One of the main responsible for adhesion failure of antireflection coatings on plastic substrates is the stress developing at the film/substrate interface during film deposition and subsequent system cooling. Coating a substrate by a different material always induces stress at the interface, because it enforces the film to copy the substrate structure. Further, non-equilibrium conditions during film growth and thermal deformations due to post-deposition cooling, also contribute to the total stress of the system. In particular, thermal stress plays an important role in adhesion failure of coated plastics, because the expansion coefficient of polymers is about an order of magnitude greater than that of hard oxides. In order to prevent delamination of the film, this strain must be balanced by bond strength between the film and the substrate. In this work, we investigate the possibility of improving the film/substrate adhesion by reducing the total stress at thin interface. In fact, the intrinsic stress of ion assisted hafnium oxide films is influenced by the assistance conditions, and consequently the total stress can be changed by varying the experimental parameters. Hafnium oxide layers were deposited on plastic substrate by dual ion beam sputtering, at different growth conditions. Stress values were obtained by measuring the curvature of the substrate before and after deposition. Practical adhesion properties were evaluated by mechanical tests.