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Automated Pump-Probe Microscope to Observe Laser Ablation on a Picosecond Scale

: Meyer, F.; Böhler, M.; Brand, A.A.; Nekarda, J.F.


Lehmann, Peter (Hrsg.) ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.; European Optical Society -EOS-:
Optical Measurement Systems for Industrial Inspection XI : 24-27 June 2019, Munich, Germany
Bellingham, WA: SPIE, 2019 (Proceedings of SPIE 11056)
ISBN: 978-1-5106-2792-5
ISBN: 978-1-5106-2791-8
Paper 110561V, 6 pp.
Conference "Optical Measurement Systems for Industrial Inspection" <11, 2019, Munich>
Conference Paper
Fraunhofer ISE ()
Photovoltaik; Silicium-Photovoltaik; Metallisierung und Strukturierung; ablation; microscopy; contact openings

Pump-probe microscopy is a well suited tool to follow the highly dynamic phenomena during laser material processing on a picosecond to microsecond scale. We present a pump-probe microscope, to monitor the phase and morphology of the silicon-dielectric interface during and after irradiation with focused high-intensity laser pulses. Laser ablation with ultrashort pulses is used to locally structure dielectric layers for contact formation on the surface of silicon solar cells. With the pump-probe microscope we can study the dynamics and the physical mechanisms of the local ablation process. A mode-locked 180 fs laser is used to illuminate the sample through a microscope objective at a wavelength of 515 nm. The same laser can be used to locally ablate the thin dielectric layer on the silicon substrate. The delay between ablation pulse (pump) and illumination pulse (probe) can be adjusted using an optical delay line with sub-picosecond precision. Alternatively other pump laser sources have been integrated using timing electronics and photodiodes, with a temporal resolution of around 1 ns. Because the system is fully automated, we can collect a large number of transient reflectance images at different points in time, which can then be combined to form a consistent video of the process. With a temporal resolution of 0.2 ps, we find a fast increase in reflectance due to melting of the surface at the beginning of the process. As the removal of the topmost layer begins, newton rings can be observed and evaluated to reconstruct the surface.