The role of laser pulse overlap in ultrafast thin film structuring applications

In this work, we demonstrate the importance of laser pulse overlap in controlling the ablation process of a thin film just above the single pulse material ablation threshold. A 532 nm, 15 ps ultrafast diode pump solid-state (DPSS) laser has been employed to pattern a gold alloy source–drain layer during the production of organic thin film transistor (OTFT) for flexible display applications. Maintaining the laser fluence constantly above and in proximity of the ablation threshold value, different process windows are identified by varying the pulse overlap. With less than four overlapping pulses per area, a debris-free removal process is achieved with almost no edge delamination. By increasing the number of pulses per area up to 20, edge delamination progressively grows stronger and eventually ablation terminates altogether. If the pulse overlap increases further, ablation starts again but with more evidence for thermal, cumulative irradiation effects. This behavior can be attributed to the progressive transition from initially a photomechanical stress-assisted ablation process, dominant at low pulse overlap to increasingly a more thermally driven removal process, at higher pulse overlaps.