Abstract:
A low-voltage wide band micro electrical mechanical (MEM) switch. The low-voltage MEM switch comprises a contact bridge. A microstrip has an impedance of about 50 Ohms, first and second portions, and a gap defined between the first and second portions. A cantilever arm supports the contact bridge. The cantilever arm has an end portion, an open state, and a closed state. The contact bridge is spaced from the microstrip at a distance of about 12 μm or greater when the cantilever arm is in the open state. The contact bridge provides electrical communication between the first and second portions of the microstrip when the cantilever arm is in the closed state. An electrically conductive coil opposes the first end, wherein the electrically conductive coil moves the cantilever arm from the open state to the closed state when a voltage of about 5 Volts or less are applied across the electrically conductive coil. A housing encloses the cantilever arm, microstrip, and electrically conductive coil. The housing has a height of about 5 mm or less, and the housing is not necessary hermetically sealed.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to integrated micro electrical mechanical switches, and more particularly, to a micro electrical mechanical switch requiring a low voltage signal to actuate.  
         BACKGROUND  
         [0002]    As electronic devices become smaller and more powerful, researchers have spent a great amount of time and resources developing and downsizing electrical components that control these devices. Components that have seen a great reduction in size over the years include discrete electrical components such as transistors, integrated circuits, memory devices, batteries, and switches.  
           [0003]    One type of switch that has been developed is the micro electrical mechanical (MEM) switch, which has a several functional uses such as opening and closing a circuit, controlling a power supply, and even switching high frequency signals between fiber optic pathways. In order to operate, typical MEM switches utilize two capacitive plates that hold a high voltage charge. Altering the capacitance causes the switch to change between states.  
           [0004]    This design has several fundamental shortcomings. For example, it requires the MEM switch to hold a relatively high voltage such as 20 Volts at all time, regardless of whether the switch is actuated. It is very difficult and is not economical to provide a 20 Volt power supply for battery-operated devices such as cell phones and mobile GPS receivers. Another significant example is that the air gap or separation between the capacitive plates must be very small—as small as 5 micrometers. Any type of contamination, whether dust particles or moisture will adversely affect operation of the MEM switch. As a result, MEM switches are typically hermetically sealed, which increases concerns regarding reliability, quality control issues during manufacturing, and cost. The current design of MEM switches also has performance issues. Additionally, MEM devices are typically made on semiconductor material, which requires a fabrication foundry and again increases cost.  
         SUMMARY  
         [0005]    In general terms, the present invention relates to a low voltage micro electrical mechanical (MEM) switch. A coil is used to actuate the MEM switch in place of capacitive plates. This switch has several advantages over the prior art. For example, a greater gap can be used between contact points for the switch. As a result, the package and related sealing requirements are not as strict. Manufacturing tolerances also are not as strict. Additionally, the switch can be formed on non-semiconductor material and thus neither dies nor a fabrication foundry is required for manufacturing. These advantages reduce the manufacturing cost, increase quality control, and increase product reliability.  
           [0006]    Other advantages relate to performance characteristics of MEM switches. For example, a coil requires a relatively low DC voltage signal to actuate the switch when compared to a switch using capacitive plates. It also consumes a relatively small amount of power. Additionally, the insertion loss and return loss of the switch using a coil is also reduced for a broad range of frequencies, including millimeter-wave frequencies. Yet another advantage is the size of a MEM switch embodying the present invention. It can have a small form factor, which allows it to be used with a variety of applications such as minturized devices and devices that have require small packaging requirements.  
           [0007]    One aspect of the present invention is directed to a low-voltage MEM switch. The low-voltage MEM comprises a cantilever arm having first and second ends. A contact bridge is connected to the cantilever arm and positioned between the first and second ends. First and second microstrips are electrically isolated from one another. An electrically conductive coil opposes the first end, wherein the electrically conductive coil moves the cantilever arm between an open state and a closed state. The contact bridge provides electrical communication between the first and second microstrips when in the closed state.  
