Molecular depth profiling with reactive ions, or why chemistry matters in sputtering

We have shown in recent papers that low-energy Cs ions can be used successfully for molecular depth profiling of polymers. This paper reviews a few key experiments that provide a better understanding of the mechanisms involved in reactive ion depth profiling. It is shown that damaged polymer (polystyrene and polycarbonate) layers, initially bombarded with high-energy gallium irradiation, can be efficiently removed by low-energy Cs sputtering so that the polymer molecular signal is restored. The role played by the primary ion energy is then studied by measuring the sputtering yields at different energies on polystyrene, poly (methylmethacrylate) and polycarbonate. Depth profiles on PC were also obtained at six different Cs energies ranging from 150 to 1 keV. The best conditions were obtained at 200 and 250 eV, and the depth profile quality degrades above 500 eV. Finally, the negative ionization enhancement due to Cs implantation is studied by means of the Cs/Xe cosputtering method. All those experiments are consistent with a polymer cross-linking inhibition model, due to reactions between the implanted Cs atoms and the free radicals generated by ion irradiation. The main effect in molecular depth profiling with reactive ions is, therefore, a chemical effect.