Abstract:
A damped micromechanical device useful for adjusting optical components, positioning transducers, and sensing motion. The micromechanical device includes a top cap that helps create an area of restricted fluid flow to increase mechanical damping of the device and minimize the response of the structure to mechanical perturbations. The micromechanical device is constructed to cause piston-like Poiseuille flow through controlled gaps within the actuator. By controlling the gap dimensions, the amount of damping can be adjusted.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority to U.S. provisional patent application No. 60/335,146 filed Feb. 8, 2002, the entire content of which is incorporated herein by this reference. 
     
    
     
       SCOPE OF THE INVENTION  
         [0002]    The present invention relates generally to micromechanical devices and more particularly to a mechanically-damped micromechanical actuator.  
         BACKGROUND  
         [0003]    Micromechanical devices such as microactuators are used for many purposes, including moving and adjusting optical components. Similar structures can be used as sensors for acceleration. An example of a linear microactuator, designed to translate a mirror in and out of a beam of light, is described in issued U.S. Pat. No. 5,998,906. In this design, the devices were activated against mechanical stops and were relatively immune from the effects of vibration. The mechanical dynamical behavior of micromechanical structures can be characterized as having an amplitude and phase response as a function of mechanical drive frequency, as displayed, for example, in a conventional Bode plot. Typically, for a micromechanical device, such mechanical response can be approximated as having a set of in-plane and out-of-plane resonances, each of which can be characterized as having a resonant frequency and a mechanical quality factor (“Q”). Since the damping of the sensor or actuator material itself is generally quite low, the overall damping is often dominated by the gas or liquid fluid environment surrounding the micromechanical structure.  
           [0004]    Micromechanical devices are prone to the effects of externally-imposed mechanical vibration, in that any acceleration imposes a force on the moving mass of the device that tends to move the device an amount dependent on the suspension stiffness in the direction of the acceleration. Designs to minimize these effects are described in U.S. Pat. No. 6,469,415 and use counterbalancing masses to minimize motion of the moving structure of the device due to external accelerations applied to the device. These balanced designs tend to reduce the motion of high-Q mechanical resonances by minimizing the drive force acting on particularly the in-plane fundamental resonance of the device, but do little to reduce the effect of electrical drive excitation of that resonance or the mechanical response of other higher modes that may not be effectively balanced.  
           [0005]    In the prior art, two general techniques have been used to damp micromechanical structures. One involves the parallel-plate motion of structures that produce “squeeze-film” damping, and the other is the lateral motion of a structure with respect to a fixed surface that generates shear forces from Couette flow in an intervening fluid and thus mechanical damping of that motion.  
           [0006]    The effects of squeeze-film damping has been recently reported by E. -S. Kim, et. al., in a paper entitled “Effect of holes and edges on the squeeze film damping of perforated micromechanical structures.” (Proceedings of the 12th IEEE Int&#39;l Conf. On Micro Electro Mechanical Systems (MEMS &#39;99), at 296-301, January 1999.) Squeeze film damping has been conventionally used to damp bulk, micromachined accelerometers, for example as described in U.S. Pat. No. 5,445,006. The effects of lateral microstructure movement have been described by Y. -H. Cho et. al., in a paper entitled “Viscous energy dissipation in laterally oscillating planar microstructures: a theoretical and experimental study.” (Proceedings of the 3rd IEEE Int&#39;l Conf. On Micro Electro Mechanical Systems, (MEMS &#39;93), at 93-98, January 1993.) In this paper, lateral surface micromachined structures were analyzed and the Couette fluid flow and sheer forces calculated, particularly as they affect resonant microsensors, where high Q and low damping are generally preferred.  
