Abstract:
Operating methods of an array of electromechanical pixels resulting in efficient and reliable operation of light modulating elements. The invention simplifies the design of electromechanical light modulators and permits the construction of large size displays with greater mechanical tolerances on glass substrates.

Description:
RELATED U.S. PATENT DOCUMENTS 
     U.S. Ser. No. 12/079,527 Mar. 27, 2008 “PLASMA ADDRESSED MICRO-MIRROR DISPLAY” which is included here as reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to displays. More particularly the invention concerns displays comprising electromechanical picture elements actuated by electrostatic force for modulating light to display motion images. More specifically the invention is directed to the operating methods of an array of electromechanical picture elements. 
     BACKGROUND OF THE INVENTION 
     Operating methods of electromechanical pixels are known from spatial light modulators (SLM) for projection displays. Prior art SLMs for projection displays comprise an array of micro-mirrors constructed on a semiconductor backplane. In prior art SLMs, each electromechanical pixel comprises a micro-mirror that is mounted on a pair of hinges above a semiconductor substrate, and a pair of address electrodes with associated drive electronics constructed on the semiconductor substrate. 
     In operation the SLM is scanned and address voltages are supplied to the address electrodes by the drive electronics. To modulate light, micro-mirrors are selectively reset to first or second deflected positions based on address voltages. 
     Large area flat panel displays based on electromechanical pixels have been proposed by present inventors and others. The prior art operating methods of SLMs require electromechanical pixels with complex mechanical structures and tight mechanical tolerances. Current manufacturing technology for large area flat panel displays that use glass substrates cannot meet these requirements. 
     Also, prior art operating methods extensively use mechanical force of micro-mirror hinges which changes with operating time and temperature. 
     The use of mechanical force, especially for selective positioning of micro-mirrors, causes problems such as resetting micro-mirrors to a wrong position. It also requires selection of operating voltages during the display production, and construction of very uniform hinges. 
     Flat panel displays usually have larger electromechanical pixels than SLMs for projection displays and need greater electrostatic actuation voltages to operate. In order to display motion images and meet the required addressing speeds, the flat panel displays need greater ratio between the actuation voltages and the address voltages than the prior art operating methods can provide. 
     SUMMARY OF THE INVENTION 
     The present invention provides efficient and reliable operating methods for an array of electromechanical light modulators. These operating methods are greatly tolerant to initial mechanical variations and changes in mechanical properties from temperature and operating time of electromechanical pixels. 
     The invention permits simpler design of electromechanical light modulators and construction of large size displays on glass substrates. Each electromechanical pixel may be addressed with one electronic switch such as a transistor or a circuit employing low-pressure gas discharge. 
     The invention also solves problems associated with electromechanical pixels such as stiction and vibration of movable elements. Additionally the invention provides operating methods which effectively dissipate residue electrostatic charges that may accumulate on surfaces of insulators and cause malfunction of pixels. 
     The foregoing as well as other advantages of the invention will be apparent by the novel operating methods illustrated in the accompanying drawings and described in the specification that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the display of the present invention. 
         FIG. 2  is a cross-sectional view taken along lines  2 - 2  of  FIG. 1 . 
         FIG. 3  is an enlarged view of the area designated as  3 - 3  in  FIG. 2 . 
         FIG. 4  is a schematic diagram of one pixel of an array of electromechanical pixels. 
         FIGS. 5 to 10  illustrate voltage waveforms of the invention supplied to the array of electromechanical pixels. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 through 3  of the drawings illustrate the construction and optical functionality of a display panel  20 . Referring to  FIGS. 1 and 2  the display  20  includes a rectangular optical waveguide  21  that is substantially wedge-shaped cross section. Waveguide  21  is preferably constructed from acrylic or other optically transparent material, having a refractive index n 1  with a value between 1.49 and 1.6 and comprises parallel first and second end surfaces  26  and  27  that are joined by parallel side surfaces  28  and  29  (see  FIG. 1 ). Waveguide  21  also includes an upper surface  30  and a lower surface  31  converging with upper surface  30 . Display  20  also includes a substrate  35  constructed from a substantially transparent material such as glass having a refractive index n 2  with a value between 1.49 and 1.6. Lower surface  36  of substrate  35  is optically coupled to the upper surface  30  of waveguide  21  via an optical layer  38  formed from a fluoropolymer or other substantially transparent material having a refractive index n 3  with a value between 1.3 and 1.4. 