           [0008]    Another aspect of the invention is a low-voltage MEM switch. The low-voltage MEM switch comprises a contact bridge. First and second microstrips, each have an impedance of about 50 Ohms and first and second end portions. The first microstrip is electrically isolated from the second microstrip. A cantilever arm supports the contact bridge. The cantilever arm has an end portion, an open state, and a closed state. The contact bridge is spaced from the microstrip at a distance of about 12 μm or greater when the cantilever arm is in the open state. The contact bridge provides electrical communication between the first and second microstrips when the cantilever arm is in the closed state. An electrically conductive coil opposes the first end, wherein the electrically conductive coil moves the cantilever arm from the open state to the closed state when a voltage in the range of about 1 Volt to about 5 Volts is applied across the electrically conductive coil. A housing encloses the cantilever arm, first and second microstrips, and electrically conductive coil. The housing has a height of about 4 mm or less and is not hermetically sealed.  
           [0009]    Another aspect of the present invention is a method of closing a circuit using a low-voltage MEM switch. The method comprises providing a low voltage MEM, the low voltage MEM including a cantilever arm, a contact bridge connected to the cantilever arm, an electrical path having first and second portions, and an electrical coil; applying a voltage of about 5 Volts or less across the electrical coil; and in response to applying the voltage of about 5 Volts or less across the electrical coil, moving the cantilever arm from a first position wherein the contact bridge is not in electrical contact with both the first and second portions of the electrical path to a second position wherein the contact bridge is in electrical contact with both the first and second portions of the electrical path.  
           [0010]    Yet another aspect of the invention is a method of closing a circuit using a low-voltage MEM switch. The method comprises providing a low voltage MEM, the low voltage MEM including a cantilever arm having first and second ends, a contact bridge connected to the cantilever arm and positioned between the first and second ends, an electrical path having first and second portions, and an electrical coil within a housing having a height of about 4 mm or less; applying a voltage of about 5 Volts or less across the electrical coil; in response to applying the voltage of about 5 Volts or less across the electrical coil, pivoting the cantilever arm around the first end, thereby moving the contact bridge a distance in the range of about 12 μm and about 2 mm from a first position wherein the contact bridge forms an open circuit between the first and second portions of the electrical path to a second position wherein the contact bridge forms a closed circuit between the first and second portions of the electrical path; and conducting an electrical signal along the first portion of the electrical path, through the contact bridge, and then along the second portion of the electrical path, the electrical signal having a frequency that is about 30 GHz or higher. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a top plan view of a MEM switch embodying the present invention, the illustrated MEM switch being in an open states, the figure having a breakout showing the details of the MEM switch underneath a lid.  
         [0012]    [0012]FIG. 2 is a side cross-sectional view of the MEM switch illustrated in FIG. 1, taken along line  2 - 2 .  
         [0013]    [0013]FIG. 3 is a bottom plan view of the MEM switch illustrated in FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0014]    Various embodiments of the present invention will be described in detail with reference to the drawings, wherein like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the invention, which is limited only by the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the claimed invention. There are alternative embodiments for all of the structures and methods disclosed herein regardless of whether specific alternatives are set forth.  
         [0015]    Referring to FIGS. 1 and 2, one possible embodiment of a low voltage micro electrical mechanical (MEM) switch, which is also called a micro electrical mechanical system, is generally illustrated as  100 . The MEM switch  100  has a substrate  102  that is made from a material on which electrical traces and components can be mounted. In one possible embodiment, the material used to form the substrate  102  is non-semiconductor material and is otherwise substantially resistant to the flow of electricity. Examples include both organic and inorganic materials such as laminates, ceramic, and PTFE (e.g., Teflon®) materials. Additionally, one possible laminate is Laminate RO4003, which is commercially available from Rogers Corporation of USA.  
         [0016]    The substrate  102  has upper and lower surfaces  104  and  106 , respectively. First and second bias ports  108  and  110 , respectively, are mounted on the upper surface  104  and are formed with an electrically conductive material upon which wires can be soldered or otherwise bonded. First and second contact traces  112  and  114 , respectively, also are formed with electrically conductive material and are mounted on the upper surface  104  of the substrate  102 . The first contact trace  112  forms a first portion of a signal or electrical path and has a first contact end  116 . The second contact trace  114  forms a second portion of a signal or electrical path and has a second contact end  118 . The first and second contact ends  116  and  118  are separated by a gap g 1 , which in one possible embodiment, is in the range of about 10 μm and about 8 mm. Other embodiments will have the first and second contact ends  116  and  118  (illustrated in phantom lines in FIG. 1) separated by other distances. In one possible embodiment, the first and second contact traces  112  and  114  are formed by microstrips having an impedance of 50 Ohms. Examples of materials that can be used to form the first and second bias ports  108  and  110 , and the first and second contact traces  112  and  114  include copper, gold, silver, and other electrically conductive material.  
