Abstract:
A micromechanical micromotion amplifier has an integrated structure formed primarily of silicon and comprises a plurality of long slender flexible beams which are connected in a predetermined manner. The beams are released from a silicon substrate for movement with respect to fixed points of reference upon the substrate. Each beam thereby has a fixed end and a relatively moveable free end. Compressive axial force induced by axial movement, applied to the moveable end of a beam, will cause that beam to deform or buckle. Buckling occurs transversely in-plane due to a high aspect ratio profile of each beam. The amount of lateral or transverse movement of a beam due to buckling is relatively large in relation to the applied axial force or axial movement which causes it. By arranging these beams in cooperating perpendicular pairs as micromotion amplifier stages, an input axial force/movement applied to the moveable free end of a first beam generates a transverse motion or buckling movement of that beam.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/115,634 filed on Jan. 12, 1999, the disclosure of which is hereby incorporated herein by reference. 
     This invention relates to U.S. Pat. No. 5,862,003, entitled MICROMOTION AMPLIFIER, the disclosure of which is also hereby incorporated herein by reference. 
    
    
     The invention was made with Government support under Grant No. DABT 63-95-C-0121, awarded by the Defense Advanced Research Project Agency (DARPA). The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to micromechanical motion amplifiers and more particularly, to integrated micromechanical structures wherein a small amount of driving force or motion translates through the device to produce a relatively large motion in a direction transverse to the driving motion. In such devices, relatively thin, elongate beams are designed to buckle in response to an applied axial compressive force induced by axial motion. The motion produced by the deformation or buckling is an order of magnitude greater than the applied axial motion which causes it. Thus, micromotion amplifiers may be provided. 
     Such prior art devices exhibit a limited amount of output and are thereby constrained with respect to a maximum amount of sensitivity with which they may operate. It follows that such devices are necessarily greatly limited in their application as sensors. Accordingly, there has been a long felt need for integrated micromotion amplification apparatus in which the amount of output deflection and hence sensitivity, is not limited or constrained by a single beam. In accordance with the present invention, the need for increased sensitivity in a micromotion device, is fulfilled by a micromotion amplifier wherein the ultimate output deflection or buckling, and hence overall sensitivity, is predetermined by the additive effect of assembling buckling beams in cooperating pairs or stages. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a buckling beam micromotion amplifier in which the ultimate output deflection, and hence sensitivity, is not constrained by the deflection of a single beam. 
     It is also an object of the present invention to provide a micromotion amplifier in which the sensitivity to an applied axial force is greatly increased over that of the prior art. 
     It is also an object of the present invention to provide micromotion amplifier stages, in the form of cooperating pairs of buckling beams, which may be cascaded. 
     It is also an object of the present invention to provide a method and apparatus by which minute amounts of movement may produce greatly increased motion within an integrated device, in orders of magnitude heretofore previously unattained, to form the basis of highly sensitive motion amplification sensors, switches and the like. Other features and advantages will be made apparent from the following description. 
     In accordance with the present invention, a method and apparatus are provided for amplifying micromechanical or microelectromechanical motion in orders of magnitude unattainable by prior micron-scale mechanical devices. Integrated buckling beams are released from a single crystal silicon substrate in cooperating pairs or stages. Each buckling beam is formed having an asymmetrical cross-section (high aspect ratio), i.e., the height of each beam is much greater than its&#39; width. This asymmetry will effectively bias or predispose each beam to bend or buckle in a predetermined direction when an applied axial force exceeds a critical value. The initial input axial force applied to a first beam can be provided by any desired source. Axial forces acting upon any subsequent beam in a pair or series of beam pairs are provided by the previous beam buckling in response to a lesser axial force. In other words, the beams are arranged in such a manner as to induce a chain reaction of buckling in one or more subsequent beams in response to an input axial force applied to the first beam. Since the amount of transverse deformation of any one beam is greater than the amount of axial motion necessary to cause it, the net deformation or buckling from a final beam in a cascade array of beam pairs or stages is significantly greater in magnitude than the initial input movement applied. Amplification of micromotion may thereby be provided as a function of a number of micromotion amplifier stages and these stages may be cascaded as desired. 
