Aspects of the disclosure provide a method. The method includes providing a substrate having a structure formed on the substrate, and forming a spacer layer on the structure. Then, the method includes forming a mask layer over the spacer layer. The mask layer includes a first layer, a second layer over the first layer, and a third layer over the second layer. Further, the method includes patterning the third layer of the mask layer, and etching the first layer and the second layer of the mask layer with a dry etching process using the third layer as an etch mask to form an opening that exposes a portion of the spacer layer. Then, the method includes removing the second layer using a wet etchant before a formation of a backfill material layer in the opening and over the first layer.