Ion energy distributions at substrate in bipolar HiPIMS: effect of positive pulse delay, length and amplitude

A bipolar HiPIMS discharge with a rectangular positive voltage pulse (with controllable amplitude, delay after the main negative pulse and positive pulse length) was systematically investigated by mass spectroscopy. The time-averaged spectra of ions measured at the substrate position exhibit a prominent high-energy peak. It is shown that the position of the peak can be varied by the positive pulse amplitude, its magnitude scales with the pulse length and its width can be slightly influenced by the length of the delay interval. Measurements of the plasma potential at the mass spectrometer position and time-resolved mass spectroscopy clearly show that the high-energy peak is formed by ions accelerated by the elevated plasma potential during the positive pulse. In addition, fine details of the ion energy distribution functions related to the plasma potential transients at the start of the positive pulse are identified. The presented results are beneficial for the optimisation of the parameters of the positive pulse in experiments implementing the bipolar HiPIMS technology.