Soft films on hard substrates — nanoindentation of tungsten films on sapphire substrates

We have studied the indentation properties of sputter deposited W thin films on single crystal sapphire substrates in an effort to understand the effects of pile-up on the indentation properties of soft films on hard substrates. Hardness and elastic modulus of these films were determined by nanoindentation using a Nano XP™. The hardness and elastic modulus were first determined using the Oliver and Pharr method to estimate the contact areas during indentation. This method does not account for the effects of pile-up. Because the film and substrate have very similar elastic properties, it was possible to determine the true contact areas directly from the measured contact stiffness and from that determine the true properties of the film. This method also provides a quantitative estimate of the pile-up height, which compares favorably with direct AFM measurement of the pile-up height.