Abstract:
A MEMS device having a movable mirror pixel supported on a substrate and coupled to a motion actuator located between the mirror pixel and the substrate so as to enable rotation of the mirror pixel about an axis lying within the mirror plane. In one embodiment of the invention, the motion actuator has a movable electrode, on which the mirror pixel is mounted. The movable electrode is supported on the substrate by a pair of upright springs, each having two parallel segments joined at one end of the spring and disjoint at the other end. One disjoint segment end is coupled to the substrate, while the other disjoint segment end is coupled to the movable electrode. The end of the upright spring corresponding to the joined segment ends points away from the substrate such that (i) the spring body protrudes through a narrow slot in the mirror pixel and (ii) the mirror plane lies at about the mid-point of the spring. Advantageously, a mirror pixel of the invention enables implementation of a segmented mirror with tightly spaced mirror pixels providing a fill factor higher than about 98%.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The subject matter of this application is related to that of U.S. patent application Ser. No. 10/772,847, also identified by attorney docket reference Greywall 31, filed Feb. 5, 2004, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to adaptive optics and, more specifically, to micro-electromechanical systems (MEMS) for implementing adaptive optics.  
         [0004]     2. Description of the Related Art  
         [0005]     Adaptive optics is a field of optics dedicated to the improvement of optical signals using information about signal distortions introduced by the environment in which the optical signals propagate. An excellent introductory text on the subject is given in “Principles of Adaptive Optics” by R. K. Tyson, Academic Press, San Diego, 1991, the teachings of which are incorporated herein by reference.  
         [0006]     A representative example of an adaptive optical element is a deformable mirror driven by a wavefront sensor and configured to compensate for atmospheric distortions that affect telescope images. Small naturally occurring variations in temperature (˜1° C.) in the atmosphere cause random turbulent motion of the air and give rise to changes in the atmospheric density and, hence, to the index of refraction. The cumulative effect of these changes along the beam propagation path may lead to beam wandering, spreading, and intensity fluctuations, each of which degrades image quality. The wavefront sensor is a device that measures the distortions introduced in the atmosphere and generates feedback for the deformable mirror. Based on the feedback, the mirror is deformed such that the beam distortions are significantly reduced, thus improving the image quality.  
         [0007]     One frequently used type of deformable mirror is a segmented mirror, in which each segment (pixel) can individually be translated and/or rotated. For many applications, a segmented mirror is required to have: (1) for each segment, translation/rotation magnitudes on the order of 1 μm/10 degrees, respectively, and (2) for the mirror as a whole, a fill factor of at least 98%. However, for many prior-art designs, these requirements are in direct conflict with each other and therefore difficult or even impossible to meet. For example, the high fill-factor requirement suggests a solution, in which mirror support elements and motion actuators are placed beneath (hidden under) the mirror. One result of this placement is that each segment typically rotates about an axis lying below the mirror surface and therefore is subjected to a lateral displacement within the mirror plane during rotation. To prevent physical interference with the neighboring mirror segments caused by this displacement, a relatively large spacing between the segments is required. The latter, however, significantly reduces the fill factor.  
       SUMMARY OF THE INVENTION  
       [0008]     Problems in the prior art are addressed, in accordance with the principles of the present invention, by a MEMS device having a movable mirror pixel supported on a substrate and coupled to a motion actuator located between the mirror pixel and the substrate so as to enable rotation of the mirror pixel about an axis lying within the mirror plane.  
         [0009]     In one embodiment of the invention, the motion actuator has a movable electrode, on which the mirror pixel is mounted. The movable electrode is supported on the substrate by a pair of upright springs, each having two parallel segments joined at one end of the spring and disjoint at the other end. One disjoint segment end is coupled to the substrate, while the other disjoint segment end is coupled to the movable electrode. The end of the upright spring corresponding to the joined segment ends points away from the substrate such that (i) the spring body protrudes through a narrow slot in the mirror pixel and (ii) the mirror plane lies at about the mid-point of the upright spring. Advantageously, a mirror pixel implemented in accordance with an embodiment of the invention has a relatively small lateral displacement during rotation while the mirror support structure takes up a relatively small surface area within the mirror plane. This enables implementation of a segmented mirror with tightly spaced mirror pixels providing a fill factor higher than about 98%.  
