Patent Publication Number: US-2007115082-A1

Title: MEMS Switch Contact System

Description:
PRIORITY  
      This patent application claims priority from provisional U.S. patent application No. 60/723,019, filed Oct. 3, 2005 entitled, “MEMS CONTACT SYSTEM USING Pt SERIES METALS AND SURFACE PREPARATION THEREOF,” and naming Mark Schirmer as the sole inventor, the disclosure of which is incorporated herein, in its entirety, by reference. 
    
    
     FIELD OF THE INVENTION  
      The invention generally relates to MEMS switches and, more particularly, the invention relates to contact systems for MEMS switches.  
     BACKGROUND OF THE INVENTION  
      A wide variety of electrical switches operate by moving one member into direct contact with another member. For example, a relay switch may have a conductive cantilever arm that, when actuated, moves to directly contact a stationary conductive element. This direct contact closes an electrical circuit, consequently electrically communicating the arm with the stationary element to complete an ohmic connection. Accordingly, the physical portions of the arm that directly contact each other are known in the art as “ohmic contacts,” or as referred to herein, simply “contacts.” 
      Contacts often are fabricated by forming an electrically conductive metal on another surface, which may or may not be an insulator. For example, a cantilevered arm may be formed from silicon, while the contact at its end is formed from a conductive metal. When exposed to oxygen, water vapor, and environmental contaminants, however, the metal may react to form an insulative surface contamination layer, such as an insulative nitride layer, insulative organic layer, and/or an insulative oxide layer. As a result, the contact may be less conductive. Larger switches nevertheless generally are not significantly affected by this phenomenon because they often are actuated with a force sufficient to “break or scrub through” the surface contamination layer (e.g., an insulative oxide layer).  
      Conversely, switches with much smaller actuation, forces often are not able to break through this surface contamination layer. For example, electrostatically actuated MEMS switches often have typical contact forces measured in Micronewtons, which can be on the order of 1000 to 10,000 times less than the comparable force used in larger switches, such as reed or electromagnetic relays. Accordingly, the insulative surface contamination layer may degrade conductivity, which, in addition to reducing its effectiveness, reduces the lifetime of the switch.  
     SUMMARY OF THE INVENTION  
      In accordance with one embodiment of the invention, a MEMS switch has 1) a first contact, and 2) a second contact that is movable relative to the first contact. At least one of the contacts is electrically conductive and has a platinum-series based material.  
      The platinum-series based material may include a platinum-series element. Alternatively, the platinum-series based material may be a platinum-series based oxide. In some embodiments, at least one of the contacts has both a platinum-series based element and a conductive passivation. For example, the platinum-series based element may be ruthenium, while the conductive passivation may be ruthenium dioxide.  
      The apparatus also may have a package containing at least a portion of the MEMS switch. To mitigate the adverse effect of contaminants, such as free oxygen, within its interior, the package may have a contaminant gettering site. For example, the package may be a wafer level package having a cap with an interior surface supporting an exposed platinum-series element. In some embodiments, the package hermetically seals the first and second contacts.  
      In accordance with another embodiment of the invention, a MEMS apparatus has a substrate, a first contact, and a movable member with a second contact that moves relative to the substrate. The substrate supports the movable member. Moreover, at least one of the contacts has a conductive platinum-series based material that provides an electrical connection when contacting the other electrical contact. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Those skilled in the art should more fully appreciate advantages of various embodiments of the invention from the following “Description of Illustrative Embodiments,” discussed with reference to the drawings summarized immediately below.  
       FIG. 1  schematically shows an electronic system a switch that may be configured in accordance with illustrative embodiments of the invention.  
       FIG. 2A  schematically shows a cross-sectional view of a MEMS switch configured in accordance with one embodiment of the invention.  
       FIG. 2B  schematically shows a cross-sectional view of a MEMS switch configured in accordance with another embodiment of the invention.  
       FIG. 3A  schematically shows a cross-sectional view of a MEMS switch configured in accordance with yet another embodiment of this invention.  
      FIG  3 B schematically shows a cross-sectional view of the MEMS switch of  FIG. 3A  in an actuated position.  
       FIG. 4  shows a process of forming a MEMS switch in accordance with illustrative embodiments of the invention. 
