Abstract:
A sensor, having a resonant frequency responsive to presence of an analyze, comprising a DC electrostatic excitation component, to produce a static force pulling a moveable element toward a backplate; an AC electrostatic excitation component, to produce an oscillation in the moveable element with respect to the backplate; and a sensor to detect contact between the moveable and the backplate.

Description:
RELATED APPLICATIONS 
     The present application claims benefit of priority from U.S. Provisional Patent Application Nos. 60/893,342 filed Mar. 6, 2007, and 60/943,303, filed Jun. 11, 2007, the entirety of which are expressly incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of microcantilever sensing devices. 
     BACKGROUND OF THE INVENTION 
     Microcantilevers that are properly functionalized with chemo- or bioselective coatings have been shown to be extremely sensitive to chemical and biological analytes in both vapor and liquid media. Microcantilevers therefore exhibit great promise as molecular and atomic recognition sensors for an extremely diverse set of applications including environmental monitoring, industrial process control, biological research, and homeland defense. Microcantilever operation is characterized by chemical reaction or adsorption of molecular species at the microcantilever surface which results in a change in the microcantelever&#39;s deflection and in properties such as its resonance frequency. While these induced changes can be very small (sub-nanometer cantilever deflection, for example), they are readily measurable with a laser beam reflection technique developed for atomic force microscope (AFM) cantilever measurements. 
     See:
     Thundat, T., G. Chen, R. Warmack, D. Allison, and E. Wachter (1995) Vapor detection using resonating microcantilevers. Analytical Chemistry 67, 519-521.   Wachter, E. A. and T. Thundat (1995) Micromechanical sensors for chemical and physical measurements. Rev. Sci. Instrum. 66, 3662-3667.   Battiston, F., J.-P. Ramseyer, H. Lang, M. Baller, C. Gerber, J. Gimzewski, E. Meyer and H. Güntherodt (2001) “A chemical sensor based on a microfabricated cantilever array with simultaneous resonance frequency and bending readout”. Sensors and Actuators B 77, 122-131.   Yang, Y. M., H. Ji, and T. Thundat (2003) “Nerve agents detection using a Cu2+/L-cysteine bilayercoated microcantilever”. Journal of the American Chemical Society 125, 1124-1125.   Wu, G. H., R. Datar, K. Hansen, T. Thundat, R. Cote, and A. Majumdar. (2001) “Bioassay of prostatespecific antigen (PSA) using microcantilevers”. Nature Biotechnology 19, 856-860.   J. D. Adams, B. Rogers, L. Manning, M. Jones, T. Sulchek, K. Murray, B. Beneschott, Z. Hu, T. Thundat, H. Cavazos, and S. C. Minne (2003) “Piezoelectric self-sensing of adsorption-induced microcantilever bending”. Appl. Phys. Lett., in press   B. Rogers, L. Manning, M. Jones, T. Sulchek, K. Murray, B. Beneschott, J. D. Adams, Z. Hu and T. Thundat, H. Cavazos and S. C. Minne. (to be published November 2003) “Mercury vapor detection with a selfsensing, resonating piezoelectric cantilever”. Rev. Sci. Instrum.   L. A. Pinnaduwage, A. Gehl, D. L. Hedden, G. Muralidharan, T. Thundat, R. T. Lareau, T. Sulchek, L. Manning, B. Rogers, M. Jones, and J. D. Adams. (accepted 2003) “Detection of trinitrotoluene via deflagration on a microcantilever”. Nature   J. D. Adams, G. Parrott, C. Bauer, T. Sant, L. Manning, M. Jones, B. Rogers, D. McCorkle and T. L. Ferrell. (to be published Oct. 20, 2003) “Nanowatt chemical vapor detection with a self-sensing, piezoelectric microcantilever array”. Appl. Phys. Lett.   J. English, Y. Shtessel, M. Yegnaraman and M. George, “MEMS Device Modeling Microcantilever Sensor via Second Order Sliding Mode Control”, Nanotech 2006 Vol. 3, Technical Proceedings of the 2006 NSTI Nanotechnology Conference and Trade Show, Volume 3, Chapter 6.   

