Selectivity is lifted through penetration by the precursor reaching reactive surface sites after diffusing through the inhibitor layer. Thus, the risk of penetration depends mostly on the density of the inhibitor layer and the size of the precursor. The factors governing the inhibitor density (i.e., surface coverage) have been discussed in section 3.4.2. Penetration risk is thereby affected twofold by the inhibitor density: (i) a higher density usually means more coverage of reactive surface groups (chemical passivation) and (ii) a more thorough blocking of the pathways to the surface (steric blocking).