Abstract:
A micro electromechanical switch has a guidepost formed upon a substrate. A signal transmission line is formed on the substrate, with the signal transmission line having a gap and forming an open circuit. The switch further includes a switch body having a via opening formed therein, with the switch body being movably disposed along an length defined by the guide post. The guidepost is partially surrounded by the via opening.

Description:
BACKGROUND 
     The present disclosure relates generally to micro electromechanical (MEM) switches and, more particularly, to a multiposition MEM switch. 
     Advances in integrated circuit technology in recent years have led to the development of micro electromechanical systems (MEMS), featuring devices of micrometer dimensions which can be actuated and controlled using mechanical, electrostatic, electromagnetic, fluidic and thermal methods. MEMS manufacturing technologies are a combination of the more established semiconductor microfabrication techniques with the newer developments in micromachining. 
     One example of a MEM device is a cantilevered beam switch having one end anchored to a substrate material, such as silicon. The free end of the beam serves as a deflection electrode which, when a voltage source is applied thereto, deflects as a result of the electrostatic forces on the beam and a field plate, thereby making contact with a stationary electrode. When the voltage source is removed, the beam returns to its “rigid” state due to the restoring forces therein and the switch contacts are opened. 
     Although advances in MEM technology have been considerable in recent years, the technology is not without its drawbacks. For example, one of the most insidious problems facing manufacturers of MEMS devices is stiction, which occurs when a surface of a micromachined part (such as a cantilever beam) becomes fused or bonded to an adjacent surface of the structure. Stiction can often result from conditions such as surface roughness, humidity, applied voltage and capillary forces during the manufacturing process. The greater the number of stiction problems occurring in a device, the greater the overall effect on the yield of the device becomes. In addition, the physical geometry of a component itself may also have an effect on its susceptibility to stiction; switches of the cantilevered type may undergo warpage due to repeated mechanical stresses on the beam. As such, it is desirable to provide a switch design which minimizes the susceptibility to stiction. 
     Other difficulties associated with beam switches may include: material fatigue, space constraints (from the requirement for anchoring points), the creation of parasitic inductances and resonant frequency problems. It is also desirable, therefore, to provide a MEM switch which addresses the aforementioned concerns. 
     SUMMARY 
     In an exemplary embodiment, a micro electromechanical switch has a guidepost formed upon a substrate. A signal transmission line is formed on the substrate, with the signal transmission line having a gap and forming an open circuit. The switch further includes a switch body having a via opening formed therein, with the switch body being movably disposed along a length defined by the guidepost. The guidepost is partially surrounded by the via opening. In a preferred embodiment, a field plate is formed on the substrate and aligned electrostatically attractably apart from the switch body. An electrostatic attraction between the field plate and the switch body causes the switch body to close the gap in the signal transmission line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a prior art, cantilever beam microswitch; 
     FIG. 2 is a top plan view of an embodiment of a micro electromechanical switch of the invention, with the upper and lower substrate levels exploded laterally to illustrate the main switch body; 
     FIG. 3 is a cross sectional view of the switch of FIG. 2, taken along the section line  3 — 3 ; 
     FIG. 4 is an alternative embodiment of the switch shown in FIG. 3; 
     FIG. 5 is a top plan view of another embodiment of the micro electromechanical switch of the invention; 
     FIG. 6 is a top cross sectional view of another embodiment of the switch body; and 
     FIGS. 7-9 are cross sectional views of the steps in fabricating a section of the switch shown in FIGS.  3  and  4 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is illustrative of a known micro electromechanical switch (MEMS). As shown, the MEMS, generally identified by reference numeral  20 , is formed on a substrate  22  with a fixed post  24  formed at one end. A flexible cantilever beam  26  is connected on one end of post  24 . The cantilever beam  26  is adapted to carry an electrical contact  28  on one end that is aligned and adapted to mate with a corresponding contact  30  on substrate  22 . The switch  20  is adapted to be activated electrostatically. A grounding plate  32  is formed on the substrate  22  while a filed plate  34  is formed on the cantilever beam  26 . The grounding plate  32  is adapted to be connected to ground while the field plate  34  is adapted to be selectively coupled to a DC voltage source (not shown). With no voltage applied to the field plate  34 , the contact  28  is separated from contact  30 , defining an open circuit state. When an appropriate DC voltage is applied to field plate  34 , the cantilever beam  26  is deflected by the electrostatic forces between plate  34  and ground plate  32 , causing electrical contact  28  to mate with contact  30 , defining a closed circuit state. When the applied voltage is subsequently removed from the field plate  34 , the cantilever beam  26  returns to its static position due to the restoring forces in the beam. 
