Abstract:
A MEMS device achieves a large angle of rotation of a plate about 2 independent axes by employing a handle portion of the plate which is isolated by respective springs coupling the handle portion to each of two actuators. A first actuator, which rotates the mirror about the same axis as done in U.S. Pat. No. 6,781,744 is essentially the same structure disclosed therein, but with the mirror plate thereof shrunken in size. This shrunken plate is coupled by a spring to the mirror plate of the instant invention. Movement of the shrunken plate causes corresponding movement of the handle portion, and hence the mirror. A second actuator, coupled by another spring to the mirror plate of the instant invention, rotates about a second axis that is perpendicular to the first axis and parallel to the substrate. The second actuator includes an actuator plate and an electrode thereunder.

Description:
TECHNICAL FIELD 
   This invention relates to micro-electromechanical systems (MEMS), and more particularly, to MEMS devices that use amplified motion to move a plate. 
   BACKGROUND OF THE INVENTION 
   Optical communication equipment often employs one or more micro-electromechanical systems (MEMS) devices. A typical MEMS device may include an array of micro-machined mirrors, with each mirror being individually movable in response to an electrical signal. Such an array may be employed in an optical cross-connect, in which each mirror in the array receives a different beam of light, for example, from an input optical fiber. The beam is reflected from the mirror and can be redirected to a different location, e.g., a location at which an output optical fiber is located. The particular output fiber that receives the redirected beam may be selected by rotating the mirror. Other optical applications for MEMS devices include wave selective switches, add-drop switches, wavelength attenuators, and wavelength blockers. Non-optical applications are also possible. 
   One problem with prior art MEMS devices having relatively large mirrors, e.g., between 100 μm and 400 μm in length and between 30 μm and 70 μm in width, is that the height of the gap between the mirror and the corresponding actuating electrode(s) has to be relatively large, i.e., greater 8 μm, to achieve relatively large, e.g., about 10 degree, rotation angles. However, an 8 μm gap height is the best that can be achieved with surface micromachine technology, which is a simple and low cost fabrication technique. 
   U.S. Pat. No. 6,781,744, which is incorporated by reference as if fully set forth herein, discloses a MEMS device having a movable mirror and a movable actuator plate mechanically that are coupled together such that a relatively small displacement of the plate results in rotation of the mirror by a relatively large angle. In one exemplary arrangement, the mirror and actuator plate are supported on a substrate. The actuator plate moves in response to a voltage difference applied between a) an electrode located on the substrate beneath the plate and b) the plate itself. One or more springs attached to the plate provide a counteracting restoring force when they are stretched from their rest positions by the plate motion. The mirror has a handle portion configured as a lever arm. A spring attached between the actuator plate and the handle portion transfers the motion of the actuator plate to the mirror such that, when the actuator plate moves toward the substrate, the spring pulls the handle portion to move the mirror away from the substrate. Advantageously, relatively large mirror rotation angles may be achieved using the relatively small displacements of the actuator plate that can be achieved using surface micromachine technology. 
   In another exemplary arrangement disclosed in U.S. Pat. No. 6,781,744, a MEMS device has first and second plates, each supported on, and positioned offset from, a substrate. The second plate is rotatably connected to the substrate. The connection defines a rotation axis and first and second portions of the second plate including its opposite ends with respect to the rotation axis. One end of the first plate is movably connected to the first portion of the second plate, while the other end of the first plate is connected to the substrate. 
   Disadvantageously, U.S. Pat. No. 6,781,744 only teaches how to achieve rotation by a relatively large angle around a single axis. 
   SUMMARY OF THE INVENTION 
   We have developed a MEMS device that can achieve a relatively large angle of rotation of a plate, which may be a mirror about two independent axes using surface micromachine technology. In accordance with the principles of the invention, a handle portion of the plate is isolated by respective springs coupling it to each of two actuators. A first actuator, which rotates the mirror about the same axis as is done in U.S. Pat. No. 6,781,744 is essentially the same structure disclosed therein as described above, but with the mirror plate thereof shrunken in size. This shrunken plate is coupled by a spring to the mirror plate of the instant invention. Movement of the shrunken plate causes corresponding movement of the handle portion, and hence the mirror. A second actuator, coupled by another spring to the mirror plate of the instant invention, rotates about a second axis that is perpendicular to the first axis but is parallel to the substrate. The second actuator is made up of an actuator plate with an electrode thereunder. This second actuator plate may be long and narrow, and its electrode may be so too. The electrode is made narrower than the actuator plate, so that if snapdown does occur, the actuator plate will not contact the electrode. 
