Investigations of high voltage plasma boundary sheaths in radio-frequency discharges operated with multiple frequencies

The complex nature of multi frequency high voltage rf plasma boundary sheaths is experimentally investigated. A mass resolved ion energy analyser is incorporated into the grounded electrode of a confined symmetric capacitively coupled rf discharge, operated either in single or dual frequency mode. Inherent difficulties in measuring ion energy distribution functions (IEDFs) are minimised by a procedure based on simulations of ion trajectories and extensive experimental checks. A novel simple calibration method for determining absolute ion fluxes is developed.\ud \ud Discharges in hydrogen, deuterium and mixtures of both are investigated. The relatively light hydrogen ions respond to temporal variations in the sheath potential at typical technologically used radio-frequencies (e.g. 13.56 MHz), allowing for detailed investigations of the sheath dynamics. Hydrogen is also well suited for comparison to simulations. It is the simplest molecular gas with the most extensive data set for collisional cross-sections.\ud \ud Experimental results of absolute ion fluxes and IEDFs are compared to 20- PIC simulations perfectly adapted to the experimental conditions. The agreement is in general excellent. Simulations yield deeper insight into the sheath dynamics and sheath chemistry. The simulations can also be used under conditions which are difficult or impossible experimentally.\ud \ud Details of the sheath dynamics and the sheath chemistry of the investigated discharges are well understood. Structures in the IEDFs caused by the complex dynamics of dual frequency sheaths can be explained and reproduced using a simple analytical model. The basic concept of separate control of ion flux and ion energy in dual frequency discharges is observed. However, the IEDFs of light ions are quite broad. This limits control and selectivity in technological processes. The concept works better for heavier ions, since the IEDFs are narrower due to more averaging over the sheath dynamics.\ud \ud Time resolved investigations in the afterglow of a pulsed mode plasma, reveal that the positive space charge sheath does not fully collapse in the post discharge. This can be attributed to electron heating by vibrationally excited molecules in the afterglow