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.