Abstract:
An acceleration-triggered or shock-triggered, smart, tunable MEMS switch that may function as both a classic accelerometer and an acceleration threshold detector. A parallel element MEMS device has a stationary and a movable element forming a capacitor. Varying acceleration moves the movable member with respect to the stationary member, thereby changing the capacitance of the device. The capacitance varying may be used, in cooperation with appropriate circuitry, to provide a signal representative of instantaneous acceleration. By applying a biasing voltage, the movable element may be positioned in a predetermined fashion such that acceleration of a predetermined magnitude causes the movable element to pull in (snap down). The movable and stationary elements may function as a switch such that when the predetermined acceleration or shock level occurs, electrodes close, a current flows between the elements so that an external device such as an air bag may be activated.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a Continuation of U.S. patent application Ser. No. 12/371,535, filed Feb. 13, 2009, now U.S. Pat. No. 8,256,291, issued Sep. 4, 2012, which is a continuation of U.S. Ser. No. 11/448,413, filed Jun. 7, 2006, now U.S. Pat. No. 7,493,815, issued Feb. 24, 2009, the entirety of which are expressly incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention pertains to acceleration sensors and, more particularly, to a MEMS switch triggered by acceleration or mechanical shock. 
     BACKGROUND OF THE INVENTION 
     Accelerometers are devices that provide an electrical signal output related to an acceleration level to which the accelerometer is subjected. Accelerometers are useful for numerous applications such as inertial navigation where continuous readings related to instantaneous acceleration are required. 
     Many types of accelerometers are known in the prior art; however, many prior art accelerometers suffer from one or more problems. They may be bulky, expensive, and/or may require elaborate support circuitry to provide their output signals. 
     While many accelerometer applications require an ongoing acceleration level from their associated accelerometers, other applications detect when a predetermined acceleration threshold has been reached. Such applications also may be satisfied by acceleration or shock-sensing switches that trigger (i.e., open, close, or otherwise signal) that a predetermined shock has occurred or an acceleration level has been reached. 
     A well-known example of a threshold detecting accelerometer relates to deploying motor vehicle air bags during a collision. While instantaneous acceleration levels of the vehicle may be useful for applications unrelated to air bag deployment, for that particular application only detection of an exceeded acceleration value due to external acceleration (i.e., not due to external forces or pressures), is required. Heretofore, elaborate systems have been required for detecting the acceleration and then deploying one or more air bags. Such systems generally require an accelerometer, amplification and/or other processing circuitry, a comparator circuit, and finally a switch to actually deploy the air bags. Such systems are relatively expensive and typically require many support components. This provides unnecessary opportunities for component failure resulting in less than optimally reliable systems. 
     DISCUSSION OF THE RELATED ART 
     Several capacitance-based accelerometers are known in the prior art. For example, U.S. Pat. No. 6,388,300 for SENSOR ASSEMBLY AND METHOD, issued May 14, 2002 to Joon-Won Kang et al. provides a device wherein a diaphragm, forming one side of a capacitor, is snapped down due to the effect of external forces/pressure plus the electrostatic forces acting on the diaphragm. The KANO et al. apparatus relies on a static instability. Static non-time-varying forces are the cause of the snapping down. In the device of the present invention, operation is based upon dynamic, not static instability. 
     United States Published Patent Application No. 2002/0008296 for INTEGRATED SENSOR HAVING PLURALITY OF RELEASED BEAMS FOR SENSING ACCELERATION AND ASSOCIATED METHODS, published Jan. 24, 2003 upon application by Tsiu Chiu Chan et al. teaches an acceleration-sensing structure. The CHAN et al. apparatus use multiple beams, each designed to operate within its own specific range of acceleration values to detect a wide range of accelerations. The CHAN et al. apparatus has no switch function but is merely an acceleration sensor. The apparatus of the present invention uses a single beam/structure to detect a wide range of accelerations. 
     In addition, CHAN et al. fail to provide an electrostatic biasing force and, consequently, there is no mechanism to provide a sudden snap down of a movable portion of the structure in response to a predetermined level acceleration as is provided in the device of the present invention. 
     U.S. Pat. No. 6,388,300 for SEMICONDUCTOR PHYSICAL QUANTITY SENSOR AND METHOD OF MANUFACTURING SAME, issued May 14, 2002 to Kazuhiko Kano et al. provides another device absent a dynamic instability-based pull-in function. KANO et al. provide no parallel plate mechanism. 
