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The role of photon self-absorption on the H (n= 2) density determination by means of VUV emission spectroscopy and TDLAS in low pressure plasmasTo cite this article before publication: Frederik Merk et al 2020 Plasma Sources Sci.

Vacuum-Ultraviolet (VUV) spectroscopy is linked to a huge effort, in particular if absolute numbers are required. To overcome this, a flexible device is developed based on a photodiode and optical filters for wavelength selection. Characterization of the diagnostic is performed against a VUV spectrometer that is radiometrically calibrated down to 46 nm. In the first instance, the latter is used for an analysis of VUV spectra of inductively coupled low-temperature plasmas for a variety of discharge gases and mixtures. The measured photon fluxes can easily have comparable magnitude as the occurring ion flux, and it is demonstrated that the photonic energy distribution can change drastically with varying external parameters (pressure, RF power). This highlights the need for energy resolution for VUV flux detection and a set of ten optical filters was selected for the VUV diode system according to the respective prominent emission ranges of typical discharge gases for plasma processing. Recommended filter combinations for each of the discharge gases and mixtures are given. The developed diagnostic is calibrated in-house against the VUV spectrometer, which makes energy-resolved absolute VUV flux measurements up to photon energies of 27 eV possible. The calibration is performed individually for all the investigated discharge gases and the corresponding filter combinations, and an accuracy of better than 25 % compared to the VUV spectrometer is attested for the investigated parameter space (0.3-10 Pa, 200-1100 W RF power). Its applicability is demonstrated by measuring VUV fluxes at two further low-temperature plasma setups.