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

摘要

This work reports a comparative study on the capabilitynof low energy primary ion beams for depth profilingnnonpolymeric molecules including amino-acid and sugarnlayers. Due to their different behavior regarding depthnprofiling, phenylalanine and trehalose molecules arenchosen as reference systems. Each molecule was dissolvednin suitable solvent prior to spin-coating on cleannsilicon wafer. The film thicknesses were in the order ofn70 and 100 nm for phenylalanine and trehalose respectively.nDepth profiling feasibility were assessed first usingnCs+ as reactive sputtering ion at various energies. Thenresults obtained under Cs+ sputtering ions are comparednafterward to those obtained under Xe+ sputteringnions which are inert and have a mass very similar tonCs+. In order to investigate the effect of oxygen, depthnprofiling are also performed using either Xe+ undernoxygen flooding or O2n+ as sputtering ions. Whilenphenylalanine could be depth profiled successfullynusing Cs+ ions, Xe+ and O2n+ ions failed to retain anyncharacteristic signal. The sputtering yields measurednas a function of the ion beam energies were highernusing Cs+, in particular at low energies. The chemicalnreactivity of the cesium atoms being implanted duringnthe sputtering process helps to prevent the loss of thenmolecular phenylalanine signal. In contrast, depthnprofiling of trehalose was more successful upon Xe+nand O2n+ compared to Cs+. In this case the sputteringnyields were higher if Xe+ primary ion is employedninstead of Cs+. The different trends observed in thisnstudy are interpreted using arguments involving thenreactivity of the sputtering ions.