Effect of native Al₂O₃ on the elastic response of nanoscale Al films

A continuous, dense aluminum oxide (Al₂O₃) layer of about 5 nm forms on the surface of Al upon exposure to oxygen or dry air. Since the elastic moduli of Al and Al₂O₃ are 69 GPa and 370 GPa, respectively, the elastic modulus of a thin Al film of sub-micron dimension (with the native oxide layer) should be much higher than that of pure Al. However, uniaxial tensile measurements on Al films with thickness down to 50 nm revealed an effective modulus close to 69 GPa. In the present paper, we investigate a plausible mechanism for this discrepancy, namely, the effect of wavy surface oxide layer. Here thin Al films are considered as Al-Al₂O₃ composites. Uniaxial tensile experiments on a free-standing, 200 nm thick Al film are performed using MEMS techniques. The surface morphology of the specimen is characterized by AFM. An analytical model is developed to estimate the effective modulus, Ē, of a wavy oxide layer. The current study shows that the model predictions using measured material parameters agree reasonably well with the experimental results, thus supporting the validity of the proposed mechanism.