Correlations between ionization radiation damage and charging effects in transmission electron microscopy

The goal of this paper is to correlate macroscopic positive charging of electron-irradiated thin insulating specimens to atomic displacements, migration and desorption effects. The leading role of the Auger process is emphasized. It is shown that the electrons coming from the surroundings cannot spontaneously compensate for Auger (and the secondary) electron emissions with the result that electrons are missing in the uppermost states of the valence bands. At the steady state, the density of missing electrons is correlated to the beam density and the electrical resistivity of the specimen. For highly resistive specimens an expression for the etching rate is also suggested. This analysis explains some experimental results such as desorption of positively charged anions into the vacuum for ionic compounds or atomic displacement for covalent compounds. The influence of the crystalline or non-crystalline state of the specimen is considered. The role of coating the specimen foil with conductive layers is analyzed. The same analysis applies also to X-ray irradiation.