           [0007]    There is a need in the art for a micromechanical device exhibiting mechanical damping of vibrations within micromechanical structures, exceeding that of damping provided by squeeze-film and Couette damping.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention, in one embodiment, is a damped micromechanical device. The device includes a body having substantially parallel first and second walls at least partially defining an internal chamber. A fluid and a movable structure are disposed in the chamber. The movable structure is movable in a direction substantially parallel to the first and second walls. The body constrains the fluid in the chamber to flow between the movable structure and the first and second walls when the movable structure is in motion within the chamber so as to mechanically damp the movable structure. In one embodiment, the damped micromechanical device further includes a dashpot, which adds additional mechanical damping to the structure.  
           [0009]    While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. As will be realized, the invention is capable of modifications in various obvious aspects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The accompanying drawings, which are somewhat schematic in many instances and are incorporated in and form a part of this specification, illustrate exemplary embodiments of the invention and, together with the description, serve to explain the principles of the invention  
         [0011]    [0011]FIG. 1 is a plan view of a damped micromechanical device having a top cap to restrict fluid flow, according to one embodiment of the present invention.  
         [0012]    [0012]FIG. 2 is a schematic sectional view of the damped micromechanical device of FIG. 1, taken along the line  2 - 2  shown in FIG. 1.  
         [0013]    [0013]FIG. 3 is an enlarged, plan view of a dashpot of FIG. 1 enclosed with a dashed line in FIG. 1 and marked FIG. 3.  
         [0014]    [0014]FIG. 4 is a schematic sectional view of the dashpot of FIG. 3, taken along the line  4 - 4  in FIG. 3. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0015]    The damped micromechanical device  10  of the present invention can be an actuator that includes a base or planar substrate  12 , first and second microactuators or motors  14  and  16 , a shuttle  18 , and a pivot assembly  20  (see FIG. 1). In one embodiment, the damped micromechanical device  10  further includes a dashpot  21  for damping vibration within the device  10 . The microactuators  14  and  16  are coupled to the pivot assembly  20  via the substantially rigid shuttle  18 , such that actuation of the microactuators  14  and  16  cause a corresponding rotation of the pivot assembly as further detailed below. The pivot assembly is connected to a movable platform  22 , which moves in an arc  24   a  or  24   b  about a pivot point  26 . A movable component is connected to the movable platform  22  and is articulated by actuation of the microactuators  14  and  16 . In one embodiment, the movable component is a collimating lens for use in a telecommunications system.  
         [0016]    The damped micromechanical device  10 , in one embodiment, is formed from the substrate  12  using an etching technique such as deep reactive ion etching. In another embodiment, the device  10  is formed using an electroplating technique, such as LIGA. The substrate  12  may be a silicon wafer and can have a thickness of between about 200 and 600 microns. In one embodiment, a second layer  30  is attached to the substrate  12  (see FIG. 2). The second layer  30  may be made from any suitable material, such as silicon, and is secured at certain points to the substrate  12  using any known technique. The second layer  30  may be fusion bonded to the substrate  12  using a silicon dioxide layer  32 . The second layer  30  can have a thickness of between about 1 and 600 microns, preferably between about 10 and 150 microns, and more preferably about 85 microns. A top layer or top cap  34  (shown in FIG. 2 and by the dotted line in FIG. 1) overlies the second layer  30  and is secured at certain points using any known bonding technique, such as an adhesive or solder. An insulating layer  35  electrically insulates the top cap  34  from the second layer  30 . The top cap  34  may be attached after completion of all etching of the second layer  30 .  
         [0017]    Each of the movable components of the damped micromechanical device  10  is formed from the second layer  30  overlying the substrate  12  using a suitable etching technique. These components, including the microactuators  14  and  16 , the shuttle  18 , and the pivot assembly  20 , are then released from the substrate  12  to allow motion across the surface of the substrate  12  (see FIG. 1).  
         [0018]    The first and second microactuators  14  and  16  can be of any suitable type known in the art, such as an electromagnetic or any other electrically-driven microactuator. In the embodiment shown in FIG. 1, the microactuators  14  and  16  are electrostatic microactuators. Although the microactuators  14  and  16  need not be identical, they are shown as substantially similar in construction and similar to microactuators disclosed in U.S. Pat. No. 6,384,510, the entire content of which is incorporated herein by this reference. Micromechanical structures similar in construction to microactuators  14  and  16  can also be used when the micromechanical device of the present invention is use for position sensing and in sensors such as accelerometers.  
