A method includes growing a plurality of parallel mandrels on a surface of a semiconductor substrate, each mandrel having at least two laterally opposite sidewalls and a predetermined width. The method further includes forming a first type of spacers on the sidewalls of the mandrels, wherein the first type of spacers between two adjacent mandrels are separated by a gap. The predetermined mandrel width is adjusted to close the gap between the adjacent first type of spacers to form a second type of spacers. The mandrels are removed to form a first type of fins from the first type of spacers, and to form a second type of fins from spacers between two adjacent mandrels. The second type of fins are wider than the first type of fins.