Degradation in (Ba,Sr)TiO3 thin films under DC and dynamic stress conditions

We investigated the electric degradation and the dielectric breakdown of 30-nm-thick (Ba,Sr)TiO₃ films under DC and bipolar stress conditions, to examine the films as a dielectric in DRAM cell capacitors. The degradation and breakdown properties were found to strongly depend on the stress type, DC or bipolar. The leakage properties for films under a small bipolar stress are more degraded than that under DC stress of the same field strength. It is considered that the degradation was caused by the deterioration of Schottky-barrier, and, under bipolar stress conditions, the dielectric relaxation current in the dielectric films probably enhances the deterioration. The breakdown time was found to be approximated by an exponential function of a reciprocal of an electric field [t_B=α exp(β/E)] for both DC and bipolar stresses. The value of the exponential factor β for bipolar stress was about a half of that of DC stress. The small value of β observed under bipolar pulse stress conditions indicates that the type of breakdown through dynamic stress should be dominant in practical device operation. This study shows that, with stoichiometric BST thin films with a equivalent thickness of 0.50 nm at 85°C, the breakdown time under an applied voltage of 1 V was expected to be more than ten years. The effect of composition ratio of Ba, Sr and Ti cations on the dielectric properties and the breakdown were also studied. It was found that the dielectric constant achieved a maximum value of 230 at 85°C for a stoichiometric film, and the breakdown time was shortened with the increase in the ratios of Ba and Sr cations. Based on these results, the degradation and breakdown mechanisms will be discussed under bipolar pulse stress conditions