     It would, therefore, be advantageous to provide an inexpensive, reliable, calibrateable, accelerometer providing both a continuous signal representative of instantaneous acceleration as well as a direct signal indicating that an acceleration limit has been reached. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided an acceleration or shock-triggered smart MEMS switch that may function as both a classic accelerometer and as an acceleration threshold detector. A MEMS device having parallel elements has both a stationary and a movable element forming a capacitor. Variation in acceleration causes movement of the movable member with respect to the stationary member, thereby changing the capacitance of the device. The variation in capacitance is used to provide a signal representative of instantaneous acceleration. An electrostatic force, provided by a bias voltage, typically but not necessarily a DC voltage, applied across the stationary and movable elements positions the movable element in a predetermined relation to the stationary element such that acceleration of a predetermined magnitude causes the movable element to pull in (i.e., snap down). When snap down occurs, the stationary and movable elements are in physical contact with one another and the combination may, therefore, function as a switch. That is, when a predetermined acceleration is experienced a current flows through the elements and an external device, for example, an air bag may be electrically activated. 
     It is, therefore, an object of the invention to provide an acceleration-triggered smart MEMS switch that may also function as an accelerometer. 
     It is another object of the invention to provide an acceleration-triggered smart MEMS switch that functions as an accelerometer that may be calibrated to provide a contact closure at a predetermined acceleration or mechanical shock threshold. 
     It is an additional object of the invention to provide an acceleration-triggered smart MEMS switch that functions as an accelerometer that may be calibrated by applying a bias voltage across a stationary and a movable element of the device. 
     It is a further object of the invention to provide an acceleration-triggered smart MEMS switch that is inexpensive to fabricate. 
     It is a further object of the invention to provide an acceleration-triggered smart MEMS switch that requires no additional circuitry to perform the function of an acceleration threshold detector. 
     It is yet another object of the invention to provide an acceleration-triggered smart MEMS switch that consumes only small amounts of electrical power. 
     It is a still further object of the invention to provide an acceleration-triggered smart MEMS switch that may be designed to be insensitive to varying damping conditions. 
     It is another object of the invention to provide an acceleration-triggered smart MEMS switch that may be designed to be sensitive to mechanical shock. 
     It is an additional object of the invention to provide an acceleration-triggered smart MEMS switch that is relatively insensitive to noise. 
     It is a yet another object of the invention to provide an acceleration-triggered smart MEMS switch that produces a clear, strong signal (i.e., indication) when activated by an acceleration in excess of the calibrated threshold acceleration. 
     It is a still further object of the invention to provide an acceleration-triggered smart MEMS switch having a self-test or self-monitoring feature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent, detailed description, in which: 
         FIG. 1  is a schematic, perspective view of a parallel plate capacitor; 
         FIG. 2   a  is a side, schematic view of the parallel plate capacitor of  FIG. 1  in an undeflected state; 
         FIG. 2   b  is a side, schematic view of the parallel plate capacitor of  FIG. 1  in a slightly deflected state; 
         FIG. 3   a  is a side, schematic view of the parallel plate capacitor of  FIG. 1  in an undeflected state; 
         FIG. 3   b  is a side, schematic view of the parallel plate capacitor of  FIG. 1  in pulled-in state; 
         FIGS. 4   a - 4   c  are magnitude vs. time plots of simple rectangular, sinusoidal, and triangular shock pulses, respectively; 
         FIG. 5   a  is a plot of deflection vs. time of the movable element of the capacitor of  FIG. 1  plate due to an applied acceleration pulse (shock force) with no bias voltage applied; 
         FIG. 5   b  is a plot of deflection vs. time of the movable element of the capacitor of  FIG. 1  plate due to an applied step voltage load with no shock load; 
         FIG. 6  is a plot of deflection vs. time of the movable element of the capacitor of  FIG. 1  plate due to both an applied voltage and a shock load; 
