Liquid crystal cells – elements and systems – Liquid crystal system – Liquid crystal writing tablet
Reexamination Certificate
2002-05-13
2004-09-28
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Liquid crystal writing tablet
C349S158000, C349S141000
Reexamination Certificate
active
06798464
ABSTRACT:
BACKGROUND OF INVENTION
The present invention concerns a liquid crystal display device, display screens based on this structure, and a method for making such display screens.
The dramatic growth of the liquid crystal display (LCD) market has been mainly fueled by the strong demand for flat and lightweight color displays in notebook computers. High quality LCD displays require an active driving of pixel electrodes with thin film transistors (TFT) which are, however, costly to produce. This prevented so far spreading of LCD displays in the monitor and television market.
A display screen is an electro optical device to make data or images appear on a monitor or on a front end device. Most display screens work with the same principle as today's television screens using a cathode ray tube (CRT). Flat panel liquid crystal displays (LCD) are thin display screens that are used in a broad range of fixed and mobile devices like appliances, mobile phones, CD players, personal digital assistants and portable computers because they have a smaller geometric form factor and lower weight. More recently the brilliance, color saturation and pixel resolution of LCD displays have been improved so that they start to challenge CRT and other types of displays used in desktop monitors and television sets.
Nearly all modern flat panel displays use LCD technologies. LCDs utilize two sheets of polarizing material with a liquid crystal (LC) solution between them. An electrical field applied between electrodes across the liquid causes the crystals to align vertically so that the polarizing plane of the light is no longer rotated by 90 degrees and thus prevents the light from passing through the crossed polarizing sheets. Each cell, defined by the top and bottom electrode therefor, is like a shutter or valve either allowing light to pass through or blocking the light. Liquid crystals generally assume a restricted set of orientations with respect to a surface. This phenomenon, called anchoring, is the result of orientation dependent interactions between the surface and the liquid crystal. The control of the orientation in LCDs is central to the operation of all displays. A substantial amount of effort has been directed to deploy reproducible methods. Rubbing of a polyimide sheet is one known approach being employed to control the orientation.
Self assembled monolayers (SAM) of linear alkanethiols on metals like gold (Au), silver (Ag), and copper (Cu) or of linear phosphoric acid molecules on conductor oxides, e.g. nickel oxide, confer a remarkable level of control over structure and chemical functionality of their surface. In a publication entitled “Orientations of Liquid Crystals on Self assembled Monolayers Formed from Alkanethiols on Gold” in ACS Symp. 695, pp. 81 102 (1998), by Abbott et al. it is shown that unlike pure conductor surfaces, SAM covered surfaces direct liquid crystals to a planar anchoring, for SAMs formed from molecules longer than twelve carbon atoms. Azimutal ordering of the liquid crystal domains is introduced by formation of SAMs on Au films that are created by oblique or tilted evaporation. An external orientation parameter, the oblique evaporation, is needed to turn the system into a macroscopic light valve for display devices. The azimutal direction of liquid crystal alignment is perpendicular to the evaporation direction for SAMs with even numbers of carbons in their chain and parallel for SAMs with odd numbers of carbons in their chain. The mechanism for the alignment is the preferential roughness created by the oblique evaporation, a result also apparent from “Alignment of liquid crystal on a Polarizing Metal Film” in Appl. Phys. Lett. 56, pp. 1723 1724 (1990) by D. Armitage. Metal layers in both referenced papers are only partially transparent thus precluding manufacturing of commercially useful displays with high brightness.
Switching of one liquid crystal cell from the planar anchored status, i.e. where the plane of polarization is rotated by 90 degrees to the homeotropic status, i.e. no rotation, uses the application of an electrical field, as stated above. This can also be extended to an array of cells as it is found in a simple small monochrome display. In such a case each pixel is addressed for a short time only by a potential difference applied to its column and row. This passive matrix approach is simple, but due to the limited time available to address each pixel it creates visible artifacts and cannot be used to drive large high quality arrays.
Large LCD arrays are better controlled by an active matrix approach. Here each pixel is driven by a thin film transistor (TFT) that charges the pixel capacitor during the time it is switched on and isolates charge during the time other pixels are addressed. This active matrix display produces images with higher quality and is also capable to drive the threefold larger arrays needed for color images. In fact, the image quality of active matrix TFT LCD color displays can easily compete with CRT displays because refresh rates are higher. The TFT technology provides the best resolution of all flat panel techniques but is also the most expensive.
Color liquid crystal displays consist of several hundred thousands to millions of pixels, and each pixel consists of three sub pixels that separately control R (red), G (green), and B (blue). A color filter is used for each sub pixel in order to display RGB, so a full color image can be obtained by combining such sub pixel displays. In such a case, only one third of the light can be utilized. Color can also be generated by separating white light into R, G, and B by a prism or a grating. To get a full color image each color is then gated separately by a subset of a pixel triplet. In SID Symposium 1998, p. 199, a method is proposed wherein, as a liquid crystal projector, arc lamp light is reflected on a diffraction grating surface to be separated into RGB light, collected by a microlens array and have its transparency controlled by a liquid crystal layer for each sub pixel.
LCD screens use backlight so that they are visible in the dark and show a higher contrast in bright environments. Space requirements of the backlight in portable displays are tight so that no far field projection is used. Instead, typical backlights are composed of a linear light source at the side of the display that feeds light into a planar light guide that illuminates the whole display area homogeneously. Backlights used to date emit light with a large opening angle which is welcome to provide a good viewing angle for the user but prevents use of the light separation protocol described above. In the Japanese patent application Nr. 011 43246, of Feb 2, 1999 entitled “Transparent Type Liquid Crystal Display”, a type of substrate is described that creates parallel light which is separated by means of a grating into RGB light. R, G, and B are emitted under slightly different angles onto a microlens array and focused onto the respective LCD pixels. Having passed a first polarizer sheet, the liquid crystal layer, and a second polarizer the light needs to be diffused again to create a homogenous angle independent color for the viewer.
The fabrication costs of displays are directly proportional to the number of the lithographic layers, the number of vacuum deposition steps, and the overall number of assembly steps. A passive matrix display needs assembly of two crossed polarizers, two transparent plates with patterned ITO electrodes possibly with an additional conductor black matrix, two rubbed layers of polyimide, and when color is needed an additional sheet containing patterned RGB color filters. For active matrix displays the fabrication costs of TFTs currently push up the overall cost by a large factor, because TFTs are fabricated using a four to seven mask process and either a Co or an Al gate metallization.
In a conventional LCD, the liquid crystal material is placed in a homogeneous electric field between the two transparent electrodes. The light propagates from a backlight source through a bottom transparent pla
Bietsch Alexander
Delamarche Emmanuel
Michel Bruno
Schmid Heinz
Dudek James A.
Tuchman Ido
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