Experimental and numerical modelling of picosecond laser ablation of thin aluminium Polyethylene Terephthalate (PET) films

An experimental and theoretical study on 10 ps single pulse ablation of 10 – 50 nm aluminium films on Polyethylene Terephthalate (PET) substrate has been carried out. Both front and rear side ablation with different film thicknesses were measured experimentally and modelled numerically. It was found that the ablation threshold varied linearly with film thickness between 10 nm and 50 nm and showed a reasonable experimental and theoretical agreement. The rear side ablation threshold is always lower than the front side, with tearing at the edges. The front and rear side ablation thresholds for a 10 nm thick film were measured to be Fth(f) = 0.021 ± 0.008 Jcm−2 and Fth(r) = 0.016 ± 0.002 Jcm−2 respectively, with a ratio of Fth (f)/Fth(r) ∼ 1.3. This ratio drops to ∼ 1.1 with 50 nm films. The maximum electron temperature of the aluminium is also related to the film thickness. The highest electron temperature appears with the thinnest film, optical density = 0.5, t ∼ 10 nm, reaching Te ∼ 7,600 K under fluence F = 0.20 Jcm−2 at a delay time t = 13 ps while the thermal equilibrium is established at ∼ 30 ps. There is also a temperature gradient at the Al/PET interface due to the electron insulation and the sharp transition of the Al and PET thermal diffusivity. With a PET damage threshold, Fth PET = 3.46 Jcm−2, a clear processing window avoiding PET substrate damage was observed in single pulse exposure, and multi-beam parallel processing with the aid of a spatial light modulator was demonstrated, accelerating film patterning significantly.