Plasma Chemistry of Octafluorocyclopentene/Argon/Oxygen Mixtures

Abstract

An octafluorocyclopentene (c-C5F8)/Ar/O2 plasma chemistry mechanism has been developed using a combination of quantum chemistry methods, a zero-dimensional plasma kinetics model, and results from quadrupole mass spectrometer (QMS) and actinometry experiments on a 200 mm capacitively coupled plasma source driven at 60 MHz. Quantum chemistry methods reveal that the degradation process of c-C5F8 under electron impact is sensitive to the characteristics of the isomeric structure of the products. We find that the primary loss process for c-C5F8 as a feed gas is electron impact dissociation into isomers of C5F7 via excitation to the triplet state of c-C5F8. Electron impact dissociation of C5F7 isomers leads finally to the production of C5F5 (an isomer with two conjugate π bonds) and C5F6 (an isomer with two π bonds and a folded ring structure). Through dissociative ionization and subsequent wall recombination C5F8 produces C4F6 less with less probability than the triplet state. C4F6 in turn produces C2F4, C2F2, and C4F4 (two double bonds) and is a gas phase pathway for the production of CF2. CF2 is also produced via electron impact dissociation of an isomer of C5F6. C5F5 is not readily dissociated by electron impact because of the existence of extra π bonds that absorb electron energy in these species. The specific isomers encountered in c-C5F8 plasmas, C4F4, C5F5, and C5F6, have additional stability because of their structure. The etch precursor, atomic fluorine, is primarily produced from electron impact dissociation of the feed-gas and its degradation products. CF is produced from dissociation of CF2. CF3 is produced primarily from the walls. Because of their stability in the gas phase, C4F4, C5F6, and C5F5 are important polymer deposition species. The rate-limiting step for fluorine production is the electron impact dissociation of the triplet state of c-C5F8. Predicted etch rates are in good agreement with experimental data examining large substrate RF bias and low pressure.