Electron Energy Loss Spectroscopy Characterization of TANOS (TaN/Al2O3/Si3N4/SiO2/Si) Stacks

The interfacial layer between the Al2O3 layer and the Si3N4 layer formed after postdeposition annealing (PDA) of TaN/Al2O3/Si3N4/SiO2/Si (TANOS) stacks was investigated using transmission electron microscopy (TEM), scanning transmission electron microscopy, and electron energy loss spectroscopy (EELS). From the result of the TEM analysis, it was found that the 2-nm-thick interface layer between Al2O3 and Si3N4 layers was amorphous. The high-loss EELS analysis showed that the phases of the interfacial layer weakly bound together instead of the substoichiometric silicon oxide and amorphous Al2O3 near the bottom interface of the crystalline Al2O3. The low-loss EELS analysis showed that aluminum existed in metallic state at the interface. Therefore, we speculated that SiOxNy could be formed by oxidation of Si3N4 during PDA and that metallic aluminum could be formed by the decomposition of weakly bound amorphous Al2O3 during electron irradiation. These complicated reactions near the interface could induce oxygen deficiency in the Al2O3 layer and finally degrade the retention properties of TANOS stacks.