Abstract:
A relay for switching an electrical signal includes switching elements, an actuator for closing the switch, and an actuator for opening the switch, the latter two of which are mechanically decoupled when the relay is in a mechanical rest position. When a relay close signal is applied, the closing actuator electrostatically drives the switching elements to complete a signal path between two terminals for the switched signal. In the process of closing the switch, the opening actuator remains stationary, i.e., no mass is displaced. Application of a switch open signal electrostatically drives the opening actuator, optionally in combination with a mechanical restoring force on the closing actuator, to open the switch to break the signal conduction path for the switched signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from U.S. Provisional Application Ser. No. 60/342,049 filed Dec. 18, 2001 and entitled METHODS FOR MICROSTRUCTURE MANUFACTURE AND APPARATUSES USING SAME, incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates generally to the field of relay switches, and more particularly to a micro-switch used to switch an electrical signal such as an RF signal. 
   BACKGROUND OF THE INVENTION 
   MEMS switches have utility in a wide variety of applications, including instrumentation equipment, optical applications, wireless communications, and telephony. There are various MEMS switches known in the prior art, but they usually have only a closing actuator but not an opening actuator, so they can become stuck when the closing signal is terminated. Another problem is that the RF signal being switched provides an electrostatic force which sometimes causes the switch to close even when no control signal is applied. Still another problem is that electrostatic pull-in is typically used, which is unstable. A further problem is a switch remaining closed after the control signal has been terminated, due to surface effects on the switch contact surfaces. 
   SUMMARY OF THE INVENTION 
   Briefly stated, a relay for switching an electrical signal includes switching elements, an actuator for closing the switch, and an actuator for opening the switch, the latter two of which are mechanically decoupled when the relay is in a mechanical rest position. When a relay close signal is applied, the closing actuator electrostatically drives the switching elements to complete a signal path between two terminals for the switched signal. In the process of closing the switch, the opening actuator remains stationary, i.e., no mass is displaced. Application of a switch open signal electrostatically drives the opening actuator, optionally in combination with a mechanical restoring force on the closing actuator, to open the switch to break the signal conduction path for the switched signal. 
   According to an embodiment of the invention, a relay switch includes a switching element having a signal make condition and a signal break condition between at least first and second signal terminals; a closing actuator operatively responsive to a close signal applied to a close terminal whereby the closing actuator moves from a closing actuator rest position to a closing actuator driven position, wherein application of the close signal to the close terminal drives the switching element into the signal make condition; an opening actuator operatively responsive to an open signal applied to an open terminal whereby the opening actuator moves from an opening actuator rest position to an opening actuator driven position, wherein application of the open signal to the open terminal drives the switching element into the signal break condition; wherein the closing and opening actuators are mechanically decoupled when in their respective rest positions; and wherein the opening actuator remains in its rest position when the close signal is applied to the close terminal. 
