Modeling of Intermediate Pressure Capacitively Coupled N₂ and Ar Plasmas

Capacitively coupled plasmas (CCPs) are widely used for etching and deposition. The pressure in CCPs varies from a few mTorr to tens of Torr. Fluid plasma models are typically used for CCPs at intermediate pressures (> 100s of mTorr). It is important to understand how well fluid plasma models capture the behavior of intermediate-pressure CCPs. 1-dimensional particle-in-cell (PIC) simulations are done for 13.56 MHz N₂ and Ar CCPs between 100 mTorr – 1.5 Torr. After reaching steady state, the particle data is analyzed to obtain all the terms in the continuity, momentum, and energy conservation equations for the charged species. The potential and current in the plasma are also processed to determine the spatially resolved impedance in the plasma. A comparison of PIC results to the fluid plasma model shows significant differences. This disparity between the two models can be linked to the assumptions about the electron energy distribution function (EEDF) and the electron transport coefficients in the fluid plasma model. In addition, the PIC simulations indicate that the EEDF is not spatially uniform. The drift-diffusion approximation appears reasonable for electron transport. However, it is necessary to solve the ion momentum equation as ion inertia effects are important.