ToF-SIMS depth profiling of Hf and Al composition variations in ultrathin mixed HfO₂/Al₂O₃oxides

Ultrathin (a few nm) mixed HfO/AlO oxides deposited by ALCVD on Si substrates were analyzed with low energy (250eV) Cs and Xe ions by ToF-SIMS in the dual beam mode. The analysis beam was 15keV Ga. Several layers were measured, with different metal ratios (Hf:Al) and different number of deposition cycles. We observed a significant enhancement of the Al (and AlO clusters) signal at the surface along with a Hf (and HfO clusters) depletion at the film surface. However, the thinnest films were found to contain much more Hf than the thickest ones. Several ion ratios were used (e.g. HfO /AlO or HfO/AlO ) in order to quantify the Hf:Al concentration ratio in the films. We found that these ratios correlate well with each other, with the metal deposition ratio, and with XPS ratios, enabling us to reconstruct the Al and Hf composition variation in the film.