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SIMS21, Poland 2017 - Andreas Amsüss abstract

Andreas Amsüss oral presentation (SN1-Fri2-1-4)

Differences between Li, Na, and K Migration in Thin SiO2 Films during ToF-SIMS O2+ Depth Profiling

Andreas Amsüss1, Stefan Krivec2, Stefan Schwab1,3, Herbert Hutter1

1 TU Wien (Vienna), Getreidemarkt 9, 1060 Vienna, Austria
2 Infineon Technologies Austria AG, Siemensstraße 2, 9500 Villach, Austria
3 KAI Kompetenzzentrum Automobil- u. Industrieelektronik GmbH - TU Wien (Vienna), Europastraße 8, 9524 Villach, Austria

Due to sputter induced charging of the sample surface, measurements of alkali metal distributions in insulating materials by time of flight secondary ion mass spectrometry (ToF-SIMS) usually encounter artefacts. Typically being present in ionic state, the alkali metals are affected by the electric field arising during depth profiling of thin SiO2 layers on conducting or semiconducting bulk material. The consequence is ion migration with the proceeding sputter front. Frequently one receives an accumulation of the alkali metals on any interface instead of the true distribution within the insulator. Amongst other influences, the severity of this effect is governed by the type of alkali metal species as demonstrated in this study by the representatives Li, Na, and K. On the example of Gaussian shaped implantation profiles within a thin SiO2 layer on Si, the influence of migration can be examined by variation of the sample temperature in a set of measurements [1]. It is shown that both Li and Na alkali metals are very fast diffusors in the insulating material, leading to a prominent accumulation at the SiO2/Si interface. In the investigated sample temperature range down to -120°C one can find heavily distorted depth profiles which do not match the simulated implantation profiles for Li and Na. Taking into account the accuracy of the applied methodology, no significant difference in ion migration behaviour can be found between these two species.

In contrast to these findings, ToF-SIMS depth profiles of implanted K show a reduced migration artefact. Although being noticeably mitigated in comparison to Li and Na, the ion migration still disables a proper analysis by ToF-SIMS depth profiling. However, the onset for significant accumulation of K on the SiO2/Si interface is shifted to higher temperatures as required for Li and Na. This effect can be referred to different mobilities of the various alkali ion species under the influence of an electric field [2].

[1] S. Krivec et al., Appl. Surf. Sci. (257), 2010, pp. 25-32

[2] G. Greeuw and J. F. Verwey, J. Appl. Phys. (56), 1984, pp. 2218-2224