Formation mechanism of subtrenches on cone-shaped patterned sapphire substrate

In this paper, the cone-shaped patterned sapphire substrates (PSS) were etched by an inductively couple plasma with BCl3 as the reacting gas. The influence of the operating pressure and the RF bias power on subtrenches of the cone-shaped PSS and the formation mechanism of subtrenches were investigated. The profiles of patterns were characterized by FESEM (field emission scanning electron microscope). It showed that the subtrench size varied with the operating pressure and the RF bias power. As the operating pressure increased from 0.2 Pa to 0.9 Pa, the subtrenches changed from narrow and deep to wide and shallow; then to narrower and shallower. When the RF bias power varied from 200 W to 600 W, the subtrenches gradually became noticeable. The FESEM results also indicated that the subtrenches were formed due to the ion scattering effect which was caused by tapered sidewalls and charges accumulation. It is discovered that the scattering effect is closely related with the operating pressure and RF bias power.