         [0017]    Referring to FIG. 3, a first and second lower traces  120  and  122 , respectively, are mounted on the lower surface  106  of the substrate  102 . The first lower trace  120  is electrically connected to the first bias port  108  by a first via  124 , and the second lower trace  122  is electrically connected to the second bias port  110  by a second via  126 . Third and fourth lower traces  128  and  130 , respectively, also are mounted on the lower surface  106  of the substrate  104 . The third lower trace  128  is electrically connected to the first contact trace  112  by a third and fourth vias  132   a  and  132   b,  and the fourth lower trace  130  is electrically connected to the second contact trace  114  by a fifth and sixth vias  134   a  and  134   b.    
         [0018]    In order to reduce insertion loss and return loss, the impedance of the vias  132   a,    132   b,    134   a,  and  134   b  can be controlled by adjusting the distance between them and their proximity to a ground plane or other grounded structure. An example of such a controlled impedance structure is illustrated in U.S. Pat. No. 6,294,966, the disclosure of which is hereby incorporated by reference.  
         [0019]    In one possible embodiment, the first, second, third, and fourth lower traces  120 ,  122 ,  128 ,  130  provide contact points for connecting the low voltage MEM switch  100  to a circuit or power source. These lower traces  120 ,  122 ,  128 ,  130  can have a variety of shapes and be connected to other electrically conductive wires and paths using conventional techniques such as solder, epoxies, and the like.  
         [0020]    Although vias and traces mounted on the lower surface  106  of the substrate  102  are illustrated for connecting the MEM switch to other circuits, signal paths, and power supplies, it is understood that other structures for interconnection of the MEM switch can be used. For example, wire bonds or similar interconnects could be used to directly connect circuits, signal paths, and power supplies to the first and second bias ports  108  and  110  or the first and second contact traces  112  and  114 .  
         [0021]    Additionally, a ground plane  136  is mounted on the lower surface  106  of the substrate  102 . The ground plane  136  is below and opposes the gap g 1  between the first and second contact ends  116  and  118  of the first and second contact traces  112  and  114 , which are mounted on the upper surface  104  of the substrate  102 . In one possible embodiment, the ground plane  136  is also beneath at least a portion of the first and second contact traces  112  and  114  to form the 50-Ohm microstrip lines.  
         [0022]    In one possible embodiment, the substrate  102  is square or rectangular and has two dimensions, d 1  and d 2 , which are perpendicular to one another and define the upper and lower surfaces  104  and  106 . The lateral dimensions are those dimensions that are orthogonal to the depth dimension discussed below. The dimension d 1  is in the range of about 1 mm and about 5 mm. The dimension d 2  is in the range of about 1 mm and about 5 mm. Other embodiments might have different lengths for the dimensions d 1  and d 2 . Additionally, the surface area for one side of the substrate  102  (e.g., upper or lower surface  104  or  106 ) is in the range of about 1 mm 2  and about 25 mm 2  although the surface area for other embodiment might be different sizes. Although the substrate  102  is illustrated as being square or rectangular, it could have a variety of other shapes. Examples of other shapes include circular and irregular shapes.  
         [0023]    Referring back to FIGS. 1 and 2, a coil  138  is formed with a core  140  and a wire  142  wrapped around the core  140 . The core has an upper end  144  and a lower end  146 . The lower end  146  is mounted on the upper surface  104  of the substrate  102 . The core  140  has an axis  143  substantially orthogonal to the upper surface  104  of the substrate  102  and is formed with a permeable material that is conducive to the flow of magnetic flux. An example of material that can be used to form the core  140  includes magnetic ferrite.  
         [0024]    The core  140  can have a variety of different dimensions. In one possible embodiment, for example, the core  140  is cylindrical, has a diameter in the range of about 1 mm and about 3 mm and a length in the range of about 0.5 mm and about 3 mm. An alternative embodiment of the core  140  might include flanges (not shown) at the oppositely disposed ends, similar to a spool of thread, to help maintain the wire wraps in place.  
