Copolymers containing polyhedral oligomeric silsesquioxane (POSS (TM)) units have been developed to be used as photoresist components in a bilayer resist scheme ford 193 nm lithography. This article reports on the behavior of these new POSS based materials under oxygen plasmas. The authors demonstrate using in situ ellipsometry and in situ x-ray photoelectron spectroscopy that during the first seconds in the plasma a silicon oxide layer is formed on the top surface of the POSS materials. This superficial, layer prevents etching and material consumption. An ion-enhanced oxidation model is proposed to describe and explain the experimental data and further. investigate POSS etching mechanisms in oxygen plasma. The model shows that the oxide formation rate is, reduced exponentially with the oxide thickness. It also predicts that thickness loss has its main roots in the layer densification that occurs when the oxide is formed and shows that the oxide formation is ion enhanced and thus favored at -100 V compared to 0 V bias. (c) 2006 American Vacuum Society.