A typical etching profile is given in Fig. 3.12. In this case the red region is the mask (protective layer) used, blue layer is to be removed selectively from regions not protected by mask and grey layer is the substrate which should not be etched. The profile after the etching indicates that there is a horizontal etching as well etching of underlying substrate. Selectivity is the characteristic of etch whereby the desired layer is etched without damaging the underlying or masking layers. The etching system's ability to do this depends on the ratio of etch rates in the two materials. The selectivity can be quantified as:

Anisotropy is the characteristic of etch whereby the desired layer is etched in one direction only. The degree of anisotropy can be quantified as: