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 combine d ramped/constant voltage stress measurements. The reliability measure ments were performed with several different bias conditions in order t o assess the effects of the measurement conditions on times to breakdo wn and breakdown fields. In the first part it was verified that the un ipolar pulsed ramp yields breakdown distributions which are identical to those of a widely used staircase ramp. In the second part the unipo lar pulsed ramp was used for pre-stress prior to a constant stress and measurement results were compared to those of a ramped/constant stres s with a staircase ramp. In several cases a ramp prior to a constant s tress increases time to breakdown. The observations made in this study imply that the time to breakdown of a constant stress in the Fowler-N ordheim tunneling regime is strongly dependent on charge trapping and, therefore, on the stressing history of the oxide. Finally, it is show n that the combined ramped/constant voltage stress is a valuable tool for monitoring extrinsic and intrinsic breakdown properties when apply ing stress parameters in the correct way. (C) 1997 Elsevier Science Lt d.