Study Of Reliability On 3D Charge Trapping Memory Based On First Principle Calculation

As early as in the 20th century,the defects in solid materials have attracted people's attention.In recent years,with the development of semiconductor technology and the emergence of new application fields,the defects have become more and more important.On the one hand,from the perspective of the application of defects,display technology,hydrogen storage technology,data storage,battery energy storage and so on are closely related to the application research of material defects.On the other hand,from the influence of defects on reliability,material defects in microelectronic devices can seriously affect the yield,performance,long-term reliability and radiation characteristics of devices and circuits.Therefore,a comprehensive understanding of defects is a crucial subject for the development of the microelectronics industry.Although various characterization techniques have been developed experimentally to study defects,they are restricted by many factors in practical application,including measurement accuracy,repeatability,reliability,and test cost.Considering the various problems that may exist in these experiments,theoretical calculation and simulation at the atomic level have become indispensable tools to study defect mechanism,and even play a crucial role in explaining reliability characteristics of semiconductor integrated circuit devices and guiding device process optimization.Because the first-principles calculation can more intuitively show the influence of defects on material properties and device reliability from the atomic level,the first-principles calculation can not only be applied to the research of materials such as semiconductors,metals,and insulators,for example,the process optimization of the highly reliable insulating dielectric layer.On the other hand,it is also an indispensable theoretical guidance tool to study transistor reliability and various memory reliability,such as the object of this paper,charge trapping 3D NAND flash memory(3D NAND).3D NAND has become the mainstream of non-volatile memory due to its high storage density,low cost,and high reliability.However,since the storage cells on the NAND String of the same Bit line share the common charge storage layer,the lateral charge diffusion between adjacent cells becomes an important issue affecting the reliability of the memory.For example,after a long time of data retention,charge diffusion will cause changes in the state of the storage cell,thus resulting in the degradation of the storage data retention characteristics.If the future storage density is further improved and the device cell size/spacing is reduced,the charge lateral diffusion problem will become more serious and become a key problem restricting the 3D memory upgrade.However,the exact defect characteristics and the physical mechanism that affect charge lateral diffusion are still unclear.There have been many experimental studies on the phenomenon of charge loss in 3D NAND.However,it is difficult to separate the vertical charge loss and the lateral charge loss in the experiment,and only the physical model can be used to extract the defect concentration,distribution,defect energy level and other parameters from the shift of threshold voltage.However,the extraction of parameters largely depends on the fitting of experimental data,which has a great correlation with the experimental environment,the device process,and the preparation environment,so the physical mechanism cannot be clearly reflected.In addition,it is impossible to get a definite conclusion about the influence of the atomic structure and impurity atoms in the charge trapping layer and the interface.Therefore,it is very important to understand the physical mechanism of charge loss at the atomic level in order to provide a theoretical basis for guiding process optimization quickly and accurately.Based on the first principles calculation method,the lateral charge diffusion mechanisms of 3D NAND are researched systemically,including shallow level defects in silicon nitride charge trapping layer,storage cell worn out in the process of P/E cycling and the stability of defects,as well as the one-dimensional model of lateral diffusion.To meet the demand for scaling,the method of improving storage density and reliability by doping different atoms is discussed.In order to fully grasp the diversity of defect characteristics in the charge storage layer of the amorphous structure,we further studied amorphous silicon nitride.Finally,the influence of defects in new materials devices is investigated.Firstly,the defect structures and the trap levels of Si3N4 charge trapping layer are studied systematically.Considering the deposition process of the Si3N4 layer and the influence of Si3N4/SiO2 interface,the influence of hydrogen atom(H)and oxygen atom(0)on the defects has also been further studied.We found that the nitrogen vacancy(VN)defect has low formation energy and is the most general defect.The existence of 0 atom at the interface between Si3N4 and SiO2 leads to the generation of extremely shallow defect energy levels,and hydrogen passivation can effectively deepen these shallow trap levels.However,in our further study,it is also found that excessive hydrogen will lead to the generation of new shallow trap levels.Therefore,the optimization of hydrogen passivation content in the actual process is very noteworthy.In the study of puckered nitrogen vacancy(VNP)defects,it is found