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS  
      In illustrative embodiments, a MEMS switch has a contact formed from a platinum-series based material. For example, the contact may be formed from ruthenium metal (hereinafter “ruthenium” alone), ruthenium dioxide, or both. This type of contact should have material properties that provide favorable resistances and durability, while at the same time minimizing undesirable insulative surface contamination layers that could degrade switch performance. Details of illustrative embodiments are discussed below.  
       FIG. 1  schematically shows an electronic system  10  using a switch that may be implemented in accordance with illustrative embodiments of the invention. In short, the electronic system  10  has a first set of components  12  represented by a block of the left side of the figure, the second set of components  14  represented by a block on the right side of the figure, and a switch  16  that alternatively connects the first and second sets of components  12  and  14 . In illustrative embodiments, the switch  16  is a microelectromechanical system, often referred to in the art as a “MEMS device.” Among other things, the system  10  shown in  FIG. 1  may be a part of a RF switching system within a cellular telephone.  
      As known by those skilled in the art, when closed, the switch  16  electrically connects the first set of components  12  with the second set of components  14 . Accordingly, when in this state, the system  10  may transmit electronic signals between the first and second sets of components  12  and  14 . Conversely, when the switch  16  is opened, the two sets of components  12  and  14  are not electrically connected and thus, cannot electrically communicate through this path.  
       FIG. 2A  schematically shows a cross-sectional view of a MEMS switch  16  configured in accordance with illustrative embodiments of the invention. In this embodiment, the MEMS switch  16  is formed as an integrated circuit packaged at the wafer level. Specifically, the switch  16  has a substrate  18  supporting and suspending movable structure that alternatively opens and closes a circuit. To that end, the movable structure includes a movable member  22  movably connected to a stationary member  24  by means of a flexible spring  26 .  
      The stationary member  24  illustratively is fixedly secured to the substrate  18  and, in some embodiments, serves as an actuation electrode to move the movable member  22 , when necessary. Alternatively, or in addition, the switch  16  may have one or more other actuation electrodes not shown in the figures. It should be noted, however, that electrostatically actuated switches are but one embodiment. Various embodiments apply to switches using other actuation means, such as thermal actuators and electromagnetic actuators. Discussion of electrostatic actuation therefore is not intended to limit all embodiments.  
      The movable member  22  has an electrical contact  28 A at its free end for alternately connecting with a corresponding contact  28 B on a stationary contact beam  29 . When actuated, the movable member  22  translates in a direction generally parallel to the substrate  18  to contact the contact  28 B on the stationary contact beam  29 . During use, the movable member  22  alternatively opens and closes its electrical connection with the stationary contact beam  29 . When closed, the switch  16  creates a closed circuit that typically forms a communication path between various elements, such as those discussed above.  
      The die forming the electronic switch  16  can have a number of other components. For example, the die could also have circuitry (not shown) that controls a number of functions, such as actuation of the movable member  22 . Accordingly, discussion of the switch  16  without circuitry is for convenience only.  
      It should be noted that various embodiments can use a wide variety of different types of switches. For example, the switch  16  could multiplex more than two nodes and thus, be a three or greater position switch. Those skilled in the art should be capable of applying principles of illustrative embodiments to a wide variety of different switches. Discussion of the specific switch  16  in  FIGS. 2A and 2B , as well as the switch  16  in  FIGS. 3A and 3B , thus are illustrative and not intended to limit a number of different embodiments.  
      In accordance illustrative embodiments of the invention, one or both of the two noted contacts  28 A and/or  28 B is formed from a platinum-series based material (also known as “platinum group” or “platinum metals”). Specifically, as known by those skilled in the art, platinum-series elements include platinum (Pt), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), and iridium (Ir). Contacts  28 A or  28 B having platinum-series based materials therefore comprise a least a platinum-series based element. For example, ruthenium dioxide (RuO 2 ) is considered to be a platinum-series based material because part of it is ruthenium.  
