Abstract:
A microelectromechanical device has a movable first electrode and stationary second and third electrodes oriented parallel with the substrate. The first electrode is separated from the substrate and includes a first hinge oriented parallel to the substrate, with spaced apart extensions extending from opposing sides of the first electrode. The second and third electrodes are fixedly formed on the substrate adjacent to the opposing sides of the first electrode and have spaced apart extensions interspersed with the spaced apart extensions of the first electrode. The first electrode can move substantially parallel to the surface of the substrate such that the extensions of one side of the first electrode substantially overlap the extensions of the second electrode in a first state and the extensions of the other side of the first electrode substantially overlaps the extensions of the third electrode in a second state. A second hinge has one or more first posts fixedly coupled to the substrate and one or more second posts coupled to the first electrode, such that movement of the first electrode causes a twisting of the first and second hinges.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. Ser. No. 10/907,992, filed Apr. 22, 2005, entitled “A Non-Contacting Electrostatically Driven MEMS Device” to Strumpell, which is incorporated by reference herein. 

   STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   This invention relates in general to microelectromechanical system (MEMS) devices and, more particularly, to an electrostatically driven MEMS device. 
   2. Description of the Related Art 
   Microelectromechanical systems (MEMS) devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching, which have been developed for the fabrication of integrated circuits. Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of MEMS device used in projection displays by controlling light through reflection. Other types of MEMS devices include accelerometers, pressure and flow sensors, and gears and motors. 
   A conventional DMD  100  is illustrated in  FIG. 1 . As shown, the DMD  100  is constructed of three metal layers: a top layer  102 , a middle layer  104 , and a bottom layer  106 . The three metal layers are situated over an integrated circuit (not shown), which provides electrical commands and signals. The top layer  102  includes a pixel mirror  108  that resides over the middle layer  104  supported via a mirror support post  110 . The middle layer  104 , in turn, resides over the bottom layer  106  supported by four hinge support posts  112 . The mirror support post  110  of the top layer  102  is attached to a yoke  114 . As the yoke  114  rotates on its torsion hinges  118 , it drives the mirror support post  110  to rotate and tilt accordingly. Consequently, as the mirror support post  110  rotates and tilts, it dictates the angle, direction, and magnitude that the pixel mirror  108  will rotate and tilt. The yoke  114 , in essence, controls the pixel mirror  108  by this relay effect. 
   One problem associated with a conventional MEMS device, such as the DMD  100 , is “stiction”, which occurs when the yoke  114  rotates on the torsion hinges  118  and the yoke landing tips  116  come in physical contact with landing sites  120  located within the underlying bottom layer  106 . In some cases, when surface adhesion forces are high enough, the yoke landing tips  116  may stick to the landing sites  120  in the underlying bottom layer  106 , and thereby adversely affect the response time of the pixel mirror  108  and the overall device performance. In other cases, the landing tips  116  may adhere to the landing sites  120  and remain stuck if an applied mechanical restoring force is not strong enough to overcome the existing surface adhesion forces. The pixel mirror  108  will then be considered permanently defective because it will remain fixated at only one angle. 
   Stiction has heretofore been addressed by applying lubrication or passivation layers to the yoke landing tips  116  and the landing sites  120  in the hopes of making these metal surfaces slippery enough to minimize sticking. In addition, reset electronics  122  have been employed to pump additional electrical energy into the yoke  114  in order to help it break free from the constraining surface adhesion forces between the yoke landing tips  116  and the landing sites  120 . These techniques require extra fabrication processes and additional cost. 
   Therefore, a need has arisen for a MEMS device which does not need special fabrication to overcome stiction forces. 
