Wrinkling of the metal-polymer bilayer: The effect of periodical distribution of stresses and strains

The viscoelastic wrinkling of aluminium films formed on silicon substrates with polystyrene interlayers (aluminium-polystyrene bilayers) is studied under thermal annealing. Compressive thermal stresses induced by the difference in thermal expansions of the bilayer and the silicon substrate are shown to lead to wrinkling instability of the Al film. The effects of annealing conditions and film thickness on the wrinkling regularities are investigated. The evolution of wrinkles, which has a three-stage character, is found to be controlled by the sign and value of in-plane stresses and those normal to the metal-polymer interface. On increasing temperature and duration of annealing, lateral extension of the aluminum-polystyrene bilayer inhibits wrinkle growth and results in smoothening of the film surface.