Electrical and physical characteristics of thin nitrided oxides prepared by rapid thermal nitridation

Ultrathin oxides (5-12 nm) were nitrided by lamp-heated rapid thermal annealing in ammonia at temperatures of 900-1150°C for 5-300 s. Elemental depth profiles were measured by Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS). Both the nitrogen concentration measured by AES and the hydrogen one measured by SIMS for a nitrided oxide are found to increase monotonically as nitridation proceeds. The AES depth profiles of oxygen show that the Si-SiO2interface does not move during nitridation. Dependences of midgap interface state density (Dᵢₜₘ) and fixed charge density (Nf) on nitridation temperature and on oxide thickness were studied. For a given temperature, bothDᵢₜₘand (Nf) are found to show turnarounds as nitridation time increases in a similar manner: at first both increase, reach respective maxima at a certain nitridation timetₘₐₓ, and then decrease gradually. The (Dᵢₜₘ) and (Nf) increase more rapidly and thetₘₐₓis shorter as the nitridation temperature is raised or the oxide film is thinner. The maximum ofDᵢₜₘincreases as the oxide film is thinner. A two-step model is newly proposed to explain the turn-around behaviors ofDᵢₜₘand Nf: the first step is defect formation as a result of nitrogen incorporation and the second step is reduction of the defects by an annealing-type process. The simulation reproduces the turnaround behaviors very well.