Interface of ultrathin HfO₂ films deposited by UV-photo-CVD

We report in this article the deposition of ultra-thin HfO₂ films on silicon substrate at 300-450 °C by photo-induced CVD using 222 nm excimer lamps. As-deposited films from 2.5 to 10 nm in thickness with refractive indices from 1.60 to 1.85 were grown. The deposition rate measured by ellipsometry was found to be 3 nm/min at a temperature of 400 °C. XRD showed that as-deposited HfO₂ films were basically amorphous. Investigation of the interfacial layer by XPS and TEM reveals that thickness of the interfacial SiO₂ layer slightly increases with the UV-annealing time and UV annealing can convert suboxide at interface into stoichiometric SiO₂, leading to improved interfacial quality. Fourier transform infrared spectroscopy (FTIR), revealed that Hf-O absorption in the photo-CVD deposited HfO₂ films is quite different at various deposition parameters. When compared to similar studies performed previously in zirconium, titanium and tantalum oxides, the interface layer of HfO₂ is generally thinner than that of other materials. UV annealing can convert a suboxide at interface between the HfO₂ and Si into stoichiometric SiO₂, leading to improved interfacial quality. The thickness of the interface layer (silicon oxide or hafnium silicate) decreases with increasing thickness of the top HfO₂ layers.