The effect of gate doping on the electrical conduction and reliability of thick gate oxides

Previous investigations have shown that excessive gate oxide leakage currents at negative gate potentials observed in MOS power products are due to the presence of phosphorus in the gate oxide. During polysilicon gate doping some phosphorus atoms may pile up at the PSi/SiO2 interface and also may diffuse into the gate oxide due to high diffusion temperatures and longer soaking times. Subsequent high temperatures and plasma processing steps result in the ionization of these atoms. The neutralization of these ions during negative gate voltage testing gives rise to abnormal gate oxide leakage currents. It is shown that the presence of any Column V element in the gate oxide, if ionized, will result in excessive gate oxide leakage currents at negative potentials. The level of excessive leakage will depend on the amount of pile up of the impurity at the polysilicon gate/SiO2 interface. Data show that abnormally high gate oxide leakages due to the presence of these impurities in the gate oxide of MOS, IGBT and MCT devices may adversely affect the yield and the long term reliability