Electrostatically driven long-microbeams for low-frequency applications

This Letter presents the design, fabrication and characterisation of an array of electrostatically actuated clamped-clamped microbeams. A large bottom actuation electrode and long beams with lengths ranging from 1 to 3.4 mm are the major features of the present device. The novelty of this Letter lies in the realisation of suspended and undeformed microstructures by controlling the process-induced stress during the fabrication process. This has been achieved by compensating the influence of the compressive and tensile stress components of the different deposited layers, resulting in ultralong beams with a relatively straight mechanical profile and an aspect ratio of approximate to 1:3400 of vertical deflection to the beam length. For the first time, ultralong microbeams of tantalum have been actuated electrostatically with AC and DC driving voltages to drive them into resonance and characterise their resonant frequencies. The lowest resonant frequency of 1.4 kHz is obtained for a 3.4 mm-long beam. The shift of the resonant frequency due to the effect of different DC biasing has been investigated experimentally. A spring softening effect has been induced through electrostatic tuning. A downward shift in the resonant frequency to 35,000 ppm for DC bias voltages increasing from 1 to 5 V has been demonstrated.