Patent Publication Number: US-8111118-B2

Title: Multi-stable micro electromechanical switches and methods of fabricating same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional application of and claims priority to U.S. Ser. No. 11/532,689 filed Sep. 18, 2006 now U.S. Pat. No. 7,688,166; which is a divisional of and claims priority to U.S. Ser. No. 10/425,861, filed Apr. 29, 2003, now U.S. Pat. No. 7,190,245; incorporated herein by reference in there entireties. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to electromechanical switches, and more particularly relates to micro electromechanical switches that have multiple stable states. 
     BACKGROUND 
     Switches are commonly found in most modern electrical and electronic devices to selectively place electrical, optical and/or other signals onto desired signal paths. Switches may be used to enable or disable certain components or circuits operating within a system, for example, or may be used to route communications signals from a sender to a receiver. Electromechanical switches in particular are often found in medical, industrial, aerospace, consumer electronics and other settings. 
     In recent years, advances in micro electromechanical systems (MEMS) and other technologies have enabled new generations of electromechanical switches that are extremely small (e.g. on the order of micrometers, or 10 −6  meters) in size. Because many micro switches can be fabricated on a single wafer or substrate, elaborate switching circuits may be constructed within a relatively small physical space. Although it would generally be desirable to include such tiny electromagnetic switches in medical devices (e.g. pacemakers, defibrillators, etc.) and other applications, several disadvantages have prevented widespread use in many products and environments. Most notably, many conventional micro electromechanical switches consume too much power for practical use in demanding environments, such as in a device that is implanted within a human body. Moreover, difficulties often arise in isolating the switch actuation signal from the transmitted signal in such environments. Further, the amount of energy (e.g. electrical voltage) typically required to actuate a conventional electromechanical switch may be too great for many practical applications, particularly in the medical field. 
     Accordingly, it is desirable to create a micro electromechanical switch that consumes a relatively low amount of power, and that can be actuated with a relatively small amount of energy. It is also desirable to create an electromechanical switch that improves electrical isolation between switch actuation signals and signals routed by the switch. In addition, it is desirable to create a micro electromechanical switch that is easily manufactured, and that is suitable for use in demanding medical device applications and the like. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY 
     In one aspect, a micro electromechanical switch including a moveable member configured to electrically cooperate with a receiving terminal is formed on a substrate. The moveable member and the receiving terminal each include an insulating layer proximate to the substrate and a conducting layer proximate to the insulating layer opposite the substrate. In various embodiments, the conducting layers of the moveable member and/or receiving terminal include a protruding region that extends outward from the substrate to switchably couple the conducting layers of the moveable member and the receiving terminal to thereby form a switch. The switch may be actuated using, for example, electrostatic energy. 
     In a further aspect, a multi-stable electromechanical switch having an open state and a closed state suitably includes a moveable member and at least one pair of receiving terminals biased to a bias position corresponding to the open state. Each terminal suitably has an outcropping configured to interface with the moveable member in the closed state. An actuating circuit provides electrostatic energy to displace the receiving terminals from the bias position, and to displace the moveable member toward the bias position. The receiving terminals then return toward the bias position when the electrostatic energy is removed to establish an electrical connection with the moveable member, thereby retaining the electromechanical switch in the closed state. 
