Effect of SiO2 passivation overlayers on hillock formation in Al thin films

Hillock formation in Al thin films with varying thicknesses of SiO2 as a passivation layer was investigated during thermal cycling. Based on the stress measurements and the number of hillocks, 250 nm thick SiO2 was thick enough to suppress the hillock formation and the suppression of hillock at 250 nm passivation and the lack of suppression at thinner passivation is related to the presence/absence of protection against the diffusive flow of atoms from the surrounding area to the surface due to the biaxial compressive stresses present in the film through the weak spots in the passivation layer. The stress state of Al films measured during annealing (the driving force for hillock formation) did not vary much with SiO2 thickness. A small number of hillocks formed during the plasma enhanced chemical vapor deposition of SiO2 overlayers at 300 °C.

► We examined the effect of SiO2 overlayers on hillock formation in Al thin films.
► Thin overlayers were not effective in suppressing diffusive flow to the surface.
► A thick overlayer suppressed the diffusive flow from the interior to the surface.
► The stress state of Al films did not vary much with SiO2 passivation thickness.
► High mechanical strength provided a large driving force for the large grain growth.