Controllable periodic structures on silicon wafer by CO ₂ laser irradiation

Laser-induced periodic microstructures on the silicon substrate have been studied using a pulsed CO ₂ laser under the backside conditions. The microstructures are formed with periodic parallel fringes and circular fringes on the front surface of silicon substrate, when the backside of which is coated with a thin layer of paints or Au. The parallel fringes have a period around 2μm with fringe orientation in parallel with the laser polarization direction. The spacing of the circular fringes is less than 2μm. The fringe period and depth can be controlled by varying the laser parameters. The microstructures are independent on the laser pulses, therefore, are re-writable. The periodic structures are considered to be formed by the thermal capillary wave effect and the laser-induced periodic surface structure effect - the LIPSS effect, during the material melting and re-solidification processes. The laser absorption is assumed to be induced by the increase of free carrier concentration, and the thermal energy resulting from the laser interaction with the backside coating materials.