Characterization of an evaporated Al/PET interface by TEM and correlation with adhesion measured by a peel test

Transmission electron microscopy (TEM) has been used to characterize thin Al films thermally evaporated (vacuum around 10-4 Torr) onto polyethylene terephthalate (PET) (Mylar™). The morphology of the Al/PET system has been correlated with the adhesion as measured by a peel test of the Al films. A special cross-section preparation revealed the exact thickness of the thin Al film, the roughness of the interfaces (Al/PET and Al/vacuum), the presence of precipitates or the diffusion of Al in the PET, the morphology of the Al layer (small grains or rod-like structure), the size of the grains or the columns (also checked with plane section), and the structure (crystalline or amorphous) of the PET at the Al/PET interface. The results suggest that the grain size and the roughness of the Al/PET interface decrease as the adhesion of the Al increases. Some spherical precipitates and diffusion of Al into PET were also observed for the films which had a low adhesion strength.