Investigation of low pressure capacitively coupled plasma behavior using PIC-MCC simulation

Summary form only given. Capacitively coupled plasmas in the sub-20 mTorr range have gained importance in recent years for advanced micro-electronic device fabrication. Due to long electron mean free path and large bias voltages in this regime, kinetic effects play an important role in the behavior of low pressure plasmas. To consider the kinetic effects, a 1D hybrid plasma model has been developed that uses particle in cell (PIC) technique [1] for charged species, and the fluid method for neutral species. The PIC model includes Monte Carlo Collision (MCC) method to calculate collision frequencies for various processes using a database of collision cross-sections. The fluid model for neutral species takes into account species transport in the plasma, chemical reactions, and surface processes. Capacitively coupled Ar plasma is simulated at low pressure (5-10 mTorr) at 60 MHz. The rf voltage (200-400 V) has been applied to the right electrode, and the left electrode is grounded. The secondary electron emission coefficient for ions is 0.3 on both electrodes. The inter-electrode gap is varied from 5.0 cm to 2.1 cm. In the plasma, electrons primarily absorb power from the external power supply at the sheath edge during sheath acceleration [2]. Energetic beam electrons are generated at the sheath edge during electron heating, which are responsible for plasma production and sustenance through collisions. With decrease in gap, electron density is first observed to decrease due to increase in surface loss to volume generation ratio. However, as the gap is further narrowed, the electron density starts increasing again. This increase in electron density occurs at a narrow gap as the beam electron transit time from one sheath to the opposite one becomes commensurate with the sheath oscillation period, resulting in enhanced power deposition. The peak in electron density is observed at the narrow gap at different pressures and rf voltages.