Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2001-05-30
2003-11-11
Wu, Shean C. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C252S299200, C252S299500, C349S143000, C349S178000
Reexamination Certificate
active
06645576
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, it relates to an active matrix liquid crystal display device of a so-called in-plane switching type.
2. Description of the Related Art
A liquid crystal display realizes display in such a manner that an electric field is applied to liquid crystal molecules in a liquid crystal layer sandwiched by a pair of substrates to change the orientation direction of the liquid crystal, so as to cause optical change of the liquid crystal layer.
The conventional active matrix liquid crystal display device is represented by a twisted nematic (TN) display system, in which the direction of the application of the electric field to the liquid crystal is set in the direction perpendicular to the substrate plane that sandwiches the liquid crystal, and the display is realized by utilizing the optical rotation of the liquid crystal layer.
On the other hand, a liquid crystal display device of a in-plane switching (IPS) system has been proposed in JP-B-63-21907, U.S. Pat. No. 4,345,249, WO 91/10936 and JP-A-6-160878, in which a comb electrode is used, and the direction of the electric field applied to the liquid crystal is set in the direction parallel to the substrate plane, whereby the display is realized by utilizing the birefringence of the liquid crystal.
The in-plane switching system has advantages, such as a wide viewing angle and a low load capacitance, in comparison to the conventional TN system, and is being rapidly developed in recent years as a new active matrix liquid crystal display device that superseding the TN system.
In the IPS system, the in-plane switching can be more perfectly realized in the case where the liquid crystal has negative dielectric anisotropy in comparison to the case of a liquid crystal having positive dielectric anisotropy, as demonstrated in J. of Appl. Phys., vol. 82, No. 4, pp. 528-535 (1997) by M. Oh-e, M. Yoneya and K. Kondo. The liquid crystal having negative dielectric anisotropy has a dielectric constant in the short axis direction of the liquid crystal molecule that is larger than the dielectric constant in the long axis direction perpendicular thereto, and the liquid crystal having positive dielectric anisotropy has a dielectric constant in the short axis direction of the liquid crystal molecule that is smaller than the dielectric constant in the long axis direction perpendicular thereto.
The perfect realization of the in-plane switching completes enhancement of the viewing angle of the liquid crystal display device including halftone. Therefore, the liquid crystal having negative dielectric anisotropy is preferred as a liquid crystal used in the IPS system from the foregoing standpoint.
The IPS system employs an opaque metallic comb electrode in a stripe form provided on an inner surface of one of the pair of electrodes.
In recent years, a modified system of the IPS system has been proposed in that the comb electrode is formed with a transparent electroconductive substance, such as ITO (indium tin oxide), instead of the opaque metallic electrode, and is arranged at a shorter interval than the conventional IPS system, and a liquid crystal material having negative dielectric anisotropy, whereby the entire liquid crystal present above the transparent comb electrode can be subjected to orientation change only with an electric field formed at the periphery of the comb electrode, so as to improve the transmittance and the opening ratio.
Literatures relating to the foregoing proposal include Asia Display 1998, pp. 371-374, by S. H. Lee, S. L. Lee and H. Y. Kim and SID digest 1999, pp. 202-205, by S. H. Lee, H. Y. Kim and T. Y. Eom.
SUMMARY OF THE INVENTION
The foregoing literatures report that in the IPS system combining the liquid crystal material having negative dielectric anisotropy and the short interval transparent comb electrode, such transmittance that is close to the TN system can be realized with maintaining such wide viewing angle characteristics that is equivalent to the IPS system.
It has been known in a liquid crystal display device that in the case where a liquid crystal driving voltage waveform having a direct current voltage superposed is applied to a liquid crystal layer, the direct current voltage (direct current offset voltage) remains in the liquid crystal layer even when the direct current voltage is removed.
As discussed in S. Matsumoto, Ekishou Display Gijutu (Liquid Crystal Display Technique), published by Sangyo Tosho Co., Ltd., Chap. 2, pp. 70-73, the application of the driving voltage waveform having a direct current voltage superposed to the liquid crystal layer may occur in an active matrix liquid crystal display device in an ordinary liquid crystal operation due to the structure of the active driving element of the liquid crystal display device, and it is difficult to completely prevent the superposing phenomenon of a direct current voltage when gradation display is conducted. The phenomenon is common to both the TN system and the IPS system conventionally employed.
The remaining direct current voltage affects the brightness in liquid crystal display devices of both the TN system and the IPS system, and difference in brightness is caused between a part applied with the direct current voltage and a part not applied therewith or between parts having different intensities of the applied direct current voltage.
Therefore, in the case, for example, where texts or graphics are displayed under ordinary driving conditions for a long period of time, such a phenomenon occurs that the texts or graphics that have been displayed are displayed for a certain period after turning off the display.
As a result, uniformity of display is impaired. Such a phenomenon is called as an after image of a liquid crystal display device, which is gradually decreased in intensity with the lapse of time after formation thereof and is finally disappeared, but there are cases where a period of 30 minutes or more is required to disappear upon viewing with the naked eye.
As a mechanism that when a direct current voltage is applied, the direct current offset voltage remains in a liquid crystal layer, a model explaining by behavior of ions in the liquid crystal layer in the conventional TN system as an example has been proposed in Shingaku Gihou (Technical Research Report of Institute of Electronics, Information and Communication Engineers), EID96-89, pp. 29-34 (1997-01).
According to the model, a direct current voltage charged in an oriented film and absorption of ions on an orientation film for directing the liquid crystal are considered as factors of the direct current voltage remaining in the liquid crystal layer, and it sums up that the remanence of the direct current voltage for several minutes is caused by charging and relaxation of the orientation film, and the remanence of the direct current voltage for a longer period is caused by absorption of ions on the orientation film.
The IPS system suffers more frequent occurrence of the after image than the TN system. In the TN system, only liquid crystal orientation controlling layers and a liquid crystal layer are present between a pixel electrode and a counter electrode, and an electric field is applied to the pixel electrode, the liquid crystal orientation controlling layer, the liquid crystal layer, the liquid crystal orientation controlling layer and the counter electrode in this order.
On the other hand, the IPS system has an insulating layer in addition to the liquid crystal layer and the liquid crystal orientation controlling layers between the pixel electrode and the counter electrode, and the electric field is applied to the pixel electrode, the liquid crystal orientation controlling layer, the liquid crystal layer, the liquid crystal orientation controlling layer, the insulating layer and the counter electrode in this order.
That is, because charging and relaxation of the orientation films and the insulating film are considered while only the orientation film is con
Iwakabe Yasushi
Kondo Katsumi
Oaku Hitoshi
Ohta Masuyuki
LandOfFree
Active matrix type liquid crystal display and liquid crystal... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Active matrix type liquid crystal display and liquid crystal..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Active matrix type liquid crystal display and liquid crystal... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3171533