Three-dimensional memory devices with lateral block isolation structures and methods of forming the same

A three-dimensional memory device, comprising:

• an alternating stack of insulating layers and composite layers that alternate along a vertical direction, wherein each of the composite layers comprises a combination of a dielectric connection plate and a plurality of electrically conductive strips that laterally extend along a first horizontal direction, are laterally spaced apart along a second horizontal direction by backside trenches that laterally extend along the first horizontal direction, and have a respective sidewall adjoined to a respective sidewall surface segment of the dielectric connection plate, wherein end portions of the backside trenches are laterally bounded by the dielectric connection plates of the composite layers;
• arrays of memory openings vertically extending through the alternating stack;
• memory opening fill structures located in the memory openings, wherein each of the memory opening fill structures comprises a vertical semiconductor channel and a respective vertical stack of memory elements located at levels of the electrically conductive strips;
• backside trench fill structures located in the backside trenches and vertically extending at least from a first horizontal plane including bottommost surfaces of the alternating stacks and at least to a second horizontal plane including topmost surfaces of the alternating stacks;
• and
• dielectric etch stop structures located within or outside a respective one of the backside trenches, wherein each of the dielectric etch stop structures comprises a respective pair of dielectric sidewalls that are located within a pair of lengthwise sidewalls of the respective one of the backside trenches, wherein the dielectric sidewalls of the dielectric etch stop structures laterally extend along the first horizontal direction and vertically extend at least from the first horizontal plane and at least to the second horizontal plane.