Effects of oxygen concentration in the Ar/O2 plasma on the bulk structure and surface properties of RF reactively sputtered zirconia thin films

Zirconium oxide is one of the most extensively studied transition-metal oxides for its several attractive properties and the variety of its technological applications. Research was especially stimulated in understanding the factors controlling the structure of ZrO2 and in identifying the relationship between bulk and surface properties of ZrO2 thin films. In the present work, ZrO2 thin films were deposited on Si, without external heating, by RF reactive sputtering from a pure ZrO2 target in Ar/O2 plasma with different O2 concentrations (0 ÷ 20%). Aim of the study was the identification of the effects of the processing parameters – mainly the O2/Ar ratio in the gas phase – on the film growth and properties. The addition of O2 was crucial to establish a good stoichiometry, as revealed by Auger depth profiling. The films obtained under O2-deficient plasma conditions have polycrystalline nanograins whose structure was consistent with either a tetragonal or a cubic phase. Adding O2 to the gas mixture turns their structure into the stable monoclinic one. Such bulk structural changes were found to affect both their mechanical behaviour and their surface properties: a chemical shift of the Zr3d and O1s core levels to lower binding energies was recorded by XPS analyses, common to all the samples deposited in presence of O2. This effect was thought to be related to the oxygen induced tetragonal/cubic-to-monoclinic transformation of the oxide structure.