   According to an embodiment of the invention, a relay switch includes a shorting bar; making means for making electrical connection between a signal-in terminal and a signal-out terminal; breaking means for breaking electrical connection between the signal-in terminal and the signal-out terminal; the making means including a first actuator which moves in a first direction to move the shorting bar into contact with the signal-in terminal and the signal-out terminal when a close signal is applied to the first actuator; and the breaking means including first spring means which moves the first actuator in a second direction when the close signal is terminated, thereby moving the shorting bar out of contact with the signal-in terminal and the signal-out terminal, the second direction being opposite to the first direction. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a top view of a relay switch according to an embodiment of the invention; 
       FIG. 2  shows details of the contact structure between a push actuator and the signal terminals of the relay switch of the present invention; 
       FIG. 3  shows details of the contact region between the shorting bar of the push actuator and the signal terminals of the relay switch of the present invention; 
       FIG. 4A  shows metallized paths on the relay switch according to an embodiment of the present invention; 
       FIG. 4B  shows metallized paths on the relay switch according to an embodiment of the present invention; 
       FIG. 5  shows details of the interlocking “T” of the relay switch of the present invention; 
       FIG. 6A  shows a cross-section of a step in constructing a silicon dioxide beam used in fabricating the relay switch of the present invention; 
       FIG. 6B  shows a cross-section of a step in constructing a silicon dioxide beam used in fabricating the relay switch of the present invention; 
       FIG. 6C  shows a cross-section of a step in constructing a silicon dioxide beam used in fabricating the relay switch of the present invention; and 
       FIG. 6D  shows a cross-section of a step in constructing a silicon dioxide beam used in fabricating the relay switch of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a relay switch  10  includes a pull actuator  12  with an open terminal  14  connected to a first end, a push actuator  16  with a close terminal  18  connected to a first end, and a signal-in terminal  20 , a signal-ground terminal  22 , and a signal-out terminal  24  connected to a second end of push actuator  16 . All of these components are preferably etched out of a silicon wafer.  FIG. 1  shows relay switch  10  in its mechanical rest position, when no signals are applied to either open terminal  14  or close terminal  18 . The second end of push actuator  16  includes a plurality of switch contacts  26  and a conductor  28  or other signal transmission portion which shorts signal-in terminal  20  to signal-out terminal  24  when relay  10  is closed. When relay switch  10  is open, switch contacts  26  disengage from signal-in terminal  20  and signal-out terminal  24 . 
   Pull actuator  12  is preferably formed as a generally rectangular frame  29 , with sides  30   a ,  30   b  and sides  32   a ,  32   b . A center backbone  34  extends from side  32   a  to side  32   b . Frame  29  and center bar  34  are preferably formed from truss-structured silicon members. At the four corners of frame  29  are four flexure arms (springs)  36   a ,  36   b ,  36   c , and  36   d  that allow motion of the pull actuator in the directions of arrow a. Arms  36   a ,  36   b ,  36   c , and  36   d  are anchored to the substrate by anchors  38   a ,  38   b ,  38   c , and  38   d  respectively. Anchors  38   a ,  38   b ,  38   c , and  38   d  are preferably part of the silicon substrate. 
   Pull actuator  12  includes a number of capacitive/electrostatic plates  40  that extend from center backbone  34  to side  30   b  in one half of frame  29  and from center backbone  34  to side  30   a  in the other half of frame  29 . These plates are preferably formed of truss-structured silicon beams. Because they are anchored at both ends against the larger frame members of pull actuator  12 , a single truss-structured silicon beam is sufficient to provide the stiffness, i.e., resistance to bending, that they require. 
   Between each pair of capacitive/electrostatic plates  40  of pull actuator  12 , a plurality of ground plates  42  extend vertically from the silicon substrate (not shown), which is grounded, in capacitive opposition to plates  40  of actuator  12 . Ground plates  42  are formed from a portion of the original silicon wafer that is neither etched away nor released from the substrate. Plates  40  of pull actuator  12  and ground plates  42  together form a parallel plate actuator. The electrostatic force between plates  40  of pull actuator  12  and ground plates  42  provides the force that displaces pull actuator  12  to the left of arrow a. Displacement of pull actuator  12  to the right of arrow a occurs when the electrostatic force between plates  40  and plates  42  disappears and the spring force of flexure arms  36   a ,  36   b ,  36   c , and  36   d  returns pull actuator  12  to its rest position as shown in  FIG. 1 . 
   Note that the spacing between each plate  40  of pull actuator  12  and the adjacent ground plate  42  to its left is considerably larger than between each plate  40  and the adjacent ground plate  42  to its right. This difference in capacitance gap, coupled with the 1/R 2  falloff in field strength with distance from the two respective ground plates, provides a differential force that causes pull actuator  12  to pull to the left when open terminal  14  is energized. 
   In similar fashion to pull actuator  12 , push actuator  16  preferably includes a generally rectangular outer frame  44  and a center backbone  46  preferably formed from truss-structured silicon members. A connecting truss  58  preferably provides support between center backbone  46  and a shorting bar  64 . Frame  44  includes opposing sides  50   a ,  50   b  and opposing sides  52   a ,  52   b.    
