Employing transistor reliability testing as an FA tool for understanding HTOL product BIST failures

We observed large Vmin shift on products produced using 28nm process during high temperature operating life (HTOL) testing. Failure analysis (FA) revealed that some of the n-channel metal oxide semiconductor field-effect (MOSFET) transistors of the failing cells have very low threshold (Vt) voltages. Those measured Vt values were unexpected and caused concern. In this study, we present the use of transistor reliability testing using nano-probing as an FA tool for understanding the built-in self-test (BIST) failures that we observed on some products during HTOL qualification. The study identified that the Vt shift was due to parametric shift under off-state stress. We found that the off-state stress (Vd = Vs = Vstress and Vg=0) caused the Vt to shift to lower value creating large imbalance in the cells, which caused Vmin failure. A correlation was found between the initial Vt value and the Vt shift. The finding led to employing additional Vmin guardband and changes in the stress pattern used during HTOL stress.