Atomic Layer Deposition of Hexagonal-Phase Ta2O5 Using TaF5 and H2O

The results of atomic layer deposition of Ta2O5 on silicon using the precursors TaF5 and H2O are reported. The films are stoichiometric Ta2O5 at the surface, becoming Ta-rich as the interface is approached. For deposition temperatures from 400 to 450DGC and chamber pressures from 0.5 to 5 Torr, the deposition rate exhibits two growth regimes, postulated to be due to differences in the densities of hydroxyl species on the chemically cleaned Si substrates and on the deposited Ta2O5 layer. Growth rate in the "converged" regime, in which Ta2O5 has completely covered the substrate, is more strongly affected by the partial pressure of the TaF5 than by that of the H2O. A comparison is made between the growth rate and the surface coverage of TaF5 calculated with a simple Langmuir model. The deposited phase changes from amorphous to crystalline (hexagonal) in the same interval of pressure and temperature. The controlling factor for formation of the hexagonal polymorph is the partial pressure of the TaF5 precursor. The extent of surface coverage of TaF5 alone does not adequately account for the dependence of the extent of crystallization on pressure and temperature, and it is likely that there are additional important surface reaction components.