Molecular Depth Profiling with Argon Gas Cluster Ion Beams

Argon gas cluster ion beams (Ar-GCIBs) are remarkable new projectiles for secondary ion mass spectrometry (SIMS) depth profiling of organic materials. However, the optimal cluster size and kinetic energy to provide the best quality of depth profiles, in terms of high ionization efficiency of the target molecules, little chemical damage, and short experiment time, for organic materials is not fully understood. Hence, the effect of cluster size and kinetic energy on the quality of molecular depth profiling is investigated on a simple platform composed of trehalose thin films to acquire more fundamental information about the ion/solid interaction. The results suggest that the sputter yield (Y/n) of argon clusters is linearly dependent upon kinetic energy per atom (En). When E/n > 5 eV/atom, normal depth profiles are obtained with relatively high sputter yields. When E/n <= 5 eV/atom, however, distorted depth profiles in the steady state region are observed, which exhibit a low sputter yield and variable ionization efficiency. As a consequence of these observations, it was concluded that high kinetic energy increases the useful molecular ion yield of trehalose and that Ar-n(+) clusters with a small E/n value minimize ion beam bombardment induced chemical damage. Hence optimal conditions for molecular depth profiling will be obtained using the highest kinetic energy with the largest clusters while maintaining a value of E/n near a threshold value of 5 eV/atom. In general, this study provides insight into selecting optimal Ar-GCIB characteristics for molecular depth profiling of organic materials.