Ultrasmooth CN_x overcoats for next-generation hard disks

Ultrathin carbon nitride (CN_x) overcoats were grown using pulsed dc magnetron sputtering. Substrates were mounted on a holder that allowed 45° tilt angle and rotation at different speeds. Effects of sputter gas composition, target power, substrate bias, substrate tilt, and rotation on film growth and properties were explored. Atomic force microscopy scans over large sampling areas show that thin CN_x films obtained at -100 V substrate bias with 45° substrate tilt and 20 r/min rotation have root-mean-square roughness almost four times lower than those prepared without substrate tilt and rotation. Several percent of helium in an Ar-5%N₂ sputter gas was found to improve surface roughness of these coatings. These 1-nm-thick ultrasmooth coatings reduced corrosion damage compared with coatings of the same thickness grown without substrate tilt and rotation. This improved performance appears to be directly related to the growth of smoother and denser coatings synthesized under the present conditions.