| Summary: | 博士 === 國防大學中正理工學院 === 國防科學研究所 === 92 === When searching the structure for next-generation Flash memory application, SONOS (Silicon-Oxide-Nitride-Oxide-Silicon) structure has attracted much attention among various choices due to its numerous advantages. However, there are still some challenges remain in the development of technology. The main issues include data retention limited by thinning tunnel oxide and endurance constrained by poor charge-trapping efficiency of silicon nitride. The reliability problem becomes even worse as devices are scaled further down. This dissertation is focused in the study of tunnel oxide and silicon nitride layer. The purpose is to improve film quality and hence device performance as well as reliability.
The dissertation has divided into two parts. The first effort is to optimize tunnel oxide by adopting various oxynitridation processes. The second part of work involves bandgap engineering of silicon nitride layer by controlling relative Si/N ratio during deposition. The impact of Si-rich composition at different film depth location is evaluated and the goal is to form a novel and efficient charge-trapping structure.
Experimental results indicate that oxynitride grown in N2O/N2 ambient leads to excellent insulation property of tunnel oxide and the nitride with a graded composition profile from Si-rich at the bottom towards N-rich at the top structure (modified nitride) gives rise to high charge-trapping efficiency and reliability. The quality improvement of silicon nitride is due to increased barrier height between nitride and tunnel oxide in addition to greater number of accessible charge trapping levels created by tapered-bandgap. As a result, the SONOS device with modified nitride structure exhibits wide operation window, excellent cycling endurance, data retention, and read disturbance. The outcome of this research is very important for future technology development of high-density Flash memories.
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