The amorphous carbon (a-C) coatings were synthesized at different substrate bias using a middle frequency pulsed unbalanced magnetron sputtering technique. Some properties of the coatings were characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), nano-indentation tests, spectroscopic ellipsometry (SE). SEM measurement shows that the deposited a-C coatings are comparatively uniform. XPS and AES measurements show that sp(3) content in a-C coatings changes with increasing substrate bias voltage. AES depth profile exhibits that the C element distributes uniformly across the coating thickness, and carbon atoms have penetrated into the Si substrate. Nano-indentation and spectroscopic ellipsometry tests indicate that the coating deposited at the substrate bias of 120 V shows the optimized synthetic properties with higher nanohardness, higher elastic modus, higher refractive index and lower extinction coefficient. These results above are useful for the practical application of a-C as antireflection protection coatings. The influences of substrate bias on the sp3 fraction can be explanted by the sub-plantation model. (C) 2012 Elsevier B. V. All rights reserved.