Experimental study of the quasi-breakdown failure mechanism in 4.5 nm-thick 8102 oxides

In this study, we have investigated the electrical properties of the failure mode referred as quasi-breakdown or soft-breakdown in MOS capacitors on p-type substrate with an oxide thickness of 4.5 nm. Quasi-breakdown appears during high field stresses as a sudden increase between two and four orders of magnitude in the gate current over the whole gate voltage range, but remains undetected in C(V) characteristics between 20 Hz and 100 kHz. Quasibreakdown was systematically triggered during negative gate voltage stresses after a threshold between 10 and 15 C/ cnr was reached in the injected charge, this threshold being independent of the stressing oxide field. A very weak temperature dependence and a low frequency noise in the gate current were also observed. The I(V) characteristics are found to follow a first-order exponential law versus the gate voltage, indicative of a direct tunneling process, which could result from a local lowering of the oxide thickness resulting from a sudden metal/isolant transition in a localized region of the oxide near the anode due to oxide defects.