     Referring now to FIGS. 2 through 4, a switch  50  of an embodiment of the invention is fabricated upon a substrate  52 , such as silicon dioxide (SiO 2 ), onto which a plurality of guideposts  54  are formed and located thereupon. Guideposts  54  are surrounded by via openings  56  formed within a moveable body  58  of switch  50 . Body  58  is comprised of a generally rectangular block  60  of conducting material, such as copper. In order to prevent oxidation, the block  60  is encapsulated within an insulating layer and capped, as is described in greater detail hereinafter. As is best seen in FIGS. 3 and 4, body  58  is movably disposed along the length of the guideposts  54 , which serve to keep the body  58  of switch  50  in proper lateral alignment as it travels vertically along the guideposts  54 . Configured in this manner, switch  50  does not require an anchor or fixed point about which to pivot or flex. 
     Body  58  is disposed in a generally horizontal alignment between an upper layer  62  of the substrate  52  and a lower layer  64  of the substrate  52 , as seen in FIGS. 3 and 4. Formed within the lower layer  64  of substrate  52  is a first field plate  66  to which a control voltage is applied. A second field plate  68  is similarly located within the upper layer  62  of substrate  52 , and is also connected to a control voltage supply (not shown). The first field plate  66  is electrostatically spaced apart from and attractable to the bottom surface  70  of the switch body  58 , whereas the second field plate  68  is electrostatically spaced apart from and attractable to the top surface  72  of switch body  58 . 
     A first signal transmission line  74  is established through the lower layer  64  of substrate  52  through contacts  76  separated by a gap  78  therebetween, and defining a open circuit in the first signal transmission line  74 . A second signal transmission line  80  is similarly established through the upper layer  66  of substrate  52  through contacts  82  separated by a gap  84 , and defining an open circuit in the second signal transmission line  80 . 
     The configuration of the switch  50  in the illustrated embodiments represents a double pole, double throw switch; however, the principals of the invention are applicable to other switch configurations as well. In the present embodiments, switch  50  can be implemented as either a two position switch or a three position switch. In order to maintain a third switch position, the body  58  of switch is maintained in position which is electrically disconnected from signal transmission lines  74 ,  80 , and between the upper and lower substrate layers  62 ,  64 . The embodiment shown in FIG. 3, for example, features a pair of hinges  90 , which are used to bias switch  50  in a neutral or “off” position. The hinges  90  may be integrated with the conducting material. 
     Alternatively, a “free floating” switch design, shown in FIG. 4, may be utilized in the absence of hinges  90 . However, in order to maintain switch  50  in a neutral third position, the first and second field plates  66 ,  68  are biased with an appropriate balancing charge such that the resulting opposing electrostatic forces exerted on the switch body  58  cancel one another out, thereby keeping switch body  58  suspended in a free floating position. In the absence of biasing electrostatic forces, switch  50  may also be used in a two position configuration, or a binary mode of operation. As an example of such a configuration, the first transmission line gap  78  is closed and the second transmission line gap  84  is open in the default or “off” position. In the energized or “on” position, the first set transmission line gap  78  is opened and the second transmission line gap  84  is closed. 
     Switch  50  is actuated by a control voltage selectively applied to one of the desired field plates. The resulting electrostatic force between the selected field plate and the switch body  58  either raises or lowers the body, depending upon which field plate is energized. If, for example, the first field plate  66  is energized, and further assuming that switch  50  is initially in a neutral position, switch body  58  will then be caused to move downward, until conducting surfaces  91  on opposite sides of the switch body  58  mate with corresponding contacts  76  on lower substrate layer  64 , thereby closing the first transmission line gap  78  and defining a closed circuit. When the first field plate  66  is subsequently de-energized, switch body  58  may be returned to a neutral position by biasing hinges  90  or by the application of balancing charges on both first and second field plates  66 ,  68 . In either case, the first signal transmission gap is reopened upon the separation of contacts  76  with the conducting surfaces on switch body  58 . 
