Structure of Velocity Distribution of Sheath-Accelerated Secondary Electrons in an Asymmetric RF-DC Discharge

Low-pressure capacitively-coupled discharges with an additional DC bias applied to a separate electrode are important for plasma-assisted etching for semiconductor device manufacturing. Measurements of the electron velocity distribution function (EVDF) impinging on the wafer and in the plasma bulk show complex structure of EVDF with multiple peaks and steps. These features in the electron energy distribution functions are possibly caused by secondary electrons emitted from the electrodes and interacting with two high-voltage sheaths: a stationary sheath at the DC electrode and an oscillating, self-biased sheath at the powered electrode. We have performed test-particle simulations where the features in the EVDF of electrons impacting the RF electrode are fully resolved at all energies, and example appearing in Fig. 1. An analytical model has been developed to predict existence of peaked and step-like structures in the EVDF. These features can be explained by analyzing the kine-matics of electron trajectories in the discharge gap. Step-like structures in the EVDF near the powered electrode appear to be due to accumulation of trapped electrons during part of the RF cycle and their subsequent release. Additional peaks, at lower energies, are formed by the electrons emitted from the RF electrode and eventually escaping to back to the same electrode. The latter electrons can be grouped by the number of bounces between the sheaths during their residence time in the discharge as shown in Fig. 1. Figure 1 – (top) Numbers of bounces electrons perform in the gap before exiting to the RF electrode. (bottom) The EVDF of electrons at the RF electrode averaged over RF period as would be seen by an energy analyzer for V DC =800V and V pp =1500V. The green line shows contribution of electrons emitted from the DC electrode and blue line shows contribution of electrons emitted from the RF electrode. The red line is the sum of the two.