         FIG. 7  is a plot of the actuation threshold of the device of the present invention as a function of shock amplitude expressed in gs; 
         FIG. 8  is a plot of the actuation threshold of the MEMS switch of the invention vs. shock amplitude for shock durations of 1.0 ms and 0.1 ms; 
         FIG. 9  is a plot of a time history of displacement of the movable element of an accelerometer/smart MEMS switch in accordance with the invention subjected to a load slightly below a desired acceleration threshold; 
         FIG. 10  is a plot of a time history of displacement of the movable element of an accelerometer/smart MEMS switch in accordance with the invention subjected to a load at a desired acceleration threshold; 
         FIG. 11  is a schematic representation of an acceleration or shock-actuated MEMS switch wherein the moving element is a cantilever microbeam; 
         FIG. 12  shows the tip deflection of the microbeam of the MEMS switch of  FIG. 11  in response to a half-sine shock pulse of 1 ms duration; 
         FIG. 13  is a time response of the cantilever microbeam of the MEMS switch of  FIG. 11  in response to a half-sine shock pulse of 0.1 ms duration; 
         FIGS. 14   a  and  14   b  are plots of DC voltage threshold vs. shock amplitude of the MEMS switch of  FIGS. 12 and 13 , respectively, having half-sine shock pulses of durations equal to 1 ms and 0.1 ms, respectively; 
         FIG. 15  is a plot of DC voltage threshold vs. shock amplitude for a cantilever microbeam having a geometry different than that of  FIGS. 14   a ,  14   b , a 5  and  16 , subjected to a half-sine shock pulse having a duration of approximately 1 ms; 
         FIG. 16  is a plot of time response of the microbeam of the MEMS switch of  FIG. 15  when biased by 0.17 V and subjected to a shock pulse of amplitude 4 g; 
         FIG. 17  shows a plot similar to that of  FIG. 16 , but with the DC bias voltage set to 0.16 V and wherein the MEMS switch does not close when subjected to a 4 g acceleration; 
         FIG. 18  is a plot similar to  FIG. 16 , but with the level of acceleration reduced to 3 g; 
         FIG. 19  is a cross-sectional, schematic view of a microstructure enclosed in a package; and 
         FIG. 20  is a plot showing the variation of the ratio D ynamic /W static  is plotted for various values of the ratio T shock /T structure . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Many micro-electro-mechanical system (MEMS) devices utilize a parallel plate capacitor consisting of a first, stationary plate and a movable element, for example, a plate that may be actuated or biased by an electrostatic DC force. As used herein, the term plate includes plates, beams, shells, diaphragms and other such structures suitable for forming a movable element in a MEMS. Referring first to  FIG. 1  there is shown a perspective view of such an arrangement, generally at reference number  100 . A movable plate  104  may be deflected relative to a stationary plate  102  by an applied DC bias voltage  106 . 
     Referring now to  FIGS. 2   a  and  2   b  there are shown side, cross-sectional schematic views of the capacitor of  FIG. 1  in an undeflected and a slightly deflected state, respectively. In  FIG. 2   a , little or no DC bias voltage  106  is applied between stationary plate  102  and movable plate  104 . V DC    106  is approximately 0. However, in  FIG. 2   b , a small DC bias voltage  106  is applied between fixed plate  102  and movable plate  104 . 
     The electrostatic force created by DC bias voltage  106  slightly deflects movable plate  104  toward stationary plate  102 . If the electrostatic force is small, the elastic restoring force of movable plate  104  is in equilibrium with the opposing, applied electrostatic force and movable plate  104  stays in the deflected position. While a DC voltage is shown for purposes of disclosure, an AC voltage, not shown, may also be used. For example, an AC voltage may be used to enhance the sensitivity of the inventive MEMS switch  110 . The use of an AC bias voltage is known to those of skill in the art and is not further described herein. 
     Referring now to  FIGS. 3   a  and  3   b , there are shown side, cross-sectional schematic views of the capacitor of  FIG. 1  in an undeflected and a collapsed state, respectively. As is typical in a MEMS switch application, as the DC bias voltage  106  increases, the electrostatic force between plates  102 ,  104  increases, resulting in increased deflection of movable plate  104 . There is an upper limit for the DC bias voltage  106 ′, beyond which the mechanical restoring force of movable plate  104  can no longer resist or overcome the applied, opposing electrostatic force. This leads to a sudden collapse of movable plate  104 , which typically impacts stationary plate as shown in  FIG. 3   b . This structural instability phenomenon is known as pull-in or snap down. 
