Patent Publication Number: US-7218193-B2

Title: MEMS-based inertial switch

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to switches and, more specifically, to motion-sensitive switches. The present invention also relates to micro-electromechanical systems (MEMS). 
   2. Description of the Related Art 
   An inertial switch is a switch that can change its state, e.g., from open to closed, in response to acceleration and/or deceleration. For example, when the absolute value of acceleration along a particular direction exceeds a certain threshold value, the inertial switch changes its state, which change can then be used to trigger an electrical circuit controlled by the inertial switch. Inertial switches are employed in a wide variety of applications such as automobile airbag deployment systems, vibration alarm systems, detonators for artillery projectiles, and motion-activated light-flashing footwear. Description of several representative prior-art inertial switches can be found, for example, in U.S. Pat. Nos. 6,354,712, 5,955,712, 4,178,492, and 4,012,613, the teachings of all of which are incorporated herein by reference. 
   A conventional inertial switch is a relatively complex, mechanical device assembled using several separately manufactured components such as screws, pins, balls, springs, and other elements machined with relatively tight tolerance. As such, conventional inertial switches are relatively large (e.g., several centimeters) in size and relatively expensive to manufacture and assemble. In addition, conventional inertial switches are often prone to mechanical failure. 
   SUMMARY OF THE INVENTION 
   Problems in the prior art are addressed, in accordance with the principles of the present invention, by an inertial switch designed as a MEMS device. In one embodiment, the MEMS device is manufactured using a layered wafer and has a movable electrode supported on a substrate layer of the wafer and a stationary electrode attached to that substrate layer. The movable electrode is adapted to move with respect to the substrate layer in response to an inertial force such that, when the inertial force per unit mass reaches or exceeds a contact threshold value, the movable electrode is brought into contact with the stationary electrode, thereby changing the state of the inertial switch from open to closed. In one implementation, the MEMS device is a substantially planar device, designed such that, when the inertial force is parallel to the device plane, the displacement amplitude of the movable electrode from a zero-force position is substantially the same for all force directions. Advantageously, an inertial switch of the invention is a monolithic device, which enables the switch to have a relatively small size (e.g., about one millimeter) and be relatively inexpensive. Due to the small size and low cost, several inertial switches of the invention may be incorporated into a corresponding switch circuit, thereby providing protection against mechanical failure and/or malfunction of any individual inertial switch in that circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A–B  show top and cross-sectional views, respectively, of an inertial switch according to one embodiment of the present invention; 
       FIG. 2A–B  show top and cross-sectional views, respectively, of an inertial switch according to another embodiment of the present invention; 
       FIG. 3  shows a beacon circuit that can employ one of the inertial switches shown in  FIGS. 1 and 2  according to one embodiment of the present invention; and 
       FIG. 4  shows a beacon circuit that can employ one of the inertial switches shown in  FIGS. 1 and 2  according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   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 of the invention. 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 mutually exclusive of other embodiments. 
     FIGS. 1A–B  show top and cross-sectional views, respectively, of an inertial switch  100  according to one embodiment of the present invention. Switch  100  is a MEMS device manufactured using a silicon-on-insulator (SOI) wafer  110 . Since manufacturing techniques for fabricating SOI MEMS structures are well developed, switch  100  can be designed to have a relatively small size. For example, modem lithographic techniques may be used to define various switch elements in wafer  110  with a sub-micron resolution, thereby making it possible to have a switch  100  that takes up less than one square millimeter of the wafer area. As a result, a relatively large number of switches  100  can be manufactured using a single wafer  110 , thereby significantly reducing the cost of each individual switch. Furthermore due to the small size and low cost, several instances of switch  100  may be incorporated into a corresponding switch circuit, thereby providing protection against mechanical failure and/or malfunction of any individual switch  100  in that circuit. 
   Wafer  110  has (i) two silicon layers, i.e., a substrate layer  112  and an overlayer  116 , and (ii) a silicon oxide layer  114  located between overlayer  116  and substrate layer  112 . Substrate layer  112  provides support for the switch structure; silicon oxide layer  114  provides electrical insulation between overlayer  116  and substrate layer  112 ; and overlayer  116  is used to define certain switch elements, each of which is described in more detail below. In particular, the following switch elements are defined in overlayer  116 : a stationary electrode  122 , a movable electrode  124 , a support structure  126 , a spring  128 , a contact pad  130 , and a conducting track  132 . 
