Abstract:
In a MEMS device employing a beam supported by transverse arms, potential bowing of the transverse arms caused by fabrication processes, temperature or local self-heating from resistive losses is accommodated by flexible terminations of the transverse arms. Alternatively, this bowing is controlled so as to provide selective biasing to the beam or mechanical advantage in the sensing of beam motion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. Patent application Ser. No. 10/001,412 filed Oct. 25, 2001, now U.S. Pat. No. 6,803,755, issued Oct. 12, 2004, which is a continuation-in-part of U.S. patent application Ser. No. 09/406,509 filed Sep. 28, 1999, now U.S. Pat. No. 6,348,788, issued Feb. 19, 2002, and U.S. Patent application Ser. No. 09/400,125 filed Sep. 21, 1999, now U.S. Pat. No. 6,417,743, issued Jul. 9, 2002, and claims the benefit thereof. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   BACKGROUND OF THE INVENTION 
   The present invention relates to microelectromechanical systems (MEMS) and in particular to MEMS devices employing beams supported for movement on flexible transverse arms. 
   MEMS devices are extremely small machines fabricated using integrated circuit techniques or the like. The small size of MEMS devices allows the production of high speed, low power and high reliability mechanisms. The fabrication techniques hold the promise of low cost mass production. 
   The parent applications to this present application describe a MEMS electrical isolator in which a beam is supported for longitudinal movement on a set of axially flexible arms, the latter of which are tied to a substrate. Motion of the beam caused by a MEMS actuator at one end of the beam, transmits a signal to a sensor positioned at the other end of the beam and separated from the actuator by an insulating segment. 
   The structure of a beam supported by transverse flexible elements provides an extremely simple and robust MEMS device. Nevertheless, the precision required for certain applications, particularly those related to sensors, may be difficult to achieve using mass-production integrated circuit processes. 
   BRIEF SUMMARY OF THE INVENTION 
   The present inventors have recognized that the complex multicomponent integrated circuit materials from which MEMS devices are constructed, have widely varying coefficients of expansion which may create distortions and stress in the MEMS beam structure (particularly in the flexible arms supporting the beam) as the MEMS device cools from high processing temperatures, or when the MEMS devices is used at different operating temperatures, or when the MEMS device is subject to local self-heating from the conduction of current. These distortions and stresses limit the beam structure&#39;s application to certain precision applications. 
   Accordingly, the present invention provides several techniques to compensate for such dimensional distortions and stress in beam-type MEMS devices, allowing mass-production of increasingly precise and accurate mechanisms. The present invention further provides methods of controlling the typical distortions in the flexible arms to provide increased functionality in beam-type MEMS devices. 
   In this regard, the invention provides improved methods of attaching the flexible arms that support the beam to the substrate. These attachment methods are augmented by enforcement of conditions of symmetry on the beam and its structure. Control of bowing of the transverse arms, discovered by the inventors in connection with their study of temperature induced distortions of the MEMS structure, is used to add bias or bi-stability or mechanical amplification to the MEMS device. 
   Specifically then, the present invention provides a microelectromechanical system (MEMS) that includes a beam supported on flexible transverse arms to move longitudinally along a substrate, where at least one of the flexible transverse arms is configured to at least one of tolerate and make use of bowing experienced by the arm. 
   It is one object of the invention, therefore, to provide an attachment system for the transverse arms that accommodates transverse dimensional changes in the arms caused by temperature changes and which, if uncorrected, can cause buckling of the arms, stress stiffening of the arms, or offset of the beam from its null position. 
   In certain embodiments, then, the present invention relates to a microelectromechanical system (MEMS) that includes a beam supported on flexible transverse arms to move longitudinally along a substrate, where ends of the arms removed from the beam are connected to the substrate by flexible longitudinally extending wrist elements. 
   The wrist elements may attach to the transverse arms via arcuate sections. 
   Thus, it is another object of the invention to eliminate points of concentrated stress at the arm ends. 
   The wrist elements may include serpentine sections, and/or the serpentine sections may be placed at the ends of the transverse arms where they are attached to the wrist elements. 
   Thus, it is another object of the invention to provide an attachment mechanism for the transverse arms that is both transversely and rotationally unrestrained so as to mimic a “free beam” whose ends are unrestrained. Transverse arms that approximate a free beam provides a less stiff bending force with movement of the supported beam and avoid stress stiffening such as may change the dynamic characteristics of the MEMS device. 
   The beam may be supported at longitudinally opposed ends by pairs of transverse arms extending from either side of the beam and the wrist elements for the transverse arms may either all extend toward the center of the beam or all extend away from the center of the beam. 
