Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1998-08-28
2001-01-23
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S172000, C349S143000
Reexamination Certificate
active
06177968
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an optical modulation device, particularly a liquid crystal device, for use in a display apparatus for displaying characters and images.
There have been known conventional liquid crystal (optical modulation) devices of, e.g., a simple matrix-type using a combination of stripe-shaped electrodes.
FIG. 14
shows an embodiment of such a conventional simple matrix-type liquid crystal device.
Referring to
FIG. 14
, a color liquid crystal device principally includes a pair of oppositely disposed transparent substrates
1
and
11
one of which is provided with color filters
13
(
13
a
,
13
b
and
13
c
), and a liquid crystal
16
disposed between the substrates
1
and
11
.
On the substrate
1
, the color filters
13
or red (R)
13
a
, green (G)
13
b
and blue (B)
13
c
and a black matrix (e.g., a stripe-shaped light-interrupting layer)
12
disposed between the respective color filters
13
a
,
13
b
and
13
c
are formed and further thereon, a protective (flattening) layer
14
is formed. On the protective layer
14
, a plurality of stripe-shaped electrodes
142
each provided with an auxiliary electrode
141
of a low-resistance material (e.g., metal) and are coated with an alignment film
143
contacting the liquid crystal
16
.
On the other transparent substrate
11
, a plurality of stripe-shaped electrodes
145
each provided with an auxiliary electrode
144
of a low-resistance material and intersecting the stripe-shaped electrodes
142
(on the opposite substrate
1
) at right angles to form an electrode matrix and are coated with an alignment film
146
contacting the liquid crystal
16
.
The transparent substrates
1
and
11
are applied to each other at the periphery thereof with a sealing agent (not shown) while leaving a prescribed call gap together with spacer beads
15
within the cell structure. The cell gap is filled with the liquid crystal
16
, thus preparing a color liquid crystal device.
Such a conventional color liquid crystal device may, e.g., have a matrix electrode structure and may be driven by, e.g., a set of drive waveforms as described
FIG. 3.6
on page 90 of “Liquid Crystals-Application Book (Ekisho-Oyo Hen in Japanese)” edited by Koji Okano and Shunsuke Kobayashi (1985) (K. K. Baihukan).
Generally, in the above liquid crystal device, one (group) of the stripe-shaped electrodes
142
and
145
is supplied with a scanning signal and the other (group) is supplied with a data signal.
As a display region size of the liquid crystal device is enlarged, the number of pixels thereof is correspondingly increased, thus resulting in an increase in number of scanning signal lines and data signal lines. For instance, there are various large-area liquid crystal devices in accordance with, e.g., a VGA (video graphics array) standard (scanning lines x data lines=480×600), an XGA (extended video graphics array) standard (768×1024) and an SXGA (super extended graphics array) standard (1024×1280).
As a result, such large-area liquid crystal devices have an increased capacitance between opposite electrodes and an increased amount of a charging current, thus requiring a larger current-carrying capacity of a driver IC (integrated circuit) used.
This problem is more noticeable in a surface-stabilized ferroelectric liquid crystal (SSFLC) display device using a ferroelectric liquid crystal.
This may be attributable to the following two factors (1) and (2).
(1) The ferroelectric liquid crystal device generally has a smaller cell gap (a distance between opposite electrodes) of 1&mgr;2 &mgr;m when compared with a conventional twisted nematic (TN) liquid crystal device having a cell gap being several times that of the ferroelectric liquid crystal device, thus having a larger capacitance between opposite electrodes. In the ferroelectric liquid crystal device, it is necessary to set a smaller cell gap in order to suppress a twisted alignment state of the ferroelectric liquid crystal molecules intrinsic thereto by constraint forces of the substrates to realize bistability as described in, e.g., N. A. Clark et al., “MCLC”, vol. 94, pp. 213-234 (1983).
(2) The ferroelectric liquid crystal has a spontaneous polarization on which an external electric field acts, thus effecting switching of two stable states (bistable stables). During the switching, a polarization inversion current passes across the liquid crystal layer and becomes larger with an increasing spontaneous polarization. When the switching speed is increased by using a higher frequency of line-sequential scanning, the ferroelectric liquid crystal used is required to a larger spontaneous polarization.
The above factors (1) and (2) affect propagation delay of an input voltage (signal) waveform and an amount of heat evolution (generation) and a temperature distribution within a panel (liquid crystal device).
The heat evolution within the panel is (directly) proportional to a capacitance of the panel and an amount of a current at the time of fluctuation of liquid crystal molecules by the action of a non-selection signal application (application of a data signal to a selected data signal line on a non-selection pixel). The applied non-selection signal interacts with the spontaneous polarization passes through the panel. The degree of heat evolution due to the capacitance of the panel is comparable to that due to the fluctuation of a spontaneous polarization of 35 nC/cm
2
. The heat evolution is also closely related to a drive frequency. As the drive frequency is decreased (i.e., a drive pulse width is increased), a current consumption due to the fluctuation of the spontaneous polarization becomes larger.
As a result, a heat evolution amount distribution within the panel becomes large because a temperature dependence of a threshold value of the ferroelectric liquid crystal is large (e.g., a degree of the threshold value change is about 10% by the temperature charge of about 1-2° C.) when compared to the nematic liquid crystal, thus largely affecting display qualities.
In the conventional cell structure of the liquid crystal device, each of opposite two substrates is provided with a plurality of stripe-shaped electrodes to be electrically connected with an external drive circuit, thus requiring lead-out portions to be connected with the drive circuit for the respective substrates. As a result, the liquid crystal device is accompanied with some problems in terms of a liquid crystal injection, an electrical connection with IC circuit(s) and an incorporation of the cell into a housing of the panel. These problems also arise in the nematic liquid crystal device. In the case of a conventional device having decreased scanning signal lines for display in watches, the above problems are remedied by using a means for connecting the scanning signal lines at the substrate provided with data signal lines via a conductor of, e.g., silver paste to effect electrical connection on the same substrate side. However, in a large-area liquid crystal device having a large number of pixels (according to the above-mentioned VGA, XGA, SXGA standards), it is substantially difficult to effect such an electrical connection on the same substrate side by drawing the lead-out portions (for electrical connection) disposed on one substrate onto the other substrate.
With respect to the ferroelectric liquid crystal device, there has been also known a phenomenon such that ion impurities within a liquid crystal cell are localized on a pair of opposite substrates by an internal electric field formed by the spontaneous polarization of liquid crystal molecules, as described in Yutaka Inaba et al., “Ferroelectrics”, vol. 85, pp. 255-264 (1988).
Within the cell, an (ion) electric field formed by ions of the localized ion impurities is balanced with the internal electric field due to the spontaneous polarization. The ion electric field, however, is not removed instantly when the direction of the spontaneous polarization of the liquid crystal is inverted by external electr
Enomoto Takashi
Hachisu Takahiro
Inaba Yutaka
Munakata Hirohide
Okada Shinjiro
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Sikes William L.
Ton Toan
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