Experimental data and statistical models for bimodal EM failures

The correct model for the electromigration failure distribution is a key issue in reliability methodology. Usually, the failure times of a sample are fitted by a single log-normal distribution. However, in some cases relevant deviations can be found. In this paper two types of non-log-normal distributions observed on via-line structures are discussed. They can be modeled by two types of bimodal distributions, each composed of two log-normal distributions. Both models consider different failure mechanisms within the sample or specimen. Experimental data sets coinciding with either model will be presented. The physical failure analysis confirms the model assumptions and supports the bimodal distribution concept.