Disclosed herein is a method of forming a structure, comprising forming a mandrel layer over a substrate, masking the mandrel layer with a first mask and performing a first etch on the mandrel layer, the first etch forming a first opening exposing a first portion of the substrate. The mandrel layer is masked with a second mask and a second etch is performed on the mandrel layer. The second etch forms a second opening exposing a second portion of the substrate, and also forms a protective layer on the first portion of the substrate and in the first opening.