The Behavior of Charged Particles in the Scanning Electron Microscope

Charging phenomena of small particles have been observed in the scanning electron microscope (SEM). Polystyrene particles, mounted on gold coated hologram grating substrates, are negatively charged by the primary beam in the SEM, and a significant distortion of the grating is seen. Computer generated images have been matched to the experimental images of gratings. Parameters related to the microscope, specimen and specimen-microscope geometry such as electron beam accelerating voltage, magnification, grating spacing, grating type, specimen tilt angles, charge location and magnitude are used to generate the calculated images. It has been found that gratings tilted at large angles to the optic axis of the microscope increase the effect of charge on electron trajectories and hence the accuracy to which charge can be measured. Typical charge values in the SEM are ~10-12C/particle. A second part of the investigation involves dynamic motion of small charged particles using television scanning rates in the SEM. Several effects were observed including uphill rolling motion towards the electron beam at low incident beam energy (10 kV), downhill motion at higher energy (20 kV), charge transfer, field lines between charged particles, and electrical contrast. It is possible to describe mathematically the motion of these particles.