         [0010]     In another embodiment of the invention, a MEMS device has an upright spring supported on a substrate. The upright spring has two segments joined at one end of the spring and disjoint at another end of the spring. The upright spring is positioned with respect to the substrate such that the joined segment ends are at a greater distance from the substrate than the disjoint segment ends. One disjoint segment end is coupled to the substrate and the other disjoint segment end is adapted to move with respect to the first one via a scissor-type motion. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a three-dimensional perspective view of a MEMS device according to one embodiment of the invention;  
         [0012]     FIGS.  2 A-C show a MEMS device according to another embodiment of the invention;  
         [0013]      FIG. 3  shows a cross-sectional view of a MEMS device according to yet another embodiment of the invention;  
         [0014]      FIG. 4  shows a three-dimensional perspective view of a MEMS device according to yet another embodiment of the invention; and  
         [0015]     FIGS.  5 A-F illustrate representative fabrication steps of the device shown in  FIG. 2  according to one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 1  shows a three-dimensional perspective view of an exemplary MEMS device  100  arranged in accordance with the principles of the invention, which may be used to implement a pixel of an adaptive optics mirror. Device  100  has a movable plate  102  and connected to a motion actuator  110 . Plate  102  is supported by a pair of torsion rods  106   a - b , each connected between the plate and one of posts  104   a - b  attached to a substrate  170 . Rods  106   a - b  define an axis of rotation for plate  102  shown by the dashed line and labeled AB in  FIG. 1 .  
         [0017]     Actuator  110  is a fringe-field actuator made up of two lateral electrodes  11   2   a - b  and an intermediate electrode  114 . Lateral electrodes  112   a - b  are attached to substrate  170  and, as such, are stationary. In contrast, intermediate electrode  114  is attached to plate  102  by a link rod  116  and, as such, is movable, together with the plate, with respect to substrate  170 . When intermediate electrode  114  is moved toward one of lateral electrodes  112 , link rod  116  transfers motion of the intermediate electrode to plate  102 , thereby rotating the plate about axis AB.  
         [0018]     Each lateral electrode  112  is electrically isolated from substrate  170  by an insulation layer  180  and can be electrically biased with respect to the substrate. In contrast, intermediate electrode  114  is in electrical contact with substrate  170  via link rod  116 , plate  102 , torsion rods  106 , and posts  104 . Therefore, electrodes  112  and  114  can be electrically biased with respect to each other to impart motion to plate  102 . For example, when lateral electrode  112   a  is biased with respect to intermediate electrode  114  while lateral electrode  112   b  is not biased, the intermediate electrode is pulled toward the biased electrode, which rotates plate  102  in the corresponding direction. Plate  102  comes to rest, when spring deformation forces of torsion rods  106  balance the electrostatic attraction force between the electrodes. When the bias is removed, the spring forces return plate  102  and intermediate electrode  114  into the initial position. Similarly, when lateral electrode  112   b  is biased with respect to intermediate electrode  114  while lateral electrode  112   a  is not biased, plate  102  rotates in the opposite direction.  
         [0019]     FIGS.  2 A-C show another embodiment of the invention. More specifically,  FIG. 2A  shows a three-dimensional perspective view of a MEMS device  200 ;  FIG. 2B  shows a three-dimensional perspective view of a spring  206  utilized in device  200 ; and  FIG. 2C  is a side view of device  200  illustrating possible plate rotation.  
         [0020]     Referring to  FIG. 2A , device  200  is similar to device  100  of  FIG. 1  and may similarly be used to implement a pixel of an adaptive optics mirror. Device  200  has a movable plate  202  supported on a substrate  270  and connected to a motion actuator  210  that is similar to actuator  110  of device  100 . However, instead of torsion rods  106   a - b  of device  100 , device  200  employs upright springs  206   a - b , one of which is shown in more detail in  FIG. 2B . Referring now to both  FIGS. 2A and 2B , each upright spring  206  has two feet  226   a - b , one of which is connected to a corresponding one of support posts  204   a - b  and the other is connected to an intermediate electrode  214  of actuator  210 . Each upright spring  206  has two spring segments  228   a - b  that protrude through a slot (opening)  208  in plate  202  as shown in  FIG. 2C  without attaching to the plate. Spring segments  228   a - b  are joined at the top of upright spring  206  by a bridge  230 . In a preferred implementation, the width and thickness (W seg  and t seg ) of segments  228  are such that upright spring  206  resists compression along the direction orthogonal to the plane of substrate  270  (i.e., has high longitudinal stiffness) while it permits a relatively easy spring deformation of the “scissor” type shown in  FIG. 2C .  
         [0021]     Referring now to  FIG. 2C , when intermediate electrode  214  is displaced from its unbiased position shown in  FIG. 2A , one foot of upright spring  206  moves together with the intermediate electrode while the other foot, being rigidly attached to post  204 , remains stationary. It can be shown that, due to the longitudinal stiffness of upright spring  206 , motion of any structure attached to the movable foot of the spring is very closely approximated by a simple rotation about an axis passing through the mid-point (i.e., at half-length) of the spring, which point is labeled P in  FIG. 2C . Therefore, when link rod  216  has a length L r  equal to about half the length of upright spring  206 , plate  202  rotates about an axis lying within the plane of plate  202  similar to that for plate  102  in device  100  ( FIG. 1 ). In certain implementations of device  200 , the surface area within the plane of plate  202  taken up by slots  208  can be made significantly smaller than the corresponding area within the plane of plate  102  taken up by posts  104  and torsion rods  106  in device  100 . This increases the fill factor of device  200  compared to that of device  100 .  
         [0022]      FIG. 3  shows a cross-sectional view of a MEMS device  300  according to yet another embodiment of the invention. Device  300  is similar to device  200  ( FIG. 2 ) with similar structural elements of the two devices marked with labels having the same last two digits. However, one difference between devices  300  and  200  is that, instead of intermediate electrode  214 , device  300  has a cradle structure  334 . Similar to electrode  214  ( FIG. 2 ), cradle structure  334  can move as a whole, when upright springs  306   a - b  are deformed, thereby enabling rotation of plate  302  with respect to substrate  370 . But in addition to rotation with respect to substrate  370 , cradle structure  334  enables piston motion of plate  302  along axis Z with respect to the cradle structure.  
         [0023]     Cradle structure  334  has a movable plate  332 , on which plate  302  is mounted using link rod  316 . Plate  332  is suspended above a cradle base  338  with a pair of serpentine springs  316  that allow for out-of-plane displacements of plate  332 . An actuating electrode  342  attached to cradle base  338  beneath plate  332  forms, together with that plate, a parallel plate actuator that can be used to translate plate  302 . For example, when electrode  342  is biased with respect to plate  332 , it generates an attractive electrostatic force, which pulls plate  332  toward the electrode, thereby translating plate  302  with respect to cradle structure  334 . When the bias is removed, springs  336  return plates  332  and  302  into their initial positions.  
         [0024]      FIG. 4  shows a three-dimensional perspective view of a MEMS device  400  according to yet another embodiment of the invention. Similar to device  200  of  FIG. 2 , device  400  implements rotation of a movable plate about an axis lying within the plane of that plate. However, in contrast to device  200 , where the movable plate rotates about a single axis, the movable plate in device  400  can rotate about two different axes, thereby providing a capability for tilting the plate in any desired direction.  
