Thickness scaling limitation factors of ONO interpoly dielectric for nonvolatile memory devices

This paper describes the scaling limitation factors of ONO interpoly dielectric thickness, mainly considering the charge retention capability and threshold voltage stability for nonvolatile memory cell transistors with a stacked-gate structure, based on experimental results. For good intrinsic charge retention capability, either the top- or bottom-oxide thickness should be greater than around 6 nm. On the other hand, a thicker top oxide structure is preferable to minimize degradation due to defects. It has been confirmed that a 3.2 nm bottom-oxide shows detectable threshold voltage instability, but 4 nm does not. Effective oxide thickness scaling down to around 13 nm should be possible for flash memory devices with a quarter-micron design rule.