A study of magnetic field effect in low pressure capacitively coupled plasmas

Low-pressure (< 100 mTorr) magnetized capacitively coupled plasmas (CCP) are widely used for plasma-enhanced chemical vapor deposition and etching in the semiconductor industry. In low-pressure plasma, magnetic field affects electron transport due to comparable electron-neutral collision and electron cyclotron frequencies. Here, magnetic field is used for plasma density enhancement that improves deposition/etch rate and for on-wafer uniformity improvement using E x B drift which has been experimentally demonstrated using static magnetic field. However, high magnetic field intensity leads to possibility of instabilities forming in plasmas which could significantly change the electron transport coefficients. Particle in Cell (PIC) simulations can be used to reliably study the effect of the magnetic field on low-pressure plasma. In this paper, we present a study of instabilities caused by magnetic field in low-pressure CCP using a 2-dimensional PIC plasma simulation model where plasma particle kinetics equations are coupled with the Poisson equation solver. Argon plasma is simulated using this model at 10 mTorr pressure driven by RF voltage at 40 MHz where the applied magnetic field is varied up to 100 G. With increasing magnetic field intensity, three distinct regimes of plasma behavior are observed. At low magnetic field intensities (0 to 65 G), we see a simple drift in plasma in the E x B direction. At relatively higher field intensities (70 to 80 G), instabilities take the form of self-organized spoke-like structures stretching from the center of bulk plasma to the boundary that rotate around the drifted bulk plasma at 10s of kHz. At significantly high field intensities (> 80 G), we see more chaotic instabilities in the form of structures of different shapes and sizes moving in both rotatory and linear fashion. The magnetic field threshold values for these three regimes are observed to change with changing ambient pressure. The plasma behavior and nature of instabilities changes with changing electrode area ratio and with addition of a blocking capacitor between the RF electrode and the RF source. The simulation results are carefully analyzed to elaborate on the physical reason behind the varying nature of instabilities with varying magnetic field intensity and varying plasma process conditions.