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30 January 2012 Mechanism of selective removal of transparent layers on semiconductors using ultrashort laser pulses
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The process of ultrashort laser-assisted selective removal of thin dielectric layers from silicon substrates has a large potential for technological applications, the most straightforward one being an energy-efficient and environmentally compatible method to produce contact openings on crystalline silicon solar cells. Using photon energies above the band gap energy, ablation of such thin transparent layers is possible without noticeable damage of the silicon substrate. To understand in detail the physics behind this damage-free delamination, experiments with a variety of laser parameters were done, utilizing in particular wavelengths from UV to mid-infrared and pulse durations between 50 and 2000 fs. Experiments were also conducted using different transparent materials on silicon, e.g. SiO2 and SixNy. The ablated regions were carefully analyzed by light microscopy (LM), atomic force microscopy (AFM), Raman spectroscopy (RS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The results give evidence that the mechanism of damage-free ablation is initiated by ultrafast creation of electron-hole plasma by the ultrashort laser pulse itself followed by non-thermal decomposition of an ultrathin Si layer of a few nm thickness only. This process works best in the region of moderate substrate absorption, i.e. using laser photon energies only slightly above the band gap, and for the shortest pulses. In contrast, laser energy input into the dielectric layer by addressing either the UV absorption or a vibrational resonance (e.g. at λ = 9.26 μm for SiO2) allowed ablation only in connection with partial damage of the substrate.
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Tino Rublack, Stefan Hartnauer, Michael Mergner, Markus Muchow, Martin Schade, Hartmut S. Leipner, and Gerhard Seifert "Mechanism of selective removal of transparent layers on semiconductors using ultrashort laser pulses", Proc. SPIE 8247, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XII, 82470Z (30 January 2012);

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