The deposition and the thermal annealing of the metallic thin film were carried out. The thermal annealing
of a metallic thin film enables the creation of metal nano particles by isolating them from each other
by means of the self-aggregation of the metal. Then, samples were soaked in an aqueous etching solution
of hydrofluoric acid and hydrogen peroxide. When Si etched for 20 min using the Au thin film, the reflectance
decreased to about 8%. But, when Si etched just 3 min using the Au nano particle, the reflectance
decreased below 3%; with an added 10 min etching, the reflectance decreased almost to 0% in the whole
range from 300 to 900 nm owing to the deep and steep double tapered structure.