Influence of La on the electrical properties of HfSiON: From diffusion to Vth shifts

We investigated the dependence of the electrical properties of TiN/La2O3/HfSiON/SiO2/Si-substrate stacks with 15 nm thick HfSiON layers on annealing temperature and time. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and capacitancevoltage (CV) measurements were used to characterize the La depth profiles and the electrical properties. We found that the threshold voltage (Vth) shift after annealing correlates linearly with the total amount of La diffused into the HfSiON rather than with the La concentration at the HfSiON/SiO2 interface. This unexpected behavior can be explained by the decreased thermal stability of thick (>2 nm) HfSiON layers which probably leads to phase separation during annealing and therefore to HfO2/SiO2 interfaces all over the stack.