Patent Publication Number: US-2007097476-A1

Title: Display system having a charge-controlled spatial light-modulator

Description:
BACKGROUND  
      Display devices, such as televisions, projectors, monitors, and camcorder viewfinders employ a variety of methods for generating images onto a viewing surface. Some of the more common approaches include using spatial light-modulators, such as Digital Light Processing (“DLP”) chips and Liquid Crystal Based Panel Displays (“LCD”) to modulate light beams before projecting a resultant image onto a viewing surface. One of the more recent developments in this area of emerging technologies is a light-modulation device containing an array of pixel elements composed of micro-electromechanical system (MEMS) devices. In general, MEMS devices are microscopic mechanical devices fabricated using integrated circuit manufacturing technologies. The mechanical structures within a MEMS device are generally flexible or otherwise moveable over a limited range of motion.  
      In a known light-modulation device, MEMS pixel elements include microscopic mirrors (“micromirrors”) with spring-like mechanisms configured to define “ON” states, wherein incident light is reflected from a micromirror to a spot (pixel) on the viewing surface, and “OFF” states, wherein incident light is diverted away from the viewing surface, generally to a light dump. In this way, a micromirror is in an “ON” state when tilted toward incident light, and in an “OFF” state when tilted away from incident light. In some cases, a display device includes an electron gun that projects an electron beam onto a front side of the pixel element, perpendicular to the surface of the micromirror, or alternatively, to the back-side of the pixel element. In both cases an “ON” state is driven by an electron beam that induces a charge on the micromirror and into an “OFF” state by passive resistive elements. The electron beam induces an electrostatic charge that attracts and tilts the micromirror towards a transmissive conductive substrate beneath the micromirror. When the electron beam is removed and the charge dissipated, the spring-like mechanism restores the micromirror to its original position. The problem, however, is that by arranging the electron gun normal to the surface of the pixel and by projecting the electron beam to the back side of the pixel element, the electron path is partially obstructed by the spring-like mechanism that is often integrated into the micromirror. This obstruction reduces the optical quality of each pixel by altering the induced electrostatic charge and by limiting the available pixel area. In addition, by having the spring-like mechanism the same material as the micromirror, the MEMS designer is limited by geometry and material selection. The embodiments described hereinafter were developed in light of these and other drawbacks.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:  
       FIG. 1  illustrates an exemplary embodiment of a display system;  
       FIG. 2  illustrates another exemplary embodiment of a display system;  
       FIG. 3  illustrates a portion of an exemplary embodiment of an array of pixel elements;  
       FIG. 4  illustrates an exemplary embodiment of an enlarged partial view of a pixel element according to  FIG. 3 ;  
       FIG. 5  is a flow diagram illustrating exemplary steps for constructing the pixel element of  FIG. 4 ; and  
       FIGS. 6A-6H  illustrate portions of an exemplary embodiment of a pixel element according to the exemplary flow diagram of  FIG. 5 . 
    
    
     DETAILED DESCRIPTION  
      A display system for projecting an image-bearing light beam onto a viewing surface is provided. The system includes a device housing, an electron gun, and a spatial light-modulator that is mounted within the device housing. The spatial light-modulator is configured to project the image-bearing light beam onto the viewing surface through an optical window in the device housing. The electron gun is selectively positioned at a predetermined angle with respect to the spatial light-modulator such that a generated electron beam strikes a front face of the spatial light-modulator at the predetermined angle.  
      The spatial light-modulator includes an array of pixel elements composed of micro-electromechanical system (MEMS) devices that are configured into an array of charge-controlled micromirrors. Each pixel element includes two conducting layers (i.e., a micromirror and a hinge) and a conducting substrate. The electron gun projects a stream of electrons that impinge the surface of the micromirror inducing a charge thereon. The charged micromirror is pulled by an electrostatic force to the grounded conducting substrate thereby tilting the micromirror to a position that reflects an “ON” or an “OFF” state. While being bombarded with electrons, the charge on the micromirror slowly drains through a resistor in the conducting substrate. When the electron beam is removed, the charge eventually decays through the resistor, allowing a restoring hinge mechanism to release the micromirror to its original position.  
