Differences between Li, Na, and K migration in thin SiO2 films during ToF-SIMS O2+ depth profiling

Distribution of alkali metals within thin insulating films as recorded by time of flight secondary ion mass spectrometry (ToF-SIMS) dual beam depth profiling using O-2(+) as the sputter projectile is usually exhibiting artifacts. Positive charges appear on the sample surface upon impact of sputter ions. This leads to large electric fields within the insulator, causing alkali metal ions to migrate toward the interface with an adjacent conductive layer. Additionally, the sample temperature is a main contributor to migration processes. By varying the temperature during measurements, the mobility of alkali metal ions can be regulated. This report addresses the difference between Li, Na, and K migration during ToF-SIMS depth profiling. It is shown that there is no significant difference between Li and Na migration within a thermally grown SiO2 layer on Si. K migration, however, is reduced, leading to less distorted depth profiles. At elevated temperatures above T = 200 degrees C, there is the onset to a second artifact, resulting in surface segregation of K and superposing the migration artifact in ToF-SIMS depth profiles. It is shown that the use of low energy electron flooding has an important role in the occurrence of alkali metal ion surface segregation. Published by the AVS.