The role of energetic ion bombardment during growth of TiO2 thin films by reactive sputtering

TiO2 thin films have been deposited by several different sputtering processes: (i) dc magnetron sputtering (dcMS) employing various geometrical conditions, (ii) ion-assisted dc magnetron sputtering where additional ion bombardment of the growing films was performed with an auxiliary ECR ion source and (iii) high power impulse magnetron sputtering (HiPIMS). Films have been investigated mainly by grazing incidence x-ray diffraction and atomic force microscopy. It is shown that the highly energetic oxygen ions inherent in reactive sputtering of metal oxides are the dominant energetic species governing structure formation of TiO2 films by their kinetic impact. The trajectories of these energetic oxygen ions strongly depend on the shape of the erosion trace and hence on the age of the target, which therefore has a strong influence on structure formation. Furthermore, in a HiPIMS discharge the role of this energetic oxygen ion bombardment is strongly intensified due to the increased target voltage and the lower deposition rate compared with a dcMS discharge. It is also demonstrated that films with pure rutile structure which are stable under a post-deposition thermal treatment can be deposited under intense energetic ion bombardment at low temperatures either by HiPIMS at high peak power densities or by ion-assisted dcMS.