Effects of ion-induced electron emission on magnetron plasma instabilities

Abstract

Some magnetron sputtering systems experience rapid oscillations in the current and voltage of the plasma discharge after several hours when equipped with certain targets. These oscillations often lead to the plasma becoming extinguished, a condition known as ‘‘flame-out.’’ This article details the study of two 90% W–10% Ti magnetron targets which differed in density. The higher density targets sometimes experienced flame-out after approximately 3 h of sputtering. The less dense material could be sputtered for the entire 15 h life of the target. Scanning electron microscopy pictures and atomic composition depth profiles were obtained using Auger electron spectroscopy. In addition, a Colutron-based ion source with a high vacuum system was used to measure ion-induced secondary electron emission coefficients as a function of energy, ion specie, and gas coverage. Analysis of the sample from the group that suffers flame-out showed large regions of pure titanium in the interior of the sample and higher levels of oxygen contamination. These oxide regions act as insulators in the material which cause the secondary electron emission to decrease, the plasma current to drop and the voltage to rise. The less dense targets had a surface topography which helped overcome these decreases is electron emission. Experiments showed that an increase in the voltage with respect to the surface significantly increased electron emission, for the less dense targets, counteracting any drops in plasma current.