유체 및 입자 시뮬레이션을 활용한 저온 플라즈마 특성 연구

Abstract

Low-temperature plasmas at low pressure (~ mTorr) are widely used for dielectric etching, deposition and surface modification. With this success in the semiconductor industry, atmospheric-pressure discharges have received considerable attention in recent years for their potential economic impact in a broad range of fields. Their nonequilibrium characteristics, chamber-free operation capability, and portability enable new applications of plasmas. Due to the high collisionality, the plasma is far from equilibrium, i.e. the electron temperature is high enough to sustain a discharge but the heavy particle temperature remains at room temperature. In an effort to overcome the experimental challenges and understand the physics of low-temperature nonequilibrium discharges, numerical simulations are often used to estimate plasma parameters.We have investigated the characteristics of low- and high-pressure discharges by using following fluid and particle-in-cell Monte Carlo collision (PIC-MCC) models: i) one-dimensional fluid model, in which the electron velocity distribution function is assumed to be Maxwellian and the energy equation is solved to determine the spatial profile of the electron temperature and ii) PIC-MCC model providing self-consistent kinetic information which is not available in fluid model. The time-averaged density, potential and power consumption profiles of helium (or argon) microdischarges driven at RF and microwave frequencies are obtained with the both models and are compared. The agreement between two models depends on the driving frequency. The kinetic information indicates that the shape of electron energy probability function (EEPF) depends on the heating mechanism sustaining the discharges and changes with discharge conditions such as pressure, frequency and gap size. The average ion energy near the electrodes is found to be lowered significantly due to frequent ion-neutral collisions and electrons play an important role in the plasma-surface interaction at atmospheric pressure. It also found that argon plasma has higher electron temperature and a large number of low-energy electrons with energy less than ~ 5eV, than helium plasma. For a given input power, the onset of field emission from the cathode surface caused by the strong electric field leads to reduction of the breakdown voltage and an increase in plasma density.