Mandrel structures and methods of fabricating the same in semiconductor devices

A method includes forming a semiconductor substrate, forming hard mask layers (HMs) over the semiconductor substrate, forming first mandrels over the HMs, forming second mandrels along sidewalls of the first mandrels, forming a protective layer over the first mandrels and the second mandrels, removing a portion of the protective layer to expose portions of the first and the second mandrels, removing the exposed portions of the second mandrels with respect to the exposed portions of the first mandrels, removing remaining portions of the protective layer to expose remaining portions of the first and second mandrels, where the exposed portions of the first mandrels and the remaining portions of the first and second mandrels form a mandrel structure, patterning the HMs using the mandrel structure as an etching mask, and patterning the semiconductor substrate to form a fin structure using the patterned HMs as an etching mask.