Abstract:
A micro-electro-mechanical system (MEMS) slotline switch includes a slotline transmission line structure defined on top of substrate, a doubly-anchored conductive beam disposed perpendicular to, and above slotline so that there is a certain spacing between the beam and the slotline, a second conductive contact attached to the beam directly above the slot of the slotline a bottom conductive contacts defined on bottom surface of substrate and forming parallel-plate capacitor with conductive beam, conductive traces defined on the bottom surface of the substrate forming a microstrip-to-slotline transition for coupling signals in microstrip line to the slotline, and beam and bottom conductive contacts being spaced apart, and the beam being continuously movable when a voltage is applied between the beam and the bottom conductive contacts.

Description:
PRIORITY 
   The present invention claims priority under 35 USC 119 for the provisional application filed Feb. 17, 2004, Ser. No. 60/545,032 

   TECHNICAL FIELD 
   The present invention relates generally to micro-electro-mechanical systems (MEMS) devices and methods. More particularly, the present invention relates to a switch apparatus and method utilizing MEMS technology. 
   BACKGROUND ART 
   Micro-electro-mechanical systems (MEMS) devices and methods are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Specifically, a MEM switch can be fabricated utilizing MEMS technology. MEM switches known in the prior art are of two types, namely, the series and shunt types. The series type  10 ,  FIG. 1 , consists of a beam  16  cantilevered from a switch base, or substrate  24 . The beam  16  has an electrode  14  disposed on it, acts as one plate of a parallel-plate capacitor and contains under its tip a contact  20 . A voltage, known as an actuation voltage, is applied between the beam  16  and an electrode  22  on the switch base  24 . In the switch-closing phase, or ON-state, the actuation voltage exerts an electrostatic force of attraction on the beam  16  large enough to overcome the stiffness of the beam. As a result of the electrostatic force of attraction, the beam  16  deflects and the contact under its tip  20  makes a connection that bridges the gap in a transmission line  18  running under it, closing the switch. Ideally, when the actuation voltage is removed, the beam  16  will return to its natural state, breaking its connection with the signal line  18  and opening the switch. 
   The shunt type MEM switch  30 ,  FIG. 2 , consists of a doubly-anchored beam (bridge) or membrane  32  anchored on a substrate  42  and disposed across a set of ground-signal-ground (GSG) traces  40 ,  38 ,  34 , respectively, known as a coplanar waveguide (CPW) transmission line. In its normal state, the “pass” or ON-state, the bridge  32  is undeflected and the amplitude of the signal propagating down the CPW line and entering at its input  44 , is minimally attenuated by capacitive coupling to the bridge  32  and, through it, to ground  40 ,  34 , after passing exiting at its output  46 . An actuation voltage applied between the bridge  32  and an insulation-protected electrode  36  disposed on the CPW&#39;s signal conductor underneath it  38 , exerts an electrostatic force of attraction on the bridge  32  large enough to overcome the stiffness of the beam. As a result the bridge deflects and substantially increases the capacitive coupling of the signal to the bridge  32  and ground  40 ,  34 . The amplitude of the signal propagating down the signal line  38 , which enters at the input  44 , after it passes the deflected bridge  32  and exits at the output  46 , is now maximally attenuated and the switch may be said to be in its “blocking” or OFF-state. Ideally, when the actuation voltage is removed, the beam  32  will return to its natural state, breaking its connection with the signal line  38 . 
   One problem with these switches is that the deflected-to-undeflected phase, or OFF-state in the series type, and ON-state in the shunt type, is not directly controlled, however, and relies on the forces of nature embodied in the spring constant of the beam to bring the beam to the undeflected state. However, the forces of nature are not always predictable and therefore unreliable. 
   For instance, in some cases once the actuation voltage is removed, stiction forces, (forces of attraction that cause the beam to stick to the contact electrode), between the beam and the contact electrode overcome the spring restoring forces of the beam. This results in the beam sticking to the contact electrode and keeping the beam down when, in fact, it should be undeflected. Prior art cantilever/bridge type switches have no mechanism to overcome stiction forces upon deflecting down. 
