Gas effect on the emission and detection of the backscattered electrons in a VP-SEM at low energy

Highlights: • Increasing pressure makes backscattering coefficient to increase at specimen top surface and decrease at the detector. • Increasing pressure makes the BSE exit zone to extend at the specimen top surface. • At 5 kV accelerating voltage and up to 1000 Pa, no effect is found under helium. • A new approach is proposed to determine the operating pressure range for the best resolution. - Abstract: The effect of the electron beam skirting on the emission and detection of the backscattered electrons (BSE) in a low vacuum scanning electron microscope is investigated at low energy regime. Monte Carlo computed dependencies of the BSE distribution on the water vapor and air pressure shown a significant increase of the extent of the BSE exit zone. The pressure variation has however a little effect when helium gas is used. A new approach based on the comparison between the sizes of the skirt and the BSE exit zone on the specimen surface provides a useful tool to determine the operating pressure range that ensures minimal degradation of the lateral resolution in BSE imaging mode.