Investigation of coupling between chemistry and discharge dynamics in radio frequency hydrogen plasmas in the Torr regime

We present the results of a study of a capacitively coupled hydrogen discharge by means of a one-dimensional numerical fluid model and experiments. The model includes a detailed description of the gas-phase chemistry taking into account the production of H? ions by dissociative attachment of H2 vibrational levels. The population of these levels is described by a Boltzmann vibrational distribution function characterized by a vibrational temperature TV. The effect of the dissociative-attachment reaction on the discharge dynamics was investigated by varying the vibrational temperature, which was used as a model input parameter. Increasing the vibrational temperature from 1000 to 6000?K affects both the chemistry and the dynamics of the electrical discharge. Because of dissociative attachment, the H? ion density increases by seven orders of magnitude and the H? ion density to electron density ratio varies from 10?7 to 6, while the positive ion density increases slightly. As a consequence, the atomic hydrogen density increases by a factor of three, and the sheath voltage drops from 95 to 75?V. Therefore, clear evidence of a strong coupling between chemistry and electrical dynamics through the production of H? ions is demonstrated. Moreover, satisfactory agreement between computed and measured values of atomic hydrogen and H? ion densities gives further support to the requirement of a detailed description of the hydrogen vibrational kinetics for capacitively coupled radio frequency discharge models in the Torr regime.