An increase in compressive stress was observed in rf magnetron sputtered yttria-stabilized zirconia thin films upon exposure to ambient conditions (25 degrees C and 75% relative humidity). This increase was attributed to absorption of water molecules into intergranular pores. It was shown that increasing substrate bias power disrupted columnar grain growth and reduced the percent change in compressive stress when exposed to ambient environments. Transmission electron microscopy confirmed a reduction in intergranular porosity for substrate bias depositions but an increase in lateral defects. These defects are hypothesized to be stress-induced microcracks caused by a tetragonal to monoclinic phase transformation. (c) 2006 American Vacuum Society.