Thin AlN films are oxidatively treated in a plasma to form AlO and AlON films without causing damage to underlying layers of a partially fabricated semiconductor device (e.g., to underlying metal and/or dielectric layers). The resulting AlO and AlON films are characterized by improved leakage current compared to the AlN film and are suitable for use as etch stop layers. The oxidative treatment involves contacting the substrate having an exposed AlN layer with a plasma formed in a process gas comprising an oxygen-containing gas and a hydrogen-containing gas. In some implementations oxidative treatment is performed with a plasma formed in a process gas including CO2 as an oxygen-containing gas, H2 as a hydrogen-containing gas, and further including a diluent gas. The use of a hydrogen-containing gas in the plasma eliminates the oxidative damage to the underlying layers.