Effect of voltage waveform tailoring and an additional 60 MHz frequency on the ion flux energy distribution function in a low pressure capacitively coupled radio frequency plasma*

Low pressure capacitively coupled radio frequency plasmas are widely used for etching and coating of large-area substrates, e.g., photovoltaic solar cell panels and coated glass windows for skyscrapers. The main challenges are to control the ion energy distribution at the electrode and to achieve a high uniformity of the coating properties at high process rates. In this work we use a multi frequency CCP apparatus. The plasma is ignited between two plane parallel electrodes with diameter of 490 mm and a gap of 75 mm. The discharge is operated in an Ar/N₂ mixture at pressures ranging from 0.5 Pa -2 Pa and driven by a tailored voltage waveform (13.56 MHz and 27.12 MHz) with an additional 60 MHz frequency to control the ion flux. We use an array of retarding field energy analysers to measure the ion energy distribution function (IEDF) and the ion flux at the grounded electrode and study the uniformity over the surface. Utilizing the electrical asymmetry effect via voltage waveform tailoring, the uniformity and the properties of the coatings can be controlled, as confirmed by simulations of the gas-phase transport and the surface kinetic processes. Furthermore, by applying the additional 60 MHz voltage, further control over the ion flux at the electrode can be achieved without noticeably influencing the uniformity of the IEDF.