     Structures such as those shown in  FIGS. 2   a ,  2   b ,  3   a , and  3   b  may be used to create MEMS accelerometers and switches. In such accelerometers, movable plate  104  is biased by a small DC bias voltage  106 . If the deflected movable plate  104  as seen in  FIG. 2   b  experiences a downward acceleration, it deflects further in accordance with Newton&#39;s second law: F=ma. Consequently, the acceleration exerts a force on movable plate  104 . This deflection changes the capacitance of the capacitor formed by plates  102 ,  104 , the capacitance change being related to the magnitude of acceleration experienced by movable plate  104 . 
     MEMS accelerometers are designed to operate away from the pull-in instability ( FIG. 3   b ); otherwise movable plate  104  collapses into stationary plate  102  and the accelerometer fails. However, devices known as MEMS switches  110  depend on the pull-in phenomenon to operate. In MEMS switches ( FIGS. 3   a ,  3   b ) the movable plate  104  is actuated by large DC bias voltage  106 ′. Movable plate  104  is deliberately deflected beyond pull-in to snap it down in a large stroke (relative to the small deflection of the structure due to electrostatic force when it is in equilibrium state) within a short time. For example, the deflection of movable plate  104  when it is in the equilibrium with the electrostatic force reaches a maximum value of approximately 30-45% of the gap width. However, when movable plate  104  collapses, there is no equilibrium and movable plate  104  rapidly moves the total gap distance. 
     When movable plate  104  is unactuated ( FIG. 3   a ), the MEMS switch  110  is in a normally open (n.o.) or “off” position. However, when movable plate  104  is actuated and impacts stationary plate  102  ( FIG. 3   b ), the switch is in a closed, “on” position. It will be recognized, however, that MEMS switches in accordance with the present invention may be configured as normally closed (n.c.) devices wherein current flows before activation. Activation by shock or acceleration then opens the switch thereby interrupting current flow. Of course it will be recognized that to function as an electrical switch, appropriate conductors, not shown, must be electrically connected to fixed plate  102  and movable plate  104 . 
     MEMS switches may also be mechanical. The motion of movable plate  104  upon pull-in may be utilized to perform a mechanical function. Such mechanical functions are well known to those of skill in the art and are not described herein. 
     The MEMS switch  110  triggered by shock and/or acceleration of the present invention combines characteristics of both a traditional accelerometer ( FIGS. 2   a ,  2   b ) and a MEMS switch ( FIGS. 3   a ,  3   b ). In the inventive configuration, a parallel plate capacitor as shown in  FIG. 2   b  is utilized. Movable plate  104  is only slightly deflected by an applied DC bias voltage  106  to a point below the instability limit (i.e., pull-in). However, as discussed hereinabove, when subjected at a downward acceleration, movable plate  104  is further deflected toward stationary plate  102 . 
     When the MEMS switch  110  functions both as an accelerometer and a switch, movable plate  104  remains deflected as long as the applied acceleration remains, assuming the acceleration is less than the designed collapse point of the device  110 . This deflection can be detected by the change of capacitance. When the acceleration ceases, movable plate  104  returns to its original position. If a new acceleration occurs, again assuming that the acceleration is less than the designed switching level, movable plate  104  is again deflected, indicating a new acceleration level. Consequently, the switch  110  monitors and records the acceleration, but does not actuate. This allows implementation of a “self test” to check that the MEMS switch  110  is properly functioning. This feature is desirable in many applications where it is important to confirm that the MEMS switch  110  is still properly functioning. For example, while instantaneous acceleration may be of no particular interest in an application such as air bag deployment, a varying signal responsive to changing acceleration from the switch can assure a monitoring circuit that the MEMS switch  110  is, indeed, functional. Once the design acceleration collapse level is experienced by the MEMS switch  110 , movable plate  104 , however, remains in this pulled in, collapsed position. 
     Novel accelerometer structures, of course, may be designed to operate within predetermined ranges of acceleration. However, if an accelerometer is subjected to an acceleration beyond its upper design limit, movable plate  104  deflects excessively. Due to this deflection, as the electrostatic force is proportional to the inverse of the distance (d) squared between the two plates  102 ,  104 , the electrostatic force becomes very large. Consequently, the electrostatic force overcomes the restoring force of movable plate  104  leading to its collapse (pull-in). 
