Characterization of a 2 MHz-radiofrequency-driven magnetically expanding plasma source

A radio frequency (rf) plasma source is operated at 2 MHz under an expanding magnetic field. The source consists of a 14-cm-diameter and 25-cm-long source tube wound by a seven-turn rf loop antenna powered via an impedance matching circuit. The efficient rf power transfer to the plasma is demonstrated, and the high-density plasma above 1012 cm-3 is successfully obtained for a kW level of the rf power. The radial density profile having a center peak is observed both in the source and in the expanding magnetic field; it does not show the conical density structure previously observed in 13.56 MHz rf sources. This implies that the electron heating layer is changed by lowering the driving frequency. Even in the source operated at 2 MHz, the measurement of the ion energy distribution shows the presence of the supersonic ion beam as well as the sources operated at higher frequencies, e.g., 13.56 MHz in the previous studies.