Abstract:
A micro-electro-mechanical (MEMs) devices including a compound hot electrode, which increases the tilting range of the MEMs device. A substantially vertical hot electrode is mounted adjacent to the end or the sides of a pivoting ground electrode, formed of the underside of a pivoting mirror, and combine with a conventional horizontal hot electrode to make up the compound hot electrode.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a pivoting MEMs device, and in particular to a pivoting MEMs device with a compound ground electrode for eliminating unwanted snapping. 
       BACKGROUND OF THE INVENTION 
       [0002]    The micro electro-mechanical (MEMs) device of the present invention is an electrostatically actuated tilting micro mirror with a torsional spring used for optical switching. When used in fiber optic networks, the MEMs mirrors redirect light signals carrying data from one optical fiber to another in order to reach a desired destination. 
         [0003]    In optical switching applications, a micro mirror needs to satisfy three requirements. The first is to enable precise and controllable orientations of the micro mirror, which stems from the fact that imprecise mirror tilt angles might cause the light signals to miss the small fiber cores of the various output optical fibers in the switch causing loss of data during switching. In particular, when the distance between the micro mirror and the fiber is increased, as the demand for higher capacity switches grows, the need for precision becomes paramount. 
         [0004]    The second requirement is related to the dynamic response of the mirror to the step voltages used to actuate the mirror. In this aspect, the mirror is required to have minimal overshoot and settling time, which are necessary for minimizing the time between two successive switching operations. 
         [0005]    Finally, the magnitude of the step voltage required to drive the micro mirror to the desired tilt angle needs to be minimal to minimize the power requirements of the electric circuits. 
         [0006]    Electrostatic parallel-plate actuators are widely used in MEMS mirror designs because of their simplicity and lateral-force-free property. However, their usable angle range is severely limited by the well-known “snapping” phenomenon, as shown in  FIG. 1 . 
         [0007]    The root cause of the snapping is that the electrical driving torque on the mirror with a constant voltage 
         [0000]    
       
         
           
             
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         [0000]    increases when mirror angle increase because dC/dθ, i.e. the change in capacitance to the change in mirror angle, is a monotonically-increasing function of mirror angle as shown in  FIG. 1 . 
         [0008]    As the result, the mirror response to a external disturbing torque ΔT becomes 
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         [0000]    where K is inherent mechanical stiffness K and K ef  is the effective stiffness of the mirror. 
         [0009]    When the driving torque increase rate reaches the level that the effective stiffness of the mirror becomes zero, the mirror will continue to rotate without an increase of the driving voltage, i.e. “snapping” occurs, i.e. when K ef  reaches zero, the actuator offers no resistant to any driving force increment. Approaching the snap point, the effective stiffness become so small that the mirror tilt is very sensitive to voltage variation and external turbulence. Depending on the stability and control resolution requirements of the application, a large portion of the tilt range near the snapping point become unusable. 
         [0010]    An object of the present invention is to overcome the shortcomings of the prior art by providing a parallel-plate actuator having a dC/dθ, which is a monotonically-decreasing function of θ, so that the effective stiffness of the actuated mirror remains greater than zero over the whole tilt range, whereby snapping is avoided, the usable tilt angle range is expanded, and tilt stability is improved. 
       SUMMARY OF THE INVENTION 
       [0011]    Accordingly, the present invention relates to a micro-electro-mechanical device comprising:
       a substrate;   a pivoting member mounted above the substrate via a hinge, defining a first axis, for tilting about a tilt range, an underside of the pivoting member defining a ground electrode;   a horizontal hot electrode mounted on the substrate below the pivoting member for attracting the ground electrode towards the substrate, thereby pivoting the pivoting member about the first axis; and   a first vertical hot electrode extending upwardly from the substrate adjacent to and along an edge the pivoting member with a gap therebetween, for increasing an effective stiffness of the pivoting member, whereby the effective stiffness of the pivoting member remains greater than a mechanical stiffness of the pivoting member over the tilt range.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0017]      FIG. 1  illustrates plots of voltage and dC/dθ vs tilt angle for a conventional MEMs parallel plate actuator; 
           [0018]      FIG. 2  illustrates the design parameters for a parallel plate actuator; 
           [0019]      FIG. 3  is an isometric view of an end section of the MEMs pivoting device in accordance with the present invention; 
           [0020]      FIG. 4  is an isometric view of a MEMs pivoting device in accordance with an alternative embodiment of the present invention; 
           [0021]      FIG. 5  is an isometric view of a MEMs pivoting device in accordance with an alternative embodiment of the present invention; 
           [0022]      FIG. 6  illustrates plots of driving voltage and dC/dθ vs tilt angle for a conventional MEMs pivoting device; and 
           [0023]      FIG. 7  illustrates plots of driving voltage and dC/dθ vs tilt angle for a MEMs pivoting device in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The design parameter definitions for an angular parallel plate actuator are illustrated in  FIG. 2 , in which g 0  is the original distance between a hot electrode on the substrate and a ground electrode on pivoting mirror, x is the distance along the ground electrode of the pivoting mirror, and θ is the angle of the mirror between horizontal and the current position. 
