Elastic deformations in thin freestanding ferroelectric films

We investigated mechanical stress in thin freestanding BaTiO₃ films prepared on bare silicon and on silicon, covered by a 120 nm thick, randomly oriented Al₂O₃ buffer. Films prepared on bare silicon by RF sputtering are essentially stress-free. However, they disintegrate after substrate removal. In contrast, the films prepared on the Al₂O₃ buffer have high tensile stress, but retain their structural integrity after separation from the substrate. Substrate removal is accompanied by film corrugation; at the same time, the freestanding films resonate mechanically. This seeming contradiction can be understood on the basis of a recently developed theory of 2D clamping in thin ferroelectric films.