Ultrathin tantalum pent-oxide films for ULSI gate dielectrics

Ultrathin tantalum pent-oxide (Ta2O5) films with a high dielectric constant over 20 and which are thinner than 10 nm have been extensively studied to realize small area capacitors for memories and high performance MOSFETs. The author and his colleagues started working on ultrathin Ta2O5 films with an equivalent oxide film thickness less than 4 nm for small capacitors for bipolar memories and DRAMS in 1984, and investigated extensively the effects of dry oxygen annealing of sputter-deposited thin Ta2O5, films on Si. We found that there was a process window (dry O-2, 800 degrees C) where defects causing initial and latent breakdown were significantly reduced and where at the same time the reduction in the capacitance inevitably caused by the growth of interfacial SiO2 is kept small. This dry O-2 annealing has been widely used for many years due to its effectiveness. This treatment was called "weak spot oxidation." The interfacial SiO2, with a smaller dielectric constant of 3.9, underneath the Ta2O5 films causes a reduction of the capacitance and an increase of the equivalent film thickness. Annealing of the films after deposition and suppression of the interfacial SiO2 growth are the keys to realizing high performance and reliable Ta2O5 capacitors and gate insulators. In this paper, these fundamental processes are described, along with more recent developments of Ta2O5 thin film technologies, and the potential of the Ta2O5, films as an alternative for a future ULSI gate insulator are also discussed.