         [0025]    The wire  142  is wrapped around the core  140  and has oppositely disposed ends  148  and  150 . End  148  is soldered or otherwise bonded to the first bias port  108 , and the oppositely disposed end  150  is soldered or otherwise bonded to the second bias port  110 . The diameter of the wire  142  is in the range of about 1 μm and about 25 μm, although other sizes of wire are possible. The wire  142  is wrapped around the core  140  between about 100 turns and about 2000 turns, although the coil  138  may have more or less turns. One possible embodiment includes only a single layer of turns. Other embodiments have two or more layers of wire turns.  
         [0026]    Many alternative embodiments of the coil  138  described herein are possible. For example, one possible embodiment has a printed coil in place of a wound coil.  
         [0027]    A support member  152  has an upper end  162  and a lower end  160 . The lower end  160  is mounted on the upper surface  104  of the substrate  102 . The support member  152  and the coil  138  are positioned on opposing sides of the gap g 1  between the first and second contact traces  112  and  114 . An arm  154  has first and second oppositely disposed ends  156  and  158 , respectively. The first end  156  of the arm  154  is connected to the upper end  162  of the support member  152  and forms a cantilever. The arm  154  is formed with a material that is magnetically susceptible and thus is attracted to magnetic fields.  
         [0028]    In one possible embodiment, the arm  154  has five segments  164   a - 164   e  extending between the first and second oppositely disposed end  156  and  158 . The five segments  164   a - 164   e  are positioned end-to-end relative to one another. The first segment  164   a  extends from the first end  156  of the arm  154  and is in a substantially parallel to the upper surface  104  of the substrate  102  when the coil  138  is not energized. The second segment  164   b  extends from the first segment  164   a  in a downward direction toward the substrate  102 . The third section  164   c  has a bottom surface  166 , extends from the second segment  164   b,  and is substantially parallel to the upper surface  104  of the substrate  102  when the coil  138  is not energized. The fourth segment  164   d  extends from the third segment  164   c  in an upward direction away from the substrate  102 .  
         [0029]    The fifth segment  164   e  extends from the fourth segment  164   d  and terminates in the second end  158  of the arm  154 . Two flanges or members  168  and  170  extend in opposite directions from oppositely disposed sides of the fifth segment  164   e.  The two flanges  168  and  170  are substantially perpendicular to the fifth segment  164   e.  When viewed from the top, as illustrated in FIG. 1, the flanges  168  and  170  give the arm  154  the appearance of a T-shape. The flanges  168  and  170  and the fifth segment  164   e  are substantially parallel to the upper surface  104  of the substrate  102 . At least a portion of the fifth segment  164   e  and the flanges  168  and  170  are suspended above the upper end  144  of the core  140  when the coil  138  is not energized.  
         [0030]    A contact bridge  172  is connected to the bottom surface  166  of the third segment  164   c  and is positioned so that it opposes the ground plane  136 , although the substrate  102  is between the ground plane  136  and the contact bridge  172 . Additionally, when the coil  138  is not energized, the contact bridge  172  is suspended over the first and second contact ends  116  and  118  of the first and second contact traces  112  and  114 , and over the gap g 1  between the first and second contact ends  116  and  118 .  
         [0031]    The MEM switch  100  has an activated or closed state and a non-activated or opened state. The MEM switch  100  is in the non-activated state when the coil  138  is not energized and the fifth segment  164   e  of the arm  154  is suspended over the coil  138 , and the contact bridge  172  is suspended over the first and second contact ends  116  and  118  of the first and second contact traces  112  and  114 .  
         [0032]    The MEM switch  100  is in the activated state when a voltage potential is applied across the first and second bias ports  108  and  110  and the coil  138  generates a magnetic field strong enough to move the fifth segment  164   e  of the arm  154  toward the upper end  144  of the core  140 . When in the MEM switch  100  is in the activated state, furthermore, the contact bridge  172  lies against or is otherwise in electrical contact with the first and second contact ends  116  and  118  of the first and second contact traces  112  and  114 . When the circuit is closed, the contact bridge  172  closes the circuit, and the contact bridge  172  and the first and second contact traces  112  and  114  form a signal path  174 .  
