Abstract:
According to the present invention, an assembly and method is provided for preventing beams or switch contacts from overheating due to high power environments. A MEMS switch is packaged so that the beam and switch is surrounded by an inert, low viscosity, dielectric fluid. Utilizing such a construction conductively and convectively dissipates heat generated by resistive heating of the MEMS beam.

Description:
BACKGROUND  
         [0001]    Many conventional micromechanical switches use a deflecting beam as the actuating means for switching electrical signals. These beams are usually cantilevered beams or beams that are fixed at both ends. The beams are conventionally deflected electrostatically. However, deflection by other means, such as magnetically or thermally, is also used. Electrical contact for signal passage is made via conductive contacts closing or by bringing together capacitively coupled plates. For high power applications, capacitively coupled plates are normally used in order to prevent microwelding of metal contacts.  
           [0002]    Another issue arises due to resistive heating of the beams during high power applications. High power applications can be of sufficient power to cause switch degradation through annealing of the beams or due to changes in the stress state in the beams. Further, losing heat from the beams is an additional issue due to the long length of the beams relative to their thickness. For instance, a beam can be approximately 300 μm long and 1-6 μm thick. Moreover, the beams are generally surrounded by gases which do not conduct heat adequately.  
         SUMMARY  
         [0003]    The present invention is directed to a microelectromechanical system (MEMS) actuator assembly. Moreover, the present invention is directed to an actuator assembly and method for improving the power handling capacity of MEMS switches.  
           [0004]    According to the present invention, an assembly and method is provided for preventing beams or switch contacts from overheating due to high power environments. A MEMS switch is packaged so that the beam and switch is surrounded by an inert, low viscosity, dielectric fluid. Utilizing such a construction conductively and convectively dissipates heat generated by resistive heating of the MEMS beam. Further, surrounding the beam with an inert, low viscosity, dielectric fluid allows local cooling of switch contacts during opening and closing thus preventing overheating and microwelding of the contacts.  
           [0005]    The MEMS beam and associated structures (e.g. capacitive and actuator plates) may have perforations to allow fluid passage and to provide less hydrodynamic drag as the beam and associated structures move through the fluid. These perforations act to minimize any time penalty associated with operating in a fluid medium.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0006]    The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention.  
         [0007]    [0007]FIG. 1 shows a cross sectional side view of a MEMS switch in accordance with the invention.  
         [0008]    [0008]FIG. 2 shows a bottom view of the long arm of a piezoelectric beam with perforations in accordance with the invention.  
         [0009]    [0009]FIG. 3 shows an alternate cross sectional view of a MEMS switch in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    The MEMS switch  100  shown, shown in FIG. 1, includes a substrate  110  which acts as support for the switching mechanism and provides a non-conductive dielectric platform. The MEMS switch  100  shown in FIG. 1 also includes deflecting beam  120  connected to the substrate  110 . In common fashion, the deflecting beam  120  forms an L shape with the short end of the deflecting beam  120  connecting to the substrate. The deflecting beam  120  is constructed from a non-conductive material. The deflecting beam  120  has an attracted plate  140  and a first signal path plate  150  connected to the long leg. An actuator plate  160  is connected to the substrate directly opposing the attracted plate. A second signal path plate  170  is connected to the substrate directly opposing the signal path plate  150 .  
         [0011]    During operation of the MEMS switch shown in FIG. 1, a charge is applied to actuator plate  160  causing attracted plate  140  to be electrically attracted thereto. This electrical attraction causes bending of the deflecting beam  130 . Bending of the deflecting beam  120  causes the first signal path plate  150  and the second signal path plate  170  to near each other. The nearness of the first and second signal path plates  150 ,  170  causes capacitive coupling, thus allowing the switch  100  to achieve an “on” state. To turn the switch off, the voltage difference between the actuator plate  160  and the attracted plate  140  is removed and the deflecting beam returns to its undeflected position.  
         [0012]    A dielectric pad  180  is commonly attached to one or both of the signal path plates  150 ,  170 . A dielectric pad is not shown attached to the signal plate  150  in FIG. 1. The dielectric pad prohibits the signal path plates  150 ,  170  from coming in contact during the bending of the deflecting beam. It is understood by those skilled in the art that electrostatically actuated micromachined high-power switches pass the signals capacitively because conduction by metal-to-metal can cause the contacts  150 ,  170  to micro-weld. Further, the high heat present in a high power capacitive MEMS switch can cause annealing of the deflecting beam  130  also resulting in a short circuited MEMS switch.  
         [0013]    It is understood by those skilled in the art that high power capacitive MEMS switches can be constructed in a variety of manners. Any capacitive MEMS switch is susceptible to annealing, melting, welding or other heat induced phenomena.  
