The imaging conditions and microscope settings used in the latest ultra-high resolution FEG-SEMs for investigating the morphology and structure of the smallest nano-materials and layers include very low kV, minimum beam current and short working distances. These conditions are incompatible with current conventional EDS detectors due to limits in geometry and sensitivity.
We introduce a new design approach for EDS detectors, which specifically provides elemental characterisation under these imaging conditions.
The detector front end has been re-designed to minimise sample to crystal distance and operating working distance. The electron trap volume has been significantly reduced to stop low kV backscatter electrons to minimise footprint. A windowless approach is used to maximise the sensitivity to low energy X-rays. Improvements in sensitivity of 10x for sub 1000eV X-rays have been achieved over conventional large area SDD’s.
This detector is used with accelerating voltages in the range 500eV to 3kV to maximise spatial resolution (<10nm) and surface sensitivity (<1nm). Under these conditions the X-ray lines available for characterisation are different from conventional micro-analysis. This unique approach makes use of previously poorly characterised lines including Li K lines, Al – Zn L lines and Sr – Lu M lines. These peaks can easily be detected, but are often overlapped due to the small energy difference in transitions in this part of the spectrum, despite the excellent detector resolution achieved (typically 35eV for Be K). Therefore these lines have been rigorously characterised with this detector with the aim of providing accurate peak deconvolution.
With its capability to provide elemental information with spatial and depth resolution close to that achieved by electron imaging, this new detector helps bridge the characterisation gaps between FEG-SEM and higher resolution TEM and surface science tools such as Auger and XPS.
