X-Ray microanalysis of stratified specimens

This paper presents a description of the state of the art methodology and software for the application of x-ray microanalysis to thin surface layers and more generally to the characterization of stratified specimens. The sensitivity of the technique to near-surface segregation is demonstrated. Some emphasis is given on the eS(pz) function (the distribution in depth of the primary generated x-ray intensity), which is the key to all advanced quantitative procedures presently available. The main characteristics of the recently developed software packages Strata and Multifilm are briefly described (physical basis, incorporation of fluorescence by lines and continuum, graphically assisted modes of operation, iterative mode of operation for the automatic determination of compositions and thicknesses). The capability of the technique is illustrated by three different applications: a bilayer on a substrate (case with no common element), a coated fibre (case with common elements and very light element analysis), a coated substrate analyzed in various geometrical conditions and by an “as soon as possible” procedure. Krywor& Stratified specimens; Thin surface layers; X-Ray microanalysis For 25 years, electron probe x-ray microanalysis (XRMA) has been conventionally applied to bulk specimens to obtain elementary chemical analyses of microvolumes of the order of 1 pm3. Contrary to other analytical techniques, XRMA has the advantage of providing quantitative results in an absolute way, i.e. it is actually not necessary to refer to standards similar to the specimen. For a given characteristic line, any standard (pure or compound) containing the element of interest permits to define an experimental relative intensity k (usually called k-ratio in the case of a pure standard), which can be converted into a mass concentration by use of a “correction” procedure. However, the conventional quantitative procedures used for 20 years (the most popular was the ZAF method) had a Corre.spo&ence to: J.-L. Pouchou, Office National d’Etudes et Recherches Mrospatiales (O.N.E.R.A.), Dept. of Materials, 29 av. de la Division Leclerc, 92320 Chatillon (France). limited field of applicability: low absorption situations (less than 30% of the x-rays emitted in the spectrometer direction should be absorbed by the specimen itself), and homogeneous microvolumes. During the nineteeneighties, several groups have made a real effort to promote new procedures in order to overcome the above limitations, extend quantitative analysis to very light elements (low energy, i.e., strongly absorbed lines), and characterize specimens with a depth-dependent composition. The key to those problems is the &z) function, defined by Castaing [l] in his pioneering work, and representing the distribution in depth of the primary ionizations generated per incident electron in the target. New models based on accurate +(pz> descriptions are now superseding the conventional procedures, namely the gaussian MSG model of Packwood and Brown [2] and Packwood et al. [3] and the PAP and XPP models of Pouchou and Pichoir [4-61. Although they use different starting points 0003~2670/93/$06.00