     In a preferred form of the present invention, a first micromechanical beam has a free first end and a second end fixed to a reference point on the substrate. A second micromechanical beam has a first end connected to a middle or buckling region of the first beam and a second end fixed to another reference point on the substrate. The first and second beams are arranged to be substantially coplanar and perpendicular to each other. The first end of the first beam may be acted upon by an actuator to induce an input axial force or movement upon the first beam and thereby produce an output buckling of the first beam. The output buckling of the first beam provides an input axial force or movement upon the second beam, thereby producing an output buckling of the second beam. Accordingly, the first and second beams arranged to function in this manner comprise a micromotion amplifier stage and any number of such stages may be cascaded. 
     Suitable actuators for inducing an input axial force may comprise devices having physical properties which are responsive to temperature, pressure, humidity, impact, acceleration or other parameters. Suitable actuators may also comprise active devices such as capacitive comb-drive actuators. Preferably, one or more integrated tunneling tips are provided for detecting, measuring and indicating an amount of buckling produced by any or all beams in a stage or cascade array. In addition, integrated capacitive or resistive sensors or other non-integrated external devices such as atomic force microscopes may be used for detecting the motion of the beams. The preferred form may further include adjunct beams provided at one or more beam ends in one or more stages, for the purpose of prestressing a beam and thereby reducing the amount of axial force necessary to induce buckling. Sensitivity of the device in any one or more stages is thereby greatly enhanced. 
     An alternate embodiment of the present invention provides micromotion amplifier stages each comprising a first beam having first and second ends fixed to reference points on the substrate; a second beam having a first end connected to the first beam with a second end fixed to another reference point on the substrate; and an actuator. The exact point along the first beam where the first end of the second beam is connected, is chosen so as to influence the direction in which the second beam will buckle. The actuator is provided at this point of connection, acting transverse to the first beam and coaxial to the second beam, so as to induce a buckling force in the second beam in the predetermined direction. In this embodiment, the beams may be made electrically conductive. Two of such stages, electrically isolated, may be disposed in parallel opposing relationship such that one or both of the second beams of each stage will buckle into or out of contact with one another, in response to energization of one or both of the actuators. Beam stages arranged according to this alternate embodiment may thereby function as a micromechanical switch. Such switches may be made highly sensitive by prestressing the first beam in each stage. Sensitivity may be enhanced even further by the addition of one or more micromotion amplifier stages of the preferred embodiment for amplifying buckling forces transmitted by one or more of the actuators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and additional objects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings in which: 
     FIG. 1 illustrates in diagrammatic form the transverse buckling response of a long slender beam due to a compressive axial force; 
     FIG. 2 illustrates in diagrammatic form the interaction of two beams which comprise a micromotion amplifier stage according to the present invention; 
     FIG. 3 illustrates in diagrammatic form two cascaded micromotion amplifier stages; 
     FIG. 4 illustrates a three dimensional perspective view of the micromotion amplifier stage of FIG. 2, fabricated from a single crystal silicon substrate; 
     FIG. 5 illustrates in diagrammatic form a two stage cascade micromotion amplifier wherein input axial compressive force is provided to a first beam by a structure which is responsive to ambient conditions or other measurable parameters; 
     FIG. 6 illustrates in diagrammatic form an adjunct beam for prestressing the first beam of FIG. 5 near the point of buckling; 
     FIG. 7 illustrates in diagrammatic form an adjunct beam in combination with an actuator for prestressing the first beam of FIG. 5 near the point of buckling and for inducing compressive axial force upon the first beam; 
