Plasma parameters in positive voltage pulses of bipolar HiPIMS discharge determined by Langmuir probe with a sub-microsecond time resolution

Abstract

We have determined the local plasma parameters using the Langmuir probe measurements with a sub-microsecond time resolution during positive voltage pulses of a bipolar high-power impulse magnetron sputtering discharge using an unbalanced magnetron with a titanium target. The effects of the positive voltage pulse amplitude and the delay between the negative voltage pulse end and the positive voltage pulse initiation are investigated as well as the spatial dependence of the plasma parameters at three distances from the target. From the results, the values of the average energy flux of ions during the positive voltage pulse to the substrate are estimated. We have found that the time evolution of the plasma parameters has similar developments which are independent of the positive voltage pulse parameters and the distance from the target, although the values of the plasma parameters are different. During the initial part of the positive voltage pulse, a large difference (up to 200 V) between the plasma and the floating potential accompanied by a high electron temperature (up to 150 eV) and a significant decrease of electron density (up to one order of magnitude) is registered. After this part, the difference of the potentials and the electron temperature are low (<2 V and ≲1 eV, respectively). The short delays between the negative voltage pulse end and the positive voltage pulse initiation as well as the higher positive voltage amplitudes have a beneficial effect on the average energy flux of ions during the positive voltage pulse to the grounded and insulated substrates.