In situ measurement of the electron elastic reflection coefficient at a plasma-exposed silicon dioxide surface

Plasma simulations require accurate input data that describe the interactions of electrons with plasma-exposed surfaces. One important, rarely studied interaction is elastic reflection. This article presents measurements of the elastic reflection coefficient made in situ, i.e., during plasma exposure, in an inductively coupled plasma (icp) reactor that has an azimuthally asymmetric, rf-biased substrate electrode. The rf current and voltage across the sheath adjacent to this electrode were measured, along with the ion current density and electron temperature, for argon plasmas at 0.67 and 1.33 Pa. Using the measurements and a numerical sheath model, the currents contributed by electrons that are emitted from the rf-biased electrode are determined. Some emitted electrons are elastically reflected from the opposing surface, the fused quartz window below the icp source. Deflection of these electrons by 13.56 MHz electric and magnetic fields in the plasma can prevent them from returning to the rf-biased electrode, producing a second-harmonic current at 27.12 MHz. The transport of the emitted electrons is simulated by a plasma model that includes electric and magnetic effects, transit-time effects, elastic reflection at the electrode as well as the window, and multiple passes through the plasma. From the plasma model and the measured 27.12 MHz current, the elastic reflection coefficient at the quartz window was determined, for electron energies from 25 eV to 1.2 keV. The measured reflection coefficient was higher than literature values for clean Si, but it had a similar energy-dependence. An approximate analytical form for its energy-dependence is presented, as well as an uncertainty analysis.