Analytical field emission gun scanning electron microscopes (FEG-SEMs) provide essential morphological and compositional data for the analysis of a broad range of materials.  However, the characterisation of certain materials can be problematic under “conventional” (i.e., > 10 kV) FEG-SEM imaging conditions.   In particular, complex multiphase materials, oxides and polymers can exhibit pronounced charging effects.   However, low voltage operating conditions (< 5 kV) precluded the generation of X-ray energy dispersive spectroscopy (XEDS) data for microchemical analysis.  The need to perform XEDS analysis at higher operating voltages made it difficult to acquire structural and compositional data from the same feature for materials that charge or degrade under the electron beam.  The ability to perform both high resolution imaging and XEDS spectrum imaging at low voltage in the FEG-SEM provides new opportunities for the evaluation of many materials.  In this study, we demonstrate how low voltage XED spectrum imaging in the FEG-SEM can rapidly provide data on two-layer oxides that form in the cracks present in an austenitic stainless steel corrosion specimen.

The Oxford Instruments X-Max^N 150 silicon drift detector (SDD) and an X-Max Extreme windowless SDD with the Oxford Instruments AZtecEnergy acquisition and XEDS analysis software were used for the XED spectrum imaging experiments in a Zeiss Merlin FEG-SEM with a Gemini-II column.  All SEM images were obtained with a parallel on-axis in-lens secondary electron (SE) detector with primary electron beam energy of 1.5 kV, 3 kV, 5 kV and 15 kV, respectively.  The SEM imaging conditions were fixed with the expection of the accelerating voltage and the working distance. All XED spectrum images were acquired with a spectrum image resolution of 512 x 512 pixel and a pixel dwell time of 1000 µs.   The Oxford Instruments TruMap processing software was used for peak deconvolution and background subtraction.  

Analyses were performed on a small oxide-filled crack in the austenitic stainless steel sample, which had been metallographically polished to a 1 μm diamond finish.  In-lens secondary electron images and corresponding XED spectrum images revealed the effect of accelerating voltage on the oxide microstructure.  Fe XED spectrum images acquired at 1.5 kV and 3 kV indicated the presence of a 2-layer oxide structure: an Fe-poor oxide adjacent to the oxide/metal interface, whereas these details are absent for the 15 kV spectrum image. Complementary Cr XED spectrum images confirmed that the oxide adjacent to the metal was Cr-enriched.  TEM and selected area electron diffraction confirmed the presence of a Cr₂O₃ innermost layer and an Fe-rich M₃O₄ in the centre of the oxide-filled crack.  The ability to generate viable XEDS datasets at low voltages provides significant improvements in spatial resolution of the analysis due to the significantly reduced depth of x-ray generation in the sample.  Thus, low voltage XEDS is providing new insights into materials analysis and new options for microstructural characterization.

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EMC Abstracts - https://emc-proceedings.com/abstract/high-spatial-resolution-spectrum-imaging-in-the-feg-sem-at-low-voltages-a-new-option-for-materials-characterisation/