Statistical mechanics based model for negative bias temperature instability induced degradation

摘要

A physics based model for negative bias temperature instability (NBTI) induced degradation is proposed. Like previous models, this model attributes NBTI to depassivation of Si-H bonds at the SiO_2/Si interface. The two distinguishing features of the proposed model are: (ⅰ) statistical mechanics is applied to calculate the decrease in interfacial Si-H density as a function of stressing conditions, and (ⅱ) hydrogen diffusion in the oxide is assumed to be dispersive and the diffusing species is identified with the positively charged hydrogen ion (H_i~+). The model assumes that as H_i~+ diffuses away from the interface into the oxide, interfacial and bulk traps are created. Based on these model assumptions, an equation for the threshold voltage shift (ΔV_t) is derived as a function of stressing time, oxide field, temperature, oxide thickness, and initial Si-H density at the interface. The model predicts that ΔV_t would increase with a power law dependence at earlier stressing times and would saturate at longer times. The power law increase at earlier stress times is attributed to dispersive hydrogen diffusion and ΔV_t saturation at longer stress times is ascribed to occur when all bonded hydrogen (Si-H) has been removed from the interface. These and other model predictions are verified using published NBTI data from various research groups for p-channel field effect transistors (pFETs). In addition, the model is shown to be compatible with NBTI data for HfO_2 pFETs.