Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-12-01
2003-07-15
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S049000, C349S051000, C257S030000
Reexamination Certificate
active
06593991
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display apparatus comprising a two-terminal nonlinear device such as an MIM (Metal Insulator Metal) device.
2. Description of the Related Art
In recent years, liquid crystal display apparatuses have widely been used for displaying purposes in personal computers, word processors, terminal displays of office-automation equipment, television image display apparatuses and like applications by virtue of their advantageous characteristics such as low power consumption, thinness and lightness. Liquid crystal display apparatuses are expected to find wider use, particularly, as image displays of portable information terminal devices. An electronic book, which serves as a substitute of a conventional book formed by binding printed paper sheets, is one such information terminal device. According to the aimed specifications of a liquid crystal display apparatus for use in this device, the screen size is about 6 to about 7 inches, the definition is about 1024×768 dot XGA, and the operating temperature range is about −20 to 70° C. An active matrix type liquid crystal display apparatus using an MIM drive has been disclosed in, for example, Japanese Unexamined Patent Publications JP-A 59-83190 (1984) and JP-A 9-54344 (1997).
FIGS. 9 and 10
illustrate part of the configuration of a conventional MIM-drive active matrix type liquid crystal display apparatus.
FIG. 9
is a plan view of a partial configuration associated with one pixel, and
FIG. 10
is a sectional view taken on line X—X in FIG.
9
. On an electrically insulating glass substrate
1
is formed a thin tantalum (Ta) film having a thickness of 3000 Å which will form a signal line
2
and a lower electrode
3
by sputtering or a like process. The thin tantalum film is patterned into a desired configuration to form the signal line
2
and the lower electrode
3
by photolithography. Subsequently, the surface of the lower electrode
3
is subjected to anodizing to form a 600 Å-thick insulating film
4
comprising tantalum pentoxide (Ta
2
O
5
). On the entire surface of the substrate in this state is stacked a titanium (Ti) film, which will form an upper electrode
5
, to a thickness of 4000 Å by sputtering or a like process, followed by patterning into a desired configuration by photolithography to form the upper electrode
5
. In this way, there is formed a single MIM device
6
comprising the lower electrode
3
, insulating film
4
and upper electrode
5
.
Further, in the case where the liquid crystal display apparatus to be constructed is of the transmissive type, a transparent electrode film of ITO (Indium Tin Oxide) or a like material is stacked on the resulting structure and then patterned into a pixel electrode
7
. Alternatively, in the case where the apparatus is of the reflective type, a reflective electrode film comprising aluminum (Al) or a like material instead of ITO or the like is stacked on the resulting structure and then patterned into a reflective pixel electrode, or, alternatively, a transparent electrode
7
of ITO or a like material is formed on the resulting structure, followed by affixing a reflective plate to the whole reverse side of the glass substrate
1
. A plurality of such pixel electrodes are arrayed in a matrix shape, and signal lines
2
are routed to associated parts so that each pixel electrode
7
should be selectively driven through the associated MIM device
6
. Similarly, pixel electrodes are formed on a counterpart glass substrate. The pair of substrates are mated with each other with their respective surfaces formed with respective pixel electrodes facing each other, and then a liquid crystal layer is placed between the pair of substrates to form the liquid crystal display apparatus.
FIGS. 11A and 11B
illustrate an equivalent electric circuit configuration per pixel of an active matrix type liquid crystal display apparatus using an MIM drive and the voltage-current characteristic of an MIM device, respectively. In the equivalent circuit per pixel as shown in
FIG. 11A
, a parallel circuit including a resistor RMIM comprising the MIM device and a capacitor CMIM is serially connected to a parallel circuit including a resistor RLC comprising the liquid crystal layer and a capacitor CLC. When the liquid crystal layer is applied with a driving voltage V through the MIM device
6
, a voltage VLC and a voltage VMIM are applied to the liquid crystal layer and the MIM device, respectively. The MIM device has the voltage-current characteristic as shown in FIG.
11
B. As shown, the MIM device
6
exhibits a very large resistance and hence hardly allows a current to pass therethrough until the voltage VMIM at opposite ends of the MIM device
6
reaches a threshold voltage VTH. When the absolute value of the applied voltage VMIM exceeds the threshold voltage VTH, the MIM device
6
exhibits a decreasing resistance, while the voltage VLC applied to the liquid crystal layer increases to give rise to an electric field that changes the alignment of liquid crystals in the liquid crystal layer.
As described above, a liquid crystal display apparatus for use in an electronic book has a panel screen size of 5 to 7 inches and a definition as high as XGA, and operates within an operating temperature range of −20 to 70° C. according to the specifications thereof. In implementing a liquid crystal display apparatus with a screen having such a size and such an XGA-grade definition, the wiring resistance of the routed electrodes and the charge addressing time raise a problem. With increasing wiring resistance, a signal applied is rounded to a greater extent and, hence, a higher driving voltage becomes required. As the location of an MIM device associated with each pixel becomes remoter from a terminal for driving the active matrix type display apparatus, the resistance of the wiring from such a terminal to the MIM device increases. Therefore, the lighting characteristic of the panel used as a liquid crystal display apparatus varies at different points of the panel which correspond to points at which differences in resistance arise. This results in a non-uniform display and like inconveniences. In the liquid crystal display apparatus described in Japanese Unexamined Patent Publication JP-A 59-83190 (1984), a pair of signal lines extending in opposite directions from a pair of terminal electrodes, respectively, are placed opposite to each other, and an MIM device is disposed between and connected to each of the signal lines and each pixel electrode. This arrangement described in this Gazette, however, aims to correct a pixel defect and, therefore, a driving signal is delivered to the pixel electrode from only one of the pair of signal lines via the associated MIM device in a normal state and, in case of the presence of a defective MIM device connected to the usually used signal line, the other signal line is used to deliver such a driving signal to the pixel electrode. This means that the Gazette does not disclose any arrangement to deliver driving signals to a pixel electrode from both of the pair of signal lines and, accordingly, a non-uniform display and like inconveniences cannot be prevented.
As a duty ratio increases with a higher definition, the charge addressing time per pixel is shortened. This results in degraded ON characteristic of MIM device
6
in particular. An active-matrix type panel in which one pixel electrode is provided with one MIM device
6
is usable within the operating temperature range of from about 0 to about 60° C., or from about −20 to about 40° C., and cannot be used within a wider temperature range above 60° C. Japanese Unexamined Patent Publication JP-A 9-54344 (1997) discloses a liquid crystal display apparatus in which two MIM devices having different I-V characteristics are connected to one pixel electrode. This apparatus described in this Gazette, however, is configured to separately apply an on voltage and an off voltage f
Akkapeddi P. R.
Birch Stewart Kolasch & Birch, LLP.
Kim Robert H.
Sharp Kabushiki Kaisha
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