Physical failure analysis (PFA)
Beginner
Opening up a failed chip and looking inside with powerful microscopes to find the actual flaw.
Novice
Finding and photographing the actual flaw in a failed chip: removing layers one at a time, cutting a cross-section through the suspect spot with a focused ion beam, and imaging it with an electron microscope.
Expert
Destructive and slow, so only a few dies get it; scan diagnosis and volume analysis decide which dies, and where to cut. Its confirmation closes the yield-learning loop.