Ionising radiations have been observed to produce significant improvements in thin film adhesion by several experimental groups. We present the results of an exhaustive and conclusive series of experiments on the effect of clean processing and heavy ion irradiation on the adhesion of metal films to substrates of silicon and tantalum. The experiments were performed in a unique research-scale Ultra High Vacuum Cluster tool, to gain control of the all important surface and interface compositions.
Our results show that adhesion is greatest for films deposited on atomically clean surfaces. Such films adhere better than conventionally deposited films subjected to a post deposition irradiation treatment. Clean processed samples show no benefit from subsequent radiation processing. Our results are consistent with the radiation enhanced adhesion phenomenon being due to the radiolysis of interfacial contaminant layers, producing an interface with lower interfacial energy and hence better bonding. Where adhesion enhancement is observed, the process is consistent with a semi-empirical model of the process using an activation energy of some 5 eV per atom.