Redeposition mechanism on silicon oxide layers during selective etching process in 3D NAND manufacture

3D NAND flash memory with vertically stacked cells has been developed to break through the limits of technology nodes. However, during the selective etching process, it is difficult to ensure the byproducts diffuse away from the trenches in the multistacked layers. Once saturated, the byproduct causes abnormal redeposition on the SiO2 layers. This problem has restricted the development of high-density 3D NAND memory. To solve this problem, the composition and formation mechanism of the redeposited layer must be clarified. In this study, a ternary-wafer system comprising a Si3N4/SiO2/Si3N4 stack was fabricated to study the redeposition mechanism, and the morphology, elastic properties, and chemical composition of the redeposited layer were clarified. The redeposited layer consists of spherical particles with elastic surfaces (average Young’s modulus of 24.17 GPa). The particles were confirmed to comprise colloidal silica gel covered by silanols. By considering the chemistry of silica, the redeposition mechanism was proposed as follows: colloidal silica gel is formed by the aggregation of silicic acids from Si3N4 etching, which adsorb onto the SiO2 layer through oxide bridges and hydrogen bonding. Our work will contribute to the development of high-density 3D NAND memory.