Abstract:
A spatial light modulator includes a two-dimensional array of hexagonal mirror plates disposed in a honeycomb pattern over a substrate. Each of the hexagonal mirror plates is supported by one or more structural members. There is a gap between adjacent hexagonal mirror plates. The structural members are not located in the gap.

Description:
BACKGROUND 
       [0001]    The present disclosure relates to micro-mirror based spatial light modulators. 
         [0002]    A spatial light modulator can include an array of tiltable micro mirrors. A micro mirror built on a substrate can include a tiltable mirror plate that can be tilted by electrostatic forces. The mirror plate tilts to an “on” position, wherein the micro mirror plate directs incident light to a display device, and to an “off” position, wherein the micro mirror plate directs incident light away from the display device. The mirror plate can be stopped by mechanical stops at the “on” or the “off” positions so that the orientation of the mirror plate can be precisely defined at these two positions. For the micro mirror to properly function, the mirror plate must be able to promptly change between the “on” or the “off” positions without any delay. The mirror plates in a spatial light modulator can be selectively tilted to “on” or “off” positions to form a display image. A desirable performance for the spatial light modulator in a display application is to provide bright and high contrast display images. 
       SUMMARY 
       [0003]    In one general aspect, the present invention relates to a spatial light modulator, including a two-dimensional array of hexagonal mirror plates disposed in a honeycomb pattern over a substrate, wherein each of the hexagonal mirror plates is supported by one or more structural members, there is a gap between adjacent hexagonal mirror plates and the structural members are not located in the gap. 
         [0004]    In another general aspect, a spatial light modulator is described that includes a two-dimensional array of hexagonal mirror plates disposed in a honeycomb pattern over a substrate, wherein each of the hexagonal mirror plates is supported by one or more structural members, there is a gap between adjacent hexagonal mirror plates, the structural members are not located in the gap and the distance between a structural member of a mirror plate and the upper surface of the mirror plate is less than 1 micron. 
         [0005]    In yet another aspect, a spatial light modulator is described having a two-dimensional array of hexagonal mirror plates disposed in a honeycomb pattern over a substrate, wherein each of the hexagonal mirror plates is supported by a structure, wherein the structure is located between two corners of the hexagonal mirror plate and the structure is at least partially under the hexagonal mirror plate. 
         [0006]    In another general aspect, a spatial light modulator is described having a two-dimensional array of hexagonal mirror plates disposed in a honeycomb pattern over a substrate, wherein adjacent hexagonal mirror plates in the two-dimensional array are separated by gaps less than 2 micron. 
         [0007]    In another general aspect, a spatial light modulator is described having a two-dimensional array of hexagonal mirror plates disposed in a honeycomb pattern over a substrate, wherein each of the hexagonal mirror plates is supported by one or more structural members, wherein at least one of the structural members is located in the vicinity of the middle of an edge of the hexagonal mirror plate and the structural member is at least partially under the hexagonal mirror plate. 
         [0008]    Implementations of the system may include one or more of the following. The one or more structural members can each be supported by a support post in connection with the substrate. The support post can be hidden under the hexagonal mirror plate At least one of the hexagonal mirror plates can configured to tilt around the one or more structural members supporting the hexagonal mirror plate. At least one of the hexagonal mirror plates can include a cavity having an opening on the lower surface and at least one of the structural members associated with the hexagonal mirror plate extends into the cavity. At least one of the mirror plates does not include a hole in its upper surface. The spatial light modulator can further include a mechanical stop over the substrate, wherein the mechanical stop is configured to contact one of the hexagonal mirror plates to stop the movement of the hexagonal mirror plate when the hexagonal mirror plate tilts around the one or more structural members supporting the hexagonal mirror plate. Adjacent hexagonal mirror plates in the two-dimensional array can be separated by gaps less than 2 micron. Adjacent hexagonal mirror plates in the two-dimensional array can be separated by gaps less than 1 micron. Adjacent hexagonal mirror plates in the two-dimensional array can be separated by gaps less than 0.5 micron. The upper surfaces of the hexagonal mirror plates can occupy at least 85% of the area of the two-dimensional array. The upper surfaces of the hexagonal mirror plates can occupy at least 90% of the area of the two-dimensional array. 
         [0009]    The upper surfaces of the hexagonal mirror plates can occupy at least 95% of the area of the two-dimensional array. The distance between a structural member and the upper surface of the associated hexagonal mirror plate can be less than 1 micron. The distance between a structural member and the upper surface of the associated hexagonal mirror plate can be less than 0.5 micron. 