   Capacitive/electrostatic truss-structured silicon plates  48  extend from center backbone  44  to sides  50   a  and  50   b . These plates are preferably formed of truss-structured silicon beams; because they are anchored at both ends against the larger frame members of push actuator  16 , a single truss-structured silicon beam is sufficient to provide the stiffness, i.e., resistance to bending, that they require. 
   At the four corners of frame  44  are four flexure arms (springs)  54   a ,  54   b ,  54   c , and  54   d  that allow motion of push actuator  16  in the directions of arrow b. Arms  54   a ,  54   b ,  54   c , and  54   d  are anchored to the substrate by anchors  56   a ,  56   b ,  56   c , and  56   d  respectively, which are preferably part of the silicon substrate. As with pull actuator  12 , ground plates  42  extend up from the silicon substrate (not shown) to provide opposing electrostatic plates so that electrostatic forces may be developed that cause push actuator  16  to generate a force to the right as shown by arrow b in  FIG. 1 . Displacement of push actuator  16  to the left of arrow b occurs when the electrostatic force between plates  48  and plates  42  disappears and the spring force of flexure arms  54   a ,  54   b ,  54   c , and  54   d  returns push actuator  16  to its rest position which is shown in  FIG. 1 . 
   The spacing between each plate  48  of push actuator  16  and the adjacent ground plate  42  to its right is considerably larger than between each plate  48  and the adjacent ground plate  42  to its left. This difference in capacitance gap, coupled with the 1/R 2  falloff in field strength with distance from the two respective ground plates, provides a differential force that causes push actuator  16  to push to the right when close terminal  18  is energized with a close signal from about 0 to about 100 V depending on the particular embodiment design. 
   When relay switch  10  closes, push actuator  16  moves from its rest position to the right to close switch contacts  26  against signal-in terminal  20  and signal-out terminal  24 . When opening switch  10 , pull actuator  12  moves from the rest position to the left, dragging push actuator  16  with it. Thus, in normal operation, the range of motion of pull actuator  12  is from the rest position shown in  FIG. 1  to a pull off position slightly to the left of the rest position. The range of motion for push actuator  16  is from the rest position shown in  FIG. 1  to a closed position slightly to the right, sufficient to close the switch contacts. In some embodiments, push actuator  16  may in addition move slightly to the left of the rest position during the time pull actuator  12  is pulling off. Signal-in terminal  20 , signal-out terminal  24 , and signal-ground terminal  22  remain stationary. An electrical shield  60  is part of the grounded silicon substrate. Shield  60  is made of two silicon structures that project up from the silicon substrate, analogous to ground plates  42  interleaved with actuator plates  40 ,  48  to provide a ground isolation plane which electrically shields the drive part of push actuator  16  from the contact region. 
   The movement of pull actuator  12  and push actuator  16  are controlled at their respective four corners by flexure arms  36   a–d  for pull actuator  12  and flexure arms  54   a–d  for push actuator  16  as described above. One end of each flexure arm is anchored to the underlying silicon substrate by anchors  38   a–d ,  56   a–d , while the other end is affixed to the respective corner of the respective actuator. The flexure arms operate in bending to allow pull actuator  12  and push actuator  16  to move through their respective ranges of motion and to restore the two actuators to their rest positions. The corner members are stiff enough to apply a restoring force to return actuators  12 ,  16  to the rest position when the open or close signal is removed, but compliant enough to allow the electrostatic forces developed between actuator plates  40 ,  48  and ground plates  42  to move actuators  12 ,  16  in the desired direction to make or break the contacts with the signal-in and signal-out terminals  20 ,  24 . 
