Abstract:
A representative embodiment of the invention provides a thermal actuator for a MEMS-based relay switch. The thermal actuator has an “active” arm that is movably mounted on a substrate. The “active” arm has (i) a thermal expansion layer and (ii) a resistive heater that is electrically isolated from the thermal expansion layer. The thermal expansion layer is adapted to expand in response to a temperature change induced by a control current flowing through the resistive heater, thereby bending the “active” arm and moving that arm with respect to the substrate. Due to the fact that mechanical and electrical characteristics of the “active” arm are primarily controlled by the thermal expansion layer and the resistive heater, respectively, those characteristics can be optimized independently to obtain better operating characteristics for MEMS-based relay switches of the invention compared to those attained in the prior art.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to micro-electromechanical systems (MEMS) and, more specifically, to MEMS-based relays for direct-current (DC) and radio-frequency (RF) electrical cross-connects. 
         [0003]    2. Description of the Related Art 
         [0004]    MEMS-based relays serve as a viable alternative to conventional mechanical relays. More specifically, MEMS-based relays are more compact and more cost effective than conventional mechanical relays. For RF applications, MEMS-based relays offer relatively low series resistance, substantially no power consumption in ON and OFF states, and relatively low intermodulation distortion compared to that, e.g., in field-effect-transistor (FET)-based relays. As a result, MEMS-based relays using electrical, magnetic, or thermal actuation, with both mono-stable and bi-stable designs are being actively developed. 
       SUMMARY OF THE INVENTION 
       [0005]    A representative embodiment of the invention provides a thermal actuator for a MEMS-based relay switch. The thermal actuator has an “active” arm that is movably mounted on a substrate. The “active” arm has (i) a thermal expansion layer and (ii) a resistive heater that is electrically isolated from the thermal expansion layer. The thermal expansion layer is adapted to expand in response to a temperature change induced by a control current flowing through the resistive heater, thereby bending the “active” arm and moving that arm with respect to the substrate. Due to the fact that mechanical and electrical characteristics of the “active” arm are primarily controlled by the thermal expansion layer and the resistive heater, respectively, those characteristics can be optimized independently to obtain better operating characteristics for MEMS-based relay switches of the invention compared to those attained in the prior art. 
         [0006]    According to one embodiment, a device of the invention comprises first and second arms movably supported on a substrate. A first end of each of the first and second arms is attached to a respective anchor affixed to the substrate. Second ends of the first and second arms are mechanically connected to one another. The first arm comprises (i) a first thermal expansion layer and (ii) a first resistive heater that is different from the first the first thermal expansion layer. The first resistive heater is adapted to increase temperature of the first thermal expansion layer in response to a first electrical current driven through the first resistive heater. The first thermal expansion layer is adapted to expand in response to the temperature increase induced by the first resistive heater and move the second ends of the first and second arms with respect to the substrate due to said expansion of the first thermal expansion layer. 
         [0007]    According to another embodiment, a method of the invention comprises driving a first electrical current through a first resistive heater of a device. The device comprises first and second arms movably supported on a substrate. A first end of each of the first and second arms is attached to a respective anchor affixed to the substrate. Second ends of the first and second arms are mechanically connected to one another. The first arm comprises (i) a first thermal expansion layer and (ii) the first resistive heater, wherein the first resistive heater is different from the first thermal expansion layer. The first resistive heater is adapted to increase the temperature of the first thermal expansion layer in response to the first electrical current. The first thermal expansion layer is adapted to expand in response to the temperature increase induced by the first resistive heater and move the second ends of the first and second arms with respect to the substrate due to said expansion of the first thermal expansion layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Other aspects, features, and benefits of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which: 
           [0009]      FIGS. 1A-B  show top views of a prior-art MEMS-based thermal actuator in “cold” and “hot” states, respectively; 
           [0010]      FIGS. 2A-D  show top views of a prior-art relay switch having two thermal actuators, each of which is analogous to the thermal actuator shown in  FIG. 1 ; 
           [0011]      FIGS. 3A-C  show a MEMS-based relay switch according to one embodiment of the invention; and 
           [0012]      FIGS. 4A-E  illustrate representative fabrication steps for a MEMS-based relay switch according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIGS. 1A-B  show top views of a prior-art MEMS-based thermal actuator  100  in “cold” and “hot” states, respectively. Actuator  100  has two cantilevered arms  110  and  130 , each anchored to a substrate at one end and linked to the other arm at the other end. The plane of the substrate is parallel to the plane of  FIGS. 1A-B . Arm  110  is a “passive” arm that is attached to the substrate at an anchor  112 . Arm  110  is (i) generally parallel to the substrate, (ii) detached from the substrate along the arm&#39;s length, and (iii) movable with respect to the substrate. Arm  130  is an “active” arm that has two beams  134   a - b . Each of beams  134   a - b  is (i) generally parallel to the substrate, (ii) attached at one end to a respective one of anchors  132   a - b , each of which is similar to anchor  112 , (iii) detached from the substrate along the beam&#39;s length, and (iv) movable with respect to the substrate. 
