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.

When etching structures by using masks for example by fotolithographic processes it is most desired to etch an exact copy of the mask structure. This can be achieved by ion etching ahich is completely anisotropic and etches only in direction of the ion impact.

However ion etching is quite a slow process.

Any other etching process such a etching by liquid solvents, chemical dry etching but also RIE which is not completely anisotropic causes undercutting of the mask.