Selective removal of silicon nitride passivation layers is of major importance in failure analysis of semiconductor devices. Typical applications are: cleaning of the die surface for optical microsccjpy and for removal of superficial contamination, electron microscopy, liquid crystals, voltage contrast, electron beam testing, mechanical microprobing, and . selective layer-by-layer strip. A new wet-etch for silicon nitride passivation layers has been developed, which is fully selective! over aluminum metallization and which preserves full device functionality after passivation removal. For the first time in the failure analysis literature, the chemical recipe and the etching procedure are given in details. This etchant has been experimented for more than two years in many failure analysis laboratories on a wide spectrum of discrete and integrated semiconductor devices, always with excellent results. Its capability and efficiency are illustrated by two failure analysis case histories.