         [0014]    Arms  110  and  130  are mechanically connected to one another by a suspended dielectric tether  140 . Tether  140  is movable with respect to the substrate and supports two conducting structures  142  and  144  that are electrically isolated from one another by a trench between them and due to the fact that the tether does not conduct electricity. Structure  142  electrically connects a beam  114  of arm  110  to a tip  146  to create a continuous electrical path between anchor  112  and the tip. Structure  144  electrically interconnects beams  134   a - b  of arm  130  to create a continuous electrical path between anchors  132   a - b.    
         [0015]    Each of beams  134   a - b  is typically made of a nickel alloy or other suitable electrically conducting material having a relatively large thermal expansion coefficient. If a control current is passed through arm  130  between anchors  132   a - b  (see  FIG. 1B ), then the current resistively heats up beams  134   a - b . The resulting thermal expansion of beams  134   a - b  causes the beams to bow, thereby bending a neck portion  116  of beam  114  and moving tether  140  and tip  146  with respect to the substrate as shown in  FIG. 1B . If the control current is turned OFF, then beams  134   a - b  cool down and contract, thereby returning tip  146  to the initial position shown in  FIG. 1A . 
         [0016]      FIGS. 2A-D  show top views of a prior-art relay switch  200  having two thermal actuators  202   a - b , each of which is analogous to thermal actuator  100  of  FIG. 1 . More specifically,  FIGS. 2A and 2D  show switch  200  in OFF and ON states, respectively.  FIGS. 2B-C  show two transition configurations of switch  200  between the OFF state shown in  FIG. 2A  and the ON state shown in  FIG. 2D . The circular insets in each of  FIGS. 2A-D  show respective enlarged views of the contact area of switch  200  having tips  246   a - b  of actuators  202   a - b , respectively. 
         [0017]    If no currents flow through the “active” arms of actuators  202   a - b , then tips  246   a - b  are separated from one another by an air gap, as shown in  FIG. 2A . Because of the air gap, there is no continuous electrical path between anchors  212   a - b , and switch  200  is in the OFF state. To transition switch  200  to an ON state, first, a first control current is driven through the “active” arm of actuator  202   b . The resulting heating and deformation of that “active” arm causes a displacement of tip  246   b  of actuator  202   b  in the positive Y direction, as shown in  FIG. 2B . Second, a second control current is driven through the “active” arm of actuator  202   a . The resulting heating and deformation of that “active” arm causes a displacement of tip  246   a  of actuator  202   a  in the negative X direction, as shown in  FIG. 2C . Third, the first control current is turned OFF, which causes the “active” arm of actuator  202   b  to cool down and return tip  246   b  to the initial position. Finally, the second control current is turned OFF. As the “active” arm of actuator  202   a  cools down, it attempts to return tip  246   a  to its initial position. However, tip  246   b  now blocks the return path of tip  246   a . As a result, the contracting “active” arm of actuator  202   a  pushes a surface  248   a  of tip  246   a  against a corresponding surface  248   b  of tip  246   b  to interlock the two tips as shown in  FIG. 2D . After tips  246   a - b  have interlocked, the air gap between the tips has closed to create a continuous electrical path between anchors  212   a - b . Thus, switch  200  is now in the ON state. Note that no control currents are needed to keep switch  200  in the ON state because the elastic return force generated by actuator  202   a  is substantially orthogonal to surfaces  248   a - b . Consequently, the return force lacks a tangential component that is needed to disengage surfaces  248   a - b  from one another. 
