ABSTRACT
One of the most critical challenges that floating-gate and nitridebased charge storage nonvolatile memory (CS-NVM) devices face is the conflicting requirements of the bottom oxide layer due to the incessant technology scaling. This conflicting requirement emerges because of the need to reduce the overall critical dimensions of the CS-NVM cell, while maintaining (if not enhancing) the charge retention performance. The overall critical dimensions of CS-NVM are consistently miniaturized to fit more transistors in a chip, as predicted by Moore’s law. By 2015, CS-NVM was predicted to be scaled down to 16 nm. With this aggressive technology scaling trend, the device fundamental limit of CS-NVM is fast approaching because of the scale down of the critical dimensions of the CS-NVM
cell, especially in the tunnel oxide layer. This has also caused the reconsideration of keeping the practical limit to balance the economic gain and the gross investment required to resolve technology issues arising because of aggressive scaling. To improve the scalability of CS-NVM devices, another promising candidate of CS-NVM that is being heavily researched is nanocrystal-based CS-NVM, which will be elucidated comprehensively in this chapter. Along the technology scaling trend, these variants of CS-NVM devices have significant advantages in terms of compatibility with the standard CMOS fabrication process (and thus cost effective) and scaling capacity, while they maintain high-quality and high-reliability performance. In this review chapter, the evolution of charge-trapping nanocrystal CS-NVM is elucidated thoroughly. Furthermore, the enhancement in the reliability performance of nanocrystal CS-NVM devices in endurance and data retention is discussed comprehensively in a later section. In view of reliability, the key challenges for nanocrystal CS-NVM are also explained, with an emphasis on the charge loss mechanism, which critically impacts data retention. Moreover, in a later section, the innovation of tunnel barrier engineering with variants of metal nanocrystals and its major potential advantages are discussed comprehensively. This chapter will serve as a good reference to understand the key advantages, reliability challenges, and technical mitigations of nanocrystal CS-NVM and the future innovation trend in nanocrystal CS-NVM to overcome the critical challenges due to technology scaling in adherence to Moore’s law.
