Impact of inside spacer process on fully self-aligned 250 GHz SiGe:C HBTs reliability performances: a-Si vs. nitride

A critical process aspect of the bipolar device is the oxide isolation between the emitter and the extrinsic base. Indeed, it is a well known fact that the emitter–base junction degradation is mainly due to interface states generation underneath oxide spacer. This study demonstrates the large impact of the inside spacer process in fully-self aligned high speed HBT on its reliability. Several types of electrical stress have been investigated and the stress-induced degradation compared for nitride and a-Si (amorphous silicon) spacers. Aging results coupled with noise measurements and TCAD simulations allowed to explain the different observed behaviors, finally concluding on the significantly higher reliability performances of devices processed with a-Si spacers.