Abstract:
A spatial light modulator (SLM) is disclosed that increases speed and efficiency of an SLM through the use of a perforated hinge. The hinge may take many forms, but generally includes an elongated body having holes therein along its longitudinal direction. One or more electrodes are charged with a predetermined voltage in order rotate a reflecting surface towards the electrodes through electrostatic attraction. The perforations within the hinge results in the hinge being made from less material so that it is less sensitive to hinge thickness variations that can occur during the manufacturing process. Consequently, the perforated hinge allows for more stable, uniform mirror rotation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to U.S. provisional patent application 60/702,139 filed Jul. 25, 2005, entitled “Spatial Light Modulator with Perforated Hinge” in the name of Jonathan David Mohn, which is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention generally relates to spatial light modulators and, more particularly, to the rotational aspects of a spatial light modulator. 
   BACKGROUND 
   A Spatial light modulator (SLM) is one of an array of optical elements (pixels) in which each pixel acts independently as an optical “valve” to adjust or modulate light intensity. The modulation of pixels is accomplished electronically based on image data, essentially converting image information from the electronic domain into light. An SLM does not create its own light, but rather modulates (either reflectively or transmissively) light from a lamp or laser source to create a dynamically reconfigurable digital image. 
   SLMs are used in many technical areas in order to control light on a pixel-by-pixel basis, such as optical data processing, adaptive optics, optical correlation, machine vision, image processing and analysis, beam steering, holographic data storage, displays and MEMS devices (e.g., digital micromirror device (DMD) or the grating light valve (GLV) device). The most typical applications of SLMs are for digital displays, consumer TVs, and office projectors. 
   The basic structure of an SLM includes a reflecting surface, a hinge member, and one or more electrodes. One electrode can be charged with a voltage while the reflecting surface is oppositely charged causing an electrostatic attraction between the underside of the reflecting surface and the electrode. This electrostatic attraction is enough to deform the hinge, upon which the reflecting surface sits, and to tilt the reflecting surface in the desired direction. 
   To return the reflecting surface to its original position, the electrode and the reflecting surface are driven to the same voltage level (e.g., ground) releasing the electrostatic attraction. The hinge member acts as a spring and returns to its natural, at-rest position, much like a bent tree limb springs back to its natural position. 
   A cross-sectional view of a SLM  100  is shown in  FIG. 1 . The SLM includes a base  102  and two opposing electrodes, shown generally at  106 ,  108 . Opposing spring fingers  110 ,  112  are mounted above the electrodes and generally stop the rotation of a reflecting surface  114  as it rotates towards one of the electrodes  106 ,  108 . The reflecting surface  114  rotates by means of a hinge, shown generally at  116 . The hinge is a solid, elongated piece (not shown) having the ability to deform under force and return to its natural position once the force is removed.  FIG. 1  shows a spatial light modulator in the activated or “ON” position. To activate the spatial light modulator, the electrode  106  is positively charged, while the underside of the reflecting surface  114  is negatively charged causing the reflecting surface to rotate about hinge  116  until the reflecting surface contacts spring finger  110 . In this position, deflected light  122  has an angle of reflection desirable to project the light beam  118  from a light source  120  towards, for example, a pixel  124  on a display, 
     FIG. 2  shows the same spatial light modulator  100  in a deactivated or “OFF” position. In this view, the reflecting surface  114  is rotated toward the opposing electrode  108  until the reflecting surface  114  contacts spring finger  112 . In this position, the directional light  118  is deflected into a light absorber  126 . 
   Current designs of the hinge member are not ideal for the proper operation of the SLM. For example, the SLM may required a high actuation voltage in order to deform the hinge. Likewise, the hinge member may have a slow spring-back time to return to its natural position hindering the speed of the SLM. The hinges are also vulnerable to hinge thickness variations during the manufacturing process that can cause non-uniform rotation. The thickness variations also affect the ability to further reduce the size of the hinge, which ultimately limits its speed. 
   Thus, it is desirable to increase the speed and efficiency of an SLM. 
   SUMMARY 
   The present invention increases speed and efficiency of an SLM through the use of a perforated hinge. The perforated hinge may take many forms, but generally includes an elongated body having one or more holes therein along its longitudinal direction. 
   The perforated hinge can be formed by using a number of different materials. Preferably, the perforated hinge may be made of material whose fatigue lifetime exceeds 1.0e 9  hinge deflection cycles. For example, the hinge may be made from one or more of the following: copper, copper alloy, aluminum, aluminum alloy, single crystal silicon, amorphous silicon, polysilicon, silicon nitride, titanium, and titanium alloy. 
   The perforations within the hinge results in the hinge being made from less material than prior art hinges. Less material within the hinge means that it will be less sensitive to hinge thickness variations that can occur during the manufacturing process. Consequently, the perforated hinge allows for more stable, uniform mirror rotation. Additionally, because it is less sensitive to hinge thickness variations, the size of the hinge can be further reduced, which results in more chips per wafer and a lower cost per chip. This reduced size of the perforated hinge also allows for faster mirror flipping speeds, such as less than 800 nano-seconds. 