     Pull-in occurs as a result of two factors: the electrostatic force and the acceleration of the plate  104 . It should be noted that if movable plate  104  were not biased by a DC voltage  106 , movable plate  104  would deflect only slightly due to the acceleration to which it is subjected and would not collapse. Likewise, if movable plate  104  is biased by a small DC voltage  106  and not subjected to acceleration, movable plate  104  again deflects slightly but does not collapse. However, the additive result of the electrostatic force imposed by the DC bias voltage  106 ′ and the acceleration causes movable plate  104  to collapses or pull in. In other words, the pull-in threshold is determined not only by the DC bias voltage  106 ′, but by the level of acceleration on movable plate  104 . 
     Structures can be subjected to large forces applied suddenly and over a short period of time. These forces are known as mechanical shocks or impacts. A shock pulse is characterized by its maximum amplitude, its duration, and its amplitude over time shape. Referring now to  FIGS. 4   a - 4   c  there are shown graphs of force vs. time of three shock pulses that approximate actual shock load profiles.  FIGS. 4   a - 4   c  are a rectangular pulse, a half-sine pulse, and a triangular pulse, respectively. 
     When a microstructure is subjected to a mechanical shock, it can experience the shock load as a quasi-static load varying slowly over time. This occurs if the microstructure has a large natural frequency (resonance frequency). Hence, its natural period T structure  (the inverse of frequency) becomes small compared to the duration of the shock pulse T shock . Notice here that to the “big” world (i.e., non-microstructure devices), the shock is still a sudden force acts over a short period of time. However, this is not what the microstructure experiences. A microstructure experiences the shock as a slow force (quasi-static force). Therefore, the response of the microstructure in this case to the shock force (W dynamic ) is close to its response to an equivalent static force (W static ). In  FIG. 20 , the variation of the ratio W dynamic /W static  is plotted for various values of the ratio T shock /T structure . As may readily be seen in  FIG. 20 , when T shock /T structure  gets large, W dynamic /W static  becomes close to unity indicating quasi-static response. This quasi-static response is not affected by the damping conditions of the microstructure. In one version of the present invention, this phenomenon is utilized to design a switch that responds quasi-statically to shock loads. Such a switch is very robust and insensitive to variations in damping and packaging conditions, shock pulse profile, and shock duration. 
     If, on the other hand, a microstructure has a natural period T structure  that is close or larger than the shock duration T shock , the microstructure experiences the shock load as a dynamic fast varying load (essentially it experiences the shock as a “true shock”, similar to the case in the macro and bigger world). Because of this dynamic experience, the ratio W dynamic /W static  is amplified, as shown in  FIG. 20 . This amplification of response can be used to design a microstructure that is more sensitive to shock. In the present invention, this dynamic experience by the structure is used to increase the sensitivity of the switch and to lower its activation threshold compared to quasi-static cases. 
     When a microstructure, for example, the MEMS switch  110  of the present invention, is subjected to a mechanical shock, the microstructure can experience the shock load as a quasi-static load that varies slowly over time. This occurs if the microstructure has a large natural frequency (resonance frequency). Under this condition the structure&#39;s natural period (the inverse of frequency) is small compared to the duration of the shock pulse. 
     In a larger structure, the shock is still a sudden force that acts over a short period of time. However, the microstructure experiences the shock as a slow force. In one embodiment of the present invention, this phenomenon is utilized to design a switch that responds quasi-statically to shock loads. Such a switch is typically insensitive to variations in damping and packaging conditions, shock pulse profiles, and shock durations. 
     If a microstructure, on the other hand, has a natural period that is close to or greater than the shock duration, it experiences the shock load as a fast varying load. In other words, a true shock is experiences, similar to that experienced by larger structures in the big world. The present invention utilizes this dynamic experienced by the structure to increase the sensitivity of the switch and to lower its activation threshold compared to quasi-static cases. 
     The inventive switch  110  can be designed to be triggered by a shock force transmitted to the switch in the form of large acceleration acting over a short period of time. In the proposed use, the switch  110  is placed inside a package. Referring now also to  FIG. 19 , there is shown a cross-sectional, schematic view of a microstructure, for example, switch  110 , in a package  150 . Package  150 , when desired, may be under vacuum to enhance the sensitivity of the switch  110 . If the package  150  is subjected to a mechanical shock force, such as that due to an impact with ground  152  or a wall, not shown, an acceleration pulse is transmitted to the switch  110 , particularly the movable element  104  of the switch  110 . The movable element  104  responds to this level of acceleration by striking the other stationary element  102  underneath it to close or break an electric circuit. The level of acceleration that the switch  110  experiences in such situations can range from hundreds to thousands of gs depending on the level of impact and the nature of surfaces involved in the impact. 