   Another problem associated with prior art switches is a problem intrinsic to the beam&#39;s change of state from undeflected to deflected. The operation of the beam is inherently unstable. When deflecting, the beam deforms gradually and predictably, up to a certain point, as a function of the actuation voltage being applied to the switch. Beyond that point, control is lost and the beam&#39;s operation becomes unstable causing the beam to pull-in, i.e., to come crashing down onto the secondary electrode. This causes the beam to stick as described above, or causes premature deterioration of the contact electrode. Both conditions impair the useful life of the switch and result in premature failure. 
   There is a need for a MEM switch that overcomes the problems associated with prior art cantilevered- and bridge-type switches. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention will now be explained with reference to the accompanying drawings, of which: 
       FIG. 1  is a perspective view of a prior art series type MEM switch  10 ; 
       FIG. 2  shows an end view of a prior art shunt type MEM switch  30 ; 
       FIG. 3  shows a top view of a prior art shunt type MEM switch  50 ; 
       FIG. 4  is a perspective view of a satellite system  60  having microwave circuits  66  that utilize slotline MEM switches in accordance with one embodiment of the present invention; 
       FIG. 5  is an end view of a slotline MEMS switch  70  in accordance with an embodiment of the present invention; 
       FIG. 6  is a top view of  200  slotline MEMS switch  70  switch in accordance with an embodiment of the present invention; 
       FIG. 7  is a top view  300  of slotline MEMS switch  70  switch with tapered beam  304  in accordance with an embodiment of the present invention; 
       FIG. 8  is a close-up view  400  of the bridge  72  of  FIG. 5  in its down position; 
       FIG. 9  is top and side views  700  of a second embodiment of this invention; 
       FIG. 10  is top and cross-section views of a blocking contact and beam of this invention; 
       FIG. 11  is a top view  500  of a single-pole double-throw switch using the slotline MEM switches  508 ,  510  of this invention; 
       FIG. 12  is a top view  600  of a fundamental switched-line phase shifter bit with propagation via the reference path  606 ,  610 ,  614 , making use of the slotline MEM switches  608 ,  612 ,  626 ,  632  of this invention; 
       FIG. 13  is a cross-sectional view of another switch of the present invention; 
       FIG. 14  is a cross-sectional view of yet another switch of the present invention; 
       FIGS. 15-21  illustrate a process of forming the switch of the present invention; 
       FIG. 22  shows a flow chart of the process of forming the switch of the present invention; and 
       FIG. 23  illustrates a cross-sectional view of a further switch of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 4 , a perspective view of a satellite system  60  in accordance with one embodiment of the present invention is illustrated. The satellite system  60  of comprised of one or more satellites  62  in communication with a ground station  64  located on the Earth  68 . Satellite  62  relies upon wireless communication to send and receive electronic data to perform attitude and position calculations and other functions. Without accurate wireless communication, proper satellite function is hindered and at times adversely affected. Each satellite  62  contains one or more switches  66  to effect signal routing. 
