Study of unipolar pulsed ramp and combined ramped/constant voltage stress on mos gate oxides

MOS gate oxide capacitors over a wide range of oxide thicknesses (10.9–28 nm) were stressed using a unipolar pulsed voltage ramp and combined ramped/constant voltage stress measurements. The reliability measurements were performed with several different bias conditions in order to assess the effects of the measurement conditions on times to breakdown and breakdown fields. In the first part it was verified that the unipolar pulsed ramp yields breakdown distributions which are identical to those of a widely used staircase ramp. In the second part the unipolar pulsed ramp was used for pre-stress prior to a constant stress and measurement results were compared to those of a ramped/constant stress with a staircase ramp. In several cases a ramp prior to a constant stress increases time to breakdown. The observations made in this study imply that the time to breakdown of a constant stress in the Fowler-Nordheim tunneling regime is strongly dependent on charge trapping and, therefore, on the stressing history of the oxide. Finally, it is shown that the combined ramped/constant voltage stress is a valuable tool for monitoring extrinsic and intrinsic breakdown properties when applying stress parameters in the correct way.