     FIG. 8 illustrates in diagrammatic form a two stage cascade micromotion amplifier wherein input compressive axial force is provided by structure which is responsive to pressure; 
     FIG. 9 illustrates in diagrammatic form a two stage cascade micromotion amplifier wherein input compressive axial force is provided by structure which is responsive to inertia; 
     FIG. 10 illustrates in diagrammatic form an alternate embodiment of the present invention comprising a micro-switch; 
     FIG. 11 illustrates in diagrammatic form the alternate embodiment of FIG. 10 when energization of the actuators induces buckling to a point where contact is made between the two buckling beams of the micro-switch; 
     FIG. 12 illustrates in diagrammatic form an alternate embodiment of the micro-switch illustrated in FIG. 10; and 
     FIG. 13 illustrates in diagrammatic form the alternate embodiment of FIG. 12 when energization of the actuator induces buckling to a point where electrical contact is made. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a long slender beam  10  has a movable first end A, a fixed second end B, a middle region  15  and a length L 1 . When the beam  10  is compressed axially along its length L 1  by a force P applied to the first end A which exceeds a critical value, the first end A displaces axially by an amount δ 1  causing the beam  10  to buckle and deform transversely within a plane. The planar or transverse deformation of the beam  10  upon buckling, results in a relative displacement of the middle region  15  by a distance D1 which is usually much larger than the relative axial displacement δ 1  of the first end A. For a beam AB fixed at one end; 
     
       
           D 1=2/π{square root over (L 1 +L δ 1 +L )}  (Eq. 1)  
       
     
     For example, if the length of the beam L 1 =1000 μm and the axial displacement δ 1 =0.1 μm, then the buckling displacement D 1 =6.37 μm. The motion is therefore amplified by a factor of 60. 
     Referring to FIG. 2, if another beam  20  has a fixed end D, a middle region  16  and a moveable end C attached to the middle region  15  of beam  10  in the manner shown, then the displacement D 1  of beam  10  will induce axial end displacement upon beam end C of beam  20 . This will cause transverse deformation or buckling in the middle region  16  of beam  20  over a distance D 2  . The amplified motion or net output displacement D 2  of beam  20  for a given initial input displacement δ 1  of beam  10  is                D   2     =         2   /   π              L   2          D   1           =       2   /   π              L   2          2   π              L   1          δ   1                         (Eq. 2)                                
     The connection of beam  10  and beam  20  may be referred to herein as a micromotion amplifier stage, generally indicated at  26 . As illustrated in FIG. 3, a second micromotion amplifier stage  28 , comprising a third beam  30  and a fourth beam  40 , may be connected to amplifier stage  26  to provide a cascaded micromotion amplifier. In the illustrated embodiment, the third beam  30  has a moveable end E, a fixed end F and a middle region  17  displaceable over a distance D 3 . The moveable end E is connected to the middle region  16  of beam  20 . The fourth beam  40  has a moveable end G, a fixed end H and a middle region  18  displaceable over a distance D 4 . The moveable end G is connected to the middle region  17  of beam  30 . Similar to that explained above with respect to beam  20 , the third beam  30  is driven axially by the buckling displacement D 2  of beam  20 , causing beam  30  to buckle in its middle region  17  over the distance D 3 . The fourth beam  40  is in turn driven axially by the buckling displacement D 3  of beam  30 , causing beam  40  to buckle in its middle region  18  over the distance D 4 . A tunneling tip  60  is fixed in proximity to a maximum deflection point of region  18  of beam  40  for the purpose of measuring and indicating the buckling displacement distance D 4 . For the two stage cascaded amplifier illustrated in FIG. 3, an input force P applied to end A of beam  10  will axially displace end A by the amount δ 1  which will thereby induce a chain reaction of buckling through beams  10 , 20 , 30  and  40  and produce a measurable net output displacement D 4 . It is important to note that this net output displacement D 4  of beam  40  is significantly greater in magnitude than the initial input axial displacement δ 1  applied to beam  10 . Displacement outputs of greater magnitude can be obtained by cascading additional micromotion amplifier stages. 
