On Recycling in High Power Impulse Magnetron Sputtering Discharges

The high power impulse magnetron sputtering (HiPIMS) discharge is a recent addition to plasma assisted physical vapor deposition (PVD) methods. Then a high density plasma is created by applying high power pulses at low frequency and low duty cycle to a magnetron sputtering device. Here we discuss the large discharge currents observed in HiPIMS discharges. We discuss the current composition and the role of self-sputter (SS-) recycling and working gas recycling within the discharge. We find that above a critical current density
$\mathrm{J}_{\text{crit}}\approx 0.2\mathrm{A}/\text{cm}^{2}$
, a combination of self-sputter recycling and working gas-recycling is the general case [1]. For high self-sputtering yields, above
$\mathrm{Y}_{\text{SS}}\approx 1$
, the discharges become dominated by SS-recycling, contain only a few hot secondary electrons that have been accelerated across the cathode sheath and have a relatively low effective electron temperature. Here, stable plateau values of the discharge current develop during long pulses, and these values increase monotonically with the applied voltage. For low self-sputtering yields, below
$\mathrm{Y}_{\text{SS}}\approx 0.2$
, the discharges operated above
$\mathrm{J}_{\text{crit}}$
are dominated by working gas recycling, acceleration of secondary electrons has a significant role and the effective electron temperature is higher, while the current evolution is generally less stable. For intermediate values of
$\mathrm{Y}_{\text{SS}}$
the discharge character gradually shifts between these two types. In addition, these new insights on ion recycling have been applied to a series of selected sputter materials. For high currents a discharge with Al target develops almost pure self -sputter recycling, while a discharge with Ti target exhibits close to a 50/50 combination of self-sputter recycling and working gas-recycling [2]. However, if the Ti target is operated in a reactive Ar/O
2
gas mixture, it is found that working gas-recycling is dominating [1] and that the Ar+- ions contribute almost solely to the discharge current [3].