         [0018]    To transition switch  200  back to the OFF state, the above-described sequence is performed in the reverse order. More specifically, first, the second control current is turned ON to move tip  246   a  in the negative X direction from the position shown in  FIG. 2D . Second, the first control current is turned ON to move tip  246   b  in the positive Y direction to arrive at the configuration shown in  FIG. 2C . Third, the second control current is turned OFF to arrive at the configuration shown in  FIG. 2B . Finally, the first control current is turned OFF to arrive at the configuration shown in  FIG. 2A , which represents an OFF state of switch  200 . More details on the structure and operation of switch  200  can be found, e.g., in U.S. Pat. No. 6,407,478, which is incorporated herein by reference in its entirety. 
         [0019]    One problem with actuator  100  and switch  200  is that the material of an “active” arm, e.g., arm  130  ( FIG. 1 ), performs two functions. First, the material serves as an electric conductor and resistive heater for the “active” arm. Second, the material serves as a mechanical elastic member that flexes, expands, and contracts to generate the desired tip displacements. Due to this dual functionality, the mechanical and electrical properties of the “active” arm cannot be optimized independently. For example, if beams  134   a - b  are made of a nickel alloy, then the beams have good thermal expansion and elastic characteristics, but relatively low electrical resistance. As a result, a relatively high control current has to be applied to “active” arm  130  to resistively heat the arm to a temperature that is sufficient, e.g., for implementing the configuration sequence shown in  FIGS. 2A-D . The relatively high control currents might disadvantageously cause the power consumption in switch  200  to be relatively high. In addition, the relatively low electrical resistance of the “active” arms forces the use of special low-resistance wiring for feeding the control currents to the “active” arms because, otherwise, the wiring becomes disadvantageously hot as well. 
         [0020]    Problems in the prior art are addressed by embodiments of a thermal actuator of the present invention, in which electrical and mechanical characteristics of an “active” arm are controlled by two separate structures. The first structure primarily functions as a resistive heater for the “active” arm, without significantly affecting the mechanical characteristics of the arm. The second structure primarily functions as a mechanical elastic member that does not affect the electrical characteristics of the arm. Advantageously over the prior art, the mechanical and electrical properties of the “active” arm can now be optimized independently. As a result, switch designers have more flexibility to attain desired switch characteristics. 
         [0021]      FIGS. 3A-C  show a MEMS-based relay switch  300  according to one embodiment of the invention. More specifically,  FIG. 3A  shows a top view of switch  300 , and  FIGS. 3B-C  show cross-sectional side views of the switch along the planes labeled BB and CC, respectively, in  FIG. 3A . 
         [0022]    Referring to  FIG. 3A , switch  300  has two thermal actuators  302   a - b  that are oriented substantially orthogonally to one another. To display a sufficiently detailed view of the actuator structure,  FIG. 3A  shows thermal actuator  302   b  in full, while showing thermal actuator  302   a  only partially. The omitted portion of actuator  302   a  is similar to the corresponding portion of actuator  302   b.    
         [0023]    Each of actuators  302   a - b  has a respective cantilevered “passive” arm  310  and a respective cantilevered “active” arm  330 . Arm  310  is attached to a substrate  304  at an anchor  312 , and arm  330  is attached to substrate  304  at two anchors  332 . Each of arms  310  and  330  is (i) generally parallel to substrate  304  (also see  FIGS. 3B-C ), (ii) detached from the substrate along the arm&#39;s length, and (iii) movable with respect to the substrate. Arms  310  and  330  are mechanically connected to one another by a tether  340 . Tether  340  supports structures  342  and  344  that are separated from one another by a trench between them. Structure  342  is an electrically conducting structure that electrically connects a beam  314  of arm  310  to a tip  346  to create a continuous electrical path between anchor  312  and the tip. Structure  344  can be made of an electrically conducting material or a dielectric material and primarily serves to anchor arm  330  to tether  340 . 
         [0024]    Referring to  FIGS. 3A-C , arm  330  has a dielectric layer  352  and a thermal-expansion layer  354 . In one embodiment, layer  354  is made of a nickel alloy and is generally similar to beam  134  of actuator  100 . Layer  352  is a dielectric layer that encapsulates a resistive heater  350  and electrically isolates the heater from layer  354 . In one embodiment, layer  352  is made of silicon nitride, and heater  350  is made of poly-silicon. 