   In making and breaking contacts  26 , it is desirable to minimize the amount of mass that has to be moved by the either push actuator  16  or pull actuator  12 , so that the make or break occurs as quickly as possible. An interlocking “T” interface  66  centered near center backbone  34  of pull actuator  12  and center backbone  46  of push actuator  16  allows push actuator  16  to make contacts  26 , while pull actuator  12  remains stationary and unconnected electrically. In the preferred embodiment, “T” interface  66  includes a center arm  70  extending from center backbone  34  of pull actuator  12 . At one end of center arm  70 , two side arms  72  extend perpendicular to the axis of center arm  70 , thus forming the upper part of the “T”. Two “L” arms  68  extend from side  52   a  of frame  44  of push actuator  16 , positioned to either side of center backbone  46  of push actuator  16 . “L” portions  74  of “L” arms  68  extend towards each other and slightly overlap side arms  72 . In an alternate embodiment, the “L” arms are replaced by “T” arms so that a plurality of “T” arms on side  32   b  of frame  29  of pull actuator  12  interlock with a plurality of “T” arms on side  52   a of frame  44  of push actuator  16 . The last arms on either side of the series of “T” arms are “L” arms. 
   The space between side arms  72  and “L” portions  74  is preferably slightly more than the travel distance of push actuator  16 . Thus, at the time that contacts  26  are closed, the gap between side arms  72  and “L” portions  74  is nearly, but not entirely, closed. When it is time to break the contact between signal-in and signal-out terminals  20 ,  24 , the close drive signal on push actuator  16  is turned off. Thus, the electrostatic force generated between ground plates  42  and capacitor/electrostatic plates  48  of push actuator  16  ceases, while the restoring force of the four corner flexure arms  54   a–d  urges push actuator  16  to its rest position, thereby carrying shorting bar  64  to the left to break contacts  26 . 
   In addition to removing the close signal to push actuator  16 , an open signal may be applied to pull actuator  12 . When this is done, the electrostatic forces generated between plates  40  of pull actuator  12  and ground plates  42  tend to move pull actuator  12  to the left in  FIG. 1 . If any gap remains between side arms  72  and “L” portions  74 , the first part of the motion of pull actuator  12  closes the gap between side arms  72  and “L” portions  74 . Further motion of pull actuator  12  provides additional force to push actuator  16  to urge push actuator  16  leftward, back into its rest position, and to break contacts  26  at signal terminals  20 ,  24 . 
   In the preferred embodiment, interlocking “L” arms  76  at the adjacent corners of frames  29  and  44  provide balance to the interaction between pull actuator  12  and push actuator  16 . 
   The gaps in the mechanical coupling interface are preferably adjusted to optimize the performance of pull actuator  12 . The gap is preferably large enough so that the coupling interface does not engage while push actuator  16  closes contacts  26 . In this manner, none of the force of push actuator  16  is used to displace springs (flexure arms)  36   a–d  of pull actuator  12 . However, once pull actuator  12  is energized and push actuator  16  is de-energized, pull actuator  12  should only move a short distance to engage the coupling interface, thus shortening the time for pull back and minimizing the amount of force that pull actuator  12  uses to displace flexure arms  36   a–d.    
   Referring to  FIG. 2 , details of the contact structure of push actuator  16  and signal-in terminal  20 , signal-ground terminal  22 , and signal-out terminal  24  are shown. Conductor  28  is preferably affixed to shorting bar  64 . Shorting bar  64  is preferably made from silicon dioxide to provide significant isolation for the electrical contact between signal-in terminal  20  and signal-out terminal  24 . Shorting bar  64  is connected to frame  44  by two arms  62 . Arms  62  are each preferably made from a single silicon dioxide beam. Arms  62  provide stability for shorting bar  64  in the left-to-right dimension of  FIG. 1 . Because shorting bar  64  and frame  44  of push actuator  16  are potentially made of different materials that may have different coefficients of thermal expansion, the relatively thin arms  62  at the two ends of shorting bar  64  allow for differential expansion of shorting bar  64  relative to push actuator frame  44 . 
   Silicon dioxide beams may be formed using an alternative method. Trenches in the pattern of the ultimately desired beam pattern may be etched into the silicon wafer, converted to silicon dioxide, and then etched using the same techniques that are used to etch and release the remainder of the device. This technique may also be used to form the beams of shorting bar  64  and connecting truss  58 . 