     In general for a given number of n beams;                D   n     =         (     2   π     )       1   +     1   /   2     +     1   /     2   2       +   …   +     1   /     2     n   -   1                  L   1     1   /     2   n              L   2     1   /     2     n   -   1              …                   L   n     1   /   2            δ   1     1   /     2   n                   (Eq. 3)                                
     If L1=L2=L3= . . . L n =L, then                D   n     =         (     2   π     )       1   +     1   /   2     +     1   /     2   2       +   …   +     1   /     2     n   -   1                  L       1   /   2     +     1   /     2   2       +   …   +     1   /     2   n                δ   1     1   /     2   n                   (Eq. 4)                                
     Referring again to FIG. 3 for example, when four beams  10 ,  20 ,  30  and  40  each 1000 μm in length, are connected as illustrated, and beam  10  is subjected to an axial end displacement of 1 Angstrom (10 −4  μm), then n=4, L=1000 μm, δ 1 =10 −4  μm, then D 4 =156.66 μm and the net motion amplification is D 4 /δ 1 =156.66×10 4 . 
     For n beams arranged in this manner, the relative change in transverse deformation D n  (of the last beam) due to buckling, as induced by a small change in δ 1 , is given by the derivative:                             D   n              δ   1         =                    (     2   π     )       1   +     1   /   2     +     1   /     2   2       +   …   +     1   /     2     n   -   1                      L       1   /   2     +     1   /     2   2       +   …   +     1   /     2   n                (     1   /   2     )       n          1   /     δ   1     1   -     1   /     2   n                           =                  1     2   n              D   n       δ   1                       (Eq. 5)                                
     Again with n=4, δ 1 =10 −4  μm, D 4 =156.66 μm; 
     
       
           dDn/dδ   1 =9.79×10 4 .  
       
     
     i.e., if δ 1  increases by 0.0001 Angstrom (10 −8  μm), then D 4  will increase by approximately 0.001 μm or 1 nm, thereby providing nanometer scale measurements of motion. 
     In the preferred form of the present invention, thin elongate high aspect ratio flexible beams, suitable for use as micromotion amplifiers and sensors, are fabricated as coplanar cooperating pairs or stages within a single crystal silicon substrate using the SCREAM (Single Crystal Reactive Etching and Metallization) process disclosed in U.S. Pat. No. 5,198,390, U.S. Pat. No. 5,316,979, both to MacDonald et al. and in U.S. Pat. No. 5,719,073 to Shaw et al., the disclosures of which are incorporated herein by reference. Although the SCREAM process is preferred, other processes such as the polysilicon process could be used as well. Devices fabricated in accordance with the SCREAM process may also be referred to as MEMS devices. 
     Etched and released beams are formed in coplanar cooperating pairs connected in the manner illustrated diagrammatically in FIGS. 2 and 3 and in a perspective view in FIG.  4 . Cooperating beam pairs connected as illustrated diagrammatically in FIG. 2, comprise a micromotion amplifier stage  26  and additional stages may be cascaded, as illustrated in FIG. 3, to achieve the amount of desired amplification and corresponding sensitivity. In the preferred form of the invention, the beams are constructed to have high aspect ratio profiles to control their direction of buckling. Thus for example, each beam may be 12 micrometers deep, 1 to 2 micrometers wide and 3 to 5 millimeters or more in length. As illustrated in FIG. 4, the beams may be released, moveable structures fabricated within a cavity  50  of a substrate  52 , preferably of single crystal silicon. One or more beam ends are unitary with a wall of the cavity  50  in which the beams are located to provide fixed reference points. In a similar manner, one or more beam ends such as beam end A in FIG. 4, may incorporate an element which is part of an actuator  54  fabricated simultaneously with beam fabrication. Such an element could be the moveable fingers  56  of a MEMS capacitive comb-drive actuator which interact with stationary fingers  58  on the substrate wall. Application of a voltage across the fingers  56  and  58  imparts an axial compressive force P or motion δ 1  to the beam end A. Other actuator elements, including those which exhibit physical properties responsive to temperature, pressure, humidity, impact or acceleration, for example, may also be used. 
