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

Abstract

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 τ neg. The magnetron sputtering discharge was equipped with a Nb target. Based on the knowledge of the IEDF’s, the bombarding ion flux density Γi 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 Γi 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 τ neg (from 130 to 40 μs) at a constant discharge power leads to a modest increase in Γi, 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.