Abstract:
An LCD having the liquid crystal in at least a portion of the interpixel region displaced by a spacer material. The spacer material has a low dielectric constant relative to the liquid crystal, thus further impeding the formation of a bend deformation in the liquid crystal in the interpixel region when pixels on opposite sides of the spacer material are operating in the inversion mode. This elimination of the bend deformation in the interpixel region eliminates the reverse tilt disclination.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention generally pertains to the field of liquid crystal displays (LCDs). In particular, the invention relates to high density reflective LCDs and improving their quality and efficiency. 
     High density reflective LCDs are generally known. FIG. 1 shows a partial cross-section of a generic embodiment of a prior art high density reflective LCD. Generally, the LCD is supported by a silicon substrate  10 . Secondary electrodes  12   a ,  12   b ,  12   c  are fabricated on or within substrate  10 . Above substrate  10  is liquid crystal  14 , which is contained by transparent electrode  16 . Above transparent electrode is a second substrate  18 , also transparent. 
     Fabricated below secondary electrode  12   a ,  12   b ,  12   c  in substrate  10  are electronic circuits  20   a ,  20   b ,  20   c , which interface with secondary electrodes  12   a ,  12   b ,  12   c . Such electronic circuits  20   a ,  20   b ,  20   c  are also known in the art, and provide a voltage level at the respective secondary electrode  12   a ,  12   b ,  12   c  that alters the state of the liquid crystal  14  adjacent the electrodes  12   a ,  12   b ,  12   c . (Such secondary electrodes are referred to as “pixels” by those skilled in the art.) 
     It is the state of the liquid crystal  14  that determines whether and how much light is transmitted. As noted above, the embodiment shown in FIG. 1 is a reflective LCD. Thus, polarized light is projected downward through substrate  18  and transparent electrode  16 . If the liquid crystal  14  above the secondary electrode  12   a , for example, is in a transmissive state, then the polarization of the light is altered as it passes through the liquid crystal  14  and reflected by secondary electrode  12   a . This change in polarization allows the light to transmit through a polarizer positioned externally (not shown in FIG. 1) and, consequentally, the pixel appears bright. 
     On the other hand, if the liquid crystal  14  above secondary electrode  12   a  is not in a transmissive state, then the polarization of the light incident on secondary electrode  12   a  is unaltered and no light passes the external polarizer. Consequently, the pixel corresponding to electrode  12   a  is dark. 
     The LCD, of course, is made up of an array of many secondary electrodes (or pixels), such as electrodes  12   b  and  12   c  shown in the cross-section of FIG.  1 . The states of these electrodes, and the corresponding state of the liquid crystal  14  above each one, will determine the state of the corresponding pixel. Also, as described further below, the state of the liquid crystal  14  may also provide for partial transmission of the light, resulting in a lower intensity glow of the respective pixel. 
     FIG. 2 is a top view of the array of an LCD such as that shown in partial cross-section in FIG.  1 . The pixels of the display corresponding to secondary electrodes  12   a ,  12   b ,  12   c  are marked in FIG.  2 . From this perspective, the driving electronics  20   a ,  20   b ,  20   c  would be beneath the electrodes  12   a ,  12   b ,  12   c  and, because of this, the pixels can be positioned closer together on the substrate, resulting in a high “fill” factor. (Fill is defined as the area of the secondary electrodes or pixels divided by the area of the supporting substrate. In FIG. 2, this would be equivalent to the square of the width of an electrode (w) divided by the square of the pitch.) A high density reflective LCD can have a fill factor on the order of 0.9 and higher. 
     Also visible in FIG. 2 are a series of “spacer beads.” The spacer posts are not visible in the cross-sectional illustration of FIG. 1, but serve to set the liquid crystal cell gap between secondary electrodes  12   a ,  12   b ,  12   c  and common electrode  16 . The spacer beads shown in FIG. 2 are comprised of a series of plastic beads that are randomly positioned between the substrates  10 ,  18 . The spacer posts can be constructed by depositing and patterning an insulating later on the substrate  10 . The beads set the liquid crystal cell gap. The beads can be seen when the display is in operation, so reduction of the number of beads needed to set the liquid crystal gap has been pursued. 
