Sputter-etching characteristics of polycrystalline strontium–titanate–oxide (STO) thin films are investigated using a high-density argon plasma. STO thin films were grown by means of a reactive pulsed-laser deposition technique in which the buffer oxygen pressure was varied to change the structural properties of the films. The sputter-etch rate of the rf-biased films is found to linearly increase with the oxygen deposition pressure. This result is shown to be related to the corresponding decrease of the film density. This dependence of the etch rate on the structural properties of the films has very important consequences on etching studies and on the optimization of etching processes. © 2004 American Institute of Physics.