Patent Publication Number: US-6985058-B2

Title: Lorentz force assisted switch

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/411,377, filed Sep. 17, 2002, the contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a capacitive microelectromechanical switch based on utilization of the Lorentz force. 
   2. Description of the Related Art 
   There now exists a small but growing number of microelectromechanical systems (MEMS) including micro-actuators; examples of which are switches, resonant magnetometers, micro mirrors, micro valves, etc. A typical MEMS shunt switch  10 , as illustrated in  FIG. 1 , includes a beam bridge  12  of length L, width w, and thickness t, and a pull-down electrode  14  having a length W and spaced from the beam bridge  12  to form a gap  16  of width g. When a voltage V is applied, the electrostatic force F causing the bridge to deflect toward a substrate  18  is given by the following equation: 
             F   =           ɛ   0     ⁢   Ww       2   ⁢   g       ⁢       V   2     ⁡     (   N   )                 (   I   )             
 
where ε 0 =8.854×10 −12  C 2 /N−m 2 , where C is coulombs and N is Newtons. As the gap  16  decreases, the electrostatic force increases. When the deflection is greater than approximately ⅓ of the initial gap  16 , this force exceeds the restoring force of the bridge and causes the switch to snap closed. The minimum voltage that causes this to happen (pull-down voltage, V p ) is given by the following equation: 
               V   p     =             8   ⁢   k       27   ⁢     ɛ   0     ⁢   Ww       ⁢     g   3         ⁢   V             (   II   )             
 
where k is the spring constant.
 
   Accordingly, to actuate a MEMS-based switch having the gap  16  of from 1.5 to 5 micrometers, typically it is required that a pull-down voltage be from 30 to 90 V. In the context of MEMS, these voltages are high enough to create problems associated with energy losses, processing and reliability. 
   A need therefore exists for a MEMS-based switch actuateable by a relatively low pull-down voltage. 
   SUMMARY OF THE INVENTION 
   This need is met by an MEMS-based capacitive switch of the present invention utilizing the Lorentz force, which is produced as a result of coupling between magnetic and electric fields applied across the switch. Accordingly, since the switch actuation is a function of the Lorentz force combined with an actuation voltage, as the Lorentz force increases, the actuation electrostatic pull-down voltage decreases. 
   Structurally, the MEMS-based switch of the present invention is configured with a source generating a magnetic field across the switch, and an electrical conductor carrying a current and extending transversely to the magnetic field. Coupling the electric and magnetic fields produces the Lorentz force sufficient to assist in displacement of the electrical conductor between two positions corresponding to the on- and off-states of the switch in accordance with a direction of current flow through the electrical conductor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features, as well as advantages and objects of this invention will become more readily apparent from the following description of the preferred embodiment accompanied by the attached drawings, in which: 
       FIG. 1  is a schematic diagram of a MEMS-based switch configured in accordance with the known prior art; 
       FIG. 2  is a schematic side view of a MEMS-based switch configured in accordance with the invention; 
       FIG. 3  is a top view of the MEMS-based switch of  FIG. 2 ; 
       FIG. 4  is a sectional top view of the embodiment of the inventive MEMS-based switch of  FIGS. 2 and 3 ; 
       FIG. 5  is a cross-sectional view of the inventive MEMS-based switch taken along lines A—A of  FIG. 4 ; 
       FIG. 6  is a sectional view of the inventive MEMS-based switch taken along lines B—B, as shown in  FIG. 4 ; and, 
       FIG. 7  is a graph illustrating magnetic fields required to produce the Lorentz forces in a 0–40 μN range for drive currents of 0.5, 1.0, and 5.0 Amps in the MEMS-based device of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 2 and 3 , a microelectromechanical (MEMS) switching device  20  of the present invention is formed on a substrate  26 , and includes a MEMS capacitive switching assembly  32  operative to couple spaced apart contacts by utilizing Lorentz force. The switching assembly  32  includes a beam bridge  22  and a fixed pull-down electrode  24  supported by the substrate and spaced from the bridge  22 . A dielectric layer  30  separating the bridge and the pull-down electrode, which both are made from a metal, polysilicon or a combination of these, prevents shorting therebetween in the off-state of the switch  20 , as shown in  FIG. 2 . 
   To provide the bridge  22  with the desired flexibility, only its opposite ends  34 ,  36  are supported by the substrate  26 , whereas an inner span  38  of the bridge is separated from the substrate by, for example, undercutting or underetching. As a consequence, the unsupported span  38  of the bridge  22  is capable of flexing towards the substrate  26  to contact the pull-down electrode  24  and, thus, to define the on-state of the device  20  once a voltage applied to the switch overcomes the restoring force of the bridge  22 . 
   In accordance with the present invention, the bridge  22  is juxtaposed with an electrical conductor  28  made from flexible conducting or semi-conducting materials and coupled to an electric field generating source  40  to conduct a current I ( FIG. 3 ) along the direction of arrow A. To produce the Lorentz force F L , the conductor  28  is placed within a magnetic field B generated by a source  33  and extending coplanar with but transversely to the electric field. As a consequence, the Lorentz force F L , as better seen in  FIG. 2 , extends in a plane perpendicular to the plane of the electric and magnetic fields and is applied to the bridge  22  so that the latter flexes towards the pull-down electrode  24  formed on the substrate  26 . Assuming that the direction of arrow A indicates the direction of current associated with the on-state of the switch  20 , reversing the direction of the current I along the conductor  28  in the presence of the magnetic field B would generate the Lorentz force directed away from the pull-down electrode  24 . Accordingly, once the direction of the current I is changed, the bridge  22  and the pull-down electrode  24  are decoupled to define the off-state of the switch  20 . 
   The source  40  is preferably an electric pulse generator, which is coupled to a pulse duration modulator  42  operative to control the duration of pulses, which are preferably relatively short to minimize Joule heating that, if not controlled, can lead to overheating of the bridge  22  and the pull-down electrode  24 . The source  33  generating the magnetic field B may include permanent magnets capable of generating high magnetic fields, a coil or a thin film deposited on the substrate  26 . 
   Referring to  FIGS. 4–6 , showing the layout and cross-sections of the exemplary embodiment of the inventive switch  20  operative to couple contacts  50 ,  52  provided on the substrate  26  to transmit and output a signal  54  in both the RF and millimeter bands. Consonant to the inventive concept, the switch has a beam bridge  62  displaceable towards a pull-down electrode  60  in response to the Lorentz force produced upon coupling transversely extending magnetic and electric fields. To provide a reliable contact between the bridge  62  and the pull-down electrode  60 , the latter may have one or multiple components. For example,  FIG. 4  illustrates four pull-down electrodes  60  positioned equidistantly from one another to form an imaginary square. The bridge  62  is configured to have a central body  64  located above and configured to overlap all four pull-down electrodes  60  to ensure a reliable electrical contact therewith. The shape of the central body  64  may have a circular, polygonal or even an irregular shape as long as the body is sized to form overlapping regions with the pull-down electrodes  60 . To facilitate displacement of the bridge  62  in response to application of the actuation voltage and the Lorentz force, its central body  64  further has multiple legs  66  each provided with a width substantially smaller than the body  64 . The legs  66 , each terminating in a respective pad  65 , which is supported by the substrate  26 , act as hinges bent by the Lorentz force exerted by a conductor  68 , which lies in transversely extending magnetic and electrical fields and is coupled to the bridge  62 . 
   While the conductor  68  does not necessarily have to contact the bridge  62  directly, preferably, the latter provides a support top surface  70  ( FIG. 6 ) directly contacting the conductor  68 . As illustrated in  FIG. 4 , the conductor  68  has a frame made from a low resistance material and including a pair of spaced apart flat strips or circular wires  72  bridging supports  76 , which are provided on the substrate  26 . Reliable coupling between the bridge  62  and the conductor  68  is realized by engagement between formations  78  and  80  provided on the inner side of the strips  72  of the conductor  68  and the pads  65  of the bridge  62 . These formations may include protrusions and indentations provided on the opposing surfaces of the bridge and the conductor and shaped and dimensioned to extend complementary to one another. Such a connection between the bridge  62  and the conductor  68  provides for their synchronous displacement towards and away from the pull-down electrode  60  in response to the application of the Lorentz force. 
   The Lorentz force generated by a current in a magnetic field B, which is applied in the plane of and perpendicular to the longitudinal direction of the bridge, is given by the following equation:
 