   Conductor  28  is preferably a metal coating  84  ( FIG. 4 ) on both the top side of shorting bar  64  and the side of shorting bar  64  with contacts  26  on it. In similar fashion, signal-in terminal  20  and signal-out terminal  24  have metal coatings on their top sides and on the sides with contacts  26  on them. 
   Signal-ground terminal  22  is disposed between signal-in terminal  20  and signal-out terminal  24 . Signal-ground terminal  22  is a grounded region of metal that serves much the same purpose as a ground plane in a printed circuit board, i.e., to isolate any stray capacitances and/or inductances between signal-in terminal  20  and signal-out terminal  24 . The signal lines are preferably kept a certain distance apart to prevent coupling and cross-talk. In an embodiment of this invention, this distance is 385 microns and is shown in  FIG. 2  as distance c. 
   Referring to  FIG. 3 , the contact region between shorting bar  64  and either signal-in terminal  20  or signal-out terminal  24  is shown. Contacts  26  on shorting bar  64  and on signal terminals  20 ,  24  are projections that extend from shorting bar  64  and signal terminals  20 ,  24 . Contacts  26  extend toward each other and preferably meet at a slightly angled pairs of faces. The angling of the faces allows small misalignments to be accommodated, and may provide some “scrubbing motion” as the contacts close to improve electrical contact. 
   Shorting bar  64  and signal terminals  20 ,  24  are preferably formed of silicon dioxide beams with metal on the top and sides. The silicon dioxide construction provides good electrical isolation between the RF signal being switched and the actuator control voltage. Having silicon in shorting bar  64  instead of silicon dioxide could permit a capacitive coupling of the RF signal into the close signal, which would be detrimental to the performance of the switch. However, because it is essential that contact resistances be very low in a relay or switch, metal is preferably deposited on the silicon dioxide beams either by vapor deposition or by sputtering to form a metal conductive layer. High conductivity metals such as gold are preferably used. After the RF signal passes through the first switch contact, through shorting bar  64 , and through the second switch contact, it is routed through a set of hybrid beams and then back onto a planar surface. At this point is can be routed off of the chip through an output bond pad. In an alternate embodiment, portions of signal terminals  20 ,  24  are made of a composite beam having a silicon core with an oxidized silicon layer on the top and sides of the beam. 
   Referring to  FIG. 4A , the metallized portions of push actuator  16  and pull actuator  12  are shown. The close signal comes in on close terminal  18  and for convenience is routed along the top of flexure arm  36   b , the top of part of side  30   b  of frame  29 , then along the top of flexure arm  36   c  and the top of flexure arm  54   b . Metallic paths  80  and  81  continue along part of the top of sides  50   b  and  50   a  of frame  44  to provide voltage to plates  48 . In like fashion, the open signal comes in on open terminal  14  and is routed along the top of flexure arm  36   a  and then portions of sides  30   a  and side  32   a  of frame  29 . Metallic paths  82  and  83  continue along a portion of the top of sides  30   a  and  30   b  of frame  29  to provide voltage to plates  40 . Optional metal paths  77 ,  79  on center backbones  34 ,  46 , respectively, provide increased conductivity between plates  40  and likewise between plates  48 . When the silicon being used is of low conductivity, optional metal paths  77 ,  79  become preferable. In  FIG. 4B , another embodiment of wiring is shown in which plates  48  are electrically connected across center backbone  46  by jumpers  78 , while plates  40  are electrically connected across center backbone  34  by jumpers  75 . Note that metal paths are preferably laid down along the tops of all flexure arms so that the deformation characteristics are the same for all arms. 
