Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within smectic phase
Reexamination Certificate
2001-11-19
2004-09-07
Flynn, Nathan J. (Department: 2826)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within smectic phase
C349S123000, C349S155000, C349S156000, C349S184000
Reexamination Certificate
active
06788381
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal device and, more particularly, to a liquid crystal device in which bulkheads maintain a desired clearance between a pair of substrates retaining a liquid crystal therebetween.
2. Related Background Art
Conventionally, CRTs are known as displays that have most commonly been used heretofore, and the CRTs are now widely used as monitors for output of moving picture of TV, VTR, or the like, or for personal computers. However, the CRTs have such characteristics as to degrade the visibility by flicker, stripes due to insufficient resolution, etc. and deteriorate a phosphor by image persistence in the case of still images. Further, they have a large volume behind the screen because of their structure, which impedes space saving at offices and homes.
A solution to such imperfections of the CRTs was liquid crystal apparatus and among the known liquid crystal apparatus was one provided with a liquid crystal device using a twisted nematic (TN) liquid crystal, for example, as described in M. Schadt and W. Helfrich, “Applied Physics Letters Vol. 18, No. 4, p127-128 (Feb. 15, 1971).”
One of such liquid crystal devices using the TN liquid crystal was of the passive matrix type holding superiority in cost, but the liquid crystal devices of this type had the problem that crosstalk occurred during time-sharing addressing in matrix electrode structure of a high pixel density, and thus had a limit to the number of pixels.
On the other hand, the liquid crystal devices called TFT devices, different from the passive matrix type devices, have been developed in recent years. Since a transistor is fabricated at every pixel, these TFT liquid crystal devices solve the problems of crosstalk and slow response speed on one hand but have the following drawbacks on the other hand, however: it becomes harder to fabricate the liquid crystal device without defective pixels as the area increases, and, even if possible, the cost becomes enormous.
For overcoming the drawbacks of the conventional liquid crystal devices as described above, Clark and Lagerwall proposed the liquid crystal device of the type utilizing the refractive index anisotropy of ferroelectric liquid crystal molecule and controlling transmitted light rays by combination with a polarizing element (Japanese Patent Application Laid-Open No. 56-107216, U.S. Pat. No. 4,367,924, and so on).
In general, this ferroelectric liquid crystal (FLC) has a chiral smectic C phase (SmC*) or H phase (SmH*) in a specific temperature region and in this condition, it has such a property that it takes either of a first optically stable state and a second optically stable state in response to an applied electric field and it maintains either state in the absence of application of an electric field, i.e., bistable memory nature. Moreover, it undergoes inversion switching because of spontaneous polarization and thus demonstrates a very fast response speed. Further, it is also excellent in viewing angle characteristics and is thus suitable, particularly, for high speed, high definition, and large screen display devices.
Incidentally, such ferroelectric liquid crystal devices in an initial orientation stage are in a state in which liquid crystal molecules oriented in a first stable state and liquid crystal molecules oriented in a second stable state are mixed in a domain. Namely, since the chiral smectic liquid crystal in the bistable state has almost equivalent energy levels of orientation regulating force to orient the liquid crystal molecules into the first stable state and orientation regulating force to orient the liquid crystal molecules into the second stable state, the liquid crystal molecules oriented in the first stable state and in the second stable state are mixed in each domain in the initial orientation stage, on the occasion of alignment under a condition of sufficiently thin alignment layers for the chiral smectic liquid crystal to demonstrate bistability.
On the other hand, among the ferroelectric liquid crystals is a &tgr;Vmin mode liquid crystal, which has negative dielectric anisotropy (&Dgr;{dot over (a)}<0) and positive biaxial dielectric anisotropy (&Dgr;{dot over (a)}>0) and which exhibits a &tgr;Vmin characteristic, because the dielectrically anisotropic torque to stabilize the liquid crystal is greater than the reversing torque of ferroelectric liquid crystal.
The &tgr;Vmin characteristic is such a characteristic that the response speed of liquid crystal (&tgr;) a certain minimum (&tgr;Vmin) with increase in the applied voltage (V), and possession of this &tgr;Vmin characteristic makes it feasible to implement achievement of high luminance, high contrast, and high speed.
Liquid crystals demonstrating the antiferroelectric property are also known as the technology of constructing the display devices by making use of the refractive index anisotropy and spontaneous polarization of like liquid crystal molecules. Here the antiferroelectric liquid crystals (A-FLCs) generally have a chiral smectic CA phase (SmCA*) in a specific temperature region and in this condition, they have such a property that an average optically stable state is a direction normal to the smectic layer in the absence of the electric field but the average optically stable state is inclined from the direction normal to the layer in the presence of the electric field. In addition, the antiferroelectric liquid crystals also undergo switching because of coupling of spontaneous polarization with the electric field, thus exhibit very fast response speeds, and are expected to realize fast display devices.
Meanwhile, in order to uniformly drive the liquid crystal device employing the ferroelectric liquid crystal or the antiferroelectric liquid crystal, in the plane of the liquid crystal panel, it is necessary to keep glass substrates, which are an example of a pair of transparent substrates provided with transparent electrodes, uniform with a small fixed clearance (cell gap) between them.
The liquid crystal devices are normally constructed in such structure that the liquid crystal is filled in the small gap between two glass substrates and a voltage not less than a certain fixed threshold is applied between the transparent electrodes provided on the respective glass substrates to drive the liquid crystal. Because of this structure, if the gap between the glass substrates is nonuniform, different electric fields will be applied in plane to the liquid crystal panel, so as to cause in-plane (longitudinal) dispersion during driving of the liquid crystal.
Particularly, in use of the ferroelectric liquid crystal (FLC) or the antiferroelectric liquid crystal (A-FLC), the clearance between the pair of glass substrates needs to be as narrow as about 1 to 3 &mgr;m, and production of the thin and uniform cell gap in plane is a hard technique while being also a very important constituent.
Methods of uniformly maintaining a pair of glass substrates with a small fixed clearance between are generally categorized into methods of placing spherical spacers between the substrates and methods of forming stripe bulkhead structures on at least one of a pair of substrates retaining the liquid crystal between, by employing flexible printing, photolithography, dry film, and so on.
FIG. 6
is a cross-sectional view of a liquid crystal device in which the cell gap is retained by use of the conventional spherical spacers. It is possible to form even a relatively narrow cell gap by use of the spherical spacers
50
as long as the spacers
50
can be made in uniform size. However, since a number of spacers
50
are scattered over one substrate
52
out of a pair of glass substrates
51
,
52
in a liquid crystal device fabrication step, some spacers
50
are also placed within the pixel display areas. As a result, alignment defects occurred around the spacers
50
and posed a problem of failure in achieving satisfactory contrast of the liquid crystal device.
In order to maintain the cell gap, granular adhesive p
Hachisu Takahiro
Miura Seishi
Flynn Nathan J.
Sefer Ahmed N.
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