SONOS device with tapered bandgap nitride layer

A unique, band-engineered, configuration of the charge-trapping layer in silicon-oxide-nitride-oxide-silicon (SONOS) devices is proposed for high-density Flash memory applications. In this paper, a varying Si-N ratio in modified silicon nitride is obtained by controlling reaction gas flow-rate during deposition. This generates a graded composition profile from Si-rich at the bottom to N-rich at the top in a nitride film. The nonuniform composition profile of the silicon nitride layer corresponds to a tapered bandgap and results in significant improvement in device performance and reliability characteristics including operation window, cycling endurance and data retention. The dramatic improvement can be attributed to increased charge-trapping efficiency of the nitride layer since a significant number of highly accessible trapping levels are created in the tapered bandgap. In addition, the increased barrier height between the nitride and tunnel oxide layers also reduces back-tunneling probability and assists charge trapping. The SONOS device designed in this paper is suitable for next-generation Flash memory applications.