Endurance and Data Retention Improvement of Silicon–Oxide–Nitride–Oxide–Silicon Nonvolatile Semiconductor Memory Devices with Partially Bottom-Silicon-Rich Nitride Structure

Abstract

A significant reliability improvement in silicon–oxide–nitride–oxide–silicon (SONOS) flash memory devices by band-gap engineering of the nitride layer has been attained. The gradually varied reaction gas flow rate during deposition has generated special nitride films with non uniform composition profiles and band gaps. As a result, SONOS devices with partially Si-rich nitride structures have exhibited superior cycling endurance, radiation hardness, and data retention compared with devices with a uniform standard nitride. The marked improvement can be attributed to the increased charge-trapping/detrapping efficiency of the nitride layer since a significant number of highly accessible trapping levels have been created in the nitride that has a graded band gap. In addition, the deepened barrier heights between the nitride and its surrounding oxides may also reduce undesirable charge-loss probability and assist in charge storage. Because the dimension of flash memory cells is continuously shrinking, the proposed technique will be valuable for mass storage applications.