M Rohde

An integrated Silicon Drift Detector System for FEI Schottky Field Emission Transmission Electron Microscopes

Silicon Drift Detectors (SDD) [1] are rapidly replacing Si(Li) detectors for EDX microanalysis in SEM, but have yet to have an impact in the S/TEM world. Main reason for this difference is the low count rate created by thin S/TEM samples compared to the bulk samples in SEM . These low count rates make EDX mapping a very slow process in S/TEM. However, the recent introduction of higher brightness electron sources [2] and probe Cs-correctors has led to significantly increased beam currents in small electron probes and, potentially, to higher EDX count rates. Since a key advantage of the SDD is the high count rate capability, the throughput improvement compared to the Si(Li) detectors will be considerable in these new instruments. Compared to SEM, the smaller excited volumes obtained with the atomic-scale probes in the new S/TEM instruments can lead to radiation damage of beam-sensitive materials before the analysis is completed. Therefore S/TEM microanalysis needs not only the higher count rate capability, but also higher collection efficiency of the X-rays generated, in order to reduce the dose on the sample. In this paper we present a new prototype EDX detector system for an FEI 200kV TEM/STEM, in which FEI has integrated a detector system consisting of multiple SDDs, placed symmetrically around the electron beam axis in the objective lens chamber without affecting the S/TEM resolution. The SDDs with a total active area of 120 mm were designed by PN Sensor to fit into the FEI design to achieve a quantum leap in solid angle of collection compared to previous designs in S/TEMs. The SDDs are cooled to achieve the optimum energy resolution, typically below 130 eV. The windowless design allows for better sensitivity for light-element detection than conventional thin-window detectors. The specially designed front-end electronics and ultra fast multi-channel pulse processor are provided by Bruker AXS MA in collaboration with FEI. The processor is capable of fast mapping with pixel dwell times down to a few microseconds and >100 kcps count rates per channel. Compared to currently available Si(Li) detectors the anticipated count rates will be an order of magnitude higher with the new detector. Additionally the new high brightness gun of FEI (X-FEG) [2] increases the brightness of the electron source compared to conventional Schottky sources, leading to a further increase in count rate, and an equivalent significant decrease in mapping time at the same spatial resolution. This improvement is illustrated in Fig. 1 where the relative minimum detectable mass MDM ~ (t.P.P/B) (t=analysis time, P=elemental peak counts, P/B = peak-to-background ratio) [3] is shown for conventional and new EDX detector count rates at the same spatial resolution. Fig.1 also compares the MDM with EELS and, for the specific case of strontium titanate, shows that the new EDX detector is expected to be more sensitive than EELS. Further results will be reported at the conference. Microsc Microanal 15(Suppl 2), 2009 Copyright 2009 Microscopy Society of America doi: 10.1017/S1431927609094288 208

The Determination of the Efficiency of Energy Dispersive X-Ray Spectrometers by a New Reference Material

A calibration procedure for the detection efficiency of energy dispersive X-ray spectrometers (EDS) used in combination with scanning electron microscopy (SEM) for standardless electron probe microanalysis (EPMA) is presented. The procedure is based on the comparison of X-ray spectra from a reference material (RM) measured with the EDS to be calibrated and a reference EDS. The RM is certified by the line intensities in the X-ray spectrum recorded with a reference EDS and by its composition. The calibration of the reference EDS is performed using synchrotron radiation at the radiometry laboratory of the Physikalisch-Technische Bundesanstalt. Measurement of RM spectra and comparison of the specified line intensities enables a rapid efficiency calibration on most SEMs. The article reports on studies to prepare such a RM and on EDS calibration and proposes a methodology that could be implemented in current spectrometer software to enable the calibration with a minimum of operator assistance.

An integrated multiple silicon drift detector system for transmission electron microscopes

A new EDX system, consisting of multiple SDDs has been developed for an FEI 200kV TEM/STEM in which the SDDs, designed by PN Sensor with a total collection angle approaching 1 srad has been obtained by placing 4 SDDs symmetrically around the electron beam axis in the objective lens chamber. The massive increase in solid angle of collection compared to previous designs in S/TEMs leads to a huge reduction in the time for EDX mapping. First results from the detector are reported.

Element Distribution in Novel Hedgehog-Like Magnetic Nanostructures Studied by, Cs-Corrected STEM-EELS and Uncorrected STEM-XEDS Using SDD-Technology

An EDS (energy dispersive x-ray spectroscopy) system incorporating a liquid nitrogen free XFlash 5030 SDD detector covering a solid angle of 0.12 sr on a conventional TEM/STEM was used to complement atomic resolution HAADF images from a NION MarkII Cs-corrected dedicated STEM and the elemental maps achieved using Gatan EELS on the Cs-corrected instrument [1]. The element distribution in the active magnetic layers of novel magnetic nano-structures was characterised and the results were used to interpret their magnetic switching behaviour and support micromagnetic simulations of the necessary energy barrier.