Development of a Practicable Digital Pulse Read-out for Dark-field STEM

TL;DR

This paper introduces a digital pulse-counting method for dark-field STEM imaging that reduces artefacts and improves image quality at low electron doses, outperforming traditional analogue detection methods.

Contribution

The authors develop and demonstrate a pulse-counting detector system for dark-field STEM that enhances image quality and reduces artefacts at low beam currents, compared to conventional analogue detectors.

Findings

Digital pulse-counting yields higher SNR images at low doses.

The method reduces streaking and afterglow artefacts.

Performance is validated on two different STEM instruments.

Abstract

When characterising beam-sensitive materials in the scanning transmission electron microscope (STEM), low-dose techniques are essential for the reliable observation of samples in their true state. A simple route to minimise both the total electron-dose and the dose-rate is to reduce the electron beam-current and/or raster the probe at higher speeds. At the limit of these settings, and with current detectors, the resulting images suffer from unacceptable artefacts including; signal-streaking, detector-afterglow, and poor signal-to-noise ratios (SNR). In this manuscript we present an alternative approach to capture dark-field STEM images by pulse-counting individual electrons as they are scattered to the annular dark-field (ADF) detector. Digital images formed in this way are immune from analogue artefacts of streaking or afterglow and allow clean, high-SNR images to be obtained even at low beam-currents. We present results from both a ThermoFisher FEI Titan G2 operated at 300kV and a Nion UltraSTEM200 operated at 200kV, and compare the images to conventional analogue recordings. ADF data are compared with analogue counterparts for each instrument, a digital detector-response scan is performed on the Titan, and the overall rastering efficiency is evaluated for various scanning parameters.