     A plurality of equally spaced-apart micro-prisms  32  are constructed at upper surface  37  of substrate  35  and, as shown in  FIG. 1 , extend between side surfaces  28  and  29  of waveguide  21 . Micro-prisms  32  may be molded or constructed using lithography from a UV curing polymer having a refractive index n 4  with a value between 1.49 and 1.6. LED light sources  25  are installed proximate the wide edge  26  of waveguide  21  and a plurality of tilting micro-mirrors  33  are constructed between micro-prisms  32 . In  FIG. 2 , one column of tilting micro-mirrors is designated as  33   a ,  33   b , and  33   c.  A section of a cover assembly  34  is illustrated in  FIG. 1  and detailed construction is shown in  FIG. 3 . 
     Now referring to  FIG. 3  of the drawings where details of multi-layer optical coatings are shown. The first layer is a light-reflecting layer  39  constructed from metal on upper surface  37  of substrate  35 . The light-reflecting layer  39  is patterned to form plurality of light reflecting regions  45  and light transmitting regions  46 . The second optical layer is a light-absorbing layer  40  formed on light-reflecting layer  39  and is patterned to partially cover light reflecting layer  39 . 
     Further illustrated in  FIG. 3  are micro-prisms  32 . Each micro-prism  32  comprises a light input facet  41 , which is optically coupled to the upper surface  37  of substrate  35 , and a light exit facet  42 , which is inclined with respect to the upper surface  30  of waveguide  21 . A reflective mirror film is deposited on the third facet  43  of micro-prisms  32  to form a light-deflecting facet  47  which is also inclined with respect to the upper surface  30  of waveguide  21  and in opposite direction to light exit facet  42  of micro-prisms  32 . 
       FIG. 3  also illustrates one of tilting micro-mirrors  33   b , which typifies the construction of each of the micro-mirrors of display panel  20 . Micro-mirror  33   b  comprises a thin aluminum alloy elastic film that is affixed to the upper portion of the light exit facet  42  of micro-prism  32 . Micro-mirror  33  is attached to micro-prisms  32  with a pair of hinges and it tilts at about axis  50  that is substantially parallel to upper surface  30  of the waveguide  21 . 
     To prevent micro-mirror stiction a small gap  58  ( FIG. 2 ) is required between the edge of micro-mirrors and the landing surfaces. Constructing small spacers from polytetrafluoroethylene or extending small portions of micro-mirrors along the edge so the entire edge of the micro-mirrors does not touch the landing surfaces may realize this. 
     Further illustrated in  FIG. 3  is cover assembly  34 , which is affixed to the upper facets of micro-prisms  32 . Cover assembly  34  comprises a substrate  44  made of glass or other substantially transparent material. A light-absorbing layer  51  constructed on the lower surface of substrate  44  from a conductive light absorbing film or a multilayer film that includes a conductor layer. The light-absorbing layer  51  is patterned and located directly above micro-mirrors  33 . The cover assembly  34  further includes a light shaping elliptical diffuser  52  formed on the upper surface of substrate  44 . 
     In display panel  20  tilting micro-mirrors  33  actuate by electrostatic attraction force. When a suitable voltage is applied between conductive light reflecting layer  39  and a micro-mirror  33 , the micro-mirror tilts down by electrostatic attraction force. When a suitable voltage is applied between conductive light absorbing layer  51  and a micro-mirror  33 , the micro-mirror tilts up by electrostatic attraction force. 
     As best seen in  FIG. 2  of the drawings, light rays  55  entering from the wide edge  26  of waveguide  21  reflect from upper surface  30  and lower surface  31  by total internal reflections and change angles towards normal with respect to the upper surface  30 . Light rays  55  exit waveguide  21  from upper surface  30  when the incident angle is less than the critical angle  54  defined by refractive index n 1  of waveguide  21  and refractive index n 3  of light transmitting layer  38 . Light rays passing through substrate  35  enter micro-prisms  32  from light input facet  41 . Light rays exit micro-prisms  32  from light exit facets  42 . 
