1. Etch Rate:

   Etch rate defines how fast is the etching processi.e how fast the material is etched (removed) during etching process. Mathematically rpresente as

$$\text{Each rate} = \frac{\text{Thickness before etching - Thickness after etching}}{\text{Etch time}}$$

1. Selectivity:

   Ability of the etched to have very high etching sale for the selected material while leaving the underlying material unharmed. Fig. a and fig. b depicts highly selective etch and poor selective etch respectively.

$$\text{Selectivity} = \frac{\text{Etch rate of wp material to be attached}}{\text{Etch rate of underlying material}}$$

1. Isotropy and Anisotropy

   If the etchant removes the material equally in all directions then it in called isotropic etching (horizontal and vertical direction) and slow in lateral direction then it is called anistropic etching.

   A perfectly anistropic etch produces vertical side walls / zero lateral direction etch rate. Below fig. a and fig. b shows isotropic etching and anistropic etching respectively.