         [0025]     Device  400  has a movable plate  402  supported on a substrate  470  and connected to a motion actuator  410 . Plate  402  is mounted using a link rod  416  on a gimbal structure  450  having an outer ring  452  and an inner disk  454 . Outer ring  452  is supported by a pair of upright springs  406   a - b,  each attached between the outer ring and one of posts  404   a - b  attached to substrate  470 . Inner disk  454  is supported by another pair of upright springs  406   c - d,  each attached between the inner disk and outer ring  452 . Each of upright springs  406   a - d  is similar to upright spring  206  shown in  FIG. 2B  and protrudes through a corresponding slot  408  in plate  402 . In a preferred implementation, the length of link rod  416  is about half the length of spring  406 , which puts the axes of rotation defined by springs  406   a - b  (axis AB in  FIG. 4 ) and springs  406   c - d  (axis CD in  FIG. 4 ) within the plane of plate  402 . Although, in the embodiment of  FIG. 4 , axes AB and CD are mutually orthogonal, other axis orientations may also be used.  
         [0026]     Actuator  410  is a fringe-field actuator comprising three lateral electrodes  412   a - c  and an intermediate electrode  414 . Each lateral electrode  412  is similar to, e.g., lateral electrode  212  of  FIG. 2A , while intermediate electrode  414  is similar to intermediate electrode  214  of  FIG. 2A . When intermediate electrode  414  is deflected from its initial position toward lateral electrodes  412 , a link rod  456  transfers motion of the intermediate electrode to inner disk  454  of gimbal structure  450 , thereby rotating the disk as further described below.  
         [0027]     Direction, in which intermediate electrode  414  is deflected, is determined by voltages applied to lateral electrodes  412   a - c . In general, intermediate electrode  414  can be deflected in any chosen direction by applying an appropriate combination of bias voltages. For example, suppose that the plane orthogonal to substrate  470  and passing through axis AB is a plane of symmetry for lateral electrode  412   b.  Then, when lateral electrode  412   b  is biased with respect to intermediate electrode  414 , while the other lateral electrodes  412   a  and  412   c  are not biased, the intermediate electrode is pulled toward electrode  412   b  along the projection of axis AB onto substrate  470 . This rotates disk  454  and therefore plate  402  about axis CD. Similarly, when electrodes  412   a - c  are biased such that intermediate electrode  414  is pulled along the projection of axis CD, disk  454  and plate  402  rotate about axis AB. One skilled in the art will appreciate that deflection of intermediate electrode  414  in an arbitrary direction will generally produce rotation of disk  454  and plate  402  about both axis AB and axis CD.  
         [0028]     Different fabrication techniques may be used to fabricate devices of the present invention. In one embodiment, a fabrication process similar to that disclosed in the above-referenced U.S. patent application Ser. No. 10/772,847 may be used. Briefly, the fabrication process begins with a silicon-on-insulator (SOI) wafer and proceeds with a sequence of patterning, etching, and deposition steps known to one skilled in the art. The patterning steps are carried out using lithography. The etching steps are carried out using material-specific etching, e.g., reactive ion etching (RIE) for various silicon layers and fluorine-based etching for various silicon oxide layers. The deposition steps are carried out using, e.g., chemical vapor deposition. Additional description of various fabrication steps may be found in U.S. Pat. Nos. 6,201,631, 5,629,790, and 5,501,893, the teachings of which are incorporated herein by reference.  
         [0029]     U.S. patent application Ser. No. 10/772,847 also discloses fabrication of flexible vertical beams that are similar to upright springs in certain embodiments of the present invention (e.g., springs  206  of  FIG. 2 ) in that both structure types extend substantially perpendicular to the plane of the substrate. However, an upright spring protrudes through the corresponding movable plate while a flexible vertical beam is confined to the space between the movable plate and the substrate. In view of this difference, fabrication steps related to the realization of the protrusion feature of upright springs are described in more detail below.  
         [0030]     FIGS.  5 A-F schematically illustrate representative fabrication steps of device  200  according to one embodiment of the invention. More specifically,  FIGS. 5A, 5C , and  5 E show top views of device  200  during those fabrication steps, whereas  FIGS. 5B, 5D , and  5 F show the corresponding cross-sectional side views of the device.  
