Dissociation Channels of c-C4F8to CF2Radical in Reactive Plasma

It has been generally assumed that octafluorocyclobutane (c-C 4 F 8 ) is mainly decomposed to CF 2 via C 2 F 4 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-C 4 F 8 is dissociated via the first triplet excited state T 1 ( 3 A 2 ), the fourth triplet excited state T 4 ( 3 2E) and the fourth singlet excited state S 4 ( 1 2E). One of the degenerate excited states of T 4 and S 4 is constituted by antibonding combination of two π bonding orbital of C 2 F 4 . T 1 state is constituted by antibonding combination of b 1u antibonding σ orbital of C 2 F 4 . Therefore, in the case of the dissociation via S 4 and T 4 excited states c-C 4 F 8 may dissociate to two C 2 F 4 , and in the case of the dissociation via T 1 excited state c-C 4 F 8 may dissociate to four CF 2 radicals. It is also found that C 3 F 5 + ion observed as the main peak in c-C 4 F 8 process plasma is produced by electron collision with the slightly larger energy than the ionization threshold value. The main dissociation path of C 2 F 4 is a vertical electron attachment. However, it is also found that dissociation pathways via 1 B 2g , 3 B 1u , and 3 B 2g excited states are very important and should not be ignored.