The effects of different boundary surface materials on electron power absorption dynamics in capacitive RF plasmas

In technological high frequency plasmas, the boundary surface materials are known to affect process relevant discharge characteristics, such as the production of charged species and radicals. However, the scientific origin of these effects is often unclear. In this work, we experimentally study the effect of different electrode surface materials on the spatio-temporally resolved dynamics of energetic electrons by Phase Resolved Optical Emission Spectroscopy in a symmetric low pressure capacitive RF discharge (13.56 MHz). Measurements are done as a function of pressure and driving voltage amplitude in argon gas with a small admixture of neon. We find that changing the electrode material can induce an ⍺- to ??-mode transition due to the different ion induced secondary electron emission coefficients. Using different surface materials at the powered and grounded electrode can lead to the presence of different modes at each electrode. This breaks the symmetry of the discharge and, thus, causes the generation of a DC self bias. With the obtained results it is shown experimentally that to more precisely model the plasma physics in simulations, material dependent electron emission coefficients need to be included, which is often not done.