Patent Publication Number: US-2003227361-A1

Title: Microelectromechanical rf switch

Description:
STATEMENT OF GOVERNMENT INTEREST  
     [0001] The Government has rights in this invention in accordance with a contract with the Department of Defense. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The invention in general relates to miniature switches, and more particularly, to a MEMS switch useful in radar and other microwave applications.  
       [0004] 2. Description of Related Art  
       [0005] A variety of MEMS (microelectromechanical systems) switches are in use, or proposed for use, in radar, as well as other high frequency circuits for controlling RF signals. These MEMS switches are popular insofar as they can have a relatively high off impedance and a relatively low on impedance, with a low off capacitance, leading to desirable high cutoff frequencies and wide bandwidth operation. Additionally, the MEMS switches have a small footprint and can operate at high RF voltages.  
       [0006] Many of these MEMS switches generally have electrostatic elements, such as opposed pull down control electrodes, which are attracted to one another upon application of a DC control signal. One of these DC control electrodes is on a substrate and an opposing electrode, having a dielectric coating, is positioned on the underside of a moveable bridge above the substrate. Upon application of the DC control signal the bridge is drawn down and an electrical contact on the underside of the bridge completes the electrical circuit between first and second spaced apart RF conductors on the substrate.  
       [0007] As will be described, for this type of design there is a possibility of stiction. Stiction is a condition wherein a charge is built up in the dielectric upon touching the opposed control electrode. When the control voltage is removed there may be enough charge built up such that there is still an attraction and the switch will remain closed, even though it is supposed to be open. Further, under such condition, at the point of closure of the control electrodes an ultrahigh field exists which can lead to contact erosion.  
       [0008] It is an object of the present invention to provide a MEMS switch which eliminates the possibility of stiction. It is a further object to provide a MEMS switch which is highly reliable, has low RF losses and a high operating bandwidth.  
       SUMMARY OF THE INVENTION  
       [0009] A MEMS switch is provided which has a substrate member with first and second spaced-apart conductors deposited on the substrate. A bridge structure, including a central stiffener portion, is disposed above the substrate and has a plurality of flexible arms connected to respective ones of a plurality of support members. At least one control electrode is deposited on the substrate for receiving a DC control signal to activate the switch to a closed position. The bridge structure has an undersurface including at least one metallic area for forming an opposed electrode portion facing the control electrode, for electrostatic attraction upon application of the DC control signal. The bridge structure, upon application of the DC control signal, is drawn down, by the electrostatic attraction, to complete an electrical circuit between the first and second conductors. The central stiffener portion is of a material to resist bending in a manner that, when said bridge structure is drawn down completing the electrical circuit, there is no contact between the control electrode and the opposed electrode portion. Additionally, the switch is fabricated such that there is no dielectric material in the area of the opposed electrode facing the control electrode.  
       [0010] Further scope of applicability of the present invention will become apparent from the detailed descriptions provided hereinafter. It should be understood, however, that the detailed descriptions and specific examples, while disclosing the preferred embodiments of the invention, is provided by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art, from the detailed description. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] The present invention will become more fully understood from the detailed description provided hereinafter and the accompanying drawings, which are not necessarily to scale, and are given by way of illustration only. In addition, the use of spatial terms such as top, bottom, above, below etc. is for ease of explanation and not as structural or orientation limitations.  
     [0012]FIG. 1A is a plan view of a prior art MEMS switch.  
     [0013]FIG. 1B is a view of the switch of FIG. 1A along lines  1 B- 1 B, in the open position.  
     [0014]FIG. 1C is a view of the switch of FIG. 1A along lines  1 B- 1 B, in the closed position.  
     [0015]FIG. 2A is a plan view of a MEMS switch in accordance with one embodiment of the present invention.  
     [0016]FIG. 2B is a view of the switch of FIG. 2A along lines  2 B- 2 B, in the open position.  
     [0017]FIG. 2C is a view of the switch of FIG. 2A along lines  2 B- 2 B, in the closed position.  
     [0018]FIG. 3A is a plan view of a MEMS switch in accordance with another embodiment of the present invention.  
     [0019]FIG. 3B is a view of the switch of FIG. 3A along lines  3 B- 3 B, in the open position.  
     [0020]FIG. 3C is a view of the switch of FIG. 3A along lines  3 B- 3 B, in the closed position.  
     [0021]FIG. 4A is an isometric view of some basic components of a switch with a contact member above two conductors.  
