A method of fabricating a semiconductor integrated circuit (IC) is disclosed. A first layer is deposited over a substrate. A plurality of mandrels is formed over the first layer. Guiding-spacers are formed along sidewalls of the mandrels. Then the mandrels are removed. A neutral layer (NL) and a block copolymer (BCP) layer are deposited over the first layer and the guiding-spacers. A anneal is applied to the BCP layer to form a first polymer nanostructure between the guiding-spacers and being surrounded by a second polymer nanostructure. The first polymer nanostructures locate at a same distance from the first layer. Polymer nano-blocks are formed by selectively etching the second polymer nanostructure and the NL. By using the polymer nano-blocks and the guiding spacer as etch masks, the first layer is etched to form openings. The substrate is etched through the openings to form substrate trench and substrate fin.