Electrostatically actuated resonant microcantilever beam in CMOS technology for the detection of chemical weapons

The operation of electrostatically actuated MEMS devices involves an inherent phenomenon termed as the pull-in instability which reduces their useful travel range. Accurate prediction of pull-in parameters (pull-in displacement and pull-in voltage) is hence vital in the design of such microdevices. In this article, we present a complete nondimensional formulation and implementation of an efficient numerical scheme based on the Rayleigh-Ritz energy technique to determine the static pull-in parameters of an electrostatically actuated narrow microcantilever beam. Deflection of the beam is approximated by an admissible polynomial function that satisfies its mechanical boundary conditions. The principle of stationary potential energy and the condition of instability are then applied to obtain a highly nonlinear algebraic equation which is solved using the Fibonacci minimization algorithm. An optimum number of Gauss quadrature points is used to integrate the nonlinear electrostatic terms. Two mathematical models accounting for the fringing field capacitance are examined in turn. A comparison is made between the normalized values of pull-in parameters obtained by considering the two aforementioned fringing field models. Two examples of electrostatically actuated narrow microcantilevers are then solved using the proposed scheme for the validation purpose. For these examples, a comparison is made between the values of pull-in parameters obtained using the proposed scheme and those previously published in the literature. An excellent agreement between the two establishes the utility of the proposed scheme.

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Modelling and measurements of a composite microcantilever beam for chemical sensing applications

Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science ◽

10.1243/09544062jmes150 ◽

2006 ◽

Vol 220 (10) ◽

pp. 1601-1608 ◽

Cited By ~ 9

Author(s):

I R Voiculescu ◽

M E Zaghloul ◽

R A McGill ◽

J F Vignola

Keyword(s):

Resonant Frequency ◽

Cantilever Beam ◽

Gas Sensing ◽

Laser Doppler Vibrometer ◽

Beam Theory ◽

Wheatstone Bridge ◽

Cmos Technology ◽

Oxide Semiconductor ◽

Sensing Applications ◽

Microcantilever Beam

A resonant microcantilever beam gas sensor was designed and fabricated in Carnegie Mellon University using complementary metal oxide semiconductor (CMU-CMOS) technology. The cantilever beam modified with a suitable sorbent coating was demonstrated as a chemical transducer for monitoring hazardous vapours and gases at trace concentrations. The design of the cantilever beam included interdigitated fingers to allow electrostatic actuation of the device and a piezoresistive Wheatstone bridge design to read out the deflection signal. The cantilever beam resonant frequency was modelled using the Euler-Bernoulli beam theory and ANSYS. The beam resonant frequency was measured with an optical laser Doppler vibrometer. Good agreement was obtained among the measured, simulated, and modelled resonant frequencies. A custom sorbent polymer layer was coated on the surface of the cantilever beam to allow its operation as a gas-sensing device. The frequency response as a function of exposure to the nerve agent simulant dimethylmethylphosphonate (DMMP) at different concentrations was measured, which allowed a demonstrated detection at a concentration of 20 ppb or 0.1 mg/m3. The air-polymer partition coefficient K, for DMMP was estimated and compared favourably with the known values for related polymers.

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