Time and space resolved microscopy of induced ablation with ultra-short laser pulses

Laser lift-off processes have been observed during structuring CIS thin film solar cells with ultra-short laser pulses, if a Mo film on glass is irradiated from the glass substrate side. To investigate the underlying physical effects, ultrafast pump-probe microscopy is used for time-and space resolved investigations. The setup utilizes a 660 fs-laser pulse at a wavelength of 1053 nm that is split up into a pump and a probe pulse. The pump pulse ablates the thin film, while the frequency doubled probe pulse illuminates the ablation area after an optically defined delay time of up to 4 ns. For longer delay times, a second electronically triggered 600 ps-laser is used for probing. Thus, the complete ultra fast pulse initiated ablation process can be observed in a delay time range from femtoseconds to microseconds. First experiments on the directly induced ablation of molybdenum films from the glass substrate side show that mechanical deformation is initiated at about 400 ps after the impact of the pump laser pulse. The deformation continues until approximately 15 ns, then a Mo disk shears and lifts-off with a velocity of above 70 m/s free from thermal effects.