Increasing laser pulse overlap restricts picosecond laser ablation of thin metal films near ablation threshold

摘要

We present experimental evidence and a tentative explanation for a recently observed phenomenon in 532 nm picosecond laser selective thin-film ablation. By gradually increasing the laser pulse overlap in thin metal patterning at fluences just above the single-pulse ablation threshold, the ablated feature quality degrades steadily until an overlap limit beyond which ablation, surprisingly, terminates altogether. The results are attributed to a combination of (1) diminishing laser-induced thermoelastic stress in the metal film which weakens layer spallation of the progressively exposed areas due to laser spot geometrical constraints and (2) the gradual change in the layer's physical properties (optical reflectivity, film brittleness, and layer adhesion) with repetitive pulse exposure.