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.