With etching of structures in silicon or etching of printed circuit boards by means of etching masks, primarily with photolithography processes, the goal usually is to make the etched structure an accurate image of the cover mask. This goal is achieved when the etching attack takes place purely anisotropic and exclusively vertical to the substrate surface. Physical or ion etching has a practically completely anisotropic effect. Chemical wet etching processes or also chemical dry etching (plasma etching), on the other hand, have an isotropic effect. With isotropic etching by means of cover masks, the etching attack proceeds from the trench not only vertical to the substrate surface, but in all other directions as well, also parallel to the substrate surface. For this reason, etching under the etching mask takes place during isotropic etching. This effect is referred to as “underetching” The term “overetching“ refers to a completely different process and is certainly not the opposite of underetching.

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