FREQUENCY DEPENDENCE OF DEGRADATION AND BREAKDOWN OF THIN SiO_2 FILMS

摘要

Thin oxide MOS capacitors are subjected to bipolar voltage stresses of different amplitudes and frequencies. According to a previously proposed breakdown model, the evolution of the current with the stress time has been considered to be due to the degradation of the oxide, i.e. to the generation and partial occupation of electron traps. When log(J) is represented versus the stress time, the slope of the plot and the magnitude of the current (which tends to decrease during constant-voltage tests) are taken as indicators of the oxide degradation rate and degradation level, respectively. Our results suggest lower degradation rates, and consequently lower degradation levels for the same stress times, at high frequencies. This is consistent with the increase of time-to-breakdown with stress frequency observed by other authors, and confirms that, also for dynamic stresses, the relation between degradation and breakdown is fundamental to understand the physics of these phenomena. The slower degradation rates confirm the improvement of oxide reliability under dynamic AC stress conditions.