     As illustrated in FIG. 3, a suitable detector  60  is provided adjacent the buckling or middle region  18  of the beam  40  where motion is to be detected. The detector  60  measures the deflection D 4  of beam  40 . As illustrated in FIG. 4, the detector  60  is adjacent the middle region  16  of beam  20  to measure the deflection D 2 . In a preferred form of the invention, the detector  60  includes one or more tunneling tips  62 , emitter tips, or the like, integrally formed upon an upstanding pillar  64  on a floor of the substrate cavity  50  for conducting a current proportional to the amount of buckling movement of the corresponding beam. An example of tunneling tip fabrication is disclosed in U.S. Pat. No. 5,235,187 to Arney et al. Optionally, these tunneling tips may be formed integral to one or more of the beams or sidewalls of the substrate cavity. In either case however, it is preferred to have at least one tunneling tip arranged in proximity to a final or last stage beam as shown in FIG. 3, to facilitate the detection of motion amplification D 4  produced by the device. Amplified motion may also be detected by non-integrated devices having a resolution greater than 1 nm. Atomic force microscopes may be used, for example. Thus, by measuring a change of 1 nm in the amplified motion output deflection D 4  of a final beam  40  in a cascade series of micromotion amplifier stages  26 ,  28 , it is possible to sense a minute change in motion δ 1  of 10 −8  μm at the input of beam  10 . 
     Sensors for a wide variety of measurable parameters may be made using the micromotion amplification described above and specific examples of these are described below. 
     Temperature Sensor 
     Referring to FIG. 5, the moveable end A of beam  10  may be connected to the substrate  52  by a rigid beam  70  of a material (a) such as silicon dioxide. Four flexible beams  10 ,  20 ,  30 , and  40  are preferably fabricated of a material (b) which is the same as the substrate; for example, single crystal silicon. The beams are released from the substrate  52  for motion with respect to the substrate  52  and are arranged in two cascaded amplifier stages as described and illustrated previously with respect to FIG.  3 . The beam ends B, D, F and H are formed integrally with corresponding points along the substrate cavity walls and comprise fixed references for the relative motion of the beams. If the thermal coefficients of expansion for the material (a) of beam  70  and the material (b) of the beams  10 ,  20 ,  30  and  40  are α a  and α b , respectively, and the beams are fabricated at a temperature higher than room temperature such that α a  is greater than α b , then the rigid beam  70  will apply a compressive axial force upon flexible beam  10  as the entire assembly cools and one or more of the beams  10 ,  20 ,  30  and  40 , depending upon their relative lengths, will buckle. By proper design of the lengths of the beams and if necessary, by the provision of an adjustable mounting for the beam  10  at end B, the beams  10 ,  20 ,  30  and  40  will be slightly prestressed such that the beams will buckle with only a very small amplitude of applied force or motion. For example, by careful selection of the length of beam  70  with respect to the length of beam  10 , the cooling of the MEMS amplifier stages  26  and  28  will produce a shift in the location of beam end A by a distance on the order of δ 1 =10 −4  μm. This will buckle beams  10 ,  20 ,  30  and  40  to produce a prestressed shift at the output from beam  40  of dD 4 /dδ 1 =156.66×10 4 . 
     Preferably, the fixed mounting of beam  10  at end B, illustrated in FIG. 5 as being on the wall of cavity  50  in substrate  52 , is modified for example as illustrated in FIG. 6 or FIG. 7, to provide an adjustable mounting. Such a mounting provides control of the amount of prestressing in the beam  10  and thus, in beams  20 ,  30  and  40 . As illustrated in FIG. 6, beam end B may be secured to, or integral with, a point  72  on a relatively short flexible support beam  74  perpendicular to beam  10  and fixed at its ends  76 ,  78  to the substrate  52 . If desired, an actuator  80  such as a comb-type capacitive actuator can be secured to, or integral with, support beam  74  at point  72 , as illustrated in FIG.  7 . The beam  74  in FIG. 6, or optionally, the combination of beam  74  and actuator  80  in FIG. 7, provide a small axial prestress to the amplifiers  26  and  28  to provide in beam  10  a shift on the order of δ 1 =10 −4  μm as discussed above. Therefore, with the structure of FIG. 5 modified in accordance with FIG. 6 or  7  to prestress the beams  10 ,  20 ,  30  and  40  at or near the point of buckling, an ambient temperature change of ΔT will cause a displacement Δδ 1  in the location of end A of beam  10 , due to the different temperature coefficients of the materials X and Y of beam  70  and beams  10 ,  20 ,  30  and  40 . 
