Interfacial reaction in the poly-Si/Ta₂O₅/TiN capacitor system

For tantalum pentoxide capacitors in 1 Gbit dynamic random access memory, titanium nitride is adopted as the top electrode. After postannealing at temperatures higher than 750°C, the capacitance of Ta₂O₅ reduces due to an interfacial reaction between TiN and Ta₂O₅. This was studied by high resolution electron microscopy and energy dispersive spectroscopy with a 1 nm electron probe. It is found that voids form along the interface between TiN and Ta₂O₅ and a large proportion of Ta dissolves into the TiN film. The origin of the reaction is concluded to be a large solid solubility of Ta in TiN. After Ta outdiffusion into TiN, vacancies agglomerate and form voids in Ta₂O₅ and thereby reduce its capacitance. Since the driving force of the reaction is the solid solubility of Ta in TiN, the amount of the reaction is affected by the thickness of the TiN films.