Abstract
The impact of linear correlation between lognormal distribution grain size mean and sigma along the polysilicon channel on threshold voltage (Vth) variability has been investigated in three dimensional (3D) NAND flash. The variety of grain size mean and sigma results in the unstable Vth variability. To obtain a stable Vth distribution with various grain size mean, the grain size mean dependent Vth variability sensitivity to the grain size sigma was used to optimize the linear correlation between grain size mean and sigma via TCAD simulation. The optimized linear correlation with stable Vth variability is obtained except for the “unbalance region” affected by the combination of grain boundaries and positions with these grain size mean and sigma values resulting in the slightly shrinking Vth variability. Our results strongly suggest that this approach could guide the direction of polysilicon crystallization optimization to obtain stable Vth distribution with the predicted linear correlation between grain size mean and sigma.
Highlights
Three dimensional NAND (3D NAND) is the preferred solution combing device performance and high capacity storage to replace planar NAND flash memory [1]–[3]
In this work, we have investigated the impact of the linear correlation between grain size mean and sigma with lognormal grain size distribution on Vth variability in vertical channel 3D NAND flash
The normalized value of grain size sigma to grain size mean is almost the same in the linear correlation, while the impact of grain conditions with different grain size sigma on device Vth variability is inconsistent under different grain size mean
Summary
Three dimensional NAND (3D NAND) is the preferred solution combing device performance and high capacity storage to replace planar NAND flash memory [1]–[3]. It is critical to improving the device performance and variability by polysilicon channel engineering for 3D NAND flash memory. A promising approach to improve device performance and variability is polysilicon grain size engineering [8], [9]. There is limited insight focusing on the correlation between grain size mean and sigma in 3D NAND flash memory. We found that a robust linear correlation between grain size mean and sigma in the vertical channel with lognormal grain size distribution. The impact of this linear correlation on Vth variability was investigated by TCAD simulation. The Vth variability sensitivity to the grain size sigma at each grain size mean was extracted and used to optimize the linear correlation between grain size mean and sigma to target Vth variability with better device performance and stable Vth distribution
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