Molecular depth profiling of polymers with very low energy reactive ions

This work follows previous studies in which we demonstrated the feasibility of molecular depth profiling with time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), using a low‐energy Cs+ source for sputtering. Poly(methyl methacrylate) (PMMA), polycarbonate (PC) and polystyrene (PS) thin films were investigated with both 250 eV Cs+ and Xe+ ions with the aim of assessing the role of the ion's reactivity in polymer depth profiling. A more detailed analysis of PS layers was given, completed with X‐ray photoelectron spectroscopy (XPS) measurements, since this polymer is known to degrade rapidly under ion irradiation. Indeed, no depth profile is possible on PS with conventional polyatomic sources. The three polymers were amenable to depth profiling with low‐energy Cs+. On the other hand, they were quickly degraded by the low‐energy Xe+ beam, although some specific molecular ions remained detectable throughout the depth profile on PMMA. Sputtering yield measurements provided valuable information on the chemistry involved during the sputtering process: the sputtering yields measured on PMMA, with both Cs+ and Xe+, were much higher than on PC and PS, probably due to depolymerization processes. On PC and PS, the sputtering yield was much higher with Cs+ than with Xe+. This is due to an inhibition of the cross‐linking reactions by the implanted Cs atoms, leading to the formation of caesium carbides and anions. Low‐energy caesium depth profiling is successful, thanks to the combination of three factors: (i) low energy means less damage; (ii) Cs enhances the negative ionization; and (iii) the Cs reactivity hinders cross‐linking reactions. Copyright © 2010 John Wiley & Sons, Ltd.