         [0019]    In one suitable embodiment, each of the microactuators  14  and  16  includes first and second stationary comb drive members  36   a  and  36   b , which also serve as first and second sidewalls of an internal chamber  38  further defined by the substrate  12  and the top cap  34 . The stationary comb drive members  36   a  and  36   b  are formed from the second layer  30 , but remain secured to the substrate  12 . A movable comb drive member  40  is disposed between the stationary members  36   a  and  36   b  and within the chamber  38 . The movable member  40  is formed from the second layer  30  and released to allow movement of the movable member  40  with respect to the substrate  12 .  
         [0020]    The stationary members  36   a  and  36   b  are disposed generally parallel to each other and each include a longitudinally-extending stationary truss  42  and a plurality of stationary comb fingers  44  extending from the stationary truss  42  toward the movable member  40 . The stationary comb fingers  44  are disposed at generally equally-spaced positions along the truss  42 . The stationary comb fingers  44  are substantially similar in construction and can each have a length of between about 15 and 150 microns. The movable member  40  includes a longitudinally-extending movable structure or movable truss  48 , a plurality of movable comb fingers  50   a  extending from the movable truss  48  toward the stationary member  36   a , and a plurality of movable comb fingers  50   b  extending from the movable truss  48  toward the stationary member  36   b . In the embodiment of FIG. 1, the microactuators  14  and  16  share the common movable truss  48 , which is connected to the shuttle  18 . In one embodiment, the movable truss  48  has a height, measured in a direction normal to the plane of substrate  12  of between about one and 200 microns, and preferably between about 10 and 150 microns. The height of the truss  48  is substantially equal to the height of the second layer  30  from which it is formed. In one embodiment, the comb fingers  44  and  50  are spaced apart from one another at a distance of between about 10 and 40 microns. The comb fingers  44  and  50  each have a height approximating the height of the corresponding truss  42  or  48  and a proximal portion with a width that is greater than the width of the corresponding distal portion of the comb finger.  
         [0021]    As discussed above and shown in FIG. 2, the movable truss  48  of the movable member  40  is disposed within an internal chamber  38 , which in one embodiment is defined by the stationary members  36   a  and  36   b , the substrate  12  and the top cap  41 . For simplicity, comb fingers  44 ,  50   a , and  50   b  are not shown in FIG. 2. The movable truss  48  is spaced from the top cap  34  and from the substrate  12  leaving flow restricted regions in the form of a top gap  41   a  and a bottom gap  41   b , respectively. In one embodiment, the top gap  41   a  and the bottom gap  41   b  are from about 0.5 to about fifteen microns each. In one embodiment, the top gap  41   a  is from one to about ten microns and the bottom gap  41   b  is from about 0.5 to about five microns. In the illustrated embodiment, a cavity or recess  55  is formed in the substrate  12  and opens onto the top surface of the substrate. This cavity  55  can serve to enlarge the size of the bottom gap  41   b . The cavity  55  may be etched into the substrate  12  to a depth of between about 0.5 and five microns in a portion of the substrate located below the movable truss  48 . In one embodiment where the gaps  41   a  and  41   b  are substantially equal, the height of the movable truss  48  is greater than one-third of the top gap  41   a  or the bottom gap  41   b.    
         [0022]    The shuttle  18 , which couples the microactuators  14  and  16  to the pivot assembly  20 , has a first portion that extends between the microactuators  14  and  16  at an approximately right angle to the movable truss  48 . The shuttle  18  is coupled to the substrate by a first flexural member  56  and a second flexural member  58 . The first and second flexural members  56  and  58  permit movement of the movable comb drive member relative to the substrate and provide the movable components of the damped micromechanical actuator with linear stiffness along a longitudinal axis of the microactuators  14  and  16 . The flexural members  56  and  58  also bias the movable comb drive member  40  to a generally central location between the stationary comb drive members  36   a  and  36   b . Although the flexural members  56  and  58  can have any suitable structure, in one embodiment each is formed from an elongate beam-like member or flexural beam  62  having a first end  62   a  coupled to the substrate  12  and a second end  62   b  connected to the shuttle  18 . Thin, elongate sacrificial beams  66  are provided for each flexural beam  62  to facilitate etching of the flexural beam  62 . A third flexural member  68  connects the shuttle  18  to the pivot assembly  20 . The shuttle  18  further connects to the optional dashpot  21  and to an optional balancing mass platform  69 .  
