Methods of preventing photoresist scum formation for etch processes for patterning material layers of semiconductor device material layers are disclosed. A treatment of N2 and O2 is used to prevent the formation of photoresist scum. The treatment may be performed in-situ, and may be performed during the etch process, after the etch process, or both. The treatment is particularly beneficial when implemented during the patterning of low dielectric constant material layers, and when used for the formation of isolated via patterns.