Spatially resolved measurements of ground state atomic oxygen and its anion in a low pressure ICP using cavity ringdown spectroscopy

Abstract

This paper presents spatially resolved measurements of the absolute ground state atomic oxygen and oxygen anion densities close to the grounded electrode in an inductively coupled oxygen plasma obtained using cavity ringdown spectroscopy. Measurements on two spin–orbit states of atomic oxygen (3P2,3P1) confirm that their populations are in thermal equilibrium and are well described by the translational temperature, such that the total atomic density can be deduced using only a single spin–orbit resolved spectroscopic transition (1D2 ← 3P2). Atomic densities range from 0.4 to 2.9 x 1014cm-3 at a height of ≈20 mm above the electrode and over the pressure range 10 − 100 mTorr at fixed power of 300 W. Under conditions of 300 W and 100 mTor, the atomic density shows a moderate increase of ≈45% as the electrode surface is approached, while the line-of sight average translational temperature falls from ca. 450 K towards 300 K. By contrast, the oxygen anion density falls sharply with increasing pressure and towards the electrode. The maximum anion density (ca. 1.5 x 1010 cm -3) occurs in the 10–20 mTorr range and falls by a factor of >5 over a range of 20 mm. This change is broadly consistent with a one-dimensional model for positive ion flux in an electronegative plasma. Further laser absorption measurements on the metastable O(5S) state are used as a proxy for plasma power density and show that the small changes in geometry induced by vertically translating the ground electrode have only a minor effect.