Large O2 Cluster Ions as Sputter Beam for ToF-SIMS Depth Profiling of Alkali Metals in Thin SiO2 Films.

A sputter beam, consisting of large O2 clusters, was used to record depth profiles of alkali metal ions (Me+) within thin SiO2 layers. The O2 gas cluster ion beam (O2-GCIB) exhibits an erosion rate comparable to the frequently used O2+ projectiles. However, because of its high sputter yield the necessary beam current is considerably lower (factor 50), resulting in a decreased amount of excess charges at the SiO2 surface. Hence, a reduced electric field is obtained within the remaining dielectric layer. This drastically mitigates the Me+ migration artifact, commonly observed in depth profiles of various dielectric materials, if analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) in dual beam mode. It is shown, that the application of O2-GCIB results in a negligible residual ion migration for Na+ and K+. This enables artifact-free depth profiling with high sensitivity and low operational effort. Furthermore, insight into the migration behavior of Me+ during O2+ sputtering is given by switching the sputter beam from O2+ to O2 clusters and vice versa. K+ is found to be transported through the SiO2 layer only within the proceeding sputter front. For Na+ a steadily increasing fraction is observed, which migrates through the unaffected SiO2 layer toward the adjacent Si/SiO2 interface.