Introduction

To tear down the “memory wall”, the solution of Storage Class Memory (SCM) was first proposed by Intel Corporation. SCM acts as the bridge between DRAM and NAND, which has a higher storage density than DRAM and a higher access speed than NAND FLASH. For the vertical 3D structure of 3D VRRAM, the number of lithograph does not increase significantly with the number of stacking layers increased, which significantly reduces the number of lithographs. Compared to other multi-layer stacked structures, the costs can be significantly diminished for 3D VRRAM. For 3D X-point structures, the integration of the selector with the memory cell can be achieved easily by a planar process, whereas for 3D VRRAM structures, the integration of the selector is very difficult because in vertical arrays, the bottom electrode of each memory cell column is formed by a trench filling process and the selector cannot be integrated separately.