Simulation and Characterization of Capacitively-Coupled Argon Plasma at Intermediate Pressure

Radiofrequency (RF) capacitively-couple plasmas (CCP) at intermediate pressure (a few Torr) are widely used in the semiconductor industry. However, the plasma behavior in this pressure regime is not well characterized.  Argon plasma physics is investigated using a one-dimensional fluid-MCS hybrid plasma model in this pressure regime, and compared with experimental data. Owing to the simple chemistry of Argon plasma, validated results in this study can provide insights into plasma physics at intermediate pressures.  Our model includes continuity equations for charged and neutral species, drift-diffusion approximation for electron flux, the momentum conservation equation for ions, energy conservation for electrons, and the Poisson equation for electric potential. While a fluid model is valid for bulk species in this pressure regime, secondary electrons emitted from the surface can exhibit kinetic behavior as they accelerate through the sheath. Therefore, a Monte Carlo model is used for secondary electron collisions to compute production rates of species, which are coupled to the fluid plasma model. The voltage-current characteristics of the plasma model are compared with the electrical measurements in a symmetric CCP chamber for model validation. Electron energy distribution function (EEDF) and the gas temperature are varied in the model to achieve good agreement between the simulations and experiments. The plasma simulations are performed at different pressures and powers. Validated results show that the EEDF transitions from a Maxwellian distribution at lower pressures of 1 Torr, to a Druyvesteyn distribution at higher pressures. It is also seen that at pressures of around 8 Torr, ionization source is concentrated in the presheath region, decaying exponentially in the bulk. At low pressures, the secondary electrons from the electrodes penetrate into the bulk plasma enhancing ionization in that region. The gas temperature is assumed to be uniform and increases from 350K to 600K as the pressure increases from 1 to 8 Torr, while the bulk electron temperature decreases. Our simulation results compare well with the experimental trends that can be useful in CCP based semiconductor processing.