Nitrogen diffusion and accumulation at the Si∕SiO2 interface in SiO2∕Si3N4∕SiO2 structures for nonvolatile semiconductor memories

Time-of-flight secondary ion mass spectrometry (SIMS) depth profiling was used to study nitrogen distribution in SiO2∕Si3N4∕SiO2 (ONO) structures employed in advanced semiconductor memories. We have investigated different factors that affect nitrogen accumulation at the Si∕SiO2 interface of the ONO structure. To isolate the impact of ion beam enhanced nitrogen diffusion towards the Si∕SiO2 interface in SIMS measurements, the top silicon oxide and silicon nitride layers were chemically etched before the SIMS procedure. Thermal diffusion effects were investigated by comparing specimens with different thermal budgets and different thickness of layers in the ONO stack. Our results unambiguously suggest that nitrogen can be accumulated at the Si∕SiO2 interface during ONO fabrication.