         [0031]     Referring to FIGS.  5 A-B, in one embodiment, fabrication of device  500  begins with a silicon-on-insulator (SOI) wafer having (i) two silicon layers, i.e., a handle layer  562  and an overlayer  566 , and (ii) a silicon oxide layer  564  located between overlayer  566  and handle layer  562 . Plate  502  is defined in overlayer  566  using reactive etching, which stops at the silicon oxide layer. Openings  208   a - b  (see also  FIG. 2 ) are created by etching away the corresponding portions of overlayer  566  and silicon oxide layer  564 . Then a timed etch is applied to handle layer  562  to create wells having a depth corresponding to the length of future upright springs  206 , by which length the springs extend above plate  202  (see  FIG. 2 ).  
         [0032]     Referring to FIGS.  5 C-D, first, a relatively thick (e.g., 5 μm) silicon oxide layer  568  is deposited over the structure of FIGS.  5 A-B. Second, layer  568  is patterned and etched to form an opening  580  for link rod  216  connecting plate  202  and intermediate electrode  214 . Then, a thin (e.g., 1 μm) poly-silicon layer  572  is deposited over layer  568 . The part of layer  572  that fills opening  580  creates link rod  216 . Finally, layer  572  is patterned and etched to remove poly-silicon from the wells corresponding to openings  208 .  
         [0033]     Referring to FIGS.  5 E-F, first, a thin (e.g., 0.5 μm) poly-silicon layer  574  is deposited over the structure of FIGS.  5 C-D. This layer covers all exposed surfaces of that structure including the vertical walls of the wells corresponding to openings  208 . Then, the composite silicon layer comprising layers  572  and  574  is patterned and etched to form intermediate electrode  214 , lateral electrodes  212 , and upright springs  206 . In particular, bridge  230  of upright spring  206  (see  FIG. 2B ) is formed from the portion of layer  574  located at the bottom of the corresponding well; spring segments  228   a - b  of upright spring  206  (see  FIG. 2B ) are formed from the portion of layer  574  located at one of the vertical walls of the well, and two feet  226   a - b  of upright spring  206  are formed from the portion of layer  574  deposited near the top circumference of the well. Note that silicon oxide layer  568  prevents upright springs  206  from making contact with plate  302 .  
         [0034]     Further fabrication steps are straightforward and proceed to form support posts  204   a - b  and substrate  270  over the structure of FIGS.  5 E-F (see also  FIG. 2 ). The final structure of device  200  is released by removing (e.g., etching away) all oxide layers. Note that handle (silicon) layer  562  will fully detach from the final structure once silicon oxide layers  564  and  568  are removed. Also note that the views shown in  FIGS. 5B, 5D , and  5 F are inversed (flipped) with respect to the view shown in  FIG. 2A .  
         [0035]     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims.  
         [0036]     Although fabrication of MEMS devices of the invention has been described in the context of using silicon/silicon oxide SOI wafers, other suitable materials, such as germanium-compensated silicon, may similarly be used. The materials may be appropriately doped as known in the art. Various surfaces may be modified, e.g., by metal deposition for enhanced reflectivity and/or electrical conductivity or by ion implantation for enhanced mechanical strength. Differently shaped plates, springs, segments, rods, posts, actuators, electrodes, and/or other device elements/structures may be implemented without departing from the scope and principle of the invention. Springs may have different shapes and sizes, where the term “spring” refers in general to any suitable elastic structure that can recover its original shape after being distorted. Spring segments of an upright spring may or may not be parallel to each other. An opening in a mirror segment (e.g., slot  208  in  FIG. 2 ) may or may not be fully surrounded by said mirror segment. Alternatively, a mirror segment may be shaped such that an upright spring passes outside the perimeter of said mirror segment. The length of a link rod (e.g., link rod  216  in  FIG. 2 ) may be chosen such that the axis of rotation for the corresponding mirror segment is not within the plane of that segment. Various MEMS devices of the invention may be arrayed as necessary and/or apparent to a person skilled in the art.