Fabrication of Phase Change Microstring Resonators via Top down Lithographic Techniques: Incorporation of VO₂/TiO₂ into Conventional Processes

The electronic, optical, and mechanical properties of vanadium dioxide (VO₂) can be significantly altered by external stimuli such as heat, electric field, and so on, due to its metal-insulator transition (MIT). While the properties and potential applications of this material have been widely studied, minimal efforts were pursued to integrate VO₂ into existing microfabrication processes. Here, in this paper, we present a process for fabricating VO₂/TiO₂-coated microstring resonators, where the stability of both films was monitored during the entire process. RIE, vapour HF, ablation, and ICP etching were used throughout the process and their effects on the VO₂/TiO₂ films, as well as the structural and masking layers was studied. Certain processes required the use of a metal hard mask rather than photo resist. VO₂ shows excellent etch selectivity over the metal mask layer with RIE. Of all metals examined, Cr was found to be the most stable as a hard mask. Once fabricated, the mechanical and electrical characterizations of the microstring were carried out by measuring resonance frequency (f_r) and electrical resistance which show over 6 % alteration in f_r and two orders of magnitude change in resistance during the MIT, with a reversible transition and minimal hysteresis. This report provides a foundation for future processing of VO₂ based MEMS/MOEMS devices.