Abstract:
The present invention provides a pixel array and a process flow for forming an array of pixel cells that features pixel electrodes having overlapping edges. This overlapping pixel configuration precludes absorption of light in inter-pixel regions that could give rise to the appearance of dark lines between bright reflective pixel electrodes. This pixel arrangement also prevents the disruption of charge stored in underlying capacitor structures due to the penetration of incident light through inter-pixel regions into the underlying substrate.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to light valves, and in particular, to pixel cell arrays for silicon light valves having pixels whose edges overlap in order to maintain reflectance in inter-pixel regions.  
           [0003]    2. Description of the Related Art  
           [0004]    Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These display devices typically include a light source which passes light through a light valve.  
           [0005]    One of the methods for producing colors in a liquid crystal display is to sequentially project light having a wavelength corresponding to a primary color onto a single light valve. Color sequential light valves create a spectrum of color within the range of the human perception by switching between a set of discrete primary colors. Typically, red, green, and blue are the primary tri-stimulus colors used to create the remaining colors of the spectrum.  
           [0006]    Specifically, during projection of each primary color, the light intensity is modulated such that combination of the intensities of the primary colors in sequence produces the desired color. The frequency of switching between the primary wavelengths by the light valve should be sufficiently rapid to render discrete primary states indistinguishable to the human eye.  
           [0007]    Two factors dictate the minimum frequency necessary for switching. The first factor is the ability of the human eye to detect the discrete primary colors (e.g., red, green, blue). At slower than ideal switching speeds, the human eye will detect a flicker and the primaries may not blend.  
           [0008]    The second factor determining the frequency of switching is the video refresh rate. During display of video images, the individual frames must be refreshed at frequencies undetectable to the human eye.  
           [0009]    The net frequency of switching demanded by the combination of sequential color blending and video refreshing is beyond the capabilities of light valves that utilize thick (&gt;1 μm) liquid crystal (LC) transducers. However, thin (&lt;1 μm) liquid crystal transducers have been successfully fabricated. These thin LC transducers demonstrate adequate color sequential blending at video refresh rates. One example of such a thin LC transducer pixel cell structure is disclosed in U.S. Pat. No. 5,706,067, to Colgan et al.  
           [0010]    In general, the conventional thin LC transducer pixel cells possess enhanced responsiveness due to the decreased volume of liquid crystal material between the top and bottom plates. A smaller volume enables the liquid crystal to shift orientation more quickly and in response to a lower applied voltage.  
           [0011]    [0011]FIG. 1 shows a cross-sectional view of adjacent thin LC transducer pixel cells in a conventional light valve. Light valve portion  100  comprises adjacent pixel cells  110   a  and  110   b  having liquid crystal (LC) material  111  sandwiched within gap  106  between a top plate and a bottom plate. Top plate  102  is composed of a translucent material, typically glass. The bottom plate is formed by the reflective metal pixel electrodes  112   a  and  112   b  of adjacent pixels  110   a  and  110   b,  respectively.  
           [0012]    Pixel electrodes  112   a  and  112   b  are separated and electrically isolated by trench  118 . Pixel electrodes  112   a  and  112   b  lie on top of an upper intermetal dielectric layer  128  that is one component of interconnect scheme  104 . Interconnect  104  overlies capacitor structures  118   a  and  118   b  formed within underlying silicon substrate  105 . Underlying capacitors  118   a  and  118   b  are in electrical communication with pixel electrodes  112   a  and  112   b,  respectively, through metal-filled vias  140  and middle interconnect metallization layer  124  and lower interconnect metallization layer  122 .  
           [0013]    The conventional pixel array described above in FIG. 1 functions adequately in many applications. However, this design suffers from a number of disadvantages.  
           [0014]    One problem is that light incident to array  100  may penetrate through trench  118  between adjacent pixel electrodes  112   a  and  112   b.  Intermetal dielectric layer  128  below trench  118  is substantially transparent to this incident light, which next encounters interconnect metallization layer  124 . Metallization layer  124  likely bears an anti-reflective coating as a result of prior photolithographic steps. As a result, light incident to inter-pixel regions is absorbed rather than reflected, and is perceived by a viewer as a dark line. This dark inter-pixel region stands in stark contrast to the bright surrounding reflective pixel electrodes. Projection displays can in turn magnify the light reflected from pixel array to such an extent that non-reflective space between pixels is readily observable and may distort the image.  
