Electrostatically actuated nonconductive polymer microresonators in gaseous and aqueous environment

We present nonconductive polymer microresonators actuated by the Kelvin polarization force. This technique allows the direct electrostatic actuation of nonconductive all-polymer microstructures in gaseous and aqueous environment. Freestanding single and double-clamped SU-8 microbeams were fabricated with a sacrificial layer process using lift-off resist as sacrificial layer material. The gold electrodes were covered with a 120-nm thick insulating layer of aluminum oxide in order to eliminate electrolysis reactions in water. The relatively low quality factors (Q) of polymer microresonators are increased by Q-enhancement which is based on a positive feedback of the oscillation velocity signal. The behavior of the feedback was determined by a Matlab/Simulink simulation. The feedback loop was further used to drive the microresonators in a self-excitation mode. SU-8 cantilevers and bridges were driven in air at atmospheric pressure and in DI water. With Q-enhancement, effective quality factors of 115 and 31 were obtained in air and in water, respectively. Applying the self-excitation technique, the effective quality factors are enhanced to more than 40,000 and 4200 in air and in water, respectively.