      In one embodiment, one contact (e.g., contact  28 A) is formed from a platinum-series based material, while the other contact (e.g. contact  28 B) is formed from another type of material, such as a gold based material. In preferred embodiments, however, both contacts  28 A and  28 B are formed from a platinum-series based material. In some embodiments, this material simply may be a conductive oxide, such as ruthenium dioxide. In other embodiments, however, one or both of the contacts  28 A and  28 B have at least two layers; namely, a base layer  30  and a conductive passivation layer  32  (also referred to simply as “passivation layer  32 ” or more generally as “conductive passivation”). For example, the base layer  30  may be a platinum-series element, such as ruthenium, while the passivation layer  32  is a conductive oxide. Among others, the conductive oxide may be a platinum-series based material, such as ruthenium dioxide. In other embodiments using this two layer approach, however, the conductive oxide is not a platinum-series based material. Moreover, this two layer approach can have additional layers, such as an adhesion layer between the two layers  30  and  32 .  
      Platinum-series based elements provide a number of advantages when used to form contacts  28 A and/or  28 B. Specifically, in the MEMS context, thin layers of such materials (e.g., on the order of angstroms) provided a relatively low resistivity while being hard enough to withstand repeated contact. During experiments, however, contacts formed from platinum-series elements alone undesirably formed an insulative surface contamination layer. It subsequently was discovered that application of an appropriate conductive oxide both passivated the base layer  30  and substantially mitigated formation of an insulative surface contamination layer. Moreover, the conductive oxide permitted sufficient conductivity. It also was discovered that rather than using a two layer approach, a single conductive oxide comprised of a platinum-series based material also provided satisfactory results. Consequently, when applied as discussed herein, certain materials, such as platinum-series based materials, can be used to form the contacts  28 A and/or  28 B without the significant risk of formation of an insulative surface contamination layer.  
      As noted above, the switch  16  in  FIG. 2A  is packaged at the wafer level. To that end, the switch  16  also has a cap  34  for protecting the sensitive internal microstructure. In illustrative embodiments, the cap  34  forms a hermetically sealed chamber  36  that protects the internal components of the switch  16 .  
      It is anticipated that the conductive passivation layer  32  may deteriorate or degrade to some extent during the lifetime of the switch  16 , or have some kind of imperfection that adversely affects its passivation capabilities. For example, although it serves its purpose as a satisfactory passivation element, the discussed conductive oxide still may have some permeability to oxygen remaining in the chamber  36  from fabrication processes. Specifically, semiconductor packaging processes often seal the chamber  36  in the presence of oxygen. In one such process, glass frit wafer-to-wafer bonding processes may require bonding in the presence of oxygen to facilitate organic burn off of volatile solvents in the glass paste. In addition, if the glass contains lead, oxygen may be required to oxidize any metallic lead to prevent subsequent surface contamination.  
      As noted above, exposure to these contaminants can cause formation of an insulative surface contamination layer. For example, when at least one of the contacts  28 A or  28 B is formed from ruthenium, sufficient exposure to oxygen may cause formation of an insulative oxide layer, such as a ruthenium oxide (RuO) layer, or a ruthenium tetraoxide (RuO 4 ) layer.  
      Accordingly, to further protect the contacts  28 A and  28 B, illustrative embodiments provide a gettering system  38  for attracting and trapping much of the residual contaminants, such as oxygen, if any, within the hermetically sealed chamber  36 . For example, among other ways of gettering, the switch  16  may have a coating of deposited platinum-series metal, such as ruthenium, innocuously located within the chamber  36 . To that end,  FIG. 24A  shows ruthenium coated on portions of the interior facing surface of the cap  34 , and on innocuous, inactive, “white” areas of the die surface. To provide maximum efficiency, the exposed gettering material preferably has a surface area that is substantially greater than the surface area of the contacts  28 A and  28 B. For example, the contacts  28 A and  28 B may have a total area of 3-12 microns squared, while the area of the gettering material could have an area of 500-1000 microns squared. Although not optimal, some embodiments do not passivate the contact  28 A and/or  28 B (e.g., with a conductive oxide if the contact  28 A and/or  28 B is a metal, such as ruthenium) and simply use the gettering system  38 . It should be noted that the gettering system  38  can be formed to attract contaminants other than oxygen. Accordingly, discussion of an oxygen gettering system is illustrative.  
