Characterization of process-induced mobile ions on the data retention in flash memory

Data retention is a major issue that significantly affects the reliability of nonvolatile memory. In this paper, data retention behaviors due to mobile ions are extensively characterized by using several test methods. The source of mobile ions during the sputtering of Ti/TiN layers for forming salicide inside array was identified by secondary ion mass spectroscopy (SIMS) analysis. Different test sequences clearly reveal the data retention caused by mobile ions. Salicide block material that includes silicon nitride effectively blocks the impact of mobile ions. Bitmapping patterns on an array indicate that the amount of charge loss spatially depends on its neighbor´s high-V_th cells via the Coulomb´s r-square field. The spatial effects of neighboring cells on the charge loss are then estimated. Beyond 1.98 μm, the charge loss caused by mobile ions of a individual cell becomes unaffected by the V_th state of its neighboring cells. Plasma charging cells also attract mobile ions. A considerable charge gain, by as much as 0.8 V with a radial distribution on fresh wafer, was observed after a UV-bake test. The interaction between plasma charging effect and mobile ions movement during wafer processing explains this interesting feature.