THERMAL AND MECHANICAL COUPLING BETWEEN SUCCESSIVE PULSES IN KRF-EXCIMER-LASER ABLATION OF POLYIMIDE

Multiple-shot effects in laser processing and ablation of polyimide ar e examined and are found to be the dominant phenomena for processes in volving several hundreds or thousands of pulses. For fluences less tha n 260 mJ/cm(2), it was found that it is impossible to cut through 75 m u m polyimide foils for an arbitrarily large number of excimer pulses even though this fluence is more than ten times the single-shot ablati on threshold. The halt in etching is due to the formation, over a numb er of shots, of a robust carbon matrix with a deep surface roughness w hich is also responsible for laser-induced electrical conductivity. Th e effect of thermal coupling between successive shots is shown to be a dominant factor in determining the electrical properties of the carbo n layer. Differences in electrical conductivity of up to 12 orders of magnitude were found for only small differences in repetition rate. Tr ansmission electron microscopy revealed the changes in microstructure responsible for the dramatic differences in electrical properties.