Impact of Random Soft Oxide Breakdown on SRAM Energy/Delay Drift

Reliability issues have been traditionally considered only for functional yield problems. Among them, the progressive gate-oxide soft breakdown (SBD) in the negative-channel field-effect transistors is foreseen to be a big issue for the deep-deep submicron technologies, where ultrathin gate oxide and low supply (stress) voltage will be massively deployed. Device-level investigation has largely confirmed that the SBD defect will not lead to an issue for the transistor functionality. In addition, simple gate-level characterization also indicates that the SBD defect in those gates only leads to performance degradation. Up to now, the impact of the random nature with such effect is not yet well understood at a complex circuit block or component level. In this paper, we have focused our analysis in providing insights on the SBD impact on sensitive static random access memory (SRAM) components as well as the complete matrix. Results have shown a significant shift in the performance figures of the blocks, which endangers the SRAM parametric yield. The drift becomes even worse when the process variability effect is taken into account. All the results indicate urgent needs to develop effective countermeasures by both the technology and design sides.

I. Introduction

Technology scaling has traditionally enabled improvements in increasing performance, lowering energy consumption, and reducing the die cost. Along with feature miniaturization, process technologists have also introduced a number of novel materials and process steps in leading-edge manufacturing processes. Examples include the high- [1], [2] materials that are used for the transistor gate insulation from the channel, the low- materials for the implementation of the dielectrics in the metal stack, and the reintroduction of copper for the implementation of interconnect wires. Characterizing these materials and their interactions for reliability degradation mechanisms is an extremely complex task [3], [4]. Typically, they are used in commercial processes before full understanding of the physical degradation mechanisms is available. At the same time, the supply voltage scaling has been saturating to keep enough headroom between the transistor threshold voltage and the supply voltage, hence increasing the electric fields and stress conditions for these devices [3], [17]. Furthermore, effects that, in the past, have been considered second order are now becoming a clear threat for the parametric and functional operation of the circuits and systems in near-future technologies. Examples include progressive gate-oxide soft breakdowns (SBDs) of transistors (particularly dramatic in high- oxides) [3], negative bias temperature instability issues in the threshold voltage of the positive-channel metal–oxide–semiconductor transistors [5], electromigration problems in copper interconnects [6], and breakdown of dielectrics in porous low- materials [7].