Impact of Hydrogen on Recoverable and Permanent Damage following Negative Bias Temperature Stress

By subjecting selected split wafers to a specifically adapted measure-stress-measure (MSM) procedure, we analyze negative bias temperature stress (NBTS) and recovery characteristics of PMOS devices with respect to the impact of hydrogen. We control the hydrogen incorporation within the gate oxide during Back End of Line (BEOL) fabrication by varying the titanium barrier thickness below the routing metallization. Differences in the initial passivation degree of the gate oxide are verified electrically by Charge Pumping (CP) measurements and physically by time-of-flight secondary ion mass spectrometry (TOFSIMS). Our results indicate that the total VTH shift is the sum of quasi permanent and recoverable damage which are of comparable scale but have completely different physical and electrical characteristics. While the permanent component seems to be strongly linked to hydrogen release from the interface (increase in CP current), the recoverable component is widely independent of hydrogen and its recovery can be controlled via carrier exchange with the silicon substrate. Hence, our results suggest different trap precursors for the individual components which challenge some predictions of the classical reaction-diffusion (RD) model and support the concepts of an alternative model based on permanent interface state creation via hydrogen transfer to recoverable E' centers which have their origin in oxygen vacancies, whose density is roughly independent of the hydrogen concentration.