Research On The Application Of Si Nanocrystals In The Irradiation Hardening Of SIMOX Materials

Si implantation have been proved to be an effective method to harden the buriedoxide of SOl materials, which limits the application of SOI devices in total-doseenvironment. Si implantation will produce Si nanocystals which can be electron trapsduring the irradiation. My research is focused on the application of Si nanocrystals inthe irradiation hardening of SIMOX materials, which can be devided into two mainparts. One is the total-dose irradiation effect of MOSFET and pseudo-MOS structureon Si-implanted SIMOX, with MEDICI simulation to verify the bias dependence. Theother part is the physical research of Si-implanted thermal oxide to understand theformation of Si nanocrystals and implantation-induced defects, and their relation withimplantation and annealing conditions.â… -â…¤characteristics of partially-depleted NMOSFETs with various gate structuresfabricated on SIMOX which is hardened by silicon ions implantation were studiedunder total-dose irradiation of three bias conditions. It has been found experimentallythat back gate threshold shift and leakage current was greatly reduced during irradiationfor hardened transistors, comparing to NMOSFETs fabricated on unimplanted wafers,especially for the enclosed gate and H gate structure. Subthreshold charge separationtechnique is employed to estimate the buildup of oxide charge and interface trapsduring irradiation, showing that the reduced⊿V_th for NMOSFET/SIMOX (implanted)is mainly due to the much less buildup of oxide charge than NMOSFET/SIMOX(unimplanted). The total-dose irradiation effect of SIMOX pseudo-MOS structuresare also investigated. This easy and convenient method testified that Si-implantationcan significantly decrease the irradiation-induced⊿V_th and the buildup of oxide charge and interface traps. MEDICI simulation confirmed that⊿V_th is relevant to the bias stateand PG is the wprst bias condition.We also studied the photoluminescence (PL), X-ray Photoelectro-Spectroscopy(XPS), Fourier Transform Infra-Red (FTIR) and Electron Spin Resonance(ESR) of Sinanocrystals embedded in thermal oxide implanted with Si. PL analysis proved theexistence of Si nanocrystals and demonstrated the effect of Si implantation dose, energyand the annealing time on the growth of Si nanocrystals and its emission intensity. Thesize of Si nanocrystals and the PL intensity increase with the Si implantation dose. Thenanocrystals can trap electrons to compensate the positive charge buildup in the buriedoxide during irradiation, thus reduce the threshold voltage negative shift. The XPSresults clearly show the evolution of various chemical structures and the formation of Sinanocrystals in the oxide of our samples. It is found that the concentration of Sinanocrystals increases with Si implantation dose. The 1h annealing at 1000℃willproduce the most Si nanocrystals among all the annealing conditions. FTIRmeasurement showed that the absorption intensity decreases with the Si implantationdose, indicating that increasing the Si implantation dose leads to the reduction of theSiO₂ structure. It is also found that the redshift of stretching frequency of the Si-O bond,which could be interpreted as the narrowing of the O-Si-O bond angle as a result of thereplacement of O atoms by the implanted Si atoms and the formation of Si-Si bonds.ESR research determined the g factors and the density of suspending bonds of samplesimplanted with different energy or dose. It proved that existence of P_b center, i. e., Sisuspending bonds located in the Si-SiO₂ interface.