The Reversed Intrinsic Curve and Voltge Dependence for Ultra-Low k Dielectrics

The reversed intrinsic phenomenon/curve and voltage dependence of ultra-low k IMD (Inter-Metal Dielectric) were investigated by experiments with large sample sizes using high and low voltage stresses for both 40nm and 28nm ultra-low k dielectrics. Using large sample sizes, we observed that the reversed intrinsic curve only exists for the stresses at high voltages, but not for the low voltage stress tests. This phenomenon should be caused by the difference in percolation paths at high and low electric fields. At low voltage stresses, the dielectric breakdown is dominated by the accumulative percolation path as what in the traditional non-low k dielectrics. On the other hand, at high voltage stresses, the geometric IMD spacing-dominate breakdowns happen and it leads to the breakdowns showing the "reversed intrinsic curve" (which the earlier portion of the fitting curve with a larger slope). We propose that the reversed intrinsic curve does not represent a true failure mechanism at the field operation conditions. From our experiments, we also found, in the tests with small sample sizes at high voltages, we cannot see a clear reversed intrinsic curve, which will result in bi-modal distributions with small R squares (coefficient of determination). The low voltage PLR (Package-Level Reliability) test will be a better solution to assess ultra-low k dielectric reliability even at a small sample size.