Pulse length selection in bipolar HiPIMS for high deposition rate of smooth, hard amorphous carbon films

Amorphous carbon films were deposited by bipolar HiPIMS, as a function of negative and positive voltage pulse lengths (50-175 μs and 0-175 μs respectively), using argon as sputter gas. The deposition rate, compressive stress, sp3 fraction and mechanical properties of the films were investigated and the results compared with those of amorphous carbon films deposited by conventional unipolar HiPIMS. We found minimum threshold positive and negative lengths are required in bipolar HiPIMS to enhance the sp3 fraction above 45 % and reduce the argon content. In addition to increasing the flux and energy of depositing ions by electrostatic control, bipolar HiPIMS also increases the flux ratio of depositing ions to sputter ions and thus reduces the probability of sputter gas incorporation into the growing amorphous carbon layers. Reduced argon content in the coatings correlates with high residual stress, high hardness and evidently enhanced tool cutting functionality.