Optical emission in magnetrons: Nonlinear aspects

Abstract

Optical emission measurements have been made on the plasmas generated in conventional, planar magnetrons used for high-rate sputtering of metals. The measurements were made as a function of distance from the 20-cm-diam cathode for Al, Cu, and Ti cathodes in Ne, Ar, and Kr plasmas. The emission from the plasma was coupled through a movable collimator and window assembly to a monochromator and diode array detector. The emission intensity was strongly peaked near the sheath, a few millimeters from the cathode. The intensity of the emission of the neutral rare-gas atoms was approximately linear with discharge current at low discharge power, and proportional to the square root of current at high power. This is consistent with previously observed gas rarefaction effects caused by heating by the sputtered atoms. The emission intensity from the sputtered atoms depended on discharge current to approximately the second power at low discharge current, falling to the 1.4 power in some cases at high power levels. The emission intensity for the ionized, sputtered atoms was observed to depend on the discharge power to the 2.6–3.0 power. These latter observations are consistent with a simple model of excitation dependent on the species density and the electron density, coupled with predicted changes in the electron density and temperature as a function of increasing discharge power.