Multi-tier three-dimensional memory device containing dielectric well structures for contact via structures and methods of forming the same

A method of forming a three-dimensional memory device, comprising:

• forming a first vertically alternating sequence of first insulating layers and first spacer material layers and a first-tier retro-stepped dielectric material portion overlying first stepped surfaces of the first vertically alternating sequence over a substrate, wherein the first spacer material layers are formed as, or are subsequently replaced with, first electrically conductive layers;
• forming a second vertically alternating sequence of second insulating layers and second spacer material layers over the first vertically alternating sequence and the first-tier retro-stepped dielectric material portion, wherein the second spacer material layers are formed as, or are subsequently replaced with, second electrically conductive layers;
• forming second stepped surfaces within a first opening through the second alternating stack by patterning the second alternating stack;
• forming a second-tier retro-stepped dielectric material portion over the second stepped surfaces in the first opening through the second vertically alternating sequence;
• forming a second opening through the second vertically alternating sequence over the first-tier retro-stepped dielectric material portion;
• and
• forming a dielectric well structure through the second vertically alternating sequence on a top surface of the first-tier retro-stepped dielectric material portion.