Ion energy analysis of a bipolar HiPIMS discharge using a retarding field energy analyser

The time evolution of the positive ion energy distribution functions (IEDF's) at the substrate position in an asymmetric bipolar high-power impulse magnetron sputtering (HiPIMS) system was determined using a gridded energy analyser. This was done for a range of operating conditions, namely the positive voltage U (rev) and 'on-time' negative pulse duration tau (neg). The magnetron sputtering discharge was equipped with a Nb target. Based on the knowledge of the IEDF's, the bombarding ion flux density Gamma(i) and energy flux density Q (i) to a grounded surface were calculated. Time-resolved IEDF measurements showed that ions with energies approaching the equivalent of the positive pulse voltage U (rev) were generated as the reverse positive voltage phase developed. On time-average, we observed that increasing the set U (rev) value (from 0 to 100 V), resulted in a marginal decrease in the ion flux density Gamma(i) to the analyser. However, this is accompanied by a five-fold increase in the ion energy flux density Q (i) compared to the unipolar, U (rev) = 0 V case. Reducing the negative HiPIMS pulse duration tau (neg) (from 130 to 40 mu s) at a constant discharge power leads to a modest increase in Gamma(i), but a four-fold increase in Q (i). The results reveal the benefit of the bipolar HiPIMS technique, in which it is possible to control and enhance the power density of ions bombarding a grounded (or fixed bias) substrate, for potentially better tailoring of thin film properties.