High mass resolution time‐of‐flight secondary ion mass spectrometry : Application to peak assignments

The negative time‐of‐flight secondary ion mass spectrometry spectrum of a trimethylsilylated silicon oxide surface and positive ion spectra of polymethylmethacrylate (PMMA) surfaces before and after treatment with an oxygen plasma are presented. The spectra were recorded with a SIMS spectrometer equipped with a Reflectron‐type mass analyser. This instrument exhibits a mass resolution high enough to be able to determine the chemical composition of the secondary ions directly from the spectra and to resolve peaks of oxygen and non‐oxygen‐containing ions with mass differences of about 0.030 amu at masses well above 200 amu. This information on the chemical composition makes it possible to determine structures of the ions originating from the surfaces, which increases insight into secondary ion formation processes. On the basis of the results from the plasma‐treated PMMA surface, a tentative mechanism is given for the interaction of an oxygen plasma with PMMA in which formation of new polymer chain‐ends containing acetyl groups plays an important role.