           [0015]    Therefore, there is a need in the art for a pixel array and a process of forming a pixel array where inter-pixel regions exhibit reflectance comparable to reflectance in pixel regions.  
           [0016]    Another problem is that the penetration of light into inter-pixel regions can cause distortion of the image displayed by the light valve. Specifically, incident light can travel through a variety of paths in the interconnect and finally enter into the underlying silicon. Penetration of incident light into the silicon substrate induces electrical currents that disturb charge stored in the underlying capacitors. As a result of fluctuation in charge stored in these capacitors, luminance of the pixel cells may change between succeeding write states, causing the image to “flicker.” This flickering reduces image quality, and may cause eye strain in a viewer.  
           [0017]    Therefore, there is a need in the art for a pixel array and a process of forming a pixel array that substantially blocks the penetration of incident light through inter-pixel regions into the underlying substrate.  
         SUMMARY OF THE INVENTION  
         [0018]    The present invention provides a pixel array and a process flow for forming an array of pixel cells which features pixel electrodes having overlapping edges. This overlapping electrode configuration precludes absorption of light in inter-pixel regions that could give rise to the appearance of dark lines between bright reflective pixel electrodes.  
           [0019]    A process flow for forming a pixel cell array in accordance with a first embodiment of the present invention comprises the steps of forming an intermetal dielectric layer over an interconnect metallization layer, and forming a first dielectric layer over the intermetal dielectric layer. A second dielectric layer is then formed over the first dielectric layer, the second dielectric layer different than the first dielectric layer. Next, a trench photoresist mask is patterned over the second dielectric layer, the trench photoresist mask masking pixel regions and exposing inter-pixel regions. A trench is then created by etching the second dielectric layer and the first dielectric layer in inter-pixel regions to stop on the intermetal dielectric layer. The trench photoresist mask is removed, and a reflective metal layer is formed over the second dielectric layer and within the trench. A pixel photoresist mask is patterned over the reflective metal layer, the pixel photoresist mask masking first alternative pixel regions and exposing second alternative pixel regions. The reflective metal layer and the second dielectric layer are etched in second alternative pixel regions to stop on the first dielectric layer. A third dielectric layer is formed over the reflective metal layer in first alternative pixel regions and over the first dielectric layer in second alternative pixel regions. A second reflective metal layer over the third dielectric layer, the second reflective metal layer having a raised portion in first alternative pixel regions and a lower portion in second alternative pixel regions. A fourth dielectric layer is formed over the over the second reflective metal layer. Finally, chemical mechanical polishing is performed through the fourth dielectric layer, the raised portion of the second reflective metal layer, and the third dielectric layer to stop on the first reflective metal layer and the lower portion of the second reflective metal layer, such that first pixels are formed in first alternate pixel regions from the first reflective metal layer and second pixels are formed in the second alternate pixel regions from the lower portion of the reflective metal layer, the first and second pixels separated and electrically isolated from one another by the third dielectric layer.  
           [0020]    A process flow for forming a pixel cell array in accordance with a second embodiment of the present invention comprises the steps of forming a layer of interconnect metallization, forming an intermetal dielectric layer on top of the interconnect metallization, and forming a plurality of electrically conducting vias extending through the intermetal dielectric to contact the interconnect metallization. Next, a first reflective metal layer is formed over the vias and the intermetal dielectric, and a first pixel photoresist mask is formed to cover first pixel regions and to exclude second pixel regions. The first reflective metal layer is etched in second pixel regions exposed by the first pixel photoresist mask, and the first pixel photoresist mask is removed. A first dielectric layer is formed over the intermetal dielectric, the vias, and first reflective metal layer remaining in the first pixel regions. A second reflective metal layer is formed over the first dielectric layer. A second pixel photoresist mask is formed over the second reflective metal layer to cover the second pixel regions and to expose the first pixel regions. Finally, the second reflective metal layer is etched in exposed first pixel regions selective to the first dielectric layer, and then the first dielectric layer is etched in exposed first pixel regions selective to the first reflective metal layer.  
