Aspects of the disclosure provide a method for fabricating semiconductor device. The method includes characterizing an etch process that is used to etch channel holes and dummy channel holes in a stack of alternating sacrificial gate layers and insulating layers upon a substrate of a semiconductor device. The channel holes are in a core region and the dummy channel holes are in a staircase region. The stack of alternating sacrificial gate layers and insulating layers extend from the core region into in the staircase region of a stair-step form. The method further includes determining a first shape for defining the dummy channel holes in a layout based on the characterization of the etch process. The first shape is different from a second shape for defining the channel holes.