Three-dimensional flat NAND memory device including concave word lines and method of making the same

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

• forming a vertically alternating sequence of insulating layers and sacrificial material layers over a substrate;
• forming line trenches laterally extending along a first horizontal direction through the vertically alternating sequence, wherein alternating stacks of insulating strips and sacrificial material strips are formed by remaining portions of the vertically alternating sequence;
• forming recessed concave sidewalls on the sacrificial material strips by laterally recessing the sacrificial material strips selective to the insulating strips, wherein the line trenches become vertically undulating trenches having vertically undulating widths within vertical planes perpendicular to the first horizontal direction;
• forming an interlaced two-dimensional array of memory stack assemblies and dielectric pillar structures located in the vertically undulating trenches, wherein each of the vertically undulating trenches is filled with a respective laterally alternating sequence of memory stack assemblies and dielectric pillar structures;
• and
• replacing the sacrificial material strips with electrically conductive strips,
• wherein each memory stack assembly comprises a vertical semiconductor channel and a pair of memory films including a respective pair of convex outer sidewalls that contact, or are spaced by a uniform distance from, concave sidewalls of the electrically conductive strips.