Low Z total reflection X-ray fluorescence analysis - challenges and answers

Low Z elements, like C, O,... Al are difficult to measure, due to the lack of suitable low-energy photons for efficient excitation using standard X-ra y tubes, as well as difficult to detect with an energy dispersive detector, if the entrance window is not thin enough. Special excitation sources and special energy dispersive detectors are required to increase the sensitivit y and to increase the detected fluorescence signal and so to improve the de tection limits. Synchrotron radiation, due to its features like high intens ity and wide spectral range covering also the low-energy region, is the ide al source for TXRF, especially of low-Z elements. Experiments at a specific beamline (BL 3-4) at SSRL, Stanford, designed for the exclusive use of low -energy photons has been used as an excitation source. Detection limits < 1 00 fg for Al, Mg and Na have been achieved using quasimonochromatic radiati on of 1.7 keV. A Ge(HP) detector with an ultra-thin NORWAR entrance window is used. One application is the determination of low-Z surface contaminatio n on Si-wafers. Sodium, as well as Al, are elements of interest for the sem iconductor industry, both influencing the yield of ICs negatively. A detect ion capacity of 10(10) atoms/cm(2) is required which can be reached using s ynchrotron radiation as excitation source. Another promising application is the determination of low-Z atoms implanted in Si wafers. Sodium, Mg and Al were implanted in Si-wafers at various depths. From measuring the dependen ce of the fluorescence signal on the glancing angle, characteristic shapes corresponding to the depth profile and the relevant implantation depth are found. Calculations are compared with measurements. Finally, aerosols sampl ed on polycarbonate plates in a Battelle impactor were analyzed with LZ-TXR F using multilayer monochromatized Cr-K alpha radiation from a 1300-W fine- focus tube for excitation. Results are presented. (C) 1999 Elsevier Science B.V. All rights reserved.