Fiber PROES: Phase resolved optical emission spectroscopy via optical fibers for knowledge-based plasma process development and monitoring.

High-frequency technological low-temperature plasmas play a key role in various industrial processes of high societal relevance, such as semiconductor manufacturing and gas conversion. Due to their complexity, the fundamentals of their operation are typically not understood and process development is done empirically. The continuous increase in process requirements with respect to precision and reproducibility, however, necessitates knowledge-based approaches toward process development and monitoring. Diagnostic techniques used for this should be non-invasive, have short measuring times, and have low equipment costs. A valuable tool to understand plasma processes is to measure the spatio-temporally resolved dynamics of energetic electrons with phase resolved optical emission spectroscopy (PROES), as these electrons generate the plasma through ionization and reactive radicals through dissociation of the neutral gas. However, PROES is typically performed based on expensive intensified charge-coupled device (ICCD) cameras, is slow, and requires large windows for optical access to the plasma, which do not exist in commercial reactors. To overcome these limitations, we present a modified version of this diagnostic, Fiber PROES, which is based on an optical fiber in combination with a photo-multiplier tube operated in a photon-counting mode. Compared to classical PROES, only a small fiber access port is required, which is typically available in commercial plasma reactors, the costs are strongly reduced, and the measurement speed is increased. We demonstrate that Fiber PROES yields similar results compared to classical ICCD-camera-based PROES by comparing measurements taken in geometrically symmetric capacitively coupled radio frequency plasma based on both PROES variants.