Improvement of Fluorine Attacking Induced Word-Line Leakage in 3D NAND Flash Memory

Abstract

This study explores the impact of fluorine (F) erosion on the tungsten (W) gate process and proposes measures to mitigate the word line (WL) leakage resulting from F erosion in 3D NAND flash memory. As the number of layers in 3D NAND increases, the tungsten (W) gate word line (WL) layer fill process becomes more challenging during the post-gate process. As the fill path length increases, the tungsten gates become more susceptible to voiding during deposition, resulting in the accumulation of fluorine (F) by-products, and causing fluorine attack issues. In particular, under the influence of subsequent high-temperature processes, the by-products containing fluorine can diffuse into the surrounding structure and corrode the surrounding oxide layer. This leads to WL leakage, thereby affecting device yield and reliability. This paper begins by analyzing the microscopic principles of fluorine erosion in 3D NAND. We also propose a low-pressure annealing method to address the issue of fluorine erosion. Then, we conducted annealing experiments on planar thin film stacks and 3D filled structures under both atmospheric and low-pressure conditions. We used various methods to characterize the concentration and distribution of residual fluorine elements. The experimental results demonstrate that residual fluorine elements in the tungsten gate can be effectively discharged by low-pressure annealing under appropriate conditions, thus reducing the leakage index of the word line. Additionally, as the outer CH is closer to the fluorine discharge channel, the impact of low-pressure annealing on the outer CH is more pronounced than on the inner CH. Low-pressure annealing can significantly reduce the fluorine content in the tungsten gate. This method can also mitigate the issue of fluorine attack oxides and decrease WL leakage. Using low-pressure annealing treatment can also enhance the quality of 3D NAND flash technology.