     [0022]FIG. 4B is an isometric view of some basic components of a switch with a contact member above one conductor and electrically integrated with the other conductor.  
     [0023]FIGS. 5A to  5 H are figures to illustrate the advantages and disadvantages of the switch designs of FIGS. 4A and 4B.  
     [0024]FIGS. 6A and 6B are side views of a contact member, as in FIG. 4B, making contact with a conductor.  
     [0025]FIG. 6C is a view of asperities of the actual contact surfaces.  
     [0026]FIG. 7A is an exploded view of another embodiment of the present invention.  
     [0027]FIG. 7B is a view along line  7 B- 7 B of FIG. 7A.  
     [0028]FIG. 7C is a view along line  7 C- 7 C of FIG. 7A.  
     [0029]FIG. 8A is an exploded view of another embodiment of the present invention.  
     [0030]FIG. 8B is a view along line  8 B- 8 B of FIG. 8A.  
     [0031]FIG. 8C is a view along line  8 C- 8 C of FIG. 8A.  
     [0032]FIG. 9A is an exploded view of another embodiment of the present invention.  
     [0033]FIG. 9B is a view along line  9 B- 9 B of FIG. 9A.  
     [0034]FIG. 9C is a view along line  9 C- 9 C of FIG. 9A.  
     [0035]FIG. 10A is an exploded view of another embodiment of the present invention.  
     [0036]FIG. 10Aa is a plan view of a component of FIG. 10A.  
     [0037]FIG. 10B is a view along line  10 B- 10 B of FIG. 10A.  
     [0038]FIG. 10C is a view along line  10 C- 10 C of FIG. 10A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0039] Referring to FIGS.  1 A-C, there is illustrated an example of one type of MEMS switch  10 . The switch  10 , shown in an open position in FIG. 1B, includes first and second spaced-apart conductors  12  and  13  for conduction of current when the switch is activated to a closed position. The particular activation mechanism includes a flexible bridge, supported at anchors  17 , and comprised of a metal top  21  and a dielectric undersurface  22 .  
     [0040] The bridge  20  includes a contact  24  on its undersurface for making electrical contact with both conductors  12  and  13  to complete the electrical circuit for signal transmission. This is accomplished with the provision of pulldown, or control electrodes. More particularly, the arrangement includes electrodes  26  and  27  to which is applied a DC control signal. Metal portions of the bridge  20  act as respective opposed electrodes, i.e., a DC return.  
     [0041] Upon application of the DC control signal, the switch  10  closes, as bridge  20  is pulled down to the position shown in FIG. 1C by electrostatic attraction of the control electrode arrangement. Bumpers, or stops  28  and  29  limit further movement of the bridge  20 .  
     [0042] In the operation of such switch, a problem may arise in that when in a closed position, as in FIG. 1C, a dielectric,  22 , is positioned between metals  21  and  26 , and  21  and  27 , potentially leading to a stiction situation. Stiction is the condition wherein the switch remains in a closed position for a period of time after the control signal has been removed. This condition is caused by a charge build-up in the dielectric  22 , and which charge build-up continues the electrostatic attraction, even after the control signal has been removed.  
     [0043] In addition, extremely high fields exist at the point of closure of the control electrodes. This can lead to high field erosion of the metal constituting the control electrode arrangement.  
     [0044] FIGS.  2 A-C illustrate one embodiment of the present invention which completely eliminates these problems. The improved MEMS switch  40  illustrated in FIGS.  2 A-C includes first and second spaced apart RF conductors  42  and  43  deposited on a substrate  44 , such as alumina or sapphire, by way of example.  
     [0045] Positioned above the substrate  44 , and above the first and second conductors  42  and  43 , is a bridge structure  46  having a central stiffener portion  48 . The central stiffener portion  48  is vertically moveable by virtue of metallic flexible spring arms  50  connected to respective support members  52 .  
     [0046] The central stiffener portion  48  includes depending edge segments  54  and  55 , as well as a depending middle segment  56 . The metallized portion of the bridge structure  46  forming spring arms  50 , extends partially across the undersurface of central stiffener portion  48 , forming respective electrode sections  60  and  61 . In addition, the undersurface of depending middle segment  56  includes an electrical contact  64  which completes the electrical connection between first and second RF conductors  42  and  43  when the switch  40  is activated. This contact  64  which completes the RF electrical circuit may be either metallic or a capacitive type connection.  
