Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1998-03-06
2001-04-24
Malinowski, Walter J. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S051000, C349S052000
Reexamination Certificate
active
06222596
ABSTRACT:
This invention relates to thin film diode elements (e.g. MIM diodes), such as diodes used in an active matrix liquid crystal display (AMLCD) or x-ray imaging apparatus.
BACKGROUND OF THE INVENTION
In known electronic matrix systems, an array of storage elements, each having a unique address, is utilized for storing electric charge and can include, for example, memory arrays and/or LCDs. In LCDs, the storage elements take the form of picture elements or pixels. The pixels generally include a pair of spaced apart electrodes having liquid crystal material disposed therebetween. Thus, each pixel constitutes a capacitor in which electric charge can be stored. The charge stored in a pixel results in a voltage potential across the opposing electrodes and an electric field across the liquid crystal material. By controlling the amount of charge stored in pixels across the array, the properties of the liquid crystal (LC) material can be controlled to obtain a desired light influencing effect or image which is displayed to a viewer.
In LCDs, it is necessary to update the condition of each pixel at regular intervals, i.e. at a given frame rate. This is required because the pixels can retain or store the applied charge potentials for only a finite time. Updating is further required in order to change the image to be displayed (e.g. when the image is changing or moving). Accordingly, the ability to rapidly transfer to, and store electric charge in, pixels and to efficiently retain the stored charge therein for a frame period is essential.
Thin film diodes (TFDs), including metal-insulator-metal (MIM) diodes, are easier to fabricate than FET/TFT LCDs and conventional diode LCDs. A typical e.g. MIM electronic matrix array requires between two and four thin film layers and photomask steps, as compared to 6-9 thin film layers and photomask steps for TFT arrays. Patterning of most MIM arrays can be achieved with less stringent overlay accuracy and resolution requirements, then is required for TFT arrays. As a result, less expensive photo-exposure equipment, such as scanning projection aligners, can be used, that have more than twice the throughput and cost less than half as much as flat panel steppers.
Despite their lower production costs, MIM driven LCDs are not widely used. This can be attributed to the inferior performance of typical MIM LCDs with regard to gray shade control, image retention, response time, and maximum size and resolution as compared to TFT LCDs. Accordingly, there exists a need in the art for an improved MIM LCD drive scheme, and MIM or TFD diode, which results in a display (or imager) which is cheaper to manufacture, less susceptible to image retention and gray scale problems, and has good resolution.
It is apparent from the above that there exists a need in the art for an improved MIM diode driven LCD or other electronic matrix array, which (i) is designed so as to require less complex and less expensive circuitry; (ii) has improved gray shade control; (iii) is less sensitive to image retention than previous MIM LCDs and has had good resolution characteristics; and (iv) is capable of being used with row inversion systems that are commonly used in LCDs and the like.
With regard to insulator (same as semi-insulator) materials for TFD and MIM diodes, U.S. Pat. No. 5,142,390 discloses a MIM diode with a doped hard carbon film as an insulator. Unfortunately, the doping in the '390 patent may not be homogeneous over the area of the MIM (i.e. not uniform over the area of the MIM) which can cause non-uniformities in electrical characteristics (e.g. permittivity or dielectric constant) of the MIM. In the '390 patent, the insulator material is only 0.1-0.03% nitrogen, which is too low achieve the frankel-poole effect, and is also too low to achieve carbon nitride “alloy” because such trivial amounts are simple dopants and these dopants may not be activated. Also, the insulator of the '390 patent could not include substantial amounts of nitrogen (the '390 patent does not disclose adding substantial amounts of nitrogen to the carbon) without becoming a conductor, as adding substantial amounts of nitrogen (which the '390 patent does not do or suggest) to the MIM of the '390 patent will cause the film to have an ohmic conduction and it will no longer be an insulator and will no longer show a frenkel-poole or space charge limited current flow mechanism. The '390 patent does not add substantial amounts of nitrogen to the carbon, because in the '390 patent the type of hard carbon films being used would not enable the nitrogen to be bonded into the matrix of carbon atoms. Thus, no alloy is formed in the '390 patent using carbon and nitrogen. An “alloy” is a compound that is formed from at least two elements, and the amounts of at least two of the elements must be substantial (e.g. at least about 5% and preferably more). In other words, simple doping does not create an alloy.
It will be apparent to those of skill in the art that there exists a need in the art for an improved TFD or MIM device which uses an “alloy” as a semi-insulator. Alloys have more hardness, improved I-V characteristics, less of a dielectric constant, are more stable, etc. than non-alloys. Also, alloys can be grown over large areas with homogeneous stochiometric. It is also desirable to have a non-photoconductive alloy which does not react to light. Display or imager performance is improved as a result of the above.
It is a purpose of this invention to fulfill the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.
SUMMARY OF THE INVENTION
Generally speaking, this invention fulfills the above-described needs in the art by providing a pixel for a liquid crystal display, the pixel comprising:
a capacitor formed by a liquid crystal layer sandwiched between first and second electrodes;
first and second nonlinear thin film diodes;
a node coupling together (i) said first diode, (ii) said second diode, and (iii) said capacitor;
first and second parallel select lines, said first select line connected to said first diode and said second select line connected to said second diode;
a data line oriented perpendicular to said first and second select lines; and
each of said thin film diodes including first and second diode electrodes sandwiching an insulating layer therebetween to form a diode, and wherein said insulating layer includes a carbon nitride alloy.
This invention further fulfills the above described needs in the art by providing a thin film diode for use in an LCD or an imager (e.g. x-ray imager of the direct or indirect type), the diode comprising:
a first conductive electrode supported by a substrate;
a second conductive electrode supported by the substrate;
a semi-insulating material including one of carbon nitride alloy, nanocrystalline carbon, and polycrystalline carbon, the semi-insulating material being sandwiched between said first and second electrodes.
This invention further fulfills the above described needs in the art by providing a thin film diode comprising:
a first conductive electrode supported by a substrate:
a second conductive electrode supported by the substrate;
a semi-insulator alloy layer located between said first and second electrodes; and
wherein said semi-insulator alloy layer includes amorphous carbon nitride alloy, and from about 20-60% of the atoms in said alloy are nitrogen atoms.
In certain embodiments, the alloy is substantially transparent to visible light and from about 40-90% of the atoms in the alloy are carbon atoms.
This invention will now be described with reference to certain embodiments thereof as illustrated in the following drawings.
REFERENCES:
patent: 4731610 (1988-03-01), Baron et al.
patent: 5117299 (1992-05-01), Kondo et al.
patent: 5142390 (1992-08-01), Ohta et al.
patent: 5576728 (1996-11-01), Maeda
patent: 0217466 (1987-04-01), None
patent: 0434627 (1991-06-01), None
patent: 0447077 (1991-09-01), None
patent: 047
Laff, Whitesel & Saret, Inc.
Malinowski Walter J.
OIS Optical Imaging Systems, Inc.
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