Spatial evolution of the electron energy distribution function in a low-pressure capacitively coupled plasma containing argon and krypton

The spatial evolution of the electron energy distribution function (EEDF) in the axial direction of a capacitively coupled plasma with two parallel plate electrodes is investigated using an optical emission line-ratio method for Ar/Kr discharges. When the rf power is increased from 25 to 400?W at a pressure of 400?mTorr, we observe a transition from convex EEDFs to concave ones and a sharp increase in electron density, due to an ??? mode transition, which is believed to be caused by the high-energy electrons originating in the high-voltage sheath. We also investigate the spatial evolution of the EEDF when the pressure is increased from 45 to 500?mTorr at a power of 100?W. The EEDF is uniform at pressures below 180?mTorr and becomes non-uniform at higher pressures, owing to the decrease in the energy relaxation length of the high-energy electrons.