           [0021]    An apparatus in accordance with a first embodiment of the present invention comprises a plurality of first reflective metal pixel electrodes, and a plurality of second reflective metal pixel electrodes, the second electrodes having edges which project out of a plane of the first pixel electrodes, the edges of the second electrodes overlapping and separated from the first electrodes by a dielectric layer.  
           [0022]    The features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 shows a cross-sectional view of adjacent pixel cells in a conventional light valve.  
         [0024]    FIGS.  2 A- 2 B show plan and cross-sectional views of adjacent pixel cells in a pixel array in accordance with a first embodiment of the present invention.  
         [0025]    FIGS.  3 AA- 3 HB show plan and cross-sectional views of the process steps for forming an array of pixel cells in accordance with the first embodiment of the present invention.  
         [0026]    [0026]FIG. 4 shows a plan view of a masking step in a process flow in accordance with an alternative embodiment of the present invention.  
         [0027]    FIGS.  5 A- 5 B show plan and cross-sectional views of adjacent pixel cells in a pixel array in accordance with a second embodiment of the present invention.  
         [0028]    FIGS.  6 AA- 6 FB show plan and cross-sectional views of the process steps for forming an array of pixel cells in accordance with the second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0029]    A pixel cell array in accordance with the present invention features pixels having overlapping edges. These overlapping edges provide a reflective metal surface to reflect incident light and thereby prevent absorption of light in inter-pixel regions that can give rise to the appearance of dark lines between the reflective pixel electrodes.  
         [0030]    FIGS.  2 A- 2 B show plan and cross-sectional views, respectively, of adjacent pixel cells in an array in accordance with a first embodiment of the present invention. Pixel array portion  200  includes adjacent pixel electrodes  202   a  and  202   b  separated by inter-pixel regions  204 . Electrodes  202   a  and  202   b  are formed over intermetal dielectric layer  206 , and communicate with underlying interconnect metallization layer  212  through metal-filled vias  214 .  
         [0031]    The underlying edges of first reflective pixel electrodes  202   a  underlie inter-pixel regions  204 . The overlapping arrangement of adjacent electrodes  202   a  and  202   b  promotes the uniform brightness of the array. This is because the underlying edge of first reflective pixels  202   a  reflect incident light that has penetrated through dielectric layer  206  remaining in inter-pixel region  204 . This ensures that dark lines indicating absorption of incident light do not appear in the light valve display.  
         [0032]    FIGS.  3 AA- 3 IB illustrate the process steps for forming an array of pixel cells in a light valve in accordance with the first embodiment of the present invention. For purposes of convention, all FIGS.  3 XA illustrate a plan view of the pixel cell array, and all FIGS.  3 XB illustrate a cross-sectional view of the pixel cell array along line A-A′ of the FIG. 3XA.  
         [0033]    FIGS.  3 AA- 3 AB illustrate the starting point for the process. Intermetal dielectric layer  300  is formed over interconnect metallization layer  302 . First dielectric layer  304  is formed over intermetal dielectric  300 , and second dielectric layer  306  is formed over first dielectric layer  304 . First dielectric layer  304  (typically silicon nitride) is formed from a different material than second dielectric layer  306  (typically silicon oxide).  
         [0034]    First vias  312  are etched in center portions  320   a  of first alternate pixel regions  320  through second dielectric layer  306 , first dielectric layer  304 , and intermetal dielectric layer  300  to stop on interconnect metallization layer  302 . First vias  312  are then filled with electrically conducting material.  
         [0035]    In FIGS.  3 BA- 3 BB, first photoresist mask  308  is patterned over pixel regions, and second dielectric layer  306  and first dielectric layer  304  are etched to form trench  310  in inter-pixel regions. In FIGS.  3 CA- 3 CB, first photoresist mask  308  is removed, and reflective metal layer  316  is formed over the entire surface, including within trench  310 .  
         [0036]    In FIGS.  3 DA- 3 DB, second photoresist mask  318  is patterned over first alternate pixel regions  320 , including the bottoms of trench  310 . Second photoresist mask  318  forms a checkerboard pattern, with corners  320   a  of masked first alternate pixel regions  320  omitted in order to ensure adequate electrical isolation between diagonally-situated pixels.  
