The influence of oxygen on the analysis of a Pt/Si structure with secondary ion mass spectrometry

Abstract

The introduction of oxygen during analysis with secondary ion mass spectrometry (SIMS) is an important tool to reduce ion beam induced topography, to enhance positive ion yields, and to remove transient effects during shallow and multilayer profiling. Its main drawback, however, is that due to the tendency of some elements to segregate towards the internal SiO2/Si interface (formed by an oxygen primary beam), large profile distortions can occur. In this work, the influence of oxygen pressure on the measurement of the Pt/Si structure is investigated and its relation to the observed SIMS depth profile is established. In the low pressure regime (leading to incomplete oxidation of Si), the profile disturbances occur in the interface region and are totally the result of ionization variations. The depth at which these disturbances are seen, as well as as the magnitude of the variations, are strongly dependent on the oxygen pressure in relation to the primary current density. The latter can be explained by the competition for the Si atoms between the formation of a Pt-Si alloy and an oxide. For high pressures where Si is completely oxidized, two regimes are observed. In the first regime, when the sample still contains a large amount of Pt, a large decay length is observed, representative of the strong segregation toward the SiO2/Si interface. In a later stage, when the amount of Pt has been reduced, the decay length decreases significantly suggesting that the segregation disappears. Internal depth profiling has shown that the two decay lengths can be correlated with two different internal Pt distributions, whereby the longest decay length corresponds to a Pt accumulation in the interface region. This correlation is in agreement with theoretical predictions about the role of the internal distribution on the SIMS decay length. The work also revealed that the polarity of the detected ions has a pronounced influence on the segregation caused by field-induced migration. This segregation depended strongly on the oxidizing conditions, suggesting the formation of higher quality oxides under higher oxygen pressures.