Abstract:
Liquid metal microrelays may be made where a contact is formed by constraining a quantity of liquid metal at the end of a contact support suspended over a substrate. Movement of the contact support typically drags the liquid metal along the surface of the substrate and allows the liquid metal to bridge contacts located on the substrate. Coplanar waveguides may be used for the switched signal instead of microstrip transmission lines to reduce transmission line discontinuities due to impedance changes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application relates to the co-pending application Ser. No. 10/857,205, filed on the same day, entitled “A Liquid Metal Contact Reed Relay With Integrated Electromagnetic Actuator” by Simon and Rosenau owned by the assignee of this application and incorporated herein by reference. 
   BACKGROUND 
   A reed relay is a common type of relay. The reed relay includes one or more thin cantilevered metal arms or reeds made of paramagnetic material such as permalloy (typically 80% nickel, 20% iron). In the presence of a magnetic field, the reeds experience a force and move to make contact with one another or another electrode to complete a circuit. While these relays can be used to switch DC signals for powering devices, AC signal switching applications dominate the areas of application for small reed relays. Reed relays have the ability to handle large currents, have long lifetimes, typically more than 1×10 8  cycles, relatively low cost, moderate contact resistance and good isolation. 
   However, reed relays typically have a number of drawbacks. There is typically a lower bound on the size of reed relay because of the space occupied by the winding of the electromagnetic actuator. The presence of the lower bound on the size of the reed relay typically limits switching speed and unless designed to mechanically latch, the inductive nature of the relay requires that significant power is typically required for the relay to remain latched. Electromechanical bounce issues may exist that impart noise into the switched signal along with contact wear issues for reed relays. Additionally, reed relays cannot operate at frequencies greater than about 5 GHz. 
   A way to improve the performance of a reed relay is to coat the electrodes with liquid mercury, thereby replacing a solid—solid contact with a liquid—liquid contact. A liquid—liquid contact provides a number of advantages by removing the electromechanical bounce issues associated with solid—solid contact; eliminating most of the contact wear issues because the liquid is refreshed every time the relay is actuated; the actuating force required to make a good contact is typically reduced; and the contact resistance and insertion loss is reduced. However, in a liquid—liquid contact the surface tension forces which need to be overcome typically tend to dominate as the relay size is scaled down, thereby setting a limit on switching speed. 
   MEMS (MicroElectroMechanical Systems) techniques have been introduced to improve the speed, lower the cost and provide multiple relays in a compact package, allowing reed relays to be made at sizes on the order of a few square millimeters. Reduced size allows some reed relays to be capable of operating at switching speeds greater than about 1 kHz. However, typically contact resistance is high due to the low contact forces that are possible with the typical electrostatic actuation. Some MEMS relays have higher force actuators but typically sacrifice speed and lifetime. 
   Some contact related limitations have been addressed by liquid metal microrelays, such as those disclosed, for example, in U.S. Patent Publication 20030201855 A1, which are latching MEMS relays that depend on a thermal actuator and all liquid contact to lower the insertion loss. Because the relay is latching, no power is required for the relay to remain actuated. The liquid metal microrelays are suitable for radio frequency (RF) signals and typically provide a bandwidth of 18 GHz. However, liquid metal microrelays have several problems. The thermal actuator cannot typically be repeatedly operated without heat buildup because the thermal actuator heats up much more quickly than it cools and this places an upper bound on how rapidly the relay may be cycled. Liquid metal microrelays are also typically sensitive to the amount of liquid mercury they contain and the volume of mercury involved is typically relatively large compared to the relay volume. 
   SUMMARY OF THE INVENTION 
   In accordance with the invention, a contact made by constraining a quantity of liquid metal at the end of a contact support suspended over a substrate is used to make liquid metal microrelays. Movement of the contact support typically drags the liquid metal along the surface of the substrate and allows the liquid metal to bridge contacts located on the substrate. Coplanar waveguides may be used for the switched signal instead of microstrip transmission lines to reduce transmission line discontinuities due to impedance changes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an embodiment in accordance with the invention. 
       FIG. 2  shows an embodiment in accordance with the invention. 
       FIG. 3  shows a side view of an embodiment in accordance with the invention. 
       FIG. 4  shows a side view of an embodiment in accordance with the invention. 
       FIG. 5  shows an embodiment in accordance with the invention that reduces discontinuities in signal transmission. 
       FIG. 6  shows an embodiment in accordance with the invention that reduces discontinuities in signal transmission. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment in accordance with the invention using an electrostatic MEMS cantilever relay with a dragged contact is shown in  FIG. 1  in a first position. Dragged contact  109  is typically mercury but may also be a gallium alloy. Microrelay  100  provides for small scale, high switching speed (greater than about 1 kHz) operation and is suitable for switching radio frequency switching operations. Use of typical lithographic techniques such as those disclosed in “Fundamentals of Microfabrication: The Science of Miniaturization”, Marc Madou, CRC Press, 2002, allows many thousands of liquid metal microrelays  100  to be fabricated in parallel and multiple microrelays  100  may be integrated into a single package allowing for added capabilities. Alternative embodiments in accordance with the invention include comb drive structures typically requiring larger die but lower operating voltages. 
