Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-12-07
2004-07-27
Whitehead, Jr., Carl (Department: 2813)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S133000, C349S172000
Reexamination Certificate
active
06768527
ABSTRACT:
This application claims the benefit of Korean Patent Application No. P2000-80735, filed on Dec. 22, 2000, the entirety of which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a ferroelectric liquid crystal display that is capable of keeping a stable alignment state against external temperature variation and external impact. The invention also is directed to a method of fabricating the above-mentioned liquid crystal display.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls light in accordance with a liquid crystal alignment state to thereby display a desired picture on the screen. A liquid crystal used for such an LCD is in a neutral phase between a liquid state and a solid state, which has both a fluidity and an elasticity. In a thermodynamic phase transition process of the liquid crystal, a liquid crystal having a smectic C phase is rotated along a smectic layer taking a layer structure having the same electrical and magnetic property. In other words, the smectic C phase liquid crystal is rotated along an outer line of a virtual cone.
Such a smectic C phase liquid crystal has a characteristic of having a spontaneous polarization regardless of an external electric field. This liquid crystal is usually referred to as a “ferroelectric liquid crystal” (FLC). The FLC has been actively studied in light of its fast response speed according to its spontaneous polarization characteristic and an ability to realize a wide viewing angle without a special electrode structure and a compensating film. The FLC includes a deformed helix FLC mode, a surface stabilized FLC mode, an anti-FLC mode, a V-type FLC mode and a half V-type FLC mode, etc. Hereinafter, the V-type FLC mode and the half V-type FLC mode will be described.
FIG. 1
shows an alignment state of a liquid crystal cell in the V-type FLC mode.
Referring to
FIG. 1
, the liquid crystal cell in the V-type FLC mode includes an upper substrate
1
on which a common electrode
3
and an alignment film
5
are disposed; a lower substrate
11
on which a TFT array
9
including a pixel electrode and an alignment film
7
are disposed; and a liquid crystal
13
injected between the upper and lower substrates
1
and
11
. The alignment films
5
and
7
are aligned into a desired state by rubbing. The injected liquid crystal
13
forms a smectic layer having a layer structure and is arranged into a phase having a desired slope with respect to a plane perpendicular to the smectic layer. In other words, the liquid crystal
13
has a desired inclination angle with respect to an aligned direction of the alignment film and is aligned such that the adjacent smectic layers have opposite polarities with respect to each other.
FIG. 2
shows the relationship between transmissivity (T) and voltage (V) of the V-type FLC mode liquid crystal cell. The liquid crystal
13
within the V-type FLC mode liquid crystal cell responds to positive and negative voltages applied thereto. Since the transmittance is suddenly changed according to an application of positive and negative voltages, a T-V curve has a V-shape. In other words, a transmittance is increased as a positive voltage increases, whereas a transmittance is decreased as a negative voltage increases.
FIG. 3
shows an alignment state of a liquid crystal cell in the half V-type FLC mode.
In
FIG. 3
, a liquid crystal
15
within the half V-type FLC mode liquid crystal cell injected between the upper substrate
1
and the lower substrate
11
forms a smectic layer having a layer structure. The liquid crystal
15
is aligned at a desired inclination angle with respect to an alignment treatment direction of the alignment films
5
and
7
such that the adjacent smectic layers have a different polarity unlike the liquid crystal
13
in the V-type FLC mode. Such a half V-type mode liquid crystal can be implemented by applying a positive or negative electric field in advance and, at the same time, lowering its temperature into a temperature having a smectic phase. The half V-type FLC mode liquid crystal
15
formed in this manner responds to only one of the applied positive and negative voltages. Thus, as seen from
FIG. 4
, the T-V curve has a half V-shape in the half V-type FLC mode. The T-V characteristic in
FIG. 4
represents when a negative voltage is used to make an initial uniform alignment. In this case, a transmittance appears not to increase upon application of a negative voltage, whereas it is increased in accordance with an increase in a positive voltage. Otherwise, when a positive voltage is used to make an initial uniform alignment, a transmittance is increased in accordance with an increase in a negative voltage.
A thermodynamic phase transition process of the half V-type FLC mode liquid crystal
15
is as follows:
Isotropic→nematic (N*) phase→smectic C*(Sm C*) phase→crystal
Such a phase transition process expresses a liquid crystal phase resulting from a gradual decrease in temperature going from left to right. The liquid crystal
15
is aligned in parallel to a rubbing direction when its temperature is slowly lowered to reach a temperature having a nematic phase after the liquid crystal
15
was injected into the liquid crystal cell at a temperature having an isotropic phase. If an electric field is applied to the interior of the cell with a temperature being slowly lowered in this state, then the liquid crystal
15
is phase-changed into a smectic phase. A direction of a spontaneous polarization of the liquid crystal
15
generated at this time is arranged in such a manner to be consistent with that of an electric field formed at the interior of the cell. As a result, when the liquid crystal
15
within the liquid crystal cell is subjected to a parallel alignment treatment, it makes one of two possible molecule arrangements. The molecule arrangement in the spontaneous polarization direction is consistent with the direction of an electric field applied in the phase transition process, and thereby has a uniform alignment state.
This uniform alignment state will be described in detail with reference to FIG.
5
and
FIG. 6
below. First, as seen from
FIG. 5
, if a negative electric field E (−) is applied upon alignment of the liquid crystal
15
, then a spontaneous polarization direction of the liquid crystal
15
identical to the electric field direction is made to provide a uniform alignment. In such a liquid crystal cell, as shown in
FIG. 6
, a liquid crystal arrangement is changed upon application of a positive electric field E (+) while a liquid crystal arrangement is not changed upon application of a negative electric field E (−).
In order to utilize a response characteristic to an electric field of the liquid crystal
15
, polarizers perpendicular to each other are arranged at the upper and lower portions of the liquid crystal cell. At this time, a transmission axis of one polarizer is arranged to be consistent with an initial liquid crystal alignment direction. In the liquid crystal cell having the above-mentioned arrangement, the T-V curve has a half V-shape as shown in
FIG. 7
, experimentally. With respect to a negative electric field E (−), a liquid crystal arrangement is not changed to block the light. In contrast, with respect to a positive electric field E (+), a liquid crystal arrangement is changed to transmit the light. In this case, as a positive electric field E (+) increases, a transmittance also increases.
As described above, the half V-type FLC mode liquid crystal uses both a temperature and an electric field so as to obtain a uniform alignment. However, the liquid crystal cell made in this manner causes a phenomenon of breaking an initial uniform alignment when external impact is applied thereto during a grinding process of a shorting bar.
Also, when a heat stress is applied to the conventional half V-type FLC mode liqui
Choi Su Seok
Choi Suk Won
Jr. Carl Whitehead
LG.Philips LCD Co. , Ltd.
McKenna Long & Aldridge LLP
Smoot Stephen W.
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