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 Tg 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 Tg 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 Tg and the plasma characteristics, a Langmuir probe is used to measure the electron density ne, effective electron temperature Te, and ion density ni 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 Tg increases monotonically. With the increase of O2 content, ne and ni are observed to decrease due to enhanced dissociation and excitation of O2, leading to a decrease in Tg. The radial profile of Tg 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 Tg with rf power during the E-to-H mode transition is also presented and discussed.