         [0010]    Implementations may include one or more of the following advantages. The disclosed methods and systems provide a micro-mirror based spatial light modulator that can produce bright and high contrast display images. The spatial light modulator can include an array of tiltable hexagonal mirrors that are distributed in honey-comb shaped cells. The disclosed hexagonal mirrors include reflective upper surfaces that have no holes, which in some instances may be an improvement over some of the conventional mirror designs that include holes in the mirror plates. Holes in the mirror plates are known to scatter light and reduce the contrast and sharpness of the display images. The elimination of holes in the disclosed hexagonal mirrors may therefore improve the contrast and sharpness in the display image as compared to conventional mirror based display systems. 
         [0011]    The disclosed methods and systems provide an array of tiltable hexagonal mirrors with high fill-in ratios, which may more fully utilizes incident light and reduce light loss, producing brighter display images. Unlike some conventional mirror-based spatial light modulators that include flexure hinges exposed in the gaps between adjacent mirror plates, the disclosed hexagonal mirrors do not require structural members on the substrate between adjacent mirror plates. Several features of the structures of the hexagonal mirrors can enable small gaps between adjacent hexagonal mirrors and thus denser packing of the hexagonal mirrors. The hexagonal mirrors can include hinge components that extend into cavities having opening at the lower surfaces of the mirror plates. The hinge components can be completely hidden to an observer of the top of the mirror plates, because the hinge components are under the hexagonal mirror plates. The disclosed hexagonal mirror plate can tilt about a rotation axis defined by the one or more hinge components associated with the mirror plate. The positions of the hinge components can allow the rotation axis to be located in the mirror plate, which minimizes the lateral movement at the edges of the hexagonal mirror plate during the tilt movement of the hexagonal mirror. Moreover, the adjacent micro mirrors do not have to be spaced apart to leave room for any structures that are required in some conventional micro-mirror systems. These advantageous features may allow the disclosed hexagonal mirror plates to be closely disposed with small gaps between adjacent mirror plates while still providing enough clearance for the tilt movement of the adjacent hexagonal mirror plates. 
         [0012]    Although the invention has been particularly shown and described with reference to multiple embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The following drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles, devices and methods described herein. 
           [0014]      FIG. 1  is a plan view of an array of hexagonal mirrors. 
           [0015]      FIG. 2-4  are respectively a perspective top view, a perspective side view, and a perspective bottom view of a hexagonal mirror in  FIG. 1 . 
           [0016]      FIG. 5  is a side view of a hexagonal mirror when viewed perpendicular to the line A-A in  FIG. 1 . 
           [0017]      FIG. 6  is a side view of three adjacent hexagonal mirrors when viewed perpendicular to the line B-B in  FIG. 1 . 
           [0018]      FIG. 7  is a side view of a hexagonal mirror when viewed perpendicular to the line B-B in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  is a top view of an array  100  that is compatible with a spatial light modulator. The array  100  includes a plurality of hexagonal mirrors  110  distributed in a honey-comb pattern. The array  100  is bounded by a boundary  120 , which can form a simple polygonal or circular shape, such as a rectangle, as shown, or a circle, hexagon, triangle, square, or other polygon. Adjacent hexagonal mirrors  150 - 152  are separated by gaps  751  and  752  (shown in  FIG. 6 ). Each mirror plate and half the gap area between a mirror plate and its neighboring plates defines a cell. Inactive areas  130  can exist between the array  100  and the boundary  120 . 
         [0020]    The upper surfaces of the hexagonal mirrors  110  are reflective. The reflective surfaces enable the hexagonal mirrors  110  to reflect light. The mirrors are tiltable, which controls where the reflected light is directed. The total active area for the array  100  is the sum of the reflective upper surface areas of all the hexagonal mirrors  110  in the array  100 . The inactive areas within the boundary  120  include the inactive areas  130  and gaps within the boundary. Although only a few dozen mirrors are shown, the array  100  can include hundreds to thousands of micro mirrors  110  along each dimension. For example, the array  100  can include 1024 micro mirrors  110  along one dimension and 1536 micro mirrors along the other dimension. 
         [0021]    A “fill-in” ratio can be defined as the percentage of the active area within a cell in a mirror array. In the example illustrated in  FIG. 1 , the fill-in ratio is the sum of the total area of the reflective upper surfaces of all the hexagonal mirrors  110  divided by the total areas for all the cells in the array  100 . In other words, the fill-in ratio is approximately the ratio of the total reflective area in the array  100  divided by the difference between the area within the rectangular boundary  120  and the inactive areas  130 . The fill-in ratio can indicate the reflective efficiency for an array of micro mirrors. A mirror array having large gaps between adjacent mirror plates loses more incident light in the gaps than a mirror array having smaller gaps between adjacent mirrors. The former therefore has a lower fill-in ratio than the latter. 
