In an embodiment, a method includes: patterning a plurality of mandrels over a mask layer; forming an etch coating layer on top surfaces of the mask layer and the mandrels; depositing a dielectric layer over the mask layer and the mandrels, a first thickness of the dielectric layer along sidewalls of the mandrels being greater than a second thickness of the dielectric layer along the etch coating layer; removing horizontal portions of the dielectric layer; and patterning the mask layer using remaining vertical portions of the dielectric layer as an etching mask.