Comparing sputter rates, depth resolution, and ion yields for different gas cluster ion beams (GCIB): A practical guide to choosing the best GCIB for every application

Molecular gas species for gas cluster ion beams (GCIBs), such as carbon dioxide and water, were examined with a range of beam energies and cluster sizes to compare with the “universal relation” of the sputter yield, Y, per cluster atom against incident beam energy, E, per cluster atom of Ar_n cluster beam using Irganox 1010. In this work, we compare Ar_n, (CO₂)_n, and (H₂O)_n gas clusters to the universal equations for Ar_n clusters. To discuss molecular gas species for GCIBs, energy per nucleon (E/N) needs to replace energy per atom. We monitored sputter rate, depth resolution, and secondary ion yield as a function of the beam parameters: gas species, beam energy, and cluster size. (H₂O)_n GCIB shows reduced sputter rates and improved depth resolution with high sensitivity compared to Ar_n and (CO₂)_n GCIBs. These initial results indicate the potential to achieve high-depth resolution with high sensitivity and suggest that (H₂O)_n cluster ion beam has the potential to play a significant role in surface analysis techniques with organic materials. Results also show that no single set of conditions will provide the “best gas cluster ion beam” for all applications. However, it is possible to choose a set of conditions that will be more or less optimal depending on the experimental goals, such as maximizing the sputter rate, depth resolution, and molecular ion yield. In this work, we recommend the following three guidelines for GCIB users to set their own conditions: (1) to maximize the sputter rate, select a smaller cluster (higher E/N), but be aware that this will increase fragmentation and reduce molecular ion yield; (2) to maximize the depth resolution, select a larger cluster (lower E/N), and use (H₂O)_n GCIB, if possible; and (3) to maximize the molecular ion signal, use the highest beam energy available, and select a cluster with 0.15–0.25 eV/nucleon for Ar and (CO₂)_n GCIBs or around 0.1 eV/nucleon if using (H₂O)_n GCIB. These results are valid for XPS, SIMS, and any technique that utilizes GCIBs.