Current and voltage (I-V) characteristics of intermediate pressure plasma

Capacitively coupled RF discharges are often characterized by their operating pressure and the resultant 'heating mode'. The pressure regime between 1-5 Torr remains largely unexplored although this pressure is employed for a number of applications. At these higher pressures, as collision mean free path becomes much smaller than the sheath thickness, ohmic heating becomes important. Specifically, electrons in the sheath gain energy from the oscillating electric field and then heat the plasma by transferring that energy through collisional processes. To understand the heating of these collisional electrons, time-dependent current and voltage (I-V) on the powered electrode were measured for argon, nitrogen and oxygen plasma in the range of 0.5 – 2.5 Torr. Typical plasma control parameters such as electrode voltage, conduction and displacement currents are calculated as a function of frequency, pressure, deposited power and used gas. A design of experiment (DOE) was carried out to find any functional relationships between the system control parameters: electrode gap; pressure; power; and gas composition, and the resulting frequency-dependent currents and voltages. From our measurements, we find that gap has the least impact on any of the resultant I-V parameters. Pressure and power have the most effect on any of the I-V parameters while gas chemistry also plays a role but to a lesser extent. We will present these results and examine some of the mechanisms through which the control parameters impact the measured I-V.