Implications of Negative Bias Temperature Instability in Power MOS Transistors

As the device dimensions in metal-oxide-silicon (MOS) technologies have been continuously scaled down, a phenomenon called negative bias temperature instability (NBTI), which refers to the generation of positive oxide charge and interface traps in MOS structures under negative gate bias at elevated temperature, has been gaining in importance as one of the most critical mechanisms of MOS field effect transistor (MOSFET) degradation. NBTI effects are manifested as the changes in device threshold voltage (VT), transconductance (gm) and drain current (ID), and have been observed mostly in p-channel MOSFETs operated under negative gate oxide fields in the range 2 6 MV/cm at temperatures around 100°C or higher (Huard et al., 2006; Stathis & Zafar, 2006; Schroder, 2005; Alam & Mahapatra, 2005; Schroder & Babcock, 2003; Kimizuka et al., 1999; Ogawa et al., 1995). The phenomenon itself had been known for many years, but only recently has been recognised as a serious reliability issue in state-of-the-art MOS integrated circuits. Several factors associated with device scaling have been found to enhance NBTI: i) operating voltages have not been reduced as aggressively as gate oxide thickness, leading to higher oxide electric fields and increased chip temperatures; ii) threshold voltage scaling has not kept pace with operating voltage, resulting in larger degradation of drain current for the same shift in threshold voltage; and iii) addition of nitrogen during the oxidation process has helped to reduce the thin gate oxide leakage, but the side effect was to increase NBTI (Stathis & Zafar, 2006). Considering the effects of NBTI related degradation on device electrical parameters, NBT stress-induced threshold voltage shift (ΔVT) seems to be the most critical one, and a couple of basic questions, which are to be addressed now, are why the NBTI appears to be of great concern only in p-channel devices, and why the negative bias causes more considerable degradation than positive bias. The bias temperature stress-induced VT shifts are generally known to be the consequence of underlying buildup of interface traps and oxide-trapped charge due to stress-initiated electrochemical processes involving oxide and interface defects, holes and/or electrons, and variety of species associated with presence of hydrogen as the most common impurity in MOS devices (see e.g. (Schroder & Babcock, 2003)). An interface trap is an interfacial trivalent silicon atom with an unsaturated (unpaired) valence electron at the SiO2/Si interface. Unsaturated Si atoms are additionally found in SiO2 itself, along with other oxide defects, the most important being the oxygen vacancies. Both oxygen