Simulation Of High-Frequency Glow Discharges In Mixed Gas Under Atmospheric Pressure

The homogeneous plasma can be generated under atmospheric pressure by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technology, which is suitable for the manufacture of microcrystalline silicon thin film. In view of high stability, simple process and saving in materials etc, microcrystalline silicon thin film is regarded as the third generation of the silicon thin film solar cells products. At present, the microcrystalline silicon thin film solar battery has made great progress, but its development is still limited due to low deposition rate. In order to achieve the rapid deposition of microcrystalline silicon thin films, the methods such as hydrogen dilution, high frequency and very high frequency under atmospheric pressure have been proposed by experimenters and obtained some good experimental results. However, the studies on the effects of these external parameters on the gas response mechanism and film particle density distribution are not fully understood, therefore, the theoretical analysis and numerical simulation are in demand. In this thesis, one-dimensional and two-dimensional fluid models are employed to study the major seed particles, electrons and atomic hydrogen particle density distribution during thin film deposition processes, which reveals physical and chemical mechanisms of these processes and further provide a theoretical basis for improving the deposition speed and quality.Using one-dimensional fluid model, we study the influence of hydrogen dilution on densities distribution of different reactive particles during Ar/SiH4/H2 mixed gas PECVD microcrystalline silicon deposition process under atmospheric pressure RF (13.56MHz). In the simulation,16 reactions including the excitation, ionization, adsorption and recombination processes are taken into account for the reactive particles. The density continuity equation, current continuity equation and electron energy conservation equation are established for 12 reactive particles (electron, argon ions Ar+, excited state of argon Ar*, the hydrogen atom H, the hydrogen molecular ion H2+, molecular ion SiH3+, molecular ion SiH3-, neutral particles SiH3 and SiH2, and background argon atoms Ar, silane molecule SiH4, and the hydrogen molecule H2). We study the effects of hydrogen dilutability varying from 1% to 6% on density variation and spatial distribution of SiH3+, SiH3-, SiH3 and atomic H and electon. The impact of hydrogen dilutability on various particles density is analyzed through physical and chemical mechanisms. To be a summary, the importance of appropriate hydrogen dilutability on improving the speed and quality of microcrystalline silicon film is explained in theory. Based on one-dimensional self-consistent fluid model, the characteristics of the mixed gas (Ar/SiH4/H2) plasma glow discharge are studied with different high-frequency (6.78 MHz, 13.56 MHz and 27.12 MHz) under atmospheric pressures. The density continuity equation, current continuity equation and the electron energy conservation equation are established for all 12 reactive particles to study the impact of excitation frequency on current density and particles density distribution. The results indicate that there appear two discharge modes for the mixed gas (Ar/SiH4/H2) plasma glow discharge with different excitation frequencies. When the excitation frequency is less than 13.56 MHz, the discharge is in a mode; when the excitation frequency is higher than 13.56 MHz, the discharge is in y mode. We present spatial distribution curves for some reactive particles (SiH3+, SiH3-, SiH3, SiH2, H, Ar+, Ar*), electron density and electron temperature in theis two modes, and explain the variation of particle density based on physical and chemical reaction mechanism.Applying two-dimensional fluid model, the numerical simulation of atmosphere of the mixed gas (Ar/SiH4/H2) plasma glow discharge are performed in very high frequency in the parallel plate electrode structure. The influence of very high frequency (90 MHz to 150 MHz) on 2D electron density distribution are simulated. The simulation results show that:The optimum electron density distribution can be obtained with the excitation frequency of 110 MHz. With the excitation frequency being 110 MHz, the discharge current density rises very obviously as with the increasing of the silane concentration, which results in the conversion of mixed gas glow discharge modes (a and y modes). The results display there exist maximum plasma particles densities (SiH3, H, e) under certain silane concentration.