The reliability of Cu/low-k structures after a methylating treatment has been studied. The lifetime increases for structures that have undergone the treatment and the effective k is also reduced. The lifetime improvement is believed to be due to the reduction of the defect density at the trench sidewalls. A simple percolation model shows that if there are existing defects at the trench interface, then percolation requires fewer additional defects. A reliability criterion of 1.5 C/cm(2) shows that the methylating treatment decreases the defect density by a factor of 5. The energy density of the capacitor, just before failure, was calculated from the current values just before failure and the values are comparable with adhesion energies of known hard mask materials. The final failure occurs at the top horizontal interface with delamination and copper penetration. (c) 2005 American Vacuum Society.