Reliability study of ultra-thin dielectric films with variable thickness levels

The time-dependent dielectric breakdown of ultra-thin gate oxides is one of the major reliability issues facing scaled metal-oxide semiconductor technologies. As the thickness of the gate dielectric film approaches its scaling limit, process issues such as poor wafer uniformity and oxide growth control become critical for ultra-thin gate dielectric reliability. This article investigates both the physics and statistical aspects of the reliability of the ultra-thin gate dielectric when film thickness variations are present. A physics-based Spatio-Temporal Monte Carlo Simulation (STMCS) model is developed to study the effects of thickness and thickness non-uniformity on dielectric reliability. Its use allows the root cause for the non-linear characteristic of the Weibull time-to-breakdown distribution observed in experimental studies to be revealed. In addition, a Bayesian Weibull Mixture (BWM) model is proposed to analyze the time-to-breakdown data considering the existence of thickness non-uniformity. Numerical results are presented that show that the proposed BWM model is significantly superior to the basic Weibull model for use in reliability projection. Both the STMCS and BWM models can successfully reproduce the experimentally observed non-linear characteristic of the Weibull time-to-breakdown distribution and thus can be used to predict device reliability when the dielectric films in a gate are of unequal thicknesses.