         [0023]    The movable member  40  is movable over the substrate  12  relative to the stationary members  36   a  and  36   b  from an unactuated or home position, shown in FIG. 1, in which the comb fingers  44  and  50  are not substantially fully interdigitated, to a first actuated position located near the stationary member  36   a , shown in FIG. 2 with respect to movable truss  48 . Comb fingers  44  and  50   a  are substantially fully interdigitated when the movable member is in the first actuated position. The movable member  40  is also movable in an opposite direction to a second actuated position located near the stationary member  36   b , in which the comb fingers  44  and  50   b  are substantially fully interdigitated. The comb fingers  44  and  50  are shown in FIG. 1 as partially interdigitated in their home position. Although the comb fingers  44  and  50  are shown as being partially interdigitated when the movable member  40  is in the home position, the comb fingers  44  and  50  can be fully disengaged when the movable member  40  is in the home position and be within the scope of the present invention. As used herein, substantially fully interdigitated includes any position in which the comb fingers  44  and  50  are more interdigitated than in the home position. When actuated to the first position, the comb fingers  50   a  extend between the comb fingers  44  and approach but do not contact the stationary truss  42 . As discussed above, FIG. 2 is a schematic sectional view of the first microactuator  14  in which the components are not illustrated to scale and the comb fingers  44  and  50   a  are not shown.  
         [0024]    The range of motion of the movable member  40  is limited by a stop  72  formed from the second layer  30 . In one embodiment, the range of motion of the movable member  40 , between the first actuated position and the second actuated position, is between about 1 and 200 microns and preferably between about 10 and 100 microns.  
         [0025]    The stationary and movable comb fingers  44  and  50 , in one embodiment, are of the type disclosed in U.S. Pat. No. 6,384,510, referenced above. In this embodiment, the comb fingers  44  and  50  are slightly inclined from a line extending normal to the respective truss  42  and  48 . Furthermore, when the movable member  40  is in the home position, the comb fingers  50  are offset from a midpoint line extending between adjacent pairs of comb fingers  44 . When the movable member  40  moves to a fully interdigitated position, the comb fingers  50  become substantially centered between adjacent pairs of comb fingers  44 . This inclination and offset account for the shortening of the flexural members  56  and  58  during actuation.  
         [0026]    The pivot assembly  20  includes a pivot member or lever  81 , which includes the platform  22 , and the first and second flexure members  83  for coupling the pivot member to the substrate  12 . The flexure members  83  are similar in construction to the flexure member  56  and  58  described above.  
         [0027]    During operation of the micromechanical device  10 , the first and second microactuators  14  and  16  are actuated by supplying an oppositely-charged electric potential to the stationary and movable comb drive members  36  and  40 , using techniques known in the art. The extent and direction of movement of the movable member  40  is determined in part by the magnitude of voltage potential across the comb fingers  44  and  50 . Movement of the movable truss  48  of the movable member  40  causes a corresponding substantially linear movement of the shuttle  18 , in a direction generally perpendicular to the elongate movable truss  48 . The movement of the shuttle  18  causes the transfer of a force to the pivot assembly  20  through the third flexural member  68 . This force causes the pivot assembly  20  to rotate in one of opposite first and second directions about the pivot point  26 . Such rotation of the pivot assembly  20  causes the movable platform  22  to move in one of substantially opposite first and second directions as shown by arrows  24   a  and  24   b . This motion of the movable platform  22  causes motion of the movable component coupled to the platform.  
