Numerical Study On Mixture Gases RF Discharge At High Pressure

High pressure RF discharge is a main technology for producing plasma, it is also widely used to modify the surface properties of materials, etch and film deposition and other technology fields. Since argon, oxygen and nitrogen are cheap, they are usually used as working gas in discharge. Compared with the pure gas discharge, the mechanism and phenomena of mixture gases discharge at high pressure are more complicated. Besides, the parameters of high pressure RF discharge, particularly gas pressure, driven frequency and mixture ratio have important relationships with the discharge. To get these relationships can help to improve the plasma technology. This dissertation used a numerical simulation to study the mixture gases RF discharge at high pressure.In this dissertation we have mainly studied for two aspects. First of all, based on a fluid model, induced plasma discharge approach has been adopted to numerically study nitrogen discharge at high pressure. It includes as the followings:(a) the effect of the time of adding nitrogen gas and initial induced argon plasma pressure on nitrogen discharge characteristics at high pressure has been studied;(b) the effect of RF frequency on nitrogen discharge with induced argon plasma in high pressure has also been studied; Secondly, in the dissertation we also have studied mixture gases atmospheric pressure RF discharge, including the effect of the concentration of O2in N2on nitrogen and oxygen atmosphere discharge and investigating the characteristics of nitrogen and oxygen RF atmospheric pressure discharge with fixed mixture gases ratio and the effect of gap distance on the plasma characteristics.There are six chapters in this dissertation. The contents in each chapter are as follows:Chapter1is the introduction. It introduces the history and present conditions of high pressure discharge and mixture gases discharge.In chapter2, one-dimensional fluid model under drift/diffusion approximation was introduced. The model was solved by a finite difference model. A code by FORTRAN language has been developed to simulate the model. The electron density, ions densities, neutral particles densities, electron temperature and electric filed in spatial at200Torr are obtained. In this chapter we investigate the effect of the time of adding nitrogen gas and initial induced argon plasma pressure on discharge characteristics. The simulation results show that when the induced argon plasma exists, the nitrogen plasma in the relatively low conditions can be obtained. And during the process of the gas pressure increasing from1Torr to200Torr, the instability doesn’t appear. Moreover the results also show that the slower rate of varying gas pressure, the plasma density is higher and the electron temperature is smaller; with the initial induced argon plasma pressure increasing, the charged particle densities increase, and electron temperature decreases.In chapter3, based on one-dimensional plasma fluid model, the effect of the drive frequency on RF nitrogen discharge at high pressure with argon induced plasma has been further investigated. The numerical results show that through modulating the driven frequency, the discharge can obtain higher plasma density. Moreover, as the driven frequency increasing, the plasma voltage increases, the electric-field in sheath increases, the length of the plasma increases, the thickness of sheath decreases; the electron temperature in sheath increases obviously and has a small reduction in bulk plasma; the electron production rate is across the whole gap and has peak value in sheath. The discharge is in a mode.In chapter4, the nitrogen gas and oxygen gas are the main components of air; therefore it is important to investigate the nitrogen and oxygen mixture gas discharge at atmospheric pressure. In this chapter, we studied the effect of the concentration of O2in N2on nitrogen and oxygen mixture gases atmosphere discharge. The numerical results show that when the concentration of O2in N2is less than12%, as the amount of O2grows, the electron density and N4+density decrease; the main negative particle is electron. And when the concentration of O2in N2is greater than12%, the electron density and N4+density increase with the increasing of oxygen; the main negative particle is O-. Moreover, O-density, O2density, electron temperature and the mixture gases electronegativity are increasing with the growth of O2in range of20%O2.In chapter5, a one-dimensional fluid model is used for studying atmosphere discharge in mixture of nitrogen and oxygen. In the model, some particles, such as e, N2, N4+,O2+,O-, O+, NO+, N2(A1∑n+), N2(a+1∑-"), O2(a1△g),O, N, NO, are taken into account, as well as their73main reactions. The model is solved numerically and the evolutions of the spatial distributions of the particle densities are obtained. It shows that the density of negative ion increases more quickly than densities of positive ions and the densities of positive ions increase more quickly than electron density. The main charged particle is O-, whose density is much higher than other ions and electron. Discharge gap distance has effect on discharge properties. As the gap distance increasing the particles densities increase with it.Finally, a brief summary ends this thesis.