     The inventive switch  110  can be tuned to operate at any desired acceleration level by modifying its design parameters, such as the structure shape, its dimensions, its clamping and mounting conditions, its material, the gap space between the movable element  104  and the stationary element, and the vacuum condition inside the package  150 . In alternate embodiments, a proof or lumped mass may be added to the movable element  104  to enhance its sensitivity to acceleration. 
     Referring now to  FIGS. 5   a  and  5   b , there are shown graphs of deflection vs. time of movable plate  104  subjected to acceleration but no bias voltage, and bias voltage but no acceleration, respectively. In both  FIGS. 5   a  and  5   b , the deflection of movable plate  104  eventually finds a stable, non-collapsed equilibrium position. 
     Referring now to  FIG. 6 , there is shown a deflection vs. time graph of movable plate  104  subjected to the combination of bias voltage and acceleration in accordance with the present invention. As may readily be seen, movable plate  104  collapses (i.e., pulls-in) when subjected to both a DC bias voltage-imposed electrostatic force and acceleration. 
     Accelerometers in accordance with the present invention, are “abused” when activated as a switch. That is, abused accelerometers are required to operate at greater than normal values of acceleration than are accelerometers of the prior art. The inventive devices are intentionally designed so that the movable plate  104  reaches pull-in (i.e., snaps down) beyond a specific desired threshold of acceleration. At accelerations below the design threshold, movable plate  104  acts as a normal accelerometer. However, at or beyond the acceleration threshold, movable plate  104  snaps down into stationary plate  102 . This snapping down action may be utilized to make plates  102 ,  104  act as a smart switch that opens or closes an electrical circuit only upon detection of a specific, predetermined level of acceleration. 
     The inventive MEMS switch/accelerometer  110  is very sensitive to changes of acceleration. An accelerometer/smart MEMS switch in accordance with the invention is calibrated to an 84 g pull-in threshold. Referring now to  FIG. 9 , there is shown a plot of a time history of displacement of the movable element, (i.e., deflection vs. time) for example, movable plate  104  ( FIG. 2   b ) when the device  110  is subjected to an acceleration of 83 g, a value just lower than the 84 g threshold. The plot shows a maximum normalized displacement x(t)/d near 0.4 (here x(t) is the plate maximum displacement and d is the gap spacing width between the capacitor plates  102 ,  104  ( FIG. 2 ). 
       FIG. 10  is a plot similar to that of  FIG. 9  showing the time history of the plate response when the device is subjected to an acceleration of 84 g, the desired threshold of acceleration. The plot shows a maximum normalized displacement x(t)/d near 1 (unity) indicating that the plate  104  has impacted the stationary element  102  as intended. Movable plate  104  does not return to a pre-acceleration equilibrium position. 
     A specific microstructure useful for implementing the MEMS switch of the invention is now provided. Referring now to  FIG. 11 , there is shown a schematic representation of an acceleration or shock-actuated MEMS switch wherein the moving element is a cantilever beam, generally shown at reference number  800 . A cantilever microbeam  802  is, for example, made of silicon, has a length  804  of approximately 110 microns, width, not shown, of approximately 10 microns, and thickness  806  of approximately 0.1 micron. The gap spacing d  808  between beam  802  and a substrate  810  (i.e., the lower stationary electrode) is approximately 2 microns. Microbeam  802  is placed in near vacuum conditions to avoid mechanical damping from the surrounding air. The natural oscillation period of microbeam  802  is approximately 0.1 ms. 
     When microbeam  802  is subjected to a half-sine shock load with approximately a 1 ms duration and an amplitude of approximately 400 g in the absence of an applied bias voltage (i.e., V DC =0), the response of microbeam  802  is shown in  FIG. 12 . 
       FIG. 12  shows the tip deflection W max ,t)  814  of the microbeam  802  normalized to the gap width d  808  versus time t. Since the shock duration (1 ms) is ten times the natural period of the microbeam  802 , microbeam  802  experiences the shock as a quasi-static load. Hence, it may be seen in  FIG. 12  that the time response of microbeam  802  has a similar shape to that of the applied half-sine pulse. 