   Referring to  FIG. 5 , a top view of pull actuator  12  and push actuator  16  shows the beam structure of the actuators. Push actuator  12  and pull actuator  16  are both preferably formed by etching the structure out of a silicon wafer. The wafer may have been selectively oxidized so that some portions of the actuators are formed of silicon dioxide. The processes for forming the actuators and isolation joints, and releasing them from the underlying silicon substrate, are discussed in U.S. Pat. No. 6,239,473 (Adams et al.) entitled TRENCH ISOLATION PROCESS FOR MICROELECTROMECHANICAL DEVICES; U.S. Pat. No. 5,719,073 (Shaw et al.) entitled MICROSTRUCTURES AND SINGLE-MASK, SINGLE CRYSTAL PROCESS FOR FABRICATION THEREOF; U.S. Pat. No. 5,846,849 (Shaw et al.) entitled MICROSTRUCTURE AND SINGLE MASK, SINGLE-CRYSTAL PROCESS FOR FABRICATION THEREOF; U.S. Pat. No. 6,051,866 (Shaw et al.) entitled MICROSTRUCTURES AND SINGLE MASK, SINGLE-CRYSTAL PROCESS FOR FABRICATION THEREOF; S. G. Adams, et. al., “Single-Crystal Silicon Gyroscope with Decoupled Drive and Sense”, in  Micromachined Devices and Components V , Patrick J. French, Eric Peeters, Editors,  Proceedings of SPIE  Vol. 3876, 74–83 (1999); K. A. Shaw, Z. L. Zhang, and N. C. Macdonald, “SCREAM I: A single mask, single-crystal silicon process for microelectromechanical structures”,  Sensors and Actuators A , vol. 40, pp. 63–70 (1994); and Z. L. Zhang, N. C. MacDonald, “A rie process for submicron, silicon electromechanical structures”,  J. Micromech. Microeng. , v2, pp. 31–38 (1992), all of which are incorporated herein by reference in their entirety. 
   Some portions of the original silicon wafer are preferably left in place to form mechanical and electrical portions of the relay structure. For example, the underlying silicon remains as a substrate to form the mechanical foundation for the entire structure, i.e., to hold all of the moveable components and terminals in place relative to each other. 
   Referring to  FIGS. 6A–6D , a cross-section of a silicon dioxide beam  100  is shown in various stages of construction. Shorting bar  28  ( FIG. 1 ) may be formed, in part or in whole, of such silicon dioxide beams. A region of silicon dioxide is preferably formed by first cutting a number of parallel trenches  103  in a silicon substrate  101 , using techniques the same as or similar to those in U.S. Pat. No. 6,239,473 mentioned above. In  FIG. 6B , silicon substrate  101  is exposed to oxidizing agents and temperatures so that the remaining silicon oxidizes to form silicon dioxide  105 . As the silicon oxidizes, the volume increases by somewhere between a factor of two and a factor of three. Therefore, the widths of the initial trenches  103  are carved at a width relative to the remaining silicon projections, so that as those silicon projections oxidize, the trenches essentially fill themselves. To fill any residual trench between the faces of the oxide growth fronts, a layer of silicon dioxide  107  is preferably deposited, using such known techniques as plasma enhanced CVD (PECVD) or low temperature oxidation (LTO), on silicon dioxide  105  to fill in whatever trenches remain. As shown in  FIG. 6C , metal is laid atop the silicon dioxide beam  100  so formed. As shown in  FIG. 6D , silicon substrate  101  optionally remains to be used as a conductor, while metal  107  atop the silicon dioxide  105 ,  107  is used as a conductor for another signal. Because silicon dioxide is a good insulator and forms a relatively large separation between the silicon and the metal, this technique provides good electrical isolation between metal  107  and silicon substrate  101  for very high voltage stand-off capability, and reduces stray capacitances and stray inductances. 
   This technique is preferably used in relay  10  for forming large regions of silicon dioxide. For example, signal-in and signal-out terminals  20 ,  24  are preferably formed by laying a metal layer atop the thick layer of silicon dioxide, so that RF signals or any electrical signals in the metal layer of the terminals are isolated from the silicon substrate. 
