Hot-electron effects and oxide degradation in MOS structures studied with ballistic electron emission microscopy

Abstract

The application of the STM-based technique of ballistic electron emission microscopy (BEEM) to the study of transport properties of SiO2 gate oxide layers is reviewed. Oxide degradation observed on a local scale of nanometer dimensions ranges from the filling of electron traps with low-kinetic-energy electrons injected just above the oxide barrier, to trap generation and filling triggered by electrons with kinetic energies ≥2 eV. BEEM provides means to determine the spatial distributions of the traps. Only positively charged traps are observed for thin (≤4 nm) SiO2 layers upon electrical stressing. Oxide breakdown is seldom induced by local stressing, suggesting that it occurs at extrinsic defect sites that are widely separated. BEEM also provides unique opportunities to address fundamental issues. In that regard, examples of the dynamic response of the SiO2 dielectric to a moving electron are presented, as well as the determination of the dispersion of the electron mass of the conduction-band electrons. The latter is achieved by modeling quantum interference oscillations in the BEEM current.