 F   L   =B×I×L   (III)
 
where I is the current, B is the magnetic field and L is the length of the conductor. The direction of the force is defined by the direction in which the current flows. Alternatively, the direction of the force may be controlled by changing the direction of the magnetic field if the latter is generated by an external source, provided, of course, that such a structure would meet the local requirements.
 
   The magnetic fields required to produce forces comparable to electrostatic pull-down forces in the bridge of 300 μm length in the range of 1–100×10 −6  N with drive currents of 0.5, 1.0, and 5.0 A are shown in  FIG. 7 . It can be seen that in order to produce a Lorentz force of 10 μN, a field of 67 mT is required for a 0.5 A drive current and 7 mT for a 5 A drive current. Based on the empirical data, the pull-down voltage results in a force that causes the beam to deflect only ⅓ of the initial gap width. If the Lorentz force acts alone on the switch, a factor of at least 3 must be allowed to effect switch closure, i.e. 50 μN for a 1.5 μm gap and 100 μN for a 3.0 μm gap. This will increase the field requirement proportionately. 
   Thus, in the switch of the present invention, which can be integrated in, for example, micromotors, microvalves, mechanical resonators, etc., the Lorentz force is used to reduce the gap between the bridge and the pull-down electrode of the switch from its “full up” position, as shown in  FIG. 5 , to a distance close enough that a lower voltage ranging between 5 to 10 V will cause the bridge to snap down. From equation (2) given above in paragraph four (4), and assuming that 90 V is required to pull-down the bridge with a 3 μm gap, the gaps are 0.44 and 0.69 μm for pull-down voltages of 5 and 10 V, respectively. These values represent a “saving” of 15% and 23% of the Lorentz force required in the unassisted case. 
   It will be understood that various modifications may be made to the embodiments disclosed herein. Therefore, the above description should not be construed as limiting the scope of the invention, but merely as exemplifications of the preferred embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.