     Since Δδ 1 =L 1 (α b −α a )ΔT and ΔT=Δδ 1 /(L 1 (α b −α a )), then for beam  10  with a length L 1 =100 μm and (α b −α a ) is on the order of 10 −6 /C (as is the case for Silicon and Silicon Dioxide), a temperature change of 10 −4  C will produce an axial end shift of beam end A of Δδ 1 =10 −8  μm, resulting in a change at D 4  of 1 nm. This can be can be accurately sensed by a tunneling tip, for example, providing a highly sensitive temperature sensor, or thermometer. 
     Irrespective of the parameter being sensed, the sensitivity of the device can be increased significantly by cascading additional beam stages. The sensitivity may be increased even further by the provision of an additional actuator  90  acting coaxially upon beam end C of beam  120  in the manner shown in FIG. 2 for a one stage micromotion amplifier or upon beam ends C, E and G of beams  20 ,  30 ,  40 , respectively, in the manner shown in FIGS. 3 and 5 for a two stage micromotion amplifier. 
     Humidity Sensor 
     In the device illustrated in FIG. 5, the rigid beam  70  disposed between the substrate  52  and the end A of the beam  10  may be fabricated of a material which will absorb moisture from the ambient atmosphere and swell in proportion to the absorbed moisture along a longitudinal axis thereof. The axial length of the rigid beam  70  will change by Δδ 1  and thereby induce a change in the output deformation D 4  of beam  40  as described above. This change can be sensed by a detector such as the tunneling tip  62 , formed integral to the substrate as shown. Alternately, tunneling tip  62  could be formed integral to beam  40 . 
     Pressure Sensor 
     In the embodiment of FIG. 8, the rigid beam  70  of the device in FIG. 5 is replaced by a flexible membrane  90  which is secured to the substrate  52  at membrane ends  96 ,  98 . The membrane  90  is subject to pressure p 1 , p 2 , with the net force acting upon the membrane being due to a pressure differential Δp across the membrane  90 , Δp=(p 2 −p 1 ). The beams are initially buckled or prestressed by the actuator  80  as described above. If the pressure differential Δp changes from an initial value, membrane  90  will shift in one direction or the other, causing an axial displacement Δδ 1  in end A of beam  10  and thereby inducing a corresponding change in D 4  of beam  40 . This change in the final output is sensed by the tunneling tip  62  so that a small change in pressure can be detected. Such devices may also be used as acoustic sensors. 
     Accelerometer 
     In the embodiment of FIG. 9, a mass  92  is provided to act upon the axis of beam  10  as shown. A flexible support beam  94  is arranged perpendicular to beam  10  for securing the mass  92  and may be secured to or integrated with the end A of the beam  10 . In addition to securing the mass  92 , the support beam  94  may also be provided for prestressing the beam  10 . Optionally, the membrane  90  illustrated in FIG. 8, may be used instead of support beam  94  to secure the mass  92 . The beams  10 ,  20 ,  30 , and  40  are initially buckled by the actuator  80 , as discussed above. If the system experiences an acceleration or deceleration which produces a force parallel to the axis of beam  10 , as indicated by arrow  96 , the mass  92  will exert a force upon the system due to its inertia. Accordingly, beam end A will move by an amount Δδ 1  and consequently a change in D 4  of beam  40  will be sensed by the tunneling tip  62 . Thus, the above device may be used as a highly sensitive accelerometer. 
     Micro Switch 
     The embodiment of FIGS. 10 and 11 illustrates the use of the MEMS amplifiers of the present invention as micromotion activated electrical contacts. Such a device is useful as a micro-switch or micro-relay, for example. 