   BRIEF SUMMARY OF THE INVENTION 
   The present disclosure relates to a microelectromechanical device has a movable first electrode and stationary second and third electrodes oriented parallel with the substrate. The first electrode is separated from the substrate and includes a first hinge oriented parallel to the substrate, with spaced apart extensions extending from opposing sides of the first electrode. The second and third electrodes are fixedly formed on the substrate adjacent to the opposing sides of the first electrode and have spaced apart extensions interspersed with the spaced apart extensions of the first electrode. The first electrode can move substantially parallel to the surface of the substrate such that the extensions of one side of the first electrode substantially overlap the extensions of the second electrode in a first state and the extensions of the other side of the first electrode substantially overlaps the extensions of the third electrode in a second state. A second hinge has one or more first posts fixedly coupled to the substrate and one or more second posts coupled to the first electrode, such that movement of the first electrode causes a twisting of the first and second hinges. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is an exploded view of a prior-art digital micromirror device (DMD); 
       FIG. 2  is an exploded view of a DMD according to the present disclosure; and 
       FIG. 3  shows the DMD with the mirror tilted. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is best understood in relation to  FIGS. 1-3  of the drawings, like numerals being used for like elements of the various drawings. 
   Referring to the conventional digital micromirror device (DMD) of  FIG. 1 , the pixel mirror  108  tilts and rotates according to the tilt and rotation of the yoke. In practice, the pixel mirror  108  also rotates and tilts due to the electrostatic forces generated by the electric fields between the pixel mirror  108  and the mirror address electrodes  113 , as well as the fields generated between the yoke  114  and the floating electrodes  121 . Electrical signals are fed and carried through metal contact holes from the underlying integrated circuit (not shown). 
   Reference is now made to  FIG. 2 , which illustrates a digital micromirror device (DMD)  200  according to the present disclosure. The DMD  200  includes a top layer  202 , a middle layer  204 , and a bottom layer  206 . As illustrated in the figure, the top layer  202  includes a pixel mirror  208  connected to a downwardly extending mirror support post  210 . The mirror support post  210  is adapted for engagement with a corresponding post-attachment site  211  of primary torsion hinge  212  of the middle layer  204 , as will be further described below. In some embodiments, the pixel mirror  208  has a thickness of about 2,000 to 5,000 Angstroms and is constructed of an aluminum composition using known methods and techniques. Preferably, the thickness of the pixel mirror  208  of the presently disclosed embodiment has a thickness of about 3,300 Angstroms. In addition to aluminum, other materials such as silicon oxide, silicon nitride, polysilicon, and phosphosilicate glass (PSG) may also be used in constructing the pixel mirror  208   m  with the mirror  208  covered with a reflective coating if needed. In some embodiments, the mirror support post  210  has a thickness of about 500 to 1,000 Angstroms and a height of 5000 to 25000 Angstroms and may be constructed of an aluminum alloy using known methods and techniques. The mirror support post  210  may also be formed of aluminum, titanium, and silicon metal alloys. Preferably, the thickness of the mirror support post  210  of the presently disclosed embodiment has a thickness of about 700 Angstroms and has a height of about 10000 Angstroms. 
   The middle layer  204 , disposed beneath the top layer  202 , includes a primary torsion hinge  212  supported by a plurality of primary hinge posts  214 . The primary hinge posts  214  may be formed according to the same or similar materials and methods as the mirror support posts  210 . Furthermore, the primary hinge posts  214  may also have the same or similar thickness as that of the mirror support post  210 . The thickness of the primary hinge layer  212  is similar to the secondary hinge layer  217  (described below). While the primary hinge  212  is shown such in line with a diagonal through the center of mirror  208 , the hinge  212  could also be aligned with a center parallel to either side of the mirror  208  (i.e., the mirror  208  could be rotated about the center of the post  210  such that it is deflected corner-to-corner or side-to-side, as desired). 