     The various electromechanical switches described herein may be useful in a wide variety of applications, including many applications in the medical device field. Such switches may be useful in producing Y-adapter-type lead multiplexers for implantable devices, for example, as well as in producing switchable electrostimulation electrode arrays and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIGS. 1A-B  are cross-sectional side views of exemplary opposing contact members of an exemplary switch; 
         FIGS. 2A-D  are cross-sectional side views illustrating an exemplary process for producing exemplary contact members; 
         FIG. 3  is a top view of an exemplary electromechanical switch; 
         FIG. 4  is a side view of an exemplary electromechanical switch; 
         FIGS. 5A-C  are top views of an exemplary tri-stable micro electromechanical switch; and 
         FIG. 6  is a top view of an exemplary bi-stable micro electromechanical switch with an exemplary actuating circuit. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     According to various exemplary embodiments, switches suitable for use in medical devices and the like are fabricated using conventional MEMS techniques. The switches suitably include a moveable armature, cantilever or other member that is capable of selectively engaging one or more receiving terminals to place the switch into a desired state. In various embodiments, the moveable member and/or receiving terminal(s) are fashioned with a protruding region formed of a noble metal (e.g. gold) or another conductive material to improve electrical connections within the switch. In further embodiments, the switch is configured to exhibit two or more stable output states without consuming energy to maintain the switch in a desired state. Stability is provided by mechanically biasing one or more receiving terminals to a position corresponding to a first state of the switch (e.g. an open state corresponding to an open circuit), and by positioning the moveable member into the bias position when the switch is in another state (e.g. corresponding to a closed switch). In such embodiments the mechanical bias of the receiving terminals maintains contact with the moveable member even when the energy used to displace switch components is removed. Accordingly, the switch remains in the desired state without requiring continuous application of energy, thereby conserving power. The various switches described herein may be used in a wide variety of applications, including applications in the medical, industrial, aerospace, consumer electronic or other arts. Several applications in the medical field include switchable Y-adapter lead multiplexers for implantable medical devices, switchable electrode arrays, and the like. 
     With reference now to  FIG. 1A , an exemplary electromechanical switch suitably includes a moveable member  101  that electrically contacts with one or more receiving terminals  102  to complete an electrical circuit, and to thereby place switch  100  into a desired output state (e.g. open or closed). Moveable member  101  and any associated terminals  102  are collectively referred to herein as “contact members”. Moveable member  101  is suitably formed from a substrate layer  104 A, an insulating layer  106 A, a conducting layer  108 A, and a conductive coating  110 A that appropriately surrounds conducting layer  108 A to form a protruding region  116 A that extends radially outward from substrate  104 A, and that provides an appropriate electrical contact to receiving terminal  102 . Similarly, terminal  102  is suitably formed from a substrate layer  104 B, an insulating layer  106 B, a conducting layer  108 B, and a conductive coating  110 B. Conductive coating  110 B may also be formed to create a protruding region  116 B extending outward from receiving terminal  102  to interface with protruding region  116 A of moveable member  101  and to thereby form an electrical connection to close switch  100 . Although both moveable member  101  and terminal  102  are both shown in  FIG. 1A  with protruding regions  116 , the protruding portion may be removed from either of the contact members in various alternate embodiments. 
     In operation, moveable member  101  is capable of lateral movement to switchably engage receiving terminal  102 .  FIG. 1B  shows an exemplary switch  100  wherein moveable member  101  is in contact with terminal  102  to thereby complete an electrical circuit and to place switch  100  into a “closed” state. Because protruding regions  116  extend outward from substrate  104 , protruding regions  116  appropriately form an electrical connection without requiring contact between substrate layers  104 A-B and/or insulating layers  106 A-B. This separation between the non-conducting layers of moveable member  101  and terminal  102  provides an electrical isolation between the two members, which in turn assists in isolating actuation signals propagating in switch  100  from signals transmitted by switch  100 , as described more fully below. 
     Referring now to  FIGS. 2A-2D , an exemplary process for building a switch  100  suitably includes the broad steps of forming insulating and conducting layers on a substrate ( FIG. 2A ), isolating the moveable members and terminals ( FIG. 2B ), applying a conductive coating to the appropriate portions of the switch ( FIG. 2C ), and optionally etching or otherwise processing a backside of the substrate to further define terminals, moveable members and the like ( FIG. 2D ). The various steps described in the figures may be implemented using any manufacturing or fabrication techniques, such as those conventionally used for MEMS and/or integrated circuit technologies. Various switch fabrication techniques are described, for example, in U.S. Pat. No. 6,303,885. 
