PIC/MCC Simulations For Nonlocal Electron Kinetics In Capacitively Coupled Argon Plasmas

Low-temperature plasma technology plays a vital role in the semiconductor industry.In particular,capacitively coupled plasma(CCP)sources have been widely used in the etching processes of the dielectric.Electrons interact with time-varying electric fields leading to power deposition,and generating a large number of active radicals and charged particles through collisions with neutral particles in CCP.One of the most fundamental subjects of CCP research is how to effectively control the reaction process in plasma and the properties of particles reaching the material surface,for achieving nano-scale processing.Therefore,understanding the mechanism of electron heating in sustaining radio-frequency(RF)discharge is essential for the applications of plasma processing technology.CCP source usually operates at a lower pressure(0.1?10 Pa)in the etching processes,electrons are thus in a non-local state,which further leads to the non-linearity and complexity of the electron kinetics in CCP.Numerical simulation helps to understand the internal discharge mechanism of CCP,thereby providing a theoretical basis for the industrial design and optimization of CCP sources,Particle simulation,i.e.,Particle-in-cell/Monte Carlo collision(PIC/MCC)simulation,does not rely on empirical parameters,which is a kinetic method based on first principle.It is especially suitable for the simulation of non-local electron kinetics in low-pressure CCP.In this paper.based on one-dimensional explicit PIC/MCC method,the heating mechanism and the ionization dynamics of non-local electron are studied in low-pressure CCP.In Chapter 1,the introduction summarizes several common plasma sources for microelectronic process applications.Also,several theoretical methods of describing plasma are briefly introduced.Finally,the development context and the key issues in the research of CCP have been summarized.In Chapter 2,firstly,the theory of PIC model is introduced,and the algorithm flow of PIC is introduced in detail;Secondly,the MCC method based on statistical sampling to deal with particle collisions is described;Finally,the secondary electron emission(SEE)model as well as the algorithm implementation of this model are explained.In Chapter 3,firstly,by considering the SEE effect that depends on the incident electron energy,and material surface properties,a simulation study is carried out on the asymmetric electron-induced SEE caused by the different electrode materials in CCP.It is found that the asymmetric electron-induced SEE process will cause the formation of self-bias on one of the electrodes.Since ion-induced secondary electrons(SEs)can be accelerated by the electric field inside of the sheath to become high-energy electrons,when these high-energy SEs are able to bombard the opposite electrode,inducing a large amount of electron-induced SEE again,thereby enhancing the discharge efficiency.Secondly,a numerical simulation of the SEE effect in hybrid direct current(DC)and RF CCP is carried out,in which a RF power is applied to one electrode and a negative DC-bias is applied to the other one.A DC/RF sheath is formed near the DC electrode,and the ions bombard the DC/RF electrode under the acceleration of the electric field inside of DC/RF sheath,leading to a large amount of SEE.These ion-induced SEs experience the acceleration of the DC/RF sheath,their energies are thus higher than the potential of the opposite RF sheath.Therefore,they can overcome the opposite RF sheath barrier to reach the RF electrode surface,and further cause a lot of ion-induced SEE.Nevertheless,the energy of SEs emitted from the RF electrode are lower than the DC/RF sheath potential,and will be reflected back to the local RF electrode,causing SEE again.The SEs emitted from the RF electrode will repeat the above process continuously,forming a positive feedback mechanism of electron-induced SEE,which leads to more SEE,hence greatly enhancing the ionization efficiency.In Chapter 4,the nonlinear oscillation phenomenon in geometrically symmetric CCP is simulated.It is found that the non-local electron beam produced by the rapidly expanding sheath is an important origin of the nonlinear oscillation.In low-pressure CCP,the sheath expands very rapidly,which will stimulate a beam of high-energy electrons,causing an instantaneous and local charge separation in plasma.In order to ensure the quasi-neutrality,plasma will selfconsistently compensate the space charge field.Due to the existence of electron inertia,it will cause the overshoot of the back-flow electrons and interact with the expanding sheath again.Due to the continuous sheath expansion,the above process occurs repeatedly,causing the nonlinear sheath oscillation.In Chapter 5,the synergistic effects of electrical and magnetic asymmetry effects are studied.By introducing an inhomogeneous magnetic field into the electrical asymmetry discharge,the two asymmetry effects are organically combined,improving the ability of controlling ion energy and flux.The simulation results show that the two asymmetry effects can operate independently of each other to a certain extent.The self-bias induced by these two asymmetry effects can be increased in the same direction or decreased in the opposite direction by adjusting the phase angle of the electrical asymmetric waveform.In addition,an external magnetic field can reduce the loss of high-energy electrons,enhancing the ionization efficiency.Therefore,the synergistic effect of the electrical asymmetry effect and the magnetic asymmetry effect can increase the incident ion flux,and control the incident ion energy more effectively at the same time.In Chapter 6,the resonance phenomenon between the gyrating electrons and the RF sheath in weakly magnetized CCP is reported for the first time.The simulation found that there is a corresponding relationship between the RF frequency and the magnetic field strength,so that the gyrating electrons can periodically interact with the RF sheath.The gyrating electrons can be continuously accelerated by the expanding sheath,that is,resonance heating of the gyrating electron-RF sheath occurs.The efficiency of this resonance heating depends on the number of continuous collisions between the gyrating electrons and the one of the RF sheaths,hence the heating efficiency of resonance electrons is particularly significant at low pressures.Based on this new resonance heating mode,it provides a theoretical reference for the design and development of new plasma sources.