A sputter beam, consisting of large O clusters, was used to record depth profiles of alkali metal ions (Me) within thin SiO layers. The O gas cluster ion beam (O-GCIB) exhibits an erosion rate comparable to the frequently used O 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 SiO 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 O-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 O sputtering is given by switching the sputter beam from O to O clusters and vice versa. K is found to be transported through the SiO layer only within the proceeding sputter front. For Na a steadily increasing fraction is observed, which migrates through the unaffected SiO layer toward the adjacent Si/SiO interface.
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