Stability of thin copper films on mesoporous dielectrics

We report observations of the morphological changes in sputter-deposited thin copper films (<100 nm) deposited on solid SiO2 and varying porosity mesoporous silica substrates. Break-up of the copper film into islands (agglomeration) was observed using both an in situ hot-stage scanning electron microscope (SEM) system and an ex situ vacuum-annealing system followed by SEM characterization. The contributions of the underlying substrate to the film and substrate interfacial energy and the film’s elastic strain energy were evaluated. Detailed pore-size and roughness characterization of the mesoporous substrates were performed in order to evaluate the interfacial energy contribution. The intrinsic stress in the copper film was also determined as a function of underlying dielectric porosity. The stress is tensile in as-deposited films and does not dominate the interactions in our system, as the stress energy is two orders of magnitude lower than the surface/interfacial energies. We have observed an enhancement in the film stability up to a limiting porosity and pore-size due to an increase in the effective interfacial area and hence interfacial energy available for the contiguous copper film. The experimentally observed stability behavior is correlated to models that have appeared in literature, but which needed to be adapted for porous substrates.