Methods for polishing dielectric layers using an auto-stop slurry in forming semiconductor devices, such as three-dimensional (3D) memory devices, are provided. The methods include forming a stack structure in a staircase region and a core array region, the stack structure including a staircase structure in the staircase region; forming a dielectric layer over the staircase region and a peripheral region outside the stack structure; and polishing the dielectric layer using an auto-stop slurry containing a ceria-based abrasive.