A study on the charge trap distribution characteristics in nitride and the retention characteristics of TaN-Al2O3-SiN-SiO2-Si memory structures

Abstract

The charge trap distribution characteristics in nitride and the retention characteristics of TaN-Al2O3-SiN-SiO2-Si (TANOS) are investigated to understand the device’s retention characteristics as the amount of threshold voltage shift by programming operation and charge trap distribution inside nitride vary. The model for charge trap distribution is composed of exponential distribution near conduction band and two Gaussian distribution in shallow and deep energy levels. The validity of this model is confirmed by comparing simulation results from simulator we developed by numerically solving equations which deal with electron’s state in nitride layer and experimental results for the device which is same as that for simulation. The first simulation for change of threshold voltage shift by programming operation reveals that the characteristic is deteriorated as the shift amount is increased because it makes more electrons located near conduction band. The other simulation for change of trap distribution inside nitride reveals that the more charge trapping dots are located near and above intrinsic Fermi-level between two Gaussian distributions, the more retention characteristics are improved. Especially, the change of trap energy depths of both Gaussian distributions affect very much on retention characteristics, so controlling over them is very important. These results can help anticipate the device’s retention characteristics for various programming amount and control optimum amount of threshold voltage shift for multi-level operation. Also, we can anticipate the device’s retention characteristics as charge trap distribution inside nitride varies, and which types of distribution change more critically affect on the characteristics.