Quantification of VUV photon and particle fluxes in low pressure plasmas for surface treatment
Final Report Abstract
Surfaces can be treated with low-temperature plasmas in order to change their specific characteristics. In this plasma-surface-interaction, particles – i.e. molecules, atoms and their ions – as well as radiation from the plasma play an important role. For both cases the energy and the flux (impinging particles per area and time) determine the final impact on the surface and thus, their quantification is a major prerequisite for optimization of the applied plasma in view of a particular surface treatment. For radiation in the wavelength range below 200 nm, i.e. in the vacuum-ultraviolet region (VUV), this quantification is connected to a huge effort, since specialized vacuum spectrometers are required which need a direct port to the plasma vessel and whose calibration procedure is quite complex. Hence, quantified absolute fluxes of VUV photons in process plasmas was previously limited to specific purposes at scientific experiments and an easy-to-use VUV diagnostic is highly desired. In this work, a systematic study on neutral, ion and photon fluxes emerging from low-temperature plasmas of typically applied gases is performed. For this, a high-resolution vacuum spectrometer was calibrated in-house down to 46 nm, which makes energy-resolved absolute photon flux measurements for photon energies of up to 27 eV accessible. The obtained measurements showed, how the ratio between ion and photon fluxes can be tuned by a variation of discharge pressure and applied RF power. As a general trend, the flux ratio of VUV photons to ions is low at high pressures and is higher in discharges containing hydrogen. Consequently, the highest photon/ion ratio for photons with energies larger than 6 eV (VUV range) was determined in pure hydrogen plasmas, with photon fluxes exceeding the ion flux by a factor of more than three at 3 Pa discharge pressure and 700 W RF power (Lyman series, Werner and Lyman band). The respective lowest flux ratio of below 1 % is measured in argon at 10 Pa and 300 W (resonant atomic lines). The highest photon energy emerges in oxygen discharges: emission lines with energies up to 23.4 eV (53 nm) were detected (resonant oxygen ion lines). As a second objective, a portable and easy-to-use measurement device for VUV radiation was developed, based on a VUV-sensitive photodiode. Band pass or long pass filters are applied to obtain a certain spectral and thus energy resolution of the detected radiation. The determined spectral composition during the systematic studies was used to identify the dominant emission ranges and thus to define the required set of filters, resulting in five band pass and five long pass filters. The device is calibrated for absolute flux measurements against the VUV spectrometer and it was thoroughly characterized. The dynamic range is more than four orders of magnitude and short-term stability as well as long-time reproducibility was better than ± 3 %. Furthermore, no signs of severe degradation due to the VUV irradation was recognized during the course of the project. Benchmark measurements between the calibrated VUV diode system and the VUV spectrometer showed a very good agreement and deviations were typically limited to below 25 %. With the developed VUV device, measurements at three different external setups were performed, demonstrating its portability and applicability. The device thus proved to be a low-cost and easy-to-use alternative to sophisticated vacuum spectrometers with their extensive efforts regarding space and calibration. Next step is to get in touch with industrial R&D to determine application limitations. The vision is to open a window for easy-touse VUV spectroscopy, either as monitoring tool in process chains or as quantitative diagnostic at specific devices – both in context of identifying the relevance of photons in plasma surface interaction processing.
Publications
- Absolute measurements of UV/VUV photon fluxes with a portable diode system in low pressure plasmas, 24th Europhysics Conference on Atomic and Molecular Physics of Ionized Gases (ESCAMPIG XXIV), Glasgow, Scotland, July 17-21, 2018
C. Fröhler, R. Friedl, S. Briefi, and U. Fantz
- Characterization of a portable measurement device for the determination of absolute VUV emission of low pressure plasmas, DPG-Frühjahrstagung, Erlangen, 04.03. - 09.03.2018
C. Fröhler, R. Friedl, S. Briefi, and U. Fantz
- A portable diode system for the quantification of absolute VUV/UV photon fluxes in low pressure plasmas, 24th International Symposium on Plasma Chemistry (ISPC 24), Naples, Italy, June 9-14, 2019
C. Fröhler, R. Friedl, S. Briefi, and U. Fantz
- Measurements of VUV/UV photon fluxes in planar ICP discharges at low pressure, DPG-Frühjahrstagung, München, 17.03. - 22.03.2019
C. Fröhler, R. Friedl, S. Briefi, and U. Fantz
- The role of photon self-absorption on the H (n = 2) density determination by means of VUV emission spectroscopy and TDLAS in low pressure plasmas, Plasma Sources Sci. Technol. (2020)
F. Merk, R. Friedl, S. Briefi, C. Fröhler-Bachus, and U. Fantz
(See online at https://doi.org/10.1088/1361-6595/abc93d) - Absolute radiometric calibration of a VUV spectrometer in the wavelength range 46-300 nm, Journal of Quantitative Spectroscopy and Radiative Transfer 259 (2021) 107427
C. Fröhler-Bachus, R. Friedl, S. Briefi, and U. Fantz
(See online at https://doi.org/10.1016/j.jqsrt.2020.107427)
