Surface Reaction Processes of Fluorocarbon Molecule and Fundamental Research on New Etching Process

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SiO_2/Si selective etching by fluorocarbon plasma is one of fundamental processes for semiconductor fabrication. So far, it is considered that the fluorocarbon molecules do not contribute on the surface reactions. However, C_5F_8 or other fluorocarbon gases are now in use in the etching process due to recent research and development on new fluorocarbon gases. C_5F_8 has rather complicated molecular structure such as double bond in the molecular structure. We have started the study of fluorocarbon molecule contribution to the etching surface reactions, motivated by such interesting chemical structure of newly developed molecules.Firstly, we investigated the surface reactivity of C_5F_8 in the cases of damaged and passivated SiO_2 surfaces, and it was found that the C_5F_8 can be reactive only in the case of damaged SiO_2 surface by Ar^+ bombardment. This is presumably due to the reaction of surface dangling bond with the double bond in the C_5F_8 molecule. Secondly, we Ar^+ and C_5F_8 were simultaneously directed to the SiO_2 surface, and it was found that the C_5F_8 has very high etching reactivity compared with other fluorocarbon molecules such as CF_4, or C_4F_8, and that the etching yield was almost three times higher than that of physical sputtering of SiO_2. Furthermore, "etch stop" of SiO_2 was also found in the case of higher C_5F_8/Ar^+ flux ratios. From the XPS measurement, it was confirmed that the thick fluorocarbon layer influenced the decrease in the etching yield. Thirdly, ability of SiO_2/Si selective etching by Ar^+/C_5F_8 co-incidence was found by comparing the etching yields for SiO_2 and Si at various Ar^+ energies. Finally, contribution of C_5F_8 molecule to the etching rate in real semiconductor fabrication process was evaluated based on the etching yields obtained by our research. It was found that the maximum contribution of C_5F_8 to the etching rate is 〜 30%.