Benchmark for intermediate pressure RF CCP: gap between exp and simulation

The PIC/MCC method (Particle in Cell/Monte Carlo Collision), despite its high numerical cost, is popular for studying RF-driven plasmas, because it can self-consistently capture non-local and kinetic effects. Recently, we (along with other groups [1], [2]) have extended PIC simulations to pressures of several Torr, and have included some chemical processes. However, experimental benchmarks are lacking, due to the difficulty of diagnosing such weakly-ionized plasmas. Here we present experimental measurements for a highly symmetric CCP (Ø 50cm gap 2,.5cm), excited at 13.56 MHz 50-500W), at 0.5-2 Torr pressure in pure Ar. The true RF current, voltage and phase at the electrodes is measured, and the electron density is estimated using a microwave hairpin probe with collisional corrections. We used diode laser absorption around 772 nm to measure Ar metastable densities and temperatures. These experimental results are compared to our hybrid-PIC model

Our simulation results, which are in good general agreement with previous work, significantly overestimate the electron density and power absorption compared to the experiments. Several explanations can be proposed for this difference, including underestimation of gas heating, quenching of argon metastables by O from air impurities, and oversimplification of the boundary condition settings. Our measurements of Ar* densities partially confirm these hypotheses, but the difference is still surprising. This work provides an experimental benchmark of RF CCP in intermediate-pressure Ar for the testing of future models.