Study of $C4F8/N2$ and $C4F8/Ar/N2$ plasmas for highly selective organosilicate glass etching over $Si3N4$ and SiC

We report the effect of $N2$ addition to $C4F8$ and $C4F8/Ar$ discharges on plasma etching rates of organosilicate glass (OSG) and etch stop layer materials $(Si3N4$ and SiC), and the results of surface chemistry studies performed in parallel. $N2$ addition exhibits different effects in $C4F8$ and $C4F8/Ar$ plasmas, which may be explained by a higher plasma density, electron temperature, and possibly, the presence of argon metastable species in the $C4F8/Ar$ plasma, all of which serve to dissociate $N2$ more effectively. When $N2$ is added to a $C4F8/Ar$ plasma, a reduction of the steady-state fluorocarbon surface layer thickness, one of the key parameters that controls the etching rate and etching selectivity on partially etched samples, is observed. This effect leads to a loss of etching selectivity for $C4F8/Ar/N2$ discharges. Adding $N2$ to $C4F8$ plasmas without Ar enhances the steady-state fluorocarbon layer thickness. X-ray photoelectron spectroscopy analysis shows, in this case, that there is an important change in the stoichiometry of either passively deposited films or the fluorination reaction layers formed on etching samples: A significant amount of nitrogen is incorporated in the fluorocarbon film for deposited films, which implies that $CxNy$ needs to be removed to achieve an etching condition. The incorporation of nitrogen in fluorocarbon films could reduce the etchant supply for $Si3N4,$ or OSG, from the gas phase, especially for $C4F8/Ar/N2$ plasmas, but not for SiC owing to the differences of the chemical compositions. $SiO2$ and Si are also studied for comparison materials. The etching behavior of $SiO2$ is similar to that of OSG and $Si3N4,$ while Si behaves more similar to SiC during fluorocarbon etching. In addition, a comparison of $N2$ and $O2$ addition to $C4F8$ or $C4F8/Ar$ plasma in terms of consequences on etching behavior of the aforementioned materials is presented.