Abstract:
A micro electromechanical actuator has movable flaps formed on a base plate. The flaps are detached from the base plate on the ends and along one side, with the opposite side being partially attached to the base plate by flexures. The flaps can be moved upward, rotating or twisting along the axis of the flexure so that a one side contacts a movable element first, followed by the opposite side. As a result, the movable element is moved laterally with respect to the flaps. Movement of the flaps can be by application of a voltage to the flaps, followed by removal of the voltage to return the flaps to their original position. The cycle can continue to move the movable element the desired amount.

Description:
RELATED APPLICATIONS 
     The present application claims priority to provisional application Ser. No. 60/546,100, filed Feb. 18, 2004, entitled “Centipede Actuator Motion Stage”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to actuators, and in particular micro electro mechanical (MEM) actuators. 
     2. Related Art 
     There is a recent surge of interest in camera systems for cell phone and other portable devices. This market has pushed the limits of standard optical systems in providing high quality zoom lens systems in a small package and at low cost. Current state-of-the art zoom lens systems use plastic and other low cost components to assemble moving lens assemblies. The motion of these lenses is typically driven using small electric motors. These systems have a lot of components and require complicated assembly to put together. 
     There is a need for an integrated actuated stage, where the actuator, the springs, and the guides are all integrated together. Further, there is a need to achieve the integration during wafer-level processing. This type of actuator also has many other applications. 
     SUMMARY 
     In one embodiment, the invention uses a micro electro mechanical system (MEMS). In one embodiment, the stage is composed of two planar components that are mounted one on top of the other: the base plate and the mount. The base plate contains an array of flaps that are actuated to move the mount. In one embodiment, the actuation of the flaps is electrostatic. The mount is supported to a frame using flexures or guides. 
     In one embodiment, the flaps are designed to provide a leveraged motion, such that the motion of the mount is less than the motion of the flaps. As a result, the force on the mount is larger than the force on the flaps. 
     In one embodiment, the stage contains all required elements needed to provide repeatable actuated motion of the mount. This includes springs or flexures to provide a repeatable restoring force, guiding to ensure straight motion of the mount, actuator and electronics to provide electronically controllable motion of the mount, and a package to limit the motion of the mount and prevent contamination of the actuator. 
     In one embodiment, a plurality of base plates are manufactured on a silicon wafer using micromachining, a plurality of mounts are manufactured on a silicon wafer using micromachining, and the two wafers are assembled such that all stages are assembled in parallel. 
     In one embodiment, the mount contains at least one hole shaped to receive an optical element. In another embodiment, the mount contains at least one hole shaped to contain a liquid. 
     These and other features and advantages of the present invention will be more readily apparent from the detailed description of the embodiments set forth below taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1   a  is a top-view schematic illustration one portion of the stage according to one embodiment of the present invention. 
         FIG. 1   b  is a cross-section schematic illustration of one portion of the stage according to one embodiment of the present invention. 
         FIG. 1   c  is a perspective view of a portion of the stage showing two flaps according to one embodiment. 
         FIG. 2  is a schematic illustration of one embodiment of the stage at a first step in actuation. 
         FIG. 3   a  is a schematic illustration of one embodiment of the stage at a second step in actuation. 
         FIG. 3   b  is a schematic illustration of one embodiment of the stage at a second step in actuation. 
         FIG. 4   a  is a schematic illustration of one embodiment of the stage at a first step in actuation. 
         FIG. 4   b  is a schematic illustration of one embodiment of the stage at a second step in actuation. 
         FIG. 4   c  is a schematic illustration of one embodiment of the stage at a third step in actuation. 
         FIG. 4   d  is a schematic illustration of one embodiment of the stage at a fourth step in actuation. 
         FIG. 5   a  is a cross-section schematic illustration of the stage according to the preferred embodiment of the present invention. 
         FIG. 5   b  is a top-view schematic illustration of the stage according to the preferred embodiment of the present invention. 
         FIG. 6   a  is a schematic illustration of one embodiment of the stage at a first step in actuation. 
         FIG. 6   b  is a schematic illustration of one embodiment of the stage at a second step in actuation. 
         FIG. 7   a  is a schematic illustration of one embodiment of the stage at a first step in actuation. 
         FIG. 7   b  is a schematic illustration of one embodiment of the stage at a second step in actuation. 
     
    
    
     Like reference numerals are used to identify like elements illustrated in one or more of the figures. 
