Reactive Sputter Deposition of Superhard Polycrystalline Nanolayered Coatings

Abstract

Nanometer-scale multilayer nitride coatings, also known as polycrystalline nitride superlattice coatings, such as TiN/NbN or TiN/VN with hardnesses exceeding 50 GPa, are deposited by highrate reactive sputtering. The high hardness is achieved by carefully controlling several deposition parameters in an opposed cathode, unbalanced magnetron sputtering system: the target power, the reactive gas partial pressure, the substrate bias voltage and current density, and the substrate rotation speed. Target power controls the deposition rate and the thickness for each layer in conjunction with the reactive gas partial pressure at each target, which also affects the composition of each layer, and the substrate rotation speed. Split partial pressure control is necessary when each layer requires a different partial pressure to be stoichiometric. Fully dense, well-adhered coatings with the highest hardness are deposited when the negative substrate bias exceeds -130 V and the substrate ion current density is 4-5 mA cm⁻². The work on polycrystalline superlattice coatings is being extended into oxide systems. Oxide coatings can now be sputter deposited using pulsed dc power, which prevents arcing on both the target and the substrate. Pulsed dc power along with partial pressure control of the reactive gas leads to significantly higher deposition rates for the oxide films compared to sputtering with conventional rf power.