         [0025]    According to the present invention, a micro-electro-mechanical (MEMs) device having a higher effective stiffness is illustrated in  FIGS. 3 ,  4  and  5 . Any form of tilting MEMs device including a pivoting member acting as a ground electrode pivotally mounted over a substrate via a hinge and actuated by a hot electrode below one side thereof, can be used as the basis for the present invention, and the following embodiments are only meant to be exemplary. In particular, any form of hinge structure can be used, including those disclosed in U.S. Pat. No. 6,934,439, which is incorporated herein by reference. 
         [0026]    With particular reference to  FIG. 3 , a vertical hot electrode  41  is mounted on the substrate  25  beyond (not beneath), but adjacent to and along the outer free end of the tilting platform  26 , so that as the mirror platform  26  tilts, the first derivative of capacitance (dC/dθ), i.e. between the vertical hot electrode  41  and the ground electrode  27 , varies in an opposite direction as that between the ground electrode  27  and the horizontal hot electrode  36 . By appropriate selection of the geometrical parameters of the vertical electrode  41 , the combined 1 st  derivative of capacitance of the system decreases with the tilt of the mirror platform  26  in the whole required range. Typically the vertical hot electrode  41  is substantially perpendicular to the substrate  25 , the horizontal hot electrode  36 , and the ground electrode  27  of the mirror platform  26 , when the mirror platform  26  is parallel to the substrate  25  and the horizontal hot electrode  36 . Typically, the vertical hot electrode  41  extends upwardly from the substrate  25  to a height equal to or greater than the gap between the tilting ground electrode  27  and the horizontal hot electrode  36 , when the tilting ground electrode  27  is horizontal, i.e. parallel to the horizontal hot electrode  36 . The vertical hot electrode  41  can be etched onto the substrate  25  during the fabrication process of the mirror platform  26  or mounted onto the substrate  25  in a subsequent fabrication step. 
         [0027]    Another embodiment of the present invention is illustrated in  FIG. 4 , in which a uniaxially tilting MEMS device  51  includes a substrate  52  with pedestals  53   a  and  53   b  extending upwardly therefrom for supporting torsional hinge  54  extending therebetween defining an axis of rotation. A horizontal hot electrode  56  is mounted on the substrate  52  parallel thereto, while a vertical hot electrode  57  is mounted on the substrate  52  extending upwardly from the substrate  52  perpendicular to the horizontal hot electrode  56 . An insulating layer  55  is disposed between the substrate  52  and the hot electrodes  56  and  57 . A platform  58  is fixed to the hinge  54  for rotating about the axis of rotation, and is disposed above the horizontal hot electrode  56 , generally parallel thereto. The platform  58  acts like a horizontal ground electrode and is rotated to various predetermined angles under control of the horizontal hot electrode  56  by adjusting the voltage thereto, as is well known in the art. Typically, the platform  58  includes a mirrored upper surface for reflecting beams of light or optical signals, used in optical switching devices. The vertical hot electrode  57  comprises a substantially rectangular structure disposed beyond (not beneath), but adjacent to and along the outer free end of the tilting platform  58 , for example: extending at least 50% to 150% of the width of the horizontal hot electrode  56  and/or the platform  58 , preferably 75% to 125%, and most preferably 90% to 110%. Typically, the vertical hot electrode  57  also extends upwardly from the substrate  52  to a height substantially equal with the platform  58  (when horizontal) or above, i.e. the height of the hinge  54 ; however, the height can be between 50% to 150% of the height of the gap between the horizontal hot electrode  56  and the horizontal platform  58 , preferably 75% to 125%, and most preferably 90% to 110%. The gap between the end of the tilting platform  58  and the vertical hot electrode  57  (when perpendicular) is typically between 1 um and 50 um, but preferably between 1 um and 10 um. The vertical hot electrode  57  can be fabricated, e.g. etched, along with the other elements of the substrate, e.g. pedestals  53   a  and  53   b,  or it can be fabricated in a separate step and mounted on the substrate  52  separately. 
