Optimization of Tilted Profile in Ultra-High Aspect Ratio Etch Process for 3D NAND Flash Memory

In $3\mathrm{D}$ NAND flash memory fabrication, tilting issue has been exacerbated as aspect ratio of features increase to beyond 50, which constrains improvement of memory density. The distortion can be mainly attributed to tilted ion trajectory, which is caused by sheath thickness variation. This paper reports advanced methods of optimizing sheath thickness uniformity. As a result, ultra-high aspect ratio $(> 60)$ trench structures, with straight sidewall profile across a whole 12-inch wafer, have been realized successfully.