Charging of dielectrics under focused ion beam irradiation

We present a detailed experimental and theoretical study of the charging of silicon nitride and silicon oxide thin films following focused ion beam irradiation. The samples were irradiated using 30 keV Ga+ ions at different ion doses and their consequent work function changes were measured by Kelvin probe force microcopy. The surface potential of both samples increased following the ion irradiation up to a critical ion dose, and then moderately decreased. The dependence of the sample surface potential on the irradiated ion dose is analyzed by taking into account all the main factors affecting charging in dielectric thin films: electron-hole generation by the incident fast ions, secondary ion-electron emission, sputtering of surface atoms, electron-hole recombination, electron recombination with the incident stopped ions, hole leakage current to the Si substrate, and various charge trapping processes. It was found that the much larger surface potential induced in Si3N4 in comparison to SiO2 is associated with the different resistance to the Ga+ ion bombardment. Under equal ion irradiation dose, a larger concentration of shallow traps is created in SiO2 than in Si3N4. This leads to an increased hole capture in shallow traps versus deep traps, and a consequent decrease in the surface potential