This paper describes a case study of a back end of the line low-k time dependent dielectric breakdown failure analysis. Due to the extremely large size of the stressed test structure, isolation of the defect and root cause determination can be quite difficult. In this particular study the defect was determined to lie in an approximately 100 um2 area and top down SEM inspection did not indicate any obvious defect. In an effort to further isolate the defect, an image comparison analysis was performed to highlight the differences between the fail area and an assumed good area of the test structure. A local area within the failing region was identified and imaged in cross section via TEM. The source of contamination which caused the fail was identified and appropriate process actions were implemented to remove the defect mechanism.