Abstract:
A liquid crystal display cell having improved cell gap uniformity is made by depositing a plurality of spacer particles on the cell substrate then subjecting the substrate to an external energy source to selectively dislodge and remove the larger particles, such as by immersing the substrate in and ultrasonic bath. Because the larger particles will inherently have a lesser attraction to the substrate relative to their mass, subjecting the entire substrate to the ultrasonic bath will inherently preferentially remove the larger particles, resulting in a distribution having a smaller standard deviation than the initial mixture of particles deposited on the substrate as well as an asymmetric reduction in the number of gap-dominating large particles.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to liquid crystal display elements and more particularly to a method of making liquid crystal display elements having a precisely controlled, uniform cell gap. 
     Use of liquid crystal display (LCD) elements has begun to emerge as the method of choice for displaying large amounts of graphical information in displays such as computers and video displays in addition to the use of LCD&#39;s in their traditional role as a display unit for devices requiring limited display of information such as in watches, cellular telephones and the like. Traditionally, liquid crystal display elements have been manufactured by printing an array of electrodes on a first substrate, printing a corresponding transparent electrode or electrodes on a second, glass substrate, and injecting a liquid crystal material between the two substrates. The separation between the two substrates, which essentially determines the thickness of the liquid crystal layer, should be held as constant as possible over the entire area of the cell. Traditionally, the cell gap is maintained through the introduction of spacers between the substrates. Typically, the spacers comprise tiny particles such as glass or plastic beads, glass fibers, or carbon fibers. The particles may be dusted onto the substrate by exposing the substrate to an atmosphere containing a particular concentration of the particles, or may be spun onto the substrate by depositing on the substrate a solvent carrying a particular concentration of the particles and centrifuging the substrate to distribute the particles and evaporate the solvent. With either of these methods of application, a combination of electrostatic and steric forces, primarily electrostatic forces, cause the particles to adhere to the substrate. 
     For optimum performance, the cell gap should be maintained at the optimum distance with no tolerance. Unfortunately, it is not commercially practicable to manufacture liquid crystal display elements with zero tolerance on the cell gap. Commercially available spacers having a particular nominal size will, of course, in reality constitute a distribution of particles having a mean particle size and particles that are larger and smaller than the mean particle size. Since, as the substrates are brought together, the largest particles in the distribution will contact the substrates first, it is the size of the largest particles that primarily determines the cell gap. Moreover, since the substrates are usually flexible, at least where the cell gap is on the scale of the 0.4 to 10 microns and the ratio of cell span to cell gap is very large, as is required for high performance microdisplays, the distribution of particle sizes within the display element will allow the cell gap to vary across the display element allowing an inhomogeneous thickness of the liquid crystal layer. The inhomogeneous thickness results in optical path length differences (the product of the birefringence of the liquid crystal and the cell gap) across the display, resulting in a deleterious effect on the contrast ratio and the chromatic fidelity of the display. 
     New and improved liquid crystal materials and high performance substrates are being developed for high-speed, low operating voltage displays. Substrate spacings of less then five microns will be required for these new and improved displays. As the mean value of the cell gap is further and further reduced, the sensitivity of these devices to variation in cell gap becomes more and more critical. Various methods have been suggested for improving cell gap uniformity. U.S. Pat. No. 5,210,629 discloses a method of filtering the glass spacers to improve spacer uniformity. U.S. Pat. No. 4,653,864 discloses a method of forming polyimide spacers on a substrate using conventional photolithographic techniques. U.S. Pat. No. 4,626,073 discloses use of elastic spacers in lieu of conventional rigid glass or polymer spacers. What is needed, however, is a method of improving cell gap uniformity without the added expense of additional photolithographic process steps or cumbersome filtration techniques. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a liquid crystal display cell having improved cell gap uniformity is made by depositing, either by dusting, spinning, or by other conventional techniques, a plurality of spacer particles on the cell substrate then subjecting the substrate to an external energy source to selectively dislodge and remove the larger particles. According to a preferred embodiment of the invention, the substrate is immersed in an ultrasonic bath and subjected to ultrasonic energy. Since the larger particles will inherently have a lesser attraction to the substrate relative to their mass, subjecting the entire substrate to the ultrasonic bath will inherently preferentially remove the larger particles, resulting in a distribution having a smaller standard deviation than the initial mixture of particles deposited on the substrate and an asymmetric shift in the distribution of particles above and below the nominal particle size (mode) with the standard deviation above the nominal particle size being less than the standard deviation below the nominal particle size. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawing figures in which like references designate like elements and, in which: 
     FIG. 1 is a cross-sectional view of a liquid crystal display cell having non-uniform spacer members; 
     FIG. 2 is a graphical representation of the distribution of spacer member size about the nominal size for commercially available spacers; and 
     FIG. 3 is a cross-sectional view of an ultrasonic bath used to preferentially remove the larger spacer members from a substrate in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The drawing figures are intended to illustrate the methods disclosed herein and are not necessarily to scale. In the description and in the claims the terms left, right, front and the back and the like are used for descriptive purposes. However, it is understood that the embodiment of the invention described herein is capable of operation in other orientations than is shown and the terms so used are only for the purpose of describing relative positions and are interchangeable under appropriate circumstances. 
