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 T_V. 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.