Information on interface composition is frequently obtained by measuring surface composition while the specimen material is gradually removed by ion bombardment (see Guide E 1127 and Practice E 1162). In this way, interfaces are revealed and characterized by the measurement of composition versus depth to obtain a sputter-depth profile. The shape of such interface profiles contains information about the physical and chemical properties of the interface region. In order to accurately and unambiguously describe this interface region and to determine its width (see Guide E 1438), it is necessary to define the shape of the entire interface profile with a single analytic function.

Although no general physical model currently exists for describing the shape of interface sputter-depth profiles, interface profiles do have a sigmoidal shape characteristic of the cumulative logistic distribution. Use of such a logistic function is physically plausible and is superior to other functions (for example, polynomials) that have heretofore been used for interface profile analysis in that it contains the minimum number of parameters for describing interface shapes.

Many attempts have been made to characterize interface profiles with general functions (such as polynomials or error functions) but these have suffered from instabilities and an inability to handle poorly structured data. Choice of the logistic function along with a specifically written least-squares procedure (described in Appendix X1) can provide statistically evaluated parameters that describe the width, asymmetry, and depth of interface profiles in a reproducible and unambiguous way.