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Abstract
Optical emission spectroscopy (OES) of the magnetic dipole allowed O2(b1Σg+) to O2(X3Σg−) transition was investigated as a non-intrusive gas temperature diagnostic for E-mode and H-mode inductively coupled plasmas (ICP) in oxygen. It was compared to tunable diode laser absorption spectroscopy using Ar admixtures, and OES of the nitrogen Second Positive System with nitrogen admixtures. O2 OES provided accurate results for the E-mode ICP, 400–600 K for powers of 100–300 W, but in H-mode the method was unsuitable probably because of excitation of O2(b1Σg+) by metastable atomic oxygen. Rotational temperatures were measured, using N2 OES with N2 admixtures, for pulsed operation of the ICP with a 30 ms pulse duration and 15% duty cycle. It took 1–3 ms before the steady-state rotational temperatures were achieved. In addition, a small variation of matching network settings affects the plasma ignition delay time by several ms.
Highlights
Low pressure oxygen plasmas find use in several industrial applications e.g. for etching or functionalisation of materials.1) The gas temperature in such oxygen plasmas plays a key role in the plasma chemistry and in optimization of these plasmas for specific applications.2)Well-established techniques for measuring the gas temperature, from Doppler broadening or as rotational temperature, in low-temperature plasmas include tunable diode laser absorption spectroscopy (TDLAS) on argon metastable atoms3) and optical emission spectroscopy (OES) on the nitrogen band N2(C3Πu, ν′ = 0) to N2(B3Πg, ν′ = 0).4) for pure oxygen plasmas both these methods require a gas admixture for diagnostic purposes, which can be a limiting factor
Zyryanov et al used the O2 Optical emission spectroscopy (OES) diagnostic to reliably measure gas temperatures in a dc glow discharge at ∼660–6600 Pa (5–50 Torr), using a high-resolution spectrometer.5) Here, we investigate whether this technique can be applied in an industrially relevant plasma; an inductively coupled plasma (ICP) at pressures of 20–50 Pa.6) Measurements of the new O2 OES diagnostic as a function of inductively coupled plasmas (ICP) power and pressure are compared to measurements using conventional N2 OES and TDLAS to establish its feasibility and accuracy
The first consideration to make is whether the rotational temperatures can be expected to be in equilibrium with the translational temperatures (TDLAS)
Summary
The use of powermodulation, or pulsing, of plasmas limits the time-averaged power deposition, and gas temperature, but maintains high plasma densities during the plasma pulse It offers an independent control over the electron temperature and gas temperature of the plasma, both of which are important for the plasma chemistry of the discharge.7) In addition, power-modulation allows treatment of heat-sensitive materials, for example plastics, with high-power, highdensity plasma processes.8) In practice, plasma pulsing introduces transient effects when the plasma is switched on, or off,9–11) which are not fully understood, making pulse optimization often an empirical process. Temporal evolution of electron properties is regularly reported, e.g.9–11) evolution of gas temperature during a plasma pulse is not often studied or taken into account in modeling.12,13) Duten et al reports gas temperature measurements in a pulsed, medium pressure microwave discharge in hydrogen,7) while Macko and Veis measured gas temperatures in a pulsed glow discharge in oxygen.14) We measure the temporal evolution of gas temperature in a plasma pulse with the aim to inform decisions about pulse duration for a desired gas temperature as well as investigate plasma formation mechanisms in a pulsed ICP
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