     Referring to FIG. 3, a schematic of the state of the liquid crystal  14  directly above secondary electrode  20   a  is shown as a function of the voltage of the secondary electrode. (The portion of the liquid crystal  14  shown in FIG. 3 corresponds to the dashed area shown in FIG. 1.) 
     The alignment of the liquid crystal molecules when there is a low voltage (referred to as “0 V”) is shown in cross-section to be tilted with respect to the axis between the secondary electrode  12   a  and the transparent electrode  16 . (If viewed from above, the molecules would form a helical structure.) In the “relaxed” state shown, light is transmitted; thus, the liquid crystal shown in FIG. 3 is “normally transmissive.” 
     When a “high” voltage is applied, shown in FIG. 3 to be 6V or higher, the liquid crystal aligns substantially normal to electrodes  12   a ,  16 , or, equivalently, substantially parallel to the electric field between the electrodes. Such alignment of the liquid crystal corresponds to a “dark state” of the liquid crystal, where little or no light is transmitted. 
     Referring to FIG. 4, the alignment of the liquid crystal is shown for adjacent electrodes both having a “high” voltage magnitude, but where one voltage is positive and one is negative. Adjacent electrodes in this state are referred to as bring in an “inversion mode.” 
     Such application of opposing voltage is routinely practiced in the LCD arts and is for the purpose of reducing artifacts such as flicker and improving the overall uniformity of the display. 
     The liquid crystal generally designated as being in the “central” regions of both electrodes  12   a ,  12   b  in FIG. 4 are similarly aligned to give a dark state, like the alignment corresponding to 6V as shown in FIG.  3 . The normal tilt inclination of the liquid crystal is the same in the central regions above both electrodes  12   a  and  12   b , even though the potentials of the electrodes are 6V and −6V, respectively. 
     The region spanning the gap between the electrodes  12   a ,  12   b  is generally designated as the “interpixel region” in FIG.  4 . Moving from the central region of electrode  12   a  through the interpixel region and into the central region of electrode  12   b , the electric field transitions from +6V in a direction perpendicular to electrodes  12   a ,  16  to −6V in a direction perpendicular to electrodes  12   b ,  16 . As shown in FIG. 4, above the interpixel region between the pixels  12   a ,  12   b  this electric field (12V) dominates the alignment of the liquid cystal, forcing it to align parallel to the substrate  10  surface in the interpixel region. (This strong parallel electric field also removes the normal helix-like alignment of the liquid crystal. That and the lack of a reflective surface between pixels  12   a ,  12   b  leads to little or no transmission of light in the interpixel region, as shown in FIG. 4.) 
     As shown, in the interpixel region the liquid crystal tends to align with this relatively strong electric field (approximately 12V, resulting from the composite electric field from electrodes  12   a ,  12   b ). As a result, at the right side of the gap region (i.e., above electrode  12   b ) the liquid crystal tends to tilt opposite its normal tilt inclination. This corresponds to the beginning of the transition of the strong electric field parallel to the electrodes in the interpixel region to a field of −6V perpendicular to electrodes  12   b ,  16  at the central region of electrode  12   b.    
     Thus, moving from the interpixel region toward the central region of electrode  12   b , the electric field decreases in magnitude and changes direction, from parallel with respect to the substrate  10  surface to perpendicular with respect to the substrate  10  surface. At a certain point, the influence of the elastic energy of the liquid crystal to align according to its normal tilt inclination exceeds the influence of the electric field to hold it opposite its normal tilt inclination. At that distance, the liquid crystal will transition from its opposite tilt to its normal tilt. As shown in FIG. 4, separating these two regions there exists an artifact of liquid crystal alignment. It is commonly referred to as a “disclination” by those skilled in the art, or, more specifically, as the “reverse tilt disclination.” 
     This disclination is referred to as the reverse tilt disclination because it separate regions of opposing tilt. This disclination results in an unwanted transmission of light. The transmission of light results in a spurious bright line across a portion of an otherwise darkened pixel. 
