Spectroscopic optical emission tomography of direct-current magnetron sputtering discharges in argon-nitrogen gas mixtures

The spectroscopic optical emission tomography of various lines of a direct-current magnetron sputtering discharge (with cylindrical symmetry) is measured. The discharge deals with an aluminium-based alloy cathode. Ar/N-2 gas mixtures are used; their composition and the total pressure are varied with the gas composition going from pure argon to pure nitrogen. The total pressure is varied in the 50 to 150 mtorr range. Emission lines corresponding to the cathode (Al) and the gas discharge species (Ar, N-2, N-2(+) and N) are considered. The three-dimensional emission profiles I(r, z) of the lines are measured; I is the distance from the centre of the cathode, z is the height from the cathode. I(r, z) are measured as a function of the total pressure, the gas composition and the electrical power. In order to compare the results, I(r, z) is fitted with an empirical analytical mathematical function. In order to understand the origin of the experimental results, the motion of electrons in our measured magnetic-field distribution, B(r, z), is calculated. It turns out that, when ejected from a certain zone of the cathode, the electrons remain confined for a long time near the cathode. This zone for confinement of electrons is seen to be in accordance with the zone of optical emission of the plasma. (C) 1999 Elsevier Science S.A. All rights reserved.