Ultrathin films of carbon nitride were grown using d.c. magnetron sputtering in a single cathode deposition system. A high frequency pulsed bias was applied to both the target and the substrate during deposition to help sustain a stable plasma and enhance ion bombardment of the growing film. These films exhibit excellent corrosion resistance at film thickness down to 1 nm. Atomic force microscopy measurements reveal an atomically smooth surface. The role of pulsed bias and bombardment by ionized species in obtaining low defect-density coatings is discussed.