Angular impact energy distributions of argon ions at the powered electrode of a helicon plasma source

A radio frequency excited helicon plasma is generated by coupling externally generated electric and magnetic fields into the plasma confined by an axial magnetic field. The 13.56 MHz rf power provided via an antenna is varied between 100 and 500 W. The axial magnetic field strength is varied between 25 and 150 G. The substrate electrode is independently powered with 13.56 MHz radio frequency. A 100 μm orifice in the substrate electrode allows a small sample of argon ions bombarding the substrate electrode to enter a detection chamber. In the detection chamber a quadrupole mass spectrometer equipped with an energy filter is used to measure mass selected ion energy distributions and by tilting the quadrupole analyzer with the vertex lying in the orifice ion angular distributions are investigated. The ion energy distributions at the powered electrode mainly consist of either a single peak or they consist of the well known bimodal structure caused by rf splitting. For some source parameter a continuous energy distribution is detected for lower bombardment energies, which is interpreted as the result of scattering of ions with the background gas during their transport to the substrate electrode. The variation of the total flux intensities, the dc bias potentials, and the plasma potentials are evaluated and are interpreted as a result of mode transitions of the helicon plasma. The ion angular distributions are unstructured and show angular widths in the range between 2° and 3° full width at half-maximum. Some aspects concerning the application of the source for semiconductor etching processes are discussed.