     With reference to FIG. 1, a typical pixillated liquid crystal display element, be it a conventional active matrix liquid crystal display (AMLCD) or a liquid crystal on silicon (LCOS) display comprises a substrate  10  on which is printed a plurality of pixel electrodes  12 . A thin-film transistor (TFT) in the case of an AMLCD or a conventional transistor in the case of an LCOS display serving as a switching element is formed near each pixel electrode  12  with the source electrodes (not shown) connected to each of the pixel electrodes  12 . A transparent substrate  14  having disposed thereon a common electrode  16  made of a transparent material such as indium tin oxide (ITO) is bonded to substrate  10  along a common periphery with a quantity of spacer members  26  comprising particles such as particles  20 ,  22 ,  24  sandwiched between substrates  14  and  10  to maintain a uniform gap therebetween. Typically, particles  20 ,  22  and  24  are deposited on substrate  10  in the form of a powder that is dusted onto substrate  10  by exposing substrate  10  to an atmosphere containing the particles  20 ,  22  and  24  propelled from a nozzle. As the particles  20 ,  22  and  24  impinge the upper surface  18  of substrate  10 , electrostatic attraction between the particles and the substrate attract and retain the particles against substrate  10 . Additionally, a weaker steric attraction also tends to attract and bond the particles  20 ,  22  and  24  against substrate  10 . 
     FIG. 2 is a graphical representation showing a possible distribution of particle sizes in a typical commercially available powder, in which the size of the particles distributed around the nominal particle size  230  is plotted along the horizontal axis  210  and the population of the particles of the various sizes is plotted along the vertical axis  220  as the solid line  240 . With reference to FIG. 2, commercially available powders contain particles that are not perfectly uniform in size, shape, or compressibility. Instead, the powders will comprise a distribution of particles  22  that are equal to (esg within about one percent of) the nominal size, particles  24  that are larger than the nominal particle size and particles  20  that are smaller than the nominal particle size. Typical commercial powders will have a three sigma distribution of particles equal to plus and minus 18% of the nominal size. As can be appreciated from the foregoing, as initially applied to the substrate, the quantity of spacer members  26  comprising particles  20 ,  22  and  24  will include a significant number of particles  24  that are significantly larger than the mean particle size  22 . Since substrates  10  and  14  are, at least initially, substantially flat, the cell gap  30  of an assembled display device  11  will, at the outset, be larger than the nominal particle size  22 . Additionally, since it is common for a liquid crystal display to have a slight negative pressure between substrates  10  and  14  the negative pressure will tend to bow one or more of substrates  10  and  14  inward between the oversized particles  24  and the more numerous particles  22  that are closer to the nominal particle size  230 . This will result in a non-uniform cell gap  30  which, as discussed hereinbefore, will have deleterious on the contrast ratio and chromatic fidelity of the liquid crystal display cell. 
     It was recognized by the inventors of the present invention that because the principle force attracting the particles to the surface of the substrate is electrostatic, the weakest attraction would be between the largest particles in the substrate. (Van der Waals forces falloff with the sixth power of the particle size.) Accordingly, it was perceived that any kind of perturbation, be it ultrasound, mechanical agitation, fluid flow or some other external energy source would preferentially remove the larger particles, which have the largest area, the largest mass and the weakest electrostatic attraction to the substrate. Accordingly, as shown by the line  250  of FIG. 2, subjecting the substrate to an external energy source would cause the distribution of spacer members larger than the nominal particle size to be narrowed such that the statistical variance of the particles above the nominal particle size would be reduced. As shown in FIG. 2 the preferential removal of the larger particles will cause a slight shift in the arithmetic mean of the distribution of particles from the point indicated as  230  (the nominal size) to the point indicated as  232 . More importantly, as illustrated by the dashed line  250  of FIG. 2, the preferential removal of larger particles results in a lower population of particles that are larger than the nominal size  230  (reduction of the statistical variance) with the largest particles completely absent from the population. 
     FIG. 3 is a schematic cross-sectional view of an ultrasonic bath used from a substrate according to an exemplary process for preferentially removing the larger particles from a substrate  10 . The exemplary process is carried out in an ultrasonic bath  310  comprising a container  312  that is partially filled with a liquid solvent  314  such as methanol. The container is agitated by an ultrasonic horn  316  attached to the exterior surface  318  of container  312 . Substrate  10  is immersed in solvent  314  and the container  312  agitated by ultrasonic horn  316 . In an exemplary process, a substrate  10  was dusted with a powder containing spherical particles having a nominal 3.7 micron diameter. Substrate  10  was subsequently immersed in solvent  314  and subjected to ultrasonic energy at 60 kilowatts at 30 kilohertz for a period of approximately 5 minutes. Upon removal from the solvent bath, it was observed that approximately 50% of the particles had been removed from the substrate. Cell gap uniformity of a liquid crystal cell using the commercially available 3.7 micron nominal diameter particles was improved by the exemplary process from plus or minus 5% to plus or minus 2% of the nominal cell gap, a better than 50% improvement in the cell gap tolerance. A similar exemplary process was conducted in which a substrate was dusted with a powder containing 1.1 micron nominal diameter spherical particles. The second substrate was subjected to the same energy and frequency for the same duration. However, it was observed that no significant number of the 1.1. micron spherical particles were removed. 
     As shown in FIG. 3, the preferential orientation of substrate  10  in the ultrasonic bath is with top surface  18  facing downward. This increases the probability that a large particle  24  being removed will not bump into and dislodge a nominal sized particle  22 , although some inadvertent dislodging of nominal sized particles is unavoidable. Nevertheless, since the mechanical agitation or other external energy source inherently preferentially dislodges the larger particles even if some nominal sized particles are removed the overall effect will be to remove a group of particles from the surface, the majority of which are larger than the nominal sized particle, thereby reducing the overall standard deviation of the distribution of particles on the surface and, more importantly, narrowing variance of particle sizes above the nominal as evidenced by the improvement in the cell gap uniformity. 
     Although certain preferred embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the spirit and scope of the invention. Accordingly, it is intended that the invention shall be limited only to the extent required by the appended claims and the rules and principles of applicable law.