Quantitative Study of Charge-to-Breakdown of Thin Gate Oxide for a p-Poly-Si Metal Oxide Semiconductor Capacitor

The charge-to-breakdown (Qbd) for p-poly-Si MOS capacitors under positive and negative gate-bias stress was investigated. Among the various boron-implanted poly-Si samples, Qbd(+) increases with dopant concentration, butQbd(—) decreases with the boron concentration. Meanwhile a large difference was found between the Qbd(+) and Qbd(—) values. Evidence for various degree of band bending of poly-Si was observed from C-V and Fowler-Nordheim tunneling measurements. From gate-voltage shift (V8) data after constant current stress, the centroid of the generated positive trapped charge can be determined. We modified the charge-trapping model to explain the above Qbd behavior. Hole trap-ping is the cause of oxide breakdown. The observed difference between gate-positive and gate-negative Qbd is due to a polarity-dependent critical trapped charge density which depends on the critical electrical field somehow related to the boron implantation. As the generated positive trapped charge reaches a critical value, part of the localized electric field near the anode disappears and the remaining part of the electric field (E) is enhanced. This critical F field triggers ther-mal runaway and oxide breakdown. Therefore, we determine that the amount of Q5 is related to the boron implantation