         [0025]    Heater  350  is a relatively narrow conducting track that electrically connects two respective anchors  332  (see  FIG. 3A ). In the embodiment of  FIG. 3 , electrical heater  350  has a switchback-shaped part, which enables the length of the heater to be about six times (6×) longer than the length of arm  330 . One skilled in the art will understand that other conducting track layouts for heater  350  can similarly be used to obtain a desired track length. In prior-art “active” arm  130  ( FIG. 1 ), bodies of beams  134   a - b  form a conducting track. The length of that conducting track is about two times the distance between the two opposite ends of arm  130 , e.g., the first end being structure  144  and the second end being at anchors  132   a - b , and this relationship between the length of the conducting track and the length of the arm is fixed. In contrast, a switch designer can change the length of the conducting track for heater  350  with respect to the length of arm  330 . More specifically, depending on the number of switchbacks in the switchback-shaped part of the conducting track, the length of the conducting track can be about 2×, 4×, 6×, 8×, etc., the length of arm  330 . 
         [0026]    Arm  310  has an optional dielectric layer  362  and a conducting layer  364 . In one embodiment, layers  352  and  362  can be made of the same material, e.g., from a common layer of a multi-layered wafer. Layers  354  and  364  can similarly be made of the same material, e.g., from another common layer of the multi-layered wafer. Arm  310  may optionally have an electrically conductive coating  360 , e.g., made of gold, which serves to improve electrical conductivity of the arm. 
         [0027]    Referring to  FIGS. 3A and 3C , arm  310  has a neck portion  316  that enables that arm to deflect relatively easily when arm  330  thermally expands. Arm  330  also has a neck portion  336  that enables that arm to bow/bend when it is heated by heater  350 . Similar to other portions of arm  330 , neck portion  336  has layers  352  and  354 . The portion of layer  352  corresponding to neck portion  336  passes through a single section, i.e., section  350 - 1 , of heater  350 . Arm  330  further has a suspended wire  338  that is part of heater  350 . More specifically, wire  338  has section  350 - 2  of heater  350  encapsulated by the corresponding portion of layer  352 . Together, heater section  350 - 1  and wire  338  provide electrical leads from anchors  332  to the switchback-shaped part of heater  350 . In one embodiment, suspended wire  338  can be strain-relieved by having, e.g., a serpentine shape. 
         [0028]      FIG. 3A  shows an OFF state of switch  300 . To transition switch  300  into an ON state, the switch is stepped through a configuration sequence that is similar to the configuration sequence shown in  FIGS. 2A-D . More specifically, first, a first control current is driven through heater  350   b  of actuator  302   b  to move tip  346   b  in the negative Y direction. Second, a second control current is driven through heater  350   a  of actuator  302   a  to move tip  346   a  in the positive X direction. Third, the first control current is turned OFF to return tip  346   b  into the initial position shown in  FIG. 3A . Finally, the second control current is turned OFF to latch tips  346   a  and  346   b . One skilled in the art will understand that a reverse configuration sequence will transition switch  300  from the ON state to the OFF state. 
         [0029]      FIGS. 4A-E  illustrate representative fabrication steps for a MEMS-based switch  400  according to one embodiment of the invention. More specifically, each of  FIGS. 4A-E  shows a cross-sectional side view of a multilayered wafer, using which switch  400  is being fabricated, at the corresponding fabrication step. Each of the cross-sectional views is similar to the cross-sectional view of switch  300  shown in  FIG. 3B . 
         [0030]    Referring to  FIG. 4A , fabrication of switch  400  begins with a silicon substrate  480 . First, a sacrificial silicon oxide layer  482  is deposited over substrate  480 . Then, layer  482  is patterned and etched to form openings (not explicitly shown in  FIG. 4A ), e.g., for forming anchors analogous to anchors  312  and  332  of  FIG. 3 . 
         [0031]    Referring to  FIG. 4B , first, a poly-silicon layer  484  is deposited over layer  482 . Layer  484  is then patterned and etched to form a heater  450  analogous to heater  350  of  FIG. 3 . 
         [0032]    Referring to  FIG. 4C , first, a silicon-nitride layer  486  is deposited over the structure of  FIG. 4B . Layer  486  is then patterned and etched according to the layout of thermal actuators of switch  400 . The corresponding portions of layer  486  form layers  452  and  462  of the actuator arms analogous to layers  352  and  362 , respectively, of  FIG. 3 . 
