Particle simulations of monatomic and polyatomic gases for radio-frequency plasma discharges

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The objectives of this research are to develop predictive numerical models and carry out simulations of low pressure radio-frequency (RF) glow discharges for diamond-like-carbon (DLC) film deposition. Understanding the mechanisms of glow discharge such as electron and ion transport, gas phase reaction and deposition is important to optimize the glow discharge systems. Particle modeling is a powerful means to simulate such non-equilibrium phenomena. While charged particles were previously modeled, the new model included neutral motion. The model was extended to CH4 (polyatomic gas) plasma, which is an important process for DLC film deposition. Particle-in-cell Monte Carlo simulations (PIC/MC) of one-dimensional capacitively coupled RF glow discharges were carried out for low pressure Argon plasmas. The present scheme includes the motions and collisions of both neutrals and charged particles. Present results agree with results from similar studies. The model is also applied to cases with a heated electrode, for which conventional PIC models cannot be applied. PIC/MC method for polyatomic gas, which has been used in non plasma situations, was applied to low pressure heat transfer in developing flow between parallel plates. Results of this method were compared to the Graetz solution (simplified continuum solution). It indicates that the MC method is appropriate for simulating low pressure gas flow. CH4 plasma was modeled by combining the models of the PIC/MC method for RF plasma and polyatomic gas collision scheme. The model considers the motions of CH4, CH4+, CH3, C2H5, H2, H and electrons. Detailed information such as space and time dependent electric field, reaction rate and particle energy distributions was obtained. Deposition rate, which is obtained by sampling impinging particles to the electrode, shows that radicals are the major species for film formation as past literature reported. The developed CH4 plasma model was also applied to various conditions such as pressure, applied voltage, electrode gap. The effects of each operating parameter were studied. The model was also applied to CH4/H2 plasmas. Results show that hydrogen tends to suppress the formation of undesirable polyatomic hydrocarbons and decreases the deposition rate.

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