Focused ion beam irradiation - morphological and chemical evolution in PMMA

For poly(methyl methacrylate) (PMMA) as a representative of amorphous thermoplastic polymers, the milling effects, and the chemical changes due to ion bombardment with a focused ion beam (FIB) at normal incidence are studied with scanning force microscopy (SFM), scanning electron microscopy (SEM)/energy dispersive X-ray (EDX), and infrared (IR) spectroscopy for varying conditions of Ga+ treatment, including the effect of partial water pressure. Stopping and Range of Ions in Matter (SRIM) simulation results for 30 keV Ga+ at normal incidence show that the zone of primary ion - polymer interaction extends ca 100 nm into the PMMA. Accordingly, this interaction region is much wider than the original beam diameter. The width of the region where the recoiled ions interact strongly with the polymer chains is larger. Secondary processes, such as fragment diffusion and phonon transport, are expected to extend even farther into the polymer. SEM and SFM reveal distinct topologies of areas milled without or in presence of water vapour. Water vapour-assisted FIB milling produces more roughness and defects. The infrared attenuated total reflection spectroscopy (IR-ATR) spectra indicate that ion milling in PMMA damages methacrylate side groups in particular. In contrary to metals, an increase in the degree of milling is found when the beam spot overlap parameter increases. Copyright (C) 2009 John Wiley & Sons, Ltd.