Laser micro/nano machining of silicon

Abstract

This work investigate the formation of micro/nanostrucutred layers of silicon wafers having various conductivity types emphasizing the crucial role of laser in stimulating the etching reaction as well as controlling the silicon surface structures in photochemical and photoelectrochemical etching processes. A CW visible laser beam was used to synthesize silicon nanostructures of various morphologies in this work. It is found in photochemical etching that short laser wavelengths produce a thin porous layer compared to longer laser wavelengths which are appropriate for thicker porous layers. SEM investigation reveals the formation of different nanostructures that were synthesized by photochemical etching. Results show that optimum porosity of 70 %, 80 %, and 90 % are achievable for n, p, and p-n silicon respectively when using a current density of 25 mA/cm2in the photoelectrochemical etching process.AFM images confirm that very small nanostructure features with average size of 80 nm can be obtained for p-n silicon in the photochemical etching process. Energy band diagrams of the three etching processes; Photochemical (PC), Electrochemical (EC) and Photoelectrochemical (PEC) provide good knowledge of the photogenerated/injected holes affecting the nanostructured surface formation. Image analysis software was used to examine the microstructuredsilicon surface produced by pulsed Nd:YAG laser. The thermal distribution profile showed that the surface temperature rises to about 2000 °C when a short laser pulse of 0.6 ms and high laser energy of 1 J were used. The surface and sub-surface temperature were estimated using COMSOL multiphysics software.