Stabilization of SiO₂ Passivation Layers with P₂O₅

Measurements are reported on the stability of planar, npn silicon transistors with and without a phosphosilicate glass layer over the SiO2passivation layer. The phosphosilicate layer forms during the emitter diffusion from a P2O5source, and the data show that, to insure stability, it must not be removed in subsequent processing steps. The units tested were of conventional geometry except for a gate electrode over the base region, which provided additional information on the surface condition. The transistors were subjected to temperatures of 150° and 200°C with either gate-bias or junction reverse-bias. Production transistors, without gates, which had failed on life test were shown to have no phosphosilicate layer because it had been removed by excessive etching during fabrication. Additional evidence for the stabilization by P2O5has been obtained using metal-oxide-silicon capacitors with and without P2O5treatment of the SiO2layer. Both dc conduction through the insulator and stability of the capacitance-voltage characteristic were measured. These experiments suggest that the transistor degradation with unstabilized SiO2is caused by an accumulation of positive space-charge in the silicon-dioxide. This charge accumulates when an electric field (directed toward the silicon) is applied to the SiO2at temperatures in the range of 150°C.