Non-invasive plasma diagnostics using oxygen optical emissions

The optimization and control of technological plasmas is facilitated by measurements of plasma properties. Non-invasive diagnostics based on optical emission spectroscopy (OES) rely on the prediction of recorded spectral features, using an emission model accounting for processes related to the excitation and de-excitation of photon-emitting species. Here we implement an emission model for low-pressure oxygen plasmas. We have measured the emission spectra of low pressure oxygen inductively coupled plasmas in the 350-900~nm wavelength range, yielding both atomic~O lines and a few O$_2^+$ molecular ion bands. The model takes the intensities as inputs, returning values of dissociation fraction, and electron density and temperature, which we benchmark against O$_2$/Ar actinometry and Langmuir probe measurements, respectively. Trends (pressure: 1-30 mTorr, RF power: 100-2000 W) reveal that the O$_2^+$ emissions have a complicated dependence on the electron density. The dominant excitation mechanism shifts from simultaneous ionization-excitation of O$_2$ molecules at very low plasma densities to excitation of ground state O$_2^+$ ions at higher densities, for which it is also found that electron-induced quenching suppresses emissions from long-lived radiative levels.