OPTICAL INVESTIGATIONS OF EXCITATION PROCESSES RESPONSIBLE FOR IONIZED SPUTTERED SPECIES IN A LOW-PRESSURE, LOW CURRENT, COAXIAL GEOMETRY GLOW-DISCHARGE

Investigations of a low pressure (0.3-2.0 torr, 30-270 Pa), low current (1-4 mA), coaxial geometry glow discharge were undertaken to investigate the role of a charge exchange mechanism in the ionization of sputtered cathode material for argon and neon discharges. Emission from three copper ion transitions served as a probe of the charge exchange process. The three transitions monitored were the 221.81 and 224.26 nm transitions which are not accessible to a charge exchange reaction, and the 224.70 nm transition which may undergo charge exchange with an argon ion level. A significant enhancement of the 224.70 nm transition relative to the other two would provide evidence that a charge exchange process is occurring. The results of these investigations did not show a significant enhancement of the 224.70 nm transition, indicating that under these discharge conditions charge exchange is not a significant contributor to the observed ionization of copper sputtered from the cathode surface. In addition, the behavior of each transition was observed with variations in discharge pressure and with the addition of methane to the discharge. All three transitions behaved in an identical manner, with signal peaks occurring at the same pressure as the peak argon metastable population for a pure argon discharge, and signals that decrease linearly for the 10% methane/argon mixture. These studies indicate a common mechanism of ion formation for all three transitions, providing evidence that charge exchange is not a significant contributor to discharge ionization of copper at the low currents employed in these coaxial geometry glow discharges.