Wrinkling evolution of a growing bubble: The wonders of petal-like patterns in amorphous silicon membranes

A large-area amorphous silicon (a-Si) membrane on an oxidized wafer (SiO₂/Si) is detached from the substrate by dissolving the sandwiched oxide layer using hydrofluoric acid solution (HF/H₂O). Due to the atomic disorder in the structure of a-Si, the rate of molecular diffusion inside the a-Si film and etching of the oxide layer is not uniform over the film; thus, certain over-susceptible spots for etchant infiltration starts to locally detach. Using an in situ optical microscope, initially the detached region is observed to be buckled as a circular dome-shaped protuberance, which then forms wrinkles around the rim of the a-Si bubble. Around the rim, the deformation pattern was strongly dependant on the thickness h of the film, where the number of facets in the engendered petal-like patterns decreased with the increasing h. The tension-induced wrinkling in a-Si membranes was analytically and semi-quantitatively examined, and we conclude that the formation of peripheral corrugation is primarily due to the upward stretching force exerted by the underlying droplet composed of etchant and etching byproducts. The understanding of the elastic instability in ultrathin membranes could be extended to direct measurement of the fundamental properties in mechanically inferior systems.