       FIG. 1  illustrates an exemplary display device  10  including a light source  12 , an electron gun  14 , and a spatial light-modulator  16  disposed within a device housing  18 . Although shown in  FIG. 1  with an open surface, the device housing  18  is an enclosed structure that is generally constructed of glass. However, other materials such as ceramic, stainless steel, or any material capable of sustaining a high vacuum internal pressure is also suitable. The spatial light-modulator  16  includes an array of pixel elements configured to modulate incoming light  20  from light source  12  to generate an image-bearing light beam  22  that ultimately impinges onto a viewing surface  24 . The electron gun  14  is selectively positioned at a predetermined angle with respect to the spatial light-modulator  16  such that an electron beam  25 , emanating from the electron gun  14 , impinges a top surface of the pixel elements in the spatial light-modulator  16 . An optical window  26  in the glass housing  18  directs the incoming light  20  from light source  12  to the spatial light-modulator  16 . The glass housing  18  further includes another optical window  28  for projecting the modulated image-bearing light beam  22  from the spatial light-modulator  16  to the viewing surface  24 . In one embodiment, the optical window  28  may additionally include a lens system (not shown).  
      Alternatively, the glass housing  18  can be modified to accommodate a multiple colored system  30 , as shown in  FIG. 2 , wherein there are three electron guns ( 32   a ,  32   b , and  32   c ) and three spatial light-modulators ( 34   a ,  34   b , and  34   c ), one for each of the primary colors red, green, and blue. Similar to the single electron gun configuration of  FIG. 1 , a light source  36  projects a light beam  37  into the device housing  38  through an optical window  40 . The difference, however, is that the light beam  37  is directed to three dichroic filters  41   a ,  41   b , and  41   c  that separate the incoming light beam  37  into three individually colored light beams of red (R), green (G) and blue (B). Each of the colored light beams R, G, and B are then directed to dedicated spatial light-modulators  34   a ,  34   b , and  34   c.    
      Like the display device of  FIG. 1 , the electron guns  32   a ,  32   b , and  32   c  are selectively positioned at a predetermined angle with respect to each of the spatial light-modulators  34   a ,  34   b , and  34   c . The device housing  38  further includes three optical windows  42   a ,  42   b , and  42   c  for projecting the modulated image-bearing light beams  44   a ,  44   b , and  44   c  to a lens  46 , and ultimately to a viewing surface  48 . In one embodiment, each of the optical windows  42   a ,  42   b , and  42   c  may additionally include a lens or optical beam converging system (not shown). In addition, reflective or refractive optical elements such as dichroic or total internal reflection (TIR) beam splitting cubes may be included into the glass housing  38  to recombine the individual colored light beams, R, G, and B into a single full color image-bearing light beam that projects onto the viewing surface  48  through a single optical window (not shown). Although equally applicable, for purposes of explanation, the description hereinafter refers only to the exemplary components of the single gun configuration of  FIG. 1 .  
       FIG. 3  is a portion of an exemplary spatial light-modulator  16  illustrating an array of pixel elements  50  having deformable micromirrors  52  on a conducting substrate  54 .  FIG. 4  illustrates an enlarged side view of an exemplary pixel element  50  having a deformable micromirror  52  and a conducting substrate  54  with a restoring hinge mechanism  56  therebetween. The micromirror  52  and the conducting substrate  54  are electrically connected by the restoring hinge mechanism  56 . The transparent or non-transparent material for forming the conductive substrate  54  may include, but is not limited to, quartz, glass, sapphire, and silicon. The top and bottom surfaces of the conductive substrate  54  are generally coated with a dielectric material  62  such that a resistive path  64  is defined therebetween. Within the resistive path  64  is a resistor  66 . Etched into the surface of the dielectric material  62  is a conductive ground path  68  and a contact  70  that electrically connects the restoring hinge mechanism  56  to the resistive path  64 . Restoring hinge mechanism  56  includes a hinge  72  that is electrically connected to the conductive substrate  54  and the micromirror  52  by a lower post  74  and an upper post  76 , respectively. The lower and upper posts  74 ,  76  are generally formed of a conductive or semi-conductive material. The hinge  72  may be constructed as a single or multilayer film. The material used to form the hinge  72  may be a metallic conductive material, or a resistive conducting material depending on the specific application and design criteria. Specific material examples include, but are not limited to, single alloy films or multi-layer films of Ta—Al, W—Al, Ti—Al, Ni—Al, Cr—Al, Al—Cu, Mo—Al, Mb—Al, V—Al, Ta—Cu, W—Cu, Ta—Si, W—Si, Ti—Si, Ni—Si, Co—Si, Cr—Si, Mo—Cu, Mb—Cu, V—Cu, Mo—Si, Mb—Si, and V—Si.  
