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The integration of VO 2 thin films in a MEMS actuator device is presented. The structural phase transition of VO 2 was induced electro-thermally by resistive heaters monolithically integrated in the MEMS actuator. The drastic mechanical displacements generated by the large stress induced during the VO 2 thin film phase transition have been characterized for static and time-dependent current pulses to the resistive heater, for air and vacuum environments. A comprehensive and simplified finite element model is developed and validated with experimental data. It was found that the cut-off frequency of the 300 µm-long VO 2 -based MEMS actuator operated in vacuum ( f 3dB = 29 Hz) was mostly limited by conductive heat loss through the anchor, whereas convection losses were more dominant in air ( f 3dB = 541 Hz). The cut-off frequency is found to be strongly dependent on the dimensions of the cantilever when operated in air but far less dependent when operated in vacuum. Total deflections of 68.7 and 28.5 µm were observed for 300 and 200 µm-long MEMS cantilevers, respectively. Full actuation in air required ∼16 times more power than in vacuum.[2013-0042]

Silicon microcantilevers were coated by pulsed laser deposition with vanadium dioxide (VO 2) (monoclinic M 1 phase) and V 1Àx Cr x O 2 with x near 0.024 (monoclinic M 2 phase), and their mechanical characteristics were studied as a function of temperature through the films' insulator-to-metal transition (IMT). The undoped VO 2 films grew with (011) M1 planes parallel to the substrate, while Cr-doped VO 2 films grew oriented with (201) M2 and (201) M2 planes parallel to the substrate. In both cases, the films transformed reversibly through the IMT to the tetragonal (rutile, R) phase, with film (110) R planes oriented parallel to the substrate. The fundamental resonant frequencies of the cantilevers were measured as the temperature was cycled from ambient temperature, through the IMT, and up to 100 C. Very high resonant frequency changes were observed through the transition for both types of samples, with increases during heating of over 11% and over 15% for the cantilevers coated with pure and Cr-doped VO 2 , respectively. From the resonant frequencies measured at room temperature for the bare and coated cantilevers in each case, the effective Young's moduli of the films were determined. The values obtained, assuming bulk densities for the films, are 156 6 7.5 GPa for VO 2 (M 1 phase) and 102 6 3 GPa for Cr-doped VO 2 (M 2 phase). Strong curvature changes during the transition to the R phase were also observed for cantilevers coated with both types of films, but these were significantly higher in the case for the Cr-doped film. Curvature changes for temperature ranges outside the IMT region were small and attributed to differential thermal expansion between film and silicon substrate. From measured cantilever tip displacements in this post-transition range-for the undoped VO 2-coated microcantilevers-a rough estimate of 110 GPa was obtained for the effective Young's modulus for R-phase VO 2. The substantially higher changes in resonant frequency and curvature for V 1Àx Cr x O 2-coated cantilevers suggest that this material may be even more useful than M 1-phase VO 2 for prospective microelectromechanical or optomechanical device applications in which ample frequency tunability-in oscillators or filters-or large displacements-in actuators-within a small temperature range is desirable. Since M 2-phase V 1Àx Cr x O 2 with Cr composition of a few atomic percent retains other desirable properties of VO 2 , such as very high resistivity changes through the IMT and a transition temperature fairly close to ambient temperature, multifunctionality is not impaired and in fact may be enhanced for some applications. V

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