Low electric field breakdown of thin SiO/sub 2/ films under static and dynamic stress

Abstract

A comprehensive study of Time-Dependent Dielectric Breakdown (TDDB) of 6.5-, 9-, 15-, and 22-nm SiO/sub 2/ films under dc and pulsed bias has been conducted over a wide range of electric fields and temperatures. Very high temperatures were used at the wafer level to accelerate breakdown so tests could be conducted at electric fields as low as 4.5 MV/cm. New observations are reported for TDDB that suggest a consistent electric field and temperature dependence for intrinsic breakdown and a changing breakdown mechanism as a function of electric field. The results show that the logarithm of the median-test-time-to failure, log (t/sub 50/), is described by a linear electric field dependence with a field acceleration parameter that is not dependent on temperature. It has a value of approximately 1 decade/MV/cm for the range of oxide thicknesses studied and shows a slight decreasing trend with decreasing oxide thickness. The thermal activation E/sub a/ ranged between 0.7 and 0.95 eV for electric fields below 9.0 MV/cm for all oxide thicknesses. TDDB tests conducted under pulsed bias indicate that increased dielectric lifetime is observed under unipolar and bipolar pulsed stress conditions, but diminishes as the stress electric field and oxide thickness are reduced. This observation provides new evidence that low electric field aging and breakdown is not dominated by charge generation and trapping.