Weak adhesion between deposited rough films: Relation to dispersion forces

Abstract

Although the contact theory between rough surfaces is designed for adhesion energies $\ensuremath{\gtrsim}100\phantom{\rule{4pt}{0ex}}\mathrm{mJ}/{\mathrm{m}}^{2}$, microsystems are controlled by much weaker adhesion $\ensuremath{\lesssim}100\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{J}/{\mathrm{m}}^{2}$, which is critical for their operation. The weakest adhesion is related to the omnipresent fluctuation-induced dispersion forces. We develop a theory for such a weak adhesion emphasizing that the adhesion energy as a function of the average distance separating the bodies is almost entirely defined by the dispersion interaction. This dependence can be evaluated using the Lifshitz theory, but the effects of contact interaction or plastic deformations give only small contribution to the adhesion. Such a behavior is explained by a specific roughness of the deposited thin films used in microtechnologies. The films deposited on cold substrates have a much larger number of high asperities than is predicted by the Gaussian distribution and the contact occurs over a few asperities with heights much larger than the root-mean-square roughness. Finally, we discuss application of the effect for more precise determination of the distance upon contact, which is crucial for precise measurements of the dispersion forces especially at short separations in the range $5<h<50\phantom{\rule{4pt}{0ex}}\mathrm{nm}$.