Optical point-spread function via defocus phase retrieval in scanning electron microscopy

The point-spread function of the probe-forming optics (PSF_optics) is reported in an uncorrected (without multipole correctors) scanning electron microscope (SEM). In an SEM, information about the electron probe is generally lost as the beam interacts with the specimen. Here, we demonstrate an approach for recovering probe phase information from reconstructed probe intensity estimates. Controlled defocus was used to acquire a focal series of SEM images of 28.5,nm gold (Au) nanoparticles (NPs) on a carbon (C) film. These images were used to reconstruct their respective probe intensities, generating a focal series of probe intensities that served as input to the phase retrieval pipeline. Using the complete description (intensity and phase) of the electron probe wavefunction at the specimen plane, we estimate the PSF_optics for multiple data sets at a beam energy of E=20,keV. These results represent an important step in demonstrating the possibility of experimental aberration quantification in uncorrected SEMs.