A portable diagnostic system for the quantification of VUV fluxes emitted from low-temperature plasmas

Abstract

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.

Absolute measurement of vacuum ultraviolet fluxes from inductively coupled plasmas via metal surface photoemission currents

“…As with the plate irradiances, the source radiance was estimated for varying RF powers and H 2 pressures, as shown in figure 6. In all cases, the RF power to VUV conversion ratio was in the range of 12%-16% which is within the typical range of 10%-30% measured in the literature [7][8][9].…”

“…Measurement of wavelengths lower than this cutoff requires spectrometer optics to be exposed directly to ions and electrons from the plasma source, the use of a biased shield or a long line of sight. Many secondary measurement techniques exist, such as the use of phosphor coatings to convert VUV to visible wavelengths [7], calibrated VUV diodes [8] or thin Au films [9].…”

Determining absolute VUV fluxes for assessing the relevance of photon-surface interaction in ion sources

“…In this contribution, a VUV diode system with a combination of edge and interference filters [2] is presented. Compared to the solution presented in [1], the present device is absolutely calibrated against a VUV spectrometer that itself is absolutely calibrated down to 46 nm [3].…”

“…Five interference and five edge filters are available with which the prominent emission systems of a variety of discharge gases can be selected. For details it is referred to [2], where the filter selection is described specifically for Ar, H 2 , N 2 , O 2 and mixtures thereof. Figure 1 (b) shows a spectrum in the VUV spectral range of a hydrogen discharge obtained at a planar ICP setup cm diameter, 10 cm height, 2 MHz, 2 kW max.…”

“…Accordingly, absolute calibration of the detected emission is performed by relating it to the absolutely calibrated spectrum from the VUV spectrometer integrated within the transmission range of the respective filter. Like this, calibration values have been obtained for the commonly observed emission systems of the above mentioned gases and a thorough benchmarking was conducted [2]. An accuracy of better than 25 % compared to the VUV spectrometer is attested and a dynamic range of more than four orders of magnitude is accessible for each filter.…”