In a three-dimensional memory device, an interconnect structure is formed over a substrate and a first deck is formed over the interconnect structure. The first deck includes alternating first insulating layers and first word line layers, and a first channel structure extending through the first stack. The first channel structure has a first channel dielectric region and a first channel layer. The first channel dielectric region is formed along sidewalls of the first channel structure, positioned over a top surface of the interconnect structure, and in contact with the first insulating layers and the first word line layers. The first channel layer is formed along the first channel dielectric region, and includes a rounded projection that extends away from the top surface of the interconnect structure, extends outwards into the first stack at an interface of the interconnect structure, the first channel structure and the first stack.