       FIG. 2B  schematically shows a cross-sectional view of another embodiment of the invention. One primary difference between this embodiment and the switch  16  shown in  FIG. 2A  is its packaging design. Specifically, unlike the switch  16  shown in  FIG. 2A , the switch  16  in this embodiment is packaged in a conventional cavity package  38  that contains the entire switch die. To that end, the package has a base  39  forming a cavity  41 , and a lid  43  that hermetically seals the cavity  41  to form the package chamber  36  noted above. As an example, the cavity package  38  could be a conventional ceramic cavity package commonly used in the semiconductor industry. In a manner similar to the switch  16  shown in  FIG. 2A , this switch  16  also has a gettering system  38  within its interior. To that end, the chamber  36  may have several gettering sites, such as on the interior facing surface of the lid  43 , along the sidewalls of the base  39 , and on the die itself. Of course, the gettering sites could be in other locations within the interior chamber  36 . Accordingly, discussion of specific locations of the gettering sites is illustrative and not intended to limit various embodiments of the invention.  
      The switch  16  can be packaged in a number of other types of packages. Discussion of the two types in  FIGS. 2A and 2B  therefore is illustrative only.  
      Another difference between the switch  16  in  FIG. 2A  and this switch  16  is the makeup of one of its contact  28 A. Specifically, the contact  28 A on the movable member  22  is the single layer type discussed above (i.e., no passivation layer  32 ). For example, this single layer contact  28 A may be formed from a platinum-series based conductive oxide, such as ruthenium dioxide.  
      Of course, as noted above, various embodiments apply to many different types of switches. For example, rather than apply to switches having one stationary contact  28 B and another moving contact  28 A, various embodiments apply to switches having two or more moving contacts.  FIGS. 3A and 3B  show yet another example of a switch  16  that may implement illustrative embodiments in the invention.  FIG. 3A  shows the switch  16  in an open circuit position (i.e., not actuated), while  FIG. 3B  shows the same switch  16  in a closed position (i.e., in an actuated position, which closes the circuit). For simplicity, reference numbers of components in this embodiment are the same as those of like components in other embodiments.  
      Rather than having a member that moves only in the plane parallel to the substrate  18 , the movable member  22  in this embodiment moves generally perpendicular to the substrate  18 , or in an arcuate manner relative to the substrate  18 . Such a design often is referred to as a “cantilevered design.” The stationary contact  28 B of this embodiment therefore simply, is generally planar and positioned on the surface of the substrate  18 . The contacts  28 A and  28 B may be comprised of the same materials as discussed above (although schematically shown as appearing to have only one layer—they still may have two layers, which is similar to other embodiments). In a similar manner, this embodiment has other similar components, such as a movable member  22 , stationary member  24 , and substrate  18 . In a manner similar to other embodiments, this embodiment may be contained in a conventional package, such as one of the packages shown in  FIGS. 2A  or  2 B, with or without gettering.  
       FIG. 4  shows one process of forming a switch in accordance with illustrative embodiments of invention. This switch  16  may be one of those shown in the previous figures, or one having a different configuration. Because it fabricates a MEMS device, the process may use the conventional micromachining technology similar to that commonly used by Analog Devices, Inc., of Norwood, Mass.  
      It should be noted that for simplicity, the process of  FIG. 4  is discussed as forming a single MEMS device. Those skilled in the art should understand, however, that this process can be applied to batch fabrication processes forming a plurality of MEMS devices on a single base wafer. Moreover, the steps of this process are illustrative and do not necessarily disclose each and every step that should or could be used in a MEMS fabrication process. In fact, some of the steps may be performed in a different order. Accordingly, discussion of the process of  FIG. 4  is not intended to limit all embodiments of the invention.  
      The process begins at step  400 , which forms the base structure. For example, the process may begin by depositing and etching various layers of materials on a base substrate. The movable member  22  may or may not be formed at this point. For example, the process may fabricate the movable member  22  and expose its end for depositing contact material in a subsequent step. Alternatively, the process may form a recess or specific area on a sacrificial layer for first depositing contact material in a subsequent step, and then depositing material (on the contact material) that forms the movable member  22  in an even later step.  
