Correlation of stochastic and ohmic power absorption with observed RF harmonics and plasma parameters in capacitively coupled discharges

Abstract

This work attempts to correlate the plasma density and RF harmonic profiles with respect to the pressure (at 13.56, 27.12 and 40.68 MHz) with the stochastic and ohmic power absorption mechanisms in a Capacitively Coupled Discharge (CCD), over a wide pressure range (0.6–1000 mTorr). Diagnostics include calibrated capacitive probe, compensated Langmuir Probe (LP) and uncompensated floating LP for measuring plasma parameters and RF signals. Pressure profiles of stochastic and ohmic powers, P Stoch and P Ohm (at 13.56 MHz) are obtained from their ratio (ξ) and the power absorbed by the electrons. Normalized profiles of an effective power (∼ P Stoch ρ × P Ohm 1 − ρ ; ρ : pressure dependent parameter) are tuned to reproduce closely the normalized plasma density profiles from which relative contributions of stochastic/ohmic mechanisms are determined. It is shown that up to ≈30 mTorr, plasma production is stochastic while beyond that both methods contribute jointly. The RF harmonic profiles can be analysed similarly. Higher harmonics produced by the intrinsic nonlinearity of the stochastic process should appear most clearly in the plasma at low pressures where the latter operates alone. On the other hand, the fundamental RF voltage that is always present in the plasma, can also produce higher harmonics at the probe tip by driving the nonlinear probe sheath. Thus, the harmonics produced directly by the stochastic nonlinearity are inextricably mixed up with those arising due to the probe sheath. Significantly, one may conclude therefore that it is not possible to investigate the stochastic mechanism of power absorption by a study of its harmonics when the latter are measured using invasive probes.