Composition and thickness of non-functional organosilane films coated on aluminium studied by means of infra-red spectroscopic ellipsometry

Surface treatments of metals, leading to the deposition of thin films, are generally performed in order to create or change certain surface properties such as corrosion resistance or adhesion. For the past few years, the use of organosilanes for aluminium surface treatments are more and more considered as they provide both corrosion protection and adhesion properties. It has been previously shown that the silane bath concentration strongly influences the thickness of the silane films deposited on aluminium substrates. A linear increase of the layer thickness with the silane bath concentration has been underlined. In this paper, the chemistry of the deposited silane films has been studied as a function of the silane bath concentration. For this, Infra red spectroscopic ellipsometry (IRSE) has been used. It is a method similar to reflection absorption infra red spectroscopy (RAIRS), which is a non-destructive infra red reflection technique, allowing chemical characterisation of thin films on metals. It will be shown that differences in the positions and strength of absorption bands are visible when the silane concentration of the solution is varied. Such changes can result in a difference in the film chemistry or in the film thickness. Since IRSE offers chemical information along with the thickness information, modifications occurring in an IRSE spectrum can be understood more easily. It will be demonstrated that the use of an optical model, describing the absorption bands and thickness of the films, permits the determination of the chemistry and the thickness of the silane films.