Embodiments of the present disclosure provide a method for forming a hole structure in a semiconductor device. The method includes forming a first etch mask over a stack structure, and removing a portion of the stack structure exposed by the first etch mask. The first etch mask may have a first mask opening with a first lateral dimension. The method may also include forming a second etch mask from the first etch mask. The second etch mask may have a second mask opening with a second lateral dimension that is greater than the first lateral dimension. The method may further include removing another portion of the stack structure exposed by the second etch mask to form the hole structure having a first hole portion and a second hole portion connected to and over the first hole portion.