Detection of fast electrons in pulsed argon inductively-coupled plasmas using the 420.1–419.8 nm emission line pair

Pulsed rf plasmas exhibit many differences as compared to continuous wave plasmas with the same average power levels, including large temporal variations in the electron temperature, with a sharp spike when the power is applied and falling dramatically in the afterglow. We present a comparison of time-resolved measurements of the effective electron temperature in pulsed inductively-coupled plasmas by means of (i) optical emission spectroscopy (OES) using different sets of argon emission lines and (ii) Langmuir probe measurements. One OES diagnostic used six strong Ar() emission lines in the 700–800 nm wavelength range, the second used only the Ar 420.1–419.8 nm line pair. For pulsed plasmas with long afterglow periods, the line pair method reveals the presence of a significant number of hot electrons ( eV) at the start of the pulse. Under these conditions, the metastable atom density is very low, and the diagnostic using the Ar() emission lines is ineffective for determining the electron temperature. For later parts of the pulse and pulsed plasmas with short periods (i.e. 10 μs), the metastable density is high and the two OES methods yield similar results which are also in agreement with probe measurements.