Chemical Composition, Charge Trapping, and Memory Properties of Oxynitride Films for MNOS Devices

The effect of varying the amount of oxygen in the silicon oxynitride film of a metal/nitride/oxide/semiconductor (MNOS) device was investigated to determine correlations between the chemical nature, current conduction, charge trapping, and nonvolatile memory properties of the device. Nitrous oxide gas was used to introduce oxygen in the oxynitride film during the low‐pressure chemical vapor deposition process. The atomic percentage of oxygen increased to 21, nitrogen concentration decreased by 16%, and electron trap density decreased by 12%, with a corresponding decrease in film conductivity for an increase of the nitrous oxide gas rate from 0 to 80 sccm. The oxygen was determined to be incorporated into the film by replacing nitrogen atoms and by parallel introduction of silicon‐oxygen bonds during the deposition process. Oxygen impurities tie up the silicon dangling bonds, which may be responsible for memory traps in the film, and thus a subsequent reduction of trapped charge in the oxynitride was observed. The charge decay rate is then reduced as the trap density decreases, thus enhancing the nonvolatile memory properties of the MNOS devices. The retention and endurance device characteristics also improve considerably, by 60% and from 107 to 108 cycles, respectively. The useful memory lifetime of oxynitride MNOS devices with enhanced endurance under repeated switching was much greater than MNOS devices with oxygen‐free nitride films.