Fault modeling and defect level projections in digital ICs

This paper presents a new model for evaluating the defect level, DL, in VLSI circuits as a function of the yield, Y, and the stuck-at fault coverage, T. It is shown that the observed DL(T) curve can be accurately modeled using non equally probable, realistic faults predicted from defect statistics data and IC layout. The deviation of DL(T) from the one estimated by the Williams-Brown equation is shown, to be caused by two effects. First, the topology of the most likely realistic faults determine their susceptibility, which is typically lower than the stuck-at fault susceptibility. Second, the incompleteness of a given test set and the detection technique (such as static voltage testing) determine a non 100% defect coverage. The suitability of the model was assessed by means of layout fault extraction and switch-level fault simulation, and the results obtained agree with previously published DL(T) experimental results on actual ICs