         [0028]    Another embodiment of the present invention is illustrated in  FIG. 5 , in which a uniaxially tilting MEMS device  61  includes a substrate  62  with pedestals  63   a  and  63   b  extending upwardly therefrom for supporting torsional hinge  64  extending therebetween defining an axis of rotation. A horizontal hot electrode  66  is mounted on the substrate  62  parallel thereto, while a pair of vertical hot electrodes  67   a  and  67   b  are mounted on the substrate  62  extending upwardly from the substrate  62  perpendicular to the horizontal hot electrode  66 , adjacent to and along opposite edges of the platform  68 . An insulating layer  65  is disposed between the substrate  62  and the hot electrodes  66 ,  67   a  and  67   b.  A platform  68  is fixed to the hinge  64  for rotating about the axis of rotation, and is disposed above the horizontal hot electrode  67 , generally parallel thereto. The platform  68  acts like a horizontal ground electrode and is rotated to various predetermined angles under control of the horizontal hot electrode  66  by adjusting the voltage thereto, as is well known in the art. Typically, the platform  68  includes a mirrored upper surface for reflecting beams of light or optical signals, used in optical switching devices. The vertical hot electrodes  67   a  and  67   b  are each comprised of a substantially rectangular structure, and are disposed beyond (not beneath), but adjacent and parallel to and along the sides of the tilting platform  68 , extending at least half the length of the horizontal hot electrode  66 ; however, the vertical hot electrodes  67   a  and  67   b  can extend from at least 50% to 150% of the length of the horizontal hot electrode  66  and/or the platform  68 , preferably 75% to 125%, and most preferably 90% to 110%. Typically, the vertical hot electrodes  67   a  and  67   b  extend upwardly from the substrate  62  to a height substantially equal with the platform  68  (when horizontal) or above, i.e. the gap distance g 0 ; however, the height can be between 50% to 150% of the height of the gap between the horizontal hot electrode  66  and the horizontal platform  68 , preferably 75% to 125%, and most preferably 90% to 110%. The vertical hot electrodes  67   a  and  67   b  can be fabricated, e.g. etched, along with the other elements of the substrate, e.g. pedestals  63   a  and  63   b,  or it can be fabricated in a separate step and mounted on the substrate  62  separately. 
         [0029]    The 1 st  derivative of capacitance (dC/dθ) between vertical hot electrode  57  or  67   a / 67   b  and the platform  58  or  68  reduces as the platform  58  or  68  tilt increases, which is opposite to that between horizontal hot electrode  56  or  66  and the platform  58  or  68 . By appropriate selection of geometrical parameters, e.g. height and width of the vertical hot electrodes  57  or  67   a  and  67   b,  the combined 1 st  derivative of capacitance of the system  51  or  61  decreases with tilt of the platform  58  or  68  and, therefore, the effective stiffness of the system is greater than the inherent mechanical stiffness in the whole required range. 
         [0030]      FIGS. 6 and 7  shows performances before and after, respectively, vertical-electrode modification of an example design. In a standard parallel plate design with a gap (at horizontal position) of 27 um between hot  56  and ground electrode  58 , a width of the hot electrode  56  of 95 um and length of 220 um, illustrated in  FIG. 6 , the first derivative (dC/dθ) increases with tilt angle θ, and the snapping point occurs at 2.2° tilt. In the device of the modified design in accordance with the present invention, illustrated in  FIG. 7 , with the parallel plate electrodes having the same dimensions as above, and with a vertical hot electrode, e.g.  57 , having a distance to the hinge  54  of 590 um, a width of 95 um, and a gap between the vertical hot electrode  57  and the platform  58  of 5 um, the first derivative (dC/dθ) of the combined capacitance decrease, i.e. over the operating range of tilt angles after a vertical electrode is added at the end of the platform. As the result, the snapping point disappears and voltage and tilt relationship become linear. Of course, the aforementioned parameters are only meant to be exemplary, and are not to limit the scope of protection in any way.