   Advantageously, in some embodiments of the invention, each spring may be optimized to primarily allow torsion in only one direction and with relative rigidity in all other directions. Advantageously, amplified motion need only be employed for motion about the first axis, since the actuator used for motion about the second axis 1) may be narrow enough that it can rotate a considerable amount, e.g., 10 degrees, without touching the substrate, and 2) may be designed to achieve the force necessary to rotate the mirror plate, as there is no inherent geometrical restriction on its length. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing: 
       FIG. 1  shows a perspective view of exemplary MEMS device, which is arranged in accordance with the principles of the invention 
       FIG. 2  shows an embodiment of the invention, similar to that shown in  FIG. 1 , but in which the movable actuator plate and spacer of  FIG. 1  are connected together, e.g., formed of a unitary piece of material, thereby forming an actuator plate; 
       FIG. 3  shows a perspective view of exemplary MEMS device, similar to that shown in  FIG. 1 , which is arranged in accordance with the principles of the invention; and 
       FIG. 4  shows an exemplary arrangement of several of the MEMS device of  FIG. 1 , arranged in a one-dimensional array. 
   

   DETAILED DESCRIPTION 
   The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
   In the claims hereof any element expressed as a means for performing a specified function is intended to encompass any way of performing that function. This may include, for example, a) a combination of electrical or mechanical elements which performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function, as well as mechanical elements coupled to software controlled circuitry, if any. The invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. Applicant thus regards any means which can provide those functionalities as equivalent as those shown herein. 
   Software modules, or simply modules which are implied to be software, may be represented herein as any combination of flowchart elements or other elements indicating performance of process steps and/or textual description. Such modules may be executed by hardware that is expressly or implicitly shown. 
   Unless otherwise explicitly specified herein, the drawings are not drawn to scale. 
   The term micro-electromechanical systems (MEMS) device as used herein is intended to mean an entire MEMS device or any portion thereof. Thus, if a portion of a MEMS device is inoperative, or if a portion of a MEMS device is occluded, such a MEMS device is nonetheless considered to be a MEMS device for purposes of the present disclosure. 
   In the description, identically numbered components within different ones of the FIGs. refer to the same components. 
     FIG. 1  shows a perspective view of exemplary MEMS device  100 , which is arranged in accordance with the principles of the invention. MEMS device  100  includes a movable mirror  101 , which is rotatable about a first axis parallel to x-direction  109  by virtue of it being mechanically coupled to coupling plate  102 , which is in turn coupled to movable actuator plate  122 , all of which are suspended off of substrate  104 . One end of movable actuator plate  122  is attached to substrate  104  using spacer  124 , and the other end of movable actuator plate  122  is coupled to coupling plate  102 . In MEMS device  100  the end of movable actuator plate  122  adjacent to spacer  124  is fixedly attached thereto by at least one torsional element  107 . Torsional element  107  may be implemented as a spring, and it is preferably adapted to permit movable actuator plate  122  to rotate substantially only about an axis parallel to x-direction  109 . Note that x-direction  109  is not an element of MEMS device  100  but is shown for pedagogical purposes only. 
   Electrode  126  is located on substrate  104  beneath movable actuator plate  122 . Movable actuator plate  122  and electrode  126  form an electrostatic actuator of device  100 . When electrode  126  is biased with respect to movable actuator plate  122 , movable actuator plate  122  rotates about an axis parallel to x-direction  109 . Preferably, moveable actuator plate  122  remains substantially undeformed in its rest position and in all positions to which it rotates. 
   Coupling plate  102  is supported above substrate  104  by at least one torsional member  114 , e.g., one or more springs, each of which is attached between at least one of stationary posts  128  and handle portion  132  of coupling plate  102 . At least one torsional member  134  is attached between handle portion  132  of coupling plate  102  and the suspended end of movable actuator plate  122  so as to mechanically couple together coupling plate  102  and movable actuator plate  122 . 
   In operation, as the end of movable actuator plate  122  coupled to coupling plate  102  moves down toward substrate  101 , handle portion  132  of coupling plate  102  likewise moves downward. This in turn causes the non-handle portion of plate  102  to move upward. This is described in more detail in U.S. Pat. No. 6,781,744, e.g., in connection with  FIG. 3  thereof, except that coupling plate  102  of the instant invention takes the place of the mirror therein. 
   The motion induced by movable actuator plate  122  in coupling plate  102  is coupled via at least one torsional element  154  to handle portion  152  of mirror  101 . Torsional element  154 , e.g., a spring, is designed so that it preferably transfers all of the torque in the x-direction of coupling plate  102  to mirror  101 . In addition, torsional element  154  is preferably adapted to permit coupling plate  102  to rotate substantially only about an axis parallel to y-direction  129 . Note that y-direction  129  is not an element of MEMS device  100  but is shown for pedagogical purposes only. Advantageously, the motion of movable actuator plate  122  about a direction parallel to x-direction  109  is transferred as the component of motion about x-direction  109  of mirror  101 , while motion of mirror  101  about an axis in a direction parallel to y-axis  129  is not transferred to movable actuator plate  122 . 
