Proceedings of the
World Congress on Micro and Nano Manufacturing (WCMNM 2022 )
19–22 September 2022, Lueven, Belgium
doi:10.3850/978-981-18-5180-3_RP32-0059
Laser removal of SiNx Nanofilm on Si Substrate via Film Breakage Due to Thermal Expansion
1Department of Mechanical Engineering, Indian Institute of Technology Bombay
2Department of Electrical Engineering, Indian Institute of Technology Bombay
3National Centre for Photovoltics Research and Education, Indian Institute of Technology Bombay
ABSTRACT
Ablating an 80 nm thin SiNx layer coated silicon substrate with minimal subsurface damage is necessary for fabricating high performance Passivated Emitter and Rear Contact (PERC) solar cells. In this work, the use of a nanosecond pulsed laser is explored to ablate the SiNx layer with minimum damage to the substrate silicon. A green laser (λ=532 nm) is chosen since the SiNx layer is transparent while the silicon substrate absorbs the radiation. It is hypothesized that the silicon substrate in the laser-irradiated region undergoes thermal expansion leading to SiNx film breakage. However, to achieve this, the laser parameters have to be chosen such that the silicon substrate is heated just above the melting temperature. A 1D model of laser heating based on the finite difference method is used to estimate the temperatures and identify the appropriate laser parameters (pulse duration: 50 ns, spot diameter: 25 µm, laser fluence range: 2.44 – 3.25 J/cm2) for the experiments. Experiments were done in the vicinity of the threshold laser fluence that can melt the subsurface silicon layer. Based on the surface profiles obtained using an optical microscope, it is observed that the precise removal of the film is achieved for a very narrow range of laser parameters. It is also confirmed from the energy dispersive X-ray spectroscopy analysis that the removal is clean without any nitrogen content. Using a thermal expansion model, supported by the scanning electron microscopy results, the mechanism of removal is identified to occur due to film breakage caused by thermal expansion of the Silicon substrate. This work demonstrates that precise removal of a SiNx nanofilm on a silicon substrate is possible through an appropriate choice of laser parameters.
Keywords: Nanosecond Laser, Laser Ablation, Passivation Layer, Silicon Nitride, Perc Solar cell.
1Department of Mechanical Engineering, Indian Institute of Technology Bombay
2Department of Electrical Engineering, Indian Institute of Technology Bombay
3National Centre for Photovoltics Research and Education, Indian Institute of Technology Bombay
ABSTRACT
Ablating an 80 nm thin SiNx layer coated silicon substrate with minimal subsurface damage is necessary for fabricating high performance Passivated Emitter and Rear Contact (PERC) solar cells. In this work, the use of a nanosecond pulsed laser is explored to ablate the SiNx layer with minimum damage to the substrate silicon. A green laser (λ=532 nm) is chosen since the SiNx layer is transparent while the silicon substrate absorbs the radiation. It is hypothesized that the silicon substrate in the laser-irradiated region undergoes thermal expansion leading to SiNx film breakage. However, to achieve this, the laser parameters have to be chosen such that the silicon substrate is heated just above the melting temperature. A 1D model of laser heating based on the finite difference method is used to estimate the temperatures and identify the appropriate laser parameters (pulse duration: 50 ns, spot diameter: 25 µm, laser fluence range: 2.44 – 3.25 J/cm2) for the experiments. Experiments were done in the vicinity of the threshold laser fluence that can melt the subsurface silicon layer. Based on the surface profiles obtained using an optical microscope, it is observed that the precise removal of the film is achieved for a very narrow range of laser parameters. It is also confirmed from the energy dispersive X-ray spectroscopy analysis that the removal is clean without any nitrogen content. Using a thermal expansion model, supported by the scanning electron microscopy results, the mechanism of removal is identified to occur due to film breakage caused by thermal expansion of the Silicon substrate. This work demonstrates that precise removal of a SiNx nanofilm on a silicon substrate is possible through an appropriate choice of laser parameters.
Keywords: Nanosecond Laser, Laser Ablation, Passivation Layer, Silicon Nitride, Perc Solar cell.
