Effects of pulse duration in laser processing of diamond-like carbon films

Experimental studies of the interaction between amorphous hydrogenated carbon (a-C:H) film and short and ultrashort laser pulses in the near-infrared and visible spectral ranges (150 ns and 1064 nm, 10 ns and 1078 nm, 300 ps and 1078 nm, 220 ps and 539 nm, 100 fs and 800 nm.) are reported. The influence of the irradiation conditions (pulse duration, wavelength, laser intensity and the number of laser shots) on the structure and thickness of the laser-induced graphitized layer has been investigated. The effects of heat dissipation on the annealing duration and on the graphitized layer thickness are discussed for the case of laser processing with short pulses. It was found that the resulting morphology of the irradiated a-C:H film surface was determined by the concurrence of three processes: change of the mass density induced by structural transformations, multiple spallations of thin layers, and material evaporation. The laser-induced spallation is asserted to be the main factor limiting the laser microprocessing reproducibility for the examined a-C:H film; its effects were found to increase dramatically for longer (150 ns) laser pulses. The ablation (evaporation) rates of the a-C:H films and glassy carbon were revealed to be similar for femtosecond and picosecond pulses, but they essentially differed for nanosecond pulses. The ablation process demonstrated the same main features for both materials: (i) increase of the ablation rate with the pulse duration, and (ii) saturation of the ablation rate with fluence for picosecond and nanosecond pulses.