The present disclosure provides a method for forming a three-dimensional (3D) memory. In an example, the method includes forming a stack structure having interleaved a plurality of stack first layers and a plurality of stack second layers, forming a stair in the stack structure, the stair having one of the stack first layers on a top surface, and forming a layer of sacrificial material having a first portion over a side surface of the stair and a second portion over the top surface of the stair. The method also includes partially removing the first portion of the layer of sacrificial material using an anisotropic etching process and removing a remaining portion of the first portion of the layer of sacrificial material using an isotropic etching process.