Study of the ablation threshold of polyimide (Kapton H) utilizing double-pulsed XeCl excimer laser radiation

Abstract

Changes in the ablation threshold of polyimide accompanying two rapid sequential pulses of 308-nm-XeCl excimer less radiation have been studied. Assuming that subthreshold radiation couples into the polymer thermally, the heat conduction equation for the system is solved in order to estimate resulting surface and subsurface temperatures. A ‘‘threshold’’ temperature for etching is suggested below which no surface modification is observed but where thermal degradation of the surface may occur. For pairs of pulses whose combined fluence is near the single pulse fluence threshold for etching, separation of the pulses in time resulted in the initiation, cessation, and reinitiation of soot deposition on the polymer surface. It is suggested that this temporally dependent preferential etching is caused by the formation of an atmosphere above the target that is rich in molecules such as CO, CN, C2H2, and C2. This atmosphere then undergoes dissociation through multiphoton and photopyrolytic processes upon receipt of the second pulse resulting in the deposition of a carbonaceous residue on the surface. The residue then acts as sites for the initiation of conical structures.