Two-dimensional PIC/MC Simulation Of A Capacitively Coupled RF Glow Discharge In Nitrogen

Nitrogen radio-frequency glow discharge has been widely used in the synthesis of nitride and the metal surface nitriding technique. Radio-frequency plasma chemical vapor deposition (RF-PCVD) in nitrogen, is a promising technology. A deep research for the plasmas process in the nitrogen discharge from a micro point of view is important to improve the efficiency and quality of the technique. A hybrid Particle-in-cell/Monte Carlo model can describe the comprehensive movement of charged plasmas and reflect the real the plasmas process. In this paper, we study the mechanisms and plasma characteristics of the nitrogen RF glow discharge using two-dimensional PIC-MC model, and investigate the two-dimensional electron impact ionization rate and dissociation rate, and those rates vary with discharge parameters. The main works are shown as follows:1. A self-consistent PIC-MC model is developed for a capacitively coupled RF glow discharge in nitrogen, in which the move of charged particles in electric field and the self-consistent field is described by the electrostatic model of the PIC, the collisions between the particles is described by the MC. Moreover, the simulation programs are compiled based on the model.2. The discharge of RF plasma with two-electrode asymmetrical structures were simulated, the spatial distributions of self-consistent potential and the electric field and the density distribution of charged particles (e-, N+ , and N2+) in the total discharge space in different periods were calculated in the subsequent cycle after the discharge had got relatively steady. Meanwhile, we simulated distribution of RF sheath thickness with time, and obtained the relationship of RF sheath thickness and discharge voltage via adjustment of discharge parameters. The simulation results are shown as follows:(1) A remarkable sheath area appeared near the driven electrodes, within which the density of the electrons decreased rapidly tending to zero, while ion density is much higher (especially on the electrode). In the outer area of the sheath is the bulk plasma where the densities of electrons and ions are nearly equal.(2) There is fairly strong electric field in the sheath area, while there is fairly weak and even-distributed electric field in the bulk plasma. (3) Discharge voltage significantly affected sheath parameters, the width of the plasma sheath increased along with the increasing of the voltage of RF supply.3. The RF discharge in nitrogen with symmetrical structures using 2-D PIC-MC method were also simulated. PIC result shows that :(1) The alteration of spatial potential in the discharge space directly affected the distribution of the charged particles, it is just the fluctuation of potential distribution caused the density distribution of the charged particles, thus an alternating-electric field formed in the plasma space. The plasmas collectively oscillated under the effect of the sheath alternating-electric field, whose oscillation cycle is consistent with RF generator.(2) Subsequently, we focused on the discussion of the distributions of electron ionization collision and dissociation collision, and calculated the contributions of"γ"process and"α"process to the generation of electron ionization collision, and we concluded that the contribution of"α"process to electron collision ionization presents more than 90% in the total ionization, proved the conjecture that"α"ionization is the main process to maintain the low-pressure capacitively coupled RF discharge.(3) The simulation of e- N2 dissociation collision revealed that the N+ density in the discharge space could be increased by increasing the voltage properly.In summary, N2 capacitived coupled RF glow discharge is a very complex physical process, and the interacted and inter-impacted plasma parameters in the discharge space are restrained by various discharging conditions. The researches of the relationship between these parameters and their effects on the generation rate of active N+ have important significance on the Experimental research of the synthesis of nitride materials. This work provided a reference to understand the mechanism of N2 RF discharge plasma process and to the experiments of intending to generate"Activated N+ source".