The adhesion strength of sputtered aluminium films to XC 70 steel substrates has been studied using the scratch adhesion test. The influence of the application of a bias voltage on the substrate during deposition was investigated by means of a mean critical load derived from a Weibull-like statistical analysis. It was found that in the absence of bias, the interface is rather abrupt, while the interface obtained with the application of a bias voltage is much more diffuse. We can consider that the widening of the interfacial region offers a better film anchorage on its substrate, requiring a stylus load value clearly excessively high to produce an adhesion failure.