     Depending on positions of tilting micro-mirrors  33 , light rays are absorbed, or directed to the viewer. When a micro-mirror is tilted up, such as micro mirror  33   b  ( FIG. 2 ), light rays reflect from the lower surface of micro-mirrors and are directed to the viewer by reflecting from light reflecting facets  47 . When a micro-mirror is tilted down, such as micro-mirror  33   c , light rays reflect from the lower surface of micro-mirrors  33  and are absorbed in light-absorbing layer  40 . 
       FIG. 4  illustrates a schematic diagram of one pixel  70  of an array of electromechanical pixels. Pixel  70  comprises a lower electrode  72 , an upper electrode  73 , a tilting micro-mirror  74  and a pixel-addressing electrode  75 . Pixel  70  also includes an electronic switch  77 , which provides selective address voltages 0V or V 1  to the pixel-addressing electrode  75 . The pixel-addressing electrode  75  forms a capacitor with lower electrode  72  to store electrical charges supplied by electronic switch  77 . Electronic switch  77  may be a transistor or an electronic circuit employing low-pressure gas discharge that is described in U.S. Ser. No. 12/079,527 patent application. The circuit described in U.S. Ser. No. 12/079,527 patent application initiates an arc in a low-pressure discharge gas to supply voltage potential to pixel-addressing electrodes. 
       FIG. 4  also illustrates a control electronics  71 , which provides micro-mirror actuation voltages to the electrodes of pixel  70 . Control electronics  71  also provides scanning voltages to row electrodes R and synchronized data to column electrodes C to address the array of electromechanical pixels. 
     Micro-mirrors and hinges may be manufactured on a planar surface that is parallel to a surface of a glass substrate. After constructing micro-mirrors, the mechanical rest or neutral position may be changed from the flat position to a tilted down position. To realize this, a suitable voltage may be applied between micro-mirrors  74  and lower electrodes  72  to tilt micro-mirrors and, while holding micro-mirrors at tilted down position, heat may be applied at sufficiently high temperatures to micro-mirror hinges. 
     In  FIG. 4  the mechanical rest position of micro-mirrors  74  is at tilted down position and near to pixel-addressing electrodes  75 . In operation this helps to reduce the required address voltage V 1  supplied to pixel-addressing electrodes  75  and allows greater variations in the mechanical specifications of micro-mirror hinges. 
       FIG. 5  illustrates first voltage waveforms of the invention supplied to the array of electromechanical pixels  70  by control electronics  71 . Initially 0V is applied to lower electrodes  72 , a DC voltage V 3  is applied to upper electrodes  73  and a DC voltage V 2 , with a value between 0V and V 3 , is applied to micro-mirrors  74 . These applied voltages generate an electrostatic bias force between each micro-mirror and the nearest upper or lower electrodes and retain micro-mirrors  74  at selected upper and lower positions during the addressing period. Also during the addressing period, a new set of address voltages 0V or V 1  is supplied to pixel-addressing electrodes  75 . 
     During the actuation period and time interval T 1 , voltage V 2  supplied to micro-mirrors  74  is raised to value V 3  of the upper electrodes  73 . This increases electrostatic attraction force between micro-mirrors and lower electrodes  72  therefore micro-mirrors that are located at upper position tilt down to lower position. Now all micro-mirrors are settled at lower position and closer to pixel-addressing electrodes  75 . 
     During the time interval T 2 , voltage V 2  supplied to micro-mirrors is reduced to 0V. This generates a selective electrostatic force between micro-mirrors and addressing electrodes  75 . Electrostatic force between the micro-mirrors and respective pixel-addressing electrodes  75  with voltage potential V 1  is greater than the electrostatic force between the micro-mirrors and upper electrodes  73 . Therefore these micro-mirrors are held at lower position and micro-mirrors with addressing electrodes  75  having voltage potential 0V tilt to the upper position. 
     During the following addressing period the voltage potential applied to micro-mirrors  74  is raised to initial value V 2  to retain micro-mirrors at new selected positions. 
       FIG. 6  illustrates second voltage waveforms of the invention supplied to the array of electromechanical pixels  70  by control electronics  71 . The operating method is the same as described with regards to  FIG. 5  except for time interval T 1  during the actuation period. 
