Radiation Induced Charge Loss Mechanisms Across the Dielectrics of Floating Gate Flash Memories

Flash memories are one of the key microelectronics technologies today. In these devices bits are stored as charge injected into floating gate (FG) MOSFETs, where a polysilicon layer (FG) is interposed between the substrate and the control gate (CG). One or more bits can be written in the FG memory cell by injecting electrons or holes in the FG that is totally surrounded by dielectrics, thus confining the excess carriers in a potential well. Preserving the stored information is achieved if these dielectrics grant almost perfect electrical isolation. To this purpose, a sandwich of Si oxide-nitride-oxide layers (ONO) is deposited between FG and CG, while a highquality thermally grown SiO2 thin layer (tunnel oxide) separates the FG from the Si substrate. The electrical isolation properties of these dielectric layers is endangered by exposure to ionizing radiation, that may directly drive charge out of the FG, or produce leakage paths across the dielectrics surrounding the FG discharging it. In both cases a degradation of the stored charge takes place and consequently an erroneous bit may be read by the external circuit.