A method for patterning a substrate is described. The method includes receiving a substrate having a patterned layer, wherein the patterned layer defines a first mandrel pattern, and wherein a first material layer of a first composition is conformally deposited over the first mandrel pattern. The method further includes partially removing the first material layer using a first gas cluster ion beam (GCIB) etching process to expose a top surface of the first mandrel pattern, open a portion of the first material layer at a bottom region adjacent a feature of the first mandrel pattern, and retain a remaining portion of the first material layer on sidewalls of the first mandrel pattern; and selectively removing the first mandrel pattern using one or more etching processes to leave a second mandrel pattern comprising the remaining portion of the first material layer that remained on the sidewalls of the first mandrel pattern.