     [0047] Activation of the switch is accomplished with the provision of a pulldown, or DC control electrode arrangement. In FIGS.  2 A-C, this DC control electrode arrangement includes electrically connected DC electrodes  70  and  71 , deposited on substrate  44 , in conjunction with opposed electrode sections  60  and  61 , on the underside of central stiffener portion  48 , without the intervention of any dielectric. The absence of a dielectric also eliminates the problem of dielectric charging by cosmic rays, if the switch is used in an outer space application.  
     [0048] A DC voltage may be applied to electrodes  70  and  71 , via input pad  72  to activate the switch, with opposed electrodes  60  and  61  forming a connection to ground, via support members  52 . With this design the RF and DC circuits are completely isolated from one another. This isolation is further aided in this, as well as subsequent embodiments, by making the line  73  from pad  72  to electrode  70 , very thin and of a high resistance material, so as to impart a high resistance to RF currents.  
     [0049] Electrostatic attraction between opposed electrodes  60 / 70  and  61 / 71  causes the bridge structure  46  to assume the position illustrated in FIG. 2C whereby the switch is closed by contact  64  electrically connecting first and second conductors  42  and  43 . In this regard, stoppers  74  and  75  may be included to limit downward movement of the central stiffener portion  48 .  
     [0050] When the switch is activated by application of a DC control voltage, depending edge segments  54  and  55  make physical contact with respective stoppers  74  and  75  at the same time as contact  64  makes contact with the RF conductors. Because of continued electrostatic attraction between opposed control electrodes, the contact  64  is pushed further in the middle, ensuring that good resistive (or capacitive) contact is made to the RF conductors  42  and  43 .  
     [0051] The central stiffener portion  48  of bridge structure  46  is sufficiently rigid so as to prevent any significant bending, thus ensuring that opposed control electrodes never touch one another, with continued application of the DC control signal. This central stiffener portion  48  may be made of a stiff metal, however, to achieve even more rapid switching speeds, the central stiffener portion  48  is preferably made of a rigid lightweight, low density material, such as a silicon oxide in the form of silicon monoxide or silicon dioxide, by way of example. Although silicon monoxide and silicon dioxide are dielectrics, the central stiffener portion  48  is not positioned between two metals, and no charging effect can take place.  
     [0052] If size is of a critical consideration, the lateral dimension of the switch  40  may be reduced by providing spring arms  50  with undulations, as depicted by phantom lines  78 . These undulations will enable the spring arms  50  to be shorter, while still maintaining the same restoring forces on the bridge structure  46 .  
     [0053] Switch  40 , like many MEMS switches, may be fabricated using conventional integrated circuit fabrication techniques well-known to those skilled in the art. The fabrication process may be greatly simplified by utilizing a design as illustrated in FIGS.  3 A-C, generally corresponding to the views of FIGS.  2 A-C.  
     [0054] As best seen in FIGS. 3B and 3C, switch  80  includes bridge structure  82  having an essentially flat metallic bridge member  83  having a flexible flat metal arm member  84 , bifurcated on either end and extending between supports  86  which are disposed on a substrate  88 . The bridge structure  82  has a central stiffener portion  90  which is also flat and which is positioned on metal bridge member  83  above RF conductors  92  and  93  on substrate  88 .  
     [0055] The DC control electrode arrangement includes electrodes  96  and  97  electrically connected together and positioned on either side of the conductors  92  and  93 . Opposed electrodes for electrostatic attraction are constituted by respective portions  100  and  101  of the metal arm directly above respective electrodes  96  and  97 , and connected to a DC ground (not illustrated). Activation of the switch  80  to a closed position, as in FIG. 3C, is accomplished by a DC control signal applied to input pad  103  (FIG. 3A).  
     [0056] Downward movement of bridge structure  82  is limited by the presence of conductors  92  and  93 , as well as stoppers  106  and  107 , which extend above substrate  88  to a position higher than DC electrodes  96  and  97 , and substantially even with conductors  92  and  93 . With this construction, during operation, the metallic bridge member  83  never touches control electrodes  96  and  97 .  
     [0057] It is generally an object in the design of MEMS switches to provide a device that has the highest possible impedance when in the off state (switch open), and the lowest possible impedance when in the on state (switch closed). This not only provides for a higher ratio of output to input power, that is, lower loss over an operating frequency range, but also allows for a higher ratio of cutoff frequency-to-operating frequency.  
