Jui-Che Chen¹, Ping-Yen Hsieh¹, Ying-Hung Chen¹, Ralf Bandorf^1,2, Kai Ortner¹, Ju-Liang He^1
1Feng Chia University, Taiwan
²Fraunhofer Institute for Surface Engineering and Thin Films IST, Braunschweig, Germany
Gas Flow Sputtering (GFS), evolved from the hollow cathode discharge concept, offers distinct advantages such as stable operation under low vacuum, high ionization efficiency, and elevated film growth rates. However, the transient plasma excitation mechanisms, particularly under pulsed-DC operation, remain insufficiently understood. In this study, time-averaged and time-resolved optical emission spectroscopy (OES) was employed to elucidate the temporal and spatial evolution of plasma species in metal-mode GFS using a copper linear source in an argon atmosphere. The effects of gas flow rate, discharge power, and cathode spacing were systematically investigated to reveal correlations between process parameters and plasma behavior along the target–substrate axis.
Experimental results show that a clear power threshold was observed: Cu emission appeared prominently in only when the average input power exceeded 100 W. The intensity ratio ICuI/IArI increased with both gas flow and discharge power, reaching a saturation plateau at high-power and high-flow conditions (e.g., 7 kW and 14 slm), indicating a saturation of excitation efficiency beyond the optimal gas flow range. The ionization ratio (I_CuII/I_(CuI+CuII₎) analysis revealed enhanced Cu ionization at 6 kW, but a gradual decline with increasing Ar flow rate, suggesting collisional quenching effects at higher gas densities. Furthermore, the dependence of plasma characteristics on the product of pressure and cathode spacing (p·d) was examined to clarify its influence on discharge stability and excitation dynamics. Overall, the OES diagnostics provided a comprehensive understanding of the transient plasma behavior in GFS, offering valuable insights into the optimization of discharge conditions for efficient metallic film deposition.