Influence of VO₂ Thin Film Coatings on the Performance of Integrated MEMS Bridges and Cantilevers

Vanadium oxide (VO₂) shows a solid-solid phase transition at around 68 °C where its mechanical and electrical properties start to change dramatically. During the phase transition, the large compressive stress generated by the VO₂ thin film coating could induce a significant frequency shift in the bimorph structures such as clamped-clamped (C-C) beams and clamed-free (C-F) beams. However, for regions outside the phase transition, the stress produced by the thermal expansion coefficient difference between VO₂ and the cantilever or bridge structural material dominates the frequency shift. Therefore, the two ends of the frequency hysteresis curve exhibits a pattern or behavior that can be different than the one shown during the phase transition. It was observed that opposite trends have been found before and after the phase transition region. This phenomena can be explained by the two competing mechanisms present in VO₂-based MEMS: frequency shift due to thermal coefficient difference between VO₂ and silicon dioxide (SiO₂) (the materials in the bimorph), and the frequency change induced by the phase transition of VO₂.