   The conceptual structure of the new MEM switch is shown in  FIGS. 5 through 8 , and its operation is described as follows: A doubly anchored cantilever beam  72  is disposed across the slot of a slotline  82 ,  78 ,  74 . The distance d 0  ( 76 ) from the beam  72  to the slot  78  is chosen such that d 0 &lt;(d 0 +h 1 −h 2 )/3, where h 1  is the substrate thickness  84 , and h 2  is a minimum substrate thickness  88  so that the beam deflection may be controlled continuously without the occurrence of pull-in [Senturia, S. D.,  Microsystem Design  (Kluwer Academic Publishers: Boston, Mass., 2001). Beam  72  width at its center L 1  ( 208 ) and slot width W ( 78 ) set the beam-to-slot parasitic capacitance, which determines insertion loss in the UP state (the thru or passing state) and the shunt capacitance in the DOWN state (the blocking state). L 2  ( 210 ,  212 ) and W r  ( 92 ) set the electrode area, which partly determines the actuation voltage. W b  ( 204 ) adds a degree of freedom to shaping the beam  72 . Thus, the beam may be caused to approach the slot to an arbitrarily close distance without it pulling-in/snapping. In the down position, a part of the beam, the “slot-blocking structure”  90 , blocks the electric field lines across the slot, thus determining the isolation. Notice that, since in the DOWN state the slot-blocking structure  90  intrudes between the two metal stripes  74 ,  82  defining the slot  78 , it is this action that effects the slot field shielding/blocking and not any contact between the beam  72  and the metal stripes  74 ,  82 . The capacitance between the beam  72  and the slotline stripes  74 ,  82 , whose interpolate gaps are  404  and  406 , also contribute to the shunting of the slot and therefore, to the blocking state. In the embodiment of  FIGS. 5 through 8 , the incoming signal is coupled to the slot via a well-known microstrip-to-slotline transition  98 ,  202 , [S. B. Cohn, “Slot Line on a Dielectric Substrate,”  IEEE Trans. Microwave Theory Tech ., Vol. MTT-17, NO. 10, OCTOBER 1969, pp. 768-778], [M. M. Zinieris, R. Sloan, and L. E. Davis, “A Broadband Microstrip-to-Slot-Line Transition,”  Microwave and Optical Tech. Letts . Vol. 18, No. 5, Aug. 5, 1998, pp. 339, 342.] so there is drop-in compatibility with current systems that employ microstrip lines. 
   The maximum capacitance and, thus, the C DOWN /C UP  ratio is determined by the gaps g o ,  404 ,  406  shown in  FIG. 8 , to which one chooses to position the beam  72  upon controlled actuation, and the gaps  410 ,  412  of dimension xW S , where x&lt;&lt;1, between the metal stripes  74 ,  82  and the slot-blocking structure  90 . For d 0 &gt;&gt;W S , ( 76 &gt;&gt; 78 ) C UP  corresponds approximately to the characteristic impedance of the slot  78 . 
   In another embodiment  300  of this invention,  FIG. 7 , the beam  304  is tapered to deal with potential stresses during actuation. 
   Yet, in another embodiment  700  of this invention,  FIGS. 9 and 10 , the beam  714  is disposed longitudinally along the slot  708 , and a recess  722  is made under the slot  708 . The relationship among the beam-to-substrate distance  728 , recess  722  depth, and secondary substrate thickness  730 , are chosen such that no pull-in/snapping of the beam is experienced. A blocking contact  802 ,  FIG. 10 , shunts the slot upon actuation. 
     FIG. 11  shows the implementation of a single-pole double-throw switch using the slotline MEM switch of this invention. The incoming signal entering at the microstrip input  504  is coupled to the slotline  506 .  502  is a slotline an open circuit stub and  524  is a microstrip open circuit stub whose size is adjusted to optimize the properties of the microstrip-to-slotline transisition. Similar function is played by  520  and  528 , and  522  and  520 . When the slotline switches  508  and  510  are UP (in the passing state), the input signal divides equally between slotlines  512  and  514 , and couples back to the microstrip lines, exiting through terminals  516  and  518 , respectively. When switch  508  is DOWN (in the blocking state) and switch  510  is UP (in the passing state), the signal propagating via slotline  506  proceeds to slotline  514  and exits via microstrip terminal  518 . When switch  508  is UP and switch  510  is DOWN, the signal propagating via slotline  506  proceeds to slotline  512  and exits via microstrip terminal  516 . 
