Ionization of sputtered Ti, Al, and C coupled with plasma characterization in HiPIMS

The ionization of sputtered Ti, Al, and C has been investigated in non-reactive high-power impulse magnetron sputtering discharges using Ar as a process gas. Two complementary techniques, time-resolved Langmuir probe diagnostics and a recently developed gridless ion meter, have for the first time been used to estimate absolute values of the ionized fractions of the sputtered material. To cover a range of commonly used discharge conditions we have carried out measurements for different process gas pressures, discharge current densities, and pulse lengths. It is found that by increasing the current density from 0.5 to 2.0 A cm⁻² there is a general increase of $n_{\text{e}}$ independently of target material and position in time with maximum plasma densities of about 1   ×   10¹⁸–5   ×   10¹⁸ m⁻³ above the target race track. Also the ionized flux fraction, measured by ion meter, is increased when increasing the current density and reaches a maximum value of 78% in the Al discharge. By using the recorded $n_{\text{e}}$ and $T_{\text{e}}$ values to calculate the ionization probability of the sputtered material, and benchmark these results using the ion meter, we also show that Langmuir probe diagnostics is a useful tool to estimate trends and changes in the degree of ionization for different process conditions. (paper)