         [0014]    A dielectric packaging  190  surrounds the MEMS switch  100  in FIG. 1. The packaging connects to the substrate  110  and provides an airtight chamber  195  around the MEMS switch  100 . The chamber  195  is filled with a suitably inert (non-reactive with the components of the MEMS switch  100  and chamber  195 , and electrochemically unreactive in the chemical and electrical environment existing within the switch chamber  195 ), low viscosity (e.g. 0.4-0.8 cs), dielectric fluid. In a preferred embodiment of the invention, the chamber  195  is filled with a low molecular weight (e.g. m.w. 290-420) perfluorocarbon. In a more preferred embodiment of the invention, the chamber  110  is filled with Fluorinert™ FC-77. Fluorinert™ is a register trademark of 3M. Heat generated by resistive heating of the MEMS switch  100  is dissipated to the fluid contained in the chamber  195 . The presence of the fluid in the chamber also allows local cooling of the signal path plates  150 ,  170  during opening and closing thus preventing overheating and microwelding of the signal path plates  150 ,  170 .  
         [0015]    The MEMS deflecting beam  120 , attracted plate  140  and signal path plates  150  may have perforations  198  to allow fluid passage therethrough. FIG. 2 shows a bottom view of the long arm of a piezoelectric beam  120  with perforations  198  in accordance with the invention. The perforations allow for increased cooling of the affected structures of the MEMS switch  100  and provide for less hydrodynamic drag as the perforated structures  120 ,  140 ,  150  move through the fluid. The switching time penalty for operating in a fluid is thus minimized. As is understood by those skilled in the art, perfluorocarbons generally have good lubricity so that friction is minimized.  
         [0016]    [0016]FIG. 3 shows an alternate cross sectional view of a MEMS switch  200  in accordance with the invention. The MEMS switch  200  shown, shown in FIG. 3, includes a substrate  210  which acts as support for the switching mechanism and provides a non-conductive dielectric platform. The MEMS switch  200  shown in FIG. 1 also includes deflecting beam  220  connected which is fixed at each end to a beam support  225 . The beam supports  225  are attached to the substrate  210 . The deflecting beam  220  is constructed from a non-conductive material. The deflecting beam  220  has an attracted plate  240  and a first signal path plate  250  connected to the long leg. An actuator plate  260  is connected to the substrate directly opposing the attracted plate. A second signal path plate  270  is connected to the substrate directly opposing the signal path plate  250 .  
         [0017]    During operation of the MEMS switch shown in FIG. 3, a charge is applied to actuator plate  260  causing attracted plate  240  to be electrically attracted thereto. This electrical attraction causes bending of the deflecting beam  220 . Bending of the deflecting beam  220  causes the first signal path plate  250  and the second signal path plate  270  to near each other. The nearness of the first and second signal path plates  250 ,  270  causes capacitive coupling, thus allowing the switch  200  to achieve an “on” state. To turn the switch off, the voltage difference between the actuator plate  260  and the attracted plate  240  is removed and the deflecting beam returns to its undeflected position.  
         [0018]    A dielectric pad  280  is commonly attached to one or both of the signal path plates  250 , 270 . A dielectric pad is not shown attached to the signal plate  250  in FIG. 3. The dielectric pad prohibits the signal path plates  250 , 270  from coming in contact during the bending of the deflecting beam. It is understood by those skilled in the art that electrostatically actuated micromachined high-power switches pass the signals capacitively because conduction by metal-to-metal can cause the contacts  250 , 270  to micro-weld. Further, the high heat present in a high power capacitive MEMS switch can cause annealing of the deflecting beam  220  also resulting in a short circuited MEMS switch.  
         [0019]    It is understood by those skilled in the art that high power capacitive MEMS switches can be constructed in a variety of manners. Any capacitive MEMS switch is susceptible to annealing, melting, welding or other heat-induced phenomena.  
         [0020]    A dielectric packaging  290  surrounds the MEMS switch  200  in FIG. 1. The packaging connects to the substrate  210  and provides an airtight chamber  295  around the MEMS switch  200 . The chamber  295  is filled with a suitably inert (non-reactive with the components of the MEMS switch  200  and chamber  295 , and electrochemically unreactive in the chemical and electrical environment existing within the switch chamber  295 ), low viscosity (e.g. 0.4-0.8 cs), dielectric fluid. In a preferred embodiment of the invention the chamber  295  is filled with a low molecular weight (e.g. m.w. 290-420) perfluorocarbon. In a more preferred embodiment of the invention, the chamber  110  is filled with Fluorinert™ FC-77. Fluorinert™ is a register trademark of 3M. Heat generated by resistive heating of the MEMS switch  200  is dissipated to the fluid contained in the chamber  295 . The presence of the fluid in the chamber also allows local cooling of the signal path plates  250 , 270  during opening and closing thus preventing overheating and microwelding of the signal path plates  250 , 270 .  
         [0021]    The MEMS deflecting beam  220 , attracted plate  240  and signal path plates  250  may have perforations  298  to allow fluid passage therethrough. FIG. 2 shows a deflecting beam  220  and signal plates  240 , 250  with perforations. The perforations allow for increased cooling of the affected structures of the MEMS switch  200  and provide for less hydrodynamic drag as the perforated structures  220 , 240 , 250  move through the fluid. The switching time penalty for operating in a fluid is thus minimized. As is understood by those skilled in the art, perfluorocarbons generally have good lubricity so that friction is minimized.  
         [0022]    While only specific embodiments of the present invention have been described above, it will occur to a person skilled in the art that various modifications can be made within the scope of the appended claims.