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

Abstract

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.

Hybrid Organic/Inorganic Materials Depth Profiling Using Low Energy Cesium Ions

“…Intensity oscillations are also noticed at the organic/ metal interface. This behavior is common with low energy Cs + depth profiling as was shown in previous works [18,24]. The initial drop is caused by rapid fragmentation of the Tyr molecules induced by the Cs + ions, leading to the formation of free radicals.…”

“…Cs high reactivity ensures a negative ion signal enhancement, along with free radicals scavenging [18]. This source already proved its efficiency on both inorganics [19][20][21][22] and organics [23][24][25].…”

Identifying exposition to low oxygen environment in human macrophages using secondary ion mass spectrometry and multivariate analysis

“…The pre‐sputtering of the surface before analysis should remove the preparation medium and any surface contamination as well as removing some of the cell wall to gain access to intracellular material. Several ion sources have been shown to be suitable for the sputtering of organic materials with low damage accumulation, of these, argon clusters, C 60 and low‐energy Cs are among the most popular. In this study we chose to use low‐energy (250 eV) Cs sputtering as this sputter source is present in the majority of TOF‐SIMS instruments (unlike argon clusters and C 60 ).…”

Chemical Analysis of Plasma‐treated Organic Surfaces and Plasma Polymers by Secondary Ion Mass Spectrometry

“…A decade ago, cluster ions started to be used in a quite general fashion for SIMS analysis, and it was found that with clusters such as SF 5 + and C 60 + , many polymers could be depth‐profiled with retention of the molecular information, i.e., with minimal damage to the new surface uncovered by the sputtering event, paving the way to molecular depth‐profiling and 3D molecular imaging . In parallel, the erosion of the sample with low energy (100–500 eV) Cs + ions was proposed as an alternative for molecular depth‐profiling, capitalizing on the combination of the limited damage induced by the low energy ions together with the ionization probability increase provided by the presence of Cs implanted in the subsurface …”