Thermally induced stress hysteresis and co-efficient of thermal expansion changes in nanoporous SiO₂

The thermomechanical response of electron beam deposited nanoporous silicon dioxide is examined using substrate curvature measurements and nanoindentation. Analysis of the thin film bond angle strain distributions versus temperature indicates that low temperature (T < 100 °C) stress hysteresis and tensioning are primarily attributed to hydrogen bonded water desorption. However, at higher temperatures, the absence of water desorption suggests that the thermomechanical behaviour is related to thermally induced bond angle strain redistributions towards the local bonding environment of quartz and thermally grown silicon dioxide. This is supported by the co-efficient of thermal expansion data that trend lower with higher annealing temperatures. The re-absorption of water into the thin film accounts for the reproducibility of the open-loop stress hysteresis and tensioning observations.