Investigation of Damage Mechanisms in PMMA during ToF-SIMS Depth Profiling with 5 and 8 keV SF5+ Primary Ions

Cluster secondary ion mass spectrometry (cluster SIMS) has been proven to be a useful technique for the surface and in-depth characterization of molecular films. In this study, an SF5+ polyatomic primary ion source is utilized for depth profiling in poly(methyl methacrylate) (PMMA) bulk and thin films (200 nm). The effects of SF5+ ion beam energy are discussed in detail. Both 5 and 8 keV ion beam energies are utilized for depth profiling experiments, where the chemistry of sputtering is investigated using surface analytical tools such as X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) in conjunction with SIMS. Thin film depth profiles acquired with 5 keV SF5+ display evidence of significant damage accumulation at the interface in the form of a highly cross-linked polymer gel. There is very little evidence of similar damage accumulation at the interface for the corresponding 8 keV SF5+ depth profile. AFM and XPS analysis of the sputtered crater bottoms also indicates that very different chemistries and morphologies are present at the interface when employing 5 keV vs 8 keV SF5+. For PMMA bulk samples, greater erosion depths were achieved when employing higher beam energies, similar to what has been observed previously with C-60(n+) depth profiling.(1) These increased erosion depths are attributed to the increased sputter rates of the PMMA at 8 keV SF5+ as compared to 5 keV SF5+, thus allowing for increased amounts of material to be removed prior to the approach of the gel point of the PMMA (dose at which a 3-D cross-linked structure is formed). Despite these increased erosion depths, the 8 keV SF5+ beam imparts greater initial structural damage, as indicated by decreased C=O contents in the C1s XPS spectra and increased amounts of graphitic type peaks in the corresponding SIMS spectra. Overall, the results indicate that, for thicker samples, one should employ higher beam energies for optimum results. However, for thinner films, in which the gel effect does not play a significant role, lower beam energies are preferred.