Nucleation and growth behavior of atomic layer deposited HfO2 films on silicon oxide starting surfaces

Growing nanometer-thin HfO2 films by atomic layer deposition (ALD) for implementation in advanced transistor structures is controlled by the density of reactive OH sites on the surface. The impact of thin SiO2 starting surfaces, grown by wet chemical processes and by wetting a thermal oxide, on the nucleation and growth of ALD HfO2 has therefore been evaluated. Our results demonstrate that both surface pretreatments display the same dependence of the initial HfO2 growth on the interfacial layer thickness. This correlation is first characterized by a linear increase, which can be interpreted in terms of increasing OH surface concentration. Once an ellipsometric oxide thickness of approximately 0.8 nm is reached, saturation of the HfO2 deposition occurs. Maximal OH coverage of the surface or steric hindrance of the adsorbed precursor molecules could explain this observation. However, the increased growth-per-cycle at lower deposition temperatures can be attributed to an improved hydroxylation of the surface, excluding steric hindrance as the primary factor causing saturation. Furthermore, electrical characterization revealed that both interfacial oxides show identical leakage scaling behavior down to an equivalent oxide thickness of 0.8 nm.