Pressure dependence of Ar$_{\mathrm{2}}^{\mathrm{+}}$, ArTi$^{\mathrm{+}}$, and Ti$_{\mathrm{2}}^{\mathrm{+}}$ dimer formation in a magnetron sputtering discharge.

Formation of Ar$^{\mathrm{+}}$ and Ti$^{\mathrm{+}}$ monomer and of Ar$_{\mathrm{2}}^{\mathrm{+}}$, ArTi$^{\mathrm{+}}$, and Ti$_{\mathrm{2}}^{\mathrm{+}}$ dimer ions in a magnetron sputtering discharge with a Ti cathode and in an argon atmosphere was investigated by means of energy-resolved mass spectrometry. Energy spectra of Ar$^{\mathrm{+}}$ and Ti$^{\mathrm{+}}$ ions show distinct features which are related to the specific formation processes taking place in the plasma region and during ion bombardment of the Ti cathode. Our observations proved that low-energy Ar$^{\mathrm{+}}$ ions are produced inside the plasma region. Ti$^{\mathrm{+}}$ ions originate from sputtering events. The measured energy distribution of titanium ions does not follow Thompson's formula, however. At the lowest gas pressure the measured energy distribution can be modelled by a shifted Maxwellian distribution. Formation of Ar$_{\mathrm{2}}^{\mathrm{+}}$, ArTi$^{\mathrm{+}}$, and Ti$_{\mathrm{2}}^{\mathrm{+}}$ dimer ions show pronounced pressure dependencies which are attributed to various formation and loss processes.