Fabrication and Transformation Properties of Vanadium Dioxide Bimorph Nanoactuators

We report on the fabrication of bimorph beam nanoactuators of vanadium dioxide-(VO₂-) based films on silicon thermal oxide with critical dimensions down to 100 nm and the investigation of their electrical resistance and mechanical deflection characteristics. VO₂ is a multifunctional ceramic material that exhibits a reversible phase transformation between the tetragonal (rutile structure) and monoclinic phase, which is accompanied by abrupt changes in electrical, optical and mechanical properties. As the Young’s modulus is rather large, about 140 GPa, large forces and high work densities can be achieved. Here, our interest is in the development of free-standing VO₂ structures at the nanoscale, which will open up new opportunities for using the multi-functional material properties in nanoactuators and -sensors. Starting materials are VO₂ films that are deposited by magnetron sputtering [1]. Addition of third elements, e.g., molybdenum (Mo) is used to tune the phase transformation temperatures. For nanofabrication, a top-down process has been developed based on electron beam lithography and reactive ion etching. Electrical resistance measurements of V_0.99Mo_0.01O₂ exhibit a semiconductor-metal transition near 60 °C. An extra small jump is observed in the temperature range of -10…-30°C, which is attributed to an intermartensitic transition caused by internal stress. The resistance change at the semiconductor-metal transition tends to increase for decreasing width indicating a size-dependent reduction of carrier scattering as the absolute number of grain boundaries decreases. In-situ measurements of electrical power-dependent deflection show that nanoactuation is dominated by the thermal bimorph effect.

[1] X. Wang et al., J Inorganic Mater. 30 (2015).