The TDDB of gate oxides is one of the most important reliability issues for ULSIs. Although phenomena have been extensively discussed, the physical mechanism of hard breakdown (HBD) has not yet clarified. Recent works have reported that the shape factor (/spl beta/) of the Weibull distribution softens as SiO/sub 2/ film thickness decreases, and the oxide-thickness dependent /spl beta/ reflects the intrinsic statistical characteristic of breakdown events (Degraeve et al, 1995; Stathis and DiMaria, 1998). These studies indicate that an accurate determination of the Weibull slope /spl beta/ is significant to consider equally reliability estimation and breakdown process of SiO/sub 2/ films (Sune et al, 2000). Although pulsed constant current stress (CCS) experiments have already been carried out to investigate breakdown events in past studies, the meaning of the unstressed interval has not been considered in depth (Fong et al, 1986; Wang et al, 2001). In this paper, we statistically analyse the time-to-breakdown (T/sub bd/) of SiO/sub 2/ films based on a new pulsed CCS method proposed here. This paper addresses the extra-trap creation within the unstressed interval; the degradation in SiO/sub 2/ films continues over the unstressed interval.