Evidence of two distinct degradation mechanisms from temperature dependence of negative bias stressing of the ultrathin gate p-MOSFET

A detailed investigation of the negative-bias temperature instability (NBTI) of the ultrathin nitrided gate p-MOSFET over a wide temperature range reveals two different activation energies, indicating the coexistence of two distinct degradation mechanisms. One mechanism is linked to the incorporation of nitrogen while the other is the classical mechanism responsible for the degradation of conventional SiO₂ gate devices. Eliminating the contribution of the former consistently yields an Arrhenius plot that matches excellently with that obtained through direct measurement of SiO₂ gate devices. This finding shows that heavy nitridation or, in the extreme case, the adoption of Si₃N₄/SiO_x gate stack does not change the nature of the classical NBTI mechanism but introduces a new degradation mechanism of an order-of-magnitude lower activation energy, which dominates over typical operating temperature range. This new mechanism is related to the spontaneous trapping of positive charges at nitrogen-related precursor sites near the Si-SiO₂ interface.