     With reference to  FIG. 2A , the switch fabrication process suitably begins by preparing a substrate assembly  200  that includes a substrate  104 , an insulating layer  106  and a conducting layer. Substrate  104  is any material such as glass, plastic, silicon or the like that is capable of supporting one or more switches  100 . In an exemplary embodiment, substrate  104  is formed from doped silicon, and has a thickness on the order of 35-75 m, although the actual dimensions will vary widely from embodiment to embodiment. Similarly, the optional dopants provided in substrate  104  may be selected to improve the connectivity of the switch, and will also vary widely with various embodiments. Substrate  104  may be prepared in any manner, and in an exemplary embodiment is prepared using conventional Silicon-on-Insulator (SOI) techniques. Insulating layer  106  may be formed of any electrically insulating material such as glass, silicon oxide, or the like, and may be placed on or near an exposed surface of substrate  104  using any technique such as sputtering, deposition or the like. Similarly, conducting layer  108  may be any metal such as aluminum, copper, gold or silver, and may be placed according to any technique. In an exemplary embodiment, insulating layer  106  and conducting layer  108  are deposited on substrate  104  using conventional liquid-phase epitaxy and/or low pressure chemical vapor deposition techniques, as appropriate. 
     With reference to  FIG. 2B , the various electrically conducting and insulating regions of switch  100  may be suitably isolated in substrate assembly  200 . Conducting layer  108  may be patterned or otherwise processed using conventional etching, lithography or other techniques, for example, to create gaps  201  between separate electrical nodes. Patterning appropriately delineates moveable members  101 , actuating circuitry, receiving terminals  102  and the like from each other. An exemplary pattern for a switch  100  is discussed below in conjunction with  FIG. 3 . In alternate embodiments, conducting layer  108  may be eliminated entirely, with conducting and/or insulating regions on substrate assembly  200  provided by selective doping of substrate  104 , as described more fully below. 
     Referring now to  FIG. 2C , an additional conducting layer  110  of gold or another appropriate material may be grown, electroplated or otherwise formed on conducting layer  108 . In one embodiment, substrate assembly  200  is further formed with an additional non-conducting layer of oxide or the like that is applied after etching or patterning. Electroless gold or another conductor can then be “grown” or otherwise applied on portions of substrate assembly that are unprotected by the additional non-conducting layer. Alternatively, conductive material can be evaporated or sputtered selectively on conductive areas using a shadow mask or the like. In yet another embodiment, gold or another conductive material is suitably electroplated, as described in conjunction with  FIG. 3  below. In such embodiments conducting layer  108  may not be present, with silicon dioxide or another insulator providing electrical insulation between parts of switch  100  used for electrostatic actuation and parts used for signal conduction. In various embodiments, protruding region  116  is formed of conductive material as appropriate to engage other contact members while maintaining electrical isolation between substrate portions  104 . Protruding regions  116  may be formed as a consequence of the additional exposed surface near the corners of conducting layer  108 , for example, or by any other technique. 
     In a further embodiment, the various components of switch  100  may be physically separated from each other using conventional MEMS techniques. An anisotropic etchant such as Tetra-Methyl Ammonium Hydrate (TMAH) or Potassium Hydroxide (KOH), for example, may be used to separate moveable member  101  from terminal  102  as appropriate. In further embodiments (and as shown in  FIG. 2D ), additional insulating layers  206 A,B and/or conducting layers  208 A,B may be formed after separation but before formation of the outer conducting layer  110  to improve coverage by layer  110 / 210 A-B. Such layers may be formed following additional etching or processing from the front or back side of substrate  104 , as appropriate. Accordingly, the various contact members and other components of switch  100  may take any shape or form in a wide variety of alternate but equivalent embodiments. 
       FIGS. 3 and 4  are top and side views, respectively, of an exemplary switch assembly  300 , with  FIG. 4  being a cross-sectional side view taken along line A-A′ in  FIG. 3 . Referring now to  FIG. 3 , an exemplary switch assembly  300  suitably includes one or more cantilevers or other moveable members  101 A-B that are capable of interacting with any number of receiving terminals  102 A-D, as appropriate. In the exemplary switch assembly  300  shown in  FIG. 3 , two tri-stable switches corresponding to moveable members  101 A and  101 B are shown. One switch, for example, has a first state corresponding to contact between moveable member  101 A and terminal  102 A, a second state corresponding to contact between moveable member  101 A and terminal  102 B, and a third state corresponding to no contact between moveable member  101 A and either terminal. Similarly, the other switch shown has a first state corresponding to contact between moveable member  101 B and terminal  102 C, a second state corresponding to contact between moveable member  101 B and terminal  102 D, and a third state corresponding to no contact between moveable member  101 B and either terminal. Accordingly, each of the two switches are capable of three separate output states. Alternate embodiments of switch fabric  300  may include any number of moveable members  101  and/or terminals  102 . Similarly, each switch may have any number of available output states such as two, three or more. 