     DETAILED DESCRIPTION 
       FIG. 1   a  shows a top view of a base plate  10  according to one embodiment of the present invention. The base plate  10  is composed of a substrate  56  and an array of movable elements or flaps  12 . Five flaps  12  are shown, although more or less flaps may be suitable for different applications. Each flap  12  is supported to the substrate through two flexures  14  (see also  FIG. 1   c ). The flexures  14  are designed in such a way as to be very soft in rotation about the axis of the flexure and motion in a direction perpendicular to the plane of the substrate  56 . All other degrees of freedom are relatively stiff. This can be achieved, for example, by a flexure  14  that is about 1 micron to 5 microns thick in the direction orthogonal to the plane of the substrate  56 . Each flexure  14  may be 5 microns to 10 microns wide and 20 microns to 200 microns long. The length of the flexure is measured from the end  20  of the etched portion to the side  21  of the etched portion (see  FIGS. 1   a  and  1   c ). Clearly, one of ordinary skill in the art may design flexures  14  of various shapes to achieve the necessary stiffnesses. 
       FIG. 1   b  shows a cross-section view of the base plate  10  according to one embodiment of the present invention. From this view, it is clear that the flaps  12  are separated from the substrate  56 , so that they are free to move and only restricted in motion by the flexures  14 . 
       FIG. 1   c  is a perspective view of two flaps  12  formed from the substrate  56 . As seen from the figure, the substrate  56  is etched with a C-shape along the complete length of one side of the flap, along the sides, and partially in the interior portion (shown by line  23 ) to form boundaries of the flap. The partial etching along with an etching (shown by line  24 ) parallel and adjacent to the length of the other side of the flap forms two flexures  14 . Note that due to the angle, the etch along the length for the C-shaped etch is not shown, but that can be seen from  FIG. 1   a . Each flap  12  is supported on or connected to the substrate  56  by two flexures  14 . 
       FIG. 2  shows a motion stage  11  according to one embodiment of the current invention. The base plate  10  is assembled with a movable mount  52  such that there is a small gap  32  between the base plate  10  and the mount  52 . The movable mount  52  is substantially free to move as long as the gap  32  is not closed and it is substantially restricted from moving towards the base plate  10 . The flaps  12  are electrically connected to an electronic voltage signal, such that their electrostatic potential may be changed as desired. The movable mount  52  is electrically connected to electrical ground. 
       FIGS. 3   a  and  3   b  show a method of actuating the motion stage  11  according to one embodiment of the current invention. In a first step, the voltage signal on the flaps  12  is changed from ground to a positive potential between 1 Volt and 300 Volts. The flaps  12  are designed such that the torque on the flaps  12  dominates the linear force. As a result, the flaps  12  rotate about the flexures  14  until one edge contacts the movable mount  52 , as shown in  FIG. 3   a . This edge is shown as the left edge in  FIG. 1   a  and the bottom edge in  FIG. 1   c . Once this contact happens, the friction between the flaps  12  and the movable mount  52  causes the movable mount  52  to move laterally (to the left) as the flaps  12  continue to move towards the movable mount  52 , as shown in  FIG. 3   b . When the first edges contacts the moveable mount  52 , the field strength increases to pull up the other side of the flap with the flexures. In a second step, the voltage on the flaps  12  is returned to 0 Volts and the flaps  12  separate from the movable mount  52 , as shown in  FIG. 2 . This cycle is repeated to continue to move the movable mount  52  laterally to a desired position. 
     In the method of actuating the motion stage  11  described above, the movable mount  52  may move between each cycle, since there is a time in which the flaps  12  are not in contact with the movable mount  52 .  FIGS. 4   a ,  4   b ,  4   c , and  4   d  show a method of actuating the motion stage  11  according to a second embodiment of the current invention. In a first step, the voltage signal on a first group of flaps  12   a  is changed from ground to a positive potential between 1 Volt and 300 Volts, while the voltage signal on a second group of flaps  12   b  is maintained at 0 Volts. As a result, only the first group of flaps  12   a  rotate about the flexures  14  until one edge contacts the movable mount  52 , as shown in  FIG. 4   a . Once this contact happens, the friction between the flaps  12   a  and the movable mount  52  causes the movable mount  52  to move laterally as the flaps  12   a  continue to move towards the movable mount  52 , as shown in  FIG. 4   b . In a second step, the voltage signal on the second group of flaps  12   b  is changed from ground to a positive potential between 1 Volt and 300 Volts, while the voltage signal on said first group of flaps  12   a  is maintained at their previous potential between 1 Volt and 300 Volts. As a result, the second group of flaps  12   b  rotate about the flexures  14  until one edge contacts the movable mount  52 , as shown in  FIG. 4   c . In a third step, substantially at the same time that the second group of flaps  12   b  contact the movable mount  52 , the voltage signal on the first group of flaps  12   a  is returned to 0 Volts. As a result, the first group of flaps  12   a  return to their undeflected position and the second group of flaps  12   b  continue to move and displace the movable mount  52  laterally, as shown in  FIG. 4   d . The timing between the actuation of the first group of flaps  12   a  and the second group of flaps  12   b  can be tuned to modify the amount of time that the movable mount  52  is allowed to move between said second and third steps. 
