On uniformity and non-local transport in low pressure capacitively coupled plasmas

Abstract

Low pressure (<30 mTorr) capacitively coupled plasmas (CCP) are important for many plasma processing applications. Due to high energy of electrons in these plasmas and long electron mean free path, kinetic phenomena dominate the electron behavior. A multi-dimensional particle-in-cell (PIC) model is utilized to study the physics of sub-100 mTorr Ar CCPs in this article. A one-dimensional model for helium plasma in the 30–300 mTorr pressure range is first used to benchmark the PIC code against published results by Turner et al [2013 Phys. Plasmas 013507]. The article then focuses on two-dimensional modeling of the gaseous electronics conference (GEC) reference cell in cylindrical geometry. Computed ion density and DC bias voltage at 100 mTorr are compared to available experimental data for a range of radio frequency voltages, demonstrating good agreement. The validated model is used to examine Ar plasmas in the GEC reference cell in the 12.5–100 mTorr pressure range. It is found that the peak in ion and electron densities is near the electrode edge at 100 mTorr due to strong electric field there. As the gas pressure decreases, the peak in plasma density moves to the chamber center. This change in plasma spatial profile is linked to kinetic behavior of electrons, both regarding plasma production and transport in the plasma. Time-dependent results are used to illustrate that electron current at 25 mTorr is mainly kinetic and carried by beam electrons generated by expanding sheaths.