Physical-chemical evolution of Hf-aluminates upon thermal treatments

This study presents an investigation on physical-chemical stability of (HfO2)(x)(Al2O3)(1-x) alloys upon prolonged post-deposition annealings. Two different Hf-aluminates were deposited by ALCVD(TM), containing 34% and 74% Al2O3 mol% respectively. Post-deposition annealings (PDA) were carried out in O-2 or N-2 atmosphere, at 850degreesC and 900degreesC for 30 minutes. Interfacial layer (IL) increase after PDA was detected on all the samples, but with small differences between N2 and 02 treatments. Stack composition was characterized by means of XRP, XRF, RBS and TOF-SIMS. Growth of interface layer was justified by limited oxygen incorporation from external ambient. Silicon diffusion from the substrate into high-k material and aluminum/hafnium redistribution were observed and associated to annealing temperature. XRD and planar TEM analysis evidenced first grain formation and then, in the case of Hf-rich samples, almost complete crystallization. Overall, Hf-aluminates were found to remain XRD amorphous during high temperature prolonged treatments up to 900degreesC for 74% and 850degreesC for 34% alloys respectively. Differently from HfO2, (HfO2)(0.66)(Al2O3)(0.34) alloy was observed to crystallized in orthorhombic phase. Hf-aluminates were also electrically characterized by means of C(V) and I(V) measurements on basic capacitors. Variations in material electrical properties were found consistent with change in physical-chemical film structure. Increase in k value up to 30 was observed on Hf-rich samples crystallized in orthorhombic phase.