Zihan Zhou,Yunwen Wu,Huiqin Ling,Jie Guo,Su Wang,Ming Li
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 SiO 2 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 Si 3 N 4 /SiO 2 /Si 3 N 4 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 Si 3 N 4 etching, which adsorb onto the SiO 2 layer through oxide bridges and hydrogen bonding . Our work will contribute to the development of high-density 3D NAND memory.
Zihan Zhou,Yunwen Wu,Silin Han,Tao Hang,Huiqin Ling,Jie Guo,Su Wang,Ming Li
3D NAND is one of the most promising storage devices due to its high storage density and low energy consumption. Selective wet etching of Si 3 N 4 by hot-concentrated phosphoric acid (PA) solution is a key process for 3D NAND fabrication. However, the channel material, polycrystalline silicon (poly-Si), is unexpectedly dissolved due to its inevitable exposure to the PA etching solution, which causes device failure and thus limits the stacking density of 3D NAND. Herein, to avoid the detrimental dissolution of poly-Si, we unveil the mechanism of poly-Si dissolution in PA solution from kinetic and thermodynamic perspectives. We find that the etch process shows a two-stage reaction, in which H 2 O plays a key role in lowering the activation barrier for poly-Si etching. Further thermodynamic analysis demonstrates that poly-Si prefers to be oxidized by H 2 O to suboxide rather than to SiO 2 . Combined with surface chemistry characterization, we propose the dissolution mechanism as a two-stage process involving the dissolution of suboxide interface and bulk poly-Si, where the nucleophilic substitution of Si–H with Si–OH on bulk poly-Si surface determines the overall etch rate. This study advances the 3D NAND development by establishing a theoretical foundation for identifying and implementing efficacious strategies to impede detrimental dissolution.
Zihan Zhou,Silin Han,Yunwen Wu,Tao Hang,Huiqin Ling,Jie Guo,Su Wang,Ming Li
The SiO2 layers, as the dielectric in the multistack storage cells of 3D NAND, are detrimentally dissolved by phosphoric acid (PA) during the selective Si3N4 etching. The thinning of SiO2 layers leads to the failure of subsequent processes; thus, an etching inhibitor for SiO2 is crucial for 3D NAND fabrication. However, most inhibitors are currently limited to neutral or alkaline solutions. In this work, the γ-ureidopropyltriethoxysilane (UPTES), compatible with a hot-concentrated PA solution, was applied to the wet-etching process for the first time and exhibited an inhibitory effect on SiO2 etching by retarding the etch rate over 60% without affecting the Si3N4 etching. The saturation adsorption of UPTES on the SiO2 substrate is confirmed to occur around the critical concentration, according to the Langmuir–Freundlich adsorption isotherm. The adsorption behavior of UPTES exhibits a reliance on its concentration. Around the critical concentration, the UPTES molecules chemisorb on the SiO2 surface by Si–O–Si covalent bonds to form ordered UPTES-siloxane layers, avoiding contact between SiO2 and the etchant; whereas oversaturated UPTES polymers physisorb not only on SiO2 but also on Si3N4, impeding the Si3N4 etching. This work contributes to a step forward in the effective wet-etching process in high-density 3D NAND fabrication.
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