A method of patterning a target layer on a substrate is described. A patterned photoresist layer is formed over the target layer, wherein the patterned photoresist layer has unexposed parts as separate islands and each unexposed part has a low proton concentration at least in its sidewalls. Acid-crosslinked polymer layers are formed only on the sidewalls of each unexposed part. A flood exposure step is performed to the substrate. A baking step is performed to the patterned photoresist layer. A development step is performed to remove the previously unexposed parts. The target layer is etched with the acid-crosslinked polymer layers as a mask.