The effect of pulse repetition rate on laser ablation of polyimide and polymethylmethacrylate-based polymers

The effect of pulse repetition rate on polymer ablation was studied experimentally and theoretically for polyimide and Vacrel(TM) 8230 photoresist, a polymethylmethacrylate (PMMA)-based polymer. Both the experimental data and the theoretical model showed a distinct tendency for increased repetition rate to decrease the ablation threshold, without substantially altering the absorption coefficient. This was attributed to heating of the sample sequentially by the laser pulses and the magnitude of the effect is proportional to the square of the absorption coefficient. Polyimide, with an absorption coefficient of 10(5) cm(-1), is expected to exhibit the repetition rate effect in the range of tens to hundreds of kilohertz; hence the effect has not heretofore been observed. The PMMA-based photoresist, on the other hand, exhibits the effect at a much lower repetition rate, owing to the smaller effective absorption coefficient.