     Each moveable member  101  and terminal  102  may be formed from a common substrate  104  as described above, with one or more hinges  304  providing flexible mechanical support for each moveable member  101 . Each moveable member  101 A-B suitably includes two conducting regions  312  and  314  that are capable of electrically interfacing with terminals  102 A-D as described above. In the exemplary embodiment shown in  FIG. 3 , member  101 A has a first conducting region  314 A that interfaces with terminal  102 A and a second conducting region  314 B that interfaces with terminal  102 B. Similarly, member  101 B has a first conducting region  312 A that interfaces with terminal  102 C and a second conducting region  312 B that interfaces with terminal  102 D. 
     Each moveable member  101  may also include another conducting region  310  that may be used to actuate the member  101  between the various states of switch  300 . In the exemplary embodiment shown in  FIG. 3 , for example, each conducting region  310  is integrally formed with a comb-type portion  316  that is sensitive to electrostatic energy or other stimulus provided by actuators  308 A-D. In the exemplary embodiment shown in  FIG. 3 , each portion  316  includes a series of comb-like teeth that include metal, permalloy or other material capable of being actuated by one or more actuators  308 A-D. In practice, each moveable member  101  may include multiple portions  316  that are sensitive to electrostatic force, and portions  316  may take any shape and/or may be located at any point on or near moveable member  101 . Although not shown in  FIG. 3  for purposes of simplicity, in practice each member  101  may include two or more portions  316  on opposing sides of conducting region  310 , for example, to increase the response to applied electrostatic force and to thereby more easily actuate the member between the various states of switch  300 . 
     In practice, each moveable member  101  is displaced by one or more actuating circuits  308 A-D as appropriate. In the exemplary embodiment shown in  FIG. 3 , for example, moveable member is suitably displaced toward terminal  102 A by providing an electrostatic charge on actuator  308 A that attracts comb portion  316 . Similarly, an electrostatic charge provided by actuator  308 B appropriately attracts comb portion  316  toward terminal  102 B. Providing an electrostatic charge to both actuators  308 A-B appropriately attracts comb portion  316  to the central location such that member  101 A is electrically separated from each terminal  102 A and  102 B to place the switch into an open circuit-type state. Similar logic could be applied to member  101 B, which is appropriately displaced between the three states by actuators  308 C and  308 D. In alternate embodiments, electrostatic attraction could be replaced or supplemented with electrostatic repulsion, RF signals, inductance of electromagnetic signals, or any other actuating force. 
     As briefly mentioned above, the various conducting regions  310 ,  312  and  314  are appropriately isolated from each other by electrically insulating portions  306 , which may be exposed portions of insulating layer  106  discussed above, or which may be made up of an additionally-applied insulating material. Alternatively, insulating portions  306  (as well as some or all of the conducting portions on switch assembly  300 ) may be formed by injecting or otherwise placing dopant materials in the appropriate regions of substrate  104 . In practice, hinges  304  and conducting regions  312  and  314  may be laid out on substrate  104  ( FIGS. 1 and 4 ) in a pattern that allows for convenient electroplating. In such embodiments, an electrical charge applied at contact  302  has electrical continuity through conducting layer  108  ( FIGS. 1-2 ) across each hinge  304  and conducting region  312  and  314 . When such a charge is applied, outer conducting layer  110  can be readily electroplated to the desired locations on switch  300 , as appropriate. Insulating regions  306  suitably provide electrical isolation for those parts of switch  300  that are not desired to become electroplated, thereby improving the manufacturability of switch  300 . Electroplating may also provide appropriate protruding regions  116  as described above, and as best seen in  FIG. 4 . 