   In an RF switch, high frequency signals must be well separated from ground and from each other to prevent loss and a high switch-off impedance, respectively. Starting with the entry of a high frequency signal to the input terminal, the signal must contact a bond pad to interface with the outside world. This bond pad and the wires leading to the relay mechanism are preferably constructed from metal on top of an oxide that in turn is on top of the silicon substrate. This metal-oxide-silicon structure forms a large capacitor that introduces losses to ground. The loss can be reduced by using a high resistivity substrate and/or dramatically increasing the thickness of the oxide. The high resistivity substrate works well in some applications and is common in the industry. Increasing the oxide thickness is not easy to do beyond a few microns. By using closely spaced oxidized trenches, with or without additional deposited oxide, a very thick oxide layer can be effectively produced to reduce the capacitance. An added benefit is that the breakdown strength is also increased dramatically, so that much higher voltages can be placed on this layer than in the case of a high resistivity substrate. 
   This reduced capacitance using the oxidized trenches has application in capacitive sensors. When this parasitic capacitance is reduced, so is one of the primary noise sources in capacitance based displacement sensors. 
   When the signal is on-chip, it can be routed as needed. When the signal path needs to go on top of released structures, a transition needs to be made. On flat surfaces, the metal is easy to pattern and align with other structures because the tolerances are large, e.g., greater than a few microns. On released structures the tolerances need to be submicron and therefore are dealt with in another fashion. Typically metal is patterned on release structures at the same time that the silicon structure is defined; this is referred to as a self-aligned process. The problem with using the typical process in the present invention is that the traditional method described in the referenced patents and publications has very little voltage stand-off protection along the sides of the oxide beams. 
   To avoid this problem, a blend of the traditional silicon beam fabrication method and the trench isolation process is used. By placing the oxide trenches in a location coincident with the sides of the silicon beam to be fabricated, a silicon beam structure with very thick sidewalls is obtained which has a very high voltage standoff capability. Furthermore, the metal can be patterned on top of this hybrid beam as a self-aligned part of the fabrication process. At this point, the metal can be routed on top of a hybrid silicon-thick-oxide-sidewall beam that is released from the substrate. 
   To form contact surfaces for an in-plane relay switch, metal must be deposited on the sides of a beam. One could try to do this using conventional techniques by attempting to blanket coat the wafer with metal and then pattern the metal on released structures. In all but a few special cases, this method is futile in that the released structures are destroyed. One aspect of the present invention is the use shadow-masking techniques to deposit metal only in the switch contact region. This coats the released structures and the silicon floor underneath the structures. Any metal or stack of metals that one chooses can be deposited without the need for further patterning. 
   The hybrid beams preferably run into the switch contact region to contact electrically the metal traces that run on top of the hybrid beams and the metal deposited using the shadow mask. The metal deposited through the shadow mask then contacts the metal traces on the hybrid beams and coats the contact surfaces for the relay switch. 
   The length of some of the trenches is preferably adjusted to prevent the sputtered contact metal from shorting. There are trenches in the contact region which run alongside the three signal lines  20 ,  22 ,  24 . These trenches provide electrical isolation between the signal lines when the contact metal is deposited through the shadow mask. If the trenches aren&#39;t there to provide a physical barrier, the metal runs along the top surface of the wafer and shorts the signal lines. Unfortunately, the shadow mask deposition process does not provide a well-defined edge to the deposited metal pattern. The thickness of the deposited metal tapers off to zero beyond the edge of the shadow mask opening. How sharply it tapers off depends on the physical separation between the shadow mask and the wafer. Therefore, if the trenches do not extend far enough beyond the edge of the shadow mask opening, the electrical isolation is compromised by this “over spray” metal. Thus, the length of the trenches must be adjusted to prevent this shorting. 
   In alternate embodiments, the contacts are arranged so that they do not make physical contact, and thus do not pass current, when the contacts are closed, but instead have a very small gap. Alternatively, a thin dielectric film is deposited on the metal contacts to prevent electrical contact. Either configuration allows for the signal to be capacitively coupled to the contact bar, especially at higher frequencies. This arrangement may reduce losses through the switch. 
   In the switch contact, the opposing structures are typically made of the oxidized silicon beams. This permits excellent electrical separation between the actuators used to close the switch and the RF signals that run through the switch contact region. 
   While the present invention has been described with reference to a particular preferred embodiment and the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the preferred embodiment and that various modifications and the like could be made thereto without departing from the scope of the invention as defined in the following claims.