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 in a number of applications. At these higher pressures, as the collision mean-free path becomes much smaller than the sheath thickness, high energy ions entering the sheath at the Bohm velocity generate a substantial number of electrons within the sheath. These electrons gain energy from the oscillating (RF) electric field and then heat the plasma by transferring that energy through the 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; gas composition, and the resulting frequency-dependent currents and voltages. From our measurements, we find that the 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.