The fast initial threshold voltage shift: NBTI or high-field stress

Abstract

Recent negative bias temperature instability (NBTI) studies have come to involve very high electric fields, yet these same studies are used to criticize the lower field ldquoNBTIrdquo models. This study examines both high- and low-field degradation phenomena by monitoring the initial threshold voltage shift (DeltaVTH) as a function of stress time and stress voltage. We demonstrate that the initial DeltaVTH is recoverable and decays rapidly as the stress voltage is reduced. We also monitor the transient transconductance (GM) degradation which surprisingly indicates the presence of an electron trapping/de-trapping component. We argue that the initial DeltaVTH and GM degradation behaviors are consistent with high-field stress degradation. The electron trapping component of the ldquorecoverablerdquo degradation is unexpected and must be addressed to insure accurate NBTI lifetime predictions.