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

Abstract

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.