     [0058]FIGS. 4A and 4B illustrate basic components of two types of MEMS switch configurations, and FIGS. 5A to  5 H illustrate the resistive and capacitive effects during operation of the switches.  
     [0059] The switch of FIG. 4A includes first and second spaced apart RF conductors  108  and  109  on a substrate  110 , with a contact member  111  disposed over both conductors. This structure is basically of the type described in FIGS.  2 A-C and  3 A-C.  
     [0060] The switch of FIG. 4B affords some advantages in reducing RF losses and is of the type to be subsequently described in FIGS.  7 A-C to  9 A-C. The switch of FIG. 4B includes first and second RF conductors  112  and  113  on a substrate  114 , with a contact member  115  disposed over conductor  112  and being electrically integrated with conductor  113 .  
     [0061]FIG. 5A illustrates the switch of FIG. 4A in a closed position and FIG. 5B is the corresponding resistive electrical representation. Let it be assumed that, between conductor  108  and contact  111 , and between contact  111  and conductor  109 , there is the series connection of two resistors, each of a resistance R, as depicted in FIG. 5B. The total resistance therefore, between points A and B is 2R.  
     [0062] With the arrangement of FIG. 4B, and as illustrated in FIG. 5C, the two resistors are now connected in parallel, as depicted in FIG. 5D. With two resistors in parallel, the resulting resistance between points A and B is R/2, a fourfold reduction in resistance as compared with the structure of FIG. 4A. This reduction in resistance significantly reduces RF losses.  
     [0063] With respect to the capacitive aspects of the two arrangements, FIG. 5E illustrates the switch of FIG. 4A in an open condition, with the capacitive electrical representation being shown in FIG. 5F. It is seen that two capacitors each of a value C are connected in series resulting in a total capacitance of C/2 between points A and B.  
     [0064] With the arrangement of FIG. 4B, and as illustrated in FIGS. 5G and 5H, the capacitors are now in parallel resulting in a total capacitance of 2C between points A and B. This increase in capacitance leads to an undesired decrease in open circuit impedance, however this is offset in the present invention by designing the MEMS switches with extremely small contact areas, which has the effect of reducing fringe capacitance.  
     [0065] Another benefit of the arrangement of FIG. 4B is illustrated in FIGS.  6 A-C. In FIG. 6A a DC control signal has been applied and a contact member  116  is drawn down to the point of just touching conductor  117 . During the application of the control signal, contact member  116  is drawn down further so as to move to the left, as in FIG. 6B, thus providing a wiping action. This wiping action provides a continuous cleaning of the mating surfaces and assures good electrical contact.  
     [0066] It is to be noted that in actuality, the mating surfaces are not totally flat but rather, on a microscopic level, include asperities as illustrated in FIG. 6C. The surfaces of both the contact member  116  and conductor  117  include asperities or protrusions  118  preventing a desired totally flat surface-surface contact. The wiping action of the design, as in FIG. 4B, aids in smoothing the surfaces during continued operation, thus reducing resistive losses of the switch.  
     [0067] FIGS.  7 A-C illustrate an embodiment of the present invention based upon the principles of the switch of FIG. 4B. In FIGS.  7 A-C, switch  120  includes first and second RF conductors  122  and  123  deposited on a substrate  125 . Suspended above the conductors is a metallic bridge structure  127  having a plurality of arms  128 ,  129  and  130 , connected to respective support members  131 ,  132  and  133 , with this latter support member  133  being formed on the end of conductor  123  which faces conductor  122 . In accordance with the present invention, the bridge structure  127  includes a central stiffener portion  136 , which may be of a silicon oxide, as previously described.  
     [0068] In order to impart greater flexibility to the bridge structure  127 , at least the laterally extending arms  128  and  129  may be bifurcated, as illustrated. The support members  131  to  133 , to which the arms are connected, are electrically conducting members such that the bridge structure  127  is suspended over conductor  122 , but is electrically integral with conductor  123 , by virtue of electrically conducting support member  133 .  
     [0069] The DC control electrode arrangement includes separated electrodes  140  and  141  on substrate  125  with the electrodes being electrically connected together by conducting trace  142 . Electrodes  140  and  141  are positioned on either side of conductor  122  at the end thereof. Opposed electrodes for electrostatic attraction are constituted by respective portions  144  and  145  of the metal arms directly above respective electrodes  140  and  141 , and connected to a DC ground via trace  147  by the path including arm  128  and support member  131 . Activation of the switch  120  to a closed position is accomplished by a DC control signal applied to input pad  148 .  
