Comparison of degradation modes in 1.2–2.1 nm thick SiO 2 oxides submitted to uniform and hot carrier injections in NMOSFETS

Abstract

In this work, we have studied the evolution of the electrical properties of 1.2–2.5 nm-thick SiO 2 oxides in N-channel metal–oxide–semiconductor field effect transistors submitted to either uniform electron injections in the direct tunneling regime or localized channel hot carrier injections. The degradation of the oxide is investigated through the classical electrical parameter set of the transistor, the gate leakage current, and charge pumping measurements. Constant current stresses at J inj=34 mA/cm 2 did not induce any damage. In contrast, constant voltage stresses at V G=3.5 V led to an increase up to a factor of 10 in the gate current, I G, in the [−0.4,−1 V] range, while small variations of transistor parameters were detected. Hot carrier stresses were performed in 50/1 μm transistors at the condition of maximum of substrate current for drain voltages, V D=3.5 and 4 V, with or without a substrate bias. We find an increase in the interface state density measured by charge pumping practically independent of the oxide thickness for times >100 s, which is correlated to transconductance and drain current reduction depending on substrate bias. But no correlation is found with the evolution of the gate current, which indicates that the I G increase is not directly linked here to the increase in interface state density revealed by charge pumping. An explanation of this apparent contradiction using the different energy levels and the localization of the defects induced in both cases is proposed.