Sub-Ångström resolution imaging at 20 keV in a scanning electron microscope from ptychography with integrated projection lens distortion correction

Session

EMAG - 4D STEM

Authors

Dr Arthur Blackburn (2), Dr Cristina Cordoba (2), Dr Matthew Fitzpatrick (2), Dr Robert McLeod (1)

Affiliations

1. Hitachi High-Tech Canada
2. University of Victoria

Keywords

Ptychography

Electron Microscopy

Coherent Diffractive Imaging

Scanning Electron Microscopy

4D-STEM

Abstract text

The desire to obtain sub-Ångström (< 10^-10 m) information from general purpose, low energy (<30 keV) scanning electron microscopes (SEMs) has existed for many years [1]. Imaging at this resolution at these energies has previously only been possible through using complex aberration correctors, which introduce additional expense, labour to operate, and installation space. Here, through combination of adding a simple diffraction projector lens to a non-aberration corrected SEM operating in transmission mode; incorporating calibration and correction of projector lens pincushion distortion with a multi-slice ptychographic reconstruction algorithm; and using un-coated hybrid direct electron detector [2], we show a resolution of at least 0.7 Å with a 20 keV beam in experimental sample image reconstructions. Test samples include Au particles on amorphous carbon, with an example reconstructed phase and amplitude given in Figure 1(a). Analyzing the Fourier ring correlation between independent reconstructions shows that features of 0.7 Å are resolvable (Figure 1(b)). Furthermore, we see excellent alignment between the experimental image fast Fourier transform (FFT) and model diffraction patterns, showing that distortion from our simple projector lens has been successfully corrected. Analyzing reconstructed images of MoS₂ based samples shows this resolution is achievable in more practical materials even in the presence of some sample contamination, through using background removal routines. Thus, low-energy sub-Å resolution imaging, previously only possible in aberration corrected S/TEM instruments, is now demonstrated as feasible at typical SEM instrument acceleration voltages. 

Figure 1 – (a) Reconstructed exit wave for electron transmission through a gold particle supported on carbon imaged at 20 keV; (b) Fourier ring correlation characteristic with experimental and model radial FFT profiles of a reconstructed image containing many particles.

References

[1] MJ Humphry, B Kraus, et al. (2012), Nature Comms, 3:730

[2] G Tinti, H Marchetto, et al. (2017), J. Synchrotron Rad. 24, 963