Flexible formation of coherent probes on an aberration-corrected STEM with three condensers

We have used geometric optics calculations and experiments to investigate the probe-forming capability of an aberration-corrected, three-condenser scanning transmission electron microscope (STEM). Large, minimally convergent and coherent electron probes are useful for a variety of electron diffraction measurements. A three-condenser lens STEM can form a probe either using a virtual aperture below the sample and a virtual source on the sample plane or using a virtual aperture on the sample and a virtual source in the front focal plane. Adding a hexapole probe aberration corrector greatly increases the range of aperture demagnification and thus probe size and convergence angle. We have created probes 0.1 to approximately 12 nm in diameter in the simplest operating mode of our STEM, and we calculate that probes as large as 5000 nm that are almost perfectly parallel may be possible in more exotic lens configurations. We have also measured the spatial coherence of some of these probes.