Investigation On The Effects Of Heavy Ion Irradiation In High-k HfO₂ Gate Dielectric Materials And Devices

Advanced nanoscale CMOS devices have been widely used in onboard electronic systems for space application.The high-k gate dielectrics,emerges as a leading candidate to replace SiO₂,has been utilized for CMOS mass production below the 45nm technology node as the integrated level of electronic device increases continuously.It was reported that heavy ions in space can induce leakage current,breakdown,life reduction,reliability degradation and other phenomena of high-k HfO₂ devices.Thus,it is very important to understand the degradation mechanism of HfO₂ high-k gate dielectric materials and devices induced by heavy ion irradiation for its space application.In this thesis,the HfO₂ film specimens and HfO₂ MOS devices were irradiated with 6.3 MeV/u ⁵⁶Fe,25 MeV/u ⁸⁶Kr ions and 12.5 MeV/u ¹⁸¹Ta ions accelerated by the Heavy Ion Research Facility in Lanzhou?HIRFL?and 4 MeV ¹³²Xe ions offered by 320kV experimental platform respectively.With a variety of high-resolution characterization methods,the latent tracks in amorphous HfO₂ were observed for the first time.The reasons for the ion irradiation induced reliability degradation of high-k devices can be explained.The main research results are as follows:Firstly,the mechanism of swift heavy ion irradiation induced degradation of electrical properties of high-k HfO₂ MOS devices was investigated.It was found that the relative dielectric constant decreased from 18 to 4 with the irradiation fluence and electron energy loss?dE/dx?_e,and the radiation-induced leakage current was clearly increased with increasing fluences and?dE/dx?_e until a leakage current density of 10^-4A/cm² was achieved.The microstructure was characterized by high resolution transmission electron microscope and the fine structure of latent track in amorphous HfO₂ was crystalline phase along the ion trajectory.The quantitative relationship between microstructure changes and electrical properties indicated that the crystallization of the amorphous HfO₂ film affected the value of the relative dielectric constant and the magnitude leakage current through the dielectric layer along the grain boundaries.We provided a new method to estimate the latent track area.The deduced diameter of the latent track in amorphous HfO₂ were 4.4 nm at 12.6 keV/nm,5.6 nm at15.0 keV/nm,6.7 nm at 19.5 keV/nm,and 13.8 nm at 44.8 keV/nm respectively.A new method to estimate the threshold of the?dE/dx?_e for crystallization was proposed,the predicted threshold of the?dE/dx?_e for crystallization to be about 10 keV/nm.This method allows for the prediction of device sensitivity to swift heavy ion irradiation.Secondly,we focus on the influence of heavy ion irradiation on charge trapping effect in HfO₂ MOS devices.After irradiated by 4 MeV ¹³²Xe ions,it was found that the capacitance in the accumulation region decreased,the threshold voltage shifted in the negative gate bias direction and there was a significant charge trapping effect.It was expected that the sharp reduction in capacitance and the shift of the C-V curves result from the increase of interface traps and the irradiation induced oxygen vacancy is the dominant defect in HfO2 MOS devices.During charging process,the oxygen vacancies produced by irradiation became the trap centers and capture electrons,resulting the parallel shift of the C-V curves and degrade the reliability of device.Finally,the effect of swift heavy ion irradiation on the microstructural properties of amorphous HfO₂ films was investigated.We analyzed the crystal structure,the degree of crystallinity,grain size and atomic chemical environment.It was found that the stoichiometric ratio of Hf/O changed from 2.481:1 to 2.154:1 after irradiated to2.13×10¹⁴ ions/cm²,which is closer to the stoichiometric ratio?2:1?of HfO₂,and the ratio of completely oxidized Hf and lattice oxygen were increased as well.The analysis of X-ray diffraction indicated that the amorphous HfO₂ films crystallized after irradiation.When the fluence the ⁵⁶Fe ions increased from 5×10¹³ ions/cm² to 2.13×10¹⁴ions/cm²,the crystallinity of the film increased from 10.5%to 38.7%,the average grain size increased from 12 nm to 14 nm,and the monoclinic phase transformed to the cubic phase.The mechanism of the effects of energetic heavy ion irradiation in HfO₂ gate dielectric materials and devices was investigated in the thesis.It has provided basic data for reference of nanodevices in space.