3D NAND is built by depositing dozens of alternating layers, then etching tall, narrow vertical channels down through the stack. Those channels are filled in wi

3D NAND is built by depositing dozens of alternating layers, then etching tall, narrow vertical channels down through the stack. Those channels are filled in with other materials to form vertical strings of memory cells. The ability to make many layers of memory cells without needing lithography and etch per layer is the main reason why 3D NAND is so cost-effective and able to continue scaling in density far beyond what was achievable with planar NAND flash memory (which could only improve by making memory cells smaller, but they were getting too small to actually work). Intel's 3D XPoint memory (used in Optane products) was doomed in part by the fact that it did need lithography for every single layer, so it couldn't scale up in layer count cheaply.