   Movable electrode  124  includes an annular mass detached from substrate layer  112  and supported on the substrate layer by spring  128  and support structure  126 . Support structure  126  is located within an inner opening of the annular mass. In one embodiment, spring  128  has three planar spiral segments  128   a–c , each attached between the outer circumference of support structure  126  and the inner circumference of the annular mass. The ends of spiral segments  128   a–c  that are attached to support structure  126  lie approximately on a circle and are separated from each other by an angle of about 120 degrees. Similarly, the ends of spiral segments  128   a–c  that are attached to the inner circumference of the annular mass of movable electrode  124  lie approximately on another circle and are also separated from each other by an angle of about 120 degrees. The circles are substantially concentric with each other and, for each of segments  128   a–c , an angle between (i) a line passing through the circles center and the segment end attached to movable electrode  124  and (ii) a line passing through the circles center and the segment end attached to support structure  126  is about 240 degrees. As such, each of spiral segments  128   a–c  extends around support structure  126  for an angle of about 240 degrees. This configuration of spiral segments  128   a–c  produces an angular overlap between each pair of spiral segments of about 120 degrees. The presence of this overlap causes spring  128  to have a spring constant that is substantially isotropic within the plane defined by overlayer  116 , i.e., substantially independent of the direction of the spring deformation within that plane. Consequently, a force of any particular magnitude acting within the plane of overlayer  116  produces a displacement for movable electrode  124 , the amplitude of which displacement is substantially the same for all force directions. 
   In one embodiment, the annular mass of movable electrode  124  is an axially symmetric grid structure composed of radial and circular beams as indicated in  FIG. 1A . One purpose of having this grid structure is to facilitate the detachment of movable electrode  124  from the underlying portion of substrate layer  112  during the fabrication process of switch  100 . More specifically, a wet etchant that is typically used to remove selected portions of silicon oxide layer  114  during the fabrication process can be brought into good and sufficient contact with the portion of the silicon oxide layer initially present underneath electrode  124  by infiltrating the openings of the grid structure (see also  FIG. 1B ). Once in contact with that portion of silicon oxide layer  114 , the wet etchant first removes the silicon oxide located directly under the grid openings and then undercuts the silicon oxide located under the beams of the grid structure, thereby detaching electrode  124  from substrate layer  112 . 
   In one embodiment, stationary electrode  122  is a circular electrode that surrounds movable electrode  124 . When movable electrode  124  is in its zero-force equilibrium position, the movable electrode is separated from stationary electrode  122  by a circular gap  140 , the width of which defines a contact threshold value for switch  100 . A contact threshold value for switch  100  is defined as the lowest possible value of an inertial force per unit mass that can bring movable electrode  124  into contact with stationary electrode  122 . An increase in the designed width of gap  140  will result in an increase of the contact threshold value for switch  100  because, to make contact with stationary electrode  122 , movable electrode  124  now has to cross a wider gap, which requires a greater displacement of the movable electrode from the initial equilibrium position and therefore an application of a greater inertial force to overcome the increased force generated by spring  128  due to the greater displacement. 
   Note that the thickness of overlayer  116  has substantially no effect on the contact threshold value because both the mass of movable electrode  124  and the spring constant of spring  128  change linearly with the thickness. In contrast, the spring constant of spring  128  for out-of-plane displacements of movable electrode  124  (i.e., displacements in the direction orthogonal to the plane of overlayer  116 ) is proportional to the cube of the thickness of overlayer  116 . As a result, one can restrict out-of-plane displacements of movable electrode  124  by using in switch  100  an SOI wafer  110  having a relatively thick overlayer  116 . 
   Movable electrode  124  is electrically connected to outside circuitry (not shown in  FIG. 1 ) via spring  128 , support structure  126 , and a wire lead  134   a  attached to the top of the support structure. Similarly, stationary electrode  122  is electrically connected to the outside circuitry via conducting track  132 , contact pad  130 , and a wire lead  134   b  attached to the top of the contact pad. 
   Switch  100  may operate, for example, as follows. When switch  100  is at rest or moving at a constant velocity, gap  140  separates movable electrode  124  and stationary electrode  122 , thereby keeping the switch in the open state. When switch  100  is accelerated such that the absolute value of the acceleration projection onto the plane of overlayer  116  exceeds the contact threshold value (discussed above), the resultant inertial force causes movable electrode  124  to cross gap  140  and come into contact with stationary electrode  122 , thereby changing the state of switch  100  from open to closed. When the absolute value of the acceleration projection falls below the contact threshold value, the spring force of spring  128  causes movable electrode  124  to become separated from stationary electrode  122 , thereby returning switch  100  into the open state. 
