Characterisation of the heating mechanisms in a capacitively coupled argon RF discharge

Abstract

A capacitively coupled rf plasma is investigated in the context of the heating mechanisms that sustain it under various conditions. These mechanisms are critically dependent on gas pressure, applied rf potential, rf current and discharge gap. Pressure ranges of 10 to 300 mTorr and rf potentials from a few Volts to several hundred Volts are investigated. The argon plasma is generated in two capacitively coupled rf systems. Plasma parameters are measured using a Langmuir probe. A microwave interferometer is used to compare density measurements with that of the probe. A current voltage monitor is used to measure the voltage, current and phase for the purpose of relating the control parameters to the plasma parameters. The design and construction of a retarding field energy analyser is presented. Plasma potential measurements using the analyser are compared with that of the Langmuir probe. Use of the analyser in electron collection mode has revealed a higher electron temperature behind the rf sheath than that measured by the Langmuir probe in the plasma. Transitions in the heating modes are investigated via plasma parameter and Electron Energy Probability Function (EEPF) measurement in the centre of the discharge. Particular attention is paid to the a-y transition. The relationship between the current voltage characteristic and the plasma parameters is considered. Pressure effects on the transition are determined. A model of the diffusion process in the capacitive rf discharge is devised and the resulting density profiles checked against known analytic solutions of the diffusion equation. A procedure to extract the ionisation profiles from experimental density measurements is devised using this model and a reconstruction algorithm. The twodimensional evolution of density and ionisation profiles is presented at low and high discharge powers over the 10 mTorr to 300 mTorr range. The total ionisation rate is compared with the theoretical ionisation rate calculated using simple particle balance equations. Disagreement in the ionisation rates is attributed to the use of unrealistic models and the assumption of uniform electron energy in the discharge. Spatially resolved EEPF measurements are made along the discharge axis. Rate coefficients for ionisation are calculated by integrating the product of the measured EEPF and the cross section for ionisation. Spatially resolved ionisation rates obtained from the coefficients are in good agreement with the measured ionisation rate profiles generated using the diffusion model and reconstruction algorithm