   Movable mirror  101  is also rotatable about a second axis perpendicular to x-direction  109 , e.g., y-axis  129 , by virtue of it being mechanically coupled to movable actuator plate  172 , all of which are suspended off of substrate  104 . One side of movable actuator plate  172  is attached to substrate  104  using posts  164  and torsional elements  167 , e.g., springs, while the opposite side of movable actuator plate  172  is suspended above substrate  104 . Torsional elements  167 , e.g., springs, are preferably arranged to permit movable actuator plate  172  to rotate substantially only about an axis parallel to y-direction  129 . 
   Electrode  176  is located on substrate  104  beneath movable actuator plate  172 . Movable actuator plate  172  and electrode  176  form an electrostatic actuator of device  100 . Preferably, when electrode  176  is biased with respect to movable actuator plate  172 , movable actuator plate  172  rotates substantially undeformed about an axis parallel to y-direction  129 . 
   Movable actuator plate  172  is coupled via torsional element  174 , e.g., a spring, to handle portion  152  of mirror  101 . The motion induced by movable actuator plate  172  is coupled via torsional element  174  to handle portion  152  of mirror  101 . Torsional element  174  is designed so that it preferably transfers all of the torque in the y-direction to handle portion  152  of mirror  101 . In addition, torsional element  174  is preferably adapted to permit movable actuator plate  172  to rotate substantially only about an axis parallel to y-direction  129 . Advantageously, the motion of movable actuator plate  172  about a direction parallel to y-axis  129  is transferred as the component of motion about y-axis  129  of mirror  101 , while motion of mirror  101  about an axis in a direction parallel to x-direction  109  is not transferred to movable actuator plate  172 . 
   Thus, mirror  101  may be rotated about axes in either of the x and y directions, or about both simultaneously and independently. 
   Optionally, to reduce the possibility of snapdown of movable actuator plate  172 , optional electrode  186  may be placed on substrate  104  beyond the footprint of movable actuator plate  172 . Advantageously, optional electrode  186 , which may be coupled to the same source as electrode  176 , counters the tendency toward snapdown as the potential difference between movable actuator plate  172  increases, while being located so that in the event that snapdown does occur, a short circuit will not result between movable actuator plate  172  and optional electrode  186 . See, for example, U.S. Pat. No. 6,600,851 B2, which is incorporated by reference as if set forth fully herein. 
     FIG. 2  shows MEMS device  200 , an embodiment of the invention, similar to MEMS device  100  ( FIG. 1 ), but in which movable actuator plate  122  and spacer  124  of MEMS device  100  are connected together, e.g., formed of a unitary piece of material, thereby forming actuator plate  222  in  FIG. 2 . However, in such an embodiment of the invention, it is more likely that actuator plate  222  will bend or deform, e.g., similar to a springboard. 
   One end of movable actuator plate  222  is attached to substrate  104 , while the other end of movable actuator plate  222  is coupled to coupling plate  202 . 
   Coupling plate  202  is supported above substrate  104  by at least one torsional member  214 , e.g., one or more springs, each of which is attached between at least one of stationary posts  228  and handle portion  232  of coupling plate  202 . At least one torsional member  234  is attached between handle portion  232  of coupling plate  202  and the suspended end of movable actuator plate  222  to mechanically couple together coupling plate  202  and movable actuator plate  222 . As the end of movable actuator plate  222  coupled to coupling plate  202  moves down toward substrate  101 , handle portion  232  of coupling plate  202  likewise moves downward. This in turn causes the non-handle portion of plate  202  to move upward. 
   The motion induced by movable actuator plate  222  in coupling plate  202  is coupled via at least one torsional element  154  to handle portion  152  of mirror  101 . The remaining elements of  FIG. 2 , and their operation, are the same as for  FIG. 1 . 
     FIG. 3  shows a perspective view of exemplary MEMS device  300 , similar to MEMS device  100  ( FIG. 1 ), which is arranged in accordance with the principles of the invention. MEMS device  300  ( FIG. 3 ) includes a movable mirror  301 , which is rotatable about a first axis parallel to x-direction  109  by virtue of it being mechanically coupled to coupling plate  302 , which is in turn coupled to movable actuator plate  122 , all of which are suspended off of substrate  104 . One end of movable actuator plate  122  is attached to substrate  104  using spacer  124 , and the other end of movable actuator plate  122  is coupled to coupling plate  302 . In MEMS device  300 , the end of movable actuator plate  122  adjacent to spacer  124  is fixedly attached thereto by at least one torsional element  107 . Torsional element  107  may be implemented as a spring, and it is preferably adapted to permit movable actuator plate  122  to rotate substantially only about an axis parallel to x-direction  109 . 
