Investigating Instabilities in Magnetized Low-pressure Capacitively-Coupled RF Plasma using Particle-in-Cell (PIC) Simulations

TL;DR

This study uses PIC simulations to explore how a magnetic field influences plasma stability and structure in low-pressure RF CCP systems, identifying regimes of behavior and thresholds for stable operation.

Contribution

It characterizes distinct plasma regimes under varying magnetic fields and provides thresholds for instability onset, aiding in optimizing magnetic field use in semiconductor processing.

Findings

Identified three regimes of plasma behavior based on magnetic field strength.

Discovered self-organized spoke structures rotating in the plasma.

Threshold magnetic field values vary with gas pressure, not reactor geometry.

Abstract

The effect of a uniform magnetic field on particle transport in low-pressure radio frequency (RF) capacitively coupled plasma (CCP) has been studied using a particle-in-cell (PIC) model. Three distinct regimes of plasma behavior can be identified as a function of the magnetic field. In the first regime at low magnetic fields, asymmetric plasma profiles are observed within the CCP chamber due to the effect of E x B drift. As the magnetic field increases, instabilities develop and form self-organized spoke-shaped structures that are distinctly seen within the bulk plasma closer to the sheath. In this second regime, the spoke-shaped coherent structures rotate inside the plasma chamber in the - E x B direction, where E and B are the DC electric and magnetic field vectors, respectively, and the DC electric field exists in the sheath and pre-sheath regions. The spoke rotation frequency is in the MHz range. As the magnetic field strength increases further, the rotating coherent spokes continue to exist near the sheath. The coherent structures are, however, accompanied by new small-scale incoherent structures originating and moving within the bulk plasma region away from the sheath. This is the third regime of plasma behavior. The threshold values of the magnetic field between these regimes were found not to vary with changing plasma reactor geometry (e.g., area ratio between ground and powered electrodes) or the use of an external capacitor between the RF-powered electrode and the RF source. The threshold values of the magnetic field between these regimes shift toward higher values with increasing gas pressure. This paper provides guidance on the upper limit of the magnetic field for instability-free operation in low-pressure CCP-based semiconductor deposition and etch systems that use the external magnetic field for plasma uniformity control.