     The gap in the second signal transmission line  80  is closed in the same manner by energizing the second field plate  68 . This time, the electrostatic forces generated cause switch body  58  to move in an upward direction until conducting surfaces  91  mate with contacts  82  on upper substrate layer  62 . The second signal transmission line  80  is in a closed circuit condition until the second field plate  68  is deenergized and the switch body  58  is returned to a neutral position. It should also be noted that the polarity of the charge applied to either field plates may be reversed, thereby creating a repulsive force on switch body  58 . The repulsive force provided by one field plate may also be used in conjunction with an attractive force provided by the other field plate, thereby creating a push-pull actuation mechanism. 
     Again, as an alternative to a three position embodiment, switch  50  can be configured in a two position mode such that one field plate is energized when the other is de-energized and vice versa. In this manner, either the first or the second signal transmission line gap is continuously opened at any given time, but not both gaps simultaneously. In other words, switch body  58  is not statically maintained in a neutral position. 
     FIG. 5 illustrates yet another embodiment of the switch configuration, adaptable for use with a cantilever beam. In this embodiment, the main switch body  58  is integrally formed upon the end of a lever arm  92  which, in turn, is affixed to a stationary post  94  formed within the substrate. Lever arm  92  does not entirely support the weight of switch body, as hinges  90  are also used in this configuration. 
     FIG. 6 illustrates another embodiment of main switch body  58 . As is shown, switch body  58  may be fabricated in a generally circular shape  100 . Thus configured, switch body  58  travels vertically upward and downward within a cavity  96  formed within the substrate  52 , while only frictionally engaging the substrate walls at four tangential surfaces  102  on switch body  58 . Although guideposts (not shown) keep switch body  58  in a relatively horizontal orientation within cavity  96 , via openings (not shown) do allow for slight lateral shifting of switch body  58  while in operation. Accordingly, with a circular design, there would be a minimal amount of surface contact between the outer edges of switch body  58  and the substrate walls defining cavity  96 . 
     Referring now to FIG. 7, the details for fabrication of the switch are illustrated. The guideposts  54  are formed from the silicon dioxide (SiO 2 ) substrate  52  by known masking, deposition and etching techniques. A sacrificial layer  200 , such as diamond-like carbon (DLC) or other conformal organic polymer, is deposited upon the substrate  52 , including the side and top surfaces of the guideposts  54 . A liner  202  is thereafter deposited upon the sacrificial layer  200 , in order to prevent the diffusion of the electroplated copper  204  which is subsequently deposited upon the liner  202 . Liner  202  is preferably comprised of a refractory metal such as titanium, titanium nitride, tantalum nitride or tungsten. Due to the poor corrosion resistance of copper  204 , a cap  206  of cobalt-tungsten-phosphide (CoWP) is electrolessly formed upon the top surface of the copper layer, as shown in FIG.  8 . It should be noted, however, that other materials may be used for cap  206 , including tantalum nitride or nickel. The top of the cap  206  is planarized with the top surface of the guideposts  54 , following chemical-mechanical polishing. A second sacrificial layer  208  of DLC is then deposited upon the caps  206  and the guideposts  54 . Next, a top cap  210  of insulating material, preferably silicon nitride, is deposited upon the second layer  208  of DLC. 
     Finally, FIG. 9 illustrates the switch following the removal of the sacrificial layers  200 ,  208  of DLC. Upon forming a number of perforations in the top cap  210 , the switch  50  is then heated in an oxygenated environment, thereby resulting in the removal of the sacrificial layers  200 ,  208  and producing carbon dioxide and carbon monoxide as waste gases. The removal of the DLC thus creates the via openings  56  in the switch body  58  through which guideposts  54  guide the vertical movement of switch body  58 . 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.