   These features and others of the described embodiments will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a side-profile view of a prior-art SLM in an activated state. 
       FIG. 2  shows a side-profile view of a prior-art SLM in a deactivated state. 
     FIG:  3  shows a perspective view of a portion of an SLM array; one SLM having its reflecting surface removed to expose a perforated hinge according to the invention. 
       FIGS. 4A-D  show perspective views of an SLM design having a perforated hinge and a centrally mounted support post for a reflecting surface. 
       FIGS. 5A-D  show perspective views of an SLM having a perforated hinge and a centrally mounted reflecting surface positioned upon a yoke. 
       FIGS. 6A-D  show perspective views of an SLM having a perforated hinge and two opposing support posts positioned at opposite ends of the hinge. 
       FIG. 7  shows a side-profile view of an SLM with a single-level electrode. 
       FIG. 8  shows a side-profile view of an SLM with a double-level electrode. 
       FIGS. 9A-9D  show a top view of an SLM having a perforated hinge having a single spring finger for each electrode. 
       FIG. 10  shows a perspective view of a side-mounted perforated hinge design. 
       FIG. 11  shows a perspective view of a single-electrode design with a perforate hinge. 
       FIG. 12  shows a top view of other perforated hinge designs. 
   

   DETAILED DESCRIPTION  
     FIG. 3  shows a perspective view of an array  140  of SLMs with the tops of various reflecting surfaces  142  having a square shape. Although an array of only nine SLMS is shown, in practice the array can include any desired number of SLMs depending on the application. An example array is a 1920×1080 array of SLMs. Normally, the array is formed on a single silicon wafer to be used in a chip. One reflecting surface is removed to expose an SLM  144  according to the invention. A perforated hinge  146  is shown and described further below in relation to other figures. The hinge  146  is diagonally positioned along a direction connecting two opposing corners of the square-shaped reflecting surface. The perforated hinge  146  may take a variety of shapes and configurations. Additionally, the perforated hinge  146  can be used in any desired SLM design. 
     FIGS. 4A-4D  show various examples of an SLM with a perforated hinge in more detail. The SLM  160  includes a base  162  that houses two sets of spring finger support posts  164 ,  165 . Spring-fingers support plates  166 ,  167  are positioned on the posts  164 ,  165 , respectively. Spring fingers  168 ,  169  are located on the ends of the support plates  166 ,  167  and are used to overcome stiction force (Vanderwaals electrostatic force) encountered during normal operation of the mirror (not shown). Electrodes  170 ,  172  are positioned between the spring fingers  168 ,  169  and are used to tilt the reflecting surface in a desired direction. The electrodes  170 ,  172  are driven by CMOS transistors (not shown) coupled to a source voltage to charge the electrodes in a well-known manner. Other methods of charging electrodes may also be used. An elongated perforated hinge  174  is coupled at its ends to opposing support plates  166 ,  167 . Centrally located on the perforated hinge, midway between the support plates  166 ,  167 , is a support post  176  for mounting of the reflecting surface. Instead of being a solid piece, the perforated hinge  174  includes non-uniformly sized holes  178  (also called perforations). Cross supports  180  may extend across the holes to further strengthen the hinge  174 . 
   Turning briefly to  FIG. 12 , several examples of perforated hinges are shown and may be used with any of the SLMs described herein. Perforated hinge  200  is shown having an elongated body  201  with opposing ends  202 ,  203 . The width of the body  201  is non-uniform, as it is wider at a center point  204  (W 1 ) than at its ends  202 ,  203  (W 2 ). The elongated body  201  has opposing side walls  206  and  207  that together with the ends  202 ,  203 , form the perimeter of the body. Between the side walls  206 ,  207 , there are a plurality of non-uniformly sized, hexagon-shaped holes  208 . Although six hexagons are shown, any number of hexagons could be included in the design. Additionally, any geometric shape or combinations of geometric shapes can be used in place of the hexagons. For example, the perforated hinge shown at  210  uses circles for perforations instead of hexagons. The holes or perforations may also be uniformly sized. Perforated hinge  212  is another example wherein the holes take the form of a plurality of parallelograms  214  having non-uniform size. The combination of parallelograms form two longitudinal members  216 ,  217  that run parallel to the axis of rotation of the reflecting surface with cross members  218  extending there between for additional support. In general, it is desirable that the holes in the perforated hinge represent greater than five percent of the total surface area of the hinge (the total surface area being measured as if the hinge was a solid piece without holes). Alternatively, it is desirable that one or more of the holes are greater in width (or diameter) than ten percent of the width of the hinge at the location of the hole. In some cases, such as perforated hinge  212 , the holes  214  represent more than eight-five percent of the total surface area of the hinge. Thus it can be seen that the shape and the form of the perforations within the hinge is not of importance and may vary depending on the application. 
   The material from which the hinge is made varies depending on the application. In one application, it is desirable that the material used has a fatigue lifetime that exceeds 1.0e 9  hinge deflection cycles. Example materials include copper, copper alloy, aluminium, aluminium alloy, single crystal silicon, amorphous silicon, polysilicon, silicon nitride, titanium, and titanium alloy or combinations thereof. Other materials may also be used. 