   One skilled in the art will understand that switch  100  reacts in a substantially analogous fashion to equal levels of acceleration and deceleration. This property of switch  100  can be understood from the following analysis. Suppose that switch  100  is triggered (i.e., changes its state from open to closed) by a certain amount of acceleration in a particular direction, denoted hereafter in this analysis as direction X. Then, due to the isotropic properties of spring  128  and substantial axial symmetry of electrodes  122  and  124 , switch  100  will also be triggered by the same level of acceleration in the opposite direction, i.e., in direction −X. Then, by noting the fact that the inertial force generated by acceleration in direction −X is the same as the inertial force generated by equal deceleration in direction X, one arrives at the conclusion that, if switch  100  is triggered by acceleration in direction −X, it will also be triggered by equal deceleration in direction X. Comparing this conclusion with the initial supposition, one finally concludes that, if switch  100  is triggered by a certain level of acceleration in direction X, it will also be triggered by the equal level of deceleration in the same direction X. In view of this property of switch  100 , said switch can be used in inertial sensors designed to detect an inertial force exceeding a selected threshold value regardless of the force origin or direction. 
   Different etching techniques may be used to fabricate switch  100  from the initial SOI wafer. It is known that silicon etches significantly faster than silicon oxide using, e.g., selective reactive ion etching (RIE). Similarly, silicon oxide etches significantly faster than silicon using, e.g., fluorine-based etchants. Various surfaces may be metal-plated using, e.g., chemical vapor deposition. Various parts of switch  100  may be mapped onto the overlayer of the SOI wafer using lithography. Additional description of various fabrication steps suitable for the fabrication of switch  100  may be found in U.S. Pat. Nos. 6,201,631, 5,629,790, and 5,501,893, the teachings of which are incorporated herein by reference. 
     FIGS. 2A–B  show top and cross-sectional views, respectively, of an inertial switch  200  according to another embodiment of the present invention. Switch  200  is a MEMS device analogous to switch  100  of  FIG. 1 . Accordingly, analogous elements of switches  100  and  200  are marked in  FIGS. 1 and 2  with labels having the same last two digits. Certain differences between switches  100  and  200  are outlined below. 
   Switch  200  has a movable electrode  224  that is substantially similar to movable electrode  124  of switch  100 . However, instead of stationary electrode  122  that surrounds movable electrode  124  in switch  100 , switch  200  has a stationary electrode  222  located within an inner opening of the annular mass of movable electrode  224 . Consequently, a gap  240  that separates the stationary and movable electrodes in switch  200  is located between the inner circumference of the annular mass of movable electrode  224  and the outer circumference of stationary electrode  222 . In addition, unlike spring  128  in switch  100  that is placed at the inner circumference of the annular mass of movable electrode  124 , a spring  228  in switch  200  is placed at the outer circumference of the annular mass of movable electrode  224 . 
   In one embodiment, spring  228  has three planar spiral segments  228   a–c  similar in design to spiral segments  128   a–c  of spring  128  ( FIG. 1 ). However, each of segments  228   a–c  is attached between the outer circumference of the annular structure of movable electrode  224  and a portion  226  of overlayer  216  located around the movable electrode. As such, portion  226  in switch  200  serves a function analogous to that of support structure  126  in switch  100 . Similar to spring  128  in switch  100 , spring  228  in switch  200  is designed to have a spring constant that is substantially isotropic within the plane defined by overlayer  216 . 
   Movable electrode  224  is electrically connected to outside circuitry (not shown in  FIG. 2 ) via spring  228 , portion  226 , and a wire lead  234   a  attached to portion  226 . Similarly, stationary electrode  222  is electrically connected to the outside circuitry via a wire lead  234   b  attached to the top of the stationary electrode. 
   When switch  200  is at rest or moving with a constant velocity, gap  240  separates movable electrode  224  and stationary electrode  222 , thereby keeping the switch in the open state. When switch  200  is accelerated/decelerated such that the absolute value of the acceleration/deceleration projection onto the plane of overlayer  216  exceeds the contact threshold value of the switch, the resultant inertial force causes movable electrode  224  to cross gap  240  and come into contact with stationary electrode  222 , thereby changing the state of switch  200  from open to closed. When the absolute value of the acceleration/deceleration projection falls below the contact threshold value, the spring force of spring  228  causes movable electrode  224  to become separated from stationary electrode  222 , thereby returning switch  200  into the open state. 