   Microrelay  100  is typically a single poll, double throw relay although other configurations are possible such as, for example, single poll, single throw; double poll, single throw and double poll, double throw relays. Microrelay  100  includes substrate  101 , signal electrodes  103 ,  104 ,  105 , switching electrode  106 , cantilever  107  and stator  108 . Signal electrodes  103 ,  104 ,  105  and switching electrode  106  are fabricated on upper surface  102  of substrate  101 , typically silicon or other suitable dielectric. Cantilever  107  is typically made of nickel by electroplating and is electrically coupled to signal electrode  105  and has a typical linear dimension on the order of 1 mm, a typical height on the order of 25 μm and a width on the order of 10 μm. Cantilever  107  has well region  115  at one end with a typical inner diameter on the order of 25 μm that holds dragged contact  109 , typically a drop of mercury that makes contact with upper surface  102  of substrate  101 . Well region  115  may have a circular, elliptical or other suitable shape in accordance with the invention. Stator  108  is typically fabricated from electroplated nickel and typically has dimensions on the order of cantilever  107 . Stator  108  is electrically coupled to switching electrode  106 . 
     FIG. 2  shows microrelay  100  in a second position. Application of a switching voltage, typically on the order of 100 V, between cantilever  107  and stator  108  causes cantilever  107  to move towards stator  108  due to an electrostatic force on the order of 200 μN. Dragged contact  109  is moved until dragged contact  109  couples signal electrodes  104 ,  105  to create a closed circuit. Stop  110  prevents cantilever  107  from contacting stator  108 . Removal of the switching voltage results in the return of cantilever  107  to its non-actuated position because of the elastic restoring force in bent cantilever  107  which is on the order of 200 μN. 
     FIG. 3  shows a cross-section of microrelay  100  in a first position in accordance with an embodiment of the invention.  FIG. 4  shows a cross-section of microrelay  100  in the second position. Ground plane  120 , typically a thin metal layer of aluminum (Al) or aluminum silicide, gold (Au), copper (Cu) or other suitable conductor covers the bottom surface of substrate  101 . A barrier/adhesion layer on the order of hundreds of angstroms, such as a Ti—Pt, Ti—W or Cr layer, is typically used between ground plane  120  and substrate  101 . 
   If signal electrodes  103 ,  104 ,  105  and cantilever  107  are microstrips, large discontinuities resulting in impedance variations are typically present at each end of cantilever  107  because of the changing distances to ground plane  120  (see  FIGS. 3–4 ) as the signal transitions from signal electrodes  104  or  103  and signal electrode  105  on substrate surface  102  to cantilever  107 . Additionally, the proximity of stator  108  to cantilever  107  produces an additional discontinuity in the transmission in cantilever  107  and this additional discontinuity depends on whether cantilever  107  in microswitch  100  is in the first or second position. 
     FIGS. 5 and 6  show an embodiment in accordance with the invention to reduce the discontinuity problems that result in impedance variations, particularly at frequencies higher than about 2 GHz. The numerical parameters for the embodiments in  FIGS. 5 and 6  are on the order of those discussed above in connection with the embodiments shown in  FIGS. 1–4 . Making signal electrodes  503 ,  504 ,  505  along with cantilever  507  co-planar waveguides by introducing second stator  511  as shown in  FIG. 5  avoids the large discontinuities due to transmission line impedance. Stators  508  and  511  can both be part of the RF ground while carrying the DC voltage required to electrostatically switch the position of cantilever  507  between signal electrodes  503  and  504 . Stators  511  and  508  are dimensionally sized on the order of magnitude of cantilever  507  dimensions. Cantilever  507  has well region  515  at one end with a typical inner diameter on the order of 25 μm that holds dragged contact  509 , typically a drop of mercury. The use of stators  511  and  508  as RF ground ensures that the distance between the signal trace and RF ground does not significantly change. Use of dual stators  508  and  511  also allows forced switching of microrelay  500  to avoid stiction instead of relying on the spring constant, typically about 1 N/m of cantilever  507  to return cantilever  507  to the first position. Stop  510  prevents cantilever  507  from contacting stator  508 . If forced switching is used, a second stop (not shown) is typically introduced to prevent stator  511  from contacting cantilever  507 . Either the first or second position of mircorelay  500  can be obtained by appropriate biasing of stators  508  and  511 . However, only stator  508  is necessary for microrelay  500  while stator  511  may serve only as part of the RF ground to avoid discontinuities. 
   Stators  511  and  508  are typically designed to ensure that the transmission line characteristic impedance along cantilever  507  is substantially independent of whether microrelay  500  is in the first or second position. This is typically accomplished by appropriately adjusting the curvature of stators  511  and  508  to adjust the distance between cantilever  507  and stators  511  and  508  to achieve an approximately constant transmission line characteristic impedance. For example, for purposes of illustration,  FIG. 5  shows stator  511  having no curvature to match the lack of curvature of cantilever  507  in the first position and  FIG. 6  shows stator  508  having a curvature to match the curvature of cantilever  507  in the second position. Typically, however, the curvature of stators  511  and  508  are not selected to match the curvature of cantilever  507  in the first and second positions, respectively, but rather the curvatures of stators  508  and  511  are selected to provide for an approximately constant transmission line characteristic impedance along the signal path for microrelay  500 . 
   While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.