Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1998-01-05
2002-05-07
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S052000
Reexamination Certificate
active
06384880
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a two-terminal type non-linear element used as a switching element, a method of manufacturing the two-terminal type non-linear element, and a liquid crystal display panel containing the two-terminal type non-linear element.
2. Description of Related Art
In an active matrix type liquid crystal display device, the space between an active matrix substrate containing switching elements provided for respective pixel regions to form a matrix array, and an opposite substrate containing, for example, color filters, is filled with a liquid crystal, and the alignment state of the liquid crystal in each of the pixel regions is controlled to display predetermined image data. For each of the switching elements, a threeterminal element such as a thin film transistor (TFT), or a two-terminal type element such as a metal-insulator-metal (MIM) non-linear element (referred to as a “MIM element” hereinafter) is generally used. The switching element having the two-terminal type element is excellent in that no cross-over short circuiting occurs, and the manufacturing process can be simplified, as compared with the three-terminal element.
In a liquid crystal display device containing the MIM elements, in order to realize a liquid crystal display panel which exhibits high image quality and high contrast in which unevenness in the display, after image phenomenon and sticking image phenomenon are not observed, it is important to satisfy the following conditions for the characteristics of the MIM element:
(1) The capacitance of the MIM element is sufficiently smaller than that of the pixel of the liquid crystal display panel;
(2) Changes in the current-voltage characteristics of the MIM element with respect to time are sufficiently small;
(3) The current-voltage characteristics of the MIM element have good symmetry;
(4) The current-voltage characteristics of the MIM element have sufficiently high steepness; and
(5) The element resistance of the MIM element is sufficiently uniform in a wide voltage range.
Namely, in order to increase the contrast, it is necessary that the capacitance of the MIM element be sufficiently small, as compared with the capacitance of the liquid crystal display panel which contains capacitances of one pixel electrode, a liquid crystal which is provided in the region driven by the pixel electrode, and a signal line provided opposite to the pixel electrode.
Also, in order to increase the contrast, the current-voltage characteristics of the MIM element should have sufficiently high steepness. In order to make the unevenness in the display unrecognizable, it is necessary for the MIM element to have sufficiently uniform resistance in a wide voltage range. In order to make the after image unrecognizable, it is necessary for the MIM element to show sufficiently small changes in the current-voltage characteristics with respect to time. Further, in order to make the sticking image unrecognizable, it is necessary for the MIM element to exhibit current-voltage characteristics having sufficiently small changes with respect to time and having good symmetry.
The “after image” is the phenomenon in which a first displayed image is observed when the display screen is switched to display a second image, and the first displayed image disappears in a short time. The “image sticking” is the phenomenon in which a first image is displayed over a long period of time, the display screen is switched to display a second image, and the first image is observed for a long time. The observed first image of the latter phenomenon will be referred to as the “sticking image”. Further, the phrase “the current-voltage characteristics have good symmetry” means that there is a sufficiently small difference between the absolute values of currents at a given voltage when a current is passed from a first conductive film to a second conductive film and when a current is passed from the second conductive film to the first conductive film.
Types of MIM elements have previously been proposed. For example, Japanese Patent Unexamined Publication No. 52-149090 proposes an MIM element containing a first conductive film of tantalum, an insulation film containing a metal oxide film formed by anodization of the first conductive film, and a second conductive film of chromium formed on the surface of the insulation film. Since the insulation film is formed by anodization of the surface of the first conductive film, the insulation film is formed with a uniform thickness without pinholes. Japanese Patent Unexamined Publication No. 57-122478 proposes that a dilute aqueous solution of citric acid is used as electrolyte for anodization. In these techniques, the characteristics (2) to (5) of the MIM element are not always sufficiently good. Namely, the changes with respect to time, the symmetry and the steepness of the current-voltage characteristics are insufficient, and the element resistance is not sufficiently uniform in a wide voltage range. Therefore, a liquid crystal display panel containing the MIM elements has a problem in that it is difficult to maintain high contrast in a wide temperature range in the panel, and it easily brings about unevenness in the display.
International Patent Application PCT/JP94/00204 (International Laid-Open No. WO94/18600) proposes a structure in which a tantalum alloy film containing tungsten is used as a first conductive film of an MIM element. In this technique, since the first conductive film of the MIM element contains not a single tantalum film but an alloy film of specific elements such as tantalum and tungsten, the characteristics (2) and (3), i.e., the changes with respect to time and symmetry of the current-voltage characteristics of the MIM element, are improved as compared with the techniques previously discussed with respect to Japanese Patent Unexamined Publication Nos. 52-149090 and 57-122478. In addition, the after image can be decreased to an unrecognizable level, and the contrast can be kept high in a wide temperature range. However, this technique does not provide sufficiently high contrast at high temperatures, and, as such, cannot be used in applications requiring high contrast at high temperatures.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a two-terminal type non-linear element exhibiting the characteristics (1) to (5) required for the MIM element described above. In particular, the MIM element should exhibit steepness of the current-voltage characteristics, sufficiently small changes in the current-voltage characteristics with respect to time, and high reliability.
Another object of this invention is to provide a liquid crystal display panel containing the two-terminal type non-linear element, exhibiting high contrast and high image quality, and causing neither unevenness nor sticking image in the display.
A further object of the present invention is to provide a method of manufacturing the two-terminal type non-linear element having the above described excellent characteristics.
In accordance with the present invention, a two-terminal non-linear element (referred to as a “MIM type non-linear element” hereinafter) contains a first conductive film, an insulation film and a second conductive film, which are laminated on a substrate. The insulation film is obtained by anodization of the first conductive film in a electrolyte containing a water solution. The insulation film has a relative dielectric constant of 25.5 or less, and preferably a relative dielectric constant of 24.0-25.5.
In an MIM type non-linear element of the present invention, a hydrogen spectrum of the boundary region between the first conductive film and the insulation film is obtained by elementary analysis carried out by secondary ion-mass spectrography (SIMS) using irradiation of cesium primary ions. The hydrogen spectrum in the depth direction preferably has a width of 10 nm or more, more preferably a width of 15-50 nm, at an intensity of one tenth of the peak intensity.
In accordance with the pre
Asakawa Tsutomu
Inoue Takashi
Ito Wataru
Naono Hideaki
Takano Yasushi
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