Automated pump-probe microscope to observe laser ablation on a picosecond scale

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.

[1]  Naoteru Matsubara,et al.  Achievement of More Than 25% Conversion Efficiency With Crystalline Silicon Heterojunction Solar Cell , 2014, IEEE Journal of Photovoltaics.

[2]  Matthias Domke,et al.  Ultra-fast movies of thin-film laser ablation , 2012 .

[3]  Xiaofeng Gu,et al.  Review of status developments of high-efficiency crystalline silicon solar cells , 2018 .

[4]  Martin A. Green,et al.  Self-consistent optical parameters of intrinsic silicon at 300 K including temperature coefficients , 2008 .

[5]  Nils-Peter Harder,et al.  Laser structuring for back junction silicon solar cells , 2007 .

[6]  A. Tünnermann,et al.  Femtosecond, picosecond and nanosecond laser ablation of solids , 1996 .

[7]  S. Peters,et al.  Mass production of p-type Cz silicon solar cells approaching average stable conversion efficiencies of 22 % , 2017 .

[8]  Klaus Sokolowski-Tinten,et al.  Transient States of Matter during Short Pulse Laser Ablation , 1998 .

[9]  Joerg Schille,et al.  High-throughput machining using a high-average power ultrashort pulse laser and high-speed polygon scanner , 2016 .

[10]  Dieter Bäuerle,et al.  Laser processing and chemistry: recent developments , 2002 .

[11]  Ralf Preu,et al.  Selective laser ablation of SiNx layers on textured surfaces for low temperature front side metallizations , 2009 .

[12]  M. Green,et al.  22.8% efficient silicon solar cell , 1989 .

[13]  Michael Schmidt,et al.  Physical mechanisms of SiNx layer structuring with ultrafast lasers by direct and confined laser ablation , 2015 .

[14]  R. Preu,et al.  Reduction of picosecond laser ablation threshold and damage via nanosecond pre-pulse for removal of dielectric layers on silicon solar cells , 2014 .

[15]  R. Preu,et al.  Impact of solidification dynamics on crystal properties of silicon molten by a nanosecond laser pulse , 2018 .

[16]  A. Aberle.  Overview on SiN surface passivation of crystalline silicon solar cells , 2001 .