     FIG. 12  shows the implementation of a single-bit phase shifter using the slotline MEM switch of this invention. This is the building block of multi-bit phase shifters. The input signal enters through terminal  602  of microstrip line  604 , and exits through terminal  636  with either a minimum reference delay or with a larger delay. The reference delay is experienced through propagation via the shortest path, which consists of the branch containing lines  606 ,  610 , and  614 . The larger delay is experienced through propagation via the longer path, which consists of the branch containing lines  624 ,  628 , and  632 . Signal steering is effected by blocking its passage through one path or the other. For example, to block the passage through the longer delay path, containing lines  624 ,  628 , and  632 , a high impedance must be presented to the signal at the input to this path, namely, at point  642 . This is accomplished by choosing the length of line  624  to be one-quarter-wavelength at the frequency of interest, and terminating it with a low impedance. The low impedance termination is effected by setting switch  626  to the DOWN state. Otherwise, to block the passage through the shorter delay path, containing lines  606 ,  610 , and  614 , a high impedance must be presented to the signal at the input to this path, namely, at point  638 . This is accomplished by choosing the length of line  606  to be one-quarter-wavelength at the frequency of interest, and terminating it with a low impedance. The low impedance termination is effected by setting switch  608  to the DOWN state. To prevent the signal from entering the longer path through the point  648  when it enters through the phase shifter terminal  602  and follows the reference path,  606 ,  610 ,  614 ,  646 ,  636 , a high impedance must be established at this point. Thus, line  632  is also chosen to be one-quarter-wavelength and switch  630  is also set to the DOWN state in this case. On the other hand, to prevent the signal from entering the reference path at the point  650  when it enters the phase shifter bit at terminal  602  and follows the path  640 ,  624 ,  628 ,  632 ,  636 , a high impedance must be established at this point. Thus, line  614  is also chosen to be one-quarter-wavelength and switch  612  is also set to the DOWN state in this case. Elements  618 ,  620 ,  622  and  634  are open circuit slot stubs, and elements  616 , and  644  are microstrip open circuit stubs, which are chosen to adjust the transmission properties of the microstrip-to-slotline transitions. The length of lines  640  and  646  is chosen to minimize coupling between the two paths, and to facilitate the layout when switch size calls for it. 
   The conceptual structure and the method to form same of additional MEM switches  1300 ,  1400  is shown in  FIGS. 13-22 , and its process of fabrication is described. A doubly anchored cantilever beam  72  is disposed across the slot of a slotline  82 ,  78 ,  74 . The distance d 0  ( 76 ) from the beam  72  to the slot  78  is chosen as discussed before such that d 0 &lt;(d 0 +h 1 −h 2 )/3, where h 1  is the substrate thickness  84 , and h 2  is a minimum substrate thickness  88  so that the beam deflection may be controlled continuously without the occurrence of pull-in. In  FIG. 13 , electrodes  13100  and  1394  are located in recesses  1301  and  1302 , respectively. Comparing the switch of  FIG. 3  with the switch  1300  of  FIG. 13  to show the relative differences, the switch  1300  demonstrates improved control and no snapping as a result of a larger distance d 0 . The larger distance d 0  is a result of a larger distance from the electrodes  13100  and  1394  to the beam  72 . Comparing the switch of  FIG. 6  with switch  1300  of  FIG. 13 , the switch  1300  of  FIG. 13  requires less voltage to move beam  72  than the switch of  FIG. 6 , and demonstrates the approximately the same control of beam  72  as the switch of  FIG. 6 . 
   In  FIG. 13 , the recesses  1301  and  1302  are formed on a front side of the substrate  96 . In  FIG. 14 , recesses  1404  and  1406  are formed on the back side of substrate  96 ; the recesses  1301  and  1302  in  FIG. 14  are of a shallower depth than illustrated in  FIG. 13 . Turning back to  FIG. 14 , the electrodes  14100  and  1496  are positioned in the recesses  1404  and  1406  respectively. Comparing  FIG. 13  and  FIG. 14  the electrodes  13100  and  1394  are positioned in approximately the same location as electrodes  1404  and  1406 . 
