Multi-level storage in a nano-floating gate MOS capacitor using a stepped control oxide

A non-volatile nanocrystal floating gate MOS capacitor with multi-level function is achieved by engineering the electric field within the tunneling oxide via a stepped control oxide. A MOS capacitor containing Au nanocrystals in a stepped HfO2 and SiO2 tunneling oxide matrix was fabricated in order to demonstrate this concept. The flatband voltage shift, measured from the C-V hysteresis curves, exhibited a saturated region within a programming mode that is not observed in the conventional step free MOS capacitor. The values of the flatband voltage shift measured at the first and second saturation were slightly higher than the values predicted from the Coulomb blockade theory. However, there is a strong consensus with the flatband voltage ratios between experimental results and the predicted values, which supports successful operation. More obvious evidence of multi-level storage function was confirmed by turnaround voltage measurement.