SIMS analysis of oxynitrides: evidence for nitrogen diffusion induced by oxygen flooding

Abstract

Dual-beam time-of-flight secondary ion mass spectrometry (ToF-SIMS) and low-energy SIMS can easily give a qualitative nitrogen depth profile through oxynitride structures. However, the use of different measurement conditions (polarity, primary ions, etc.) shows that interpretation of the results is not straightforward and can lead to different artefacts, particularly with respect to the nitrogen peak position and (apparent) tailing of the nitrogen into the underlying silicon. Because concurrent ionization yield changes influence the profile interpretation, oxygen flooding in combination with Ar+ sputtering has been used in this paper to separate ion beam mixing and ionization yield effects. The results indicate that the distortions of the nitrogen profiles originate from two combined effects. First, the preferred nitrogen cluster under Ar+ profiling, Si2N+, is extremely sensitive to small variations in the oxygen content of the sample. The observed Si2N+ profile at the oxide/silicon interface is largely influenced by these ionization yield changes. Second, nitrogen in oxynitrides is mobile under primary ion beam bombardment, an effect that is enhanced by the use of oxygen flooding. Under oxygen flooding, it is demonstrated that nitrogen can diffuse through the oxide/silicon interface far into the underlying silicon substrate. This investigation reveals that some of the reported nitrogen peak positions and tails should be interpreted as SIMS artefacts rather than as sample characteristics. Copyright © 2000 John Wiley & Sons, Ltd.