Abstract

We have studied changes in the ablation threshold of polyimide by combining two subthreshold XeCl excimer laser pulses with a set time delay. Initially overlapping, the pulse pair had a combined fluence above the ablation threshold for the polymer. When the two pulses were separated in time, cessation of etching occurred in a fashion that was more intensity-dependent than fluence-dependent. For pairs of pulses whose combined fluence was near the fluence threshold quoted in the literature, separation of the pulses resulted in an unusual initiation, cessation, and reinitiation of etching of the polymer surface. In the case of the reinitiated etching, we observed a large deposition of carboneous residue about debris particles on the surface. Based on a thermal model, the appearance of the large amount of carboneous residue coincided with a heat pulse generated by the first laser pulse that was above the thermal-degradation temperature of the polyimide. We think that this temporally dependent preferential etching is caused by an atmosphere of thermal-degradation products above the target surface that photo-dissociates upon receipt of the second pulse. A comparison of subthreshold fluence etching with etch rates generated by pulses with fluences above the threshold will also be presented.

© 1990 Optical Society of America