PEEM Image Contrast:
Electric Fields at the Sample Surface I

In photoelectron emission microscopy (PEEM), electrons are photo-emitted from a sample, then collected and focused by strong electric fields to form an image, much as glass lenses are used to collect and focus photons. A grounded sample placed 3-4 mm from the PEEM anode at 10 kV sees an electric field of ~3 Volts/µm. If the sample has topographic features – steps or trenches – boundary conditions on the applied field result in locally strong lateral fields at the sample surface.

When emitted, the electrons are traveling quite slowly so that these short-range lateral fields can have an unexpectedly large effect on the electrons’ flight. The field kicks electrons away from the step edge. In the image, this results in a dark edge. Our calculations of the lateral field strength indicate that the range of these fields is comparable to step height, so the effect should be greater for a taller step.


Test Sample Structure

This increasing edge effect is apparent in PEEM images of steps etched in Titanium. At left is an SEM image of a typical step profile. On the right, PEEM images of these same steps show the darker edge effect for the taller step.