Secondary ion mass spectrometry on polymers: pathway to molecular weight determination and evaluation of surface segregation

BackgroundThe molecular weight (MW) of a photoresist can influence its properties and is an important metric for patterning. However, it is challenging to measure the MW in the solid phase, where the spatial distribution of MW can be nonuniform due to segregation effects during deposition.AimUsing surface mass spectrometry techniques such as secondary ion mass spectrometry (SIMS); the ratio and abundance of characteristic secondary ions (SI) can be used to determine the MW of the polymer and potentially the spatial distribution of molecular weights.ApproachExperiments on polymethylmethacrylate (PMMA) with different MW were analyzed with nanoprojectile SIMS (NP-SIMS) using Au4004+ as a primary ion and compared with reported values in the literature using different primary ion projectiles (Ga+, C60+, Au+).ResultsThe results are in good agreement with the literature. Using Au4004+ leads to a 5 to 100-fold increase in yields for observed PMMA peaks compared with other primary ions. We found that the sum of all ions between 0 and 200 m/z is inversely related to the MW of the polymer; however, factors related to manufacturing affect the yields significantly. A general trend of increasing MW and decreasing ratio of end-group to main chain ions was observed for all SIMS projectiles and could be applied to results obtained from different SIMS instrumentation. The experimental results were compared with theoretical end-group concentrations at the surface, and the equation for end-group segregation was fitted to the experimental values, which shows the influence of spin-casting compared with polymer melts in equilibrium.ConclusionNP-SIMS can be used to identify the MW of a sample if all other factors of the manufacturing process are carefully monitored. Furthermore, this method could be used to investigate the MW of as-cast photoresists or monitor changes in the MW of a polymer as a result of exposure. In addition, the detection of both main chain and end-group ions makes an analysis of the extent of surface segregation of end-groups feasible.