Three-dimensional flat nand memory device having curved memory elements and methods of making the same

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

• forming alternating stacks of first sacrificial material strips and second sacrificial material strips over a substrate, wherein the alternating stacks are laterally spaced apart from one another by line trenches laterally extending along a first horizontal direction;
• modifying the line trenches to provide a two-dimensional array of lateral recesses on each sidewall of the line trenches, wherein each two-dimensional array of lateral recesses is laterally bounded by a respective two-dimensional array of laterally-recessed surfaces of the second sacrificial material strips;
• forming memory stack assemblies in each volume that includes a combination of a volume of a line trench and volumes of two adjoining two-dimensional arrays of lateral recesses, wherein each of the memory stack assemblies comprises two two-dimensional arrays of lateral protrusion regions, and each of the lateral protrusion regions comprises a respective charge storage element;
• replacing remaining portions of the second sacrificial material strips with electrically conductive strips;
• and
• forming air gap strips by removing the first sacrificial material strips.