     In this embodiment, the MEMS micromotion amplifiers consist of parallel first and second high aspect ratio beams  100 , 102  and corresponding perpendicular support beams  116 ,  118 . The beams  100 ,  102 ,  116  and  118  are preferably fabricated of single crystal silicon within a cavity of a substrate  110  using the SCREAM process so that the beams are made integral to the substrate but are released from it and relatively moveable. The beams  100 ,  102 ,  116 ,  118  are coplanar and have high aspect ratio profiles. For example, each of the beams may be 1-2 μm wide, 10-20 μm deep and 3-5 mm in length. The beams  100  and  102  are parallel and spaced apart relatively far from each other, for example 50 μm. Beams  100  and  102  each have a first moveable end A and C, respectively, which is secured to or integral with a corresponding perpendicular flexible support beam  116 ,  118 . Each support beam  116 ,  118  has its ends secured to or made integral with the substrate  110  and provides an adjustable mounting to prestress its corresponding beam  100 ,  102  near the point of buckling. Each beam  100 ,  102  also has a second fixed end B and D, respectively, which is secured to or made integral with the substrate  110  to provide a fixed mounting and point of reference for the corresponding one of the beams  100 ,  102 . Actuators  120  and  122  are secured to or made integral with corresponding support beams  116  and  118  at beam ends A and C and provide axial compressive forces upon beams  100 ,  102 , respectively, when energized. The actuators  120 ,  122  may be capacitive comb-drive structures or any desired micro-actuator for supplying axial compressive forces to the beams  100  and  102 . For example, other actuator elements such as those which exhibit physical properties responsive to temperature, pressure, humidity, impact, acceleration or any other measurable parameter may be used. 
     The support beams  116 ,  118  are not only provided for prestressing of the beams near the point of buckling as previously described, but they also effectively control the direction in which a respective beam  100  or  102  will buckle by introducing asymmetry. As illustrated, the beams  100  and  102  are connected off-center of beams  116  and  118  where connection is made to beam ends A and C, respectively. The exact point of connection along the support beam will influence the direction in which the corresponding beam  100  or  102  will buckle. Thus for example, segments  114  and  115  of beam  116  are unequal and segments  117  and  119  of beam  118  are unequal. Energization of actuators  120  and  122  will therefore cause support beams  116  and  118  to buckle as illustrated in FIG. 11 toward each other. The resultant curvature of support beams  116 ,  118  will supply a resultant axial compressive force to the corresponding one of beams  100  and  102 , causing them to buckle inwardly toward each other so as to ultimately make contact. at a contact point  130 . 
     The beams  100  and  102  are made electrically conductive but in this embodiment they are electrically isolated from each other and from the substrate, such as by providing silicon dioxide segments in the beam ends A and B of beam  100  and beam ends C and D of beam  102 . As is known in MEMS devices, electrical leads may be connected to the beams  100  and  102  to enable the beams to serve as electrical contacts of a switch or relay and thereby provide a switching device which is highly sensitive to micromotion. The sensitivity of this structure can be greatly enhanced by the addition of one or more micromotion amplifier stages  26 , as described previously with respect to FIG.  2 . For example, a micromotion amplifier stage  26  may be used to amplify the axial motion provided by actuator  120  to beam  100 . Similarly, amplifier stage  26  may be used to amplify the axial motion provided by actuator  122 , of FIGS. 10,  11 , to beam  102 . 
     In an alternate embodiment illustrated in FIG. 12, a stationary electrical contact  150  may be substituted for the beams  102 ,  118  and actuator  122  of FIGS. 10,  11 . The stationary contact  150  may be fixed to or integral with the substrate  110 . The stationary contact  150  may also be fixed to or integral with any other structure in proximity to the buckling of beam  100 . Thereby, energization of actuator  120  will buckle conductive beam  100  to a point where contact is made with the stationary electrical contact  150 . 
     Although the invention has been described in terms of preferred embodiments, various modifications will be apparent to those of skill in the art without departing from the true spirit and scope thereof, as set forth in the accompanying claims.