A Tetra-Methyl-Ammonium-Hydroxide (TMAH) aqueous solution etches crystalline silicon without removing the damaged silicon and sapphire. Crystalline silicon films with 600 nm thickness on sapphire were irradiated with Si2+ Focused-Ion-Beams (FIB), and amorphous fine patterns were formed on the surfaces of the silicon films. Crystalline silicon regions were selectively etched off with the TMAH solution without removing the amorphous patterns acting as the etching mask, and silicon fine structures with the maximum feature size of 800 nm were formed. The feature size depends on the film thickness and can be optimized by the FIB irradiation and etching conditions. The technique is expected to be utilized in the nano-electronic device processing.