that VNP defects have shallow trap levels compared with VN defects,and shallower trap levels will be generated after combining with O atoms,but H passivation can also well inhibit these shallow trap levels.It is shown that the shallow trap levels of oxygen atom may be the main reason for the fast charge loss.Although hydrogen passivation eliminates the shallow defects caused by oxygen atoms,excess hydrogen atoms cause other shallow trap levels.In addition,consider the stability of hydrogen bond,we proposed an approach by replacing hydrogen with deuterium hydrogen for Si3N4 layer passivation to avoid the fracture of hydrogen bond during P/E cycling because Si-D has better stabilities.In addition,in order to understand the phenomenon of lateral charge loss,quantitative models including trap levels,electrical properties,and charge transfer mechanism need to be established.Therefore,for the trap levels by the first principle calculation,we establish a one-dimensional lateral diffusion model,which more intuitively reflects the influence of trap levels on the transverse diffusion.Secondly,we have studied the stability of the defect structure and the generation of new defects during the continuous injection of electrons and holes under P/E cycling operation.Stability mainly refers to whether the defect can be restored to the original atomic structure after the electron or hole injection-removal.For the unrecoverable defects,we discuss the change of the trap levels of the initial structure and the stable structure.In the analysis of the defect stability with charge injection in the charge trapping layer of Si3N4,we found that most defects remained stable after electron/hole injection-removal.However,for VNP defects,after hole trapping,it will be relaxed back to the VN defect and could not return to the VNP defect atomic structure.That is,after P/E cycling,the initial VNP defect turns to be a stable VN defect.In addition,in the process of study of the 3D NAND charge injection effect,we found that in most cases,the free H atoms will produce new defects.And most of these new defects are shallow electron trap center,which can enhance the lateral diffusion and degrade the data retention properties.In other words,the free H atom in Si3N4 will aggravate the generation of shallow trap levels and lead to serious lateral charge diffusion problem.These results show that process optimization is very important to improve the reliability of 3D charge trapping flash memory.In the viewpoint of the future development demand of the continuous scaling of the storage cell,the storage density in charge trapping layer needs to be further improved and the shallow level defects affecting the reliability of the storage cell also needs to be better controlled.Therefore,silicon nitride doping is studied to improve electron storage density and reduce shallow level defects.Through the study of different doping elements and doping types,we found that the doping of titanium(Ti),hafnium(Hf)and ruthenium(Ru)atoms would not produce shallow trap levels.Considering the difficulty of doping,Ru atom doping always has relatively high formation energy,which could be difficult for higher doping concentration.However,Ti and Hf have relatively low formation energies when they replace Si atoms,so it is suggested that Ti and Hf can be used as doping atoms.We also carried out further analysis on Ti doping,mainly considering whether Ti atom doping can still maintain the appropriate defect energy level in the presence of H atom and O atom at the interface.We find that although O atom would generate very shallow trap levels in Si3N4,O atom had no effect on Ti atom doping.The defect energy level generated by Ti replacing Si atomic defects is still relatively deep after O atom doping.This indicates that Ti atom doping can still maintain deep trap levels in the presence of O atom at the interface.Most of the current computational work on silicon nitride is based on the study of crystal ?-Si3N4.However,the Si3N4 material in the charge capture layer of the actual memory is amorphous,and its defect concentration is higher than that of the crystalline Si3N4 material,which is more conducive to electron storage.Although ?-Si3N4 and amorphous silicon nitride(a-Si3N4)have similar local atomic structures,their properties can be qualitatively studied by calculating the crystal P-Si3N4.However,since the structure of amorphous materials is not as periodic as that of crystal compounds,the influence of the fluctuations of the atomic environment such as bond length and bond Angle on the distribution of defect energy level is still unclear.Therefore,it is necessary to study the theoretical calculation of amorphous silicon nitride.In this paper,a model of amorphous silicon(a-Si),amorphous silicon(a-SiO2)and a-Si3N4 structures are constructed by bond-switching.Finally,considering the cross-fusion of new materials and new functional devices in the future,we have also done extensive research on the influence of defects in new material devices.Transition metal sulfide(TMDs)has attracted extensive attention due to its advantages such as monatomic layer thickness and absence of dangling bonds on the surface,especially in low-power nano-scale devices.Mono-layer TMDs materials have shown great application potential in the field of nano-electron and nano-photon.In this paper,the effects of defects in WSe2 materials,passivation of different atoms and coupling between defects on the properties of materials and TFET devices were studied.