Dynamics of the Plasma-Surface Interface in Capacitively Coupled Radio-Frequency Oxygen Plasmas: Coupling Numerical Simulations with Optical Diagnostics

Plasma processing on industrial scale is becoming increasingly complex, demanding new strategies for process control and monitoring. Of particular interest is the energy transport in the interface region between the non-equilibrium low-pressure plasma and the surface. The individual plasma components have varying properties and exhibit different dynamics, which enable numerous chemical and physical modifications of surfaces simultaneously. Measurements of the in-situ surface condition and important chemically active radical species are extremely challenging. The most promising approach to overcome these challenges to achieve advanced process control is the active coupling of numerical simulations and experiments. In this regard, numerical simulations are a well-established technique to study fundamental plasma parameters and plasma dynamics for a variety of discharge sources.

The utilised numerical simulation is an experimentally benchmarked 1D fluid model, with semi-kinetic treatment of electrons and an improved energy dependent ion mobility treatment. This model is applied for a geometrically symmetric and asymmetric capacitively coupled oxygen RF discharge. Within the investigated pressure range of 10 Pa - 100 Pa the simulations predict that changing surface conditions have a significant effect on dynamics of the plasma-surface interface. In particular, the surface loss probability and lifetime of metastable singlet delta oxygen as well as the secondary electron emission coefficient are identified to substantially influence the electronegativity and the plasma sheath dynamics on a nanosecond timescale. Phase resolved optical emission spectroscopy measurements, utilising different surface materials, confirm these predictions by comparing measured and simulated excitation features for three different optical emission lines.

Through the synergistic coupling of numerical simulations and experiments, the surface work functions as well as other key surface parameters are assessed. Furthermore, the use of an advanced actinometry technique, demonstrated by coupling simple electron kinetic simulations and optical measurements, enables measurements of the spatial distribution of radical atomic oxygen densities and local electron energies over the total discharge volume.