Chasing ghosts: How an SRAM detected the subtle impact of stray light

The ever-shrinking world of semiconductors has always challenged the interplay of tool capability, process integration, and characterization. The fine line between structural or electrical success and failure has steadily been redefined from microns to nanometers with leading edge technology using terms with the likes of angstroms and layers of atoms. Failure modes at these nodes become increasing difficult at times to even grasp, let alone “visualize” with electrical and physical analytical techniques. The complexity of these state-of-the-art problem sets demand adaptive and robust problem solving methodologies tailored to uncover and drive root cause understanding. Assembling and organizing diverse teams covering a broad range of expertise becomes paramount. In this case study, we were able to make use of a unique low voltage static random access memory fail mode and spatial fingerprint to chase down a 1 nanometer change in transistor gate line widths; the sleuthing effort concluded with the unlikely combination of mask chrome frame and stray light from lithography as the ultimate root cause.