Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-08-31
2004-03-09
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S138000, C349S139000
Reexamination Certificate
active
06704085
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display apparatus and particularly concerns a liquid crystal display apparatus using an in plane switching method.
An active-matrix liquid crystal display apparatus has been known that has a TFT (TFT: THIN FILM TRANSISTOR) element as a switching element in a display region constituting a pixel.
As for the active-matrix liquid crystal display apparatus, a structure has been adopted in which a liquid crystal layer is inserted between a pair of substrates and is sandwiched therebetween. On one of the substrates (TFT substrate), TFT elements, pixel electrodes, electrodes and wiring for a scanning signal and a data signal, and terminals connecting the wiring and an external driving circuit are formed. On the other substrate (CF substrate), a color filter and opposing electrodes are formed. A twist nematic display method has been adopted in which a longitudinal electric field is applied to provide display. The longitudinal electric field is substantially perpendicular to a substrate surface.
Meanwhile, JP-A-6-160878 specification discloses a liquid crystal display apparatus using an in plain switching method as a method for improving a viewing angle and contrast that have been problems of a liquid crystal display apparatus. In this method, a common signal electrode is disposed on a TFT substrate instead of an opposing electrode disposed on a color filter substrate, and a voltage is applied between a comb-shaped pixel electrode and the common signal electrode. Thus, electric field components, which are substantially in parallel with a surface of the substrate, are used for providing display.
The pixel electrode and the common signal electrode may be made of a metallic electrode wiring material. Further, as disclosed in JP-A-9-73101 specification, the electrodes may be made of ITO (INDIUM TIN OXIDE), which is used as a transparent pixel electrode in a twist nematic display method.
In the above liquid crystal display apparatus using an in plain switching method, a leaked electric field that is unnecessary for its display occurs between signal wiring and the pixel electrode or the common signal electrode, the signal wiring being adjacent to the pixel electrode or the common signal electrode, as well as between the pixel electrode and the common signal electrode.
The signal wiring being adjacent to the electrodes includes scanning signal wiring, which is extended in x direction and is disposed in parallel in y direction, and data signal wiring, which is extended in y direction and is disposed in parallel in x direction.
It has been known that cross talk resulted from a leaked electric field drives liquid crystal thereon and causes leakage of light. Such leakage of light results in a defect in picture quality (smear) that is a line appearing along the signal wiring.
A means for solving the above problem is specifically disclosed in, for example, JP-A-6-202127 specification.
However, the liquid crystal display apparatus is configured such that a shield electrode for shielding an electric field is disposed next to the signal wiring and a reference potential is supplied to the shield electrode from the outside. Hence, a large current is charged and discharged to a capacity between the shield electrode and a signal electrode, thereby overloading a driving circuit.
Therefore, the liquid crystal display apparatus is large in power consumption or the driving circuit is too large in size. Further, connecting means is necessary for applying a potential to the shield electrode, resulting in a larger number of steps and a connecting defect.
Moreover, the above method is seriously disadvantageous as follows: an opening area on a pixel that contributes to display is reduced by the placement of the shield electrode, resulting in lower luminance of the liquid crystal display apparatus.
Means for solving the above problem is specifically disclosed in Japanese Patent Application No. 10-543713 (WO98/47044) specification.
A structure is adopted in which corresponding to image signal wiring which is adjacent to and in parallel with a comb-shaped pixel electrode, a reference electrode is formed while being completely superimposed on a plane via an organic insulating film formed entirely on a substrate. Thus, unnecessary electric lines of force that are generated from the data signal wiring are mostly terminated on the reference electrode.
Therefore, it is possible to solve the problem of cross talk that is caused by electric field leakage peculiar to an in plane switching method.
According to the above method, it is possible to eliminate the necessity for a shield electrode, which has been conventionally disposed on both sides of the data signal wiring or on an opposing substrate to reduce cross talk. Hence, the opening area on a pixel can be increased.
The liquid crystal layer on the reference electrode acts as a self-shielding layer and it is possible to eliminate a light shield film (black matrix), which has been conventionally disposed to cover a gap between the data signal wiring and the shield electrode. Hence, an aperture ratio of a pixel can be further increased.
Also, an organic insulating film, which is provided as an interlayer film entirely on the substrate, is small in permittivity as compared with an inorganic insulating film, and the organic insulating film can be readily increased in thickness as compared with the inorganic insulating film. Thus, even in the case where the data signal wiring is entirely covered with the reference electrode, it is possible to reduce a parasitic capacity of the wiring formed between the data signal wiring and the reference electrode.
Therefore, since a load is lighter with respect to the data signal wiring, a data signal has smaller delay in transmission through wiring and a signal voltage can be sufficiently charged to a display electrode.
Consequently, it is possible to downsize the driving circuit for driving the data signal wiring.
However, the above-mentioned conventional art has another problem as follows. As described above, when a voltage is applied to liquid crystal to provide display, a potential difference applied between the pixel electrode and the common signal electrode is used.
According to the above conventional art, the common signal electrode serving as a reference electrode is formed on the top layer via the organic insulating film, which is entirely formed on the substrate, to reduce a capacity between the data signal wiring and the common signal electrode. The pixel electrode is disposed on a lower layer, so that the organic insulating film is disposed on the pixel electrode.
Therefore, between the pixel electrode and the common signal electrode, the organic insulating film forms another capacity connected in series with liquid crystal.
Hence, a potential difference applied between the pixel electrode and the common signal electrode is partially reduced by the capacity connected in series with the liquid crystal layer.
As a result, between the pixel electrode and the common signal electrode, in consideration of the voltage drop, it is necessary to apply a potential difference at or more than a desired voltage to be applied to liquid crystal.
Namely, another problem arises as follows: a capacity component connected in series with the liquid crystal layer is reduced due to the presence of the organic insulating film, which is formed to decrease a capacity between the data signal wiring and the common signal electrode, resulting in larger voltage drop caused by a capacity connected in series with liquid crystal. Consequently, liquid crystal requires a larger driving voltage.
An increase in driving voltage raises power consumption, which is inappropriate particularly for a portable liquid crystal display apparatus.
Further, when a driving voltage increases, an inexpensive low-voltage driver is not applicable. Hence, it is not possible to provide a liquid crystal display apparatus at low cost.
According to the above conventional art, in the liquid crystal display appar
Abe Makoto
Ishii Masahiro
Nakayoshi Yoshiaki
Nishimura Etsuko
Okishiro Kenji
Antonelli Terry Stout & Kraus LLP
Chowdhury Tarifur R.
Hitachi , Ltd.
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