Three-dimensional memory devices and methods for forming the same

A method for forming a three-dimensional (3D) memory device, comprising:

• forming a first dielectric layer, a first polysilicon layer, and a second polysilicon layer on a substrate;
• forming a second dielectric layer and a third polysilicon layer on the second polysilicon layer;
• forming a dielectric stack comprising interleaved third dielectric layers and fourth dielectric layers on the third polysilicon layer;
• forming a channel hole penetrating the dielectric stack, the third polysilicon layer, the second dielectric layer, the second polysilicon layer, the first polysilicon layer, and the first dielectric layer to expose the substrate;
• performing an oxidation operation to form a fifth dielectric layer on the first polysilicon layer and the second polysilicon layer exposed by sidewalls of the channel hole;
• forming a channel structure in the channel hole;
• removing the substrate, the first polysilicon layer, the second polysilicon layer, the second dielectric layer, and a bottom portion of the channel structure;
• and
• forming a fourth polysilicon layer over the channel structure.