   Thus, it is another object of the invention to balance any forces on the beam caused by a slight bowing of the transverse arms such as may be incurred by an expansion of those arms or other distortions by encouraging countervailing bowing. It is a further object of the invention to compensate for any Lorentz forces that may occur on the wrists when current is passed through the transverse arms. By facing the wrists in the same direction, a transverse balancing of Lorentz forces from the wrists is obtained. 
   The beam may be supported at its center by a pair of transverse arms extending from the beam on opposite sides of the beam and the wrist elements for the center transverse arm may extend in opposite longitudinal directions. 
   Thus it is another object of the invention to promote an S-shape bending for a transverse arm centered on the beam such as prevents any longitudinal biasing of the beam as would occur with an uninflected bowing. Such a central beam may have no current flowing through it to eliminate any issues with Lorentz forces. 
   The beam may be designed to stabilize at a dimension that places the respective pairs of transverse arms on either end of the beam in equal and opposite flexure: either bowing in or bowing out. Thus, it is another object of the invention to balance any of the forces that may be placed on the beam by distortions in the lengths of the flexible arms. 
   In certain embodiments, therefore, the present invention relates to a microelectromechanical system (MEMS) that includes a beam supported on flexible transverse arms to move longitudinally along a substrate, where the beam is supported at longitudinally opposite ends by respective pairs of transverse arms extending from the beam on opposite sides of the beam and where the beam is sized to place the respective pairs of transverse arms in equal and opposite flexure. 
   The transverse arms may also be made of equal length. The points of attachment of the transverse arms to other than at ends of the beam may be centered between the points of attachment of the transverse arms at the end of the beam. The actuator and biasing structures for the beam may be placed at the end of the beam. 
   Thus, it is another object of the invention to enforce a longitudinal and transverse symmetry on the MEMS device so that other effects of dimensional distortion in the transverse arms and beam are balanced out. 
   In some embodiments, the beam may be supported on at least one pair of flexible transverse arms, which are bowed to present a force that increasingly resists longitudinal motion of the beam in a first direction up to a snap point after which the force abruptly decreases. The force may change direction after the snap point or keep the same direction. 
   Thus, in certain embodiments, the present invention relates to a microelectromechanical system (MEMS) that includes a beam supported on at least one pair of flexible transverse arms to move longitudinally along a substrate extending in a bow to present force increasingly resisting longitudinal motion of the beam in a first direction up to a snap point after which the force abruptly decreases. 
   Thus, it is another object of the invention to provide a bistable or monostable mode of operation of the beam device. 
   After the snap point, the bow may increasingly resist longitudinal motion of the beam in a second direction opposite the first direction up to a second snap point at which the force abruptly decreases. The second snap point may be different from the first snap point. 
   Thus, it is another object of the invention to provide for a hysteresis actuation of the beam using mechanical elements. 
   In a different embodiment, the beam may be supported by at least one flexible transverse arm, which is angled to also extend longitudinally. A sensor detecting transverse motion may receive the first transverse arm at an end removed from the beam. 
   Thus, it is another object of the invention to provide for a mechanical amplification of either the force or motion of the beam as transmitted to the sensor structure. 
   In certain embodiments, therefore, the present invention relates to a microelectromechanical system (MEMS) that includes a beam supported for longitudinal motion along a substrate on at least one pair of flexible transverse arms, a first of which is angled so as to also extend longitudinally. The MEMS further includes a sensor detecting transverse motion receiving the first transverse arm at an end removed from the beam, whereby longitudinal motion of the beam may be amplified for detection by the sensor. 
