Laser spectroscopic studies of plasma

This thesis describes the application of sensitive optical absorption techniques in order to probe inductively coupled plasmas of oxygen and nitrogen. Radio frequency plasmas formed from these simple molecular species have found an increasingly important role in many industrial applications and high resolution spectroscopy provides a means to probe their chemistry with unrivalled specificity and sensitivity. In particular, this work applies the technique of cavity ringdown spectroscopy (CRDS) to detect atomic, ionic and electronically excited molecular species as a function of plasma operating conditions. The plasma probed in this work is created in a low pressure (10 − 100 mTorr) inductively coupled plasma chamber by application of up to 500 W of 13.56 MHz radio frequency power via a 1.5 turn double spiral antenna in a stove top arrangement. The optical cavity utilised in the measurements probes the plasma 120 mm below to top window (which separates the driven coil from the plasma) and 50 mm above the lower, ground electrode. CRDS results are supplemented with observations of plasma emission spectra and comprehensively interpreted by kinetic modelling.
The work is divided into two sections according to the plasma being probed. The first section concerns oxygen plasma with CRDS measurements of O(3P) and O2(a1∆g) utilising forbidden transitions. These measurements reveal dissociation fractions as high as ≈ 15%, metastable molecule fractions of ≈ 5% and translation temperatures up to ≈ 450 K. The target species, by virtue of their different threshold energies for electron impact production, provide insight into different regions of the electron energy distribution function (EEDF). As a result, measurements of O(3P) and O2(a1∆g) in combination with a volume averaged kinetic plasma model allow changes in the EEDF to be investigated as the plasma transitions from the E to the H-mode of operation. In addition, aspects of the spectroscopy of O2(a1∆g) are clarified with respect to the appropriate sum rule for Honl-London factors, necessary in order to properly deduce absolute concentrations.
The volume averaged modelling, although quantitatively useful, does not account for spatial inhomogeneity within the plasma. This inhomogeneity is investigated using measurements of O2(X3Σ−g) in the v = 0 and v = 1 vibrational states. These observations also elucidate the degree of vibrational excitation within the plasma and reveal a vibrational temperature (amongst the low v states) of ≈ 750 ± 150 K at 100 mTorr and 300 W. A 1D model utilising physically reasonable line of sight variation in plasma temperature and composition corroborates the CRDS measurements.
The second section of this thesis concerns nitrogen plasma and focuses on CRD measurements of the molecular cation, N+2(X2Σ+g ), and the electronically excited N2(A3Σ+u) state. These species can be probed using allowed transitions, but due to their low density, the sensitivity enhancement afforded by CRDS is still advantageous. Notably, the use of large intracavity radiation intensities to probe allowed transitions results in optical saturation, the effects of which must be carefully accounted for when determining species temperatures and densities.
With adjustments made for the effects of optical saturation the CRD measurements show ion (and therefore electron) densities of the order of 109−1010 cm−3 in the plasma bulk (depending on operating conditions) and metastable densities an order of magnitude higher. Interestingly the two species show rather different translational temperatures with the ions typically ≈ 1000 K and the metastables ≈ 600 K. Once again the absolute density measurements are interpreted in terms of a volume averaged kinetic model. The model reveals a limitation in the understanding of nitrogen discharges that has arisen consistently in the literature, namely, the inability to account quantitatively for the density of Nu(A3Σ+u the literature rate coefficients for the processes typically deemed most important in its production and loss. The possible reasons for the discrepancy are explored in depth.
In addition, spatially resolved measurements of the same nitrogen species are presented, with particular reference to how ion densities change as the edge of the chamber is approached (in regions known as the plasma pre-sheath and sheath). Measurements with a spatial resolution of ≈ 100 µm show that the ion density is reduced by almost an order of magnitude close to the chamber’s lower electrode.
Finally, the effects of saturation on the CRD spectra are explored and the possible contributions to the Lamb dip width are discussed in the context of spectral broadening mechanisms. The laser linewidth is measured by a self-heterodyne beat note experiment to be < 100 kHz indicating that it contributes little to the observed Lamb dip widths (> 100 MHz) and that other processes are dominant. It is concluded that, whilst power broadening plays a significant role in explaining the width of the Lamb dips, the dominant cause of the broadening is unresolved hyperfine structure arising due to the non-zero nuclear spin of 14N.

Advisor: Ritchie, G