Large solid angle racetrack-shaped SDDs for efficient EDX analysis in SEMs and TEMs down to Lithium

Abstract text (incl. figure legends and references)

The progress made over the last decade in high resolution electron microscopy (EM) has pushed the instrument abilities towards new frontiers. Higher beam currents, smaller spot sizes, more accurate and fast detectors – all these features enable sample imaging and elemental mapping at atomic scale resolution in real time. Often equipped with one or more Energy Dispersive X-Ray (EDX) detectors, the analytical capabilities of the electron microscopes have gained more and more significance over the last years.

For high resolution EMs like TEMs or HR-SEMs, the working distance between the upper pole-piece and sample is very small; thus, when using an EDX detector of a certain size, the maximum achievable solid angle for collecting the x-ray signal is mainly determined by the minimum distance at which the detector can be positioned relative to the sample point. For these cases, using a racetrack-shaped detector with its reduced vertical dimension enables a closer positioning of the detector with respect to the sample point (see Fig. 1a) and therefore a substantial increase in solid angle (Fig. 1b).

With more than 20 years of experience in developing and manufacturing of high-quality Silicon Drift Detectors (SDD), we at PNDetector have been continuously working on tailoring the detector performance to keep pace with the new application trends in EDX microanalysis. With respect to the maximum achievable solid angle, the well-established Racetrack detector series comprising sensors with active areas from 60 mm² to 200 mm² (see Fig.2) has been already introduced for some time [1]. From the various detector sizes, especially the 100 mm² SDD Racetrack is being successfully used for many years as a large solid angle EDX detector in many TEMs or in HR-SEMs. Depending on the pole-shoe configuration, solid angle values of up to 1 sr can be achieved. The solid angle can be doubled by using a dual-detector configuration.

In the research field of renewable energies, Lithium is a key element for the development of efficient batteries. Detection of Lithium in various compounds by EDX is not a trivial task and necessitates a state-of-the-art EDX detector with minimum noise level and high detection efficiency. Extending the energy detection limits of the SDDs for ultra-light elements down to Lithium (54 eV) has become one of our major development objectives over the past years [2].

Combining the large solid angle of x-ray collection with the very low-noise SDD^plus technology makes the racetrack-shaped detectors ideal for Lithium detection and mapping in an electron microscope. Figure 3 shows a Lithium spectrum measured on a SEM with a 100 mm² SDD^plus Racetrack detector in windowless configuration. In this contribution we will presents newest developments and spectroscopic results obtained with various racetrack-shaped detector configurations.

[1] A. Niculae et al., Microscopy and Microanalysis 22 (Supl3), 2016.

[2] A. Niculae et al., Microscopy and Microanalysis 25 (Supl2), 2019.

Fig. 1 (a) Schematic view of a TEM pole-piece with a round EDX detector on the left and a racetrack-shaped detector of similar area on the right-hand side; (b) Detector solid angle distribution as a function of the distance from detector to the sample and detector area.

Fig. 2: Three racetrack-shaped SDD geometries with total active areas of 200 mm², 100 mm² and 60 mm².

Fig. 3. Spectrum from a pure Li sample measured in SEM with a 100 mm² Racetrack SDD^plus detector