Particle-in-cell simulations of the ionization process in microwave argon microplasmas

Abstract

The importance of microwave device reliability and performance for microscale devices motivates a more fundamental understanding of breakdown mechanisms in this regime. Microwave breakdown theories predict breakdown when electron production balances electron loss. Electron production depends strongly on the ionization rate νi; however, previous studies either used the measured νi in macroscale gaps or the empirical formula for DC voltage, inaccurately predicting νi in microscale gaps. Alternatively, this work characterizes νi in microwave microplasmas by using particle-in-cell simulations. We calculated νi in argon gas at atmospheric pressure for 2–10 μm gaps under AC fields ranging from 1 to 1000 GHz. The behavior of νi may be separated into two regimes by defining a critical frequency fcr that depends on the amplitude of the applied voltage, gap distance, and pressure. For frequency f<fcr, the electrodes collect the electrons during each cycle and the electron number oscillates with the electric field, causing νi/f to roughly scale with the reduced effective field Eeff/p. For f>fcr, the phase-space plots indicate that the electrons are confined inside the gap, causing the electron number to grow exponentially and vi/p to become a function of Eeff/p. These results elucidate the ionization mechanism for AC fields at microscale gap distances and may be incorporated into field emission-driven microwave breakdown theories to improve their predictive capability.