Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within smectic phase
Reexamination Certificate
2000-07-07
2002-02-26
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within smectic phase
C349S184000
Reexamination Certificate
active
06351301
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a smectic liquid crystal and a liquid crystal display using the same. More particularly, the present invention relates to a smectic ferroelectric liquid crystal which enables grayscale display, and a liquid crystal display using the same.
2. Description of the Related Art
A liquid crystal display using a surface stabilized ferroelectric liquid crystal (SSFLC) is proposed as a liquid crystal display in which a wide viewing angle and a quick response can be expected. Such a liquid crystal display is disclosed in “Submicrosecond bistable electro-optic switching in liquid crystals”, N. A. Clark et al., Appl. Phys. Lett., 36, pp.899-901 (1980). The SSFLC has a spontaneous polarization which is inverted by the application of an external electric field. This inversion causes a direction of a liquid crystal molecule in the SSFLC to be switched. Two states of a bright state and a dark state are generated depending on the direction of the liquid crystal molecule in the SSFLC.
However, the SSFLC is bi-stable. The SSFLC display has the only two states of the bright state and the dark state. Thus, conventionally, the SSFLC liquid crystal display has a problem that the grayscale display is difficult.
A method for driving the SSFLC by using an AC stabilization effect is disclosed in “A Multiplexed Ferroelectric LCD Using ac Field-Stabilized States”, J. M. Geary, SID 85 Digest, pp.128-130. Also, a SSFLC display using the &tgr;−V
min
drive method is disclosed in “The ‘Joers/Alvey’ Ferroelectric Multiplexing Scheme”, P. W. H.Surguy et al., Ferroelectric, 122, pp.63-79 (1991), “Color Digital Ferroelectric LCDs for Laptop Applications”, P. W.Ross et al., SID 92 Digest, pp.217-220 (1992), and Japanese Laid Open Patent Application (JP-A-Heisei 9-318921). However, the problem can not be solved by the drive method for driving the SSFLC by using the AC stabilization effect or the &tgr;−V
min
drive method.
Therefore, a dithering method for spatially dividing pixels, a field cutting method of time-divisionally generating graylevels and an area gradation method for controlling a generation degree of a polarization inversion area within a pixel are frequently used for achieving grayscale display. However, those methods have a problem that a circuit for driving a liquid crystal is complex.
Moreover, the conventional SSFLC is hard to be driven by a TFT(Thin Film Transistor) because of its large spontaneous polarization. Thus, the liquid crystal display using the SSFLC should be driven by a simple matrix drive. However, it is difficult for the simple matrix drive to attain the liquid crystal display having a high resolution and a high image quality.
On the other hand, as the liquid crystal display in which grayscale display is achieved, a liquid crystal display which uses anti-ferroelectric liquid crystal material is disclosed in “Ferroelectric Liquid Crystal Display Using Tristable Switching”, Y. Yamada et al., Jpn. J. Appl. Phys., 29, pp.1757-1764 (1990) and “Antiferroeletctric Chiral Smectic Phases Responsible for the Tristable Switching in MHPOBC”, A. D. L. Chandani et al., Jpn. J. Appl. Phys., 28, pp.L1265-L1268(1989). The anti-ferroelectric liquid crystal material has a tri-stable property. A liquid crystal display making use of switching the anti-ferroelectric liquid crystal material under an application of a bias voltage achieves grayscale display under an application of a bias voltage.
However, the liquid crystal display using the anti-ferroelectric liquid crystal material has a problem that the bias voltage is necessary for the grayscale display and that a drive wave form is complex in a case of a display element having a high resolution and many scan lines. Moreover, in the conventional liquid crystal display using the anti-ferroelectric liquid crystal material, it is difficult to carry out a TFT drive because of a large value of a spontaneous polarization of the anti-ferroelectric liquid crystal material.
Moreover, as a ferroelectric liquid crystal display in which grayscale display can be done, a liquid crystal display device using a deformed helix ferroelectric (DHF) liquid crystal is disclosed in “Behaviour of ferroelectric smectic liquid crystals in electric field”, Ostrovski et al., Advances in Liquid Crystal Research and Applications, Oxford/Budapest (1980) pp.469-482 and Japanese Laid Open Patent Application (JP-A-Heisei, 1-152430). As shown in
FIG. 1
, in the liquid crystal display using the DHF liquid crystal, a distance d
1
between substrates
101
,
102
is set to be longer than a pitch d
2
of a helix
104
formed by liquid crystal molecules
103
of the DHF liquid crystal. In the arrangement of the substrate
101
,
102
and the liquid crystal molecules
104
, the generation of the helix is never suppressed by surface stabilization. As a result, the liquid crystal molecules
103
are arrayed such that the helix is drawn in a direction parallel to the substrates
101
,
102
.
A diffraction grating is formed when the pitch d
2
of the helix
104
is within a wave length range of a light. The diffraction grating is not formed if the pitch d
2
of the helix
104
is set to be shorter than the wave length range of the visible light. The pitch d
2
of the helix
104
is typically set to be shorter than a half wave length ½&lgr;. Apparent refractive indexes are averaged if the pitch d
2
of the helix
104
is set to be shorter than the wave length range of the light and thereby the diffraction grating is not formed. At this time, the DHF liquid crystal can be treated similarly to a medium having a uniaxial birefringence parallel to a helical axis.
That is, the DHF liquid crystal shows double refraction in a helix axis direction when a voltage is not applied. When the voltage is applied, the DHF liquid crystal is gradually deviated from the helix array in the liquid crystal orientation. Thus, it has the distorted helix structure, which causes a transmissivity to be changed. Hence, the DHF liquid crystal can carry out the continuous grayscale display. Its driving method is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 6-194625). Moreover, another DHF liquid crystal is disclosed in Japanese Patent Office Gazette (Jp-B 2532606).
Also, a polymer stabilized FLC (ferroelectric liquid crystal) is known as a ferroelectric liquid crystal material which can attain the continuous grayscale display. The polymer stabilized FLC is disclosed in “Mesogenic Polymer Stabilized Ferroelectric Liquid Crystal Display Exhibiting Monostability with High Contrast Ratio and Grayscale Capability”, H.Furue et al., Jpn. J. Appl. Phys., 36, pp.L1517-L1519 (1997) and “Fabrication of a Zigzag Defect-Free Surface-Stabilized Ferroelectric Liquid Crystal Display Using Polyimide Orientation Film”, H. Furue, et al., Jpn. J. Appl. Phys., 37, pp.3417-3421(1998). The polymer stabilized FLC liquid crystal includes monomer together with the FLC (Ferroelectric Liquid Crystal) material. The polymer stabilized FLC is stabilized by irradiating an ultraviolet light while aligning the liquid crystal molecules in one direction through the application of electric field. The continuous grayscale display can be done in the polymer stabilized FLC liquid crystal.
FIG. 2
shows a voltage-to-contrast ratio curve in this polymer stabilized FLC measured at four temperatures (25° C., 30° C., 35° C. and 40° C.). The contrast ratio is a ratio of the transmissivity to 0V at the darkest time, and substantially corresponds to a voltage transmissivity curve. As shown in
FIG. 3
, the transmissivity is gradually changed. The continuous grayscale display can be obtained in the polymer stabilized FLC display.
Also, another liquid crystal display which can attain the continuous grayscale display is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 4-212126). In this liquid crystal display, two substrates are arranged such that the orientations are substantially parallel to each other. Projection components to the su
Chowdhury Tarifur R.
Foley & Lardner
Sikes William L.
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