Three-dimensional memory devices and fabricating methods thereof

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

• forming an alternating dielectric stack and a first insulating layer on a substrate, the alternating dielectric stack comprising a plurality of first dielectric layers and second dielectric layers;
• forming a channel hole penetrating the first insulating layer and the alternating dielectric stack, a first diameter of a lower portion of the channel hole being smaller than a second diameter of an upper portion of the channel hole;
• forming a channel structure comprising a functional layer in the channel hole, the functional layer comprising a storage layer;
• forming an electrode plug in the upper portion of the channel hole;
• replacing the storage layer in the functional layer in the upper portion of the channel hole with a second insulating layer;
• and
• replacing the plurality of second dielectric layers in the alternating dielectric stack with a plurality of conductive layers.