Define selectivity and isotropy and their importance in rela

Define selectivity and isotropy, and their importance in relation to etching.

Solution

Selectivity is the characteristic of an etchant to dissolve one material and not another. It is determined by the ratios of two films to each other with a given etchant. A ratio of 1:1 would mean the etchant has no selectivity and would dissolve both film materials equally. A ratio of 100:1 means the etchant would selectively dissolve one film one hundred times faster than it will dissolve the other. This is important so that underlying layers of material are not destroyed when the present layer is being developed, and is also a key player in the resolution of lines and vertical cross-section profiles.

Isotropy is the characteristic of an etchant to want to etch every part of a film in all directions simultaneously. It can cause issues in some processes, because the mask is just on the surface of the wafer, and the etchant will dissolve down and under the mask at the same time, creating an undercut. How severely a film layer gets undercut affects the precision and detailed features possible in each layer, and limits its density