A method for forming a semiconductor structure includes forming a hard mask layer over a target layer. The method also includes forming first mandrels over the hard mask layer. The method also includes forming a first opening in the first mandrels. The method also includes depositing a spacer layer over the hard mask layer and the first mandrels. The method also includes depositing a second mandrel material over the spacer layer. The method also includes planarizing the second mandrel material. The method also includes forming a second opening in the second mandrel material. The method also includes patterning and etching the second mandrel material to form second mandrels. The method also includes etching the spacer layer. The method also includes etching the hard mask layer and the target layer.