The determination of the current-voltage characteristics of a closed-field unbalanced magnetron sputtering system

Abstract

An investigation has been carried out into the current-voltage ( I-V) characteristics of a closed-field unbalanced magnetron sputtering system (CFUBMS). CFUBMS is a versatile technique for the high-rate deposition of high quality metal, alloy and ceramic coatings. A key factor in this system is the ability to transport high ion currents to the substrate. This can enhance the formation of fully dense coatings at relatively low values of homologous temperature. A study has been made of the parameters that determine the I-V characteristics of this system, at both the target and the substrate. From this, empirical relationships have been developed to describe the substrate ion current density and the substrate self-bias potential. Measurements have been taken of the discharge voltage, the substrate self-bias voltage, and substrate current using aluminium, zirconium, tungsten, and copper targets, at target currents of up to 8 A, over the pressure range 0.5-3 mTorr, and at substrate-to-target separations of 80 mm, 110 mm and 150 mm. The discharge voltage, VT, follows the established relationship I T= β( V T− V 0) 2, where I T is the discharge current and V 0 is the minimum voltage required to strike a discharge. The variations of β and V 0 with target material and pressure are discussed. At the substrate, a linear relationship, I s= mI T, was observed between the ion current drawn at the substrate, I s, and the discharge current, I T, where the coefficient of proportionality, m, varies with chamber pressure and substrate-to-target separation. Also, self-bias voltages, in the range −17 to −30 V, were measured. The value was largely independent of target material and substrate-to-target separation, but depended strongly on chamber pressure.