     During the time interval T 1  voltage V 3  applied to upper electrodes  73  is reduced to value V 2  of micro-mirrors  74 . This reduces electrostatic bias force between micro-mirrors  74  located at upper position and upper electrodes  73  and allows micro-mirrors to tilt down. Micro-mirrors tilt down by a combination of electrostatic force between micro-mirrors and lower electrodes generated by V 2  voltage supplied to the micro-mirrors and mechanical forces stored in hinges. 
     This second operating method permits closer distance between the micro-mirrors and the pixel addressing electrodes by limiting maximum voltage difference to V 2  between the micro-mirrors and the pixel addressing electrodes. 
       FIG. 7  illustrates third voltage waveforms of the invention supplied to the array of electromechanical pixels  70  by control electronics  71 . The operating method is the same as described with regards to  FIG. 5  except for the time interval T 0 . 
     During time interval T 0 , voltage V 2  supplied to micro-mirrors  74  is reduced to −V 2 . This increases the electrostatic force between upper electrodes  73  and micro-mirrors  74  located at upper position. If micro-mirrors  74  are designed sufficiently flexible, the increased electrostatic force will cause micro-mirrors to bend or bow towards the upper electrodes  73  and release micro-mirrors stuck at the upper position. 
     As before, during the actuation period and time interval T 1 , voltage V 2  supplied to micro-mirrors  74  is raised to value V 3  of upper electrodes  73 . This increases electrostatic attraction force between micro-mirrors and lower electrodes  72  therefore micro-mirrors located at upper position tilt down to the lower position. 
     Additionally this increased electrostatic attraction force between micro-mirrors and lower electrodes  72  bows flexible micro-mirrors towards lower electrodes  72  and releases stuck micro-mirrors. 
     Now that all micro-mirrors are at lower position, voltage V 2  supplied to micro-mirrors  74  is gradually reduced from V 3  to 0V. This helps dissipate mechanical forces stored in flexible micro-mirrors during T 1 , so selective displacement of micro-mirrors during T 2  will operate mainly by electrostatic forces. 
     The above operating method helps to release micro-mirrors that are stuck due to humidity or capillary forces between micro-mirrors and landing surfaces. The described method may be used in addition to using fluorosurfactant as a lubricant in the electromechanical pixels. 
       FIG. 8  illustrates fourth voltage waveforms of the invention supplied to the array of electromechanical pixels  70  by control electronics  71 . The operating method is the same as described with regards to  FIG. 6  during time intervals T 1  and T 2  and similar as described with regards to  FIG. 7  during time interval T 0  and during transition from T 1  to T 2 . 
       FIG. 9  illustrates fifth voltage waveforms of the invention supplied to the array of electromechanical pixels  70  by control electronics  71 . The operating method is the same as described with regards to  FIG. 5 . Here supplied voltages V 2  to micro-mirrors  74  and supplied voltages V 3  to lower electrodes  72  are inverted during subsequent addressing and actuation periods. This helps to dissipate residual electrostatic charges accumulated on insulator surfaces during the operation and prevents pixel malfunction. 
       FIG. 10  illustrates sixth voltage waveforms of the invention supplied to the array of electromechanical pixels  70  by control electronics  71 . The operating method is the same as described with regards to  FIG. 5 . Here during the addressing periods alternating voltage V 2  is supplied to micro-mirrors  74  and V 3  to lower electrodes  72  in order to dissipate residual electrostatic charges accumulated on insulator surfaces. 
     Both of these methods described for dissipating residual electrostatic charges can be combined with methods described for releasing stuck micro-mirrors with regards to  FIGS. 7 and 8 . 
     Depending on the display size and resolution, each picture element of the display panel may include several tilting micro-mirrors. Reducing the size of individual micro-mirrors helps to reduce the required electrostatic actuation voltages. 
     Also, micro-mirrors for each picture element may be grouped to modulate different levels of light when suitable voltage is applied between the fixed electrodes and a selected group of micro-mirrors. This reduces the display addressing constraints. For example, each picture element may include 7 micro-mirrors grouped in quantities of 1, 2 and 4 and selectively addressed to modulate 8 levels of light. Additionally, temporal artifacts inherent in pulse-width-modulation displays are reduced. 
     Having now described the invention in detail in accordance with the requirements of the patent statutes, those skilled in this art will have no difficulty in making changes and modifications in the individual parts or their relative assembly in order to meet specific requirements or conditions. Such changes and modification may be made without departing from the scope and spirit of the invention, as set forth in the following claims.