Three-dimensional memory device containing dual-depth drain-select-level isolation structures and methods for forming the same

Abstract

A three-dimensional memory device includes an alternating stack of insulating layers and electrically conductive layers, memory openings vertically extending through the alternating stack, and memory opening fill structures located within a respective one of the memory openings. Composite drain-select-level isolation structures divide each drain-select-level electrically conductive layer into a respective plurality of electrically conductive strips. Each drain-select-level isolation structure includes a respective first drain-select-level isolation material portion vertically extending through each drain-select-level electrically conductive layers and a respective set of second drain-select-level isolation material portions vertically extending through each of the drain-select-level electrically conductive layers and at least a topmost dummy electrically conductive layer that underlies the drain-select-level electrically conductive layers.