Abstract:
A miniaturized relay with an integrated electromagnetic actuator allows scaling a reed relay to a small size to reduce the power needed to actuate it while retaining a high quality liquid metal contact. A dragged liquid metal contact is used. Coplanar waveguides may be used for the switched signal instead of microstrip transmission lines to reduce transmission line discontinuities that occur due to impedance changes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application relates to the co-pending application Ser. No. 10/857,306 filed on May 28, 2004, entitled “Liquid Metal Contact Microrelay” 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. Typical problems with reed relays are the requirements for high currents to latch and hold the connection and the high contact resistance that is present because of the relatively low force contact. Typical designs also suffer from poor radio frequency (RF) properties at greater than about 2 GHz because the un-terminated cantilevers act as antennas when the relay is open. An improvement to the typical reed relay is obtained by replacing the solid contacts with a thin mercury layer to reduce the contact resistance. This is typically known as a mercury film relay. 
   It is sometimes desirable to have a relay that can operate at speeds greater than 3 kHz. To increase the switching speed of a mechanical relay, the size of the mechanical relay typically needs to be scaled down in size to reduce inertia. MEMS (MicroElectroMechanical Systems) techniques have been adapted to produce a wide variety of small sized relays. Most such small sized relays have increased contact resistance because as the relay is scaled down in size the contact forces are also scaled down. Stiction forces increase as the relay is scaled down because surface forces scale with the area while restoring forces scale with the volume and stiction may become a problem if the devices are not handled appropriately during production and hermetically packaged. Mercury film relays typically cannot be significantly scaled down in size because of the surface tension forces that arise due to the smaller radius of the meniscus and act to prevent the relays from switching. 
   SUMMARY OF THE INVENTION 
   In accordance with the invention, a miniaturized relay with an integrated electromagnetic actuator allows for scaling a reed relay to a small size to reduce the power needed to actuate it while retaining a high quality liquid metal contact that is scalable. A dragged liquid metal contact is used. 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 
       FIGS. 1   a–c  show an embodiment in accordance with the invention 
       FIGS. 2   a–c  show an embodiment in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1   a  shows an embodiment in accordance with the invention. Liquid metal contact reed relay  100  in  FIG. 1   a  has cantilever  107  that is typically a composite made from a magnetic material such as permalloy together with a highly conductive layer of Al, Au, Cu or other highly conductive material to improve RF transmission performance. An intermetalic barrier metallization, such as, for example, Ti—Pt, Ti—W, Cr or other similar material may be used between the permalloy and the highly conductive layer. Typical thickness for the highly conductive layer is on the order of thousands of Angstroms while the thickness of the barrier layer is on the order of hundreds of Angstroms. 
   For example, cantilever  107  has a typical linear dimension of about 1 mm, a typical height of about 25 μm and a typical width of about 10 μm. Cantilever  107  is suspended over substrate surface  102 , typically made from silicon, ceramic or other suitable dielectric. Liquid metal contact reed relay  100  is 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. 
   The substrate typically has a ground plane (not shown) typically a thin layer of aluminum (Al) or aluminum silicide, gold (Au) or copper (Cu) or other suitable conductor, located on the substrate face opposite to surface  102  of the substrate. 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 the ground plane (not shown) and the substrate face opposite to surface  102 . Note that cap  125  is cutaway in  FIGS. 1   a–b  except for patterned traces  123  which are part of cap  125 . Additional sealing structures may be fabricated on the bottom of cap  125  and surface  102  of substrate  101  to provide for a hermetic seal using, for example, a gold—gold compression technique. 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 contact reed relays  100  to be fabricated in parallel and multiple liquid metal contact reed relays  100  may be integrated into a single package allowing for added capabilities. 
   Cantilever  107  is fabricated such that well  110  is positioned at one end of cantilever  107 . Well  110 , for example, typically has an inner diameter of about 25 μm Well  110  contains mercury, gallium alloy or other suitable liquid metal alloy drop  120  that is in contact with substrate surface  102 . Well  110  may have a circular, elliptical or other appropriate shape. On substrate surface  102  there are signal electrodes  103 ,  104 ,  105 . Signal electrodes  103 ,  104 ,  105  in a first switched state as shown in  FIG. 1   a  make an electrical connection from signal electrode  103  through cantilever  107  through liquid metal drop  120  to signal electrode  104 . In a second switched state as shown in  FIG. 1   b , signal electrodes  103 ,  104 ,  105  make an electrical connection from signal electrode  103  through cantilever  107  through liquid metal drop  120  to signal electrode  105 . Typical current carrying capacities for liquid metal contact reed relay  100  are on the order of 10 mA for Hg liquid metal drop  120  having about a 25 μm diameter. The diameter of liquid metal drop  120  is the limiting factor because the boiling point of liquid metal drop is typically much less than melting point of cantilever  107 . Typically, the driving signal is on signal electrode  103  and signal electrodes  104 ,  105  may be signal paths or termination. 
   Substrate surface  102  has electrical traces  122  on it that run substantially perpendicular to cantilever  107 . With reference to  FIG. 1   c , liquid metal contact reed relay  100  has cap  125  (shown partially cutaway in  FIG. 1   c ) which typically has etched cavity  126  containing patterned traces  123  at an angle with respect to traces  122  so that adjoining traces  122  are electrically connected. When cap  125  is properly aligned and brought together with surface  102  of substrate  101 , traces  122  and  123  together form inductor  130  with cantilever  107  at the center. When a current passes through inductor  130 , a magnetic field substantially perpendicular to traces  122  and  123  is created. To obtain about a 20 μT magnetic field from inductor  130 , a winding current of about 1 mA and about 16 turns/mm are required. The magnetic field places a magnetic force, for example, typically on the order of 200 μN on cantilever  107  to move liquid metal alloy drop  120  in well  110  to signal electrode  105 . When the winding current is removed from inductor  130 , the spring force on cantilever  107 , for example, typically on the order of about 200 μN, moves liquid metal alloy drop  120  in well  110  to signal electrode  104 . 