   The bottom layer  206 , situated below the middle layer  204 , includes a floating electrode  216  and mirror address electrodes  220 . Floating electrode  216  includes secondary hinge  217  and extensions  218 . Mirror address electrodes  220  flank either side of the floating electrode  216 . Each mirror address electrode includes extensions  22  which interleave with extensions  218 , separated by groove  221 . The bottom layer  206  further includes contact pads  224 , which are provided for receiving the primary hinge posts  214  at attachment sites  224   a  and  224   b . The inner hinge posts  214  are attached at attachment sites  217   a  and  217   b  of secondary hinge  217 . Electrical signals and connections from an integrated circuit (not shown) positioned beneath the bottom layer  206  are sent to the mirror address electrodes  220 . The integrated circuit may be a static random access memory (SRAM) cell or an integrated complementary metal oxide semiconductor (CMOS) device. In other embodiments, the integrated circuit may be a multi-chip module (MCM) where many devices are assembled together by stacking one on top of another into a single module for faster electronic devices with added functionalities. 
   The floating electrode  216  is formed over a sacrificial masking layer, which is later removed, while the mirror address electrodes  220  and contact pads  224  are formed on the substrate. Once the sacrificial masking layer is removed, the floating electrode  216  is separated from the substrate, suspended by the primary torsion hinge  212  and posts  214 . The floating electrode  216  can thus move laterally between the two mirror address electrodes,  220 , as described in greater detail below. 
   The floating electrode  216  generally resides in a middle portion of the bottom layer  206  and is flanked by the two outer mirror address electrodes  220 . Accordingly, the extensions  218 ,  222  are substantially interdigitated to form a comb-like structure. In some embodiments, the floating electrode  216  and the two mirror address electrodes  220  have a thickness of about 500 to about 3,000 Angstroms. Preferably, the thickness of the floating electrode  216  and the two mirror address electrodes  220  within the presently disclosed embodiment is about 1,500 Angstroms. Additionally, the interspersed extensions  218 ,  222  may have a corresponding width and length of about 20 μm and a thickness of about 500 to about 3,000 Angstroms. Still further, the spacing between the interspersed extensions  218 ,  222  can vary from about 5 to 10 μm. Preferably, the spacing between the interspersed extensions  218 ,  222  within the presently disclosed embodiment is about 7.5 μm. The thickness of the secondary hinge  217  is less than the thickness of the floating electrode and can range from 400 to 1500 Angstroms with a preferred thickness of about 600 Angstroms. 
   Although the interspersed extensions  218 ,  222  are depicted as being square in shape, they can take on a variety of polygonal shapes and sizes. For example, the interspersed extensions  218 ,  222  may be in the shape of a rectangle, a triangle, a parallelogram, a diamond, a trapezoid or any other suitable shape. In addition, the interspersed extensions  218 ,  222  may also take on plane-curve shapes such as circles, semi-circles, ellipses, semi-ellipses, lines, parabolas, or hyperbolas. Furthermore, the interspersed extensions  218 ,  222  may be uniformly spaced or non-uniformly spaced and uniform in shape and size or non-uniform in shape and size. Uniform and non-uniform combinations of shapes and sizes are also contemplated. 
   One benefit of the DMD  200  is realized through the amount of electrostatic force that can be generated between the extensions  218 ,  222 . In particular, an electrostatic force F acting upon a charged object Q 1  as a result of the presence of another charged object Q 2  can be calculated by Coulomb&#39;s law (F=k*Q 1 *Q 2 /d 2 ), where k is a constant and d is the distance between the objects. The magnitude of a charged object Q can be calculated by multiplying the surface density σ with the surface area of the charged object A (Q=σA). Accordingly, the electrostatic force F scales proportionally with the surface area of the charged object A (FαA). The interspersed extensions  218 ,  222  increase the surface area of the electrodes of the DMD  200 , thereby facilitating the generation of a greater electrostatic force than that of a conventional DMD  100 . 