   The foregoing objects and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified block diagram of a beam-type MEMS device of the present invention in which the beam is supported on three sets of transversely extending arms; 
       FIG. 2  is a detailed, top plan view of the beam-type device of  FIG. 1  for use as an electrical isolator, the device using three electrostatic motors and a capacitive sensor attached to the beam and having wrist elements attaching the transverse arms to the substrate; 
       FIG. 3  is a schematic diagram of a simplified wrist element of  FIG. 2  such as provide transverse movement of the ends of the transverse arms and balanced Lorentz forces; 
       FIG. 4  is a perspective fragmentary view of a wrist element of  FIG. 3  showing an arcuate transition to reduce stress concentration; 
       FIG. 5  is a figure similar to that of  FIG. 3  showing an exaggeration expansion of the outer transverse arms that cause an inward bowing of the outer arms such as produces countervailing forces and an S bowing of the center transverse arms that produces a torsion but no net longitudinal force; 
       FIG. 6  is a fragmentary view similar to that of  FIG. 4  showing the addition of an expansion outrigger to the wrists counteracting expansion induced stress in the transverse arms; 
       FIGS. 7 and 8  show the addition of serpentine portions to the wrists and ends of the transverse arms such as provide both additional transverse compliance and rotational freedom simulating a free beam structure; 
       FIG. 9  is a diagram similar to  FIG. 1  showing major axes of symmetry, which are preserved in the invention to counteract additional forces; 
       FIG. 10  is a figure similar to that of  FIG. 3  showing an exploitation of expansion induced bowing to create a bistable biasing on the beam; 
       FIG. 11  is a plot of force versus longitudinal displacement of the beam showing the snap action created by buckling of the bowed transverse arm of  FIG. 10 ; 
       FIG. 12  is a figure similar to that of  FIG. 10  showing a fabricated stress-free bowing of a pair of transverse arms to provide a monostable biasing of the beam; 
       FIG. 13  is a plot similar to that of  FIG. 11  showing the monostable biasing provided by the bowing of the transverse arm of  FIG. 12 ; 
       FIG. 14  is a figure similar to that of  FIG. 12  showing attachment of the bowed transverse arm to movable position sensors, the arm such as may provide a mechanical leverage increasing sensitivity of the sensors to longitudinal movement of the beam; and 
       FIG. 15  is a geometric diagram showing the mechanical amplification provided by the bowed beam of  FIG. 14  reduced to a trigonometric approximation. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , a MEMS device  10  of the present invention may include a longitudinal beam  12  supported on three pairs of transverse arms  14 ,  16  and  18 , where transverse arms  14  extend from opposite sides of the leftmost longitudinal end of the beam  12 , transverse arms  16  extend from opposite sides of the longitudinal center of beam  12 , and transverse arms  18  extend from opposite sides of the rightmost longitudinal end of the beam  12 . As supported by flexing of the transverse arms  14 ,  16  and  18 , the beam  12  is free to move along a longitudinal axis  20 . 
   This beam structure can provide a number of useful MEMS by employing a combination of an actuator  22 , sensor  24  and biasing means  26  distributed along the beam  12  and possibly separated by insulating sections  28  and  30 . Generally, the actuator  22  and biasing means  26  may be any of a Lorentz force motor, an electrostatic motor, a piezoelectric motor, a thermal-expansion motor, and a mechanical-displacement motor, and the sensor  24  may be any of a capacitive sensor, a piezoelectric sensor, a photoelectric sensor, a resistive sensor, an optical switching sensor, or inductive sensor. 
   Referring now to  FIG. 2 , a MEMS device  10  for use as an electrical isolator and constructed according to the beam structure of  FIG. 1 , provides a beam  12  divided into conductive beam portions  12   a ,  12   b  and  12   c  separated by insulating sections  28  and  30 . The actuator  22  may be a Lorentz force actuator conducting a current along the transverse arm  14  in the presence of a magnetic field  32  to produce a force along longitudinal axis  20 . Current may be provided to the transverse arm  14  through terminals  34 . 
   A sensor  24  may be provided by capacitor banks  35  having inter-digitated capacitor plates  36   a  and  36   b , where the spacing of plates  36   a  increases with rightward longitudinal movement of the beam  12  and the spacing of plates  36   b  decreases with rightward movement. A comparison of the capacitances of plates  36   a  and  36   b  accessible through terminals  38   a ,  38   b  and  38   c  provides a position measurement of the beam  12  with a null position ideally being where the capacitances of plates  36   a  and  36   b  are equal. Precise location of the beam  12  both in a longitudinal and transverse manner is desired for proper operation of the capacitor plates  36   a  and  36   b.    
   Finally, a biasing means  26  is provided by a Lorentz force motor formed by current passing through transverse arm  18  introduced by means of terminals  40  in magnetic field  32 . 
   The structure of the MEMS device  10  generally includes as many as three layers including, for example, a metal layer, a silicon layer and an oxide layer. The structure of the beam  12  and transverse arms  14 ,  16 , and  18 , shown in  FIG. 2  may include all three layers which are cut away from a substrate  42  to be free therefrom, with the ends of the transverse arms  14 ,  16 , and  18  distal to the beam  12 , connected to the substrate  42  only at the terminals  34 ,  38  and  40 . The insulating sections  28  and  30  may be produced by removing an upper layer of metal and silicon  44  leaving only a bridge of oxide, or by other similar methods. 
   In operation, a current passing through transverse arm  14  creates an actuation force via its interaction with the magnetic field  32  causing movement of the beam  12  against a biasing force created by current passing through transverse arm  18 . The net effect is sensed by capacitor banks  36   a  and  36   b . In this way, an analog or digital isolator may be produced or a sensitive magnetic field measuring or current measuring device as well as many other devices. 