         [0033]    Referring to  FIG. 4D , first, nickel-alloy layers  454  and  464  are electroplated over layers  452  and  462 , respectively. Layers  454  and  464  are generally analogous to layers  354  and  364  of  FIG. 3 . Then, a gold layer  460  is deposited over layer  464 . Layer  460  is generally analogous to layer  360  of  FIG. 3 . 
         [0034]    Referring to  FIG. 4E , sacrificial layer  482  is removed (e.g., etched away) from the structure shown in  FIG. 4D  to arrive at the final structure of switch  400 . Note that, unlike layer  352  of switch  300 , which fully encapsulates heater  350 , layer  452  of switch  400  encapsulates heater  450  only partially. Full encapsulation of the heater would be required if a trench similar to trench  306  (see, e.g.,  FIG. 3B ) had to be formed in silicon substrate  480 . Otherwise, the reactants that etch the trench in substrate  480  would also etch away the exposed material of heater  450 , also made of silicon. A trench similar to trench  306  may be useful for improving thermal isolation of the “active” arm from the substrate and expanding the accessible temperature range for the “active” arm with respect to that in the structure without such a trench. 
         [0035]    Other suitable fabrication techniques that can be used for fabricating relay switches of the invention are disclosed, e.g., in commonly owned U.S. Pat. Nos. 6,850,354 and 6,924,581, the teachings of which are incorporated herein by reference. Additional layers of material may be deposited using, e.g., chemical vapor deposition. Various parts of the switches may be mapped onto the corresponding layers using lithography. Additional description of various fabrication steps may be found, e.g., in U.S. Pat. Nos. 6,201,631, 5,629,790, and 5,501,893, the teachings of all of which are incorporated herein by reference. Representative fabrication-process flows can be found, e.g., in U.S. Pat. Nos. 6,667,823, 6,876,484, 6,980,339, 6,995,895, and 7,099,063 and U.S. patent application Ser. No. 11/095,071 (filed on Mar. 31, 2005), the teachings of all of which are incorporated herein by reference. 
         [0036]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. For example, a heater can be made of other Si compounds, such as SiGe and metal silicides; a thermal expansion can be made of metals, such as Cu and Tungsten and their alloys; and a heater-encapsulating layer can be made of silicon oxide or polymers, such as polyimide and benzocyclobutene (BCB). Various surfaces may be modified, e.g., by metal deposition for enhanced electrical conductivity, or by ion implantation for enhanced mechanical strength. Differently shaped arms, tethers, beams, latches, heaters, and/or anchors may be implemented without departing from the scope and principle of the invention. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims. 
         [0037]    It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the present invention. 
         [0038]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
         [0039]    Throughout the detailed description, the drawings, which are not to scale, are illustrative only and are used in order to explain, rather than limit the invention. The use of terms such as height, length, width, left, right, top, bottom is strictly to facilitate the description of the invention and is not intended to limit the invention to a specific orientation. 
         [0040]    For the purposes of this specification, a MEMS device is a device having two or more parts adapted to move relative to one another, where the motion is based on any suitable interaction or combination of interactions, such as mechanical, thermal, electrical, magnetic, optical, and/or chemical interactions. MEMS devices are fabricated using micro- or smaller fabrication techniques (including nano-fabrication techniques) that may include, but are not necessarily limited to: (1) self-assembly techniques employing, e.g., self-assembling monolayers, chemical coatings having high affinity to a desired chemical substance, and production and saturation of dangling chemical bonds and (2) wafer/material processing techniques employing, e.g., lithography, chemical vapor deposition, patterning and selective etching of materials, and treating, shaping, plating, and texturing of surfaces. The scale/size of certain elements in a MEMS device may be such as to permit manifestation of quantum effects. Examples of MEMS devices include, without limitation, NEMS (nano-electromechanical systems) devices, MOEMS (micro-opto-electromechanical systems) devices, micromachines, Microsystems, and devices produced using microsystems technology or microsystems integration. 
         [0041]    Although the present invention has been described in the context of implementation as MEMS devices, the present invention can in theory be implemented at any scale, including scales larger than micro-scale. 
         [0042]    Also for purposes of this description, the terms “connect,” “connecting,” or “connected” refer to any manner known in the art or later developed in which a particular type of energy (e.g., electrical or mechanical) is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the term “directly connected,” etc., imply the absence of such additional elements.