Latest Improvements on Silicon Drift Detectors for Fast, High Resolution EDX Spectroscopy in Electron Microscopy

Silicon Drift Detectors (SDD) have become the standard choice for Energy Dispersive X-Ray (EDX) spectroscopy systems integrated in analytical electron microscopes. With more than 20 years’ experience in the development and manufacturing of SDD sensors, modules and systems, PNDetector is continuously working on tailoring the detector performance to keep pace with more and more challenging requirements in EDX microanalysis. Modern electron microscopes are able to provide high beam currents, which makes possible fast and accurate EDX analysis of bulk samples. High-resolution, ultra-fast EDX applications require detectors with extremely low input capacitance, insuring optimum operation at very short processing times. With our unique ability to monolithically integrated the first FET onto the sensor chip, the total detector input capacitance can be reduced to unprecedent low values, i.e., down to 50 fF for droplet-type silicon drift detectors (SD3plus series). Correspondingly, energy resolution values down to 121 eV at Mn-Ka or 35 eV at C-Ka, as well as detection of ultra-light elements up to Lithium - very interesting for the field of applications “alternative energies” - are possible [1]. While the droplet type SD3 detectors are very powerful detectors with small and medium active areas up to 30 mm2, for larger detection areas they start to suffer from the so called “ballistic deficit” (rise-time spread of the incoming signals due to the asymmetric position of the anode). This unwanted but physically well-understood effect limits the performance at the shortest processing time, i.e., for high count rate applications. For this case further optimization of the radial symmetrically anode/FET configuration which is used for larger area detectors is needed. With the introduction of the SDDplus FET/Anode configuration in 2012, an important step in the minimization of the input capacitance for SDDs with round or racetrack geometries has already been realized [2]. In the recent years we have been working on further reducing the input capacitance for these configurations in order to come closer to the SD3plus performance independently on the detector size. The result of those optimization steps is the so-called SDDplus+ Anode/FET configuration which was now tested for the first time on a 30 mm2 SDD detector. Fig. 1 shows the energy resolution as a function of the processing time of such configuration compared to the standard SDDplus configuration of the same size. As it can be seen, a significant improvement in the energy resolution at shorter processing time has been achieved, pointing out to the further reduction of the detector input capacitance. Also, the light-element performance is profiting from the performance improvements. Fig. 2 shows a light-element spectrum of a multi-element sample featuring a noise peak of 22 eV fwhm and a Carbon peak of 38 eV of fwhm. The Al-L at 68 eV line can be also well separated from the noise. In this contribution we will present further measurement performed with the newly developed SDDplus+ FET/Anode configuration and discuss the results as well as the potential for use in larger area detectors. Energy resolution vs. processing time of a 30 mm2 SDDplus+ detector compared to a SDDplus sensor of the same size. Significant improvements are observed especially at short shaping times. Light element spectra measured with a 30 mm2 SDDplus+ detector at -40°C. Detection of Al-L line (68 eV) is possible.