Defects in the interfacial layer of SiO2-HfO2 gate stacks: Depth distribution and indentification

The defects in the SIO₂ interfacial layer (IL) in SiO₂-HfO₂ gate stacks are studied using charge pumping (CP). To that aim, conventional CP curves and interface trap density vs. energy, D_it(E), profiles measured on these devices are compared with those recorded from state of the art (S-A) MOSFETs. By doing so, specific features in the results recorded from the devices with HfO₂ are pointed out. Then, trap depth concentration, N₁(x), profiles, recorded from the Si- SIO₂ interface towards HIO₂ are reported. With regard to S-A MOSFETs, these profiles provide evidence of 1) a strong increase in N, towards HfO₂, 2) an impact of HfO₂ on the trap properties up to the Si-SiO₂ interface, and 3), a large increase in the minimum N, value. As the SiO₂-HfO₂ interface is probed for the devices annealed at the lowest temperature, T_A, the profiles show a continuous increase of N_t from SiO₂ to HfO₂. Then, the impacts on the trap profiles of both T_A, and the HfO₂ thickness deposited, d_HfO2, are dealt with. A higher T_A yields SIO₂ re-growth. This 1) pushes the increase of Nt observed towards HIO₂ further away from the Si-SiO₂ interface but 2) yields also to a deeper penetration of this increase in the IL. The first effect dominates. Also, it seems that increasing d_HfO2 slightly increases the trap penetration into the IL. Finally, the two states of the P_bo center are identified in the trap depth concentration profiles. Their characteristics differ from those in S-A MOSFETs. They are correlated for the first time in fresh and fully processed devices with the interface trap densities measured in both parts of the silicon bandgap. By adjusting the experimental conditions, the N₁(x) profiles corres- ponding to the two P^ center levels can be drawn.