ToF-SIMS depth profiling of PS-b-PMMA block copolymers using Arn+, C60++ and Cs+ sputtering ions

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS is a high performance tool for molecular depth profiling of polymer films, in particular, when they are structured in micro-phases. However, a major issue is the degradation of polymer materials under ion irradiation with reactions such as cross-linking, chain breaking or reorganization processes of polymers which have been illustrated on materials such as polystyrene (PS) and poly (methyl methacrylate) (PMMA). This work aims at comparing ToF-SIMS molecular depth profiling of structured polymers (polystyrene (PS)-b-poly methyl methacrylate (PMMA) block copolymers (BCP)) using either ultra-low energy cesium as well as more recently introduced C60++ (under NO dosing and with sample cooling) and argon cluster ion beam (using Ar1500+ ions at 5 keV). The latter improved the quality of the depth profiles, especially the argon cluster ion beam as it is characterized by a greater homogeneity for the sputter yields of PS and PMMA. No significant artifact were observed and this was confirmed by the comparison of depth profiles obtained from films with variable thickness, annealing time, and morphology (cylindrical blocks vs spherical blocks). A comparison to a theoretical model (hexagonal centered pattern) ensured that the ToF-SIMS depth profiles described the real morphology and may thus be a relevant characterization tool to verify the morphology of the films as a function of the deposition parameters.