Ion distribution functions at the electrodes of capacitively coupled high-pressure hydrogen discharges

The flux-energy distribution functions of ions at the electrodes of capacitively coupled parallel plate discharges operated in a regime of relatively high gas pressures (200–650 Pa) are investigated experimentally using (differentially pumped) energy-resolved mass spectrometry under various conditions and compared with the results of a numeric simulation and a model. It is shown that the simulated distribution function can be reproduced by a simple analytical modeling approach, which assumes a constant collision frequency and is valid for highly collisional sheaths. The comparison between experiment and simulation reveals that not the entire angular distribution is covered by the diagnostics. However, the normalized experimentally obtained distributions are close to the simulated ones, thus indicating that the measurements allow for a reasonable discussion. In a single frequency 13.56 MHz discharge, the width of the distribution increases as a function of the voltage amplitude and decreases as a function of pressure. Applying an electrically asymmetric voltage waveform (13.56 MHz + 27.12 MHz) to the powered electrode breaks the symmetry of the geometrically symmetric discharge. This allows manipulation of the shape and width of the ion flux-energy distribution functions at the electrodes by tuning the phase angle between the two frequencies. It is found experimentally that controlling the ion energy without affecting the total flux is possible via the electrical asymmetry effect under these high-pressure conditions, whereas a change in pressure or voltage amplitude affects both the energy and the flux of ions.