A new low-dose STEM imaging mode with probability driven intra-pixel beam blanking

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EMAG - EM Data Processing & Analysis
Jonathan Peters (1, 5, 6), Bryan Reed (3), Yu Jimbo (4), Alexandra Porter (2), Daniel Masiel (3), Lewys Jones (1, 5, 6)
1. Advanced Microscopy Laboratory, Trinity College Dublin
2. Department of Materials, Imperial College London
3. IDES Inc.
4. JEOL Ltd
5. School of Physics, Trinity College Dublin
6. turboTEM

low-dose STEM, adaptive beam blanking

Abstract text

Transmission electron microscopy (TEM) has proven itself as an invaluable tool in understanding material properties, physical phenomena, and biological processes. Annular dark field scanning TEM (ADF-STEM) has emerged as a prevalent imaging mode due to its ease, interpretability, and flexibility with collecting multiple simultaneous signals. However, to achieve this a highly concentrated electron beam is focussed onto the sample, potentially causing beam related damage to the sample. For beam sensitive samples (e.g. organic materials), it becomes difficult to image the sample in its pristine state without modifications due to the beam. In many cases it may even be impossible to image the sample at all before catastrophic damage.

To mitigate beam damage effects, several approaches have emerged, including dose rate control (i.e. multi-frame imaging), rapid beam blanking, and advanced scan strategies including interlacing or compressed sensing. In any case, as dose is lowered, the acquisition system becomes limited by Gaussian noise. Whilst this is improved by digital pulse counting imaging, it is perhaps over-simplistic to consider ADF-STEM imaging as a continuous signal to be measured, but instead an event stream of quantised electrons.

The core of ADF-STEM image contrast is the interaction of the electron beam with the atoms in a material, scattering a fraction of the beam onto a detector over an acquisition time (i.e. dwell time of the beam). This is, in essence, a scattering event rate measured over a fixed time. We propose a new event-driven approach whereby the time to detect a set number of electrons, n, is measured. For example, a heavy, strong scattering atom, such as lead, will scatter n electrons onto the ADF detector more quickly than a low atomic number element, e.g. oxygen. This allows imaging with the desirable number of detected electrons per pixel, where the information gained can be described by a Bayesian model, with most of the image information contained within the first few electrons.

To realise this measurement practically, we combine the electron event detection of the turboTEM Pulse digitiser with the rapid blanking (within 10 ns) of the IDES Inc. electrostatic dose modulator (EDM) system. We show experimentally that by obtaining the maximum signal information from each electron, we can control and drastically reduce dose. We demonstrate reduced beam damage for imaging biological and solid state materials whilst retaining the benefits of ADF STEM imaging in combination with simultaneous spectroscopies.