         [0028]    A suitable damping fluid such as air is disposed in the chamber  38  (see FIG. 2). Motion of the movable member  40  displaces the fluid  75 , which is restricted by the structures surrounding the chamber  38  from escaping device  10  (see FIG. 2). The top cap  34  covers a large fraction of the surface area of the micromechanical device  10  producing a number of flow-restricted regions, such as gaps  41   a , within the microactuators  14  and  16 . In these flow-restricted regions, the reduction in the cross-sectional area of the fluid passageway causes an increase in the fluid flow rate in these regions. Thus, as the movable member  40  moves, air is forced through these regions at relatively high fluid flow rates.  
         [0029]    The pressure-driven flow through the flow-restricted regions creates a damping force on the moving surfaces adjoining such regions. For example, when member  40  moves as a result of an external force applied to the device  10 , damping forces are exerted on the movable truss  48  by the pressure-driven Poiseuille flow through the top gap  41   a , the bottom gap  41   b , or both, which results in a mechanical dissipation or damping of such external forces. Further damping within the microactuators  14  and  16  is caused by the Couette flow of the fluid in the chamber  38  between adjacent comb fingers  44  and  50 .  
         [0030]    The cap  34  constrains the fluid  75  within chamber  38  to flow through the flow-restricted regions  76  during movement of the movable member  40 . As the movable member  40  oscillates or moves in the lateral direction, as shown by the arrow in FIG. 2, a volume of fluid such as air proportional to the height h of the movable truss  48  and width w perpendicular to the direction of motion is displaced. If the device  10  were not capped, as in prior art devices, this displaced air would be free to leave the chamber  38  by flowing upwards and away from the movable truss  48 . In that case, the only appreciable damping would be due to the shear, or Couette, flow in cavity  38 . In the present invention, however, the cap  34  constrains the displaced fluid  75  to travel through the restrictive gaps  41   a  and  41   b  and other flow-restricted regions  76 , substantially increasing the dissipation or damping of vibrations within the movable components of the device  10 .  
         [0031]    The magnitude of the Couette and Poiseuille damping can be directly compared in a simplified case, exemplified by a device with a cross-section similar to that shown in FIG. 2, where the pressure driven flow and shear flow are both generated by the same moving element, namely movable truss  48 . In this example, the moving element with velocity ν generates a shear flow due to that motion and generates a pressure driven flow due to that same motion. For the simplified two-dimensional case, the Couette shear force, τ c , can be approximated by:  
         τ c   =μν/g    
         [0032]    where μ is the fluid viscosity and ν is the velocity of the moving plate. A similar damping term is created for both the top gap  41   a  and bottom gap  41   b , so the total Couette shear force for configuration shown in FIG. 1 is:  
         τ c   =μν/g   1   +μν/g   2 =μν( g   1   +g   2 )/ g   1   g   2    
         [0033]    where g 1  and g 2  correspond to the top gap  41   a  and the bottom gap  41   b . For a movable truss  48  with the height “h,” the Poiseuille damping force can be approximated by:  
           τ   p =μν 6 h ( g   1   +g   2 )/( g   1   3   +g   2   3 )  
         [0034]    As can be seen, both Couette and Poiseuille damping terms are functions of the same fluid viscosity and plate velocity. In one embodiment, it is desirable for the Poiseuille damping to exceed the Couette damping. For the case where the top gap  41   a  is substantially equal to the bottom gap  41   b , the Poiseuille damping exceeds the Couette damping when h is greater than one-third the gap  41   a  or  41   b . Thus, for the capped, laterally-moving micromechanical actuator of the present invention, the Poiseuille damping dominates over Couette damping. For embodiments having relatively large heights and small gaps, this large Poiseuille damping can reduce the Q of the lateral resonant modes of the device to values between 0.5 and 10. This represents a substantial reduction over devices in which only Couette damping is present, which typically have Q values from about 10 to about 100.  