     The voltage applied to the electrode will determine where the disclination forms on the pixel surface. When the disclination is sufficiently close to the edge of the pixel, it could be masked by a dark matrix preformed on the passive plate. However, alignment of the dark matrix is difficult, especially for high fill LCDs. Misalignment of the mask will result in a loss of light transmission, thus reducing efficiency. Even if the matrix is positioned correctly, it will block transmission of some light from the pixel when it is in a “elit” state, thus reducing its efficiency. In such displays, avoiding use of a dark matrix mask is preferred. 
     It is thus an objective of the invention to reduce or eliminate the reverse tilt disclination on an LCD. It is also an objective to do so without a dark matrix mask, where there can be a significant loss of light transmission when the pixels of an LCD are in a lit state, and/or a loss of yield that can also result from misalignments of the dark mask (a loss that might otherwise be tolerable in low fill LCDs). 
     The present invention overcomes these disadvantages by eliminating the reverse tilt disclination. Because the disclination is eliminated through the internal make-up of the LCD, there is no need for a dark matrix mask and its resulting disadvantages. 
     In accordance with the present invention, the disclination is eliminated by displacing the bend deformation of the interpixel region. Displacement of the bend deformation is achieved by introducing a spacer material in the liquid crystal corresponding to the region between pixels. The spacer material displaces the liquid crystal, thus preventing development of the bend deformation and the resulting disclination when two adjacent pixels are operated in the inversion mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For better understanding of the invention, reference is made to the following drawings which are to be taken in conjunction with the detailed description to follow: 
     FIG. 1 is a illustration representing a cross section of a number of pixels of a known LCD; 
     FIG. 2 is a partial top view of the LCD represented in cross-section in FIG. 1; 
     FIG. 3 is an illustration representing various states of the alignment of the liquid crystal within the portion represented with dashed lines of FIG. 1, and a corresponding graphical representation of the transmission of light through the liquid crystal; 
     FIG. 4 is an illustration representing the alignment of the liquid crystal between the electrodes shown in FIG. 1 when the electrodes are in the inversion mode and a corresponding graphical representation of the transmission of light through the liquid crystal; 
     FIG. 5 is an illustration representing a cross section of a number of pixels of an LCD in accordance with the present invention and a corresponding graphical representation of the transmission of light through the liquid crystal; 
     FIG. 5 a  is a partial top view of the LCD represented in cross-section in FIG. 5; and 
     FIG. 5 b  is a partial top view of an alternative embodiment of the LCD represented in cross-section in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 5, a cross-section of a portion of an LCD in accordance with the present invention is shown. The portion of FIG. 5 shown is analogous to the portion of the LCD of FIG. 4 described above. As seen in FIG. 5, the LCD is constructed on a silicon substrate material  110 . A spacer  110 ′ projects from the substrate  110  into the liquid crystal in the interpixel region (between electrodes  112   a ,  112   b ). (Although the spacer  110 ′ is shown as contiguous with the substrate  110 , it will be a different material, as described further below.) 
     A spacer  110  such as that shown in FIG. 5 will serve the function of maintaining a precise thickness of the liquid crystal layer  114  (the liquid crystal cell gap). In projection systems, where magnifications on the order of 50 to 100× are common, patterning spacers into the interpixel regions will reduce viewing artifacts associated with a random spacer distribution. A high contrast ratio and light throughput (brightness) will also be achieved. 
     In order for the spacer  110 ′ to prevent formation of the bend deformation in the interpixel region, it must extend the length of the border region between adjacent pixels. Thus, as shown in FIG. 5 a , the “spacers” are actually ribs, when viewed from above the LCD. FIG. 5 a  shows only ribs extending in one direction, between columns of pixels. This will suppress the formation of reverse tilt disclination when adjacent pixels in the same row (such as pixels corresponding to electrodes  112   a ,  112   b ) are in the inversion mode. To prevent a reverse tilt disclination when adjacent pixels in the same column are in inversion mode, both the rows and columns would have to have the ribs. (In other words, each pixel would be completely surrounded with the spacer material, as shown in FIG. 5 b .) 
