From oxide breakdown to device failure: an overview of post-breakdown phenomena in ultrathin gate oxides

摘要

The time to breakdown (t{sub}(BD)) of SiO{sub}2/SiON gate oxides strongly decreases when oxide thickness (T{sub}(OX)) is scaled from 3 nm to 1 nm. However, the oxide BD does not necessarily cause an immediate circuit failure because the device performance is not always severely degraded after the BD [1-3]. This is important for circuit reliability because an extra lifetime margin can be obtained from the post-BD phase. Pursuing these ideas, at least three different approaches have been proposed to deal with the post-BD reliability: the progressive BD (PBD) approach [4,5], the hard breakdown (HBD) prevalence ratio method [6] and the successive BD approach [7,8]. The PBD approach is based on the observation that a certain time is required in ultra-thin oxides for the progressive growth of the BD current up to a value that perturbs the device performance. The HDB prevalence ratio approach is based on the existence of two BD modes, HBD and soft BD (SBD). Since, in this approach, it is assumed that only HBD causes the device failure, the failure distribution relevant to reliability is that of HBD instead of that of the first BD event, thus obtaining a lifetime increase which depends on the so called HBD prevalence ratio (α{sub}(HBD)). Finally, the successive BD approach is based on the assumption that all the BD events which occur under operation conditions in ultra-thin oxides are SBDs. The chip failure is considered to occur when a fixed number of such events have occurred in any device of the chip [7] or in the chip as a whole [8], thus obtaining a significant lifetime improvement. In this paper, we present an overview of post-BD phenomena in ultrathin (1 nm < T{sub}(OX) < 3 nm) oxides and couple this description to a discussion of the different methodologies proposed to deal with the post-BD reliability. We focus on the complete description of the statistics of the time to device failure (t{sub}(FAIL)) and of the residual time (t{sub}(RES)) from oxide BD to device failure, and on their scaling properties. Both intrinsic and extrinsic BD modes will be considered and the impact of burn-in will be briefly analyzed.