Laser ablation of thin molybdenum films on transparent substrates at low fluences

The selective structuring of thin molybdenum (Mo) films is a major challenge for the monolithic interconnection of CIS thin film solar cells during their production. Here we present the structuring of ca. 0.5 μm thin molybdenum films on glass substrates with picosecond laser pulses (pulse duration 10 ps, wavelength 1064 nm) without any visible thermal effect on both, the remaining film and the substrate material. When the molybdenum film is irradiated from the transparent substrate side with a fluence level below 1 J/cm 2 a “lift-off” process is initiated, which seems to be induced by a direct effect in the removed molybdenum film. At that fluence level, the energy input per ablated volume of ca. 30 J/mm 3 is much less than would be needed for a thermodynamic heating, melting and vaporization of the complete film with ca. 78 J/mm 3 . Therefore we conclude that the molybdenum is only evaporated partially. Parts of the ablated Mo-film can be found as structurally intact debris. We assume that partial melting and vaporization with high-pressure formation play an important role during that picosecond laser ablation without thermal side effects. Due to its remarkable physical nature we called that process “directly induced laser ablation”.