Capacitors made up of thin films of Ta2O5/TiN on polysilicon have been studied for application to 256 Mbytes dynamic random access memories. In this study, the thermal stability of poly‐Si/Ta2O5films with TiN electrode film deposited by (i) chemical vapor deposition (CVD) with TiCl4and NH3, (ii) sputtering, and (iii) metalorganic CVD (MOCVD) was compared. These samples were annealed under vacuum at 700, 800, and 900 °C for 30 min. The as‐deposited and the annealed samples were analyzed by Rutherford backscattering with 2 MeV He+ions. The CVD TiN films, which contained a small amount of Cl, provided pathways for the underlying Ta2O5to out‐diffuse during annealing at all temperatures between 700 and 900 °C. The sputtered films were resistant to the diffusion of Ta2O5up to 800 °C, but relented at 900 °C, with measurable outdiffusion of Ta. There was no noticeable interaction of the Ta2O5with the TiN layers, deposited by the MOCVD process, at temperatures up to 900 °C. However, the MOCVD TiN films were found to contain light elements such as carbon and oxygen, which were slightly reduced on annealing. Nuclear reaction analysis and secondary ion mass spectroscopy were used to detect the concentrations and depth profiles of light elements. These indicate presence of C and O contents up to at least 20 at. % in these films.