The etch gas composition DOE discussed in 3.1.2 and the Ohnishi plots in Figure 5 show that the C-N ring structure etches slower in O2 and faster in CF4 and N2/H2 than it should. Based on polymer decomposition theory, we know the C-N ring can form a stable structure and performs like aromatic carbon ring in O2. In this case, nitrogen has negative contribution to etch, which should be considered as the same function of carbon. The appropriate etch rate equation is