     Referring to  FIG. 13 , there is shown the time response of microbeam  802  subjected to a similar shock pulse but wherein the pulse duration is 0.1 ms. In other words, the applied pulse duration is approximately equal to the natural period of microbeam  802 .  FIG. 13 , however, shows a dynamic response in contrast to the quasi-static response of  FIG. 12 . It may readily be seen that the maximum deflection of the microbeam  802 , approximately 0.65 W/d, is larger than the maximum deflection, approximately 0.43 W/d, of quasi-static response seen in  FIG. 12 . This indicates that the sensitivity of a MEMS switch can be increased by designing its mechanical structure to have a natural period close to the duration of the expected shock force. 
     MEMS switch  800  which may act as a trigger may be made tunable switch by applying DC bias voltage  812  between the cantilever microbeam  802  and substrate  810 . By varying the voltage  810 , the switch may be tuned to cause microbeam  802  to impact substrate  810  (i.e., pull-in) at a desired level of shock load to close or break an electric circuit, not shown. 
     Referring now to  FIGS. 14   a  and  14   b , there are shown plots of DC voltage threshold vs. shock amplitude for a half-sine shock pulse of duration 1 ms (i.e., a quasi-static case) and 0.1 ms (i.e., a dynamic loading case), respectively. In each case, as may be seen, the shock amplitude at which microbeam  802  ( FIG. 11 ) impacts substrate  810  may be controlled by varying the DC voltage  812 . By comparing  FIGS. 14   a  and  14   b , it may be seen that the range of acceleration that triggers the switch  800  is lower for the case of dynamic load ( FIG. 14   b ). 
       FIGS. 14   a  and  14   b  demonstrate a tunable switch with an operational range of hundreds of gs. Some applications, however, require that the MEMS switch  800  be triggered at lower acceleration levels. In such applications, the MEMS switch  800  and its package may not be subjected to a shock force. Shock forces typically induce large values of acceleration. For example, in an application such as protecting a portable device (e.g., a laptop computer), when falling the MEMS switch must function once it detects free falling, which induces an acceleration of one g. If the laptop hits the ground, it is too late to protect the hard drive. So it is desired that the switch  800  be triggered at a level of one g before impact. 
     To lower the operating range of the switch  800 , the geometry of the microbeam may be modified compared to microbeam  802  of the MEMS switch of  FIG. 11 . By choosing a microbeam having a length of approximately 900 microns, a width of approximately 100 microns, and a thickness of approximately 1 micron, the trigger acceleration level is significantly lowered 
     Referring now to  FIG. 15 , there is shown a plot of DC voltage threshold vs. shock amplitude for a half-sine shock pulse having a duration of approximately 1 ms.  FIG. 15  corresponds to  FIG. 14  for the microbeam  802  of  FIG. 11  except that by modifying the microbeam dimensions the operation range of the switch has been reduced significantly to a maximum of 12 g. The switch  800  is more sensitive to variation in acceleration in this lower range. 
     The sensitivity of a MEMS switch  800  having a beam  802  with the modified dimensions to variations in the DC bias voltage and the acceleration level is now shown. For example, a desired threshold for the switch closing (i.e., the MEMS switch pulling in) is chosen to be an acceleration level of 4 g or greater. As may be seen in  FIG. 15 , a DC bias voltage of approximately 0.17 V must be applied between the microbeam and its substrate to create a pull-in at 4 g. 
     Referring now also to  FIG. 16 , there is shown the time response of the microbeam when biased by 0.17 V and subjected to a shock pulse of amplitude 4 g. As may be seem in  FIG. 16 , the microbeam impacts the substrate closing the switch when W max /d=1. 
       FIG. 17  shows a plot similar to that of  FIG. 15 , but with the DC bias voltage set to 0.16 V. As may readily be seen in  FIG. 17 , the MEMS switch does not close when subjected to a 4 g acceleration. 
       FIG. 18  shows a plot similar to  FIG. 16 , but with the level of acceleration reduced to 3 g. It should also be noted that the MEMS switch does not close (i.e., pull in) but remains in the off position. It may be concluded from  FIGS. 16-18  that the inventive MEMS switch can be accurately tuned to close at a predetermined, desired acceleration level. As may be seen in  FIG. 18 , below the desired threshold of acceleration, the switch does not close. Also, as may be seen in  FIG. 18 , reducing the voltage bias prevents the MEMS switch from closing. 