         [0033]    In one possible embodiment, a voltage potential of about 5 Volts or less and a current of about 10 mA or less actuates the MEM switch  100 . In this embodiment, the coil  138  consumes about 50 mW or less of power. In another embodiment, a voltage potential of about 3 Volts or less and a current of about 10 mA or less actuates the MEM switch  100 . In this embodiment, the coil  138  consumes about 30 mW Watts or less of power. In yet another possible embodiment, the coil  138  consumes power in the range of about 1 mW and about 50 mW when the MEM switch  100  is in the actuated state. In one possible embodiment, the impedance of the contact bridge  172  substantially matches the impedance of the first and second contact traces  112  and  114 . One possible technique to match the impedance between the contact bridge  172  and the first and second contact traces  112  and  114  is to adjust the width of the contact bridge  172 . In this embodiment, the electrical characteristics of the signal path  174  formed by the contact bridge  172  and the first and second contact traces  112  and  114  is expected to be substantially similar to the electrical characteristics of a single conductor. One possible impedance for the contact bridge is about 50 Ohms. Accordingly, it is anticipated that in one possible embodiment, the signal path  174  will conduct DC signals as well as a signal having at least one frequency component. In one possible embodiment, for example, the signal path  174  will conduct a signal having at least one frequency component of about 20 GHz or higher with a return loss of about 15 dB or higher. In another embodiment, it is anticipated that the signal path  174  will conduct a signal having at least one frequency component of about 30 GHz or higher with a return loss of about 15 dB or higher. In yet another possible embodiment, it is anticipated that the signal path  174  will conduct a signal having at least one frequency component of about 50 GHz or higher with a return loss of about 15 dB or higher.  
         [0034]    In one possible embodiment, when the coil  138  is not energized, the gap g 2  between the upper end  144  of the core  140  and the fifth segment  164   e  of the arm  154  is about 12 μm or greater, and the gap g 3  between the upper surface  116  of the signal trace  112  and the lower surface  178  of the contact bridge  172  is between about 12 μm or greater. An advantage of this embodiment is that it is not required to hermetically seal the MEM switch  100  for many applications because the gap g 3  is large enough that some moisture on the first and second contact traces  112  and  114  and the contact bridge  172  will not necessarily close the circuit.  
         [0035]    In another embodiment, the gap g 2  between the upper end  144  of the core  140  and the fifth segment  164   e  of the arm  154  is in the range of about 25 μm and about 1 mm, and the gap g 3  between the upper surface  116  of the signal trace  112  and the lower surface  178  of the contact bridge  172  is in the range of about 25 μm and about 1 mm. In yet another possible embodiment, the gap g 2  is in the range of about 12 μm and about 2 mm, and the gap g 3  is in the range of about 12 μm and about 2 mm. The gap g 2  is sized respect to the gap g 3  so that the upper end  144  of the core  140  will not stop movement of the actuator toward the substrate  116  before the contact bridge  172  closes the circuit between the first and second contact traces  112  and  114 . Additionally, the distance between the first and second ends  156  and  158  of the arm  154  is in the range of about 1.5 mm and about 4 mm.  
         [0036]    In one possible embodiment, a cover or lid  180  is connected to the substrate  102 . The lid  180  and substrate  102  form a housing that encloses the coil  138 , the arm  154 , the contact bridge  172 , the support member  152 , the first and second bias ports  108  and  110 , and the first and second contact traces  112  and  114 . The lid  180  has a top portion  182 , a first sidewall  184 , a second sidewall  186 , a third sidewall  188 , and a fourth sidewall  190 . The sidewalls  184 ,  186 ,  188 , and  190  are connected to the substrate using conventional techniques. The overall depth d 3  of the enclosed MEM switch  100  assembly from the bottom  106  surface of the substrate  102  to the top portion  182  of the lid  180  is in the range of about 1000 μm and about 4000 μm. In one possible embodiment, the lid  180  is attached but not hermetically sealed to the substrate  102 . In another possible embodiment, the lid  180  is hermetically sealed to the substrate  102 .  
         [0037]    The lid  180  can be made from a variety of materials, including materials that offer protection from environmental factors and material that is substantially non-permeable. An advantage of non-permeable materials is that it minimizes the effect of stray magnetic fields on the impedance of the contact bridge  172  and the contact traces  112  and  114 . Other embodiments might use permeable materials to form the lid  180 . An example of material that can be used to form the lid  180  includes Laminate RO4003 material. Another embodiment uses the same material as used to form the substrate  102 .  
         [0038]    The various embodiments described above are provided by way of illustration only and should not be construed to limit the invention. Those skilled in the art will readily recognize various modifications and changes that may be made to the present invention without following the example embodiments and applications illustrated and described herein, and without departing from the true spirit and scope of the present invention, which is set forth in the following claims.