Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2002-11-07
2004-06-22
Wu, Shean C. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C252S299630, C252S299660, C252S299500, C570S127000, C570S129000, C570S130000
Reexamination Certificate
active
06753046
ABSTRACT:
The invention relates to a liquid crystalline compound, especially suitable as a component of a liquid crystalline medium with positive dielectric anisotropy and high optical anisotropy. Furthermore, the invention also relates to such a liquid crystalline medium and its use in electrooptical displays and projection systems, in particular reflective displays, LCoS™ displays and displays based on a birefringence effect, such as OCB displays.
Displays of the OCB (optically compensated bend) are based on a birefringence effect and comprise a liquid crystal layer with a bend structure. The bend cell, also known as pi-cell, was first proposed by P. Bos et al., SID 83 Digest, 30 (1983) for an electrically controllable half-wave plate, whereas the OCB mode for displays was described by Y. Yamaguchi, T. Miyashita and T. Uchida, SID 93 Digest, 277 (1993), followed by papers of T. Miyashita et al. in, inter alia, Proc. Eurodisplay, 149 (1993), J. Appl. Phys. 34, L177 (1995), SID 95 Digest, 797 (1995), and C. -L. Kuo et al., SID 94 Digest, 927 (1994). An OCB cell comprises a liquid crystal cell with bend alignment and a liquid crystal medium with positive &Dgr;&egr;. Furthermore, OCB displays as reported in the above papers comprise one or more birefringent optical retardation films to eliminate light leakage by the bend cell in the black state. OCB displays bear several advantages like for example a wider viewing angle and shorter switching times than conventional displays based on twisted nematic (TN) cells.
The above mentioned papers have shown that liquid-crystalline phases must have high values for the optical anisotropy &Dgr;n and a relatively high positive value for the dielectric anisotropy &Dgr;&egr;, and preferably should have rather low values for the ratio between the elastic constants K
33
/K
11
and for the viscosity, in order to be usable for high-information display elements based on the OCB effect. The industrial application of the OCB effect in electro-optical display elements requires LC phases which must satisfy a multiplicity of requirements. Particularly important here are chemical resistance to moisture, air and physical effects such as heat, radiation in the infra-red, visible and ultra-violet regions and direct and alternating electrical fields. Furthermore, LC phases which can be used industrially need a liquid-crystalline mesophase in a suitable temperature range, a relatively high birefringence, a positive dielectric anisotropy and a low viscosity.
LCoS™ (Liquid Crystal on Silicon) displays are known in prior art and are available from Three-Five Systems Inc. (Tempe, Ariz., USA). LCoS™ microdisplays are reflective displays that typically comprise a liquid crystal layer with twisted nematic structure sandwiched by a silicon backplane and a cover glass. The silicon backplane is an array of pixels, each of which has a mirrored surface which is at the same time a conductor. Each pixel comprises a stationary mirror covered by an active liquid crystal layer with twisted nematic orientation that can be switched into homeotropic orientation by application of a voltage. LCoS™ microdisplays are small with a diagonal of typically less than 1.0″, however, they enable high resolution from ¼ VGA (78 thousand pixels) to UXGA+ (over 2 million pixels).
Due to the small pixel size LCoS™ displays also have a very thin cell thickness, which is typically about 1 micron. Therefore, liquid-crystalline phases used in these displays must in particular have high values for the optical anisotropy &Dgr;n, in contrast to conventional reflective type LC displays, which usually require LC phases with low &Dgr;n. At the same time, high reliability against UV light is essential due to the backlight system of LCoS™ displays and projection displays in general.
None of the series of compounds having a liquid-crystalline mesophase which have been disclosed hitherto includes a single compound which meets all these requirements. Generally, therefore, mixtures of from two to 25, preferably from three to 18, compounds are prepared to give substances which can be used as LC phases. However, ideal phases cannot easily be produced in this way, since liquid-crystal materials having at the same time high birefringence and low viscosity were hitherto not available.
OCB mode and LCoS™ displays can be operated as matrix displays. Matrix liquid-crystal displays (MLC displays) are known. Examples of nonlinear elements which can be used to individually switch the individual pixels are active elements (i.e. transistors). This is then referred to as an “active matrix”, and a differentiation can be made between two types:
1. MOS (metal oxide semiconductor) transistors on silicon wafers as substrate,
2. Thin-film transistors (TFT) on a glass plate as substrate.
In the case of type 1, the electro-optical effect used is usually dynamic scattering or the guest-host effect. The use of monocrystalline silicon as substrate material restricts the display size, since even the modular assembly of various part-displays results in problems at the joints.
In the case of the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect. A distinction is made between two technologies: TFTs comprising compound semi-conductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive research efforts are being made worldwide in the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, while the inside of the other glass plate carries the transparent counterelectrode. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully colour-compatible image displays, in which a mosaic of red, green and blue filters is arranged in such a manner that each filter element is located opposite a switchable pixel.
The TFT displays disclosed hitherto usually operate as TN cells with crossed polarizers in transmitted light and are illuminated from the back. In case of OCB mode displays, however, reflective displays have also been proposed by T. Uchida, T. Ishinabe and M. Suzuki in SID 96 Digest, 618 (1996).
The term MLC display here covers any matrix display containing integrated nonlinear elements, i.e. in addition to the active matrix, also displays containing passive elements such as varistors or diodes (MIM=metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications or for high-information displays in automobile or aircraft construction. In addition to problems with respect to the angle dependence of the contrast and the response times, difficulties occur in MLC displays due to inadequate resistivity of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. As the resistivity decreases, the contrast of an MLC display worsens. Since the resistivity of the liquid-crystal mixture generally decreases over the life of an MLC display due to interaction with the internal surfaces of the display, a high (initial) resistance is very important for displays which must have acceptable resistance values over a long operating period. Furthermore MLC displays, especially for outdoor use, are exposed to UV-radiation by the sunlight, a situation similar to the influence of the backlight in projection and LCoS™ displays. Compounds of the liquid crystalline medium, especially tolane derivatives, which are used to achieve a high optical anisotropy, may decompose under intensive UV radiation, also leading to a decrease of the resistivity of the medium.
The disadvantage of the MLC-TN displays disclosed hitherto is due to thei
Bremer Matthias
Manabe Atsutaka
Merck Patent GmbH
Millen White Zelano & Branigan P.C.
Wu Shean C.
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