Disclosed is an integrated circuit with multiple semiconductor fins having different widths and variable spacing on the same substrate. The method of forming the circuit incorporates a sidewall image transfer process using different types of mandrels. Fin thickness and fin-to-fin spacing are controlled by an oxidation process used to form oxide sidewalls on the mandrels, and more particularly, by the processing time and the use of intrinsic, oxidation-enhancing and/or oxidation-inhibiting mandrels. Fin thickness is also controlled by using sidewalls spacers combined with or instead of the oxide sidewalls. Specifically, images of the oxide sidewalls alone, images of sidewall spacers alone, and/or combined images of sidewall spacers and oxide sidewalls are transferred into a semiconductor layer to form the fins. The fins with different thicknesses and variable spacing can be used to form a single multiple-fin FET or, alternatively, various single-fin and/or multiple-fin FETs.