Abstract:
A micromechanical reflection phase grating may be formed of spring-like ribbon reflectors that are secured to a transparent cover positioned over a substrate such as a silicon substrate. The ribbon reflectors are formed independently of the silicon substrate. If a defect occurs in the phase grating and particularly the ribbon reflectors, the top plate assembly can be reworked or discarded without sacrificing the relatively expensive silicon substrate.

Description:
BACKGROUND 
     This invention relates generally to micromechanical diffractive phase gratings that are also known as grating light valves for display applications. 
     A micromechanical phase grating includes a plurality of ribbon-shaped reflectors that may be selectively deflected to diffract incident light. In one embodiment, the phase grating includes parallel rows of ribbon reflectors. If alternate rows of reflectors are flexed downwardly relative to the other reflectors, incident light may be diffracted. 
     When the reflectors are all in the same plane, incident light is reflected back on itself. By blocking that light that returns along the same path as the incident light, a dark spot may be produced in a viewing system. 
     Conversely, when alternate reflectors are deflected, the diffracted light may be at an angle to the incident light which may bypass the blocking element that blocks light returning along the incident light path. This diffracted light then produces a bright spot in the viewing system. 
     Thus, a phase grating may be created which selectively produces light or black spots. In addition, gray scales and color variations may be produced in some embodiments. 
     One problem with conventional designs for micromechanical reflection phase gratings is that they are formed on the silicon substrate. That substrate may include other high value components fabricated beneath the phase grating. Thus, if the phase grating is not formed properly, the entire silicon-based device may be ruined. This greatly increases yield problems and therefore fabrication costs. 
     Therefore, it would be desirable to have a way to form micromechanical reflection phase gratings in a way which does not risk the finished silicon wafer when defects occur in the formation of the micromechanical phase grating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a greatly enlarged cross-sectional view of one embodiment of the present invention; 
     FIG. 2 is a greatly enlarged cross-sectional view showing an initial stage of manufacture of the embodiment shown in FIG. 1; 
     FIG. 3 is a greatly enlarged cross-sectional view of a subsequent stage of manufacture of the embodiment shown in FIG. 2; 
     FIG. 4 is a greatly enlarged cross-sectional view of a completed top plate assembly; 
     FIG. 5 illustrates the operation of the embodiment shown in FIG. 1 when all of the ribbon reflectors are undeflected; 
     FIG. 6 illustrates the operation of the embodiment shown in FIG. 1 when alternate ribbon reflectors are deflected; and 
     FIG. 7 shows a display system in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a micromechanical reflection phase grating or light valve  10  includes a top plate assembly  12  that may be clamped or otherwise secured to a bottom plate assembly  24 . The top plate assembly  12  may include a transparent cover  14  having a stepped portion  16  in one embodiment of the present invention. Secured to the transparent cover  14  are rows of resilient ribbon reflectors  18 . The top surfaces of the ribbon reflectors  18  may have a coating  20  that makes them reflective. The ribbon reflectors  18  rest on supports  22  on the bottom plate assembly  24 . 
     Thus, the central portions  26  of the ribbon reflectors  18  may be deflected as leaf springs. That is, once deflected towards the bottom plate assembly  24 , the reflectors  18  ultimately return to the undeflected position, shown in FIG.  1 . 
     The top plate assembly  12  may be secured to the bottom plate assembly  24  using a clamping housing (not shown). The bottom plate assembly  24  includes a conductive bottom electrode  28  that in one embodiment of the present invention may be formed of tungsten. Beneath the electrode  28  is an insulator layer  30  that in one embodiment of the present invention may be formed of silicon oxide. Beneath the insulator  30  is a silicon substrate  32 , in one embodiment. The silicon substrate  32  may be formed from a wafer having electronic components integrated therein. For example, the substrate  32  may form the driving circuitry for the phase grating  10  and may also include other electronic components including a processor and memory as examples. 
     In another embodiment, the top plate assembly  12  may be hermetically sealed, using adhesive for example, to the bottom plate assembly  24 . An inert gas may be maintained in the region between the assemblies  12  and  24 . As another alternative, the cover  14  may be formed of two separate pieces, one forming the stepped portion  16  and the other forming the remainder of the cover  14 . The pieces may then be adhesively secured together, in one embodiment. 
