Ultrafast pump-probe microscopy reveals the mechanism of selective fs laser structuring of transparent thin films for maskless micropatterning

Abstract Maskless patterning of biocompatible Ta 2 O 5 /Pt/glass sensor chips can be realized by ultra-short laser pulse ablation. At a fluence of 0.2 J/cm 2 , the thin Ta 2 O 5 film is selectively lifted-off by indirectly-induced ablation at laser wavelenghts where the Ta 2 O 5 is transparent and the Pt absorbing. This enables precise and very fast structuring. Here, 660 fs laser pulses at a center wavelength of 1053 nm are applied. The driving physical effects of this ablation mechanism are revealed by pump-probe microscopy. This technique allows the observation of the whole ablation process ranging temporally from femtoseconds to microseconds. An ultrafast heat-expansion in the absorbing Pt, initiating a shock-wave to the Ta 2 O 5 within the first 10 ps, bulges the Ta 2 O 5 film after some nanoseconds. Bulging velocities of 750 m/s are determined corresponding to an extreme acceleration of about 10 10  g. Exceeding the stress limit in the Ta 2 O 5 causes film disintegration after 50 ns. A model, describing essential reaction steps, is developed. This model is also applicable to other industrial important layer systems, where thin transparent films have to be removed.