Investigations On The Ignition Process Of A Pulse Modulated Radio Frequency Capacitively Coupled Ar Plasma

Radio-frequency capacitively coupled plasma（RF-CCP）sources have been widely used in the semiconductor manufacturing due to the simple structure and the abilities of generating large-area homogeneous plasmas.With the development of the advanced technology nodes,the critical dimensions of the latest semiconductor process have been approaching atomic level and the transistors have evolved to complex three-dimension structure.Therefore,it is a great challenging to meet the requirements of the advanced technology nodes by optimizing the plasma sources.Among the several ways for optimizing plasma sources,pulse modulation continues to attract the attention of researchers and semiconductor practitioners due to its unique advantages,e.g.,flexible control of the discharge parameters,the reduced wafer surface damage,etc.In a pulsed RF-CCP,both the plasma and electrical parameters exhibit time-evolving characteristics.Especially,during the ignition phase of a pulsed RF-CCP,the applied RF power is strongly coupled into the plasma system,and meanwhile,both the system impedance and electron power absorption undergo complex mode transitions.When a pulsed plasma is ignited with extremely low initial charged particle density,the ignition process is similar to a gas breakdown process,and the parallel-plate discharge system undergoes an evolution from a“gas-filled plate capacitor”to a“fully formed plasma”.Therefore,to obtain a better exploration of the ignition process in a pulsed RF-CCP,the methods featuring high spatio-temporal resolution are required.No matter the pulse ignition process is investigated by experimental diagnostics,simulation methods or analytical model,the time-evolving characteristics of this process must be exhibited.In this thesis,argon is used as the working gas:1)the temporal evolution of the electrical and plasma parameters during the ignition phase in a pulsed RF-CCP is obtained by multifold experiment diagnostics,2)a Particle-in-Cell/Monte Carlo Collision（PIC/MCC）simulation is adopted to follow the plasma ignition process,whose results verify part of the experimental results and provide an in-depth understanding of this process,3)an analytical electrical model is developed,aiding the understanding of the complex impedance evolution during the ignition process.This thesis aims to give a comprehensive understanding of the evolution of the ignition process in a pulsed RF-CCP under moderate pressure（13.3-133 Pa）,which is expected to provide certain reference for practical process.In chapter 1,firstly,the application of plasmas in the semiconductor industry and the advantages of the pulsed plasmas are introduced.Secondly,the typical characteristics of a pulsed plasma are presented.Then,the research progress of the ignition process in a pulsed RF-CCP is introduced.Finally,the arrangement of this thesis is given.In chapter 2,the pulsed RF-CCP discharge system used in this work is firstly described.Then,the diagnostic methods used in experiment are introduced,including the time-resolved hairpin probe,time-resolved and phase-resolved optical emission spectroscopy,as well as the time-resolved Fast Fourier Transform（FFT）for the measured voltage and current waveforms.Finally,the PIC/MCC simulations are introduced.In chapter 3,by operating a pulsed plasma ignited with extremely low initial charged particle density,the ignition process of an RF-CCP,which behaves like gas breakdown,is investigated comprehensively.In experiment,the electron density,the plasma optical emission intensity and electron impact excitation dynamics,and the amplitudes and the relative phase shift of the voltage and current waveforms,as a function of time are determined by corresponding experimental diagnostics,respectively.In particular,the spatio-temporal excitation dynamic of the electron swarm driven by the homogeneous RF electric field between the inter-electrodes is firstly presented in experiment.To obtain a better understanding of the transition between different electron power absorption modes during the ignition process,the evolution of the pulse ignition process is followed by a PIC/MCC code under the same experimental conditions.The simulation reproduces the experimental results very well.An electric model is developed to analyze the rapid impedance evolution of the discharge system,also aiding the understanding of the phase evolution of the electric field at discharge center.In chapter 4,by using the same set of the diagnostic methods and PIC/MCC code as in chapter 3,the effects of the pulse-off duration(T_off),which determines the initial charged particle density(n_ini),on the ignition process of a pulsed RF-CCP are investigated.The experimental results show that the plasma and electrical parameters during the pulse ignition phase depend strongly onT_{of f}.Particularly,whenT_off is relatively long,the plasma optical emission intensity is found to change with time in the same fashion as the RF power deposited into the system,suggesting that the power is primarily absorbed by the electrons,which dissipate their energy via inelastic collisions quickly.The results of the spatio-temporal electron impact excitation rates show that the electron power absorption undergoes different mode transitions during the pulse ignition phase for differentT_{of f}.Specifically,for shortT_{of f}(highn_ini),the“α”mode dominates during the ignition process.For intermediate values ofT_off(moderate n_ini),another excitation pattern caused by an enhanced drift electric field between electrodes is observed.For longerT_{of f}(very lown_ini),the ignition of a pulsed plasma behaves like gas breakdown.Computed values of the relative phase shift between the voltage and current waveforms,φ_vi,show a similar time-dependence to the experimental results,if the PIC/MCC simulations are initialized with specificn_ini.Meanwhile,the simulation results aid understanding the temporal evolution of some electrical parameters as well as the system impedance during the ignition process.In chapter 5,the effects of the gas pressure and voltage amplitude on the ignition process of a pulsed RF-CCP have been investigated by multifold experimental diagnostics.In this work,the pulse plasma is ignited with extremely low initial charged particle density,so the ignition process behaves like gas breakdown.The spatio-temporal pattern of the electron excitation dynamic obtained at the specific values of the selected pressures（firstly displayed in experiment）,as well as other results,aid the intuitive understanding of the dependence of the RF breakdown voltage on the gas pressure.It’s found that at lower pressures,the excitation pattern exhibits shorter and tilted regions,and ends at electrodes during early ignition stage,implying a substantial electron energy loss.While at relatively high pressures,the excitation pattern becomes wider and less tilted,and the proportion of electron energy consumed by the inelastic collisions process with lower threshold energies（e.g.,some collisions leading to the generation of excitation states particles）increases.So,the RF breakdown voltage reaches a minimum value at a specific gas pressure,and increases when the value of gas pressure deviates from this one.With the increase of the applied voltage amplitude,the ignition is advanced and becomes more pronounced.Particularly,at high voltage amplitude,the electron impact excitation rate exhibits complex spatio-temporal distribution when the plasma optical emission intensity overshoots,i.e.,the excitation maximum adjacent to the electrode is much higher than that in the center due to enhanced local electric field.