Inducing locally structured ion energy distributions in intermediate-pressure plasmas

Ion energy distribution functions (IEDFs) incident upon material surfaces in radio frequency (rf) capacitively coupled plasmas are coupled to spatial and temporal sheath dynamics. Tailoring the ion energy distribution function within intermediate-pressure plasmas (≈133 Pa, 1 Torr), which find application in surface modification and aerospace industries, is challenging due to the collisional conditions. In this work, experimentally benchmarked 2D fluid/Monte-Carlo simulations are employed to demonstrate the production of structured IEDFs in a collisional (200 Pa 1.5 Torr argon) rf hollow cathode discharge. The formation of structures within the IEDFs is explained by an increase in the Ar⁺ ion-neutral mean-free-path and a simultaneous decrease in the phase-averaged sheath extension as the rf voltage frequency increases over 13.56-108.48 MHz for a constant rf voltage amplitude (increasing plasma power) and gas flow rate. 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 and 54.24 MHz and additional "high energy" (180 eV) structures between 81.36 and 94.92 MHz, with the structures within each region displaying a distinct sensitivity to the applied voltage amplitude. 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 over a wider voltage range. The capabitlity to extend the range of access to an operational regime, where the structured IEDFs are observed, is desirable for applications that require control of the ion-bombardment energy under collisional plasma conditions.