||Current requirements of nonvolatile memory (NVM) are the high density cells, low-power consumption, high-speed operation and good reliability for next-generation NVM application. However, all of the charges stored in the floating gate will leak into the substrate if the tunnel oxide has a leakage path in the conventional NVM during endurance test. Therefore, the tunnel oxide thickness is difficult to scale down in terms of charge retention and endurance characteristics. Nanocrystals (NCs) NVMs are one of the promising candidates to substitute for conventional floating gate memory since the discrete storage nodes as the charge storage media can effectively enable the improvement of data retention for the scaling down device.|
In this thesis, we try to overcome the limitation of conventional NVMs during the scaling down process and further increase the retention time by means of changing the structure of Nanocrystals NVMs. Firstly, we deposit a NiSi2 layer as the nanocrystal self-assembled layer and thereby apply it to Nanocrystals NVMs. In room temperature, we bombard NiSi2 target to form single layer and double layer charge trapping layer through sputtering system layer by layer, and the two charge trapping layers are separated by 30 Å silicon-oxide (SiO2). Next, we also deposit silicon oxide as control oxide. According to rapid thermal anneal (RTA) mix oxide gas, we improve the oxide quality and supply NiSi2 sufficient energy to reach the smallest Gibbs free energy so as to form uniform and high density NiSi nanocrystal. On account of the increasing of trapping center and the coulomb repulsion power, the double layer structure NiSi Nanocrystals NVMs has better memory window and retention than the single layer one.
In the similar process, we sputter NiSi2 target with Ar gas mixes NH3 gas to form silicon-nitride compound layers. Then, we use the same RTA process to form nanocrystal and improve the oxide quality. In the light of TEM and XPS analysis, we may infer that the nanocrystal is formed by NiSi2 and SiNX compound. Further, based on our electronic analysis, we can observe that the retention of NiSi2/SiNX compound Nanocrystal NVMs after 104 sec rises from 50% to 72% in comparison with the traditional one due of the quantum well band structure contributes by NiSi2 and SiNX compound nanocrystals. The retention of NiSi2/SiNX compound Nanocrystal NVMs after 104 sec is even better than the double layer without NH3 mixed one, 68%. Furthermore, the threshold voltage of NiSi2/SiNX compound Nanocrystal NVMs has not been subject to change after endurance with 104 programming and erasing cycles continuously.
Thus, by means of depositing nanocrystal charge trapping layer mixed with NH3 gas, we achieve the objective of simplifying the fabrication process. These fabrication techniques for the application of nonvolatile nanocrystal memory can also be applicable to the current manufacture process of the integrated circuit manufacture.