Cavity ringdown studies of the E–H transition in an inductively coupled oxygen plasma: comparison of spectroscopic measurements and modelling

Abstract

The absolute number density of ground state oxygen atoms, O(3P), present in a 100 mTorr oxygen plasma has been determined as a function of operating power using cavity ringdown spectroscopy (CRDS). The dissociation fraction increases by an order of magnitude from ∽0.8% at 50 W to 8% at 250 W and reflects a similar increase in the electron density over this power range. Emission spectra show that the E-H switchover is accompanied by increased rotational heating of O2 and this behaviour is also observed in the translational temperatures determined by fitting the Doppler limited O(3P) CRDS data. The measurements are contextualised via a volume averaged kinetic model that uses the measured absolute densities of O(3P) and O2(a1Δg, v = 0) as a function of power as its benchmarks. Despite the inherent spatial inhomogeneity of the plasma, the volume averaged model, which uses a minimal set of reactions, is able to both reproduce previous measurements on the absolute density of O− and to infer physically reasonable values for both the electron temperature and number density as the E-H switch over is traversed. Time-resolved emission measurements return a value of 0.2 for the wall loss coefficient for O2(b1Σg +); as a consequence, the number density of O2(b1Σg +) is (at least) one order of magnitude less than O2(a1Δg).

Insights into spatial inhomogeneity in an oxygen plasma from cavity ringdown spectroscopy

“…The chemistry and physics of these plasmas is heavily influenced not only by the presence of negative ions, but also high densities of ground state atoms, O( 3 P), and electronically excited molecules, particularly the low-lying metastable state, O 2 (a 1 ∆ g ). Such is the importance of these species that accurate knowledge of their absolute densities is vital to testing and refining models of both the gas-phase and the gas-surface chemistry occurring in these systems [13][14][15][16][17].…”

“…With these aims in mind, we have recently reported high resolution diode laser-based cavity ringdown measurements of the most important species in a pure oxygen plasma [17][18][19]. Ground state atomic oxygen, O( 3 P), has been probed using the O( 1 D) ← O( 3 P) absorption at 630 nm as a function of plasma pressure and power [18].…”

“…Ground state atomic oxygen, O( 3 P), has been probed using the O( 1 D) ← O( 3 P) absorption at 630 nm as a function of plasma pressure and power [18]. Measurements included the observation of wall loss kinetics and of the switchover of the plasma from the capacitive E-mode of operation to the hotter and denser H-mode indicated by an increase in atom density by an order of magnitude (from a few 10 13 to a few 10 14 cm -3 ) [17]. The measured dissociation fractions (≈10% in the H-mode) are consistently high predominantly as a result of the slow wall loss of atoms in the aluminium plasma chamber (as defined by the wall loss coefficient, γ ≈ 0.003).…”

“…These CRDS results have been supported by observations of the plasma emission, in particular from strong atomic emission lines and from the O 2 b-X system. The fitting of molecular spectra to simulations enabled determination of both rotational (350-630 K) and vibrational (905 K) temperatures as well as giving an indication of the relative density of O 2 (b) as a function of plasma power at a total pressure of 100 mTorr (13.332 Pa) [17].…”

“…In our work, all of the experimental results were interpreted using a volume averaged kinetic model based on a restricted number of important reactions [17]. This model, which included 34 kinetic processes, was successful in reproducing experimental atom, anion and metastable molecule densities and from it electron densities and temperatures were inferred.…”