   Electrode  126  is located on substrate  104  beneath movable actuator plate  122 . Movable actuator plate  122  and electrode  126  form an electrostatic actuator of device  100 . When electrode  126  is biased with respect to movable actuator plate  122 , movable actuator plate  122  rotates about an axis parallel to x-direction  109 . Preferably, moveable actuator plate  122  remains substantially undeformed in its rest position and in all positions to which it rotates. 
   Coupling plate  302  is supported above substrate  104  by at least one torsional member  314 , e.g., one or more springs, each of which is attached between one end of coupling plate  302  and at least one stationary support  328 . At least one torsional member  334  is attached between the opposite end of coupling plate  302  and the suspended end of movable actuator plate  122  so as to mechanically couple together coupling plate  302  and movable actuator plate  122 . 
   In operation, as the end of movable actuator plate  122  coupled to coupling plate  302  moves down toward substrate  301 , the end of coupling plate  302  coupled thereto likewise moves downward. This effectively rotates coupling plate  302  downward about an axis in the x-direction that passes through the top of stationary support  328 . Optional electrode  385  can be used to induce additional torque in the rotation of coupling plate  305 . Optional electrode  385  should be sized smaller than coupling plate  302  so that in the event of snapdown of a short circuit does not result. 
   The rotation of coupling plate  302  effectively rotates torsional element  354 , which is coupled to coupling plate  302 , in the same direction about the same axis. Torsional element  354 , e.g., a spring, is designed so that it preferably transfers all of the torque about the x-direction of coupling plate  302  to handle portion  352  of mirror  301 . This in turn causes mirror  301  to rotate about the same axis. Thus, when the end of movable actuator plate  122  coupled to coupling plate  302  moves down toward substrate  104 , the non-handle portion of mirror  301  rises away from substrate  104 . 
   Note that, torsional element  354  is preferably adapted to permit coupling plate  302  to rotate substantially only about an axis parallel to y-direction  129 . Advantageously, the motion of movable actuator plate  122  about a direction parallel to x-direction  109  is transferred as the component of motion about x-direction  109  of mirror  301 , while rotation of mirror  301  about an axis in a direction parallel to y-direction  129  is not transferred to movable actuator plate  122 . 
   Movable mirror  301  is also rotatable about a second axis perpendicular to x-direction  109 , e.g., y-direction  129 , by virtue of it being mechanically coupled to movable actuator plate  172 , all of which are suspended off of substrate  104 . One side of movable actuator plate  172  is attached to substrate  104  using posts  164  and torsional elements  167 , e.g., springs, while the opposite side of movable actuator plate  172  is suspended above substrate  104 . Torsional elements  167  are preferably arranged to permit movable actuator plate  172  to rotate substantially only about an axis parallel to y-direction  129 . 
   Electrode  176  is located on substrate  104  beneath movable actuator plate  172 . Movable actuator plate  172  and electrode  176  form an electrostatic actuator of device  100 . Preferably, when electrode  176  is biased with respect to movable actuator plate  172 , movable actuator plate  172  rotates, substantially undeformed, about an axis parallel to y-direction  129 . 
   Movable actuator plate  172  is coupled via torsional element  174 , e.g., a spring, to handle portion  352  of mirror  301 . The motion induced by movable actuator plate  172  is coupled via torsional element  174  to handle portion  352  of mirror  301 . Torsional element  174  is designed so that it preferably transfers all of the torque in the y-direction to handle portion  352  of mirror  301 . In addition, torsional element  174  is preferably adapted to permit movable actuator plate  172  to rotate substantially only about an axis parallel to y-direction  129 . Advantageously, the motion of movable actuator plate  172  about a direction parallel to y-direction  129  is transferred as the component of motion about y-direction  129  of mirror  301  while motion of mirror  301  about an axis in a direction parallel to x-direction  109  is not transferred to movable actuator plate  172 . 
   Thus, mirror  301  may be rotated about axes in either of the x and y directions, or about both simultaneously and independently. 
   Optionally, to reduce the possibility of snapdown of movable actuator plate  172 , optional electrode  186  may be placed on substrate  104  beyond the footprint of movable actuator plate  172 . Advantageously, optional electrode  186 , which may be coupled to the same source as electrode  176 , counters the tendency toward snapdown as the potential difference between movable actuator plate  172  increases, while being located so that in the event that snapdown does occur, a short circuit will not result between movable actuator plate  172  and optional electrode  186 . 
     FIG. 4  shows an exemplary arrangement in which several of MEMS device  100  ( FIG. 1 ), are organized as a one-dimensional array. Advantageously, mirrors  101  of each MEMS device  100 , can be packed closely together. 
   Note that those of ordinary skill in the art will readily recognize that other types of drives, e.g., comb drives may be employed in lieu of flat electorodes.