Failure analysts occasionally find themselves faced with the problem of having one of the various defect isolation techniques indicating a defect location, and yet no defect is readily visible through the various imaging methods available. Many common conventional imaging tools, such as scanning electron microscopes (SEM), display images in shades of gray. The human eye is inherently more sensitive to changes in color rather than changes in grayscale. As a result, subtle variations in grayscale which could indicate the defect location can go unobserved unless a careful examination of the image intensities is performed. One useful way to highlight these variations is through intensity profiling where a line scan is drawn through the region under investigation and compared to an identical reference location contained within the same image. Subtle variations in the intensity profiles can then be identified and defect locations can be pinpointed to specific locations for later cross sectioning and final root cause determination. This paper discusses the application of this methodology to two case studies: a resistive short and a resistive open.

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.