55 nm硅-氧化硅-氮化硅-氧化硅-硅闪存单元的γ射线和X射线电离总剂量效应研究

Total ionizing dose effects of γ and X-rays on 55 nm silicon-oxide-nitride-oxide-silicon single flash memory cell

基于^(60)Co-γ射线和10 keV X射线辐射源,系统地研究了55 nm硅-氧化硅-氮化硅-氧化硅-硅闪存单元的电离总剂量效应,并特别关注其电学特性退化的规律与物理机制.总剂量辐照引起闪存单元I-V特性曲线漂移、存储窗口变小和静态电流增大等电学特性的退化现象,并对其数据保持能力产生影响.编程态闪存单元的I_d-V_g曲线在辐照后显著负向漂移,而擦除态负向漂移幅度较小.对比两种射线辐照,擦除态的I_d-V_g曲线漂移方向不同.相比于擦除态,富含存储电子的编程态对总剂量辐照更为敏感;且相比于^(60)Co-γ射线,本文观测到了显著的X射线剂量增强效应.利用TCAD和Geant 4工具,从能带理论详细讨论了55 nm硅-氧化硅-氮化硅-氧化硅-硅闪存单元电离总剂量效应和损伤的物理机制,并模拟和深入分析了X射线的剂量增强效应.

The total ionizing dose(TID)effects on 55 nm SONOS flash cell,caused by 60Co-γray and 10 keV X-ray radiation source,are systematically investigated in this paper.The degradation of electrical characteristics is discussed while the underlying physical mechanism is analyzed.The drift of I-V characteristic curve,the degradation of memory window,and the increase of stand-by current are observed after TID irradiation separately by the two radiation sources.The data retention capability is also affected by the TID irradiation.The I-Vg curve of the programmed single flash cell significantly drifts towards the negative direction after TID irradiation,while the negative drift of erased state is much slower.Referring to the erased state,the drift directions of Id-Vg curves forγ-and X-ray radiation source are obviously different.The physical mechanism of irradiation damage in a 55 nm SONOS single flash cell is discussed in detail by the energy band theory and TCAD simulations.The storage charge loss in silicon nitride layer,the charge accumulation,and the generation of interface states all together lead to the degradation of threshold voltage and stand-by current after TID irradiation.Another cause for the increase of stand-by current is the positive trapped charges in the isolated oxide induced by irradiation,which leads to the generation of leakage paths.Significant dose enhancement effect of X-ray irradiation is observed in this paper.Device model of memory transistor c is established while the dose enhancement effect of X-rays is investigated by Geant 4 tool.The high-Z materials above the polysilicon gate lead to the dose enhancement effect of X-rays’irradiation,which results in the higher degradation.The density of electron-hole pairs produced by irradiation in W layer is much higher than in Cu layer.In particular,W layer is a critical factor regardless of the thickness,which can be obviously observed in the simulation.