     Accelerometers/smart MEMS switch devices in accordance with the invention exhibit distinctive and strong responses at accelerations beyond their desired acceleration thresholds. However, below the desired acceleration thresholds, the plates deflect by small magnitude. As shown in  FIG. 17 , the normalized deflection is approximately 0.4. Above the desired acceleration threshold, the plate  104  deflects by large magnitudes until it impacts the fixed element  102 . 
     For example, as shown in  FIG. 18 , the deflection is 1.0. Hence, the switch  110  produces a clear and strong signal that is less sensitive to noise than devices of the prior art. 
     An illustrative application for the inventive device is for an air bag sensor/activator for motor vehicles. In sensor/activation mechanisms of the prior art, an accelerometer monitors the car acceleration and sends its output signal to a decision/controller unit. If the car experiences a sharp deceleration due to a collision, the decision unit sends a signal to a switch which, in turn, deploys the air bag. This complex system requires at least three distinct components: the accelerometer, the decision/controller unit, and a switch to deploy the air bag activated upon command from the decision/controller unit. 
     A system built around the inventive sensor, however, requires no additional components; the MEMS smart switch formed by the snapped-down movable plate creates the necessary electrical connection to fire the air bags directly. The smart switch must be constructed and calibrated to fire the air bag only at the desired acceleration g force level. 
     It will be recognized that the inventive accelerometer/smart switch may be used in many other applications. For example, modern notebook computers may incorporate the inventive device to lock down the heads of a hard disk drive when an acceleration is caused by the computer falling off a surface. The concept may be extended to cell phones, PDAs, digital cameras, and other similar portable electronic devices, to perform a safety shutdown or evasive function upon detecting a predetermined acceleration value. 
     In weapons systems (e.g., missiles), the inventive device may be used to arm, disarm, or fire the weapon upon striking or missing its target. 
     The inventive accelerometer/MEMS smart switch is highly advantageous in that it is easy to both fabricate and operate. It has low power consumption and can be fabricated and calibrated to act like a switch that operates beyond a specific level of acceleration. Consequently, the inventive accelerometer/smart MEMS switch can inexpensively replace existing complex and/or expensive systems employing sensing and actuating mechanisms. 
     The devices may be designed to be insensitive to changing damping conditions. This is the case when the moving structure is designed to have a high natural frequency, and hence a short natural period compared to shock duration. Therefore, the structure responds quasi-statically. Referring now to  FIG. 7 , there is shown a plot of an activation threshold vs. the shock amplitude of an acceleration pulse. Two plots,  602  and  604 , represent, respectively, damping of approximately 0.05 and 0.7. As may be seen, there is very little difference in the response of the moving member to pull-in voltage even across this wide range of damping (relatively low to high). Accordingly, relatively low-cost packaging techniques may be used. MEMS devices typically must be well sealed and in near vacuum. As shown in  FIG. 7 , this requirement is now unnecessary. 
     Also, the accelerometer/smart MEMS switch  110  of the invention can be insensitive to the duration of the acceleration pulse, also called shock. Referring now to  FIG. 8 , there is shown a plot of the actuation threshold of the device of the present invention as a function of shock amplitude expressed in gs. Plots  702 ,  704  show shock pulse durations of 1.0 ms and 0.1 ms, respectively. As may readily be seen, plots  702 ,  704  are substantially coincident across a large portion of the operating range of the inventive device. These insensitivities to both damping ( FIG. 7 ) and shock pulse duration ( FIG. 8 ) enhance the reliability of the inventive device. 
     The accelerometer/smart MEMS switch  110  of the invention is also easy to calibrate so that snap down may be triggered across a wide range of accelerations. As may be seen, for example, in  FIGS. 7 and 8 , accelerations in an operational range from approximately a few hundred gs to 10,000 g are shown. Since the inventive MEMS switch  110  may be designed to operate in a range between approximately zero g and hundreds of thousands of gs, the invention is not limited to MEMS switches having any particular g-force operating value. Calibration may be achieved by modifying the geometry of the plate  104  and its boundary conditions (e.g., clamped, free, etc.) and by changing the applied DC bias voltage. One example is provided by changing the geometry of  FIGS. 14   a  and  15 . 
     While the microstructure of the present invention is designed to function without need for pre-stressing the movable element, it will be recognized that similar structure using a pre-stressed movable element could be constructed. Consequently, the invention includes device having either unstressed or pre-stressed moving elements. 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, this invention is not considered limited to the examples chosen for purposes of this disclosure, and covers all changes and modifications which does not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.