Measurement of neutral gas temperature in inductively coupled Ar and Ar/O₂ plasmas

In low-temperature inductively coupled radio frequency (rf) plasmas, electrons and ions that gain energy from the electric field can transfer a portion of energy to neutral particles. The resulting radial variation of the neutral gas temperature [Formula: see text] can significantly influence the radial distributions of reaction rates and radical densities on the substrate, thus affecting the etching/film deposition uniformity. In this work, we perform an experimental study on the dependence of the neutral gas temperature [Formula: see text] on external parameters (i.e., rf power, pressure, and gas component) in inductively coupled Ar and Ar/O2 plasmas by using a fiber Bragg grating sensor. To analyze the correlation between [Formula: see text] and the plasma characteristics, a Langmuir probe is used to measure the electron density [Formula: see text], effective electron temperature [Formula: see text], and ion density [Formula: see text] under the same discharge conditions. It is found that in both Ar and Ar/O2 plasmas, neutral gas heating is sensitive to plasma density. As the plasma density increases with the pressure/power, the collisions of ions and electrons with neutral particles are enhanced so that [Formula: see text] increases monotonically. With the increase of O2 content, [Formula: see text] and [Formula: see text] are observed to decrease due to enhanced dissociation and excitation of O2, leading to a decrease in [Formula: see text]. The radial profile of [Formula: see text] exhibits a parabolic distribution in pure Ar discharges, whereas it evolves through a center-flat shape into a saddle shape with the increase of O2 content. The variation of [Formula: see text] with rf power during the E-to-H mode transition is also presented and discussed.