Dissociation Channels of c-C₄F₈ to CF₂ Radical in Reactive Plasma

It has been generally assumed that octafluorocyclobutane (c-C₄F₈) is mainly decomposed to CF₂ via C₂F₄ in etching process plasma. However, the detailed mechanism for the dissociations is yet ambiguous. In this paper we have calculated the probable dissociation pathways by using ab initio molecular orbital method. The results show that c-C₄F₈ is dissociated via the first triplet excited state T₁(³A₂), the fourth triplet excited state T₄(³2E) and the fourth singlet excited state S₄(¹2E). One of the degenerate excited states of T₄ and S₄ is constituted by antibonding combination of two π bonding orbital of C₂F₄. T₁ state is constituted by antibonding combination of b_1u antibonding σ orbital of C₂F₄. Therefore, in the case of the dissociation via S₄ and T₄ excited states c-C₄F₈ may dissociate to two C₂F₄, and in the case of the dissociation via T₁ excited state c-C₄F₈ may dissociate to four CF₂ radicals. It is also found that C₃F₅⁺ ion observed as the main peak in c-C₄F₈ process plasma is produced by electron collision with the slightly larger energy than the ionization threshold value. The main dissociation path of C₂F₄ is a vertical electron attachment. However, it is also found that dissociation pathways via ¹B_2g, ³B_1u, and ³B_2g excited states are very important and should not be ignored.