     Because of the configuration of the phase grating  10 , the top plate assembly  12  may be formed independently of the bottom plate assembly  24 . In such case, if a defect or failure occurs in connection with the top plate assembly  12 , only the top plate assembly  12  may be discarded or reworked. In this way, the value incorporated into the completed bottom plate assembly  24 , that may include a large number of components not directly associated with the phase grating  10  function, may be preserved. 
     Since the bottom plate assembly  24  may be formed using conventional techniques, the ensuing discussion focuses on the formation of the top plate assembly  12  as illustrated in FIGS. 2-4. In FIG. 2, the transparent cover  14  has been defined and is inverted with respect to the orientation shown in FIG.  1 . The step  16  creates a channel that may be filled with a layer  36 . In one embodiment of the present invention, the layer  36  may be a first type of photoresist. Covering the layer  36  and the step  16  is a second layer  38 . The layer  38  may be photoresist as well. For example, in one embodiment of the present invention, the layers  36  and  38  may be positive or negative photoresist. In other embodiments, one of the layers  36  or  38  may be positive photoresist and the other layer may be negative photoresist. 
     Using photolithographic techniques for example, as illustrated in FIG. 3, a bump or hillock  40  may be defined over the layer  36  by selective removal of the layer  38  in one embodiment of the present invention. For example, the hillocks  40  may be developed so that they remain after the rest of the layer  38  is photolithographically removed. Alternatively, the hillocks  40  may be undeveloped so that they remain when the other regions are removed in a photolithographic technique where developed regions are removed. 
     Next, as shown in FIG. 4, a layer  23  of a flexible or resilient material may be formed over the FIG. 3 structure, to create the reflectors  18 . In one embodiment of the present invention, the layer  23  may be formed of deposited nitride. Before depositing the layer  23 , a reflective coating material  27 , such as an aluminum alloy, may be coated over the layer  36 . After the layer  23  is formed, the reflective coating material  27  preferentially adheres to the layer  23 . The individual reflectors  18  may be defined from the layer  23  in a conventional fashion. 
     Thereafter, the layer  36  may be photolithographically removed. The reflectors  18  are formed in a cantilevered arrangement over the gap  29  (FIG. 1) formed by the supports  22  in the reflectors  18 . The assembly  12  is then completed and may be inverted and secured over a bottom plate assembly  24  to form the structure shown in FIG.  1 . 
     Referring to FIGS. 5 and 6, the phase grating  10  operates in a conventional fashion. When the ribbon reflectors  18 , which are part of the top plate assembly  12 , are undeflected, incident light “B” is reflected back in the direction B. If the returning light is blocked, the result is a dark pixel. Thus, the arrows B and the light patterns “A” illustrate the formation of a dark pixel. 
     Conversely, alternate ribbon reflectors  18   a  may be deflected towards the bottom plate assembly  24 , as shown in FIG.  6 . For example, an appropriate electrical charge through the conductive bottom electrodes  28   a  may attract the reflectors  18   a  toward the bottom plate assembly  24 . Then, incident light, indicated as C, may be diffracted by the resulting phase grating between the ribbon reflectors  18  and  18   a . The electrode  28  may be formed of alternating elements  28   a  that are charged differently than the remaining electrodes  28 . 
     As a result, two diffracted fronts D and E are created that are angled with respect to the path of the incident illumination C. The fronts D and E, diffracted by the phase grating effect, are not blocked by the blocking element (not shown) that blocks the reflected light B in the embodiment shown in FIG.  5 . 
     For example, referring to FIG. 7, a projection system  41  may use the phase grating  10 . A white light source  42  creates light “C” that is reflected by the prism  44  towards the lens  46 . The lens  46  may be a focus lens that causes the incident light to be focused on the phase grating  10 . The light, indicated by the arrows E and D in FIGS. 6 and 7, diffracted by the grating  10  then bypasses the prism  44 . The light patterns E and D, after passing through an opening  48 , are then focused by a capture lens  50  onto a projection screen  52 . As a result, an array of light and dark pixel areas may be selectively defined to create an image for display. 
     While a simple blocked light system is illustrated, the same techniques may be utilized in multicolor systems and gray scale systems. Light that is reflected by undeflected ribbon reflectors  18  when the reflectors  18  are not deflected, may be reflected by the prism  44  out of the system  10 . 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.