Fluid Simulation Of Discharge Characteristic In Capacitively Coupled Ar Plasmas Driven By Pulse Modulated Radio Frequency

Low-temperature plasma technology has a wide range of applications in surface processes of various materials,such as semiconductor doping,ion implantation,material etching,and thin film deposition.Continuous RF capacitive glow discharges have advantages of their simple structure and uniform plasmas of high densities and large areas.However,with the more accurate applications of plasma technology in surface processes of materials,the requirements for plasma sources are becoming more and more stringent.Continuous RF capacitive glow discharges have disadvantages of high energy consumption,high ion energy damage to the substrates,positive ion charging effects,less control parameters,etc.And the pulse modulated RF capacitive glow discharge can solve the aforementioned problems very well.Therefore,studying on the physical properties of pulse modulated RF capacitive glow discharge plasmas,as well as understanding the evolution process of various particles in the plasmas and the response to external control parameters,has significant theoretical relevance and practical importance in optimizing the key parameters of the production process.Numerical simulation is one of the basic methods to study the physical properties of RF capacitive plasmas.Since the time scale of pulsed RF discharges is small,this brings challenges to both experiments and numerical simulations.Computer modeling can not only investigate systematically various physical mechanisms of the RF capacitive glow discharges,but also provide evolution details of the state parameters of the discharge.Numerous recent literatures covering low-pressure continuous RF capacitive glow discharges and pulse modulated RF capacitive glow discharges demonstrate that this research area is still a hot issue.This thesis adopts fluid simulation method to study numerically both continuous and pulsed RF capacitive discharges in low pressure.The thesis is arranged as following.In the first chapter,introduction,plasmas and their applications in early time are stated briefly,as well as the applications of cold plasmas in industry.Four kinds of low temperature plasma sources which commonly are used in industry,namely the RF capacitive coupled plasma source,the RF inductively coupled plasma source,the electron cyclotron resonance plasma source,and helical wave plasma source,are introduced.In this paper,the current research status and the urgent problems of the pulse modulated RF capacitive coupled plasma are introduced.In these foundations,the purpose and content of research in this thesis are given.In the second chapter,a plasma fluid model of the pulsed RF capacitive discharges is established.The several kinds of descriptions on plasmas are stated briefly.The numerical simulation methods of plasmas are introduced emphatically.Based on a geometric model of the parallel plate discharge device,considering the main collision processes in the discharge,the plasma is regarded as a continuous medium composed of various components such as the electrons,ions and neutral particles.The drift-diffusion approximation is utilized to describe the fluxes of the electrons,ions and metastable atoms.Poisson equation is used to govern the changing electric field.With the additional assumption of radially uniform distribution of the plasma,the fluid equations of the plasmas in the pulsed RF capacitive discharges are obtained.By self-consistently solving the continuity equations of the various particles and the energy equation of the electrons as well as the Poisson equation of the electric field by the numerical method,the spatial and temporal distributions of various macroscopic parameters,such as the plasma density,temperature and electric field,are obtained.In the third chapter,based on the model established in the second chapter,using a finite difference method,the corresponding numerical solutions are obtained.Through analysis of the numerical solutions,the transient physical properties are compared and studied between the continuous and pulse modulated RF discharges.The results show that,under the same conditions for the driving voltage,the basic physical quantities,such as the electron density and the current density,are consistently higher in the continuous RF discharge as compared to those of the pulse modulated RF discharge.Similar results are obtained under the same conditions for the power deposition density.Modeling also shows that a higher driving voltage is required in the pulse modulated RF glow discharge,in order to maintain the same plasma effective current density as in the corresponding continuous discharge.Further studies show that,in the plasma region,the peak value of the bipolar electric field,generated in the pulse modulated RF discharge,is larger than that in the continuous RF discharge.The ground state ionization coefficient in the plasma sheaths has a bimodal form.Furthermore,comparative studies of transient physical properties are performed under other discharge conditions.Through these comparisons,the similarities and differences between these two types of discharges are revealed,leading to a deeper and more comprehensive understanding of the discharge parameter regimes and transient physical properties.In the fourth chapter,based on the established plasma fluid model,the plasma dynamic characteristics of the pulse modulated RF capacitive glow discharges are systematically investigated.The results show that the electron density,ion density and metastable atom density,as well as their reaction coefficients,lag behind the driving voltage when the pulse modulation is turned on.This is independent of whether or not the pulsed RF discharge reaches a steady state.After the pulse modulation is turned off,the electron density first rises and then falls,which is especially noticeable when the pulsed discharge does not reach a steady state.Meanwhile,when the pulse modulated RF discharge reaches the steady state and during the pulse-on phase,the electron temperature at the plasma center is almost not affected by the driving voltage,nor by the secondary electron emission coefficient of the electrode material.The electron density,however,increases with increasing the aforementioned parameters.The plasma power dissipation density increases as well.On the other hand,the electron temperature in plasma center decreases with the increase of pulse modulation duty ratio and other parameters,such as the electrode spacing,the pulse modulation frequency,and the electron recombination rate.Accompanied is also a reduction of the electron density.This results in a reduction of the power dissipation density.Systematical studies are also performed for the spatial-temporal evolution of the electron energy conversion in pulsed RF discharges.In the fifth chapter,based on the plasma fluid model for the pulse modulated RF capacitive glow discharge,the influence of the secondary electron emission on the transient physical properties is investigated in detail.The results demonstrate that,with increasing secondary electron emission coefficient,the electron density and the metastable atom density increase during the pulse-on phase.The electron temperature remains basically the same,but the plasma potential and the electric field decrease.After the pulse modulation is turned off,the electron density and the electric field also vanish;the metastable atom density and the plasma potential drop to lower levels;and the electron temperature also drops.As the secondary electron emission coefficient increases,from the spatial distributions of the plasmas,including the electron energy density,the power density of electron deposition,the absolute value of the electron energy flux power density,the electron energy loss power density,the plasma power dissipation and the various reaction coefficients,all of them increase.In the pulse modulation off phase,various discharge parameters quickly decrease to a very low value.Finally,with the increase of the secondary electron emission coefficient,the pulsed RF capacitive glow plasma discharge is converted from the alpha discharge mode to the gamma discharge mode.In the sixth chapter,the main conclusions,innovations and prospects for future work are given.