The role of molecular weight on the ToF-SIMS spectra of PMMA using Au+ and C60+ primary ions

Low polydispersity (M-w/M-n < 1.08) spin-cast films of poly (methyl methacrylate) (PMMA) with molecular weights ranging from 2770 to 19 940 g/mol have been analyzed in a time of flight secondary ion mass spectrometer using 20 keV C-60(+) and Au+ primary ion sources. Differences in secondary ion yields as a function of sample molecular weight resulting from the use of the two ion sources were investigated. Under similar conditions, the ions investigated with the C-60(+) ion source produced yield increases ranging from 15- to 65-fold over yields collected with the Au+ ion source in both ion polarities. As a function of molecular weight, these yield enhancements were consistent across the molecular weight range investigated for the cations (less than 15% variation), but varied by up to a factor of 3 for the anions. Investigation of relative changes in anion yields for the primary ion beams from low- to high-molecular-weight PMMA shows similar trends in the data for end-group related fragment ions, but differences in ion yields for main-chain related fragments. Using Au+ shows a decrease in the relative yields with increasing mass but using C-60(+) shows an increase in yields suggesting that the mechanisms behind ion formation as a function of molecular weight of the sample differ somewhat between these two primary ions. This is partly attributed to the ability of the bombarding particle to overcome chain entanglement and covalent interactions of the polymers at the surface. It is concluded that C-60(+) is much more effective in generating ion yield for higher molecular weight systems, and this is supported by high oligomer ion yields for PMMA 2770. The results are discussed within the context of molecular dynamics simulations of atomic primary ion and C-60 sputtering of polymers to provide insight into the experimental observations. Copyright (C) 2010 John Wiley & Sons, Ltd.