Liquid crystal displays find use in a variety of different applications, such as in data displays in watches, calculators and the like, as well as in flat panel displays found in laptop or notebook computers. Liquid crystal displays offer many advantages over alternative technologies such as cathode ray tube displays, including low power consumption, small size, light weight, and the like. For these reasons, liquid crystal displays are expected to eventually replace present competing display devices in a wide variety of applications, including desktop computer monitors and televisions.
Liquid crystal (LC) materials are composed of rod-like molecules that may be aligned by intermolecular forces and in response to external electric fields. A liquid crystal display (LCD) device includes cells of an LC material with the LC material present between two opposing transparent glass plates with electrodes, or between a transparent glass plate with an electrode and a reflective plate with an electrode. The glass plates serve as substrates for the other elements, although a plate with its attached elements may also be called a substrate. Electrodes are commonly made of indium tin oxide (ITO), which is transparent. Polarizer plates or materials are placed on the outer surfaces of the glass plates. Onto the inner surfaces of the glass plates, on top of the electrodes, an alignment film (commonly polyimide) is coated. The alignment film may be treated or modified so that it has an alignment layer which can orient nearby LC molecules. There is a gap, a few microns wide, between the two plates, so that the alignment films are parallel but do not directly contact each other. LC molecules are filled into this gap. The entire assembly may be transparent, and is capable either of modulating light that passes through it, or, if one substrate is reflective, of modulating reflected light. In order to modulate transmitted or reflected light, and to provide contrast, it is necessary to control the orientation of the LC molecules that fill the gap between the plates, since the orientation of these molecules alters the polarization of light passing through the gap. The two polarizer plates are generally "crossed" (oriented at 90.degree. to each other), so that, unless the polarization of the light changes as the light passes between the plates, polarized light that passes through one plate will not pass through the other. It is the re-orientation of the LC molecules in response to the electric fields that are imposed by the electrodes that alters the polarization of light passing between the plates and varies the intensity of the light leaving the device. The varying light intensity of a multitude of cells operating in parallel forms the images that allow the device to operate as a display.
LC molecules stack to fill the space between the plates, with the long axis of the rod-like LC molecules nearly parallel to the plane of the surfaces of the alignment film on the substrates, and with adjacent molecules aligned in generally the same orientation by the alignment layer. At the limits of the gap, the LC molecules have to be anchored down in such a way as to align with the two crossed polarizer plates adjacent to these films. Since these alignment directions are at 90.degree. to each other, molecules on opposite sides of the gap between the plates point in perpendicular directions although in planes nearly parallel to the surfaces of the polyimide substrates. In between, across the breadth of the gap, the orientation of the LC molecules gradually shifts between the two orientations. The LC molecules thus form a twisted helix from one side to the other. However, the orientation of the LC molecules can be adjusted by the alteration of an electric field imposed between the plates.
Light entering an LCD device passes through, and is polarized by, a first polarizer before encountering the LC molecules in the gap between the plates. The first LC molecules in this gap are anchored to the nearest alignment layer, and are aligned with their long axes parallel to that of the adjacent polarizer. A second polarizer with polarization plane perpendicular to the first polarizer is in place at the opposite plate. If there were no change in the polarization of the light passing through the gap, virtually no light would emerge from the second plate since the planes of polarization are perpendicular. However, as the light progresses through the LC material to the second plate and second polarizer, the gradual helical twisting of the LC orientation changes the polarization of the light from linear to elliptical, so that part of the incoming light is transmitted by the second, perpendicular polarizer, allowing some light to emerge for viewing by an observer. Since the amount of light that is transmitted depends on the orientation of the LC molecules, rotation of the long axis of the rod-like LC molecules changes the amount of light that is transmitted. The orientation of the LC molecules is changed by the application (via the electrodes) of small voltages to each color cell within all the pixels.
In typical flat panel displays currently found in many laptop computers, the picture on the screen of the display is composed of many hundreds of thousands of pixels (so-named because each pixel is one "picture element"). Each pixel may be further divided into smaller regions ("color cells") that provide differently-colored spots within the pixel. In this way, the desired color is created in each pixel by "mixing" blue, green and red primary colors of different intensities by using patterned color filters of these three colors. The intensity of each color is adjusted by separate electrodes to orient the LC molecules to change the light intensity transmitted out of the front of the display.
As the voltage of the electrodes is increased, the LC long axis becomes increasingly parallel to the electric field direction, which is parallel to the light transmission direction, and not parallel to the planes of the substrates. Orienting the LC molecules more closely to that of the electric field reduces light transmission. Thus, it is apparent that the proper orientation of the LC molecules is critical to the operation of an LCD device.
The alignment film, with its alignment layer, that forms the boundary between LC-filled gap and the plate is the most important feature used to align the LC molecules to the substrate. The alignment layer is an insulating layer composed of molecules that are aligned in a predetermined direction, and serves to orient the LC molecules in that direction. The alignment of the alignment layer molecules is determined by the molecular structure of that layer, and is stable, the molecules being bound to the substrate and each other. The alignment layer imposes an ordered orientation upon the LC molecules directly in contact with it through non-covalent molecular interactions such as van der Waals, ionic and steric interactions. The rod-like LC molecules in contact with the alignment layer can and do orient to accommodate to the fixed orientation of the alignment layer.
