Multiple diagnostic techniques measured neutral gas temperatures in N<sub>2</sub> plasma and Ar-N<sub>2</sub> mixed plasma

Low-temperature inductively coupled radio-frequency plasma is a key plasma source in semiconductor fabrication, where the neutral gas temperature (<i>T</i><sub>g</sub>) is one of the critical parameters influencing chemical reactions and plasma characteristics. Precise control of <i>T</i><sub>g</sub> significantly influences processes such as thin-film deposition and reactive ion etching, with its synergistic interaction with plasma parameters (<i>n</i><sub>e</sub>, <i>T</i><sub>e</sub>) often determining process outcomes. Consequently, a thorough understanding of the evolution of <i>T</i><sub>g</sub> and its correlation with discharge parameters has become a critical issue for optimizing semiconductor manufacturing processes. To achieve more accurate measurements of neutral gas temperature, this work employs three temperature measurement techniques: spectroscopy, Bragg grating, and fiber optic sensing. These methods are used to systematically investigate the variation patterns of neutral gas temperature (<i>T</i><sub>g</sub>) in nitrogen plasma and nitrogen-argon mixed plasma under different radio-frequency power, gas pressure, and gas composition conditions. To elucidate the gas heating mechanism, this work combines Langmuir probe measurements of electron density, electron temperature, electron energy probability distribution with a global model simulation. The results show that as the RF power increases, the energy coupled to the plasma increases, the ionization reaction is enhanced, and the collision process and energy transfer between electrons and neutral particles increase, resulting in a monotonically increasing trend of <i>T</i><sub>g</sub>. When gas pressure initially increases, both electron density and background gas density rise together, enhancing heating efficiency and driving rapid <i>T</i><sub>g</sub> growth. However, beyond 3 Pa, electron mean free path shortens and electron density declines. In contrast, background gas density continues to increase, leading to slower <i>T</i><sub>g</sub> growth. In nitrogen/argon mixed system discharges, increasing the argon proportion significantly enhances the rate of <i>T</i><sub>g</sub> increase. This occurs because a higher argon ratio elevates the proportion of high-energy electrons and electron density, thereby strengthening ionization and neutral gas heating. At the same time, argon metastable atoms enhance the density of excited nitrogen particles through the Penning process, which promotes nitrogen molecular excitation to higher energy levels and further heats the gas. Additionally, we observe that the radial temperature distribution in pure nitrogen plasma shifts from parabolic to saddle-type with axial height increasing, due to intensified electron collision excitation near the coil under electromagnetic field effects. In this study, it is also found that the glass transition temperature at the radial edge remains virtually unchanged as atmospheric pressure increases. This is because, as pressure continues to rise, electrons beneath the coil struggle to migrate to the radial edge to collide with neutral particles, thereby limiting the heating of edge neutral particles.