Device decapsulated (and/or depassivated) – Retest ok – What happened?

One of the very first steps to enter into physical device failure analysis is the device decapsulation. In some cases, an additional depassivation follows to give access to contact needles for internal probing. However, it happens from time-to-time that the device has been cured from its failure behaviour. Such cases often end as non-conclusive analysis result. Our principle investigations and results, however, will help to understand the mechanisms and allow in many “hopeless” cases to draw useful conclusions on the root causes. These are linked in most cases to metal related failures. Besides the “classical case” of touching bond wires, typical root causes are metal filament shorts in the nanometer-order-of magnitude, which can be removed easily by mechanical and/or chemical effects of any delayering procedure. Surface metal shorts may also be generated by bump metal or pad-interface-metallisation-redeposition onto the passivation surface and by metal residue-related recombinations of trimming fuses. In both latter cases, nanometer metal films short-circuit neighbouring pads or fuses in trimming-fuse-arrays. This paper describes these and some second-order mechanisms in detail, which sometimes let the chip recover after decapsulation and/or depassivation.