Deffect profiling in the SiO2/ Al2O3 interface using Variable Tcharge-Tdischarge Amplitude Charge Pumping (VT2ACP)

A Variable T_charge-T_discharge Amplitude Charge Pumping (VT²ACP) is used to profile defect in the SiO₂ and Al₂O₃ separately in Flash Memory based devices. It is shown that by independently controlling the pulse low timing ldquodischarging timerdquo and high level timing ldquocharging timerdquo, the contribution of interface and bulk Al₂O₃ traps can be separated. By using the ellipsometry and the measured intersection time t_charge to trap in the high- k (~60 mus), SiO₂ thickness of 0.87 nm and scanning rate of 0.19 nm/dec is found. Using a slanted wafer, the result shows that in the case of thin SiO₂ (~1 nm) the trap density close to the substrate (short t_charge) is one order of magnitude higher compared to thick SiO₂ (~3nm). For t_SiO2 = 1.7nm all traps are in the SiO₂ or SiO₂/Al₂O₃ transition layer. Only for the thickest SiO₂ layers (2.7 and 3 nm) the trap density becomes low and constant. Additionally WKB-approximation is used to calculate the filling probability of the traps (f_T), the modeled scanning rate nearly doubles to ~0.29 nm/dec and 0.27 nm/dec for amorphous and crystalline, respectively. In summary, the method of trap energy/depth profiling by using VT²ACP allows scanning from ~0.5 nm up to 1.2 nm in depth and 0.1 to 0.7 eV in energy range above the E_c ^Si band depending on sample used. The results show that there exist significant interaction between SiO₂ and Al₂O₃ when processed with PDA 1000degC. For amorphous Al₂O₃ (PDA 700degC) the impact of the precursor is not reflected in the SiO₂ trap density while for crystalline Al₂O₃ no increase in trap density at 0.3 eV above the E_c ^Si band is observed.