         [0022]    Referring to  FIGS. 2-5 , a single hexagonal mirror  150  includes a hexagonal mirror plate  200 . In some embodiments, the mirror plate  200  includes a reflective upper layer  201 , a middle layer  202 , and a lower layer  203 . The reflective upper layer  201  can be made of a metallic material such as aluminum, gold, or one of their alloys. The middle layer  202  provides mechanical strength to the mirror plate. The middle layer  202  also includes a portion of a cavity  205 . The middle layer  202  can include silicon, polysilicon, amorphous silicon, aluminum, titanium, tantalum, tungsten, molybdenum, and silicides or alloys of aluminum, titanium, tantalum, tungsten, or molybdenum. The lower layer  203  can be made of an electrically conductive material such as aluminum, doped silicon, polysilicon, amorphous silicon, aluminum-silicon alloys, titanium, tantalum, tungsten, molybdenum, and silicides or alloys of aluminum, titanium, tantalum, tungsten or molybdenum. The lower layer  203  includes a portion of the cavity  205  that has an opening at the lower surface of the lower layer  203 . 
         [0023]    The hexagonal mirror  150  also includes a hinge component  210  that extends into the cavity  205 . The hinge component  210  is connected with the lower layer  203  through a connection portion and with an upper portion  215  of a hinge support post  217 . The hinge component  210 , upper portion  215  and the hinge support post  217  are under the hexagonal mirror plate  200  and can be hidden from above the hexagonal mirror plate  200 . That is, the hinge component  210 , upper portion  215  and hinge support post  217  do not extend beyond the footprint of the hexagonal mirror  150 . The hinge component  210  and the hinge support post  215  are located in the vicinity of the middle of an edge of the hexagonal mirror plate  200 , between two corners of the mirror  150 . 
         [0024]    The upper portion  215  of the hinge support post  217  is connected with an electrode  220  via a lower portion  216  of the hinge support post  217 . The hinge component  210 , the upper portion  215  of the hinge support post  217 , and the lower portion  216  of the hinge support post  217  are made of electrically conductive materials, which allow the electric potential of the lower layer  203  to be controlled by a voltage signal applied to the electrode  220 . The electrically conductive materials can include silicon, polysilicon, amorphous silicon, aluminum, titanium, tantalum, tungsten, molybdenum, and silicides or alloys of aluminum, titanium, tantalum, tungsten, or molybdenum. One or more step electrodes  230   a  and  230   b  are also disposed over the substrate (not shown for illustration clarity) under the mirror plate  200 . Each of the step electrodes  230   a  and  230   b  includes a lower layer and an upper layer. Each of the step electrodes  230   a  and  230   b  can receive voltage signals to establish electrostatic potential differences between the lower layer  203  and the step electrodes  230   a  or  230   b . As a result, an electrostatic force can be produced over the mirror plate  200 . The voltage signals applied to the electrode  220  and the step electrodes  230   s  and  230   b  can be designed to produce an electrostatic torque to tilt the mirror plate  200 . The typical diagonal dimension for the hexagonal mirror plate  200  is in the range of 1 to 100 microns. 
         [0025]    One or more landing stops  260  can be provided over the substrate. The tilt movement of the mirror plate  200  can be stopped when the mirror plate  200  comes to contact with one of the landing stops  260 . The landing stops  260  can be made of electrically conductive materials. The electric potential of the landing stops  260  can be controlled by electrodes  261 . The electrodes  261  can be connected with the electrode  220  such that landing stops  260  are at the same voltage as the lower layer  203  of the mirror plate  200 . The equal potential between the lower layer  203  and the landing stops  260  assures that the voltage of the lower plate  203  is maintained when it comes to contact with one of the landing stops  260 . 
         [0026]    The lower layer  203  can further include one or more cavities each having an opening in its lower surface. A deflectable cantilever  265  in connection with the lower layer  203  extends into the cavity  205 . The tilt movement of the mirror plate  200  can be stopped when a landing stop  260  contacts the corresponding cantilever  265  on the mirror plate  200 . The cantilever is deflectable and slightly bent by the pressure applied by the landing stop  260 . The elastic energy stored in the distorted cantilever  265  can be released to cause the mirror plate  200  to snap back during the separation of the mirror plate  200  from the landing stop  260 . The release of the elastic distortion energy can help overcome stiction between the landing stop  260  and the mirror plate  200 . Details about the structures and the fabrication of the cantilever in the mirror plate are disclosed in the commonly assigned U.S. patent application Ser. No. 11/366,195, entitled “Spatial Light Modulator Having a Cantilever Anti-Stiction Mechanism”, filed Mar. 1, 2006. 
