Characteristics of a VHF H₂plasma at high pressures for different discharge gaps

A capacitively coupled VHF H2 plasma (60 MHz) was produced at high pressures, and plasma parameters were examined as a function of pressure and power at different discharge gaps (10, 15, and 20 mm). Here, a balanced power feeding method was adopted to avoid anomalous discharges. It was found that the electron density peaked at a certain pressure. As the discharge gap decreased, the peak pressure increased. The electron temperatures at the discharge gaps of 10 and 20 mm were approximately 8 and 3 eV at high pressures, respectively. In addition, we found that the sheath potential corresponding to the ion bombardment energy with the discharge gap of 10 mm was lower than the theoretical value calculated on the basis of the sheath theory, suggesting the existence of negative ions. Estimation of negative ion concentration was attempted on the basis of the sheath theory taking into consideration negative ions.