The distribution of ion energies at the substrate in an asymmetric bi-polar pulsed DC magnetron discharge

Abstract

Using an energy-resolved mass spectrometer and a time-resolved Langmuir probe, the distribution of bombarding ion energies, their fluxes and energy fluxes at a substrate in an asymmetric bi-polar pulsed DC magnetron have been determined. The discharge was operated in Ar at a pressure of 0.53 Pa with a Ti target and pulsed DC frequencies of 100 and 350 kHz with a range of duty cycles (from 50 to 96%). At 100 kHz, the Ar+and Ti+ time-averaged ion energy distribution functions (IEDFs) reveal three peaks, which are at low energy (<10 eV), in a mid-range (20-50 eV) and at high energy (60-100 eV). We correlate these peaks with distinct phases of the discharge voltage. At 350 kHz the IEDFs show four peaks reflecting a more complex voltage waveform. The low-energy ions are generated in the `on' phase when the plasma potential is typically a few volts above ground. The Ti+ energy spectra show a remnant of the original sputter-neutral energy distribution function. The mid-range ions are produced in the quiescent region of the voltage reverse phase, when the plasma potential is raised globally a few volts above the cathode potential, typically 10-30 V. The high-energy ions are generated in a period of ~0.3 µs, during the discharge voltage overshoot, when the target potential rises to typically over +140 V. However, given the time resolution of the Langmuir probe (0.5 µs), it is not possible to determine if plasma potential is lifted globally to this high potential or only close to the cathode. At 350 kHz, these `fast' ions make up to about a quarter of the total ion flux at the substrate and an upper bound transient power flux of about 2.5 times the maximum delivered in the `on' phase. The total power flux to a substrate in the sustained phase of the discharge is found to increase with frequency and reverse time.