   In practice, an electrostatic field is generated using the integrated circuit by pulsing a charge on one the mirror address electrodes  220  or, alternatively, pulsing both mirror address electrodes  220  with opposite charges. The floating electrode  216  is not pulsed by the integrated circuit. The charge (or charges) on the mirror address electrodes causes an electrostatic field which provide a pulling and/or pushing force on the floating electrode  216 . Hence, the electrostatic field(s) between mirror address electrode(s)  220  and the floating electrode  216  causes the floating electrode  216  to travel toward one mirror address electrode  220  away from the other mirror address electrode  216 . With the primary hinge  212  having its outer posts  214  attached to the stationary contact pads  224  and the inner posts  214  connected to the secondary hinge  217 , which slides along the substrate along with the floating electrodes, the mirror  208  will rotate downward away from the attracting mirror address electrode and toward the repelling mirror address electrode  220 . Both the secondary hinge  217  and primary torsion hinge  212  will twist responsive to the movement of the floating electrode  216  towards one of the mirror address electrodes  220 . When the voltage is removed from mirror address electrodes  220  and the floating electrode  216 , the secondary hinge  217  and primary torsion hinge  212  will untwist, returning the floating electrode to an intermediary position between the two mirror address electrodes. 
   Unlike a conventional DMD  100 , wherein the pixel mirror  108  can experience stiction during tilting or rotation, the DMD  200  can generate much greater electrostatic forces thereby eliminating or at least reducing the chance that the pixel mirror  208  will stick to underlying layers of the DMD  200 . In addition, the increased electrostatic force eliminates the need for reset electronics. 
   One of the advantages of the design of the DMD  200 , is that thin film fabrication techniques may be used to produce a compact structure. For thin film fabrication techniques to be used, components of the devices that are formed by etching, such as the electrodes  216  and  220  and hinges  212  and  217 , must be oriented in a plane parallel to the substrate—i.e., the thickness of the component must be much less than its width and length. A first material layer is used to form the contact pads, mirror address electrodes and floating electrodes (not including the secondary hinge  217 ). A mask layer is formed in the interior of the floating electrode and a material layer is formed over the mask layer to produce the secondary hinge with a thickness less than the thickness of the remainder of the floating electrode  216 . A mask layer is formed over the components of the bottom layer  206 , and the primary torsion hinge  212  is formed on top of the mask layer. Holes are formed through the primary torsion hinge  212  where the posts  214  are to be formed; the masking layer is etched through the holes to reach the attachment sites  217   a - b  and  224   a - b . The posts  214  are formed by sputter deposition of the material into the holes; the posts  214  will form on the sidewalls of the masking layer beneath the holes. 
   Similarly, a masking layer is formed over the middle layer  204  and the mirrors  208  are formed over the masking layer. Again, the masking layer is etched through holes in the mirrors  208  where the posts  210  are to be formed; the masking layer is etched through the holes to the attachment sites  211 . The posts  211  are formed by sputter depositions of the material into the holes. The masking layers can then be removed, leaving the structure shown in  FIG. 2 . In some embodiments, the hollow posts  210  may be filled in order to prevent unwanted reflections from the interior of the posts. 
   It should be noted that the components that have substantial height, i.e., the posts, are formed by deposition of a material over a sacrificial masking layer, not by deep etching. 
   Since the etched components are horizontally oriented (i.e., oriented such that the sides with the largest surface area are parallel to the surface of the substrate), deep etching is not required to form the hinges or electrodes. Deep etching, which would be needed for vertically oriented components, could not be reasonably used for metallic components, such as those formed from an aluminum composite material, and could cause odd geometries which would affect performance (such as hinges that are much thicker at the bottom than at the top, due to the longer etching times at the top of a material layer). Further, deep etching at the bottom level will cause unevenness at higher levels, which may prevent flat mirrors from being formed. 
   It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. For example, the DMD  200  may be manufactured by surface micromachining, where the structures are built up in layers of thin film on the surface of a silicon wafer or any other suitable substrate. Another technique of manufacturing a DMD is bulk micromachining. In addition, the presently disclosed embodiments may also be applied to MEMS devices for useful applications in the study and understanding of biological proteins and gene functions. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and ranges of equivalents thereof are intended to be embraced therein. 
   Although the Detailed Description of the invention has been directed to certain exemplary embodiments, various modifications of these embodiments, as well as alternative embodiments, will be suggested to those skilled in the art. The invention encompasses any modifications or alternative embodiments that fall within the scope of the Claims.