     [0070] It is noted that switch  120 , as well as subsequent embodiments, does not include stoppers as in FIGS. 2 and 3. Stoppers may be used in some designs to limit downward movement of the bridge structure so as to avoid opposed DC control electrodes from touching one another and shorting out. Upon application of the DC control signal, the electric field generated force causes the bridge structure to move downward. When the voltage (and therefore the force) is sufficient, the bridge structure will snap down and make contact with the RF conductor(s). This voltage is called the pull-in voltage. To increase the speed with which the closing action takes place, the applied control voltage may be increased to typically 1.5 times the pull-in voltage, which may be considered within the normal range of applied control signal.  
     [0071] If the voltage is further increased, the force may be sufficient to bend the bridge structure to short out the control electrodes. This voltage is called the second pull-in voltage. The margin between the pull-in voltage and second pull-in voltage may be increased with the provision of stoppers, however with many designs the provision of the central stiffener portion of the bridge structure is sufficient to prevent this shorting when DC control signals within a normal range are applied.  
     [0072] When switch  120  is activated to a closed position, the metallized underportion  153  of bridge structure  127  bears down on a contact area  155  (shown stippled) of conductor  122  to complete the RF circuit between conductors  122  and  123 . In order to improve isolation, and therefore lower RF losses when the switch is open, it is desired that this contact area be as small as practical, while still being able to maintain low ON resistance and concomitantly support the power handling requirements of the application.  
     [0073] In addition, the loss associated with the contact area is a function of the force that can be exerted due to the electric field generated by the applied DC control voltage. A greater contact force will result in a lower resistance contact. This may be accomplished by providing a larger total area of DC control electrode on the substrate. The embodiment of the present invention illustrated in FIGS.  8 A-C meets these objectives of smaller contact area and larger DC control electrode.  
     [0074] Switch  160  includes first and second RF conductors  162  and  163  deposited on substrate  165 . As compared with conductor  122  in FIGS.  7 A-C, conductor  162  is foreshortened at its distal end  168 , resulting in a relatively small contact area  170  with bridge structure  172  when it is activated to close the switch.  
     [0075] The DC control electrode arrangement includes electrode  174  deposited on substrate  165  in a manner that it partially surrounds the end of conductor  162 . That is, electrode  174  is adjacent the sides of conductor  162  in the vicinity of contact area  170  and extends completely around the front of conductor  162  resulting in a greater electrode area as compared with that of FIGS.  7 A-C.  
     [0076] Since the attractive force is directly proportional to the area of the control electrode  174 , this allows either a smaller DC control voltage to be applied to pad  176  to achieve the same force, or with the same applied DC control voltage as in FIGS.  7 A-C, a greater force will be applied, lowering the contact resistance, with a consequent reduction in RF losses.  
     [0077] RF losses are further reduced by the novel design of the second conductor  163 . The conductors for these MEMS switches are actually small transmission lines having a characteristic impedance. In many RF circuits a 50 Ohm transmission line is common, and conductor  163  represents such 50 Ohm transmission line. Direct connection to an adjacent 50 Ohm transmission line may be made without any losses or the conductor may be tapered to match a higher impedance line.  
     [0078] Conductor  163 , which also serves as a DC ground, is bifurcated and includes two end segments  178  and  179  electrically connected to respective support members  180  and  181 . A third electrically conducting support member  182  is positioned on the conductor  163  at a position aligned with conductor  162 . These support members  180 ,  181  and  182  respectively support arms  184 ,  185  and  186  of bridge structure  172 , which also, in accordance with the present invention, includes a central stiffener portion  190 .  
     [0079] When switch  160  is activated to a closed position by application of a DC control signal to pad  176 , the electrostatic attraction between DC electrode  174  and opposed electrode portion  192  of the underside of metal bridge structure  172  causes bridge structure  172  to snap down to make contact with contact area  170 . RF current then flows into conductor  163  through three parallel paths comprised of segment  178 , via arm  184 , segment  179 , via arm  185  and through the central portion of conductor  163 , via arm  186 . Each path presents a certain resistance, however the equivalent resistance of three paths in parallel is smaller than any single path. Therefore the conductor design reduces resistance and lowers RF losses.  