Polyimide films are commonly used to form alignment films with alignment layers. Such films can be formed by applying a wet coat of polyimide to a substrate, as can be done by known printing or spinning techniques, baking the wet coat to form a polyimide film on the substrate that may then be run through a roller to even out the surface of the film. Then, some technique for providing alignment properties to the film must be applied.
The preferred technique for providing an alignment layer in an alignment film such as a polyimide film has been to deposit an alignment film on each transparent electrode, to rub or abrade the film with a gigged, flocked or velvet cloth in a desired direction and, subsequently to clean the film to remove debris left by the rubbing process before assembling the transparent substrates to form a LC cell. This method thus requires direct contact with the surface of the alignment film. The numerous steps required by this process are time consuming and costly. Mechanical rubbing methods introduce contamination on the rubbed surface and therefore require cleaning the surface with detergents or solvents. Such contamination is unsuitable for a clean-room environment and requires a special room within a clean-room to produce the alignment layer, adding a significant cost to manufacturing. Uneven pressure and the varying directionality of the rubbing contacts with the polymer surface lead to non-uniformities in the alignment layer. Further, if a post spacer is incorporated for maintaining an equal spacing between the two plates, mechanical rubbing will cause a shadow effect. In addition, it is difficult to implement the desired multi-domain and wide viewing angle technology that are among the objects of the present invention when contact methods are used to produce an alignment layer.
Other methods and materials for creating LC alignment layers include stretching a polymer (Aoyama et al., Mol. Cryst. Liq. Cryst. 72:127(1981)), creating a Langmuir-Blodgett film (Ikeno, et al., Jpn. J. Appl. Phys. 27:L475 (1988)), creating a grating structure by microlithography (Nakamura and Ura, J. Appl. Phys. 52:210 (1981)), depositing SiOx by oblique angle deposition (Ienuing, Appl. Phys. Lett. 21:173 (1982)), and applying polarized UV radiation to a polymer film (Schadt, et al., Jpn. J. Appl. Phys. 31:2155(1992)). All of these methods are very expensive and time consuming and have not achieved satisfactory results. Most of the aforementioned processes entail a large number of processing steps, which creates more possibility for error, lower device yields, and increases in fabrication time and device cost. Accordingly, there is a need for a more efficient and cost effective method to provide an alignment layer on a substrate.
A display is said to be "single domain" if the LC molecules have a single pre-tilt angle along one azimuthal direction of the surface plane (i.e. the long axis of the LC molecules is orientated along in-plane direction and tilted up from that direction by a well defined angle which, in the case of current 12.1 inch SVGA displays, is a few degrees) and hence the long axes of all LC molecules appear more or less parallel to each other over the whole display. A multi-domain display contains at least two differently oriented single domain regions such that the two or more single domain regions form a color sub-pixel of the display.
A drawback of many currently employed single-domain liquid crystal displays is that such devices are characterized by having a narrow or limited viewing angle. An advantage of multi-domain LCD devices is the greater viewing angle as compared to single-domain LCD devices. That is, an observer will have wider selection of viewing angles with which to perceive the display of a multi-domain LCD device as compared to a single-domain LCD device. As such, a number of different methods have been developed for producing multi-domain liquid crystal displays. Such methods include the mask rubbing two domain method (JP-106624), the fringe field two domain method (U.S. Pat. No. 5,309,264 to Lien and John), the double alignment layer two domain method (Koike et al., SID International Symposium Dig. Tech. Papers (1992) 23:798-801) and the UV treatment two domain method (Lien et al. (1995) Appl. Phys. Lett. 67:3108). Yet another approach to improving the viewing angle of liquid crystal displays has been to employ a textured alignment layer. See Nikkei, Flat Panel Display (1998) 104 to 107. The present inventors have applied for patent on yet another method for producing multi-domain alignment layers by using an electric field to change the angle of incidence of an ion beam bombarding a surface so that the ions contact the surface at non-normal incidence (U.S. patent application Ser. No. 09/185,234, filed Nov. 3, 1998).
Despite the development of the above methods for producing multi-domain LC displays, there continues to be an interest in the development of new methods of producing multi-domain displays with broader viewing angles. Ideally, such methods should have a minimal number of steps, be efficient and be adaptable to clean-room high throughput manufacturing. Thus, there is need for a controllable non-contact method of producing multi-domain displays with broad viewing angles. The present invention is addressed to this need in the art.
The following patents pertain to one or more aspects of the invention and thus provide useful background information: U.S. Pat. No. 5,757,455 to Sugiyama et al., which relates generally to alignment layers; U.S. Pat. No. 5,721,600 to Sumiyoshi et al., relating to LCD devices; U.S. Pat. No. 5,717,474 to Sarma, pertaining to multi-domain LCD devices; U.S. Pat. No. 5,657,105 to McCartney, relating to multi-domain LCD devices; U.S. Pat. No. 5,608,556 to Koma, pertaining to multi-domain LCD devices and methods for controlling the orientation of LC molecules; U.S. Pat. No. 5,576,862 to Sugiyama et al., relating to multi-domain LCDs and alignment layers therefor; U.S. Pat. No. 5,550,662 to Bos, relating to color LCDs with wide viewing angles; U.S. Pat. No. 5,508,832 to Shimada, relating to methods for producing multi-domain LCD devices; U.S. Pat. No. 5,479,282 to Toko et al., which relates to multi-domain LCDs; U.S. Pat. No. 5,410,422 to Bos, which relates to a wide-angle gray-scale LCD display; and U.S. Pat. No. 5,309,264 to Lien et al., pertains to multi-domain LCD displays.