Effects of Gaseous ion Concentration on Secondary Electron Contrast of Dielectric Materials Imaged in the ESEM

Abstract

An increasing body of experimental evidence indicates that the flux and distribution of gaseous ions significantly affects secondary electron contrast of dielectric materials imaged in low vacuum and environmental scanning electron microscopes. Unusual contrast has been identified in some insulating materials, and attributed to charging effects (1-3). However, as these contrast mechanisms have not been reported in conventional microscopes, it would seem that the ions play an important role in making the contrast manifest. The effects of ions can include the creation of space charge in the gas above the specimen, influencing the charge state of the specimen surface, and electron-ion recombination (possibly leading to energy filtering of emitted secondary electrons). Space charge and surface potential can influence the gain realised in the gas amplification cascade by altering the detector field (4). However, surface potential, and recombination affect the number of electrons injected into the amplification cascade, to give rise to contrast variations in the final image.