Point-Spread Function of the Optics in Scanning Electron Microscopes

Point-spread function of the probe forming optics ($PSF_{optics}$) is reported for the first time in an uncorrected (without multipole correctors) scanning electron microscope (SEM). In an SEM, the electron probe information is lost as the beam interacts with the specimen. We show how the probe phase information can be recovered from reconstructed probe intensity estimates. Controlled defocus was used to capture a focal-series of SEM images of $28.5\;nm$ gold ($\mathrm{Au}$) nanoparticles ($\mathrm{NPs}$) on a carbon ($\mathrm C$) film. These images were used to reconstruct their respective probe intensities to create a focal-series of probe intensities, which were the input to the phase retrieval pipeline. Using the complete description (intensity and phase) of the electron probe wavefunction at the specimen plane, we report the $PSF_{optics}$ for multiple data sets for beam energy $E\;=20\;keV\;$. This work opens up new possibilities for an alternative way of aberration correction and aberration-free imaging in scanning electron microscopy.