     Electroplating hinges  304  also provides mechanical reinforcement for supporting moveable members  101 , which are appropriately otherwise isolated from substrate  104  to promote ease of movement. With reference now to  FIG. 4 , member  101 A is suitably separated from substrate  104  by a gap  402  to permit lateral movement toward terminals  102 A and  102 B as appropriate. Gap  402  may be formed through conventional MEMS techniques, including backside etching or the like. Alternatively, substrate  104  may be formed with a sacrificial layer  404  that can be etched using conventional front side etching or otherwise removed to form gap  402 . In such embodiments, sacrificial layer  402  may be formed of an oxide (e.g. silicon oxide) or another material that may be etched through cavities formed in layers  106 ,  108  and/or  110  as appropriate. 
     With reference now to  FIGS. 5A-C , switch  500  is appropriately held in a number of stable output states through the use of mechanical energy applied by one or more receiving terminals. Switch  500  suitably includes at least one moveable member  101  that is displaceable to interface with one or more terminal arms  502 ,  504 ,  506 ,  508 . Each terminal arm  502 ,  504 ,  506 ,  508  is appropriately designed to be moveable, rotatable, deformable or otherwise displaceable to place switch  500  into different output states. In an exemplary embodiment, each arm  502 ,  504 ,  506 ,  508  is designed to bend in an elastic-type fashion about a fixed point  512 . Such deformabililty or elasticity may be provided by conventional MEMS or other techniques. In various embodiments, one or more terminal arms are designed to include an outcropping  510  that is able to electrically communicate with moveable member  101 . In the embodiment shown in  FIGS. 5A-C , terminal arms  502  and  504  cooperate to provide an electrical connection with moveable member  101  when the switch is in a first state, and terminal arms  505  and  508  cooperate to provide an electrical connection with moveable member  101  when the switch is in a second state, as shown in  FIG. 5C . A third state may be provided when moveable member  101  is electrically isolated from both sets of terminal arms, as shown in  FIG. 5A . The layout and structural components of switch  500  appropriately corresponds to those of switches  100 ,  300  and the like discussed above, or the concepts described with respect to switch  500  may be applied to any type of switch or switch architecture in a wide array of equivalent embodiments. Various equivalent embodiments of switch  500  include any number of moveable members  101 , terminal arms, terminals, or output states for each moveable member  101 . Although not visible in  FIG. 5 , each outcropping  510  or any other portion of terminal arms  502 ,  504 ,  506  and/or  508  may include a protruding region  116  as discussed above to further improve electrical connectivity between the terminal arm and moveable member  101 . 
     Referring to  FIG. 5A , switch  500  is shown in an exemplary “open” state (corresponding to an open circuit) whereby moveable member  101  is not electrically coupled to either set of terminal arms. Terminal arms  502 ,  504 ,  506  and  508  are appropriately designed such that their natural “biased” state corresponds to the open state wherein the arms are isolated from moveable member  101 . As used herein, “biased state” refers to the physical space occupied by one or more terminal arms  502 ,  504 ,  506 ,  508  when no actuation force or energy is applied and when no other object blocks or prevents natural movement of the terminal arm. 
     In operation, switch  500  is placed into a different state when moveable member  101  is moved into the bias position of one or more terminal arms such that the mechanical force applied by the terminal arm in attempting to return to the bias state holds the terminal arm in contact with moveable member  101 . In an exemplary embodiment, this movement involves moving the terminal arms out of the bias position, moving the moveable member into the space occupied by the terminal arms in the bias position, and then releasing the terminal arms to create mechanical and electrical contact between the arms and moveable member  101 . With reference now to  FIG. 5B , terminal arms  506  and  508  are appropriately actuated to move outcroppings  510  out of the way so that moveable member  101  may be displaced as appropriate. Although this movement is shown in  FIG. 5B  as a rotation about a fixed pivot point  512  on terminal arms  506 ,  508 , alternate embodiments may make use of lateral displacement in vertical and/or horizontal directions, or any other type of movement. 
