  Thin metal films are subject to the pressure caused by the zero point
oscillations of the electromagnetic field, which depends upon the film optical
properties and, in case of deposition onto a substrate, upon the substrate
reflectivity. It has been suggested that this force may be relevant in
determining the stability of deposited pseudomorphic films with respect to
buckling or island formation. We present a detailed analysis of its behaviour
as a function of the optical parameters and of the film thickness and we
illustrate the conditions under which it may play some role. For free standing
films it turns out that the film stabilization is basically due to the surface
stress, which largely overwhelms the vacuum force. For epilayers on metal
substrate the vacuum force may be important, and we give stability diagrams and
critical thicknesses for several cases, illustrating how the flat surface
growth may depend upon the film parameters. The importance of including
retardation effects into the theory for a realistic determination of the
stability conditions is also discussed.
