Microstructure of hard and optically transparent HfO2 films prepared by high-power impulse magnetron sputtering with a pulsed oxygen flow control

Reactive high-power impulse magnetron sputtering was used to deposit HfO2 films on Si substrates using a voltage pulse duration, t1, from 100 to 200 μs and an deposition-averaged target power density, < Sd >, from 7.2 to 54 Wcm− 2. The effects of these processing parameters on the microstructure and properties of the films were studied by atomic force microscopy, nano-indentation, X-ray diffraction, electron diffraction and high-resolution transmission electron microscopy. Four HfO2 films were prepared with (1) t1 = 100 μs, < Sd > = 7.2 Wcm− 2 (T100S7), (2) t1 = 200 μs, < Sd > = 7.3 Wcm− 2 (T200S7), (3) t1 = 200 μs, < Sd > = 18 Wcm− 2 (T200S18) and (4) t1 = 200 μs, < Sd > = 54 Wcm− 2 (T200S54). All films were found to be composed of an interlayer next to the Si interface followed by a nano-columnar structure layer. The interlayer structure of the films was found to contain a population of lower density nanoscale regions. A reduction in < Sd > in films T200S54, T200S18, T200S7 and T100S7 caused an increase in the interlayer thickness and a decrease in the width of the nano-columnar structures from ~ 46 nm to ~ 21 nm. This microstructural change was accompanied by a concomitant change of the grain boundary structure from tight and interlocking in films T200S54 and T200S18, to rough and thicker (~ 1 nm) boundaries in films T200S7 and T100S7. Films prepared with larger t1 = 200 μs have a monoclinic HfO2 structure and that with smaller t1 = 100 μs exhibits a mixture of monoclinic and orthorhombic HfO2. A high hardness of 17.0-17.6 GPa was shown for films with a monoclinic HfO2 structure. The films exhibited a refractive index of 2.02-2.11 and an extinction coefficient between 0.1 × 10− 3 and 1 × 10− 3 (both at a wavelength of 550 nm). High refractive index was achieved for films T200S54 and T200S18 owing to the presence of a dense microstructure with sharp and interlocking grain boundaries.