Characterization of Beam Sensitive Samples Through Dose Fractionated Spectrum Imaging

Acquisition of EELS spectrum images (SI) from dose-sensitive samples has proved technically challenging due to the relatively low critical dose thresholds above which Radiolysis occurs. Multipass SI continuously sums multiple, rapidly acquired passes together until the desired accumulated dwell time is achieved, rather than acquiring a single pass with a long pixel dwell time. This fractionates the dose over several passes, gives rapid feedback on sample integrity, and the ability to correct sample drift between frames. If passes continue to be summed above a critical dose, spectra analysis will be compromised. Due to read noise, optically coupled CCD and CMOS cameras require longer dwell times to achieve a sufficient signal-to-noise ratio. This requires the dose to spread over a larger sample area, limiting the measurement's spatial resolution [1]. <br/>Here we show how multi-pass, <i>in-situ</i> SI enables the application of EELS to dose-sensitive samples. Multi-pass <i>in-situ</i> SI saves each pass individually while fractioning the dose over multiple passes. If the acquisition of a SI continues above the critical dose, compromised spectra are removed post-acquisition, and only the pristine passes are summed together. This is particularly advantageous since the critical dose for a sample may be unknown. Combining the sensitivity of a direct electron counting detector with multi-pass <i>in-situ</i> SI decreases the pixel dwell time. This fractionates the dose over more passes and enables the SI to be acquired at a higher spatial resolution. <br/>Calcium carbonate (CaCO₃) was used as a model system since the critical dose thresholds for reduction and mass loss are well characterized [2]. We observe mass loss through the reduction of O and C concentrations. The formations of voids are observed to occur heterogeneously, likely at defects. Due to the increased sensitivity of direct detection, we can track the evolution of the Ca L_2,3 near-edge fine structure (ELNES) to pinpoint when the onset of radiolysis starts. Furthermore, by monitoring the ELNES, we confirm that reducing the dose rate increases the critical dose at which voids form, from 4.4x10⁴ e^-/Ų to greater than 1.3x10⁵ e^-/Ų.<br/>We then apply EELS Multi-pass <i>in-situ</i> spectrum imaging to a more challenging blended polymer film of polycarbonate (PC) and poly(styrene-acrylonitrile) (SAN). The PC/SAN film has a significantly lower dose threshold of 10 e^-/Ų, compared with the calcium carbonate films. By fractionating the dose over several passes, we can map the distribution of the PC and SAN phases using references from the C K-edge ELNES. Due to the direct electron detector sensitivity, we acquired phases maps with a spatial resolution of 20-30nm, which was previously impossible with optically coupled cameras [1].<br/> <br/>[1] Colby R. et al., Ultramicroscopy 246 (2023) 113688<br/>[2] R. Hooley, A. Brown, R. Brydson Micron <b>120</b> (2019)