Influence of the magnetic field on the discharge physics of a high power impulse magnetron sputtering discharge

Abstract

The magnetic field is a key feature that distinguishes magnetron sputtering from simple diode sputtering. It effectively increases the residence time of electrons close to the cathode surface and by that increases the energy efficiency of the discharge. This becomes apparent in high power impulse magnetron sputtering (HiPIMS) discharges, as small changes in the magnetic field can result in large variations in the discharge characteristics, notably the peak discharge current and/or the discharge voltage during a pulse. Here, we analyze the influence of the magnetic field on the electron density and temperature, how the discharge voltage is split between the cathode sheath and the ionization region, and the electron heating mechanism in a HiPIMS discharge. We relate the results to the energy efficiency of the discharge and discuss them in terms of the probability of target species ionization. The energy efficiency of the discharge is related to the fraction of pulse power absorbed by the electrons. Ohmic heating of electrons in the ionization region leads to higher energy efficiency than electron energization in the sheath. We find that the electron density and ionization probability of the sputtered species depend largely on the discharge current. The results suggest ways to adjust electron density and electron temperature using the discharge current and the magnetic field, respectively, and how they influence the ionization probability.

Modeling of high power impulse magnetron sputtering discharges with tungsten target

“…In figure 9(a) we see that the ionization probability α t increases while the back-attraction probability β t,pulse decreases with increased discharge voltage. Earlier we have argued that the ionization probability depends only on the peak discharge current and increases with increased peak discharge current [70] and the decreasing backattraction probability with increasing peak discharge current confirms the suggestion made by Brenning et al [71,72]. Recall that the peak discharge current increases with increased discharge voltage (see figure 2), and the peak current density J D,peak in this study is in the range 0.33-0.73 A cm −2 .…”

Modeling of high power impulse magnetron sputtering discharges with graphite target

“…In particular, α t for carbon is much lower than it is for titanium. At a peak current density of 1 A cm −2 the time averaged ionization probability for carbon is ∼13% compared to a time averaged ionization probability >80% for titanium [81]. This shows that the primary cause of the low F flux in carbon HiPIMS discharges is the difficulty of ionizing carbon.…”

“…The electron temperature is high and evolves similarly for all cases during the pulse. For comparison in a HiPIMS discharge with titanium target the cold electron temperature is below 4.9 eV [81]. In this case the presence of metal atoms and ions lowers the electron temperature due to their low ionization potential, which is not the case for graphite target.…”

Foundations of physical vapor deposition with plasma assistance

“…Ohmic heating accounts for a significant fraction of the electron power absorption in dcMS discharges, while it is believed to be the dominating electron power absorption mechanism in high power impulse magnetron sputtering (HiPIMS) discharges (Brenning et al 2016, Huo et al 2017. For discharges with a higher fraction of Ohmic heating over total electron heating (Ohmic heating plus sheath energization), the same discharge current can be maintained at a lower discharge voltage, as the discharge becomes more energy efficient (Rudolph et al 2022).…”