   Referring now to  FIG. 3 , each of the transverse arms  14 ,  16  and  18  may be connected through longitudinal wrist elements  46  to stationary pylons  48  being attached to the substrate  42 . The longitudinal wrist elements  46  allow some transverse movement of the distal ends of the transverse arms  14 ,  16  and  18  in the event of dimensional variations or expansion caused by electrical conduction. 
   Referring to  FIG. 5 , this transverse compliance provided by the wrists  46  reduces the bowing or distortion of the transverse arms  14 ,  16  and  18  (exaggerated in  FIG. 5 ) and prevents stress stiffening of the transverse arms  14 ,  16  and  18  such as would change the resonate frequency (or spring constant) of the beam  12  or the forces necessary to actuate the beam  12 . 
   In order to neutralize the effects of the Lorentz forces on the wrists  46 , the wrists  46  of current conducting transverse arms  14  and  18  are both directed in the same direction for transverse arm pairs  14  and  18 . Further, the wrists  46  of transverse arms  14  and  18  may be directed in opposite directions either both facing outward or both facing inward so as to direct any bowing in the transverse arms  14  and  18  in opposite directions so as to cancel the resulting force on the beam  12 . Judicious selection of the expansion characteristics of the beam  12  may promote an inward or outward bowing so as to ensure this balanced opposite bowing force. 
   In contrast, the wrists  46  of the conductive transverse arms  16  extending from the center of the beam  12  face in opposite longitudinal directions. This creates a more complex S shape bowing shown in  FIG. 5  with relative lengthening of the transverse arm  16  which provides a slight torsion but no net longitudinal force to the beam  12 . In this way, the null position of the beam (for example, as dictated by a midrange separation of the capacitor plates of the sensor) is preserved despite dimensional distortions caused by uneven contraction or expansion rates of the various components of the MEMS device  10 . 
   Referring now to  FIG. 4 , the wrists  46  may be attached to any of the transverse arms  14 ,  16  or  18  by means of a smoothly curving arcuate section  52  such as eliminates points of concentrated stress. 
   The above-described wrist elements  46  may accommodate dimensional changes caused by the manufacturing process or by local self-heating caused by currents used in the Lorentz actuators and biasing means. Variation in these dimensions caused by different ambient operating conditions may be reduced by the use of outriggers  54  of  FIG. 6  (one pair associated with each of transverse arms  14 ,  16  and  18 ) attached to pylons  48  adjacent to the beam  12  and extending transversely outward by nearly the full length of the transverse arms  14 ,  16  and  18 . The transverse arms  14 ,  16  and  18  may be attached by the laterally extending wrists  46  to the outboard ends of the outriggers  54  which are ideally constructed of the same materials as the wrists  46  and transverse arms  14 ,  16  and  18  to provide for compensating expansion. It will be understood that by using the outriggers  54 , expansion of the material of the transverse arms  14 ,  16  and  18  such as would cause a slackening of transverse arms  14 ,  16  and  18  is compensated for by nearly equal expansion of outriggers  54 , and vice versa. 
   Referring now to  FIG. 6 , the outriggers  54  are attached only at pylons  48  leaving the remainder of the wrists  46  and the transverse arms  14 ,  16  and  18  free above but lying in the plane of substrate  42 . 
   Referring to  FIG. 7 , the, wrists  46  may be modified to provide for a serpentine portion  51  providing both the transverse freedom shown by arrow  56  and increased rotational freedom shown by arrow  58  such as simulates a “free beam” configuration for transverse arms  14 ,  16  and  18  providing a less stiff and more uniform characteristic to their flexure. 
   Referring to  FIG. 8 , it will be seen that the serpentine portion  51  may be extended to the distal ends of the transverse arms  14 ,  16  and  18  to provide further flexure and further may be placed on the distal ends of the transverse arms  14 ,  16  and  18 , in lieu of their placement on the wrists  46  (not shown). The serpentine portions  51  may be crenellated as shown or may be a smoother curve to eliminate stress concentrations. 
   Referring again to  FIG. 2 , the wrists  46 , in an alternative embodiment particularly suited for transverse arm  16  may provide for two opposed wrist portions  46   a  and  46   b  extending in opposite longitudinal directions from the distal end of the transverse arm  16  to a T-configuration such as also may provide a neutral compensation for expansion of transverse arm  16  without the need for the S shaped bowing. 
