Material Discrimination at High Spatial Resolution using Sub-300eV X-rays

Abstract

Oxford Instruments NanoAnalysis, High Wycombe, Bucks, U.K.Conventional X-ray analysis has relied on X-rays being generated by transitions that produce X-rays at a fixed energy characteristic of the atom involved and where the generated intensity from a bulk material is proportional to the mass fraction of the element. In the decades that followed Castaing's pioneering work on quantitative microprobe analysis with a wavelength dispersive spectrometer (WDS), many research groups worked on improving the algorithms for correcting for the effects of the matrix on excitation and absorption, the technology for the energy dispersive spectrometer (EDS) improved and there is now international consensus on how reliable element compositional analysis for atomic number Z>=11 can be achieved with EDS at beam energies of 10-25keV [1]. For X-ray energies below 1 keV, quantitative analysis using L and M line emissions is less reliable because of anomalous absorption and chemical bonding effects [2] and at energies below 300 eV, the chemical environment of the atom has a significant influence even on K emissions. Such "chemical effects" were seen as a barrier to quantitative analysis and were the subject of considerable investigation in the 1960's (e.g. [3]). At the time, special purpose instruments were built for particular research projects and used electron beams that were not capable of high spatial resolution.Quantitative analysis for sub-300eV X-rays still remains an elusive goal except for some restricted areas of application. However, in the last few years, some major technology advances have enabled new spectrometers to become available commercially that offer detection of X-rays to below 50 eV in energy ("SXES" WDS [4] and "X-Max Extreme" EDS [5]) and can be mounted on electron columns that can provide high beam currents at low kV with a highly focussed beam. This has allowed investigators to discover new ways to characterise materials that are beyond the bounds of conventional microanalysis. SXES provides extremely good energy resolution (0.2 eV) revealing the full emission profile and has enabled collection of new data that surpasses what was discovered in the 1960's. X-max Extreme resolution approaches 30 eV fwhm at these energies but is much more sensitive so that good signals can be obtained at much smaller beam currents and considerably faster than SXES. Chemical state is impossible to determine from X-ray intensity alone but the shape of the X-ray emission profile can sometimes enable discrimination of materials in different chemical and crystallographic states. Figure 1 shows two spectra taken from graphite facets of different orientation that show shifts in C Kα position consistent with the variation in shoulder due to p bonding that has been shown by SXES [4]. However emission band changes in Si L between pure Si and TiSi2 and in B K between CaB6 and LaB6 that are detectable by SXES [6] are too subtle to be discriminated using X-max Extreme. Furthermore energy shifts between different chemical states can be v...