Breakdown Modes and Breakdown Statistics of Ultrathin SiO2 Gate Oxides

Abstract

The dielectric breakdown of ultra-thin silicon dioxide films used as gate insulator in MOSFETs is one of the most important reliability issues in CMOS technology. In this paper, two main aspects of oxide breakdown are considered: the modeling of the breakdown statistics and the properties of the two main breakdown modes, namely Soft Breakdown and Hard Breakdown. The most invoked models for the breakdown statistics that relate defect generation and breakdown are reviewed. Particular attention is paid to the percolation models and to a recent cell-based analytic picture. The scaling of the breakdown distribution with oxide thickness is considered and it is shown that both pictures are equivalent for ultra-thin oxides. It is shown that soft and hard breakdown show coincident statistics and this is used to conclude that both breakdown models are triggered by the formation of the same kind of defect-related conduction paths. The big differences in the post-breakdown conduction properties are attributed to phenomena occurring during the very fast breakdown current runaway that determine the area of the final breakdown spot. The properties of soft and hard breakdown are explained within the common framework of a model based on quantum-point-contact conduction. This mesoscopic approach to the post-breakdown conduction is shown to explain the main experimental results including conductance quantization after hard breakdown, the area and thickness independence of the soft-breakdown I(V) characteristics and the statistical correlation between current level and normalized conductance. Finally, we deal with some open questions and relevant issues that are now subject of intensive investigations. The fact that some breakdown events can be tolerated for some digital applications is considered. In this regard, the distinction between breakdown and device failure distributions is made and some implications for device reliability are discussed. It is argued that energy dissipation during the breakdown runaway can determine the breakdown efficiency, the prevalence ratio of soft to hard breakdown, and their variations with stress conditions, experimental setup (series impedance) and sample characteristics.