         [0027]    The lateral dimensions of the mirror plate  200  extend beyond the lateral dimensions of the hinge component  210 , and the upper portion  215  and the lower portion  216  of the hinge support post  217 . In other words, the hinge component  210 , the upper portion  215  of a hinge support post  217 , and the lower portion  216  of a hinge support post  217  are completely hidden under the mirror plate  200  when viewed from above the mirror plate  200 . The step electrodes  230   a ,  230   b  and the landing stop  260  can also be hidden under the mirror plate  200 . No support structures are needed outside the lateral dimensions of the mirror plate  200  (i.e., in the gaps  751  and  752 ) over the substrate. This is one reason that the adjacent mirror pates  150 - 152  can be closely positioned and separated by only small gaps  751  and  752 . The above disclosed mirror design is also an improvement over some conventional micro mirror systems that include structures on the substrate between adjacent mirrors. These conventional mirror systems require much larger gaps between adjacent mirrors to accommodate these structures. 
         [0028]    Details about fabricating the tiltable micro mirrors are disclosed in the commonly assigned U.S. patent application Ser. No. 10/974,468, tilted “High contrast spatial light modulator and method”, filed Oct. 27, 2004, and the commonly assigned provisional U.S. patent application Ser. No. 60/750,506, tilted “System and Method for Making a Micro-Mirror Array Device”, filed Dec. 14, 2005. 
         [0029]    Referring to  FIG. 6 , the mirror plates of the hexagonal mirrors  150 - 152  are separated by gaps  751  and  752 . Referring to  FIG. 7 , the upper portions  215  of the two hinge support posts  217  support two hinge components  210  (not visible in  FIG. 6 ) that extend into the cavities  205  in the lower layer  203  in the hexagonal mirror plate  200 . The two hinge components  210  can define a rotational axis about which the hexagonal mirror plate  200  can tilt. An advantageous feature of the disclosed hexagonal mirror is that the hinge components  210  extend into the cavity  205  in the lower layer of the mirror plate  200 . Referring back to  FIG. 7 , the rotational axis for the tilt movement of the hexagonal mirror plate  200  is therefore within the lower layer of the mirror plate  200 . As a result, the edges of the mirror plates  730 ,  731 ,  732  in the hexagonal mirrors  150 - 152  experience mostly vertical displacement during the tilting of the mirror plates  730 ,  731 ,  732 . The mirror plates  730 ,  731 ,  732  can therefore be closely spaced without interfering with one another&#39;s tilting. In some embodiments, the gaps  751  and  752  can be smaller than 2 microns, such as less than 1 micron or less than 0.5 micron. 
         [0030]    Due to the above described advantageous features of the hexagonal mirrors  150 - 152 , the gaps  751  and  752  can be kept very small. In some implementations, the fill-in ratio of the array  100  hexagonal mirrors  110  can be over 85%, 90%, 93% or 95%. 
         [0031]    Another advantageous feature of the disclosed hexagonal mirror is that the upper layer  201  does not include a hole in its reflective upper surface. Holes in the mirror plates in some conventional micro-mirror based display systems are known to scatter light, and reduce the contrast and sharpness of the display images. The elimination of holes in the disclosed hexagonal mirrors can provide improved contrast and sharpness in the display image as compared to these conventional micro-mirror based display systems. 
         [0032]    Still another advantageous feature of the disclosed array of hexagonal mirrors is that the gaps between the hexagonal mirrors are not distributed in a set of periodic straight lines across the whole array as in an array of rectangular mirrors. Moreover, the corners of a hexagonal mirror are more obtuse than the right angles in a rectangular mirror. These advantageous features allow the disclosed array of hexagonal mirrors to produce less stray light caused by unwanted diffractions and scatterings. 
         [0033]    Although multiple embodiments have been shown and described, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope. For example, the exact design of the electrodes for producing electrostatic forces on the hexagonal mirror plates can vary. Furthermore, the substrates compatible with the disclosed system can include electronic circuits for controlling the hexagonal mirror plates. Moreover, the boundaries of the array of the hexagonal mirrors can take many shapes such as rectangular, hexagonal, or round. 
         [0034]    The content of all patents and publications described herein are incorporated by reference for all purposes.