   For microstrip transmission lines, the impedance is determined by the scale of the respective signal conductor&#39;s dimensions to the distance to ground plane  115 . If signal electrodes  103 ,  104 ,  105  and cantilever  107  are microstrips, large discontinuities in impedance are typically present at each end of cantilever  107  because of the changing distance to ground plane  115  (see  FIGS. 1   a – 1   c ) as the signal transitions from signal electrodes  104  or  105  and signal electrode  103  on substrate surface  102  to cantilever  107 . 
   Coplanar waveguides use ground conductors that are coplanar with the signal conductors. Hence, the impedance is controlled by the width of the signal conductors and the gap between the ground conductors and the signal conductors.  FIGS. 2   a–c  show an embodiment in accordance with the invention that reduces the discontinuity problems that may result in impedance variations, particularly at higher frequencies by using coplanar waveguides. The numerical parameters for the embodiment in  FIGS. 2   a–c  are the same as that for the embodiment in  FIGS. 1   a –c above. 
   Liquid metal contact reed relay  200  in  FIG. 2   a  has cantilever  207  that is typically made from a magnetic material such as permalloy and is suspended over substrate surface  202 , typically silicon, ceramic or other suitable dielectric. The substrate typically has a ground plane, typically a thin layer of aluminum (Al), gold (Ag) or copper (Cu), located on the substrate face opposite to substrate surface  202 . Note that cap  265  is cutaway in  FIGS. 2   a–b  except for patterned traces  263  which are part of cap  265 . Cantilever  207  is fabricated such that well  210  is positioned at one end of cantilever  207 . Well  210  contains liquid mercury, gallium alloy or other suitable liquid metal alloy drop  220  that is in contact with substrate  201 . Well  210  may have a circular, elliptical or other appropriate shape. On substrate surface  202  there are signal electrodes  203 ,  204 ,  205  and RF ground electrodes  223 ,  224 ,  225 ,  226 . 
   Substrate surface  202  has metal traces  262  on it that run substantially perpendicular to cantilever  207 . With reference to  FIG. 2   c , liquid metal contact reed relay  200  has cap  265  (shown partially cutaway in  FIG. 2   c ) which typically has etched cavity  266  containing patterned traces  263  at an angle with respect to traces  262  so that adjoining traces  122  are electrically connected. When cap  265  is properly aligned and brought together with substrate surface  202 , traces  262  and  263  together form inductor  230  with cantilever  207  at the center. When a current passes through inductor  230  to actuate liquid metal contact reed relay  200 , a magnetic field substantially perpendicular to traces  262  and  263  is created. The magnetic field places a magnetic force on cantilever  207  moving liquid metal alloy drop  220  in well  210  to signal electrode  205 . When current is removed from inductor  230 , the spring force on cantilever  207  moves liquid metal alloy drop  220  in well  210  to signal electrode  204 . Additional sealing structures may be fabricated on the bottom of cap  265  and surface  202  of substrate  201  to provide for a hermetic seal using, for example, a gold—gold compression technique. 
   Signal electrodes  203 ,  204 ,  205  in the unactuated state as shown in  FIG. 2   a  make an electrical connection from signal electrode  203  through cantilever  207  to liquid metal drop  220  in well  210  and on to signal electrode  204 . In the actuated state when current is passed through electromagnet  230  as shown in  FIG. 2   b , signal electrodes  203 ,  204 ,  205  make an electrical connection from signal electrode  203  through cantilever  207  to liquid metal drop  220  and on to signal electrode  205 . Typically, the driving signal is on signal electrode  203  and signal electrodes  204 ,  205  may be signal paths or termination. 
   In an embodiment in accordance with the invention, liquid metal contact reed relay  200  as shown in  FIGS. 2   a–c  is designed to reduce the discontinuities resulting in impedance variations. Signal electrodes  203 ,  204 ,  205  are made co-planar waveguides by introducing electrode  225  next to signal electrode  205 , electrode  224  next to electrode  204 , electrodes  223  and  226  bordering electrode  203  with raised metal trace  231  electrically connecting electrode  225  to electrode  223  and raised metal trace  232  electrically connecting electrode  224  to electrode  226  reduces the discontinuity problems. Note that the dimensions of raised metal trace  231  and  232  are typically on the order of cantilever  207 . Electrodes  223 ,  224 ,  225 ,  226  are typically kept at RF ground along with raised metal traces  231  and  232 . Introducing the appropriate curvatures for raised traces  231  and  232  reduces the discontinuities resulting in transmission line impedance variations. For example, as shown in  FIGS. 2   a–c  for purposes of illustration, raised trace  231  has a curvature that matches the curvature of cantilever  207  when liquid metal contact reed relay  200  is in the actuated state and raised trace  232  has a curvature that matches the curvature of cantilever  207  in the unactuated state. Typically, however, in order to reduce the discontinuities, the curvature of cantilever  207  will not exactly match the curvature of raised traces  231  and  232  in the actuated and unactuated state, respectively. Hence, transmission line characteristic impedance of the signal path does not depend on the on/off state of liquid metal contact reed relay  200 . 
   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.