Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-04-27
2001-10-09
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S092000, C345S095000, C345S098000, C349S033000, C349S056000, C349S042000, C349S141000, C349S143000, C349S139000, C349S019000, C349S187000
Reexamination Certificate
active
06300926
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an active matrix type liquid crystal display using a horizontal electric field system.
As regards the display of information on a liquid crystal display (hereinafter, referred to as a LCD), an electric field is applied to liquid crystal (hereinafter, referred to as LC) molecules in a LC layer interposed between substrates to thereby change the direction of orientation of the LC molecules, and the display is obtained by a change in the optical quality of the LC layer resulting from the change in the direction of orientation of the LC molecules.
Particularly, an active matrix type LCD using active elements, which are represented by thin film transistor elements, has been employed as a display terminal of an OA apparatus having lower power consumption, by which the standard CRT will be replaced in terms of a response characteristic capable of coping with moving images at high precision and the like.
A conventional TN display system of the active matrix type LCD has a disadvantage in that it has a narrow visual field angle, which has posed a problem for the desired replacement of the standard CRT in terms of image quality.
On the other hand, a system (horizontal electric field system) for displaying information through the use of a change in double refraction properties of a LC by making the direction of an electric field to be applied to the LC, using comb-teeth electrodes, substantially parallel to the substrate surface, has been proposed, for example, in Japanese Patent Publication No. 63-21907 by “R. Kiefer, B. Weber, F. Windcheid and G. Baur, Proceedings of the Twelfth International Display Research Conference (Japan Display 192) pp. 547-550”.
This horizontal electric field system has advantages, such as wider visual field angle, and lower loading capacity than the conventional TN system, and this technique offers hope as a full-fledged active matrix type LCD capable of replacing the standard CRT.
SUMMARY OF THE INVENTION
As regards how to apply an electric field to the LC, a vertical electric field is applied to a LC layer using solid (transparent) electrodes in the TN system, whereas a horizontal electric field is applied using striped comb-teeth electrodes in the horizontal electric field system. In the horizontal electric field system, since the electric field is applied using comb-teeth-shaped electrodes, there was a problem in that its characteristic property greatly fluctuates depending upon the electrode finish width in the manufacturing process.
More specifically, in a case where comb-teeth electrodes are formed by a conventional process, when the electrode width varies because of the photo-mask precision, exposure uniformity of photolithography and variations of etching, fluctuation in the electrode interval caused thereby causes variations in the horizontal electric field intensity even if the same voltage is applied between these electrodes, thus causing luminance unevenness on the display surface.
Further, as regards the electro-optic characteristic including threshold, in the TN system, the electro-optic characteristic does not directly depend upon the electrode interval (=thickness of LC layer) because the interval between a pair of electrodes which apply the electric field to the LC layer coincides with the thickness of the LC layer; whereas, in the horizontal electric field system, changes in the characteristic due to fluctuation of the electrode interval become great because the electro-optic characteristic directly depends thereon independently.
In the horizontal electric field system, therefore, when the comb-teeth electrode interval fluctuates owing to variations in the electrode finish width, a large shift occurs in the electro-optical characteristic curve, which mainly causes display luminance unevenness.
In order to solve this problem, a method to secure the above-described electrode finish precision by forming paired electrodes, which apply the horizontal electric field to the LC layer, in the same layer, and by dividing a display surface to combine a small-area high precision photo-process with a stepper, has been proposed by, for example, “Y. Matsutani, S. Tahata, M. Hayashi, T. Onawa, K. Kobayashi, K. Nagata and M. Morishita, (SID97 Digest) pp. 14 to 18”. However, this method is inferior from the point of view of mass production of the display device because the divided photo-process using this stepper takes time, and this is considered to be a serious problem particularly when a large screen is increasingly used for the LCD in the future. As described above, the conventional horizontal electric field system active matrix type LCD has had a problem in that the electrode finish width is susceptible to the influence of the mass production process fluctuation, and surface luminance unevenness resulting from this fluctuation is prone to occur, resulting in inferior mass productivity.
It is an object of the present invention to provide an active matrix type LCD using the horizontal electric field system, which is excellent in mass producibility, in high image quality and which hardly causes display unevenness resulting from fluctuations in the electrode finish width.
According to the present invention, an attempt is made to solve the above-stated problems by causing other fluctuation factors at the same time which cancel and compensate for the effect of the electrode interval fluctuations to thereby reduce display luminance unevenness resulting from the electrode interval fluctuations in a horizontal electric field system of a LCD.
Hereinafter, the relationship between fluctuation factors, which cause display luminance unevenness in the horizontal electric field system, will be described.
In a translucent LCD element, a display luminance unevenness occurs because of unevenness in the transmission factor, and this is most easily recognized visually at a low transmission factor (relative transmission factor of 10% to 20%) corresponding to a low luminance. Since a transmission factor-voltage (T-V) characteristic in this low transmission factor area is considered to dominate the luminance fluctuation caused by a pixel electrode width fluctuation, we have considered a model in which the T-V characteristic in the low transmission factor area is linearly approximated. That is, the transmission factor-electric field (T-E) characteristic in the low transmission factor area is linearly approximated as follows:
T
(
E
)=&agr;(
E−Eth
)
Eth=
(Π/
d
)(
K
2
/&Dgr;
E
)
(Eth is threshold electric field, and is considered to be a constant here). When the horizontal electric field E is further approximated to E=V/1 using a pixel electrode interval
1
, the transmission factor-voltage (T-V) characteristic can be expressed as follows:
T
(
V
)=(&agr;/1)(
V−Eth
1
)
In this respect, this T-V characteristic model can be considered to be a model obtained by tangent-approximating not only limited to the low transmission factor area, but at any arbitrary point.
From the above equation, fluctuation (&Dgr;T) in the transmission factor due to minute fluctuation in each parameter is given by:
Δ
T
=
(
∂
T
/
∂
V
)
Δ
v
+
(
∂
T
/
∂
1
)
Δ
1
+
(
∂
T
/
∂
α
)
Δ
α
=
(
α
/
1
)
Δ
V
-
(
α
V
/
1
^
2
)
Δ
1
+
(
(
V
-
V
-
Eth1
)
/
1
)
Δ
α
(
1
)
This is a sensitivity analysis equation for each fluctuation factor with respect to total transmission factor fluctuation &Dgr;T. The &Dgr;
1
term is a term of fluctuation of the electrode interval
1
due to the pixel electrode width fluctuation which is the matter at issue now, and this becomes the main cause for the transmission factor fluctuation &Dgr;T in the left side, causing the display luminance unevenness. As a method of reducing the transmission factor
Ando Masahiko
Ishii Masahiro
Kondo Katsumi
Ohta Masuyuki
Tsumura Makoto
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Kovalick Vincent E.
Shalwala Bipin
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