Embodiments of semiconductor devices and methods for forming the semiconductor devices are disclosed. In an example, a method for forming device openings includes forming a material layer over a first region and a second region of a substrate, the first region being adjacent to the second region, forming a mask layer over the material layer, the mask layer covering the first region and the second region, and forming a patterning layer over the mask layer. The patterning layer covers the first region and the second region and including openings corresponding to the first region. The plurality of openings includes a first opening adjacent to a boundary between the first region and the second region and a second opening further away from the boundary. Along a plane parallel to a top surface of the substrate, a size of the first opening is greater than a size of the second opening.