Interpretation of macropore shape transformation in crystalline silicon upon high temperature processing

A physical interpretation is proposed for the sequence of transformations that macropores embedded in crystalline silicon undergo during high temperature treatments. First, cylindrical pores spheroidize by surface diffusion at constant volume. In the presence of stress and due to perturbation in the spherical symmetry stored elastic energy competes with cavity surface energy transforming large cavities by surface diffusion into an oblate shape with a major radius that continuously expands. At a critical condition close to the Griffith fracture criterion, the cavity collapses catastrophically into an ultrathin uniform slit that splits one or more thin crystalline film off the original substrate. On the other hand, if the stress is not high enough or the major radius of the cavity is not large enough the cavity does not collapse and maintains a rounded shape. Annealing in an ambient gas with a high partial pressure enhances the surface reaction which accelerates cavity growth and wall smoothening. The proposed theory agrees well with experimental observations.