Time-resolved ion mass spectrometry analysis reveal high Bi2+ and Bi3+ ion fluxes during high-power impulse magnetron sputtering of Bi target

Time-resolved ion mass spectrometry was used to investigate the ion fluxes present during high-power impulse magnetron sputtering of a Bi target in an argon atmosphere. The study focuses on the temporal evolution of singly, doubly, and triply charged Bi ions (Bi+, Bi2+, and Bi3+) under two different discharge conditions with varying peak target current densities (JT) and pulse frequencies (f). The results show that Bi2+ and Bi3+ ions dominate the ion population in the postpulse plasma, with Bi2+ exhibiting the highest flux across both discharge conditions. Interestingly, the lower peak target current density JT = 0.07 A/cm2 (f = 2000 Hz) results in higher total ion fluxes and longer ion persistence compared to those detected with JT of 0.18 A/cm2 (f = 700 Hz). Severe Ar gas rarefaction, enhanced by the high sputter yield and large mass of Bi, strongly influences the ion energy distribution functions. Both Bi2+ and Bi3+ exhibit a dominant low-energy peak (0–10 eV) from ions emitted in preceding pulses, together with broad Sigmund–Thompson-like tails extending up to ∼25–30 eV due to ions generated during the ongoing discharge. At JT = 0.07 A/cm2 (f = 2000Hz), the short off-time (470 μs) is insufficient for full ion thermalization, resulting in residual high-energy ions persisting into the early afterglow. In contrast, at 0.18 A/cm2 (f = 700 Hz), the longer off-time (∼1.4 ms) enables complete thermalization. As a result, the mean ion charge state remains above 2.0 in both cases but is slightly higher at lower JT (2.17–2.28 vs 2.07–2.15). These findings provide practical guidelines for substrate biasing: bias should be applied after ∼90 μs to minimize Ar implantation and optimized at reduced amplitudes (2–3 times lower than for Ta or W) to take advantage of the high Bi charge states without inducing excessive ion damage.