Electromagnetic Plasma Modeling of Microwave Discharge with Resonance Power Absorption

Microwave plasmas have recently gained importance in the semiconductor industry due to high plasma density and low ion energy. In a microwave plasma source, the resonant power absorption where plasma density reaches critical density can have significant effect on the plasma behavior. The mode transition from under-dense to over-dense has been confirmed earlier. In this study, microwave discharges at 2.45 GHz are investigated numerically in a cylindrical reactor with a resonator powered using a co-axial port. A fluid-based electrostatic plasma model is coupled to an electromagnetic (EM) model for microwave power deposition. The power deposition from EM model is fed back to the plasma model. Strong power deposition near the resonator plate is observed where plasma density reaches the critical density. Plasma simulation is performed for different powers, pressures, and gas mixtures of Ar, He, and N₂. The over-dense plasma is observed to spread radially with an increase in power. The over-dense plasma region spreads farther in Ar than in He. However, N density is higher in He/N₂ plasma compared to the Ar/N₂ plasma. The microwave discharge is sustained over a wide range of pressures (tens of mTorr to several Torr). Simulated results of over-dense plasma density region compare well with experimental measurements.