     FIGS. 15  through  FIG. 21  shows the process by which the switch can be fabricated, and  FIG. 22  shows the sequence of steps of the invention. While the switch maybe fabricated and implemented by a variety of methods and materials, the described method is employed for purposes of illustration. The method in general is surface micromachining, with a substrate of low resistivity silicon, the transmission line (slot line and microstrip) metallization-chrome-gold (Cr—Au) sacrificial layer-copper, structural layer-nickel (Ni) and protection or isolation coating-silicon dioxide. In  FIG. 15 , the substrate is formed in step  2202  and the microstrip Cr—Au metal traces  74 ,  82  to define the slot  78  are defined and patterned. On the top surface of the substrate  96  the slot  78  is defined and patterned while on the bottom surface of the substrate  96  the microstrip  98  is defined and patterned by opening windows in the silicon dioxide protection layer by depositing and pattering and a adhesion layer of Cr with a approximate thickness of 200 Å and followed by a layer of Au with an approximate thickness of 2 μm in step  2204 . In  FIG. 16 , the recess patterns  1601 ,  1602  are defined step  2204 . More particularly a photoresist is spun on and windows are defined where the recesses/trenches  1301 ,  1302  are to be made in the substrate.  FIG. 17  shows the process for an etching the recesses  1301 ,  1302  via the reactive ion etching (DRIE) in step  2208 . In  FIG. 18 , the recess electrodes  13100 ,  1394  are defined in the recesses. The recess electrodes  13100 , 1394  are patterned and formed by depositing a second adhesion layer of Cr with a thickness of approximately 200 Å followed by depositing a layer of gold Au with an approximate thickness of 2 μm in step  2210 . Turning now to  FIG. 19 , a copper sacrificial layer  1906  is deposited and the beam anchor windows  1902 ,  1904  are defined. More particularly, the copper sacrificial layer  1906  is deposited, and the recesses are filled in step  2212 . The surface is planarised by using a chemical mechanical polishing (CMP) operation in step  2214  and windows are open by etching to define beam anchor windows  1902 ,  1904  and to pattering slot-blocking structure. In  FIG. 20 , the beam  2002  and beam anchors  2004  are deposited by plating nickel Ni for approximately 2 μm. In  FIG. 21 , the beam  2002  is patterned and the remaining copper sacrificial layer is removed by etching to empty the recesses and form the space under the beam step  2216 . 
     FIG. 23  shows that a Photonic Bandgap Crystal (PBC)  2302  is positioned between electrodes  13100 ,  1394  to provide additional isolation for the electrodes  13100 ,  1394  and to substantially inhibit propagation of waves emanating from the slotline strips.  FIG. 23  shows that a number of PBCs could be used. While four PBCs are shown in  FIG. 23 , additional or fewer PBCs could be used. The PBC is formed in a trench along with the formation of the recesses. As shown, the PBC  2302  is formed at approximately the same depth as the recesses  13100 ,  1394 . 
   The invention disclosed is believed to be superior to prior art MEMS-based switches for the following reasons:
     1) The switch operates in the pre-pull-in voltage regime, thus, no contact-related reliability issues, such as stiction or ohmic loss, resulting from snapping, are present;   2) The beam and control electrodes are naturally well isolated, so dielectric charging issues are non-existent;   3) The switch, in addition to fabrication compatible with integrated circuits, is also amenable to microwave integrated circuit (MIC), or hybrid, fabrication, thus rendering a low cost solution;   

   4) Because of 1), the switch lifetime is only limited by fatigue of the beam, so it has the inherent potential to achieve a lifetime of 1000 Billion cycles or greater [C. L. Muhlstein, S. B. Brown and R. O. Ritchie, “High-Cycle Fatigue of Single-Crystal Silicon Thin Films,”  J. Microelectromechanical Syst ., Vol. 10, No. 4, December 2001, pp. 593-600.] 
   It will be understood that various details of the invention may be changed without departing from the scope of the invention. The above concept can be applied to varactors, variable inductors, switched or reconfigurable circuits and any other known type device known to those of skill in the art requiring placement of an element on a substrate. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation—the invention being defined by the claims.