   Returning to  FIG. 4B , a perforated hinge  220  is shown having holes formed therein by two equally sized rectangles  222 ,  224 . In this example, the perforated hinge  220  has uniform width. One difference between the embodiment of  FIG. 4B  and those of  FIG. 12 , shown at  200  and  210 , is the ratio of hole space to the overall size of the perforated hinge  220  is much greater with the perforated hinge of  FIG. 4B . Having a greater ratio of hole space in the perforated hinge allows the hinge stiffness to be less sensitive to hinge thickness. On the other hand, the greater the ratio, the weaker the hinge. 
     FIGS. 4C and 4D  show perforated hinges  230  and  240 , respectively. Hinge  230  has two parallelogram-shaped holes  232 ,  234  with opposing side walls converging as they approach a center support  236  for the reflecting surface.  FIG. 4D  has the hinge  240  with two parallelogram-shaped holes, but with opposing sides of the parallelograms diverging as they approach the center support  242 . 
     FIGS. 5A-5D  show the perforated hinge used in a different type of SLM. This type of SLM is disclosed in U.S. Pat. No. 6,819,470, which is hereby incorporated by reference. However, a perforated hinge, such as shown at  260 , is incorporated into the design. In this embodiment, a yoke  262  is coupled between opposite portions  264 ,  266  of the perforated hinge  260 . The portions  264 ,  266  each include a hole in the shape of a parallelogram. 
     FIG. 5B  shows a perforated hinge  280  including holes formed by multiple rectangles on each side of the yoke.  FIGS. 5C and 5D  show similar variations of forms that the perforated hinge may take. 
     FIG. 6A  shows yet another form of SLM where the reflecting surface (not shown) is mounted to several support posts  300 ,  302  located on opposite ends of the perforated hinge  304 .  FIGS. 6B-6D  again show variations of the form the perforated hinge may take. 
     FIG. 7  shows other variations of SLMs wherein single-level electrodes  320  are used. The perforated hinge  322  can also be used to rotate a reflecting surface  324  in this type of SLM. 
     FIG. 8  shows an SLM where double-level electrodes  340  are used. A reflecting surface  342  rotates on a perforated hinge  344  based on the state of the double-level electrodes  340 . The first level of the electrode  340  can be seen at  346  and the second level at  348 . The double levels provide a greater electrostatic attraction between the reflecting surfaces and the electrodes. 
     FIGS. 9A-9D  show other possible types of an SLM with perforated hinges. Looking first at  FIG. 9A , a perforated hinge  360  is coupled to opposing supports  362 . Opposing electrodes  364  allow for rotation of a reflecting surface (not shown) towards either of the electrodes. Spring fingers  366  are used to stop the rotation of the reflecting surface. This embodiment differs in that only a single spring finger is used per side, rather than two.  FIGS. 9B-9D  show that other perforated hinge designs may also be used with a type of SLM having a single spring finger per side. 
     FIG. 10  shows another possible SLM design. In this case, an SLM is shown that is of the type described in U.S. Patent Publication 2004/0145795, which is hereby incorporated by reference. In this case, the SLM  380  has a side-mounted perforated hinge  382  to rotate a reflecting surface  384 . All other variations of the perforated hinge may also be used. 
     FIG. 11  shows another design of an SLM of the type described in U.S. Pat. No. 6,867,897, which is hereby incorporated by reference. In this case, a single electrode  400  is used to rotate a reflecting surface  402 . A perforated hinge  404  is mounted to a base  406  in order for the reflecting surface to rotate. 
   Thus, it can be seen that the perforated hinge may be used with a wide variety of SLM types, including single electrode, double electrode, single-level and double-level electrodes, single or double spring fingers, etc. Additionally, the perforated hinge may take a variety of forms, 
   Having illustrated and described the principles of the illustrated embodiments, it will be apparent to those skilled in the art that the embodiments can be modified in arrangement and detail without departing from such principles. 
   For example, some of the potential SLM designs are illustrated in the description and figures, but every possible SLM available cannot be described for simplicity and clarity of the application. Nonetheless, those skilled in the art may easily adapt the perforated hinge described herein into different SLM designs. 
   Additionally, although square reflecting surfaces are used, the reflecting surfaces can be any desired shape. 
   Further, the perforated hinge may be mounted to the reflecting surface in any desired manner, such as using one or more support posts or a side connected hinge. 
   Still further, although a two-level electrode is shown, additional levels may be added. For example, a three-level electrode may be used. With a multi-level electrode, each successive level is shorter than the previous level so as not to interfere with mirror rotation. Thus,  FIG. 8  shows the second level electrode  348  is shorter than the first level electrode  346 . Likewise, a third level positioned on top of the second level would again be shorter. Ideally, as the mirror rotates, the bottom surface of the mirror is equal distance from the distal ends of each electrode. 
   In view of the many possible embodiments, it will be recognized that the illustrated embodiments include only examples of the invention and should not be taken as a limitation on the scope of the invention. Rather, the invention is defined by the following claims. We therefore claim as the invention all such embodiments that come within the scope of these claims.