Optical emission characteristics and mode transitions in low-frequency inductively coupled plasmas

Optical emission characteristics of the 500 kHz flat-coil inductively coupled discharges in pure argon, nitrogen, and gas mixtures of Ar+H 2 , N 2 +Ar , and N 2 +H 2 are investigated. Variation of input power and operating gas pressure lead to hysteresis in the optical emission intensity (OEI), which is associated with the transitions between the electrostatic (E) and electromagnetic (H) discharge operating regimes. The characteristics of the hysteresis loops and character of mode transitions appear to be different in pure gases and gas mixtures. It has been observed that the E→H transition are always discontinuous, while the H→E transitions appear smooth in pure nitrogen and N 2 -dominated discharges. Dependence of the E→H transition threshold on gas composition in Ar+N 2 , Ar+H 2 , and N 2 +H 2 mixtures is investigated and underlying mechanisms are discussed. It is also shown that the OEI of nitrogen species can efficiently be controlled by small Ar or H 2 admixtures. Addition of argon enhances the optical emission of N 2 , N 2 + , N, and N + species, whereas the effect of hydrogen admixture is the opposite.