Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-06-24
2002-01-01
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C349S192000
Reexamination Certificate
active
06335717
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a liquid crystal display device using a liquid crystal material having spontaneous polarization induced by application of an electric field or inherent thereto.
A liquid crystal display device has a feature of low voltage consumption, light in weight and the like, and are widely used for a display device of a word processor, personal computer or car navigation system. Particularly, a TN mode TFT-LCD having pixels connected to switching elements such as TFTs (thin film transistors) and using a nematic liquid crystal has an excellent display performance. However, the TN mode has a problem that the viewing angle is narrow and the response speed is low.
At present, a liquid crystal display element constructed by a liquid crystal material (antiferroelectric liquid crystal, ferroelectric liquid crystal or the like) having spontaneous polarization induced by application of the electric field or inherent thereto and held between two electrodes has received much attention as a display element having a wide viewing angle and high response speed.
Most types of liquid crystals having spontaneous polarization take three alignment states of no voltage application state, positive voltage application state and negative voltage application state.
Recently, liquid crystal materials such as a thresholdless antiferroelectric liquid crystal (TLAF), Deformed-Helix Ferroelectric liquid crystal (DHF), Twisted Ferroelectric liquid crystal (TFLC) or electric clinic, which could take an alignment state between the above three states according to an applied voltage in addition to the above three alignment states, were found among the liquid crystal materials having spontaneous polarization. The above liquid crystal materials have no or little memory characteristic, but a desired alignment state thereof can be held and gray scale display can be attained by using switching elements such as TFTs, TFDs (thin film diodes) or MIMs (metal-insulator-metal diodes) provided for the respective pixels in the active matrix system and holding the voltage during the non-selected period. As a result, a liquid crystal display device which can display gray scales with high speed and wide viewing angle can be attained.
The arrangement of molecules of a liquid crystal having spontaneous polarization is set in a state called a smectic phase. In the smectic phase, rod-like molecules are arranged in a layered form and set in parallel to one another as shown in
FIGS. 1A and 1B
.
If an external force is applied to the image plane of the liquid crystal display device by depressing the image plane by a finger, for example, the alignment of the liquid crystal is disturbed and the display becomes defective. In the TN mode or STN mode, since the liquid crystal has no layer structure, the alignment is naturally restored to the original state and the defective display can be cancelled when the external force is removed.
However, since the order parameter of the liquid crystal of the smectic phase is high, the disturbed layer structure cannot be restored even when the external force is removed if the alignment is once destroyed by application of the external force or the like. That is, the alignment of the liquid crystal is not restored to the original state and a portion to which the external force is applied remains semi-permanently as a display defective portion.
For example, in the case of antiferroelectric liquid crystal, if a force of 2 kg/cm
2
or more is applied to the liquid crystal display element by a finger or the like, the layer structure of the smectic liquid crystal is disturbed as shown in
FIGS. 1C and 1D
and the alignment is not restored to the original state even if the force is removed, and an alignment defective region is formed.
Since the alignment degree of the liquid crystal molecules is lowered in the alignment defective region, display of black level is made poor (the transmission factor is high when black is displayed) and the contrast is lowered so that the display quality of the liquid crystal display device will be significantly degraded. Thus, the liquid crystal of smectic phase has a serious problem that the “alignment destruction” occurs by finger-pressing or the like.
In order to restore the liquid crystal alignment which is once disturbed to a uniform state (to effect the alignment treatment), the following methods are provided.
(1) After the temperature of the liquid crystal is raised to a temperature of phase transfer to the isotropic phase or more, or a temperature approximately equal thereto, the temperature is gradually cooled to the room temperature.
(2) A relatively high AC voltage (generally, ±7V or more, preferably, ±10V or more is applied between the pixel electrode and the common electrode) which is approximately equal to the saturation voltage is applied to the liquid crystal (this method is also called a voltage application alignment treatment).
(3) A combination of the methods (1) and (2) is effected.
In the method (1), the liquid crystal display device can be carried into a electronic oven or the like and the liquid crystal can be easily heated to the phase-transfer temperature if the circuit is not yet mounted. However, if the TAB and driving circuit are mounted on the liquid crystal display device, the plastic-made casing and polarization plate are deformed or deteriorated when the whole portion of the liquid crystal display device is heated in the electronic oven and thus it is extremely difficult to heat the liquid crystal to the phase transfer temperature without giving any influence on other members.
Further, the method (1) is effective only when the liquid crystal molecules exhibit the nematic phase at a temperature higher than that for the smectic C-phase as in a certain type of DHF. However, the method is not effective when the liquid crystal makes phase transfer from the isotropic phase to the smectic phase without passing through the nematic phase as in the case of thresholdless antiferroelectric liquid crystal.
In the method (2), it is possible to apply a sufficiently high voltage by use of a function generator and amplifier if the driving circuit is not yet mounted on the liquid crystal display device. However, the inventors of the present invention studied this method and found that the following problems would occur if this method was applied to the liquid crystal display device having switching elements such as TFTs.
It is necessary to apply a voltage higher than the pixel voltage by approximately 15V or more in order to turn ON the TFT element. Therefore, in order to apply a high voltage to the pixel electrode and effect the alignment treatment, it is necessary to apply a gate voltage which is higher than usual. However, if the high voltage is applied to the gate, a problem that the reliability of the TFT element is lowered due to degradation of the insulating property of the gate insulating film has occurred.
Further, the characteristics of switching elements provided for the respective pixels slightly fluctuate and the fluctuation of the characteristic becomes significant when a voltage of ±5V or more is applied to the pixel electrode. When the voltage of ±5V or more is applied to the pixel electrode to effect the alignment treatment, the effective values of the applied voltages are slightly different depending on the respective pixels and the degree of the alignment treatment becomes different for each pixel, thereby making the display state worse.
If the TAB and driving circuit are mounted on the liquid crystal display device, only a maximum voltage of ±5V, can be applied to the pixel electrode, since the maximum amplitude of a withstand voltage of the normal driver IC is 5V, or the maximum amplitude is 10V when a special driver IC is used. Therefore, a problem that a high voltage (±7V or more) necessary for the alignment treatment cannot be applied to the pixel electrode occurs.
Further, in a case wherein the gates are driven based on the line-at-a-time scanning method
Akiyama Masahiko
Fujiwara Hisao
Hara Yujiro
Hasegawa Rei
Iida Rieko
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hjerpe Richard
Kabushiki Kaisha Toshiba
Nguyen Frances
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