Investigation On The Evolution Mechanism Of The Electron Density And Temperature In Pulsed Capacitively Coupled Plasmas

The pulsed discharge has been widely applied in industry, especially in the material surface processing area. Comparing with stationary discharges, pulsed discharges have several advantages, such as controlling the flux of different active species with more freedom thourgh adjustion of pulse frequency and duty cycle, decreasing surface charging and surface damage.The time scales of different physical processes in pulsed discharges vary in a wide range, causing a significant nonlinear transient relaxation of crutial discharge parameters(e.g., electron density and tempearture). Therefore, a time-resolved measurement and modelling analysis are required to understand and manipulate the physical characteristics of pulsed discharges. Notice that this transient relaxation closely depends on variousdischargeconditions, such as pressure, power, feed gas, pulse frequency and duty cycle. Because of the discharge complexity, sofar the relative research in literature cannot yet describe the relaxation process very well. This work attempts to reaveal the physical mechanism of the electron density and temperature evolution in pulsed capacitively coupled rf discharges, combining experimental measurements and models. The main contentsare:1. The distortion on the Lanmguir probe measurement due to the displacement current caused by the ac-coupling between the probe and the plasma is experimentally determined. A circuit model is developed for further quantitative discussion. An analytic method is proposed to amend the probe data in a strong ac-coupling situation, and the amendatory results can well agree with the results obtained by other independent measurements. 2. A model including the effects of both sheath collapse and e-e scattering within a relative wide discharge conditions is proposed to describe the electron density and temperature evolution in low pressure noble gas afterglow. The model results agree well with the measurement. As a comparison, other models in literature are applicable only in high density plasma, and there is no discussion on the spatial distribution profile of the plasma density. 3. The different relaxation characteristics of the electron density under various afterglow duration inpulsed electronegative oxygen discharges due to the memory effect is experimentally determined. An electron cycle is discovered when the afterglow duration is short, which is caused by the electron-impact dissociation and the oxygen metastable induced detachment according to modeling analysis.Besides, a significant electron reheating due to the super-elastic collisions with the oxygen vibrationally excited states and the oxygen metastables is also found in the late afterglow. 4. The influence of step-ionization, power transmission, electron temperature evolution, ion escape and molecular gas mixure on the electron density relaxation in the early power-on phase is investaged by both measurements and modelin pulsed noble gas discharges. The mechanism for the density evolution of reactive species and charged particles in pulsed CF4 discharge is revealed, and the dominant generation and loss processes for F-atom is summaried.