     Referring back to FIG. 5, even if the spacer  110 ′ does not completely displace the liquid crystal in the interpixel region, i.e., there is a small region of liquid crystal between the spacer  110 ′ and transparent electrode  116  and/or the spacer material does not span the entire width between electrodes  112   a ,  112   b , formation of a reverse tilt disclination in the region above electrode  112   b  can be prevented. As described below, selection of an appropriate material for the spacer  110 ′, particularly for cases when the spacer material does not fill the interpixel region, can further serve to impede the formation of the reverse tilt disclination. 
     Concentrating the strong electric field parallel to the electrode surfaces in the interpixel region when adjacent pixels are in inversion mode will help to eliminate the bend deformation and therefore the associated disclination. In general, the material used for the spacer  110 ′ should have a dielectric constant that is significantly lower than that of the liquid crystal. If the spacer  110 ′ has a dielectric constant greater than or equivalent to the liquid crystal, then the electric field distribution may support sufficient bend deformation in the surrounding liquid crystal to cause a disclination to form. 
     On the other hand, if the dielectric constant of the spacer is relatively low, for example, by a factor of two to ten times less than the liquid crystal, then it will further impede formation of a bend deformation in the liquid crystal surrounding the spacer  110 ′ by focusing the field within the spacer itself. Thus, the electric field above the electrode stays relatively perpendicular to the electrode surface (or, equivalently, the lines of equipotential remain parallel to the electrodes across the entire length of the electrode, even adjacent the interpixel region). Thus, the liquid crystal aligns with its normal tilt inclination above all of electrode  112   b , as shown in FIG. 5, even though adjacent pixels are in inversion mode. 
     For the case where the spacer material does not entirely fill the interpixel region, selecting a relatively low dielectric constant will eliminate the bend deformation. (With a sufficiently low dielectric constant, the spacer material does not have to entirely fill the interpixel region as shown in FIG.  5 . Thus, the spacer material does not have to be so high as to fill the entire liquid crystal gap and/or can be thinner than the space between pixels, such as between pixels  12   a  and  12   b .) 
     Patterning of spacers to form ribs such as that shown in FIG. 5 a  (or completely surrounding each pixel, as in FIG. 5 b ) can be easily integrated into the process that constructs the active matrix and thus may be done in the same foundary that is used to construct the active matrix. As noted above and shown in FIG. 1, the prior art devices also had some type of spacer material to set the liquid crystal gap. For example, as discussed above, FIG. 2 shows a layer that has been patterned into a series of posts or columns extending between the substrate and the secondary electrode to set the liquid crystal gap. 
     Patterning of an insulating layer to create the rib style spacers shown in FIGS. 5 and 5 a  (or spacers completely surrounding each pixel, as in FIG. 5 b ) may be accomplished by modifying such existing techniques. The active matrix is constructed using known methods that leaves a grid of pixels (electrodes) of, for example, Al, exposed on the top surface of the substrate. The rib spacers may then be created by deposition of an insulating material and using a mask that will leave a rib pattern extending between pixels. Accordingly, the processes for creating the LCD of the present invention are readily available in most silicon foundaries. 
     Choice of materials and processing for creating the spacers  110 ′ are very important to the quality of the device. A plasma enhanced chemical vapor deposition (PECVD) process is desired over sputtering or evaporation for adhesion, material quality, etchability and low temperature processing capability. The Al of the electrodes must remain below 250° C. during the processing of the spacer layer to avoid damage to the Al which results in reduced reflectivity and higher scattering. 
     As noted above, the material selected to construct the spacer and, in particular, one with a reduced dielectric constant, is important in eliminating the bend deformation in the interpixel region. Use of a PTEOS (plasma-tetra ethyl oxysilane) for the spacer in lieu of a plasma nitride (SiN x ) will be more effective. For example, where the liquid crystal has a permittivity of 10∈ o , a spacer using PECVD SiNx having a dielectric permittivity in the range of 4∈ o , to 8∈ o  has been found to be effective in eliminating the reverse tilt disclination. A spacer made of PECVD TEOS having a dielectric constant in the range of 1∈ o  to 3∈ o  is expected to be an improvement. 
     The above described embodiments are merely illustrations of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the present invention and the appended claims. Thus, the above description should be considered a representative embodiment of the invention and not a limitation on the scope of the invention.