Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-05-01
2003-01-14
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06507385
ABSTRACT:
BACKGROUND OF THE INVENTION
(1). Field of the Invention
The present invention relates to a liquid crystal display element, which is utilized for an optical shutter and the like, and a manufacturing method thereof.
(2). Description of the Prior Arts
A conventional and general liquid crystal display element, wherein a twisted nematic liquid crystal material (hereinafter referred to as “TN liquid crystal”) and an active matrix mode are combined, changes an alignment state of a liquid crystal molecule according to the applied voltage by holding the TN liquid crystal between two substrates with an electrode and applying voltage between the above-mentioned substrates. Light transmittance in a liquid crystal molecule changes according to an alignment state. Thus, a mode (a longitudinal electric field type) in which light transmittance is controlled by voltage is used for TN liquid crystal. A liquid crystal display element is composed of an active matrix substrate with a switching element for controlling whether voltage is applied or not, typically a thin film transistor hereinafter referred to as “TFT”), and a pixel electrode; an opposite substrate; a liquid crystal sealed between both substrates; and a polarizer disposed outside both substrates. An alignment state of a liquid crystal molecule is changed by an applied voltage between electrodes on both of the substrates, and thereby light transmittance is changed
However, in such a general liquid crystal display element, optical activity of transmitted light for an observer changes according to viewing angle, namely, an angle with a screen, and consequently brightness is affected. For instance, an image with a high contrast ratio is observed in the case of viewing from a front direction vertical to a screen with a white display, namely, a normal direction to the screen, while an image is observed dark in the case of viewing the screen from an oblique direction below the normal direction. When the screen is viewed from a much lower direction, the phenomenon of tone reversal, wherein brightness and darkness are reversed, occurs. When the screen is viewed from an upper oblique direction, an image is observed white. This phenomenon occurs for the reason that a direction, in which a liquid crystal molecule is set up, is determined in a display mode wherein optical activity is controlled by applying electric field to liquid crystal in a normal direction to a substrate and setting up a liquid crystal molecule along a direction of the electric field. Since a multitude of micro globes ‘spacer’ is scattered for obtaining the uniformity of cell thickness, the unevenness occurs easily on a screen, whereby the grade of display is badly affected.
Recently, a lateral electric field type has been noticed as a means of improving such a problem of viewing ankle. As described above, in a conventional liquid crystal display element, electric field is applied in a normal direction to a substrate and an alignment state of liquid crystal is controlled, while a lateral electric field type is a mode of controlling by an electric field in a lateral direction parallel with a substrate. This mode supplies a wide viewing angle in principle, and is considered as an idea for improving with a highest effect because of little change in color tone. Thus, although a viewing angle of a lateral electric field type liquid crystal display element is greatly wider than that of a longitudinal electric field type, the element has a wide area of shielding such as a common electrode, a source electrode and a switching element, therefore an aperture ratio of a pixel has no choice but to become lower than a conventional TN type. Consequently, a bad influence of many spacer particles, which is scattered for obtaining the cell thickness, on the grade of display is greater than a conventional TN mode.
In order to solve these problems and to obtain a fine grade of display, the following methods have been employed. a column-shaped spacer is formed in an area of shielding on an active matrix substrate with such a switching element as TFT; and a column-shaped spacer is formed in an area of shielding, typically a black matrix in a pixel part of a color filter.
In a conventional TN type, a structure, in which it is unnecessary to scatter spacer particles, has been proposed. For instance, a liquid crystal display element, wherein a spacer support is formed by joining together projections provided on both of an active matrix substrate and a substrate with a color filter, has been proposed in Japanese Unexamined Patent Publication No. 7-281295.
As regards a lateral electric field type, a liquid crystal display element using no spacer particles has been proposed in Japanese Unexamined Patent Publication No. 6-214244. In the liquid crystal display element, both of a common electrode and a pixel electrode (a source electrode) are formed while setting up vertically to a substrate, and both of the electrodes serve as a spacer (a support). Therefore, spacer particles are not necessary.
A conventional liquid crystal display element, wherein a polymer dispersed liquid crystal material (hereinafter referred to as ‘PDLC’) and an active matrix substrate with such a switching element as TFT are combined, has a constitution in which a liquid crystal layer, wherein a liquid crystal molecule is dispersed in a droplet or a network into polymer hereinafter referred to as ‘polymer matrix’) with a matrix structure, is held between two substrates with an electrode. A molecular structure of both is designed in advance so that a refractive index of a liquid crystal molecule aligned in an applied direction is equal to that of the above-mentioned polymer matrix under an application of voltage. Since refractive indexes of the liquid crystal molecule and the polymer matrix are equal under an application of voltage, an incident light into a PDLC layer becomes a transmitted light. On the other hand, the liquid crystal molecule is aligned in a disorderly direction under no application of voltage. Consequently, refractive indexes of the liquid crystal molecule and the polymer matrix are ordinarily unequal, and thereby an incident light into a PDLC layer becomes a scattered light. Thus, a transmitting state and a scattering state of an incident light are switched in PDLC by whether voltage is applied or not.
Also in a conventional polymer dispersed liquid crystal display element, a multitude of micro globes ‘spacer’ is scattered for obtaining the cell thickness to realize a fine grade of display. The spacer in the polymer dispersed liquid crystal display element is used for a projection display for enlarging and projecting on a screen an image of light projected from a lamp with a high luminance, and a reflection display for displaying an external light by utilizing a reflector provided inside the liquid crystal display element.
A dispersion density of spacers in a liquid crystal layer can be lowered in appearance by providing a spacer beforehand in an area of shielding on a substrate used for conventional TN type liquid crystal display element or lateral electric field type liquid crystal display element. However, on the contrary, the problem is a deterioration in the grade of display due to a change in temperature. This is described below referring to FIG.
12
.
The problem is a low-temperature foaming during a change from room temperature to low temperature. When a liquid crystal display element
100
is left at a low temperature, the volume of a liquid crystal molecule contracts. Then, the cell thickness needs to contract, namely, thin in the thickness direction according to the volume contraction of the above-mentioned liquid crystal molecule. A low-temperature foaming
108
is a phenomenon wherein a vacuum foam is caused in a liquid crystal layer since the cell thickness can not follow such volume contraction of the liquid crystal molecule. When a spacer
105
formed on a substrate by patterning is firm, the low-temperature foaming
108
is caused easily since the contraction of the cell thickness according to the
Nakao Kenji
Nishiyama Seiji
Uemura Tsuyoshi
Matsushita Electric - Industrial Co., Ltd.
Parker Kenneth
Parkhurst & Wendel L.L.P.
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