Fluid Simulation On Electron Heating Mechanism In RF Capacitive Argon Discharges At Low Pressure

Capacitively coupled radio frequency discharges are widely used in many industrial applications such as plasma cleaning and thin film deposition.Currently,plasmas serve as the essential technology that is indispensable for the microelectronics manufacturing industry.In practice,the efficiency and quality of the etching and deposition depend on largely plasma characteristic and working parameters of plasmas.The plasma characteristic is determined by external parameters,such as driving power,pressure,geometry,and properties of working gas.It exhibits rich nonlinear phenomena of complicated physicochemical processes over space and time,which brings us great difficulties to understand deeply the physical mechanism in the discharges.The numerical simulations can help us design plasma sources for industrial application both efficiently and economically.Aiming at simulation of complicated physicochemical processes in a real capacitively coupled radio frequency discharge geometry,the purpose of this dissertation is to develop a one-dimensional fluid model to study how the capacitively coupled radio frequency discharge operating parameters affect the plasma characteristics,especially the electron heating mechanism.In this dissertation,we first briefly review the background,recent advances,and challenges of studying capacitively coupled radio frequency argon discharge at low pressure,and also the problems we face to in Chapter 1.Chapter 2 shows a one dimensional fluid model of a geometrically symmetric capacitive radio frequency discharge in argon at low pressure based on the drift-diffusion approximation in detail.It is also presented here the comprehensive summary of the selection and determination of parameters,as well as the main numerical algorithms in the simulation.Chapter 3,based on the results of the one dimensional fluid simulation of capacitively coupled radio frequency argon discharge at low pressure,focuses on the influence of the driving frequency on plasma parameters such as the electron density,electron heating and electron temperature.The results of simulation are as follows.The cycle-averaged electric field,electron temperature,electron density and electric potential are compared by varying driving frequencies as 3.39,6.78,13.56 and 27.12MHz,respectively.The cycle-averaged electron pressure cooling,Ohmic heating and electron energy loss are discussed for the driving frequencies as 3.39,6.78,13.56 and 27.12,respectively.The results show that the electron density and the simulation time required to reach convergence to increase as a function of driving frequency.Then the net power absorption of the electrons is analyzed at different time in one cycle during the convergence of the simulation and it is splitted into Ohmic heating,conductive heat flux and the energy dissipation due to inelastic collisions.At early time during the process of convergence,the net power absorption shows local maxima at the sheath edge due to Ohmic heating.At later time,the net power absorption is center-peaked,since thermal convection and energy dissipation contribute significantly once the electron density has reached a high enough value.Chapter 4 focuses on the effect of neutral gas pressure on the plasma characteristics of capacitively coupled radio frequency argon discharges at low pressure.The results show that the strong electric fields accelerate the electrons inside the sheaths,resulting in more Ohmic heating.The strong electric fields may be caused by low electron convection in the bulk plasma region when the gas pressure increases.Thus,a peak of the dissipation in the boundary between the sheath region and the bulk plasma region is observed and the peak of the net power absorption grows in the bulk plasma region.Chapter 5 presents the strongly modulation of the electric field inside the sheath regions when driving frequency increases.Consequently,the electrons are accelerated to high energies in the sheaths and ionization generates in the bulk plasma region,which finally leads to the increase of the electron density and decrease of the sheath widths.Finally,the summary of the major results in the dissertation and the future works are presented in Chapter 6.