Investigation of damage mechanisms in PMMA during ToF-SIMS depth profiling with 5 keV and 8 keV SF5+ primary ions.
Christine M. Mahoney, James G. Kushmerick, Kristen L. Steffens
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. Here, an SF5+ polyatomic primary ion source is utilized for depth profiling in poly(methyl methacrylate) (PMMA) bulk and thin films (200 nm), where the effects of SF5+ ion beam energy are discussed in detail. 5 keV 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 indicate 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 are achievable when employing higher beam energies, similar to what has been observed previously with C60 depth profiling.[i] 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).