Laser induced femtosecond microstructuring of surfaces: formation mechanisms from LIPSS to Black Silicon

International audienceFemtosecond lasers allow one to functionalize surfaces at a micro and nanometer scale using a clean and dry process [1]. Femtosecond near-IR irradiation of Si wafers with fluence near melting threshold can produce three type of structures (i) resonant quasi-parallel periodic structures named "ripples", as a result of interfered laser scattered waves with roughness, inducing subwavelength periodic structures in the electron/hole plasma, then marking the material through electron-phonon collision processes, (ii) non-resonant "beads", or "grooves" as a result of melting, coalescence into larger ripple structures, droplet formation by capillary instability, then resolidification, (iii) spike-shaped structures named "Black Silicon", as a result of bead amplification by a strong absorption of the field in the valleys of the textured sample. A saturation process explaining the spike final shape is identified. The formation of these structures is studied numerically as a function of laser fluence and the number of the applied laser pulses. The corresponding formation mechanisms are discussed. The finally obtained "Black Silicon"'s high absorption properties in the visible and IR range open numerous application possibilities in microelectronics, photovoltaics, surface modification of wetting, calibration, and THz emission. [1] R. Torres et al, Jour. of Opto. and Adv. Mat. , Vol. 12, No. 3, pp. 621-625 (2010)