   Referring now to  FIG. 9 , improved immunity to dimensional changes occurring during the fabrication process may be obtained by providing for strict symmetry of the MEMS device  10  along a longitudinal axis  20  passing through the beam  12  along its midpoint and a transverse axis  62  cutting the beam  12  transversely into two equal segments with respect to transverse arms  14  and  18 . This provides equal length of the transverse arms  14 ,  16  and  18  causing forces induced by these arms in contraction or expansion to be roughly equal preserving the midline alignment of the beam  12  along longitudinal axis  20 , whereas positioning transverse arm  16  midway between transverse arms  14  and  18  provide that the null point measured at the midpoint of the beam  12  remain roughly at the same location with respect to the substrate despite length differences in the beam  12  itself such as may draw the transverse arms  14  and  18  into a bow or expand them outward. 
   For similar reasons the actuator  22  and biasing means  26  may be placed symmetrically on opposite sides of the beam  12  and the sensor  24  sensing the null point as close as possible to the center of the beam  12  as determined by the connections of the beam  12  to the transverse arms  14  and  18 . 
   Referring now to  FIGS. 10 and 11 , the bowing of a beam  12 , for example, of transverse arm  18  (or any of the transverse arms) may be exploited to provide a biasing force to the beam  12 . Under this construction, the actuator  22  would be positioned at one end of the beam  12  and the sensor  24  positioned at the other end of the beam  12 . The bowing creates a snap action occurring as the beam  12  is moved from left to right. As a result of the bowing of the transverse arm  18 , which in this example is to the right, the force  66  resisting the rightward longitudinal movement of the beam is positive (rightward) and increases up to a snap point  68  whereupon the bow of the transverse arm  18  buckles and reforms as a bow in the opposite direction shown by dotted line of transverse arm  18 ′. This in turn results in a reversal of the force  66  to negative (leftward) past snap point  68 . 
   Now motion of the beam  12  in the opposite direction from left to right causes the experience of an increasing negative force pushing the beam backward to the left up to a second snap point  70  whereupon the force reverts again to a positive direction and the beam moves fully to the right if unimpeded. The two snap points  68  and  70  provide a degree of hysteresis that may be desirable for certain applications and create in effect a bistable beam  12  as may be useful to provide a memory element. This mechanical memory element may be combined with other devices including accelerometers or isolators, or current or magnetic field sensors. 
   Referring now to  FIGS. 12 and 13 , the bowing created by the transverse arm  18  of  FIG. 10  was induced by exploiting the differences in expansion coefficients of the various MEMS materials and thus puts transverse arm  18  in a stressed state. However, a bowing may also be created in a stress-free transverse arm  18  by forming the transverse arm  18  into a bowed configuration during fabrication, for example, etching the transverse arm  18  in a bowed shape. In this case, the force  71  may be employed in a monotonically increasing region  72  providing a simple biasing force always in a positive direction or may be used outside of region  72  to a buckling point  74  after which the force  71  decreases returning only to an increasing mode after some additional distance is traversed, however, at no point becoming a negative force such as would create the bi-stability of the device of  FIG. 10 . In this way, a monostable device may be created. 
   Referring now to  FIG. 14 , an intentional bowing of transverse arm  16 , for example, may provide for a mechanical lever communicating between the beam  12  and a position sensor  24 ′ in this case formed of interleaving capacitor plates  75  and  76  with capacitor plate  75  being movable in the transverse direction and capacitor plates  76  being fixed. Capacitor plates  75  are attached to the distal end of transverse arm  16  removed from the beam  12  so as to be pushed outward by the transverse arm  16  with motion, in this case leftward, by the beam  12 . This transverse motion is controlled by the slight longitudinal bending of the transverse arm  16  such as approximates a triangle  80  as shown in  FIG. 15 . Via the transverse arm  16 , small longitudinal motions Δx of the beam  12  being converted to the greater or lesser transverse motions Δy acting on capacitor plates  75 . Depending on the particular angle of the transverse arm  16 , the leverage may create additional motion or additional force. The decree of additional motion or mechanical advantage was determined by the amount of longitudinal extent of the transverse arm  16  according to well-understood trigonometric principals. 
   In an alternative embodiment, the position sensors  24 ′ may be operated as electrostatic motors to change the stress in the transverse arm  16  and therefore its frequency characteristics and those of the system, where tightening the transverse arm  16  would increase the natural resonant frequency of movement of the beam  12 . In yet a further alternative embodiment, the motors could be used to adjust the bowing of the transverse arms  16  so as to move the beam  12  as a bias method or to control the amount of bias force on the beam  12 . 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but that modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments also be included as come within the scope of the following claims.