     
       
         
               
               
             
           
               
                   
               
             
             
               
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     Each of the above references is expressly incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     The present invention provides a resonant mechanical structure which is forced to escape from a potential well; in this case the potential well is electrostatic. This escape leads to dynamic pull-in and collapse of the structure. According to a preferred embodiment, the collapse results in a switching action of an electrical circuit. 
     While a preferred embodiment of the invention excites the resonant mechanical structure with an alternating electric field (AC) while simultaneously biasing it with a static electric field component (DC), the structure can be driven near its natural frequency (resonance) by any excitation method, for example, with a piezoelectric effect, an electromagnetic or magnetic effect, a thermal effect, a photoresponse effect, and acoustic methods. 
     An embodiment of the present invention combines the functions of two devices: a sensor (e.g., gas or solute) and an electro-mechanical switch. This device is capable of detecting a specific kind of gas or substance, such as an explosive gas, and then sends a strong electrical signal (e.g., low impedance output) as a sign or indication of this detection. This signal can be used to actuate an alarming system or to activate a defensive or a security system. 
     The present state of the art has many examples of gas sensors that can detect specific gases, but the signal obtained from this detection is not strong enough or usable to be utilized to do an action. In order to utilize the obtained signal from such devices, a complex system of sensors, actuators, decision units, amplifiers, analogue-to digital converters, and other electronic components, is needed. These may be expensive and complex and may not be reliable enough. If any of those single components becomes non-functional, the whole system fails. The new device, on the other hand, is reliable, simple, relatively less expensive, and can be made to be of high sensitivity. 
     An embodiment according to the present invention has high reliability. When configured as a switch, it will operate only when the concentration of the hazardous gas or material exceeds the permitted percentage, then it will send a direct electrical signal. So if there is no risk, no signal comes out; if there is any danger, there will be a signal. In some cases, it may operate in inverse as a “normally closed” output, wherein the hazardous condition is signified by an interruption of the signal. An integration of multiple sensing elements is possible, sensing the same or different effects at various sensitivities and thresholds, and it is also possible to provide a differential output in which one switch is closed and another opened to indicate the condition. 
     A sensor device according to various embodiments of the invention is simple to fabricate: for example, it may consist of only a microbeam capacitor with an appropriate coating, and a conductive path sensor. Such a device is typically excited with a DC potential to provide a pull in force, and an AC signal to generate an oscillation. 
     The present invention is not limited to functioning as an actuator, and thus can also function as a sensor having an analog and/or proportional output. In that case, a circuit is provided to analyze a characteristic of the oscillating element, such as its position, oscillation amplitude, resonance frequency or non-linearities in transfer function, to determine an amount or change in response of the sensor to the condition being sensed. It is noted that calibration of the sensor and/or switch may be achieved by altering the AC and/or DC voltages. 
     The sensor may also be made responsive to external mechanical influences, such as shock or inertia. See, Younis et al., “Characterization of the Performance of Capacitive Switches activated by Mechanical Shock”,  J. Micromechanics and Microengineeering  17 (2007), 1360-1370, expressly incorporated herein by reference. 
     The present invention has all the advantages of the MEMS sensors and actuators (low power consumption, low weight, etc.). The principals involved may also apply to nano-electromechanical sensors, formed, for example, with nanotubes, and may be scaled to larger sizes as well. Thus, while the MEMS technology is not a size or scale limitation of the invention, though devices and features obtained through application of masking, etching and/or selective oxidation of silicon wafers are presently preferred. 
    
    
     
