Fluid Simulations And Experimental Verifications For Radio-frequency Capacitively Coupled N₂ And N₂/Ar Plasmas

Low temperature plasma has been widely used in the fabrication of very large scale integrated circuits(VLSI),such as etching and thin film deposition processes.In practice,complex and multi-component gases are employed to generate plasma,which exhibits rich nonlinear phenomena full of complicated physicochemical processes over a multiscale of space and time.Gaining a good insight into the physical and chemical mechanisms in the complex discharge systems,can help us to optimize the design of plasma sources,which will provide enormous societal benefits.Aiming at having a deep understanding of the complicated physicochemical processes in a real discharge reactor,it is very important to develop a numerical model of multi-fields(including plasma,electrostatic field and chemical reactions,etc)to simulate the real plasma.The purpose of this dissertation is to study how the operating parameters affect the plasma characteristics in capacitive N2 and N2/r discharges by using a fluid model,which is coupled with electrostatic module and chemistry module.Meanwhile,various experimental diagnostics are employed to verify the simulation results.This dissertation is organized as follows.The applications of the capacitively coupled plasmas(CCPs)in the semiconductor industry are briefly introduced in Chapter 1.And also,we review the recent research progress,and the existing problems and challenges of CCPs.In Chapter 2,there is a comprehensive introduction of the two dimensional self-consistent fluid model.This model,coupled with electrostatic module and chemistry module,can simulate the plasma behavior in the capacitive discharges.Then the species,as well as the chemical reactions,considered in the model are discussed in detail.The effects of rf power and electrode gap on the plasma characteristics,especially on the plasma radial uniformity,in capacitively coupled nitrogen discharges have been investigated in Chapter 3.It is found in the simulation that with the increase of rf power or the decrease of electrode gap,the electrostatic edge effect becomes remarkable,which gives rise to an increase in the plasma density at the electrode edge and thus the worse plasma radial uniformity.In order to validate the simulation results,a floating double probe is employed to measure the radial profiles of ion density.A generally qualitative agreement between the experimental and calculated results is achieved.In Chapter 4,both the fluid model and the experimental diagnostic are employed to study the dependencies of the species concentrations,as well as the chemical mechanisms,on the gas proportion and the rf power in the capacitive N2/Ar discharges.The results indicate that the N2/Ar proportion has a considerable impact on the species densities.By adjusting the ratio of gas mixing,the species concentrations can be controlled.In addition,although all the charged and neutral species densities increase pronouncedly with the rf power,the corresponding fractional contributions of the main production mechanisms are found to be independent of the rf power under the operating conditions concerned.In the experiment,the total positive ion density and the emission intensity originating from Ar(4p)transitions are measured by the floating double probe and the optical emission spectroscopy,respectively.The simulation results are compared w'ith the experimental ones,and the general qualitative agreement indicates that the present fluid model can correctly describe the capacitively coupled discharges.In Chapter 5,we show the effect of the dielectric ring on the plasma radial uniformity in the practical 450 mm CCP reactor.It is observed that the homogeneous plasma density is obtained by using the dielectric ring with low relative permittivity and large thickness.Moreover,in order to avoid the spatial discontinuity of the dielectric materials,which could give rise to a rapid drop of ion flux,the width of the dielectric ring should be made as short as possible or the entire-layer dielectric ring structure should be utilized.Finally,all the findings presented in the dissertation are summarized and the further research work plan is proposed in Chapter 6.