      Accordingly, step  402  then deposits the contact materials; namely, the process deposits platinum-series based material on at least the location designated step  400 , and on a location that will form the stationary contact  28 B. In illustrative embodiments, the process may deposit ruthenium metal through conventional means, such as with a sputtering or plating mechanism. After it is deposited, conventional wet or dry etch processes pattern the deposited material to ensure that the ruthenium is at the correct contact locations. Alternatively, as noted above, rather than deposit ruthenium metal, this step may deposit and pattern a conductive oxide, such as ruthenium dioxide, in a conventional manner to the relevant location.  
      The process then continues to step  404 , which completes fabrication of the structure and circuitry on the switch die. As noted above, this step may employ conventional surface micromachining technologies, such as plating, deposition, patterning, etching, and release operations. For example, this step may deposit sacrificial oxides and conductive layers to form the movable member  22  and other components, and then release the movable member  22  and other suspended components (if any). In illustrative embodiments, the movable member  22  is primarily formed from gold or a gold alloy.  
      It then is determined at step  406  if the contacts  28 A and/or  28 B should be passivated (i.e., protected from the environment of the package chamber  36 , which, as noted above, could have residual oxygen or other contaminants). If step  402  deposited a platinum-series metal, such as ruthenium, then the contact  28 A and/or  28 B should be passivated to minimize formation of an insulative surface contamination layer. In that case, the process continues to step  408 , which first cleans the contacts  28 A and  28 B (e.g., removing any oxidization that occurred to that point), and then forms a conductive oxide on the platinum-series element. For example, the process may form ruthenium dioxide on a ruthenium metal contact  28 A and/or  28 B substantially entirely covering its entire area. In some embodiments, however, the entire area of the ruthenium metal contact  28 A and/or  28 B is not covered (only a portion of it is covered).  
      Among other ways, the ruthenium contacts  28 A and/or  28 B may be exposed to a thermal oxidizing environment at an elevated temperature (e.g., 200 degrees C. or greater). Alternatively, ruthenium dioxide may be directly sputtered on a surface using DC magnetron sputtering. Typical sputtering conditions, for example, may be at temperatures of 300° C., 12 mTorr pressure, with an argon/oxygen mix at 14/45 sccm. This should form a uniform a ruthenium dioxide layer that could be patterned as required by the device application. Etching materials may include O 2 /CF 4 , O 2 Cl 2 , or O 2 /N 2  plasmas. Exposure of ruthenium metal to an oxygen plasma also should result in the selective formation of a conductive ruthenium dioxide passivation layer over the existing patterned ruthenium based metal.  
      Step  408  may be entirely skipped, however, if step  406  determines that passivation is not necessary. In either case, the process continues to optional step  410 , which applies gettering material to the package or the die. For example, as noted above, this gettering material may control free oxygen (among other things), which, in some instances, can form a native, insulating oxide if exposed to the contacts  28 A and/or  28 B. As noted above, the impact of oxygen on the contacts  28 A and  28 B should be substantially mitigated if an area within the chamber  36  having a platinum-series “gettering” metal that is significantly greater than the area of the contacts  28 A and  28 B. In some embodiments, the gettering metal is the same as the metal used on the contacts  28 A and/or  28 B. Other embodiments, however, use different metals.  
      The process then concludes at step  412  by hermetically sealing the switch  16  in ambient oxygen levels that are sufficiently low so as not to saturate the gettering system  38  formed by step  410 . One of ordinary skill in the art can determine those levels based on a number of factors.  
      Accordingly, illustrative embodiments of the invention benefit from the material properties of platinum-series based materials while mitigating the contamination problems that prevented known prior art devices from using such materials. Moreover, various embodiments further protect against possible contamination with a gettering system  38  within the package chamber  36 . Among other benefits, these optimizations should improve switch performance and increase switch lifetime.  
      Although the above discussion discloses various exemplary embodiments of the invention, it should be apparent that those skilled in the art can make various modifications that will achieve some of the advantages of the invention without departing from the true scope of the invention. For example, in some embodiments, only one contact  28 A or  28 B is formed as discussed above, while the other contact  28 B or  28 A is formed by conventional means, such as with gold or a gold alloy. In other embodiments, an apparatus may have a plurality of contacts that operate in parallel.