Embodiments of methods for forming staircase structures for three-dimensional (3D) memory devices double-sided routing are disclosed. In an example, a first dielectric layer is formed on a substrate, and a first photoresist layer is formed on the first dielectric layer. A recess is patterned through the first dielectric layer to the substrate by cycles of trim-etch the first dielectric layer. A plurality of dielectric/sacrificial layer pairs filling in the recess are formed. A second photoresist layer is formed on a top surface of the dielectric/sacrificial layer pairs. The dielectric/sacrificial layer pairs are patterned by cycles of trim-etch the dielectric/sacrificial layer pairs. A second dielectric layer covering the patterned dielectric/sacrificial layer pairs is formed. A memory stack on the substrate including a plurality of conductor/dielectric layer pairs is formed by replacing, with a plurality of conductor layers, the sacrificial layers in the patterned dielectric/sacrificial layer pairs.