Application of high-spatial-resolution distributed fiber-optic sensing technique for neutral gas temperature mapping in inductively coupled Ar plasmas

Neutral gas temperature in inductively coupled plasmas (ICPs) has a great impact on the chemical reaction rates, particle transport properties, and spatial distribution of plasma parameters. Moreover, its spatial distribution directly influences the precision and uniformity of the chip fabrication process. Despite its critical importance, rapid measurement of neutral gas temperature distribution in the plasma environment remains a major challenge nowadays. In this study, we first utilize a high-spatial-resolution distributed fiber-optic sensing technique based on optical frequency-domain reflectometry (OFDR) to achieve spatially continuous measurement of the neutral gas temperature in a low-pressure Ar ICP discharge. Conventional communication optical fibers can serve as the sensing medium, which is immune to electromagnetic interference, and hundreds of spatial temperature readings can be given simultaneously within seconds, providing a powerful tool to map the temperature profile in the plasma discharge. We systematically investigate the dependence of neutral gas temperature distribution on external parameters, such as radio frequency (RF) power and pressure. Simulations are also carried out based on a two-dimensional fluid model to quantitatively analyze the contributions from different electron collision reactions to the heat source for better elucidating the heating mechanisms of the neutral gas. Our results indicate that the neutral gas temperature rises with the increased RF power and pressure. Furthermore, as the vertical distance from the substrate increases, the neutral gas temperature initially rises and then falls, and the radial temperature profile transitions from a “convex” shape to a “saddle” shape. This “saddle” shape becomes more and more pronounced as the distance is further away from the substrate, leading to a significant degradation of radial temperature uniformity. This variation is mainly due to the increasing nonuniformity in the heat source distribution, which attributes to stronger electron collision excitation reactions, as it is closer to the coil. Our work demonstrates a novel approach for the neutral gas temperature measurement in ICPs by adopting the fully distributed optical fiber sensing technique based on OFDR, providing comprehensive temperature information in plasma discharge, which is expected to play an important role in investigating plasma heating mechanisms and optimizing fabrication processes.