       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  shows a parallel-plate capacitor. 
         FIGS. 2 and 3  show a parallel plate capacitor before (a) and after (b) the application of a small and large DC voltage, respectively, producing a static force. 
         FIG. 4  shows a parallel plate capacitor showing the excitation using both a DC load and an AC harmonic load. 
         FIG. 5  shows the maximum displacement of a cantilever beam of a parallel-plate capacitor versus the frequency of excitation. 
         FIGS. 6 and 7  show a time history response of the microbeam when excited by different frequencies. 
         FIGS. 8 and 9  show the operating principle a switch in accordance with the present invention as a gas sensor. 
         FIG. 10  shows dimensional frequency-response curves for a clamped-clamped microbeam before and after 5% mass increase, and two time history simulations for the response of the microbeam before and after mass detection. 
         FIG. 11  shows the instability tongue of an electrically actuated cantilever beam. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relies on electrostatic actuation of a parallel plate capacitor, shown in  FIG. 1 , in which one plate (or any other structure of arbitrary shape) is stationary and the other plate is movable and is actuated or biased by an electrostatic DC force. Indeed, the existence of a stationary plate is not required, though it provides a simplified construction and analysis thereof. 
     The DC force deflects the moveable plate toward the other stationary plate ( FIG. 2   b ). If the electrostatic force is small, the moveable plate is maintained in a deflected position, at which the elastic restoring force of the plate is in equilibrium with the opposing electrostatic force. 
     When the DC voltage increases, the electrostatic force increases, and hence the plate deflection increases. There is an upper limit for the DC voltage, beyond which the mechanical restoring force of the plate can no longer resist the opposing electrostatic force. This leads to a “collapse” of the plate, which hits the stationary plate ( FIG. 3   b ). This structural instability phenomenon is known as pull-in. In accordance with an embodiment of the invention, the contact of the two plates provides a switching function, which is potentially relatively low impedance, and thus can provide direct drive capability, without requiring an electronic sensing circuit or amplifier in order to provide a suitable signal for external use. 
     Pull-in can also occur due to the actuation of a combination of a DC load and an AC harmonic load. The DC load deflects the movable electrode slightly and the AC load vibrates the electrode around the new deflected position. Because pull-in here occurs due to a dynamic (harmonic) loading, it is called dynamic pull-in, as opposed to static pull-in, which occurs due the actuation of DC loading only, as explain above with respect to  FIGS. 2 and 3 . 
     The AC harmonic load has the form of v(t)=V AC  cos(Ωt), where V AC  is the amplitude of the AC excitation and Ω is the excitation frequency. The maximum influence of this excitation on the movable structure (electrode) occurs when the excitation frequency Ω gets close to the natural frequency of the structure ω natural . This causes a resonant behavior. Hence, we expect the dynamic pull-in phenomenon to occur in the range of excitation frequency that is close to the natural frequency of the movable electrode. 
     To demonstrate an example of the invention, we consider a parallel plate capacitor employing a cantilever beam as its upper electrode. The microbeam is made of silicon, with length 100 microns, width 10 microns, and thickness 0.1 micron. The gap spacing between the beam the substrate (the lower stationary electrode) is d=2 microns. The quality factor of the microbeam is assumed to be 10. The natural frequency of this microbeam is equal 3.5*ω nat , where ω nat  is a universal natural frequency for beams. So ω natural /ω nat =3.5. When the beam is biased by V DC =0.4 V, ω natural /ω nat  drops to 3.3. 
     The pull-in voltage for the microbeam when actuated by a DC voltage only is V DC =0.6 V. If this microbeam is excited by V DC =0.4 V and an AC harmonic load of amplitude V AC =0.1 V, a dynamic pull-in occurs when the excitation frequency Ω is close to the natural frequency of the microbeam, that is Ω/ω nat =ω natural /ω nat =3.3. This is demonstrated in  FIG. 5 , which shows the maximum displacement of the cantilever beam W max  normalized by the gap width d underneath the beam versus the excitation frequency Ω normalized by ω nat .  FIG. 5  shows that when Ω gets close to ω natural  (Ω=3.3 ω nat ), resonance occurs and W max  reaches its peak. Because of the presence of the instability threshold pull-in, the frequency-response curve opens up. We note that there is band of frequency near this regime where there is no stable state for the microbeam exists. We can call this band the “pull-in band”. If the microbeam is excited near this range of frequency, it will snap down and go to pull-in. On the other hand, if the microbeam is excited at a frequency away from this pull-in band, the microbeam will oscillate in a stable motion and never goes to pull-in.  FIG. 6  shows these cases. 
       FIG. 5  shows the maximum displacement of a cantilever beam of a parallel-plate capacitor versus the frequency of excitation. Here, V DC =0.4 V and V AC =0.1 V. 
       FIG. 6  shows a time history response of the microbeam when excited by Ω=3, which is in the pull-in regime. It is clear that the response is unstable and it goes to pull-in, where W max/d  is equal one. 
       FIG. 7  shows a time history response of the microbeam when excited by Ω=2.6, which is away from the pull-in regime. It is clear that the response is stable and it reaches a steady-state value W max /d equal 0.3. 
