Comparative Study of the Usefulness of Low Energy Cs+, Xe+, and O2+ Ions for Depth Profiling Amino-Acid and Sugar Films

This work reports a comparative study on the capability of low energy primary ion beams for depth profiling nonpolymeric molecules including amino-acid and sugar layers. Due to their different behavior regarding depth profiling, phenylalanine and trehalose molecules are chosen as reference systems. Each molecule was dissolved in suitable solvent prior to spin-coating on clean silicon wafer. The film thicknesses were in the order of 70 and 100 nm for phenylalanine and trehalose respectively. Depth profiling feasibility were assessed first using Cs+ as reactive sputtering ion at various energies. The results obtained under Cs+ sputtering ions are compared afterward to those obtained under Xe+ sputtering ions which are inert and have a mass very similar to Cs+. In order to investigate the effect of oxygen, depth profiling are also performed using either Xe+ under oxygen flooding or O-2(+) as sputtering ions. While phenylalanine could be depth profiled successfully using Cs+ ions, Xe+ and O-2(+) ions failed to retain any characteristic signal. The sputtering yields measured as a function of the ion beam energies were higher using Cs+, in particular at low energies. The chemical reactivity of the cesium atoms being implanted during the sputtering process helps to prevent the loss of the molecular phenylalanine signal. In contrast, depth profiling of trehalose was more successful upon Xe+ and O-2(+) compared to Cs+. In this case the sputtering yields were higher if Xe+ primary ion is employed instead of Cs+. The different trends observed in this study are interpreted using arguments involving the reactivity of the sputtering ions.