SIMS of Delta Layers in Organic Materials: Amount of Substance, Secondary Ion Species, Matrix Effects, and Anomalous Structures in Argon Gas Cluster Depth Profiles

Abstract

A study is reported of the quantification of the amount of matter by secondary ion mass spectrometry (SIMS) when depth profiling a nominally 3.1 nm thick delta layer of FMOC-l-pentafluorophenylalanine in Irganox 1010. The depth profiles are made using 5 keV Ar2300+ cluster ions with analysis by 25 keV Bi3+ ions. Data for 89 negative secondary ions shows profiles whose integrated intensities as a function of depth, even when normalized to the intensity for the pure material, still vary over a factor of 12. This variation mainly arises from matrix effects that are measured here using separate samples with mixed layers of three intermediate compositions of the two materials. Matrix enhancements or suppressions vary widely from secondary ion to secondary ion and are not related to the energy of the analyzing Bi3+ ion. Strong effects can cause the delta layer signal to show structure that may be misinterpreted. The compositional profile is established by using trial Dowsett profiles, representing the composition, which are then enhanced or reduced according to the measured matrix effect with the result then fitted to the normalized intensity data. By fitting each secondary ion profile separately, the computed amount of matter still varies weakly with the enhancement. This arises as a result of a longer wavelength roughening, equivalent to a root-mean-square value of about 2.5 nm, which causes, for example, the measured maximum intensity to be lower than the actual maximum intensity appropriate for the delta layer. The effective matrix enhancement is thus reduced by 20%. When this is included, it is found that the variation with enhancement disappears and the amount of matter is found to be equivalent to 3.22 ± 0.07 nm although the scatter from individual ions has a standard deviation of 0.45 nm. It is concluded that the matrix terms used are a good description of the phenomenon and that SIMS profiles may be made quantitative if suitable secondary ions are available and the matrix terms are measured. In the absence of measured matrix terms, the measured quantities are prone to large errors.