Channel-length dependence of mechanical stress effect by hybrid shallow trench isolation on NBTI degradation of HfSiON/SiO2 p-channel MOSFETs with strained Si/SiGe channel

This paper quantifies how mechanical stress induced by the hybrid shallow trench isolation affects negative bias temperature instability (NBTI) degradation of p-type channel MOSFETs (pMOSFETs) that have a strained Si/SiGe channel. As the channel length L decreased, the NBTI degradation decreased and electrical characteristics degraded. Experimental results indicate that tensile stress σt applied to the channel region decreased the energy band-gap by increasing the intrinsic carrier concentration. NBTI degradation was not affected by application of an oxide electric field to compensate for the charge that is induced differently in the Si channel during strong inversion. The distribution of the shift ΔVth of threshold voltage before and after NBTI stress in the channel region was also examined. As L decreased, the decrease of ΔVth was greater in the gate edge region than in the center region. These results show that the observed L-dependence of NBTI degradation characteristics were caused by σt in the channel region, and affected the gate edge region dominantly. Therefore, to achieve reliable high-k pMOSFETs with the strained Si/SiGe channel as L is scaled down, methods should be developed to control mechanical stress in the gate edge region.