Soft breakdown and hard breakdown in ultra-thin oxides

Soft breakdown (SBD) and hard breakdown (HBD) events are characterised separate: of each other for a 3.4 nm gate oxide. It is shown that both breakdown events can have significantly different voltage and temperature acceleration behaviour. Further it is demonstrated by photoemission microscopy (PEM) for a 2.2 nm oxide that different types of breakdown paths exist. HBD-like and SBD-like breakdowns are found on the same gate area during constant voltage stress. PEM also points out that a structural change of a breakdown path can occur, usually referred to as thermal breakdown of SiO2. It is concluded that a separate characterisation of SBD and HBD events is correct, if the stress conditions do not cause this structural change for the first SBD event. (C) 2001 Elsevier Science Ltd. All rights reserved.