Stability of ZrO2 layers on Si (001) during high-temperature anneals under reduced oxygen partial pressures

Electron energy-loss spectroscopy and high-resolution transmission electron microscopy were used to investigate ZrO2 layers grown by electron-beam evaporation in a molecular-beam epitaxy system. ZrO2/Si layers were investigated before and after uncapped annealing at 1000 °C under different oxygen partial pressures. The thickness of a SiO2-like, low-dielectric constant layer at the silicon interface was found to depend on the oxygen partial pressure during annealing. At oxygen partial pressures of about 10−4 torr the interfacial silicon oxide thickness increased through oxygen diffusion through the ZrO2 layer and silicon consumption at the interface. At oxygen partial pressures in the range of approximately 10−5 torr, only a thin (1 nm) interfacial silicon oxide layer was present, as required for low-equivalent oxide thicknesses of gate stacks incorporating alternative oxides. Further reduction of the oxygen partial pressures (about 10−7 torr) during annealing resulted in zirconium silicide formation at the interface. ZrO2 films annealed at the optimal partial pressure for a thin interfacial oxide were found to crystallize and contain no silicon. High-resolution analytical capabilities afforded by scanning transmission electron microscopy techniques proved essential in analyzing the stability of these ultrathin layers.