         [0037]    FIGS.  3 EA- 3 EB show etching of first metal layer  316  and underlying second dielectric layer  306  in regions unmasked by second photoresist mask  318 . Where first reflective metal layer  316  has conformed to a trench sidewall, first reflective metal layer  316  will be removed all of the way down to underlying intermetal dielectric  300 , creating gap  321 .  
         [0038]    FIGS.  3 FA- 3 FB show deposition of a third dielectric layer  322  over the entire surface. Third dielectric layer  322  will underlie inter-pixel regions of the array. Incident light penetrating third dielectric layer  322  in inter-pixel regions will be reflected by underlying first reflective metal layer  316 . Light reflected in inter-pixel regions will thus experience a phase difference relative to incident light reflected by the surface of the array. This difference in phase is due primarily to the longer path length traversed by incident light crossing, and reflected light recrossing, third dielectric layer  322 .  
         [0039]    Accordingly, the thickness of third dielectric layer  322  should be specifically tailored to generate constructive interference with light reflected by the surface of the array. Generation of constructive interference between reflected light is described in detail in co-pending U.S. patent application Ser. No. 08/872,013, entitled “REFLECTANCE ENHANCING THIN FILM STACK”, filed Jun. 7, 1997 and hereby incorporated by reference.  
         [0040]    Second vias  324  are next etched in center portions  321 a of second alternate pixel regions  321  through third dielectric layer  322 , first dielectric layer  304 , and intermetal dielectric  300  to stop on interconnect metallization  302 . Second vias  324  are then filled with electrically conducting material, typically tungsten.  
         [0041]    FIGS.  3 GA- 3 GB show formation of a second reflective metal layer  326  over the entire surface, followed by formation of a fourth dielectric layer  328  over second reflective metal layer  326 .  
         [0042]    FIGS.  3 HA- 3 HB show the final step of the process flow in accordance with a first embodiment of the present invention, wherein the surface of the array is chemical-mechanical polished. Specifically, chemical mechanical polishing is performed through upper portions of second reflective metal layer  326  to stop on first reflective metal layer  316  and second reflective metal layer  326 . This reveals discrete reflective metal pixel electrodes  328  and  330 , electrically coupled with interconnect metallization layer  302  through first vias  312  and second vias  324 , respectively.  
         [0043]    First and second reflective pixel electrodes  328  and  330  are separated by remaining third oxide layer  322 . Third oxide layer  322  in inter-pixel regions  332  is not shown in FIG. 3HA, as this layer is substantially transparent.  
         [0044]    Fabrication of the liquid crystal silicon light valve is completed by adding liquid crystal material, and then sealing a translucent top plate over the structure.  
         [0045]    The thin LC transducer pixel cell and the process for forming this pixel cell in accordance with the present invention offers a number of important advantages. One advantage is that light incident to inter-pixel regions is reflected by the underlying edges of adjacent pixels rather than being absorbed into the substrate. This prevents the appearance of dark lines associated with the absorption of light, and also shields the underlying substrate from the unwanted currents generated by light leakage.  
         [0046]    Although the invention has been described in connection with one specific preferred embodiment, it must be understood that the invention as claimed should not be unduly limited to this embodiment. Various other modifications and alterations in the structure and process will be apparent to those skilled in the art without departing from the scope of the present invention.  
         [0047]    For example, as shown in FIG. 3BA, the photoresist layer forming the first photoresist mask  308  may be specifically patterned to exclude corners of pixel regions, thereby providing space for dielectric material between diagonally-situated pixels. However, there are several possible alternatives to this step. FIG. 4 shows a plan view of a photoresist masking step in a process flow in accordance with an alternative embodiment of the present invention. In FIG. 4, first photoresist mask  400  is formed by patterning a photoresist layer in a precise checkerboard pattern, followed by carefully over-exposing the photoresist layer during development. As a result of this calculated over-exposure, corners  400   a  of first photoresist mask  400  are rounded and exclude corner portions  400   a.    
         [0048]    Yet another possible embodiment of the present invention is illustrated in FIGS. 5A and 5B, which show plan and cross-sectional views, respectively, of an alternative reflective metal pixel array. Pixel array portion  500  includes adjacent pixel electrodes  502   a  and  502   b  separated by inter-pixel regions  504 . Electrodes  502   a  and  502   b  are formed over intermetal dielectric layer  506 , and communicate with underlying interconnect metallization layer  512  through metal-filled vias  514 .  
