Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-06-29
2003-01-14
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S138000
Reexamination Certificate
active
06507383
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a liquid crystal display unit and more particularly to a structure of a liquid crystal display unit using transparent electrodes as a pixel electrode and/or a common electrode.
BACKGROUND ART
Many liquid crystal display units adopt an active matrix system having a structure including thin film transistors (TFT) as switching elements in a display area in which pixels are provided. The liquid crystal display unit of this kind adopts a structure having a liquid crystal layer disposed between a pair of substrates. TFT elements, a pixel electrode, wiring and electrodes for scanning signals and image signals, terminals for connecting wiring and external driving circuits and the like are formed on one substrate (TFT substrate) and a color filter and an opposite electrode are formed on the other substrate (CF substrate). There is adopted a twisted nematic display system in which a voltage is applied between the pixel electrode on one substrate and the opposite electrode on the other substrate to drive the liquid crystal and control display.
In contrast to the above system, as a system capable of improving a visual field angle or viewing angle and contrast which are problems to be solved in a liquid crystal display unit, JP-A-6-160878 discloses a liquid crystal display unit including a common signal electrode disposed on the TFT substrate instead of the opposite electrode disposed on the substrate on which the color filter is formed and in which a voltage is applied between the pixel electrode in the form of teeth of a comb and the common signal electrode to drive the liquid crystal and control display. The pixel electrode and the common signal electrode may be made of metal for electrodes or wiring or Indium Tin Oxide (ITO) used in a transparent pixel electrode.
As an example in which the ITO electrode is used, S. H. Lee et al. describe in SID '98 DIGEST, p.371 (1998) and SID '99 DIGEST, p.202 (1999) a technique that the pixel electrode and the common signal electrode are constituted by two ITO electrodes constituting upper and lower layers between which an insulation layer is disposed and a width of the pixel electrode in the form of teeth of a comb and the common signal electrode and a distance between the electrodes are made minute to be optimized so that a voltage is applied between the two upper and lower ITO electrodes to drive the liquid crystal.
DISCLOSURE OF THE INVENTION
In the above related art, when driving of the liquid crystal and a manufacturing process are considered, it is understood that there are two problems in regard to the driving of the liquid crystal and two problems in regard to the process due to a cross-sectional structure.
The problems will be described.
(1) Problem of Increasing Writing Time of Voltage to Liquid Crystal
In the related art, since the width of the two upper and lower ITO electrodes formed into teeth of a comb and the distance therebetween are made minute to thereby control driving of the liquid crystal, a deviation in alignment of the two upper and lower ITO electrodes in a photolithographic process upon processing of the two upper and lower ITO electrodes and a deviation in processing dimensions produce uneven display characteristic as they are and more particularly scattered brightness. In order to avoid this problem, a method that the lower ITO electrode is formed on a substantially whole surface within a unit pixel area is adopted. Since the lower ITO electrode is formed on the substantially whole surface within the unit pixel area, it is not necessary to consider a deviation in alignment of the upper and lower ITO electrodes. Consequently, scattering of the display characteristic can be reduced.
In the related art, a parasitic capacitance connected in parallel to a liquid crystal layer is newly formed in a portion where the two upper and lower ITO electrodes constituting the pixel electrode and the common signal electrode overlap each other through an inter-layer insulation layer. This parasitic capacitance can be effectively utilized as a capacitance for improving the voltage holding characteristic of the liquid crystal, whereas there causes a problem that a time required until a desired voltage is applied to the liquid crystal layer, that is a writing time of a voltage to the liquid crystal is increased, so that the voltage cannot be applied to the liquid crystal sufficiently, when a voltage is applied between the two upper and lower ITO electrodes.
A method of changing a structure of the insulation layer disposed between the two upper and lower layer ITO electrodes is also one of measures for reducing the parasitic capacitance, although there is a problem that a driving voltage of the liquid crystal is increased as described later.
(2) Problem of Increasing Driving Voltage of Liquid Crystal
When a voltage is applied to the liquid crystal, a potential difference applied to the two upper and lower ITO electrodes is utilized, while an insulation layer also exists in the area where the insulation layer does not exist in a conventional structure, that is, the area which is positioned above the lower ITO electrode and where the upper ITO electrode does not exist and the insulation layer forms a capacitance connected in series to the liquid crystal between the two upper and lower ITO electrodes. Accordingly, part of the potential difference applied to the two upper and lower ITO electrodes is absorbed by the capacitance connected in series to the liquid crystal layer. Consequently, it is necessary to apply between the two upper and lower ITO electrodes the potential difference larger than a desired voltage to be applied to the liquid crystal in consideration of a voltage drop.
More particularly, there is a problem that the driving voltage is increased due to the insulation layer. When the driving voltage is increased, the power consumption is increased and the increased power consumption is not suitable to a portable type liquid crystal display unit particularly. Further, when the driving voltage is increased, a cheap low-voltage driver cannot be used and accordingly there is a problem that the liquid crystal display unit cannot be provided cheaply.
In the conventional structure, as methods of reducing the parasitic capacitance connected in parallel to the liquid crystal layer, (1) a method of increasing a thickness of an insulation layer between the two upper and lower ITO electrodes and (2) a method of changing material of the insulation layer and introducing material having a small dielectric constant or adding a new layer are considered. In other words, when the number of insulation layers is n, the dielectric constant of the k-th insulation layer ∈
k
and the thickness thereof d
k
in the portion where the two upper and lower ITO electrodes overlap each other, S
A
defined by
1
∑
k
=
1
n
d
k
ϵ
k
is made small to thereby reduce the parasitic capacitance. In the conventional structure, however, when the parasitic capacitance is reduced, S
A
of the insulation layer existing in an area which is positioned above the lower ITO electrode and where the upper ITO electrode does not exist, that is, the insulation layer forming the capacitance connected in series to the liquid crystal layer is also made small to thereby reduce the capacitance connected in series to the liquid crystal layer. Consequently, a voltage drop due to the capacitance connected in series to the liquid crystal is increased to thereby increase the driving voltage of the liquid crystal.
Conversely, in order to reduce the driving voltage of the liquid crystal, when S
A
of the insulation layer disposed in an area which is positioned above the lower ITO electrode and where the upper ITO electrode does not exist, that is, the insulation layer forming the capacitance connected in series to the liquid crystal layer is increased, S
A
of the inter-layer insulation layer, that is, the insulation layer forming the parasitic capacitance connected in parallel to the liquid crystal layer is also incre
Abe Makoto
Nakayoshi Yoshiaki
Nishimura Etsuko
Ono Kikuo
Wakagi Masatoshi
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Rude Timothy L
Ton Toan
LandOfFree
In-plane switching liquid crystal display apparatus with... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with In-plane switching liquid crystal display apparatus with..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and In-plane switching liquid crystal display apparatus with... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3022467