Reaction of Fluorocarbon Species with Si and SiO$_{2}$ Surfaces

Highly-selective high-aspect-ratio etching of SiO$_{2}$/Si is an indispensable key issue in the ULSI manufacturing processes. Furthermore, recent etching technology utilizes high density plasmas and requires complex fluorocarbon molecules such as C$_{4}$F$_{6}$ or C$_{5}$F$_{8}$ to achieve high etching speed and high etching selectivity. To improve etching performance, precise control of fluorocarbon plasmas based on deep understanding of radical reactions on SiO$_{2}$ and Si surfaces is required. Well-defined beam experiments in ultra-high vacuum are powerful for basic study of surface reactions. This paper shows elementary surface processes of fluorocarbon etching process, especially focused on the unique chemical reactivity of C$_{5}$F$_{8}$ molecule under co-incidence of Ar ion. The device was specially designed so as to enable \textit{in situ} measurements of etching yield and etched surfaces. Namely, Ar$^{+}$ beam at energies from 50 to 400 eV and various kinds of fluorocarbon neutral species (C$_{5}$F$_{8}$, C$_{4}$F$_{8}$, CF$_{2})$ are co-incident on a clean SiO$_{2}$ surface at a controlled flux. Etching yield of beam-incident surface is measured by profilometer while \textit{in-vacuuo} X-ray photoelectron spectroscopy (XPS) analysis reveals a time evolution of atomic composition of surface layer during the etching. In the case of C$_{4}$F$_{8}$/Ar$^{+}$, surface atomic composition after SiO$_{2}$ etching was almost similar to that of pure Ar$^{+}$ sputtering except for a small amount of F component. In the case of C$_{5}$F$_{8}$/Ar$^{+}$, however, formation of fluorocarbon layer after SiO$_{2}$ removal was observed as in the case of CF$_{2}$/Ar$^{+}$. The SiO$_{2}$ etching yield monotonically increased with the Ar$^{+}$ incident energy above 400 eV, and the etching yield of 2.4 was obtained at an Ar$^{+}$ incident energy of 900 eV with C$_{5}$F$_{8}$ co-incidence, which was about 3 and 1.5 times larger compared with pure Ar$^{+}$ sputtering and CF$_{2}$/Ar$^{+}$ co-incidence, respectively. These results suggest that fluorocarbon molecules themselves are important species in fluorocarbon etching plasma.