Investigating the spatial distribution of inductively coupled plasmas using spectroscopy and plasma modelling simulations Charlie Kniebe-Evans 1 , Robert Peverall 1 , Samuel Rogers 2 , Gus Hancock 1 , Grant Ritchie 1 1 Department of Physical & Theoretical Chemistry, University of Oxford, UK, 2 Wolfson Atmospheric Chemistry Laboratory, University of York, UK Cavity-enhanced absorption techniques are utilised to determine the spatial distribution of reactive etch plasmas, including number densities and temperatures for trace species N 2 + (X) and N 2 (A) in a low-pressure Inductively Coupled Plasma (ICP) chamber. Selective rotational lines and vibrational bands within the N 2 + (A 2 Π u ← X 2 Σ g + ) and the N 2 (B 3 Π g ← A 3 Σ u + ) electronic transitions have been studied with CRDS, in addition to CEAS studies on the (Δν=2) vibrational band of the N 2 (B 3 Π g ← A 3 Σ u + ) transition. Saturated cavity ringdown spectroscopy (sat-CRDS) allows sensitive measurements of absolute number densities and translational temperatures, detecting ion densities as low as ≈ 1×10 7 cm -3 . Numerically verified methods have been implemented to quantify optical saturation effects and, in the plasma bulk, experiments conducted across a matrix of pressure (10−100 mTorr) and rf power (200−400 W) plasma conditions return maximum ion densities of ≈ 1×10 10 cm -3 . Bulk translational temperatures are observed to range from 650−1400 K and 550−1000 K for N 2 + (X) and N 2 (A), respectively. CRDS has also been applied to help determine the wall loss coefficients of N 2 (A) under various plasma conditions, which is vital information for plasma modelling. To effectively probe the plasma-surface boundary region of the plasma with sub-mm resolution, a ‘wavelength switching’ experimental technique is introduced and absolute number densities as a function of height above the non-driven electrode are quantified. A clear decrease in N 2 + (X) (ν=0) number density is observed as the electrode is approached, with a weaker effect observed for the N 2 (A) metastable species. This consolidates (with greater spatial resolution) previous work by Woodcock et al.[1] where the acceleration of ions towards the driven electrode in a capacitively coupled chamber was resolved by the use of LIF. Further species height profiles are obtained to explore spatial differences in number density as a function of bias voltage, where, to realise this, different negative DC biases are applied to the non-driven electrode. Imaging of a capacitive regime plasma captures a clear darkened region which appears to extend when the negative bias is applied. Broadband cavity-enhanced absorption spectroscopy (bbCEAS) allows vibrational and rotational temperatures to be obtained by contour profiling of several complete rovibrational bands in the N 2 (B 3 Π g ← A 3 Σ u + ) electronic transition. The dependence of T vib and T rot on power, pressure, and height is investigated, with notably higher vibrational temperatures observed close to the electrode surface, in the plasma-surface boundary region. A two-dimensional axisymmetric model of the ICP chamber has been established to accompany the experimental results using Vizglow, a plasma simulation software. CRDS ion density height trends are supported by preliminary modelling results and plasma simulations within this model will build upon experimental insights to optimise a commercial nitrogen chemistry set. References 1. B K Woodcock, J R Busby, T G M Freegarde, and G Hancock 1997 Journal of Applied Physics 81 5945
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