Adhesion, Microstructure and Composition of Thermally Evaporated Aluminum Thin-layers On Polyethylene Terephthalate Films

Industrial polyethylene terephthalate (PET) films have been metallized by aluminium evaporation in two different sets of experimental conditions. In the first set, aluminium layers of 100 nm thickness were deposited at a constant deposition rate (10 angstrom s-1) for different residual pressures varying from 1 Pa to 10(-4) Pa and, in the second set, the residual pressure was kept constant (2.6 x 10(-3) Pa), while the deposition rate was varied from 5 angstrom s-1 to 40 angstrom s-1. The adherence between the aluminium layers and the PET film was measured by means of scratch and peel tests. The critical load and the peel strength exhibit a maximum at about 10(-2) Pa when the deposition rate is kept constant. The microstructure of the aluminium layers, mainly the mean grain size, was studied by transmission electron microscopy (TEM), while secondary ion mass spectrometry (SIMS) depth profiles through the aluminium layers were performed in order to provide the chemical information, mainly aluminium layer oxidation. Concerning the TEM results, the grain size increases when the residual pressure is decreased and also when the deposition rate is increased. The SIMS depth profiles show different levels for aluminium oxidation at the surface, in the bulk of the layers and at the interface, all increasing for high residual gas pressure and for low deposition rates. From these results, it appears that the oxygen content at the Al-PET interface plays a critical role in the microstructure owing to its influence on the nucleation and on the growth of the aluminium layers. It also influences the adhesion between aluminium and PET for which an optimum oxygen amount seems to be required.