Standard Practice for Analytically Describing Sputter-Depth-Profile Interface Data by an Extended Logistic Function
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.
1.1 This practice covers a systematic method for analyzing sputter-depth-profile interface data and for accurately characterizing the shape of the interface region. Interface profile data are described with an appropriate analytic function; the parameters of this function define the interface width, its asymmetry, and its depth from the original surface. The use of this practice is recommended in order that the shapes of composition profiles of interfaces acquired with different instruments and techniques on different materials can be unambiguously compared and interpreted.
1.2 This practice is intended to be used to describe the shape of depth profile data obtained at an interface between two dissimilar materials for that case in which the measured concentration of the outer material goes from 100 to 0% and the inner material goes from 0 to 100%.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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