     FIG. 3  shows a beacon circuit  300  according to one embodiment of the present invention. Circuit  300  has a power source (e.g., a battery)  350 , a crowbar circuit  360 , and a beacon (e.g., a light-emitting diode)  370 . Crowbar circuit  360  incorporates an inertial switch  362  that may be analogous to either one of switches  100  and  200  of  FIGS. 1 and 2 . Beacon circuit  300  turns on beacon  370  when inertial switch  362  is triggered (i.e., momentarily changes its state from open to closed). Beacon circuit  300  keeps beacon  370  in the on state even if, at a later time, inertial switch  362  returns to the open state. As a result, an observer can determine whether beacon circuit  300  is or has been subjected to a critical inertial force corresponding to the contact threshold value of inertial switch  362  by simply detecting a presence of the beacon signal. 
   Beacon circuit  300  operates, for example, as follows. In an initial state, inertial switch  362  is in the open state and a silicon-controlled rectifier (SCR) of crowbar circuit  360  is in the off state. This configuration holds beacon  370  in the off state. The SCR is a rectifier controlled by a gate signal. The SCR is switched from the off state (high resistance) to the on state (low resistance) by an appropriate signal applied to the gate. Once the SCR is turned on, it can remain in the on state even after removal of the gate signal as long as a minimum current, called the holding current, continues to flow through the SCR. In crowbar circuit  360 , resistors R 1 , R 2 , and R 3  are selected such that (i) the SCR is turned on when inertial switch  362  is triggered and (ii) a current greater than the holding current is maintained through the SCR, resistor R 1 , and beacon  370  when inertial switch  362  returns to the open state after the initial trigger. 
     FIG. 4  shows a beacon circuit  400  according to another embodiment of the present invention. Beacon circuit  400  comprises a power source (e.g., a battery)  450 , a breaker circuit  460 , and a beacon  470 , and is analogous to beacon circuit  300  ( FIG. 3 ). In particular, beacon circuit  400  (i) turns on beacon  470  when an inertial switch  462  incorporated into breaker circuit  460  is triggered and (ii) keeps beacon  470  in the on state even if, at a later time, inertial switch  462  returns into the open state. 
   Beacon circuit  400  operates, for example, as follows. In an initial state, both inertial switch  462  and a breaker switch  464  of breaker circuit  460  are in their open states, which holds beacon  470  in the off state. When inertial switch  462  is triggered, electrical current begins to flow through a coil  466  of breaker switch  464 , which causes a T-shaped conductor  468  to be pulled toward the center of the coil, thereby connecting (closing) the contacts of breaker switch  464 . Once breaker switch  462  is closed, electrical current continues to flow through coil  466  regardless of the state of inertial switch  462  because the breaker switch provides an electrical bypass around the inertial switch. As a result, T-shaped conductor  468  is kept in place by the electromagnetic force generated by coil  466 , thereby keeping the contacts of breaker switch  464  closed. As such, breaker switch  464  provides power from power supply  450  to beacon  470  and keeps the beacon in the on state. 
   While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. 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. 
   Although inertial switches of the invention have been described in the context of silicon/silicon oxide SOI wafers, other suitable materials, such as germanium-compensated silicon, may similarly be utilized. The materials may be appropriately doped as known in the art. Various surfaces may be modified, e.g., by metal deposition for enhanced electrical conductivity or by ion implantation for enhanced mechanical strength. Differently shaped electrodes, segments, beams, grids, pads, tracks, and support structures may be implemented without departing from the scope and principle of the invention. Springs may have different shapes and sizes, where the term “spring” refers in general to any suitable elastic structure that can recover its original shape after being distorted. A different number of segments may be used to implement an isotropic spring without departing from the scope and principles of the invention. Various types of beacons may be used in beacon circuits of the invention, wherein a beacon may be any suitable means (not limited to electromagnetic radiation-emitting devices) for enabling an observer to determine whether a corresponding inertial switch of the beacon circuit has been triggered. Beacon circuits may or may not be designed to keep the beacon in the on state when, after an initial contact, the electrodes of the inertial switch become separated. Various switches of the invention may be arrayed or integrated on a chip with other circuitry as necessary and/or apparent to a person skilled in the art. Two or more variously oriented inertial switches of the invention may be incorporated into a beacon circuit to enable the circuit to sense variously oriented inertial forces. 
   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 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. 
   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.