     [0080] Switch  196  in FIGS.  9 A-C, includes a first conductor  198 , which is bifurcated at its distal end, and a second conductor  199  deposited on substrate  200 . Bridge structure  202 , having central stiffener portion  203 , includes arms  204 ,  205  and  206  connected to respective electrically conducting support members  210 ,  211  and  212 . This latter support member  212  is electrically integral with second conductor  199 . With this arrangement bridge structure  202  is suspended above segments  214  and  215  of the bifurcated end of conductor  198 .  
     [0081] Positioned between segments  214  and  215  of conductor  198  is the DC control electrode  218  having a relatively large area, and connected to pad  219  to which a DC control signal is applied to activate the switch to a closed position. When the DC control signal is provided, the electrostatic attraction between electrode  218  and the opposed electrode portion  222  on the underside of bridge structure  202  rapidly brings the bridge structure  202  into electrical contact with contact area  224 , to thus complete the RF circuit. The relatively small contact area  224  (shown stippled), in conjunction with the relatively large area control electrode  218  ensures that fringe capacitance is small and that the closing force is sufficiently high to minimize contact resistance, so that switch  196  has low RF losses.  
     [0082] A significant increase in the ratio of DC electrode area-to-contact area is achieved with the embodiment of the invention illustrated in FIGS.  10 A-C. Switch  230  is of the type illustrated in FIG. 4A wherein a contacting member is supported and positioned over both first and second conductors.  
     [0083] More particularly, and with additional reference to FIG. 10Aa, switch  230  includes a substrate  231  upon which is deposited first and second spaced apart conductors  232  and  233 . These conductors are mirror images of one another and conductor  232  has a first section which may be a 50 Ohm section  232   a , and a tapered section  232   b . Section  232   b  tapers down to a higher Ohm section  232   c  which, in turn, tapers down to two small contact areas  234  and  235  via tapered sections  232   d  and  232   e , respectively.  
     [0084] Similarly, conductor  233  may be a 50 Ohm section  233   a , and includes a tapered section  233   b . Section  233   b  tapers down to a higher Ohm section  233   c  which, in turn, tapers down to two small contact areas  236  and  237  via tapered sections  233   d  and  233   e , respectively.  
     [0085] A DC control electrode  240  occupies the space between conductors  232  and  233  and further partially surrounds the contact areas  234  to  237 . This is accomplished with the provision of four notches  244  to  247 , in the sides of electrode  240 , as best illustrated in FIG. 10Aa.  
     [0086] Bridge structure  250 , including central stiffener portion  251  is suspended over the ends of conductors  232  and  233  by means of arms  254  and  255  connected to respective support members  256  and  257 . At least one of these support members  256  and  257  is electrically conducting to serve as a DC ground. Support member  256  is symbolically shown as the ground return, through pad  258 . When the switch  230  closes, bridge structure  250  becomes part of the RF circuit and to effect isolation and to reduce potential RF losses, line  259 , leading from support member  256  to pad  258 , is fabricated to be of extremely high resistance.  
     [0087] A DC control signal applied to pad  260  causes electrostatic attraction between electrode  240  and its opposed electrode  261 , constituted by a portion of the underside of bridge structure  260 . When the contact areas  234  and  235  are electrically connected to contact areas  236  and  237  by means of the bridge structure  250 , switch  230  will conduct RF current between the first and second conductors  232  and  234  with relatively little resistive losses. This low loss feature is attributable to the excellent contact resulting from the large attractive force created by the relatively large control electrode  240 .  
     [0088] It is to be noted that the dimensions of the components of the various switch embodiments described herein have been greatly exaggerated for clarity. Typical thicknesses for the various components are, by way of example as follows:  
     [0089] Substrate:—500 μm  
     [0090] DC electrode:—0.1 μm  
     [0091] Conductors:—1.0 μm  
     [0092] Support member:—3.0 μm  
     [0093] Bridge structure:—1.0 μm  
     [0094] Central stiffener portion:—1-2 μm  
     [0095] It is an objective of the switch design that the contacting conductors and bridge structure are fabricated of metals chosen so they have excellent wear properties and conductivity, that is, low electrical resistance. In addition these components should exhibit high thermal conductivity, resistance to oxidation, and the bridge structure metal and conductor metal should have dissimilar melting points. The basic conductor and bridge structure metals may be of silver or gold, by way of example, with suitable respective coatings such as ruthenium, tungsten or molybdenum, to name a few, so as to meet the above objectives.  
     [0096] The foregoing detailed description merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.