Abstract:

Selective etching of Si₃N₄ in a stacked structure of Si₃N₄ and SiO₂ using phosphoric acid as an etchant is one of the most critical steps in the manufacturing process of 3D NAND flash memory chips. With the increasing number of stacked layers, diffusion limitation becomes important and the concentration gradient of the etching product H₂SiO₃ becomes large, which brings the problems of low etching selectivity and serious oxide regrowth in this etching process. Facing these above-mentioned problems, this work combines characterizations and kinetics experiments to obtain the kinetics models of Si₃N₄ and SiO₂ etched by phosphoric acid with or without the influence of H₂SiO₃. The results show the etch of Si₃N₄ is the nucleophilic attack of Si by the nucleophilic reagents, like water and phosphoric acid. The activation energies for etching Si₃N₄ and SiO₂ by phosphoric acid are 60.71 kJ·mol^-1 and 66.90 kJ·mol^-1, indicating the energy required to break Si—O bond is larger than that to break Si—N bond under the attack of nucleophilic reagents. The etching kinetics of Si₃N₄ and SiO₂ with the influence of H₂SiO₃ concentration show that the etching rates of Si₃N₄ and SiO₂ decrease with the increase of H₂SiO₃ concentration, and the etching rate of SiO₂ is more sensitive to the change of H₂SiO₃ concentration. The results in this work should provide some fundamental data and theoretical understanding for the optimal design of phosphate wet etching process for manufacturing 3D NAND.

Key words: 3D NAND flash, wet etching, silicon nitride, silicon oxide, H₂SiO₃, chemical reaction, kinetics, kinetic modeling

摘要：

以磷酸为刻蚀剂选择性刻蚀Si₃N₄和SiO₂堆叠结构中的Si₃N₄，是3D NAND闪存芯片制造过程中最关键的步骤之一。随着3D NAND存储单元堆叠层数的不断升高，扩散限制越来越严重，刻蚀产物偏硅酸浓度梯度不断加大，致使刻蚀工艺面临刻蚀选择性低、氧化物回沾等问题。针对上述3D NAND湿法刻蚀问题，结合表征和动力学实验，获得了有无偏硅酸影响的磷酸刻蚀Si₃N₄和SiO₂动力学模型。结果显示Si₃N₄刻蚀过程是水和磷酸等含氧亲核试剂对Si原子的亲核攻击过程，磷酸刻蚀Si₃N₄和SiO₂的反应活化能分别为60.71 kJ·mol^-1和66.90 kJ·mol^-1，亲核试剂破坏Si—O键所需能量高于Si—N键。包含偏硅酸影响的Si₃N₄和SiO₂刻蚀动力学显示偏硅酸浓度越高Si₃N₄和SiO₂的刻蚀速率越小，并且SiO₂刻蚀速率对偏硅酸浓度的变化更敏感。本工作的研究结果可为3D NAND磷酸湿法刻蚀工艺的设计优化提供一定的基础数据和理论认识。

关键词: 3D NAND闪存, 湿法刻蚀, 氮化硅, 氧化硅, 偏硅酸, 化学反应, 动力学, 动力学模型