THE INFLUENCE OF OXYGEN ON THE ANALYSIS OF A PT SI STRUCTURE WITH SECONDARY-ION MASS-SPECTROMETRY/

The introduction of oxygen during analysis with secondary ion mass spe ctrometry (SIMS) is an important tool to reduce ion beam induced topog raphy, 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 p rofile distortions can occur. In this work, the influence of oxygen pr essure on the measurement of the Pt/Si structure is investigated and i ts relation to the observed SIMS depth profile is established. In the low pressure regime (leading to incomplete oxidation of Si), the profi le disturbances occur in the interface region and are totally the resu lt of ionization variations. The depth at which these disturbances are seen, as well as as the magnitude of the variations, are strongly dep endent on the oxygen pressure in relation to the primary current densi ty. The latter can be explained by the competition for the Si atoms be tween the formation of a Pt-Si alloy and an oxide. For high pressures where Si is completely oxidized, two regimes are observed. In the firs t regime, when the sample still contains a large amount of Pt, a large decay length is observed, representative of the strong segregation to ward the SiO2/Si interface. In a later stage, when the amount of Pt ha s been reduced, the decay length decreases significantly suggesting th at the segregation disappears. Internal depth profiling has shown that the two decay lengths can be correlated with two different internal P t 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 distribut ion on the SIMS decay length. The work also revealed that the polarity of the detected ions has a pronounced influence on the segregation ca used by field-induced migration. This segregation depended strongly on the oxidizing conditions, suggesting the formation of higher quality oxides under higher oxygen pressures.