Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-11-30
2004-12-28
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06836309
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to liquid-crystal-on-silicon (LCoS) microdisplay systems and, more particularly, to a high contrast microdisplay system using a vertically aligned nematic reflective cell design.
BACKGROUND OF THE INVENTION
Microdisplays are the most recent addition to the family of flat-panel displays. Microdisplays are based on a number of different techniques to generate modular light using microfabrication technologies to produce a rectangular array of pixels on a semiconductor back plane. Examples of microdisplays include liquid crystal displays, field emission displays, and digital micro-mirror displays.
Liquid crystal displays (LCD) using a single polarizer plate (herein referred to as single polarizer plate method) provide high resolution and high contrast. Many LCD's utilize a quarter-wave plate to produce high contrast.
Japanese Laid-Open Patent Application No. 55-48733/1980 (Tokukaisho 55-48733/1980) discloses a reflective TN-method (45° twisted) LCD having one polarizer plate and a quarter-wave plate. By using a 45° twisted LC layer and controlling the electric field applied across the LC layer, the device displays black and white by effecting two states in which the polarization plane of the incident linearly polarized light on the quarter-wave plate is parallel and twisted by 45° with respect to the optic axis of the quarter-wave plate. The LCD display includes a polarizer, a 45° twisted LC layer, a quarter-wave plate, and a reflective plate arranged in sequence from the light incident side.
U.S. Pat. No. 4,701,028 (Clerc et al.) discloses a reflective-type LCD device including a combination of a single polarizer plate, a quarter-wave plate, and a vertically aligned LC cell. Japanese Laid-Open Patent Application No. 6-337421/1994 (Tokukaihei 6-337421/1994) discloses a reflective-type LCD device including a combination of a single polarizer plate, a quarter-wave plate, and a bend vertically aligned LC cell. A New Reflective Display with High Multiplexibility and Gray Scale Capability (Euro Display '96, page 464) also discloses a reflective-type LCD device including a combination of a single polarizer plate, a quarter-wave plate, and a vertically aligned LC cell. SID 96 Digest (page 763) discloses an example of an application to a reflective projection of a display mode in which chiral-dopant-containing LC having negative dielectric anisotropy is sandwiched between upper and lower substrates with vertical alignment.
A polarizer plate, in combination with the LCD device cell on the incident side, allows only the incident light and outgoing light polarized linearly in a certain direction to pass therethrough, and blocks light that is polarized linearly in all the other directions. If no electric field is applied to the LC layer, the incident light having passed through the polarizer plate is converted into circularly polarized light by an optical retardation compensation plate such as a &lgr;/4 plate (quarter-wave plate), enters the LC layer, passes through the vertically or substantially vertically aligned LC layer, and without any further conversion or change, reaches a reflective plate. The light having reached the reflective plate is converted into circularly polarized light of reverse rotation by the reflective plate, passes through the LC layer, the &lgr;/4 plate and other components in reverse order from the incident light, and is converted to light that is linearly polarized vertical to the linearly polarized incident light. The dark state is thus effected.
In addition, if the LC inclines or tilts upon application of an electric field across the LC layer to realize a phase difference under certain conditions, the incident circularly polarized light (having passed through the polarizer plate and &lgr;/4 plate) is converted into linearly polarized light, becomes linearly polarized at the reflective plate, resulting in out-going linearly polarized light having the same polarization direction as the linearly polarized light at the polarizer plate, which is the incident light for the display device having passed through the polarizer plate. The bright state is thus effected.
There are some problems with the vertical alignment LCD device described in U.S. Pat. No. 4,701,028, U.S. Pat. No. 4,492,432, Japanese Laid-Open Patent Application No. 6-337421/1994, and A New Reflective Display with High Multiplexibility and Gray Scale Capability (Euro Display '96). Since the directions of the vertical alignment, especially those of the tilted vertical alignment, are parallel between the upper and lower substrates, the LC inclines or tilts in one direction and the viewing angle dependence of the image on the display surface is extremely large. Also, since the reflectance has a large wavelength dependence, undesirable coloring occurs.
Many LCD devices adopt a planar alignment mode where LC molecules are aligned parallel to the substrate when no voltage is applied. In a vertical alignment mode, LC molecules are aligned vertically relative to the substrate when no voltage is applied. When a LCD device operates in a so-called normally black mode, in which dark display is effected using the vertical alignment when no voltage is applied, a darker and more achromatic black display can be effected in comparison to the planar alignment mode, and therefore the display contrast is improved. Since the LC layer does not cause birefringence with light transmitted in the normal direction of the LC layer through the LC layer when no voltage is applied across the LC layer, the normally black mode has the characteristic that the required accuracy in the LC layer thickness (LC cell gap) is less.
In the vertical alignment mode, the LC layer thickness can be larger than the conventional planar alignment mode, there can be wider variations in the cell gap, and improved black display is possible. The primary factor limiting high contrast in LCD's is the amount of light which leaks through the display in the dark state. If vertical alignment normally black mode is used, there is little negative effect from light leaking as a result of the use of spacers to maintain the cell gap of the LC layer. For these reasons, vertical alignment is a preferred alignment for high contrast displays.
However, a uniform alignment state is difficult to achieve with the vertical alignment mode. It is also difficult to effect stable alignment when an electric field is applied across the LC layer and the alignment of the LC layer molecules is deformed from the vertical direction.
Publications including Enclosure Electrode Method (Japanese Laid-Open Patent Application No. 7-64089/1995 [Tokukaihei 7-64089/1995]) and Multidomain Method (Nikkei Microdevice, January 1998 Issue, page 136) disclose a proposal to change the shape of the substrate by changing the direction of the electric field, or by use of an insulating structure within the pixel area of a display.
Methods to slightly incline (tilt or pre-tilt) the initial LC alignment with respect to the normal direction of the substrate by rubbing (Japanese Laid-Open Patent Application No. 62-299814/1987 [Tokukaisho 62-299814/1987]) or by photo-induced polymeric alignment (Control of Liquid Crystal Alignment Using an Optically Active Polymer Film, by Mr. Yasushi IIMURA, Tokyo University of Agriculture and Technology, First JLCS-ALCOM Joint Conference) are disclosed to obtain uniform alignment with respect to the vertical alignment over the entire panel when voltage is applied. These methods provide alternative solutions to the problem other than the creation of microdomains. Unfortunately, this pretilt angle has been shown to reduce the effectiveness of the black state. (H. Kurogane, K. Doi, T. Nishihata, A. Honma, M. Furuya, S. Nakagaki, I. Takanashi, Victor Company of Japan, SID Dig. Tech. Pap., 29, 33-36).
The enclosure electrode method and the multidomain method sandwich the upper and lower substrates LC layer in a precise and complex structure. This results in an increased number of m
Anderson James
Stefanov Michael
Akkapeddi P. R.
Chowdhury Tarifur R.
HANA Microdisplay Technologies, Inc.
Roetzel & Andress
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