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We investigated the degradation of device reliability due to Negative Bias Temperature Instability (NBTI) of PMOSFET with ultrathin gate oxide. It was experimentally demonstrated that the chemical reactions at the gate oxide/substrate interface and/or diffusion of hydrogen related species are the major cause of the NBTI. We also found that nitridation of gate oxide enhances NBTI. In order to suppress the NBTI, the density of hydrogen terminated silicon bond at the interface needs to be minimized. Thus, the concentration of nitrogen in thin gate oxide has to be optimized in terms of the reliability reduction due to NBTI. IntroductionThe thickness of the gate oxide is aggressively reduced to achieve high speed and low power circuit operation at the same time. To meet this requirement, the electric field applied to the gate oxide increases as shown in figure 1. As reported recently [ 1-31, NBTI has become a major concem when realizing highly reliable integrated CMOS devices, because the Vth shift of the PMOSFET due to NBTI has become a limiting factor for MOSFET scaling [Z].On the other hand, nitridation technology is widely used to prevent boron penetration from p+ gate electrode. However, we found that nitridation of gate oxide degrades reliability of PMOSFET due to NBTI. In this paper, we show the origin of NBTI and the lnfluence of nitrogen incorporation. A possible mechanism for NBTI is proposed.

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