Memory openings and backside openings are formed through an alternating stack of insulating layers and sacrificial material layers over a substrate. Memory opening fill structures are formed in the memory openings, and sacrificial backside opening fill structures are formed in the backside openings. Cavities are formed in volumes of the backside openings by removing the sacrificial backside opening fill structures. Remaining portions of the sacrificial material layers are replaced with material portions including electrically conductive layers. Each electrically conductive layer is formed as a continuous material layer including holes around the backside openings. Each electrically conductive layer is singulated into a plurality of electrically conductive strips by isotropically recessing the electrically conductive layers around each backside opening. Width-modulated cavities including expanded volumes of the backside openings are formed, and are filled with width-modulated insulating wall structures.