Extending the possibilities of soft x-ray spectrometry through the etching of layered synthetic microstructure monochromators

One of the major limitations in the use of layered synthetic microstructure (LSM) monochromators for x-ray spectrometry techniques, such as electron probe microanalysis with wavelength-dispersive spectrometers or x-ray fluorescence, is the high-level background generated by the specular signal reflected by these monochromators, The specular reflection increases as the Bragg angle, the glancing angle theta of the incoming radiation with the monochromator surface, decreases, This signal is added to the conventional bremsstrahlung, leading to a strongly curved background whose shape cannot be accurately modelled, Background mis-estimations have drastic consequences for quantification of peaks associated with minor or trace elements. Etching of the LSMs in order to obtain rectangular in-relief gratings (with a period of about 1 mu m) would permitted by a factor of 25-100 the LSMs' intrinsic specular reflection at large glancing angles to be dramatically reduced, as predicted by calculations performed on an Mo-SI LSM designed for the detection of ultra-soft x-rays (<100 eV). These LSM gratings are used in the specular condition, i.e. in the zeroth diffraction order, The nominal reflectivity of the initial LSM is affected by the etching but may be restored by increasing the number of the bilayers deposited, Moreover, the etching improves the spectral resolution of the LSM, thus preventing severe peak interferences which are common in the soft x-ray region. X-ray lithography associated with a reactive ion etching process is shown to be the best compromise to realise large-size (of the order of 1-2 cm(2)), high-resolution gratings etched in thick (ca, 0.5 mu m) LSMs.