       FIG. 5   a  shows a cross-section illustration of a packaged motion stage  14  according to one embodiment of the current invention. The packaged motion stage  14  is composed of a base plate  10 , a movable mount  52 , and a cover  57 .  FIG. 5   b  shows a top view of the packaged motion stage  14 , where the movable mount  52  is visible through the cover  57 . Flexures  17  connect the movable mount  52  to the frame on the stage. In other embodiments, flexures  17  are eliminated and the frame functions as a guide for the movable mount  52 . In these embodiments, the guided stage is similar to a standard commercial stage that uses guides and ball bearings to restrict the motion of the stage. In one embodiment, the base plate  10  and the movable mount  52  are fabricated using silicon micromachining techniques. In one embodiment of the current invention, the actuator is made using bulk micromachining of silicon. One of ordinary skill in the art can recognize that the same structures could be made using injection molding, surface micromachining, LIGA, laser etching, or any other suitable method. In the current embodiment, the base plate  10  is fabricated using a silicon on insulator (SOI) wafer that is made up of a silicon epilayer on top of a silicon dioxide epilayer on top of a polished silicon wafer. The flaps  12  are shaped by etching the silicon epilayer, and they are released from the substrate by selectively etching the underlying silicon dioxide. In order to prevent capillary adhesion, oxide bumps are patterned on the surface of the flaps  12 . 
     The epilayer is electrically contacted by depositing an aluminum pad that creates ohmic contact with the epilayer. The epilayer has the required doping to make it substantially electrically conductive. The movable mount  52  is made from a double side polished silicon wafer by patterning and etching using deep reactive ion etching (DRIE). The movable mount  52  is electrically contacted by depositing an aluminum pad that creates ohmic contact with the silicon. A thin oxide may be grown on the surface of the movable mount  52  to prevent electrically shorting with the flaps  12  during actuation. In the current embodiment, the base plate wafer and the movable mount wafer are bonded together, and the devices are singulated by dicing. A cap  57  is fabricated using precision plastic injection molding, placed over the motion stage, and glued in place. 
     All current embodiments that have been described provide motion in a single direction. Clearly, flaps  12  oriented in the opposite direction could be added to provide motion in two directions, but the surface area would then have to be shared between the two types of flaps  12 , thereby reducing the force available by a factor of two. By making a flap  12  that can be actuated in two different directions, as shown in  FIGS. 6   a ,  6   b ,  7   a , and  7   b , it is possible to enable movement in two directions without reducing the force by a factor of two. The flap  12  in this embodiment is composed of three parts  15   a ,  15   b ,  16 . Two parts  15   a  and  15   b  on opposing ends of the flap  12  are supported to the central part  16  with flexures  14   a  and  14   b  that are stiff in every degree of freedom except for one rotational degree of freedom corresponding to the motion shown in  FIG. 6   a  and  FIG. 7   a . The central part  16  is supported to the substrate  56  with flexures that are stiff in every degree of freedom except for one translational degree of freedom corresponding to motion towards the movable mount  52 , as shown in  FIGS. 6   b  and  7   b.    
       FIGS. 6   a  and  6   b  show a method of actuating the motion stage  11  in one direction according to one embodiment of the current invention. In a first step, the voltage signal on the first part of the flap  15   a  and the center part of the flap  16  is changed from ground to a positive potential between 1 Volt and 300 Volts. As a result, the first part of the flap  15   a  rotates about the flexure  14   a  until one edge contacts the movable mount  52 , as shown in  FIG. 6   a . Once this contact happens, the friction between the first part of the flap  15   a  and the movable mount  52  causes the movable mount  52  to move laterally as the central part of the flap  16  moves towards the movable mount  52 , as shown in  FIG. 6   b . In a second step, the voltage on the first portion of the flap  15   a  and the center portion of the flap  16   a  is returned to 0 Volts and the flap  12  separates from the movable mount  52  to move back into its unbiased position. This cycle is repeated to continue to move the movable mount  52  laterally.  FIGS. 7   a  and  7   b  show the cycle to move the stage in the opposite direction. The central part of the flap  16  provides force independent of the direction of motion. 
     Having thus described embodiments of the present invention, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, the above description showed the flaps formed in the base plate for moving a mount located above the flaps. However, the present invention can also be used with the flaps formed on the movable mount, such that action by the flaps in the manner described above causes the flaps to contact the underlying base plate, resulting in movement of the mount relative to the base plate. Thus the invention is limited only by the following claims.