Influence of the Burst Mode onto the Specific Removal Rate for Metals and Semiconductors

For most applications, the benefit of the burst mode can easily be explained: the energy of each pulse in an n-pulse burst is n times smaller compared to single pulses with identical average power and repetition rate. Thus, the peak fluence of each pulse is nearer the optimum value and the removal rate is therefore increased. It is generally not as high as it would be if single pulses with identical peak fluence but n times higher repetition rate could be applied. However, there are situations where the burst mode can lead to higher efficiencies, i.e., specific removal rates and a real increase in the removal rate can be obtained. For copper at 1064 nm and with a 3-pulse burst, the specific removal rate amounts to about 118% of a single pulse. For silicon, a huge increase from 1.62 to 4.92 μm3/μJ was observed by applying an 8-pulse burst. Based on calorimetric measurements on copper and silicon, the increased absorptance resulting from a rougher surface is identified as an effect which could be responsible for this increase of the specific removal rate. Thus, the burst mode is expected to be able to influence surface parameters in a way that higher efficiencies of the ablation process can be realized.