Modern scanning electron microscopes (SEM) have been an excellent tool to probe nature at the nanoscale. Electron beam inspection, metrology, and lithography are some of the many applications of SEMs in the semiconductor industry, material science, etc. In an SEM, the lens system is used to form a focused scanning electron probe (SEP) which scans the specimen. In this work, we report the developments in reconstruction of the wavefunction of the SEP by performing non-interferometric phase retrieval. Firstly, we have explored phase retrieval of the SEP based on defocus variation. A through-focal image series is taken by moving the specimen (Au-C) across a fixed focal plane. These images are used to reconstruct the SEP intensity distributions which serve as the input for iterative phase retrieval. We observe that the defocus variation does not provide enough information diversity causing the reconstructed phase to stagnate. Therefore, we propose an experimental setup that uses a spiral phase plate to generate an electron vortex illumination and introduce substantial information diversity between two intensity measurements. We have shown by simulation that the phase reconstructed by this technique offers a much more robust solution to the phase retrieval problem. Aberration estimation and correction, and low-dose imaging could be some of the direct applications of knowing the complete wavefunction of the SEP. Our goal is the advancement of scanning electron microscopy as a domain where we can completely characterize the beam wavefunction as it is currently possible for transmission and scanning transmission electron microscopes (TEM and STEM).
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