Particle in cell simulation of dual frequency capacitive discharges

Summary form only given, as follows. Capacitive discharges have been used in the manufacture of microchips and in related applications for many years. A typical aim in these applications is to effect a surface modification by delivering a flux of energetic ions onto the surface. Consequently, the ion flux and ion energy are critical process parameters. For the most part, such discharges have been excited by a single frequency which has almost invariably been 13.56 MHz. Recently, there has been interest in exciting this type of discharge with two frequencies, where typically one frequency is smaller than 13.56 MHz and the other larger. The point of this complication is to separately control the plasma density and sheath voltage, and thereby independently control the ion flux and average energy. Since the impedance of the plasma is usually dominated by the capacitance of the sheaths, it is reasonable to expect that an appropriately high frequency will drive a large current through the plasma bulk without dropping a large voltage across the sheaths. Conversely, a suitably low frequency will drive a low current through the bulk while dropping a large voltage across the sheaths. Therefore, the hope is that the higher frequency will couple primarily to the electrons, while the lower frequency couples to the ions. This ideal picture is not exactly realised in practice, and the aim of the present work is to apply self-consistent kinetic simulation to the problem of discovering what conditions must be satisfied in order that a dual frequency discharge will operate in the way intended. For this purpose we use an electrostatic particle in cell simulation with Monte Carlo collisions. We do not consider electromagnetic effects, which may well be significant in some parts of the parameter space of practical interest.