A method of forming fine patterns of a semiconductor device includes double etching by changing a quantity of producing polymer by-products to etch a film with different thicknesses in regions having different pattern densities. In a first etching, reactive ion etching (RIE) is performed upon a buffer layer and a hardmask layer both in a low-density pattern region and a high-density pattern region under a first etching ambient until an etch film is exposed in the low-density pattern region using mask patterns as an etch mask. In second etching for forming the hardmask patterns, using the mask patterns as an etch mask, the hardmask layer is etched until the etch film is exposed in the high-density pattern region while accumulating polymer by-products on the etch film in the low-density pattern region under a second etching ambient having polymer by-products produced greater than in the first etching ambient.