Effect of the molecular weight on the depth profiling of PMMA thin films using low-energy Cs+ sputtering

In this work, we investigate the influence of the molecular weight of poly (methyl methacrylate) (PMMA) thin films coated on silicon wafer on the ToF SIMS (Time of Flight Secondary Ion Mass Spectrometry) sputtering mechanisms and kinetics during depth profiling using low energy monoatomic caesium ions. The sputtering yield volumes are determined as function of molecular weight, film thickness and beam energy. The results show that the sputtering yield volume decreases with increasing molecular weight Mw down to a threshold value below which it becomes nearly constant, as previously observed with argon cluster ions. The relevance of physical parameters such as the glass transition temperature Tgdetermined here from ellipsometry measurements-and the entanglement of the polymer chains to account for this behaviour is discussed. The variation of the sputtering yield was also found to vary logarithmically with the primary beam energy. In addition, preliminary experiments carried out using a low molecular weight PMMA (4 kg/mol) evidenced a nano-confinement effect similar to that observed with argon cluster sputtering but of lower magnitude.