Stairless three-dimensional memory device and method of making thereof by forming replacement word lines through memory openings

A method of forming a memory device, comprising:

• forming an alternating stack of insulating layers and sacrificial material layers comprising a dielectric material over a substrate;
• forming memory openings through the alternating stack in a memory array region;
• forming memory opening fill structures in the memory openings, wherein each of the memory opening fill structures comprises a respective vertical stack of memory elements and a vertical semiconductor channel;
• forming sacrificial via opening fill structures vertically extending through a respective subset of layers within the alternating stack and contacting a top surface of a respective sacrificial material layer within the alternating stack in a contact region;
• forming backside trenches through the alternating stack;
• forming backside recesses by isotropically etching portions of the sacrificial material layers by introducing an isotropic etchant into the backside trenches, wherein remaining portions of the sacrificial material layers comprise dielectric material plates that laterally surround the sacrificial via opening fill structures;
• forming electrically conductive layers in the backside recesses;
• forming voids by removing the sacrificial via opening fill structures;
• forming laterally-extending cavities underneath the voids by isotropically etching portions of the dielectric material plates such that surfaces of the electrically conductive layers are exposed to the laterally-extending cavities;
• and
• forming integrated line-and-via structures, wherein each of the integrated line-and-via structures comprises a conductive plate portion filling a respective one of the laterally-extending cavities and a conductive via portion filling a respective one of the voids.