     The cantilever microbeam can be coated with a sensing material, such as some “smart” polymers that are sensitive to specific gases. (See references incorporated herein by reference supra). Hence, the microbeam becomes as a chemical sensor to that gas. The coated sensitive surface layer of the cantilever beam can absorb a small amount of specific gas, which is around in the environment. This increases the weight of the cantilever beam, which leads to a decrease in its natural frequency, since it is proportional to the inverse of the square root of the mass of the beam (if the stiffness of the microbeam is denoted by k, then ω nat =√{square root over (k/m)}). This shift in frequency can be considered as an indication to the presence of the gas in the environment. This is effect is well known. A sensing is possible of any condition which directly or indirectly changes the relevant mechanical characteristics of the beam, such as its mass, stiffness, size, resonant frequency, damping, or the like. In the case, for example, of a swellable polymer, the mechanical separation of the plates may be changed in dependence on a concentration or presence of an analyte. Other configurations are possible, as well, so it should be understood that the scope of the invention is not limited to a sensor formed by an absorptive coating on a microcantilever beam altering the resonant frequency thereof. 
     The principle of operation of an embodiment of the device relies on the above principle of gas sensors and the dynamic pull-in concept demonstrated in the previous section. According to the present invention, the microbeam is excited by a combination of DC load and AC load such that the microbeam normally operates below the dynamic pull-in band of frequency. 
     When the microbeam is subject to the existence of the specific gas or other substance desired to be detected, which absorbs to the beam or a coating thereon, its mass will increase, and its natural frequency will decrease. This will decrease the ratio Ω/ω nat , and hence the operating point on the frequency response curve will shift to the right. We can calibrate this shift such that the shifted Ω/ω nat  lies in the dynamic pull-in frequency band. Hence, the microbeam collapses, to close an electric circuit to indicate the presence of the gas and at the same time to send an electrical signal, which can be used for alarming or any other useful function. In some cases, the collapse is a reversible process, and therefore a decrease in a concentration of a material can be sensed by an opening of the switch. 
       FIGS. 8 and 9  illustrate the principle of operation.  FIG. 8  shows the operating principle of a switch in accordance with the present invention. The microbeam is biased by a DC voltage equal 0.4 V and an AC load equal 0.1 V. The dashed line to the left represents the operating point of the microbeam before the mass detection. The dashed line to the right represents the operating point of the microbeam after detecting a gas, which increase its mass by 10%. 
       FIG. 9  shows the switch when biased by a DC voltage equal 0.4 V and an AC load equal 0.15. The dashed line to the left represents the operating point of the microbeam before the mass detection. The dashed line to the right represents the operating point of the microbeam after detecting a gas, which increase its mass by 5%. 
       FIG. 10  shows dimensional frequency-response curves for a clamped-clamped microbeam (i.e., one which is supported on opposite sides, and which therefore has a degree of freedom for movement between the supported sides) before and after a 5% mass increase. 
     This scenario is illustrated using dimensional quantities and plots. Consider the case in which the microbeam is initially excited by a combination of a DC and AC harmonic load of a fixed frequency below the escape band, for example at 52 kHz in  FIG. 10 . Assuming a 5% increase in mass because of external mass detection/absorption, this leads to a decrease in its natural frequency shifting it to the left. This means that the whole frequency-response curve of the microbeam shifts to the left too. By maintaining the frequency of excitation fixed at 52 kHz, while the microbeam&#39;s natural frequency shifting to smaller values, and by calibrating this shift such that the operating frequency lies in the escape band after mass detection, the microbeam will be forced to pull-in. Hence, it can act as a switch to close an electric circuit, and, for example, pass a low impedance electric signal. 
       FIG. 10  also shows a simulated time history response for the microbeam for two states, before and after mass detection. Prior to mass detection, the microbeam oscillates at a steady-state amplitude of 0.4. After mass detection, the microbeam undergoes unstable oscillation leading to its collapse after 0.25 ms. 
     The microbeam thus normally is driven to operate close to the instability tongue, and the perturbation caused by a change in mechanical properties causes the microbeam to enter the instability tongue and collapse. This collapse, in turn, permits a switching action dependent on a physical contact of the microbeam and the back plate. 
       FIG. 11  shows a calculated instability tongue for a cantilever microbeam for the case of Q=100 as a function of V AC  and Ω/ω Universal . The figure also illustrates the operating principle based on primary-resonance excitation. 
     The present invention can operate as a chemical sensor or a biosensor. In the case of a biosensor, typically the sensor component itself provides a biochemical specificity for binding or catalyzing a reaction, for example. It can be used to detect explosive, hazardous, or any other gases or substances, and to activate/actuate alarming or defensive systems. The invention also may be used to detect biological agents, such as bacteria and viruses, in the environment or in the human body and then send a signal indicating their existence, and may be to perform other functions. 
     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 example 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.