A method for fabricating a semiconductor device includes forming a target etch layer over a substrate, a first auxiliary layer over the target etch layer, an isolation layer over the first auxiliary layer, and a second auxiliary layer over the isolation layer. A first exposure process is performed, where the first auxiliary layer is in focus and the second auxiliary layer is out of focus. A second exposure process is performed, where the second auxiliary layer in focus and the first auxiliary layer is out of focus. The second auxiliary layer is developed to form first mask patterns. The isolation layer and the first auxiliary layer are etched by using the first mask patterns to form second mask patterns. The second mask patterns are developed to form third mask patterns that are used to facilitate subsequent etching of the target etch layer.