According to the dynamical diffraction theory, X-ray standing wave generated around the Bragg condition gives precise information on the atomic position of impurities in the crystal as well as of adsorbed molecule on the surface1). The technique can be applied not only to perfect crystals but also to multilayers2). The layer structure has been often evaluated by means of X-ray fluorescence or Auger electron emission from major elements. In the present study, depth position of the trace impurities in the multilayers has been analyzed by the standing wave technique.
The experiment was carried out with monochromatic X-rays (8 keV) at BL-4A. The samples measured are commercially available multilayers for X-ray optics (Osmic Inc., USA); the Ni/C (2d=97.6Å) and the W/Si (2d=43Å). The grazing incidence X-ray equipment, which is originally for total reflection experiment, was employed for the measurement, because of low glancing angle (at most 2 deg). The angular profile of X-ray fluorescence from trace metal (Fe) was investigated by a Si(Li) detector. Since W-L and Ni-K shells are not excited at 8 keV, and C and Si Ka fluorescence is very weak because of air absorption, Fe signal can be collected with excellent efficiency.
Figure 1 shows the angular dependence (theta/2theta scan) of Fe Ka fluorescence around the Bragg reflection peak. Each multilayer gives different result, and this suggests Fe impurity located at the different depth position between both samples. Intensity distribution of the X-ray standing wave was calculated and is shown in Fig.2 The experimental data and the calculation have been compared, and it has been found that Fe impurity is uniformly distributed in Ni layer in the Ni/C, but is segregated at the W/Si interface in the W/Si. The result is interesting when analyzing the origin of trace impurity in the fabrication process. That is, it is suggested that the following conditions are worth consideration; the purity of materials for the Ni/C and mechanics of the shutter and/or the rotating sample holder of the sputter equipment for the W/Si. In summary, the X-ray standing wave technique is useful for analyzing depth position of trace metals within 1 periodic unit of the multilayers.
1) B.W.Batterman et al., Phys. Rev. Lett., 22, 703 (1969); P.L.Cowan et al., Phys. Rev. Lett., 44, 1680 (1980).
2) T.W.Barbee Jr. and W.K.Warburton, Mat. Lett. 3, 17 (1984).
Figure 1 Angular dependence of Ka fluorescence from trace iron in the multilayers.
Figure 2 X-ray standing wave inside the multilayer (Calc.)
