Fluid Simulation Of Electrical Asymmetry Effect And Electromagnetic Effects In Capacitively Coupled H₂ Plasmas

Capacitively coupled plasmas have been extensively used within microelectronics industry.How to obtain uniform plasma distributions at high reaction rate for large area thin film deposition and etching process,is one of the key factors.The research shows that if the discharge is driven by multiple harmonics,the symmetry of the two sheaths can be adjusted by varying the phase angle between the fundamental frequency and the second harmonic,i.e.,the so-called electrical asymmetry effect?EAE?.When the highest harmonic is in the very high frequency?VHF?regime,the electromagnetic effects become more obvious,which will affect the plasma radial uniformity.Therefore,further research about the interaction between EAE and electromagnetic effects is needed to optimize the plasma processing.In this thesis,a two-dimensional fluid model combined with the full set of Maxwell equations has been employed to investigate the plasma properties in capacitively coupled hydrogen discharges driven by different voltage waveforms.The simulation results show that:?1?The amplitude of DC self-bias voltage((1_{?9?8??increases with the number of harmonics6}in discharges sustained by consecutive harmonics?equation?1.1??.The evolution of(1_{?9?8?with phase angle1 is different for various6?,and the period is changed from?to 2?.In addition,under the same6},the evolution of(1₍(9?8? with the phase angle of other harmonics is different,and the variation of(1₍(9?8? becomes less obvious by changing the phase angle of the highest harmonic.?2?In discharges sustained by the fundamental frequency,the second and the highest harmonics?equation?3.5??,the evolution of(1_{?9?8? with phase angle1 is the same as that of6?=2.When6}increases,the variation of(1₍(9?8? becomes less obvious.In discharges sustained by the fundamental frequency and the highest harmonic?equation?3.2??,the amplitude of(1₍?9?8?decreases with increasing6?,and the evolution of(1₍(9?8? with₁ also becomes different.?3?In discharges sustained by multiple consecutive harmonics?equation?1.1??,the best radial homogeneity is obtained at the phase angle₁=180°,when the fundamental frequency is 13.56 MHz,(1₀=100 V and the pressure is 200 mTorr.Within a period,three positive peaks ofare observed,which also causes substantial ionization at similar places.is characterized by a pronounced peak at the end of the period,and the value is lowest at₁=180°.The results indicate that the nonuniformity caused by the electromagnetic effects can be effectively suppressed by adjusting the phase angle between harmonics.Moreover,the influence is different under different discharge conditions.For instance,the modulation of the uniformity becomes obvious when the voltage amplitude(1₀ increases,while it becomes less evident with the increase of the pressure and the fundamental frequency.?4?In discharges sustained by multiple harmonics,it is found that the plasma radial uniformity becomes quite different by changing the ratio of the voltage amplitude of different harmonics,or employing different voltage waveforms.However,the amplitude of(1₍(9?8?decreases when changing the optimal structure of voltage waveform,and the radial uniformity become worse when the fundamental frequency increases.The results obtained in this thesis show that the influence of the voltage waveform on the amplitude and variation trend of(1₍(9?8? is different.In capacitive coupled hydrogen plasmas,the plasma uniformity can also be adjusted by the electrical asymmetry effect,and the effect is different under different discharge conditions.The results obtained in this work are important for improving the plasma processes by EAE,especially when the higher order harmonics are included.