Tomography of Optical Emission Line Intensities of a Radio-Frequency Magnetron Discharge

Abstract

An experimental setup for spectroscopic optical tomography of glow discharges is presented. It combines a fiber-optics system with a monochromator coupled to a charge-coupled device (CCD) camera. The light emitted by the discharge is sent to the input of 10 optical fibers through a collimating system. The outputs of these optical fibers are coupled to the entrance slit of a monochromator. The CCD camera is in the output slit. This arrangement allowed us to acquire 10 spectra simultaneously, each being associated with a small zone of the discharge. By translation of the optical fibers, the whole discharge was studied. The data were then analyzed by means of a tomographic reconstruction program in order to obtain the three-dimensional spectroscopic optical tomography, for a large number of lines. This experimental setup was used to deduce the three-dimensional optical emission of various lines of a radio-frequency (rf)-powered magnetron discharge. The discharge deals with an aluminum base alloy cathode, sputtered by Ar. Emission lines corresponding to the cathode (Al: 394.4- and 396.2-nm lines) and the gas discharge (Ar: 763.5 nm-line) were considered. The three-dimensional emission profiles, I( r, Z), of the lines were measured; r is the distance from the center of the cathode, and Z is the distance from the cathode. I( r, Z) profiles are different for the Al and Ar lines. The effects of the electrical power and the pressure were also studied. It was found that the corresponding I( r, Z) profiles behave differently with the pressure. In order to compare the results, I( r, Z) was fitted with an empirical analytical mathematical function, with parameters depending on pressure. At high pressure, the shape of I( r, Z) for the Al line tends towards the shape of the Ar line.