there arises a difference in pretilt angles caused in the respective regions.
More specifically, when the alignment films
13
aa
and
13
bb
are subjected to a uniaxial aligning treatment, such as rubbing in a direction A as shown in
FIG. 25
, an elevated portion MP is subjected to a stronger rubbing and a depressed portion WP is subjected to a weaker rubbing, respectively compared with a pixel region, thus resulting in a high and low pretilt distribution and a discontinuous alignment characteristic.
When such a pixel P accompanied with a discontinuous alignment characteristic is driven for display by applying drive signals to the transparent electrodes
12
aa
and
12
bb
, a portion
17
A of the liquid crystal above the auxiliary metal electrode
19
s
is supplied with a stronger electric field to cause an earlier memory state disorder or premature switching than a portion
17
B of the liquid crystal at the pixel P. Further, the disorder generates a domain in a state opposite to the stable state in the pixel, which domain is

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