Capacitively coupled discharges
In a glow discharge used for analytical purposes, the sample to be analysed forms one of the two electrodes. Usually the sample is held at negative potential in order to cause ablation of material by cathodic sputtering. As the electrodes are an integral part of the electrical circuit maintaining the discharge, insulating materials can not be analysed in this configuration using a continuous direct power supply. If the sample-electrode is non-conductive, applying a DC voltage across the two electrodes will only lead to a short breakdown followed by the creation of a surface charge; no current can flow through the non-conductive sample. A way around this is to apply an alternating voltage, for example RF, between the two electrodes which then alternate between cathodic and anodic behaviour in each half-cycle; the most commonly used frequency is 13.56 MHz (a band allowed by the International Telecommunications Union). This results in a capacitively coupled discharge, so-called because the electrodes and their sheaths form a capacitor (Fig.). High plasma densities can be achieved with sufficient RF input power. The electric field in the chamber transfers energy to both electrons and ions. At the usual RF frequencies and pressures used, because of collisional heating in main plasma body and collisionless heating across the sheath, ions and electrons have different behaviour according to their masses and are therefore not at thermal equilibrium. The resulting high electron temperature sustains the discharge via electron impact ionisation of the fill gas. Capacitively coupled RF plasmas are more efficient in converting the power from the supply into the plasma. RF discharges, compared to equivalent DC discharges also produce much lower energy ions at the cathode, thus minimising sputtering damage at the surface. This is important for example in some microelectronic applications. When the surface area of the two electrodes of an RF discharge is very different a significant negative DC bias voltage may develop at the smaller electrode. Due to the high mobility of electrons the time average of the current, measured over an rf-cycle, remains zero. This asymmetric configuration allows a stable plasma to be sustained and causes continuous ion bombardment of only one electrode, the sample. It thus allows the efficient sputtering of both conductive and insulating material.
First published on the web: 15 March 2008
Author: Hakan Candan . The text is based on a lecture given by Philippe Belenguer at the first GLADNET training course in Antwerp Sept. 2007.