Interaction of 157-nm excimer laser radiation with fluorocarbon polymers

摘要

Two important fluoropolymers, polytetrafluoroethylene [PTFE-(C_2F_4)_N] and polyvinylidene fluoride [PVDF-(C_2H_2F_2)_N], respond to 157-nm laser radiation in dramatically different ways. At fluences sufficient to produce rapid etching, the volatile emissions from PTFE are dominated by (CF_2)_n fragments. The velocities of the fastest (CF_2)_N molecules at each mass are consistent with kinetic energies on the order of an electron volt-and change little with fluence. This fluence independence suggests that the velocities are not affected by collisions after emission. To account for the high kinetic energies and the unusual, half-monomer mass distribution, we propose that these fragments are produced by photochemical scission of the polymer backbone, and that a fraction of the excitation energy is delivered to each fragment as kinetic energy. In contrast, the principle neutral species from PVDF is HF. HF is produced by the scission of C-F bonds, followed by chemical reactions with nearby hydrogen. This process is accompanied by the conjugation of backbone C-C bonds. The photochemical cleavage of C-C bonds in PTFE and C-F bonds in PVDF is consistent with the lower C-C bond energy of PTFE.