Characterization and properties of amorphous carbon coatings prepared by middle frequency pulsed unbalanced magnetron sputtering at different substrate bias

► a-C coatings were synthesized by middle frequency pulsed unbalanced magnetron sputtering technique. ► The coating deposited at 120 V exhibited the optimized synthetic properties. ► The AES depth profile exhibited that the C element distributes uniformly and has penetrated into the Si substrate. ► The influences of substrate bias on the sp3 fraction can be explanted by the sub-plantation model. ► The coatings may be used as antireflection protection coatings.