In the burst mode the reported removal rates were often higher than the ones achieved with single pulses at identical repetition rate and average power. But this effect is mainly caused by the reduced energy per single pulse in the burst and the corresponding fluence which is then nearer its optimum value showing highest specific removal rate. But there exist special situations where the burst mode shows a higher efficiency and therefore an increased specific removal rate. For copper e.g. it was found that a 3-pulse burst with a time spacing of 12 ns at a wavelength of 1064 nm leads to an about 15% higher specific removal rate.
We extended the burst investigations to semiconductors and isolators and measured the specific removal rate as a function of the applied peak fluence for different materials, number of pulses in the burst and time spacing. For 1064 nm silicon e.g. shows a maximum specific removal rate which amounts about 1.7 µm3/µJ for single pulses and a 2 pulse burst as well. Then it almost linearly increases up to about 5 µm3/µJ when the number of pulses in the burst is raised to 8. A similar effect is found for machining grooves into diamond-like nanocomposite films with single pulses and a 2- and 3-pulse burst, respectively. In contrast, for silicon and 532 nm wavelength where the photon energy exceeds the bandgap, only a small difference in the maximum specific removal was observed.
Heat accumulation is assumed to cause the higher specific removal rates but further experiments are needed gain a clearer picture.
Beat Neuenschwander, Beat Jaeggi, Stefan M. Remund, and Sergei M. Pimenov, "Machining of semiconductors and dielectrics with ultra-short pulses: Influence of the wavelength and pulse bursts (Conference Presentation)," Proc. SPIE 10519, Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXIII, 1051904 (Presented at SPIE LASE: January 29, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291909.5751440559001.
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