         [0049]    The raised edges of first reflective pixel electrodes  502   a  overlie inter-pixel regions  504 . The overlapping arrangement of adjacent electrodes  502   a  and  502   b  promotes the uniform brightness of the array. This is because the overlying edge of first reflective pixels  502   a  reflect incident light that would otherwise penetrate through dielectric layer  506  in inter-pixel regions  504 . This ensures that dark lines indicating absorption of incident light do not appear in the light valve display.  
         [0050]    FIGS.  6 AA- 6 FB illustrate the process steps for forming an array of pixel cells in a light valve in accordance with the first embodiment of the present invention. For purposes of convention, all FIGS.  6 XA illustrate a plan view of the pixel cell array, and all FIGS.  6 XB illustrate a cross-sectional view of the pixel cell array along line A-A′ of the FIG. 6XA.  
         [0051]    FIGS.  6 AA- 6 BA show the starting point for the process. Intermetal dielectric layer  600  is formed over interconnect metallization layer  602 . Vias  604  are etched through intermetal dielectric  600  to stop an interconnect metallization layer  602 . Vias  604  are then filled with electrically conducting material, and the electrically-conducting material is removed outside of vias  604 .  
         [0052]    FIGS.  6 BA- 6 BB show the next step, wherein first reflective metal layer  606  is formed over the entire surface. First photoresist mask  608  is then patterned such that first pixel regions  610  are masked, while second pixel regions  612  are exposed.  
         [0053]    FIGS.  6 CA- 6 CB show etching of first reflective metal layer  606  to stop on intermetal dielectric layer  600  in second pixel regions  612  unprotected by first photoresist mask  608 . First photoresist mask  608  is then stripped, and first dielectric layer  614  is formed over the entire surface, including portions of first reflective metal layer  606  remaining in first pixel regions  610 .  
         [0054]    FIGS.  6 DA- 6 DB show patterning of second photoresist mask  616  over portions of dielectric layer  614  overlying first pixel regions  610 . Portions of first dielectric layer  614  exposed by second photoresist mask  616  are then etched to stop on underlying intermetal dielectric layer  600 . Where both first dielectric layer  614  and intermetal dielectric  600  are formed from silicon oxide, a thin silicon nitride layer formed prior to dielectric layer  616  can serve as an etch stop.  
         [0055]    FIGS.  6 EA- 6 EB show the removal of second photoresist mask  616 , followed by the formation of second reflective metal layer  618  over the entire surface.  
         [0056]    FIGS.  6 FA- 6 FB show the patterning of third photoresist mask  620  such that first pixel regions  610  are unmasked. Exposed portions of second reflective metal layer  618  are then etched selective to first dielectric layer  614 , and exposed portions of first dielectric layer  614  are then etched selective to underlying first reflective metal layer  606 . Subsequent removal of third photoresist mask  620  reveals the complete pixel array shown in FIGS.  5 A- 5 B.  
         [0057]    The second embodiment of the present invention continues to offer the advantage of blocking penetration of incident light into the underlying substrate in inter-pixel regions, thereby preventing unwanted flickering. The second embodiment also presents the appearance of dark lines in light-absorbing inter-pixel regions.  
         [0058]    However, the second embodiment depicted in FIGS.  5 A- 5 B and  6 AA- 6 FB is less favored than the first embodiment for several reasons. First, the overlapping edges of the pixels create unwanted surface topography. Incident light can scatter off of these raised features, degrading the overall reflectance of the array.  
         [0059]    Additionally, the process for forming the second embodiment calls for etching through the second reflective metal layer and first dielectric layer to stop on the first reflective metal layer. This first metal layer ultimately serves a reflective pixel electrode, and exposing it to etchant during this step can roughen the surface of the metal and degrade its reflectance.  
         [0060]    However, it should also be recognized that the second embodiment of the present invention offers certain advantages over the first embodiment. For example, the alignment tolerance for the third photoresist masking step is relatively large, as etching can take place at any point within the first pixel regions.  
         [0061]    Given the plurality of possible specific embodiments of the present invention, it is intended that the following claims define the scope of the invention, and that methods and structures falling within the scope of these claims and their equivalents be covered thereby.