Octafluorocycropenten (C₅F₈) is expected as one of the potential plasma sources which can meet the demands of the future ULSI manufacturing in two ways: higher etching selectivity of SiO₂ to Si₃N₄[1] and higher deposition rate of fluorocarbon film on Si and Si₃N₄ surfaces[2]. The molecule has shorter atmospheric lifetime (one year) than other perfluorocarbon molecules (CF₄, for example, has 50,000 years) and is expected far more friendly to the global environment than other popular etching gases. It, however, is rather recently industry manufactured and its electron collision cross section data and the related electron transport data are scarce.
We measured electron transport parameters, the electron drift velocity (W) and the product of the gas number density and the longitudinal diffusion coefficient (ND_L) in pure C₅F₈ and C₅F₈-Ar mixtures. We used the double-shutter (four-gauze) drift tube, whose electron drift distance was variable from 1 to 10 cm. The electron arrival time spectra (ATSs) were observed at several drift distances, and the drift velocity and the longitudinal diffusion coefficient were determined from the positional variations of the mean arrival time and the characteristic width of the ATSs. From this the hydrodynamic equilibrium of the parameters were confirmed. Measurement was carried out over the E/N (where E is the applied electric field and N the gas number density and 10^-17 Vcm² = 1 Td) range from 50 to 4000 Td in pure C₅F₈ and from 5 to 100 Td in C₅F₈-Ar mixtures.
The ATSs observed in pure C₅F₈ usually showed a dim component of delayed electrons following immediately after the main electrons. The more precise analysis including electron detachment may be needed, and that may alter the present ND_L. The present W and ND_L had a broad minimum (2.0x10⁷ cm/s and 1.0x10²² cm^-1s^-1, respectively) at 150-300 Td. The total electron cross section of C₅F₈ over the electron energy range of 5-3000 eV is about 50% larger than c-C₄F₈ which also has a cyclic structure but with a single bond[3]. The present drift velocity, however, was higher than that in pure c-C₄F₈, and the latter increased rather monotonously with increasing E/N[4].

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