Role of vacuum fluctuation forces in thin metal film stability

TL;DR

This paper investigates how vacuum fluctuation forces influence the stability of thin metal films, highlighting their significance mainly in films on substrates and emphasizing the role of optical properties and retardation effects.

Contribution

It provides a detailed analysis of vacuum fluctuation forces on thin metal films, including stability conditions and the importance of retardation effects, extending understanding beyond surface stress effects.

Findings

Vacuum forces are negligible for free-standing films due to surface stress.

Vacuum forces can significantly affect the stability of films on substrates.

Stability depends on optical parameters, film thickness, and retardation effects.

Abstract

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.