Depth profiling preferential sputtering

Other recent investigations involving AES, often with depth profiling, deal with the surface segregation of Ag in Al-4.2 % Ag [2.163], of Sn in Cu and formation of superficial Sn-Cu alloy [2.164], of Mg in Al-Mg alloy [2.165], and of Sb in Ee-4% Sb alloy [2.166]. Note the need to differentiate between, particularly, segregation, i. e. original sample properties, from the artifact of preferential sputtering. 

Compared with XPS and AES sputter depth profiling After achieving sputter equilibrium, and until a layer with different sputtering behavior is reached [3.59], the SN flux represents stoichiometry and not altered layer concentrations evolving because of preferential sputtering effects. 

The atomic concentration profile shows an O/Si ratio of about 1.5, which is lower than the expected O/Si ratio of 2. Standard relative sensitivity factors (RSFs) were used in the calculation in order to demonstrate a drawback of ion sputtering, where one species is removed at a faster rate as compared to another. This effect referred to as preferential sputtering usually results in the lighter atoms being removed from the material by the ion beam as compared to the heavier ones. When this occurs, the composition of the surface exposed by the ion beam is altered until a composition is reached where all of the atoms present in the material are removed at an equal rate. Such a layer is referred to as an altered layer. Provided that the relative composition of the original film does not vary as a function of depth, once the altered layer is reached, the relative ratios of the elements should then remain constant throughout the profile. When this is the case and it is possible... 

In spite of the many processes which can induce artefacts into depth profiling, the technique can often provide an excellent picture of the subsurface region. Careful selection of experimental parameters can minimize or eliminate artefacts. Generally, it is desirable to use high ion-sputter rates, low temperatures, and sample rotation. The influence of incident current density is shown in Fig. 38 [110], Unfortunately, some problems such as that of preferential sput-... 

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