Liquid crystal display element having a precisely controlled cell gap and method of making same

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.

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'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.

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.