E- and √E-model too conservative to describe low field time dependent dielectric breakdown

Extremely low field time dependent dielectric breakdown measurements were performed on single damascene structures with 90 and 50nm ½pitch integrated in a porous low-k material (k=2.5). We found with statistical significance that the E-model and the √E-model were too conservative to extrapolate our high field data to these low fields. Also, while soft breakdown does not occur at higher fields and wider spacings, we demonstrate that the time difference between soft and hard breakdown becomes significant in the 50nm ½pitch structures at normal operating fields. Besides this, we detected a change in distributional shape at these low fields and we argue that an extrinsic failure mode could be driving these failures or that the role of spacing variations across the wafer becomes more significant at lower fields.