         [0035]    Where the top gap  41   a  and bottom gap  41   b  are not equal, the original equations can be used. For a movable truss  48  with a top gap  41   a  of 5 microns, a bottom gap  41   b  of 15 microns, and a height h of 85 microns, the Poiseuille flow condition would produce a damping force approximately 12 times larger than the Couette flow condition, and the Q of the lateral mode providing that motion would be reduced by the same factor. If the larger gap is taken to be a factor of n times the smaller gap g, then the Poiseuille damping dominates when h is greater than about n 2  g/6.  
         [0036]    In one embodiment of the present invention, for example, experimentally measured Q values have been obtained for uncapped and capped configurations. With a bottom gap  41   b  measured to be approximately 5 microns and a height measured to be about 83 microns, the Q of the moving components of the device  10  was measured to be about 41 without a top cap. In the same device including a cap  34  defining a top gap  41   a  of about 5 microns, the Q was measured to be about 2.4.  
         [0037]    One embodiment of the present invention includes optional dashpot  21  coupled to the shuttle  18 . The dashpot  21  acts to provide further mechanical damping to the moving portions of the micromechanical device  10  by, among other things, providing a plurality of additional flow-restricted regions  76  to the device  10 . The dashpot  21  is formed from the second layer  30  overlying the substrate  12  and includes both a movable structure  82  and fixed posts  84  (see FIGS. 3 and 4). The movable structure  82  and fixed posts  84  are surrounded by the substrate  12 , the top cap  32 , and a side wall  86 . These components form a dashpot chamber  88  holding fluid  75 . The fluid can enter or exit the dashpot chamber  88  through a restricted-flow region, such as a flow channel  90 , or any other suitable channel. By enclosing the dashpot from below, on the sides, and above, a region of high damping capacity is created by again creating pressure-driven or Poiseuille flow in the flow-restricted regions created for example by the closely spaced-apart surfaces of the movable structure  82  and the fixed posts  84 .  
         [0038]    In this embodiment, actuation of the microactuators  14  and  16  causes a corresponding lateral translation of the movable structure  84 , which is attached to the shuttle  18 . As the movable structure  84  translates from an initial home position (shown in FIGS. 3 and 4) to an actuated position in which a leading surface  92  approaches the fixed post  84 , the fluid within the dashpot chamber  88  is forced to flow through the various flow-restricted regions  76  within the dashpot. For example, desirable Poiseuille flow is created in a top gap  94   a  between the movable structure  82  and the top cap  34  and a bottom gap  94   b  between the movable structure  82  and the substrate  12 . These gaps  94   a  and  94   b  are each flow-restricted regions. Such fluid flow creates a damping force on the movable structure  82 , as described above, which mechanically damps vibrations within the movable structure  82 , as well as the shuttle  18  and the movable portion of the microactuators  14  and  16 .  
         [0039]    This dashpot  21  can also function to damp out-of-plane vibrations due to the squeeze-film damping between the plate regions of the top and bottom of these structures and the top cap  34  and substrate  12 . In one embodiment, the total damping is increased by increasing the combined top and bottom surface area of those portions of the movable structure  82  over which fluid flows. In this embodiment, the overall damping of the movable components of the micromechanical device  10  is a summation of the damping action within the microactuators  14  and  16  and the damping action of the dashpot  21 .  
         [0040]    While the movable component has been described as an optical element such as an optical lens, a skilled artisan will appreciate that any other element can be carried by the holder and thus the damped micromechanical actuator. Other optical elements that are suitable as movable components include optical filters, prisms, and attenuators. In addition, the damped micromechanical actuator or device  10  of the present invention can also function to position transducing heads in data storage devices, transducer element, and motion sensing elements, including lateral accelerometers. The damping techniques of the present invention can also be applied to a micromechanical device having two degrees of motion. One example of such a device is provided in co-pending U.S. patent application Ser. No. 09/938,871 filed Aug. 24, 2001, the entire content of which is incorporated herein by this reference.  
         [0041]    Although the present invention has been described with reference to exemplary embodiments, 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.