Analysis of hydrogen plasma in MPCVD reactor
The aim of this work is to build a numerical model that can predict the plasma properties of hydrogen plasmas inside a Seki Technotron Corp. AX5200S MPCVD system so that it may be used to understand and optimize the conditions for the growth of carbon nanostructures. A 2D model of the system is used in the finite element high frequency Maxwell solver and heat trasfer solver in COMSOL Multiphysics, where the solvers are coupled with user defined functions to analyze the plasma. A simplified chemistry model is formulated in order to determine the electron temperature in the plasma. This is used in the UDFs which calculate the electron number density as well as electron temperature. A Boltzmann equation solver for electrons in weakly ionized gases under uniform electric fields, called BOLSIG+, is used to obtain certain input parameters required for these UDFs. The system is modeled for several reactor geometries at pressures of 10 Torr and 30 Torr and powers ranging from 300 W to 700 W. The variation of plasma characteristics with changes in input conditions is studied and the electric field, electron number density, electron temperature and gas temperature are seen to increase with increasing power. Electric field, electron number density and electron temperature decrease and gas temperature increases with increasing pressure. The modeling results are compared with experimental measurements and a good agreement is found after calibrating the parameter gamma in Füner's model to match experimental electron number densities. The gas temperature is seen to have a weak dependence on power and a strong dependence on gas pressure. On an average, the gas temperature at a point 5 mm above the center of the puck increases from about 1000 K at a pressure of 10 Torr to about 1500 K at 30 Torr. The inclusion of the pillar produces an increase in the maximum electron number density of approximately 50%; it is higher under some conditions. It increases the maximum electron temperature by about 70% and at 500 W and 30 Torr, the maximum gas temperature is seen to increase by 50%. The effect of susceptor position is studied and it is seen that the only condition favorable to growth would be to raise it by less than 25 mm from the initial reference position or to maintain it at the same level.
Alexeenko, Purdue University.
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