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- EPMA (5)
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Organisationseinheit der BAM
Energy dispersive and wavelength dispersive X-ray spectrometry are used to determine the composition of a specimen in a scanning electron microscope (SEM) by electron probe microanalysis (EPMA). A valuable completion for the SEM is analysis by X-ray fluorescence (XRFA). The main advantage of this method consists in its low detection limit. XRFA with a SEM uses the same spectrometers as for EPMA, necessary are only an additional X-ray source for the excitation of fluorescence spectra and software for their evaluation. Recent developments of smart low-power X-ray tubes and X-ray optical components enabled the construction of compact X-ray sources with focussing properties as add-on for a SEM. For quantitative XRFA the source spectrum has to be known. For its calculation the X-ray production yields Y have been measured for the commonly used tube target materials Mo, Rh and W. The excitation spectra were calculated for a variety of source types and compared with respect to the achievable photon flux and its spectral distribution.
X-ray Fluorescence (XRF) with a scanning electron microscope (SEM) is a valuable completion of the analytical capabilities of SEMs. Small and compact micro-focus x-ray sources are mounted to the microscope chamber, and the x-ray spectra are monitored with conventional EDS systems. Up to now the x-ray tubes used for the micro-focus x-ray sources are equipped with beryllium windows about 100 µm thick. The poly-capillary x-ray lenses have their transmission maximum at photon energies around 10 keV. It drops down in both low- and high-energy ranges. Hence, L-radiation from an Mo or Rh target will be strongly attenuated, and the excitation of fluorescence in the soft x-ray range becomes very ineffective. A new micro-focus x-ray source was developed. It is characterised by a lower self-absorption in the tube target, thin beryllium windows and an x-ray optics having a large distance between its foci and the maximum of transmission at about 5 keV. Thus K line fluorescence of light elements becomes effectively excited by the L-radiation from Mo or Rh tube targets. The detection limit for sodium oxide in glass was found to be below 1 mass%.
A method is proposed to determine the effective detector area for energy-dispersive X-ray spectrometers (EDS). Nowadays, detectors are available for a wide range of nominal areas ranging from 10 up to 150mm2. However, it remains in most cases unknown whether this nominal area coincides with the “net active sensor area” that should be given according to the related standard ISO 15632, or with any other area of the detector device. Moreover, the specific geometry of EDS installation may further reduce a given detector area. The proposed method can be applied to most scanning electron microscope/EDS configurations. The basic idea consists in a comparison of the measured count rate with the count rate resulting from known X-ray yields of copper, titanium, or silicon. The method was successfully tested on three detectors with known effective area and applied further to seven spectrometers from different manufacturers. In most cases the method gave an effective area smaller than the area given in the detector description.
Electron probe microanalysis (EPMA) measurement of thin-film thickness in the nanometre range
(2002)
The thickness of thin films of platinum and nickel on fused silica and silicon substrates has been determined by EPMA using the commercial software STRATAGEM for calculation of film thickness. Film thickness ranged in the order 10 nm. An attempt was made to estimate the confidence range of the method by comparison with results from other methods of analysis. The data show that in addition to the uncertainty of the spectral intensity measurement and the complicated fitting routine, systematic deviation caused by the underlying model should be added. The scattering in the results from other methods does not enable specification of a range of uncertainty, but deviations from the real thickness are estimated to be less than 20%.
The results of an interlaboratory comparison of energy dispersive X-ray microanalysis of TiN0.84 and ZrN were presented. The microprobe group of the German Physical Society (DPG) and the Federal Institute for Materials Research and Testing (BAM) had initiated the interlaboratory comparison. The primary aim was to test modern EDX systems equipped with ultrathin windows concerning the accuracy and reliability of the analysis of compounds containing light elements. The participants from 23 laboratories performed the analysis at different primary energies, in the standard-less mode as well as on the base of own standards, and considering Ti-K or Ti-L in case of TiN0.84. The results show a slight overestimation of the nitrogen content and a large standard deviation from the mean value. Reasons for the scattering of the results are discussed.