       FIG. 5  is a flow diagram illustrating a set of exemplary steps for constructing a pixel element  50  according to the exemplary structure shown in  FIG. 4 . References to physical components refer to the exemplary components illustrated in  FIGS. 1 and 4 , and  FIGS. 6A-6H . Referring first to  FIG. 6A , at step  100  the conductive substrate  54  is constructed by first building a resistor  66  into a conducting material  78 , and then by applying a coating of dielectric material  62  to both the upper and lower surfaces of the conducting material  78 . In addition, a ground path  68  and a contact  70  are etched into the dielectric material  62 . Accordingly, a resistive path  64  is defined between the layers of dielectric material  62  and the conductive substrate  54 . At step  102 , and referring to  FIG. 6B , the lower post  74  makes a conductive contact to the contact  70  of the conductive substrate  54 . At step  104 , and as shown in  FIG. 6C , a fill material  80  is deposited and planarized onto the conductive substrate  54  and lower post  74 . The fill material  80  is subsequently etched to expose the surface of the lower post  74 .  
      Referring to  FIG. 6D , at step  106  the material for the hinge  72  is deposited, imaged, and etched onto the lower post  74 . At step  108 , the upper post  76  is conductively bonded to the hinge  72  as shown in  FIG. 6E . At step  110 , and referring to  FIG. 6F , additional fill material  80  is deposited and planarized over the upper post  76 . Subsequently, the fill material  80  is etched to expose the surface of the upper post  76 .  
      Further, with reference to  FIGS. 6G and 6H , at step  112  the material for the micromirror  52  deposited, imaged, and etched onto the surface of the upper post  76 , and the fill material  80  is removed.  
      Referring to  FIGS. 1 and 4 , in operation the electron gun  14  and the light source  12  cooperatively project an electron beam  25  and a light beam  20 , respectively, onto the surface of the spatial light-modulator  16 . The electron beam  25  induces a charge on the micromirror  52  such that the micromirror  52  becomes electrostatically drawn to, or tilted toward, the conductive substrate  54  causing the pixel element  50  to be in one of either an “ON”, or an “OFF” state. When in the “ON” state, the micromirror is titled toward the light beam  20  thereby reflecting the incident light beam onto the viewing surface  24 . Conversely, when in an “OFF” state, the micromirror  52  is generally tilted away from the incident light beam  20 , reflecting no light back to the viewing surface  24 . While being bombarded with electrons, the charge on the micromirror slowly drains through the resistive path  64  and the resistor  66  to the ground path  68  in the conducting substrate  54 . After the electron beam  24  is removed the micromirror discharges over time and the restoring hinge mechanism  56  releases the micromirror  52  to its original neutral position. The amount of time it takes for the micromirror  52  to fully discharge is determined by the amount of charge induced on the micromirror  52  by the electron beam  25 , and the resistance value of the resistive path  64 .  
      While the present invention has been particularly shown and described with reference to the foregoing preferred embodiment, it should be understood by those skilled in the art that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention without departing from the spirit and scope of the invention as defined in the following claims. It is intended that the following claims define the scope of the invention and that the method and system within the scope of these claims and their equivalents be covered thereby. This description of the invention should be understood to include all novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. The foregoing embodiment is illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application. Where the claims recite “a” or “a first” element of the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements.