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 temperatur e anneals was investigated. By controlling the oxygen partial pressure duri ng heat treatment, lanthana and zirconia films could be protected against r eaction with the underlying Si substrate and against the growth of low-epsi lon interface layers. The electrical thickness of the dielectrics could be maintained after a 900 degreesC exposure. The critical oxygen pressure at 9 00 degreesC for low-epsilon interface formation beneath ZrO2 and La2O3 diel ectrics was similar to 2e(-4) Torr. The interfaces that formed beneath the ZrO2 and La2O3 layers are distinctly different. The sub-ZrO2 interface, inf luenced primarily by phase separation, tends towards pure SiO2, while the s ub-La2O3 interface, influenced primarily by silicate formation, tends towar ds a La-Si-O alloy. For both materials, reducing the oxygen pressure to val ues below 10(-7) Torr resulted in rapid degradation of the metal oxide. Thi s dielectric degradation is believed to be linked to SiO evaporation. These results suggest that at high temperatures, a window of optimal oxygen part ial pressure exists in which the stability of many oxides in contact with s ilicon can be achieved. (C) 2001 American Institute of Physics.