Microstructural origin of compressive in situ stresses in electron-gun-evaporated silica thin films

Abstract

Room-temperature electron-beam deposition of transparent silica produces thin films that are typically under large compressive stresses (a few hundreds of megapascals), yet of smaller density than tempered glass. The physical origin of these stresses remains unknown as all prior experimental studies were performed in manufacturing conditions where it is impossible to disentangle effects arising from vaporization versus residual gases. Here, we tackle this problem using a combination of experimental and numerical approaches. Our key experiment is performed after having achieved a high vacuum that allows us to approach the ideal case where residual gas effects are negligible. It shows that, even in this case, films are strongly compressive, which rules out in situ reactive processes as a possible origin of these stresses. Another test allows us to show that collisional slowing down causes films to grow much less dense and less compressive (even tensile). Our numerical model, designed to represent deposition in the absence of reactive processes, produces films with reasonable stress and density values, yet only when assuming that vaporized particles reach the growing film with kinetic energies of order a few electron volts, which is an order of magnitude higher than literature estimates. We argue that such values are plausible because, silica being an electric insulator, its surface may concentrate charges when irradiated by electrons, and hence its e-beam vaporization, which is a much more rapid process than its evaporation, is likely to involve Coulombian effects. Finally, the microstructural analysis of numerical systems shows that these compressive stresses result primarily from the fact that, in usual deposition conditions, films tend to be oxygen-deficient, and thus contain a significant fraction of coordination defects akin to partial oxygen vacancies, which cause their Si-O covalent network to be less cohesive and tensile than that of relaxed silica.