Reaction chemistry in reactive microplasma jets
Final Report Abstract
The main achievements are summarised in the following: The MBMS sampling system for neutrals has been optimised for the analysis of depositing plasmas (the sampling orifice can be changed and aligned without breaking the vacuum in the mass spectrometer). The plasma chemistry in the He/HMDSO/O2 plasma is now better understood based on a combined analysis with MBMS with rotating substrate holder experiments. Surface reactions have been identified as a main removal mechanism of carbon from the layer. These are oxidation reactions in He plasmas, where addition of O2 molecules is necessary. This is confirmed by a fluid model of a He/HMDSO/O2 plasma. A ratio of 30 – 50 oxygen atoms to deposition radical was determined, to obtain a carbon free surface. However, we could show that active species generated in an Ar plasma without admixture of O2 (probably argon metastables, ions or impurities) are also capable of removing carbon from the growing film. The MBMBS sampling system for ions has been implemented and the contribution of ions to the effluent of a microplasma jet has been quantified. A detailed numerical analysis of gas expansion into a region of low pressure has been carried out by a kinetic particle simulation while the results were used for the subsequent determination of the ion energy and transmission by trajectory calculations in the system. The determination of the ion energy distribution function by numerical and experimental methods reveals a mean ion energy of the order of 0.1 eV. This indicates that the analysed ions are originating from the plasma rather than being a product of a secondary discharge triggered by the APP within the first pumping stage of the sampling system. Thus, ion densities in the effluent of a micro-scaled APP jet (μ-APPJ) are estimated to be of the order of 10^15 m^−3 in good agreement with the expectations from corresponding numerical simulations of the plasma chemistry. The ion composition of non-thermal APPs is heavily influenced by impurities which typically leads to the formation of water cluster ions of the form H+(H2O)n. This finding from previous studies could be confirmed in this work by investigation of different APP sources, such as the μ-APPJ, a plasma jet based on a dielectric barrier discharge (DBD-jet, or ’bullet jet’) and a static negative corona discharge. The high abundance of H+(H2O)n is reasonable due to the inherent presence of water as a polar gas at atmospheric pressure in combination with a large number of collisions between primary ions and neutrals before sampling. Even a small amount of remaining impurities with an estimated concentration of the order of 0.1 ppm leads to significant formation of NO+ by charge transfer reactions due to the low ionisation energy of NO even in the highly purified gas. The contribution of ions is typically in the ppm range and they are less relevant to any surface process in comparison to the metastable species impinging on the surfaces. Depending on the impurity level of the microplasma source, water ion clusters may dominate as the main positive ions in the effluent of the plasma. These ions originate in secondary reactions (Penning ionisation, photoionisation). He+ ions or He2+ in He plasma or Ar+ or Ar2+ ions in Ar plasma can only be detected, when the precursors gas is purified to avoid quenching of the metastable species and charge transfer reactions.
Publications
- Phase Resolved Optical Emission Spectroscopy of Coaxial Microplasma Jet Operated with He and Ar, Eur. J. Phys. 60, 539 (2010)
J. Benedikt, S. Hofmann, N. Knake, H. Böttner, R. Reuter, A. von Keudell, and V. Schulz-von der Gathen
(See online at https://doi.org/10.1140/epjd/e2010-00246-9) - Surface reactions as carbon removal mechanism in deposition of silicon dioxide films at atmospheric pressure, Appl. Phys. Lett. 98, 111502 (2011)
R. Reuter, D. Ellerweg, A. von Keudell, and J. Benedikt
(See online at https://doi.org/10.1063/1.3565965) - Quadrupole Mass Spectrometry of Reactive Plasmas (Review), J. Phys. D: Appl. Phys. 45 (2012) 403001
J. Benedikt, A. Hecimovic, D. Ellerweg, and A. von Keudell
(See online at https://doi.org/10.1088/0022-3727/45/40/403001) - The Role of Oxygen and Surface Reactions in the Deposition of Silicon Oxide like Films from HMDSO at Atmospheric Pressure, Plasma Processes and Polymers (2012)
R. Reuter, K. Rügner, D. Ellerweg, T. de los Arcos, A. von Keudell, and J Benedikt
(See online at https://doi.org/10.1002/ppap.201100146) - Unexpected O and O3 production in the effluent of He/O2 microplasma jets emanating into ambient air Plasma Sources Science and Technol. 21, 034019 (2012)
D. Ellerweg, A. von Keudell, and J. Benedikt
(See online at https://doi.org/10.1088/0963-0252/21/3/034019) - Insight into the Reaction Scheme of SiO2 Film Deposition at Atmospheric Pressure Plasma Processes and Polymers 10, 1061-1073 (2013)
K. Rügner, R. Reuter, D. Ellerweg, A. von Keudell, and J. Benedikt
(See online at https://doi.org/10.1002/ppap.201300059) - Mass pectrometry of positive ions and neutral species in the effluent of an atmospheric pressure plasma with hexamethyldisiloxane and oxygen, J. Phys D. 46 (2013) 464017
J. Benedikt, D. Ellerweg, S. Schneider, K. Rügner, R. Reuter, H. Kersten, and T. Benter
(See online at https://doi.org/10.1088/0022-3727/46/46/464017) - Mapping of properties of thin plasma jet films using imaging spectroscopic reflectometry, Meas. Sci. Technol. 25 (2014) 115201
D. Nečas, V. Čudek, J. Vod’ak, M. Ohlídal, P. Klapetek, J. Benedikt, K. Rügner, and L. Zajíćková
(See online at https://doi.org/10.1088/0957-0233/25/11/115201) - The effect of surface reactions of O, O3 and N on film properties during the growth of silica-like films J. Phys. D: Appl. Phys. 47 224005 (2014)
K Rügner, R Reuter, A von Keudell, and J Benedikt
(See online at https://doi.org/10.1088/0022-3727/47/22/224005) - Mass spectrometry of atmospheric pressure plasmas Plasma Sources Science and Technology 24, 044008 (2015)
S. Große-Kreul, S. Hübner, S. Schneider, D. Ellerweg, A. von Keudell, S. Matejcik, and J. Benedikt
(See online at https://doi.org/10.1088/0963-0252/24/4/044008)
