Transparent electrical–optical probe for noninvasive plasma diagnostics

Abstract We present a wall-mounted indium tin oxide (ITO)–glass probe that enables co-located, minimally invasive electrical–optical plasma diagnostics suitable for industrial semiconductor reactors. Since ITO is both transparent and highly conductive, this probe allows for simultaneous electrical and optical measurements from the chamber wall. Electrically, the primary challenge is that the ITO glass behaves as a series resistor–capacitor ( RC ) load. To address this, we generalize the floating harmonic method by developing a comprehensive RC load model. Our analysis reveals that even a modest series resistance (∼tens of ohms) can attenuate harmonic currents and alter their phase, causing conventional, purely capacitive models to significantly overestimate the electron temperature. Our modified model corrects for these distortions, yielding accurate electron temperature and plasma density. Optically, the argon metastable density and the relative oxygen radical density were measured using the line-ratio method and actinometry, respectively. Finally, photoresist etching experiments were performed in an oxygen plasma, and the etch rate was directly measured. By combining the electrical and optical plasma parameters obtained from our probe, the observed variations in the etch rate could be consistently interpreted. This integrated approach provides a robust and practical method for noninvasive process monitoring, enabling combined electrical and optical diagnostics using a single wall-mounted probe.