High temperature stability in lanthanum and zirconia-based gate dielectrics

Gate dielectrics composed primarily of lanthana and zirconia were prepared by reactive evaporation. The stability of the layers during high temperature anneals was investigated. By controlling the oxygen partial pressure during heat treatment, lanthana and zirconia films could be protected against reaction with the underlying Si substrate and against the growth of low-ε interface layers. The electrical thickness of the dielectrics could be maintained after a 900°C exposure. The critical oxygen pressure at 900°C for low-ε interface formation beneath ZrO₂ and La₂O₃ dielectrics was ∼2e^-4 Torr. The interfaces that formed beneath the ZrO₂ and La₂O₃ layers are distinctly different. The sub-ZrO₂ interface, influenced primarily by phase separation, tends towards pure SiO₂, while the sub-La₂O₃ interface, influenced primarily by silicate formation, tends towards a La-Si-O alloy. For both materials, reducing the oxygen pressure to values below 10^-7 Torr resulted in rapid degradation of the metal oxide. This dielectric degradation is believed to be linked to SiO evaporation. These results suggest that at high temperatures, a window of optimal oxygen partial pressure exists in which the stability of many oxides in contact with silicon can be achieved.