Student Excellence Award Finalist: Control of ion energy distribution functions in intermediate pressure plasmas

Ion energy distribution functions (IEDFs) incident upon material surfaces in radio-frequency capacitively coupled plasmas (rf CCPs) are coupled to the spatial and temporal sheath dynamics. Tailoring the ion energy distribution function within intermediate-pressure plasmas ($\approx$~133~Pa, 1~Torr), finding application in surface modification and aerospace industries, is challenging due to the collisional conditions. In this work, experimentally benchmarked fluid/Monte-Carlo simulations are employed to demonstrate control of the shape of IEDFs in a collisional (200~Pa 1.5~Torr argon) rf hollow cathode discharge through the application of high frequency ($\geq$~13.56~MHz) voltage waveforms. Two distinct transitions in the shape of the IEDF are observed at 450~V, corresponding to the formation of mid-energy (60~-~180~eV) structures between 40.68~-~54.24~MHz and additional high energy ($\gtrsim$~180~eV) structures between 81.36~-~94.92~MHz. Transitions between these energy ranges occurred at lower applied voltages for increased applied voltage frequencies, providing increased control of the mean and modal ion energy, varying by 106~eV and 280~eV, respectively. Structured IEDFs are of interest to applications requiring control of ion-bombardment energy under collisional conditions.