     After the terminal arms are moved out of the bias position, moveable member  101  is appropriately actuated to place at least some portion of member  101  into the space occupied by at least some portion of terminal arms  506 ,  508  in the bias position. This actuation may be provided with electrostatic force as described above and below, or with any other conventional actuation techniques. In the embodiment shown in  FIGS. 5A-C , moveable member  101  is laterally displaced using electrostatic force or the like so that a portion of moveable member  101  occupies space corresponding to the bias positions of outcroppings  510  of terminal arms  506 ,  508 . 
     As actuating force is removed from terminal arms  506  and  508 , potential energy stored in the arms is converted to kinetic energy to thereby produce a torque that attempts to return arms  506 ,  508  to their bias positions. Because the bias position is now occupied by moveable member  101 , however, arms  506  and  508  impact upon member  101  and are suitably prevented from further movement. Because potential energy remains in the arms until they are placed in the bias position, a mechanical force is provided that maintains arms  506 ,  508  against moveable member  101  to thereby hold switch  500  in the closed state (corresponding to a closed circuit). Accordingly, switch  500  will remain in the closed state even though no further electrostatic or other energy is expended. Although  FIGS. 5A-C  have concentrated on actuation of terminal arms  506  and  508 , similar concepts could be employed to actuate terminal arms  502 ,  504  and to place moveable member  101  in contact with arms  502 ,  504 . Switch  500  is therefore capable of several stable output states, and may be considered to be a multi-stable switch. 
     Additional detail about an exemplary actuation scheme is shown in  FIG. 6 . With reference now to  FIG. 6 , each terminal arm  506 ,  508  is fabricated with an electrostatic-sensitive area  606  that is receptive to electrostatic energy provided by actuators  602 ,  604 , respectively. Electrostatic energy from actuators  602 ,  604  appropriately attracts a metal, permalloy or other material in areas  606  to displace the arms away from their bias position. Although actuators  602 ,  604  and areas  606  are shown as comb-type actuators in  FIG. 6 , any time of electrostatic or other actuation could be used in alternate but equivalent embodiments. Similarly, moveable member  101  may be actuated into position using any actuation technique or structure  308 . Although a simple block actuator  308  is shown in  FIG. 6 , in practice moveable member  101  may be displaced with a comb-type or other actuator such as that discussed in conjunction with  FIG. 3  above. 
     In various embodiments, the relative positions of outcropping  510  and areas  606  may be designed so as to increase the amount of leverage applied by terminal arms  506  and/or  508  upon moveable member  101 . In the embodiment shown in  FIG. 6 , arms  506  and  508  appropriately pivot about a relatively fixed base  512 . If the actuation force is applied to the arms at a position on arms  506 ,  508  that is relatively far from the pivot point, the amount of displacement realized from the actuation force can be increased or maximized. Similarly, by locating outcropping  510  to be relatively nearer to pivot point  510 , the amount of leverage applied by arms  506 ,  508  upon member  101  can be increased. This increase in leverage appropriately provides improved mechanical force to thereby maintain arms  506 ,  508  in position against member  101 , and serves to increase the efficiency of force applied for a given duration or magnitude of actuating force. Of course other physical layouts of arms  506 ,  508  and member  101  could be formulated, with outcropping  510  and/or areas  606  being relocated, eliminated or combined in other equivalent embodiments. The efficiency of the actuating force can be further increased by providing a dielectric material in the spaces surrounding and/or in close proximity to actuators  602 ,  604  and/or areas  606 . Examples of dielectric materials that may be present in various exemplary embodiments include ceramics, polymers (e.g. polyimides or epoxies), silicon dioxide (SiO 2 ), dielectric liquids and/or any other organic or inorganic dielectric material. 
     Accordingly, there is provided a micro electromagnetic switch that is capable of providing enhanced electrical connectivity, and that is capable of remaining in a selected output state even when actuation energy is no longer provided to the switch. Such switches have numerous applications across many fields, including medical, aerospace, consumer electronics, and the like. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.